zno nanoparticles phd thesis

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Synthesis of ZnO nanoparticles by two different methods & comparison of their structural, antibacterial, photocatalytic and optical properties

Md Jahidul Haque 1 , Md Masum Bellah 1 , Md Rakibu Hassan 1 and Suhanur Rahman 1

Published 16 March 2020 • © 2020 The Author(s). Published by IOP Publishing Ltd Nano Express , Volume 1 , Number 1 Citation Md Jahidul Haque et al 2020 Nano Ex. 1 010007 DOI 10.1088/2632-959X/ab7a43

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1 Department of Glass & Ceramic Engineering, Rajshahi University of Engineering & Technology (RUET), Rajshahi-6204, Bangladesh

Md Jahidul Haque https://orcid.org/0000-0001-7945-5937

• Received 23 December 2019
• Revised 3 February 2020
• Accepted 26 February 2020
• Published 16 March 2020

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Method : Single-anonymous Revisions: 1 Screened for originality? Yes

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In this work, two different methods (sol-gel and biosynthesis) were adopted for the synthesis of zinc oxide (ZnO) nanoparticles. The leaf extract of Azadirachta Indica (Neem) was utilized in the biosynthesis scheme. Structural, antibacterial, photocatalytic and optical performances of the two variants were analyzed. Both variants demonstrated a wurtzite hexagonal structure. The biosynthesized variant (25.97 nm) exhibited smaller particles than that of the sol-gel variant (33.20 nm). The morphological analysis revealed that most of the particles of the sol-gel variant remained within the range of 15 nm to 68 nm while for the biosynthesized variant the range was 10–70 nm. The antibacterial assessment was redacted by using the agar well diffusion method in which the bacteria medium was Escherichia coli O157: H7. The zone of inhibition of bacterial growth was higher in the biosynthesized variant (14.5 mm). The photocatalytic performances of the nanoparticles were determined through the degradation of methylene blue dye in which the biosynthesized variant provided better performance. The electron spin resonance (EPR) analysis revealed that the free OH · radicals were the primary active species for this degradation phenomenon. The absorption band of the sol-gel and biosynthesized variants were 363 nm and 356 nm respectively. The optical band gap energy of the biosynthesized variant (3.25 eV) was slightly higher than that of the sol-gel variant (3.23 eV). Nevertheless, the improved antibacterial and photocatalytic responses of the biosynthesized variants were obtained due to the higher rate of stabilization mechanism of the nanoparticles by the organic chemicals (terpenoids) present in the Neem leaf extract.

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1. Introduction

As a rapidly growing sector in materials science, nanotechnology and nanoscience deal with materials that have particles within a size range of 1 to 100 nm and a high surface-to-volume ratio [ 1 ]. In general form, these particles are termed as nanoparticles (NPs) which exhibit highly controllable physical, chemical and biological properties in the atomic and sub-atomic levels. However, these unique features create opportunities to use them in different sectors such as electronics, optoelectronics, agriculture, communications, and biomedicine [ 2 , 3 ].

Although, several NPs are showing their effectiveness in different sectors of technology, but zinc oxide (ZnO) NPs have gained much more importance in the recent years due to their attractive and outstanding properties such as high chemical stability, high photostability, high electrochemical coupling coefficient and a wide range of radiation absorption [ 4 ]. Again, ZnO NPs are also recognized as n-type multi-functional semiconductor materials that have a wide band gap of 3.37 eV and exciton binding energy up to 60 meV even at room temperature [ 1 ]. Nowadays, ZnO NPs are predominantly used as antimicrobial agents, delivering systems vaccines and anti-cancer systems, photocatalyst, biosensors, energy generators and bio-imaging materials [ 5 – 7 ]. Among themselves, the photocatalytic application of ZnO NPs is significant. However, the photocatalytic performance of ZnO NPs can be significantly enhanced by adopting two ways. The first one involves the reduction of particle sizes by using efficient synthesis methods, while the second one involves the change of structural morphology by the incorporation of several elements (such as metal, non-metal, noble metal, transition metal, etc) into the crystal structure of ZnO NPs. However, in this work, we will proceed by adopting the first one.

Several fabrication techniques are used to produce ZnO NPs such as thermal hydrolysis techniques, hydrothermal processing, sol-gel method, vapor condensation method, spray pyrolysis and thermochemical techniques [ 8 ]. Nevertheless, recently a new synthesis method has been introduced and that is called biosynthesis scheme in which the NPs are prepared by using biological materials having significant reducing and stabilizing features. Moreover, NPs with variable size and shape can be achieved through this process.

Researchers proposed several possible plant extracts and fungal biomasses that were used in the green synthesis of ZnO NPs such as Aloe Barbadensis Miller (Aloe Vera) leaf extract [ 9 ], Poncirus trifoliate leaf extract [ 10 ], Parthenium hysterophorus L. (Carrot grass) leaf extract [ 11 ], Aspergillus aeneus [ 12 ], Calotropis procera latex [ 13 ], Sedum alfredii Hance [ 14 ], Physalis alkekengi L. [ 15 ], etc. However, the smaller particle size of ZnO NPs was observed by using Poncirus trifoliate leaf extract (8.48–32.51 nm), while for others, the results were satisfactory. In addition, another potential element for the preparation of ZnO NPs through the biosynthesis method is considered to be a leaf extract of Azadirachta indica (Neem leaf). The leaf extract contains highly active phytochemicals and enzymes that participate in the oxidation or reduction reactions that occur during the fabrication method and manipulate the bulk ZnO to convert into ZnO NPs [ 16 ]. Moreover, Neem leaf provides significant biological restrictions against bacterial growth and fungal growth [ 17 ].

The present study focused on the preparation of ZnO NPs by two different methods. The first one is the sol-gel method, while the second one is the biosynthesis method in which the Neem leaf extract was used as a mandatory element. A comparison of the properties (structural, antibacterial, photocatalytic and optical) between the two variants of ZnO NPs was performed. Here, the sol-gel synthesized and biosynthesized ZnO nanoparticles were nominated as ZnO A NPs and ZnO B NPs respectively.

2. Methodology

2.1. materials.

All the starting raw materials including zinc acetate dihydrate [Zn(CH 3 COO) 2 .2H 2 O, Merck Specialties, India], sodium hydroxide [NaOH, Merck Specialties, India] and absolute ethanol [CH 3 CH 2 OH, Merck Specialties, Germany) were maintained at a high purity level (>99%). However, in the biosynthesis method, another raw material was also used and that was the leaf of Azadirachta indica (Neem leaf).

2.2. Synthesis of ZnO nanoparticles (ZnO A NPs) by sol-gel method

At first, 20 gm Zn(CH 3 COO) 2 .2H 2 O was mixed into 150 ml distilled water and stirred for 20 min at 35 °C to produce a zinc acetate solution. Again, 80 gm NaOH powder was weighed, mixed into 80 ml water and stirred for around 20 min at 35 °C for producing NaOH solution. After mixing both solutions, the titration reaction was performed by the addition of 100 ml ethanol into the drop-wise manner accompanied by vigorous stirring. The stirring was continued for around 90 min to complete the reaction for obtaining a gel-like product. Then the gel was dried at 80 °C overnight and calcined in an oven at 250 °C for 4 h. Finally, ZnO nanoparticles were prepared. However, the overall chemical reaction for the preparation of ZnO nanoparticles by using NaOH can be expressed as:

2.3. Synthesis of ZnO nanoparticles (ZnO B NPs) by biosynthesis method

At first, the neem (A. Indica) leaves were collected from the Azadirachta Indica trees on the campus of Rajshahi University of Engineering and Technology, Bangladesh. After washing with distilled water, the leaves were dried into a dryer for 24 h. Then 20 gm dried leaves were smashed and mixed with 50 ml distilled water. After that, the mixture was stirred by a magnetic stirrer and heated at 60 °C for 1 h. As the mixture displayed a yellow color, it was filtered using the Whatman TM filter paper. However, the extract solution was used for further preparation of ZnO nanoparticles. The overall process for the preparation of Neem leaf extract is stereotyped in figure 1 .

Figure 1.  Process flow diagram for the preparation of Neem leaf extract.

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The next step included the preparation of the zinc acetate solution. For this, 21.94 gm Zn(CH 3 COO) 2 .2H 2 O was mixed into 50 ml water and stirred for 20 min at 35 °C. Similarly, in order to prepare a NaOH solution, 4 gm NaOH powder was added into 50 ml distilled water and simultaneously stirred for 20 min at 35 °C. Both solutions were then mixed by vigorous stirring. During this stirring process, the neem leaf extract was drop-wise mixed with the solution. As the addition of neem leaf continued, white precipitation of nanoparticles appeared. Then the solution was filtered and the filtered product was dried at 80 °C for 4 h. After that, the dried powder was calcined at 250 °C for 4 h and grounded to obtain the desired ZnO nanoparticles.

2.4. Characterization of ZnO NPs

X-ray diffraction was performed for structural analysis employing 40 kV-40 ma (scanning step of 0.02°) and Cu- K α radiation having wavelengths of K α 1  = 1.54060 Å, K α 2  = 1.54439 Å (Bruker Advance D8, Germany). Morphological characterization was accomplished by scanning electron microscopy (ZEISS EVO 18, UK). The optical properties were determined through UV–vis spectroscopy (SHIMADZU UV/Vis-1650 PC, Japan) into a range of 200–800 nm.

2.5. Antibacterial analysis of ZnO NPs

Escherichia coli bacteria were mainly involved in the determination of the antibacterial performance of ZnO NPs. Initially, the bacteria was stock-cultured in brain heart infusion (BHI) growth medium at −20 °C. Around 3 ml of BHI broth was added to 300 ml of stock-culture and preserved the culture overnight at 36 °C ± 1 °C for 24 h. After 24 h of incubation, dilution of the bacterial suspension (inoculum) was accomplished by using sterile saline. To indicate the bacterial growth during the test, a solution of 2-(4-iodophenyl)−3-(4-nitrophenyl)−5-phenyltetrazolium chloride (INT) in ethanol was added to the bacterial inoculum. Then the inoculum was distributed on a Mueller Hinton Agar Petri Dish in a consistent manner. After that, ZnO A NPs and ZnO B NPs were placed into the wells (prepared by cutting the agar gel) and the systems were preserved at 36 °C ± 1 °C for 24 h to allow successive incubation. After 24 h, the growth of bacteria was monitored and finally, the zone of inhibition for bacterial growth was determined in mm scale.

2.6. Photocatalytic analysis of ZnO NPs

The photocatalytic analysis was performed by monitoring the degradation of Methylene Blue (MB) dye due to ZnO NPs under the influence of UV radiation (having intensity ∼120 μ W cm −2 and wavelength ∼300–400 nm). At first, 5 gm NPs were added into MB solution and mixed properly. The mixture was placed in the dark for 2 h and then irradiated with UV rays with subsequent stirring action and at a variation of time (0, 40, 80, 120, 160, 200 min). The absorbance of the mixture was measured by UV–vis spectroscopy (SHIMADZU UV/Vis-1650 PC, Japan). The efficiency of photodegradation was measured by the following equation:

Where C 0 is the absorption of MB solution before the addition of ZnO NPs and C 1 is the absorption of the mixture solution with respect to time t.

ESR (electron spin resonance) analysis was performed using the EPR spectrometer (Bruker EMX MicroX, Germany) for the identification of the major factor that provides effective photocatalytic performance. During this characterization, DMPO (5,5-dimethyl-1-pyrroline-N-oxide) was used as a spin-trapped reagent in methanol and aqueous state. Moreover, the analysis was performed both in the presence and absence of light irradiation.

3. Results and discussion

3.1. effect analysis of neem leaf extract.

Neem leaf extract contains various phytochemicals such as flavones, quinines, organic acids, aldehyde and ketones which act as reducing agents and significantly reduces the particle sizes. After the successive reduction of particle sizes, the NPs are also affected by the terpenoids. Because of the interaction between the terpenoids and the ZnO NPs become stabilized as terpenoids are effective capping and stabilizing agents. The corresponding mechanism is graphically abstracted into figure 2 . Moreover, the possible seven types of terpenoids that are present in Neem leaf extract are stereotyped in figure 3 .

Figure 2.  Schematic representation of the mechanism of size reduction and stabilization of ZnO NPs during the biosynthesis fabrication scheme using Neem leaf extract.

Figure 3.  Chemical structures of different types of terpenoids subsisting in the Neem leaf extract.

3.2. X-ray diffraction analysis

Figure 4 represents the corresponding X-ray diffraction patterns of ZnO nanoparticles synthesized by sol-gel and bio-synthesis schemes respectively. The intense peaks at the crystal faces (100), (002), (101), (102), (110) assure the emergence of hexagonal wurtzite structure (as shown in figure 5 ) which belong to the space group of P6 3mc (JCPDS card no. 36-1451) [ 18 ]. The bio-synthesized ZnO nano-particles show more acute diffraction peak value introducing the appearance of the high percentage of crystalline phases. In addition, no impurity phases are present in the samples.

Figure 4.  XRD patterns of ZnO A and ZnO B NPs.

Figure 5.  Schematic wurtzite crystal structure of ZnO NPs.

However, considering the most severe diffraction peak (101), the crystallite size (D) can be calculated in accordance with the Debye Scherer formula [ 19 ]:

Hither, β is the Full Width at Half Maxima of the corresponding peak, k is a dimensionless shape factor (∼0.94), while λ is the wavelength of Cu K α radiation (1.54 Å) and ϴ is the Bragg angle. D is mainly the mean size of the ordered domains which is considered to be equal to the particle size (applicable for only particles less than 100 nm). So, the average particle size of ZnO A NPs and ZnO B NPs is 33.20 nm and 25.95 nm respectively [ 19 ]. Again, there remains an inverse relationship between the β and the D which means that narrower peaks are resulted due to larger particles while broader particles are obtained because of smaller particles. The ZnO NPs showed a good agreement with this statement.

Since the crystallite size can be further employed for the determination of defect concentration within the specimen which is designated as the dislocation density ( δ ) and the leading formulae is adopted for this purpose [ 20 ]:

From the exploration of diffraction data, the lattice constant (a & c), inter-planar spacing (d) and unit cell volume (V) of the specimens (table 1 ) can also be enumerated by utilizing the following formulas respectively [ 21 ]:

Where, h, k, l belong to Miller indices.

Table 1.   Structural information on ZnO A and ZnO B NPs.

Besides, the lengthening of the stricture (L) between Zn and O can be enumerated by the following equation [ 20 ]:

Where u corresponds to parameterized constant belonging to wurtzite structure and can be expressed as:

In accordance with the Williamson-Hall proposition, the lattice strain was calculated by adopting the undermentioned equation [ 20 ]:

Figure 6.  W-H plot of (a) ZnO A NPs and (b) ZnO B NPs for the measurement of lattice strain.

3.3. Morphological analysis

Figures 7 (a) and (b) shows the scanning electron micrographs of ZnO A and ZnO B NPs respectively. From the previous section, we have learned that the average particle size of ZnO B NPs (25.97 nm) is smaller than that of ZnO A NPs (33.20 nm). This can be also caused due to the presence of terpenoids in the Neem leaf extract. The terpenoid act not only as a stabilizing agent but also as a powerful reducing agent that interacts with ZnO NPs and reduces its size significantly [ 8 , 17 ]. Moreover, the maximum particles of ZnO A NPs remain between the range of 15 nm to 68 nm, whereas for ZnO B NPs the range lies from 10 nm to 70 nm.

Figure 7.  SEM micrographs of (a) ZnO A NPs and (b) ZnO B NPs.

3.4. Antibacterial activity

Antibacterial activity of ZnO A NPs and ZnO B NPs was analyzed by adopting the agar well diffusion method using Escherichia coli O157: H7 as the bacterial medium. Generally, there involve three mechanisms behind the interaction between the bacteria and the NPs. The first one involves the formation of extremely active hydroxyls and the second one involves the deposition of NPs on the bacteria surface. In addition, for the last one, the NPs accumulates in the cytoplasm or in the periplasmic region of bacteria cell which disrupts the cellular operations and simultaneously disorganizes the membrane. However, in consideration of E. coli , ZnO NPs firstly disorganize the membrane of E. coli and enters into the cytoplasmic region. Positioning themselves into the cytoplasm, the NPs neutralizes the respiratory enzymes and increases the emersion of cytoplasmic contents into the outward direction which impairs the membrane and finally kills the E. coli bacteria resulting in a zone of inhibition of bacterial growth around itself [ 3 , 23 ].

From figure 8 , it is observed that the zone of inhibition of bacterial growth due to ZnO A NPs is different from the zone of inhibition that is caused by ZnO B NPs. However, ZnO B NPs introduce a higher zone of inhibition than ZnO A nanoparticles and the measurements of the inhibition zone of bacterial growth are tabulated in table 2 . According to Krishna R Rangupathi, the antibacterial activity of nanoparticles is a size-dependent property and the property enhances with the reduction of particle size [ 23 ]. As the ZnO B NPs have smaller particle size as well as higher surface area, they show more antibacterial potential than that of ZnO A NPs [ 2 ].

Figure 8.  Antibacterial analysis of ZnO NPs showing the zone of inhibition of the growth of Escherichia coli O157: H7.

Table 2.   Antibacterial measurements of ZnO A NPs and ZnO B NPs.

3.5. Photocatalytic activity

Figure 9.  Degradation mechanism of MB dye by ZnO NPs under the influence of UV irradiation.

However, the corresponding reactions in the photodegradation scheme can be summarized as below [ 24 , 25 ]:

Figure 11 displays the discoloration of MB dye due to the photocatalytic action of ZnO NPs at different times (0, 40 and 120 min). However, figures 12 (a) and (b) illustrates the absorption spectra of MB dye as a function of wavelength under the influence of UV radiation at a variation of time i.e. 0, 40, 80, 120, 160, 200 min. From the graph, it is observed that the absorption rate of MB containing ZnO B NPs decreases more rapidly than that of ZnO A NPs. Moreover, the degradation efficiency ( η ) of ZnO NPs (biosynthesized and sol-gel synthesized) with respect to time is illustrated in figure 13 . The degradation of MB for sol-gel synthesized ZnO are 35.3%, 45.7%, 56.1% 62.4%, 68.9% at 40, 80, 120, 160 and 200 min respectively. Again, the values for biosynthesized ZnO are 36.9%, 47.5%, 62.7%, 72.1%, and 80.2% at 40, 80, 120, 160 and 200 min respectively. So, MB dye degraded more rapidly in the presence of ZnO B NPs backing the reason for smaller particle sizes than that of ZnO A NPs. As the particles become smaller, the active surface area for the photocatalysis increased which results in enhanced degradation of MB [ 26 ]. Moreover, there remain terpenoids in the neem leaf extract which stabilizes the nanoparticles by capping themselves which also causes in the increment of photocatalytic action [ 27 ].

Figure 11.  Visual inspection of the degradation phenomenon of MB dye by ZnO NPs.

Figure 12.  Absorption spectrum of (a) ZnO A NPs and (b) ZnO B NPs as a function of wavelength at 0, 40, 80, 120, 160, 200 min.

Figure 13.  The degradation efficiency of ZnO NPs for methylene blue dye with respect to time.

3.6. Optical analysis

Figures 14 (a) and (b) displays the room temperature absorption spectrum of ZnO nanoparticles fabricated by sol-gel and biosynthesis methods correspondingly. Here, the absorption wavelengths are remaining within the maximum allowable limit of the absorption band of bulk ZnO (∼373 nm). Although the absorption slightly increases up to a wavelength of 363 nm for ZnO A NPs, the maximum incremental value for ZnO B NPs is 356 nm. The slight shift of the absorption peak may be caused due to the variation of particle size and their configuration [ 28 ]. However, this phenomenon results in the presence of a wide range of particle size distribution of ZnO [ 29 ]. Moreover, the redshift of ZnO A NPs compared to ZnO B NPs corresponds to the formation of agglomeration in the specimens significantly. Furthermore, in accordance with Gunanlan Sangeetha et al the shifting of absorption band to the higher wavelength as well as higher energy was associated with the increment of the size of nanoparticles [ 30 ]. Moreover, considering the direct interband transition between the valence band and the conduction band, the absorption band gap energy was measured by adopting the following Tauc's formula [ 31 ]:

Where A is an energy-independent constant, α is the absorption coefficient, h υ is for the photon energy, and E g is the optical band gap energy. The E g of the ZnO NPs was obtained from the ( α h υ ) 2 versus h υ plot (as shown in the inset of figures 14 (a) and (b). Where the extrapolation of the linear segment of the graph to (α h υ ) 2  = 0 provides the value of E g for ZnO NPs. It is observed that the optical band gap energy of ZnO B NPs (3.25 eV) is higher than that of ZnO A NPs (3.23 eV). This incremental phenomenon is mainly attributed to the quantum confinement effect. According to this theory, as the particle size decreases, the electrons in the valence band and the holes in the conduction band confine themselves within a space having a dimension of the de Broglie wavelength. However, this confinement influences the quantization of the energy and the momentum of the corresponding carriers and also enhances the optical transition energy between the valence band and the conduction band resulting in a broad band gap [ 32 ].

Figure 14.  Absorption spectra of (a) ZnO A NPs and (b) ZnO B NPs (inset shows ( α h υ ) 2 versus h υ plot for the determination of band gap energy.

Figure 15 displays the UV visible transmittance spectrum of ZnO A NPs and ZnO B NPs. Here, the transparency of ZnO B NPs is greater than that of ZnO due to the reduced particle size of ZnO B NPs. From the research of Takuya Tsuzuki, it is clear that smaller particles are capable to show higher transparency at the visible range of spectrum [ 33 ]. However, the UV blocking characteristics are almost similar for each of the variants of NPs.

Figure 15.  Typical transmittance spectra of ZnO NPs.

4. Conclusion

In summary, ZnO NPs were synthesized by two different methods i.e., sol-gel and biosynthesis method. The green synthesis of ZnO NPs allows avoiding the toxic chemical agents that are used in the sol-gel method for the size reduction. However, the Neem leaf extract possesses some phytochemicals which not only performs in the reduction of the particle sizes but also provide sufficient stabilization. Although, the average particle size of ZnO B NPs (25.97 nm) was smaller than that of ZnO A NPs (33.20 nm), the optical band gap energy of ZnO B NPs was higher than that of ZnO A NPs due to the quantum confinement effect. In addition, the antibacterial and photocatalytic properties of ZnO B NPs were greater than that of ZnO A NPs. Where, the zone of inhibition of bacterial growth for ZnO B NPs was 14.5 mm and for ZnO A NPs, it was 9.3 mm. Moreover, the degradation efficiency of ZnO B NPs at 200 min was 80% while for ZnO A NPs, the corresponding efficiency was 68%. Again, from the ESR analysis, it was proved that the OH · radicals were the main contributing factor for the degradation of MB dye. So, based on the comparison between the properties of the two variants, it is concluded that the biosynthesis method shows more effectiveness than the sol-gel method for the synthesis of ZnO NPs.

Acknowledgments

The authors are grateful to Rajshahi University of Engineering & Technology (RUET) for providing the opportunity to perform various tests. Special thanks go to Tasmia Zaman, Assistant Professor, Department of Glass & Ceramic Engineering, Rajshahi University of Engineering & Technology, Bangladesh for her cordial assistance.

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• Biochem Biophys Rep
• v.17; 2019 Mar

Synthesis and characterization of zinc oxide nanoparticles by using polyol chemistry for their antimicrobial and antibiofilm activity

Pranjali p. mahamuni.

a Centre for Interdisciplinary Research, D.Y. Patil University, Kolhapur, India

Pooja M. Patil

Maruti j. dhanavade.

b Department of Microbiology, Shivaji University, Kolhapur, India

Manohar V. Badiger

c CSIR, National Chemical Laboratory, Pune, India

Prem G. Shadija

Abhishek c. lokhande.

d Department of Materials Science and Engineering, Chonnam National University, Gwangju, Republic of Korea

Raghvendra A. Bohara

e CURAM, Center for Research in Medical Devices, National University of Ireland Galway, Ireland

Associated Data

The present investigation deals with facile polyol mediated synthesis and characterization of ZnO nanoparticles and their antimicrobial activities against pathogenic microorganisms. The synthesis process was carried out by refluxing zinc acetate precursor in diethylene glycol(DEG) and triethylene glycol(TEG) in the presence and in the absence of sodium acetate for 2 h and 3 h. All synthesized ZnO nanoparticles were characterized by X-ray diffraction (XRD), UV visible spectroscopy (UV), thermogravimetric analysis (TGA), fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy(FESEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) technique. All nanoparticles showed different degree of antibacterial and antibiofilm activity against Gram-positive Staphylococcus aureus (NCIM 2654)and Gram-negative Proteus vulgaris (NCIM 2613). The antibacterial and antibiofilm activity was inversely proportional to the size of the synthesized ZnO nanoparticles. Among all prepared particles, ZnO nanoparticles with least size (~ 15 nm) prepared by refluxing zinc acetate dihydrate in diethylene glycol for 3 h exhibited remarkable antibacterial and antibiofilm activity which may serve as potential alternatives in biomedical application.

• • Synthesis of Zno NPs of different size & shape by tuning polyol/catalyst/reaction time.
• • Shape and size control were possible by varying these parameters.
• • Antibacterial and antibiofilm activity were studied against Staphylococcus aureus and Proteus vulgaris.
• • Comparative study revealed DEG synthesis for 3 h in absence of sodium acetate showed maximum antibacterial/biofilm activity.

1. Introduction

Biofilms are the complex communities of microorganisms attached to any biological or non-biological surface that remain enclosed in self-produced hydrated polymeric matrix [1] , [2] . Microorganisms in biofilm transcribe genes that are different from the genes transcribed by planktonic bacteria [3] . The cells in the biofilm are inherently protected from phagocytosis, develops high resistance to antibiotics which make them difficult to treat [4] , [5] , [6] , [7] . Both Gram-positive and Gram-negative bacteria can form the biofilm on various medical devices such as catheters, prosthetic joints, endotracheal tubes, heart valves, contact lenses and ortho-dental instruments [8] . In this regard, Staphylococcus aureus and Proteus vulgaris are biofilm-forming pathogens on medical implants able to produce severe biofilm-associated infections such as urinary tract infection, musculoskeletal infection and respiratory tract infection [9] . It has been estimated that the maximum bacterial infections treated in hospitals are associated with bacterial biofilm [6] . In fact, the number of implant-associated infections near about 1 million/year in the US alone and their direct medical costs exceed $3 billion annually [10] .

The problem of biofilm-related infections could be resolved by removal of biofilm physically or removal of implants which is not feasible economically. Other methods like use of depolymerase enzyme and the use of bacteriophages could be used to control biofilm formation [11] . Recent reports suggest that several synthesized antimicrobial peptides (AMPs) are able to interact with the membrane through penetration or dissolving the biofilms [12] , [13] . Alternatives to these conventional methods which recommend, recent developments in nanotechnology that have been proven to be an efficient approach to control biofilm formation [14] .

The ability of nanomaterials for biofilm disruption has been reported. For example, Simona and Prodan et al investigated the effect of glycerol iron oxide nanoparticles for biofilm inhibition produced by Pseudomonas aeroginosa [15] . Among nanosized metal oxides, zinc oxide (ZnO) has gained much more attention due to its interesting properties such as high surface to volume ratio, low cost and long-term environmental stability [16] , [17] . According to Sirelkhatim et al. and Dhillo et al., it is already reported by several studies that ZnO nanoparticles are non-toxic to human cells and toxic to bacterial cells. Toxicity studies showed that DNA in human cells do not get damaged by zinc ions. This fact made ZnO nanoparticles biocompatible to human cells [16] , [18] , [19] .

Various methods have been used to prepare zinc oxide nanoparticles suchas hydrothermal [20] , [21] , [22] , [23] , solvothermal methods [24] , [25] ,microemulsion [26] , sol-gel [27] , [28] and thermal decomposition of precursors [29] , [30] .

According to Raghupathi et al. and Applerot et al., ZnO nanoparticles exhibit a maximum degree of antibacterial activity with the decrease in particle size [7] , [31] . Method of synthesis of nanoparticles strongly affects the size and shape of nanoparticles, which determines the properties of nanoparticles [32] , [33] .

Fievet, Lagier, and Figlarz first introduced the use of polyols for the synthesis of small particles termed as “polyol process” or “polyol synthesis.” The polyol synthesis allows the formation of ZnO nanoparticles with excellent crystalline quality and controlled morphology. Its peculiarity lies in the properties of polyols like high boiling point (up to 320 °C), high dielectric constant, the solubility of simple metal salt precursors and coordinating properties for surface functionalisation preventing agglomeration [34] , [35] . Also, the presence of weak base sodium acetate in the reaction controls the nucleation process and assembly process through which nanoparticles with different morphology can be obtained [36] .

In the present investigation, we have synthesized ZnO nanoparticles by applying different approaches, (i) regular synthesis in polyols, (ii) in presence of sodium acetate, (iii) increasing reaction time. We have employed different strategies to synthesize ZnO nanoparticles. The synthesis method mainly involves reflux of zinc acetate dihydrate precursor in diethylene glycol (DEG) and triethylene glycol (TEG) in the presence and in absence of weak base sodium acetate for varied reaction time. The effect of these two polyols, presence and absence of sodium acetate and reaction time on size and morphology of synthesized ZnO nanoparticles is presented. These nanoparticles were studied for their antimicrobial and antibiofilm activity against Staphylococcus aureus (NCIM 2654) and Proteus vulgaris (NCIM 2813).

2. Materials and methods

2.1. materials.

All chemicals used here were of analytical grade and used without further purification. All chemicals were purchased from Loba fine chemicals, Mumbai, India. The media have been procured from Himedia Laboratories Pvt. Ltd, Mumbai, India. Distilled water was used in the all experiments. The microorganisms, Gram-positive ( Staphylococcus aureus NCIM 2654) and Gram-negative ( Proteus vulgaris NCIM 2613) were collected from the National Collection of Industrial Microorganisms (NCIM), Pune, India.

2.2. Synthesis of ZnO nanoparticles

ZnO nanoparticles were prepared by refluxing precursor zinc acetate dihydrate (0.1 M) in diethylene glycol and triethylene glycol at 180 °C and 220 °C respectively. Reaction time varied for 2 and 3 h with and without sodium acetate (0.01 M). Before refluxing, the solution was kept on a magnetic stirrer at 80 °C for 1.5 h. After completion of reflux action, the samples were centrifuged at 8000 rpm for 15 min and washed with distilled water and ethanol for three times. Further, it was dried at 80 °C for overnight ( Table 1 , Table 2 ).

TGA results of ZnO samples (1) DEG 2 h, (2) DEG 2 h with sodium acetate, (3) DEG 3 h, (4) DEG 3 h with sodium acetate, (5) TEG 2 h, (6) TEG 2 h with sodium acetate, (7) TEG 3 h, (8) TEG 3 h with sodium acetate.

% weight loss and remaining residue for all ZnO samples are given in Table 3 . From table listed above it was observed that, DEG 3 h(3) and TEG 3 h with sodium acetate (8) shows minimum weight loss and maximum final residue.

Reaction conditions used for synthesis of Zinc oxide nanoparticles.

Calculated crystallite size of ZnO NPs are listed below.

Where, D = crystallite size, λ = X-ray wavelength, β = FWHM of diffraction peak and θ = .

angle of diffraction.

2.3. Reaction mechanism of ZnO formation

By considering the chemicals involved in the hydrolysis process, the mechanism of the ZnO nanoparticles formation is proposed as follows.

Formation of metal oxides proceeds in 2 steps: hydrolysis reaction and condensation reaction. Hydrolysis reaction is water dependent, absence of water in the reaction leads into failure of occurrence of next step of reaction that is condensation reaction which will not form any product. Also, due to presence of excess amount of water, particles start to agglomerate and give large sized particles with large distribution. So the hydrolysis ratio is considered as an important factor which affects the size and morphology. ( Scheme 1 ).

Schematic representation of synthesis of DEG and TEG mediated ZnO nanoparticles.

Hydrolysis ratio is the ratio of number of moles of metal ions to number of moles of water. Alkaline ratio also considered an important factor affecting size and morphology. Amel Dalklaoui et al reported the effect of increasing alkaline ratio on morphology which showed the change in morphology from irregular and anisotropic forms to spherical form. Alkaline ratio is the number of moles of sodium hydroxide to metal which is attributed to the effect of OH - ions on morphology. Also the concentration of precursor and temperature of the reaction affects the morphology of particles. In the present investigation, concentration of precursor, hydrolysis ratio and alkaline ratio is kept constant throughout the all synthesis processes of ZnO.

First, the reaction between zinc acetate dihydrate and DEG/TEG leads to esterification that forms (Zn-OH) 2 . Further dehydration of (Zn-OH) 2 results into formation of ZnO nanoparticles. The basic approach for addition of sodium acetate was the addition of excess acetate ions that gives different particle morphologies than the particles synthesized in absence of sodium acetate. Sodium acetate causes a weak hydrolyzation, which controls the release rate of OH − [36] , [37] , [38] , [39] , [40] , [41] , [42] .

2.4. Characterization of nanoparticles

The X-ray diffraction studies of ZnO NPs were carried out using Rigaku 600Miniflex X-ray diffraction instrument (XRD) with Cukα radiation (λ = 1.5412 Å) in the scanning range of 10 0 -80 0 . To confirm the absorbance of ZnO NPs and to observe the changes in the absorbance caused due to variations in reaction conditions, UV–visible (UV–vis) spectra were carried in the wavelength range of 200–600 nm using Agilent Technologies Cary 60 UV–vis. In order to identify the characteristic functional groups present on the surface of the ZnO, Fourier transform infrared (FTIR) spectra of all samples were recorded by using JASCO INC 410,Japan,in a range of 400–4000 cm −1 . Thermal gravimetric analysis(TGA) was carried out to observe thermal stability of ZnO on instrument PerkinElmer STA-5000. All samples were heated from 50 to 900 °C at the rate of 10 °C/min. The surface morphology of all synthesized ZnO were studied by field emission scanning electron microscopy(FESEM) and transmission electron microscopy(TEM). Elemental analysis was performed by energy dispersive X-ray (EDX) spectroscopy (JSM-6701F, JOEL, Japan).

2.5. The antimicrobial assay

Antimicrobial study of different ZnO NPs was performed by agar well diffusion method. The relative activities of these samples were studied against both Gram-positive Staphylococcus aureus (NCIM 2654) and Gram-negative Proteus vulgaris (NCIM 2613) bacteria. In this method, in each well 1 mg/ml concentration of all ZnO NPs was inoculated on nutrient agar plates which were previously seeded by 100 µl of 24 h old bacterial inocula. ZnO samples were sonicated for 15 min in distilled water before inoculation. Then the plates were incubated at 37 °C for 24 h for the growth of microorganisms. Antimicrobial activity was observed by measuring the inhibition zone diameter (mm).

2.6. Determination of minimum inhibitory concentration

The determination of minimum inhibitory concentration was performed in sterile Muller –Hinton broth at concentration of nanoparticles ranging from 10 mg to 50 mg/ml against two pathogens Gram positive Staphylococcus aureus (NCIM 2654) and Gram negative Proteus vulgaris(NCIM 2613) bacteria. The assay was carried out in 96 well plates by using tryptic soy broth medium. In brief, 200 µl volume of tryptic soy medium was added in each well and inoculated with 24 h old 10 µl of bacterial inocula. One well was maintained without addition of nanoparticles, used as a control. The microplates were incubated at 37 °C for 24 h. After incubation OD was recorded at 600 nm. From graph, minimum inhibitory concentration and % of inhibition at each concentration was determined.

2.7. Antibiofilm activity

Antibiofilm activity was done by using microtiter plate method. For this, Staphylococcus aureus (NCIM 2654) and Proteus vulgaris (NCIM 2613) were inoculated in sterile tryptic soy broth and incubated for 24 h at 37 °C. Then samples were centrifuged at 5000 rpm and pellet was suspended in phosphate buffer(pH 7.0) 1 mg/ml stock of all ZnO samples were prepared. In brief, 200 µl medium with known concentrations of ZnO were inoculated with 10 µl of bacterial suspension and incubated for 24 h at 37 °C. After incubation, the wells were drained, washed with phosphate buffer saline(PBS),fixed with cold methanol, and then stained with 1% crystal violet for 30 min. Biofilm formed in wells was resuspended in 30% acetic acid. The intensity of suspension was measured at 570 nm and % of biofilm inhibition was calculated by using equation given below [8] .

3. Results and discussion

3.1. x-ray diffraction studies.

Fig. 1 A and B represents diffractograms of ZnO NPS.The XRD of all the samples having 2θ values with reflection planes at 31.72° (100), 34.39° (002), 36.23° (101) and 47.44° (102) corresponds to JCPDS Card No. 36-1451. So,all diffraction peaks fit well with hexagonal wurtzite structure of ZnO, which proves that ZnO was successfully synthesized by polyol hydrolysis method [43] .

(A) XRD of DEG 2 h(a), DEG 2 h with sodium acetate(b), DEG 3 h(c), DEG 3 h with sodium acetate(d), (B) TEG 2 h(a), TEG 2 h with sodium acetate(b), TEG 3 h(c), TEG 3 h with sodium acetate(d).

The crystallite sizes of ZnO NPs were calculated from FWHM of the most intense peak using the Debye–Scherrer formula (Eq. (1) ), given below:

3.2. UV–vis spectroscopy analysis

In order to observe the UV spectroscopy of synthesized ZnO nanoparticles, they were sonicated in distilled water for about 15 min and UV spectra were recorded Supplementary data Fig. 1 A and B shows the UV–vis absorption spectra of the ZnO nanoparticles synthesized by using DEG and TEG. The absorption peak was recorded in each spectrum in range of 360–380 nm which is a characteristic band for the pure ZnO.Absence of any other peak in the spectrum confirms that the synthesized products are ZnO only [17] . ( Fig. 2 , Fig. 3 ).

FESEM micrographs of (a) DEG 2 h, (b) DEG 2 with sodium acetate, (c) DEG 3 h, (d) DEG 3 h with sodium acetate, (e)TEG 2 h, (f) TEG 2 h with sodium acetate, (g) TEG 3 h, (h) TEG 3 h with sodium acetate.

Representative TEM images of (a) DEG 2 h, (b) DEG 2 h with sodium acetate, (c) DEG 3 h, (d) DEG 3 h with sodium acetate, (e)TEG 2 h, (f) TEG 2 h with sodium acetate, (g) TEG 3 h, (h) TEG 3 h with sodium acetate.

It is reported that the intensity of absorption peak in UV–visible spectrum is related with particle size of nanoparticles. As the particle size decreases, absorption peak shifts towards lower wavelength that is blue shift. As in case of DEG mediated synthesized ZnO nanoparticles, DEG 2 h sample shows absorption peak at 366 nm while DEG 2 h sample with sodium acetate show absorption peak at 368 nm. Similarly remaining all samples show blue shift with decrease in particle size which interpret that the intensity of the absorbance peak shows slight blue shift with decrease in particle size. The type of polyols used, temperature and reaction time have effect on absorption peak [44] , [45] .

3.3. Field emission scanning microscopy (FESEM)/energy dispersive X-ray spectroscopy (EDX)

Morphology of all ZnO nanoparticles synthesized by using DEG and TEG were studied by images obtained by FESEM and TEM. Fig. 4 , Fig. 5 clearly shows that the zinc oxide nanoparticles obtained by refluxing diethylene glycol and triethylene glycol for 2 h and 3 h in presence and in absence of sodium acetate have uniform shape and size with different morphology. Image depicts addition of sodium acetate, use of different polyol and change in reflux time from 2 h to 3 h offers difference in morphology from oval to rod shape with average particle size of ~ 15 to 100 nm. FESEM and TEM analysis reports DEG refluxed for 3 h in absence of sodium acetate exhibited least particle size of ~ 15 nm.

Antibacterial activity of DEG and TEG mediated synthesized ZnO NPs (1 mg/ml) against Gram-positive Staphylococcus aureus(NCIM 2654) (A)and Gram-negative Proteus vulgaris(NCIM 2613) (B), In plate (I) and (III) samples inoculated are(1)DEG 3 h, (2) DEG 3 h with sodium acetate, (3) DEG 2 h, (4) DEG 2 h with sodium acetate and in plate (II) and (IV) samples inoculated are(1)TEG 2 h with sodium acetate, (2) TEG 3 h, (3) TEG 3 h with sodium acetate, (4) TEG 2 h.

% of inhibition of all ZnO samples at different concentrations of all ZnO nanoparticles against Staphylococcus aureus(NCIM 2654) (A) and Proteus vulgaris(NCIM 2613) (B), (1) DEG 3 h, (2) DEG 3 h with sodium acetate, (3) TEG 3 h, (4) TEG 3 h with sodium acetate, (5) TEG 2 h, (6)TEG 2 h with sodium acetate, (7) DEG 2 h, (8) DEG 2 h with sodium acetate.

The difference observed in the morphology of the ZnO nanoparticles depends upon release rate of OH – ions. In presence of sodium acetate release rate of OH - ions becomes slow due to its weak hydrolyzing ability of acetate ions, which affects on condensation and nucleation process. So particles show elongated rod shaped morphology [38] .

The elemental analysis of all ZnO nanostructures was performed by EDX spectroscopy. The Supplementary Fig. 2 shows the EDX of all synthesized ZnO nanoparticles which reveals presence Zn and O that indicate the synthesis of pure ZnO nanoparticles. The impurity free nanoparticle exhibits the promising anti-microbial and antibiofilm activity.

3.4. Fourier Transform Infrared Spectroscopy (FT-IR) analysis

In Supplementary data Fig. 3 A and B , FTIR spectrum of ZnO nanoparticles synthesized in DEG and TEG showed characteristic peak at ~ 3443 cm −1 , which was assigned to stretching vibrations of hydroxyl group [46] , [47] and the peaks at ~ 2922 cm −1 were assigned to –CH stretching showing presence of CH 2 ,CH 3 groups [48] . The 2 peaks at about ~ 1586 cm −1 and ~ 1412 cm −1 were assigned to symmetric and asymmetric C˭O stretching [49] . The peak position at 1125 cm −1 were assigned to –CH deformation showing –CH 2 , CH 3 bending. Due to inter atomic vibrations, metal oxides generally exhibit absorption bands in fingerprint region below 1000 cm −1 . [50] . In the infrared region, the peaks at around 415–480 cm −1 corresponds to ZnO which show the stretching vibration of Zn-O [51] . This observation indicate that, DEG/TEG molecules get adsorbed on synthesized ZnO nanoparticles [48] . The differences in the particle sizes may lead to different wavenumber and frequencies are consistent to the reported literature [52] .

3.5. Thermogravimetric analysis

The thermal decomposition behaviour and presence of adsorbed polyols of all ZnO samples were observed by TGA analysis. All samples were heated from 50 to 900 °C at the rate of 10 °C/min. The Supplementary data Fig. 4A and B shows the thermal decomposition of DEG and TEG mediated synthesized ZnO nanoparticles respectively. The two successive decompositions were observed in all samples. The initial weight loss observed was due to the evaporation of surface adsorbed water and moisture occurred in range of 145–270 °C [53] and further 2ndstage of decomposition was observed in the range of 452–490 °C due to loss of adsorbed DEG/TEG molecules in all samples and which was confirmed by FTIR [54] .

3.6. Applications of ZnO NPs

3.6.1. antimicrobial activity.

From the results in Table 4 , it was observed that among all ZnO nanoparticles the smallest ZnO nanoparticles synthesized in DEG for 3 h showed significant zone of inhibition against Staphylococcus aureus(NCIM 2654) and Proteus vulgaris(NCIM 2613).

Diameter of zone of inhibition by ZnO against Staphylococcus aureus and Proteus vulgaris .

The intensity of antibacterial activity is size dependent. Intensity of antibacterial activity is inversely proportional to the size of nanoparticles, so nano-sized ZnO show good antibacterial activity than bulk ZnO [55] , [56] . The intensity of inhibition by nanoparticles depends upon small size, shape and large surface area to volume ratio, as it affects on the interaction with membrane of microorganisms. Yamamoto et al reported, study of antibacterial activity of different sized ZnO nanoparticles (10–50 nm), which showed better antimicrobial property than bulk ZnO (2 µm) [57] , [58] . According to Pratap et al., ZnO synthesized by using green route Coriandrum sativum leaf extract exhibit antibacterial activity at concentration more than 100 mg/ml [59] . Sharmila et al., demonstrated antibacterial activity of ZnO nanoparticles (22–93 nm) synthesized through green route Bauhinia tomentosa leaf extract, which showed antibacterial activity against Gram positive and Gram negative bacteria [60] . Several reports suggest that the action of ZnO on bacterial species is due to release of reactive oxygen species (ROS) species and zinc ions. Generated ROS species, that is, hydrogen peroxide (H 2 O 2 ), OH - (hydroxyl radicals), O 2 −2 (peroxide) and zinc ions from ZnO nanoparticles bind to the negative surface of the cell membrane, leading to disruption of the cells followed by leakage of inner cellular material that causes cell death [61] .

In the present study, our interest was to synthesize particles with different morphologies and to study their size dependent antibacterial activity. Out of all synthesized ZnO nanoparticles, DEG 3 h sample with least particle size (~ 15 nm) exhibited comparatively remarkable antibacterial activity against both bacteria. It’s small size and it’s high surface area to volume ratio may helped for more interaction with bacterial cell, than other ZnO NPs with greater size, this could be the reason why these nanoparticles exhibited significant antibacterial activity than other synthesized nanoparticles.

3.6.1.1. Quantitative antimicrobial assay

From the above results, it was concluded that minimum inhibitory concentration for all samples was in range of 10–20 µg/ml. It was revealed that among all samples DEG 3 h sample showed significant % of inhibition for Staphylococcus aureus(NCIM 2654) as compared to Proteus vulgaris(NCIM 2613).  For Staphylococcus aureus and Proteus vulgaris it showed 32.67% and 22.38% of inhibition at 50 µg/ml concentration respectively. ( Fig. 6 , Fig. 7 )

% of biofilm inhibition of all ZnO samples at different concentrations of all ZnO nanoparticles against Staphylococcus aureus(NCIM 2654) (A) and Proteus vulgaris(NCIM 2613) (B), (1) DEG 3 h, (2) DEG 3 h with sodium acetate, (3) TEG 3 h, (4) TEG 3 h with sodium acetate, (5) TEG 2 h, (6) TEG 2 h with sodium acetate, (7) DEG 2 h, (8) DEG 2 h with sodium acetate.

Antibacterial and antibiofilm action of ZnO on bacteria.

3.6.1.2. Antibiofilm activity by microtiter plate

Effect of all synthesized ZnO nanoparticles on biofilm formation on Staphylococcus aureus (NCIM 2654) and Proteus vulgaris(NCIM 2613) was shown in figure 11 A and B. These graphs indicate that all ZnO samples synthesized by using DEG and TEG inhibited the activity of biofilm formation. Out of all synthesized ZnO nanoparticles, ZnO synthesized by refluxing DEG for 3 h without sodium acetate showed significant % of inhibition in Staphylococcus aureus as compared to Proteus vulgaris at each concentration. All ZnO samples showed increased % of inhibition with increase in concentration. At 250 µg/ml concentration of ZnO synthesized by DEG refluxed for 3 h exhibited maximum 67.3% and 58.18% biofilm inhibition against Staphylococcus aureus and Proteus vulgaris.

Staphylococcus aureus and Proteus vulgaris are pathogens that have ability to form biofilm on medical implants associated with chronic infections. These infections are difficult to irradicate due to resistant nature of biofilm [62] . Action of antimicrobial agents against biofilm associated infections is not that much effective due to inability of penetration into network of biofilm. To overcome this problem application of nanoparticles for inhibition of antibiofilm is efficient [4] , [63] .

In present study, by using different strategies we have synthesized ZnO nanoparticles with different morphologies in which ZnO nanoparticles synthesized by refluxing DEG for 3 h in absence of sodium acetate proved to be efficient nanoparticle with remarkable antibiofilm activity than other synthesized ZnO nanoparticles with size greater than these particles. These results revealed that smaller nanoparticles exhibited significant inhibition of biofilm than larger nanoparticles.

4. Conclusion

In the present investigation, we have synthesized ZnO nanoparticles by applying different approaches, i) regular synthesis in polyols, ii) In presence of sodium acetate, iii) increasing reaction time. We showed that it is possible to control shape and size of nanoparticles through these approaches. XRD analysis revealed the phase purity. The synthesized nanoparticles have crystallite nature having hexagonal wurtzite structure. UV spectroscopy showed that absorption edges was shifted to a shorter wavelength showing blue shift due to decrease in crystal size. FTIR and TGA analysis presented that DEG and TEG molecule adsorbed on ZnO nanoparticles. The prepared all ZnO nanoparticles posses antibacterial and antibiofilm activity against Staphylococcus aureus and Proteus vulgaris. The most interesting observation found in present study is that, all synthesized nanoparticles showed nicely organized oval and rod shaped morphology with different size. In case of nanoparticles synthesized by using polyol DEG, it was observed that, addition of sodium acetate and increase in reflux time from 2 h to 3 h changes morphology of nanoparticles from oval to rod shape, while in case of nanoparticles synthesized by using polyol TEG all particles show rod shaped morphology and increase in size with addition of sodium acetate and increase in reflux time from 2 h to 3 h which highlights the role of sodium acetate in change of morphology. Out of all particles, ZnO synthesized by refluxing zinc acetate precursor in DEG for 3 h in absence of sodium acetate with particle size ~ 15 nm showed maximum activity against Staphylococcus aureus and Proteus vulgaris than other synthesized ZnO nanoparticles. This study showed that the antimicrobial and antibiofilm efficacy of ZnO nanoparticles increases with decreasing particle size. We have demonstrated that applying different approaches affects on the size and shape of nanoparticles, these findings provide better understanding of ZnO nanoparticles that can serve as a potential antibacterial and antibiofilm agent in biomedical application.

Acknowledgements

The corresponding author is thankful for D.Y. Patil University for financial support (DYPU/R&D/190) and financial support from the Irish Research Council under the Government of Ireland Postdoctoral fellowship Grant GOIPD/2017/1283. The funding agencies are highly acknowledged.

Appendix A Supplementary data associated with this article can be found in the online version at doi:10.1016/j.bbrep.2018.11.007

Appendix B Supplementary data associated with this article can be found in the online version at doi:10.1016/j.bbrep.2018.11.007

Appendix A. Transparency document

Supplementary material

Appendix B. Supplementary material

ZnO nanostructured materials and their potential applications: progress, challenges and perspectives

First published on 9th March 2022

Extensive research in nanotechnology has been conducted to investigate new behaviours and properties of materials with nanoscale dimensions. ZnO NPs owing to their distinct physical and chemical properties have gained considerable importance and are hence investigated to a detailed degree for exploitation of these properties. This communication, at the outset, elaborates the various chemical methods of preparation of ZnO NPs, viz. , the mechanochemical process, controlled precipitation, sol–gel method, vapour transport method, solvothermal and hydrothermal methods, and methods using emulsion and micro-emulsion environments. The paper further describes the green methods employing the use of plant extracts, in particular, for the synthesis of ZnO NPs. The modifications of ZnO with organic (carboxylic acid, silanes) and inorganic (metal oxides) compounds and polymer matrices have then been described. The multitudinous applications of ZnO NPs across a variety of fields such as the rubber industry, pharmaceutical industry, cosmetics, textile industry, opto-electronics and agriculture have been presented. Elaborative narratives on the photocatalytic and a variety of biomedical applications of ZnO have also been included. The ecotoxic impacts of ZnO NPs have additionally been briefly highlighted. Finally, efforts have been made to examine the current challenges and future scope of the synthetic modes and applications of ZnO NPs.

1. Introduction

ZnO has a slew of unique chemical and physical properties, viz. , high chemical stability, high electrochemical coupling coefficient, broad range of radiation absorption and high photostability, which make it among all metal oxides a key technological material and confer upon it its wide applications in varied fields. ZnO is categorized as a group II–VI semiconductor in materials science because zinc belongs to the 2 nd group while oxygen belongs to the 6 th group of the periodic table. Its covalence is on the borderline demarcating ionic and covalent semiconductors. Besides, it has good transparency, high electron mobility, an outsized exciton binding energy (60 meV), wide band gap (3.37 eV), 1 strong room temperature luminescence, high thermal and mechanical stability at room temperature, broad range of radiation absorption and high photostability that make ZnO the most favorite multitasking material. 2,3,5,6 As a result of its distinctive optical and electrical properties 4 it is considered to be a possible material in electronic applications, optoelectronic applications and laser technology. ZnO among nano-sized metal oxides has also been further extensively exploited to derive possible benefits from its antimicrobial and antitumor activities. 7 Because of its blocking and absorbing capabilities ZnO finds inclusion in some cosmetic lotions. 8 ZnO can also be used in human medicine as an astringent (for wound healing), and to treat hemorrhoids, eczema and excoriation. 9 ZnO nanoparticles have recently attracted attention owing to their unique features. There are numerous promising applications of ZnO nanoparticles in veterinary science due to their wound healing, antibacterial, antineoplastic and antigenic properties. Recently, many research studies and experimental analyses have improved the efficiency of zinc oxide (ZnO) materials by producing nano-structures where each nano-dimension is reduced to generate nanowires, thin films and other structures for plenty of applications including defense against intracellular pathogens and brain tumors. 10 One-dimensional structures include nanorods, 11–13 nanoneedles, 14 nanohelixes, nanosprings, nanorings, 1 nanoribbons, 15 nanotubes, 16–18 nanobelts, 19 nanowires 20–22 and nanocombs. 23 Nanoplates/nanosheets and nanopellets 24,25 are their two-dimensional forms while flowers, dandelions, snowflakes, coniferous urchin-like structures, etc. 26–29 count as the three-dimensional morphologies of ZnO nanoparticles. Nevertheless, the challenges in terms of the potential toxic effects of ZnO nanoparticles do require special attention.

2. Chemical methods for synthesis of zinc oxide nanoparticles

2.1 mechanochemical process.

Ao et al. 32 carried out a mechanochemical process of synthesizing ZnO NPs by exploiting the reaction between ZnCl 2 and Na 2 CO 3 and using NaCl as a diluent. 32 The pure nanocrystalline ZnO was obtained by removing the by-product NaCl and finally drying in a vacuum. TEM images showed moderately aggregated ZnO nanoparticles of size less than 100 nm which were prepared by a 6 h milling followed by a thermal treatment at 600 °C for 2 h. The effect of milling time and annealing was carefully investigated in the study. A decrease in nanocrystallite size from 25 nm to 21.5 nm was observed as the milling time increased from 2 to 6 h after which it attained steadiness. This phenomenon was chalked up to a critical effect prevailing in the course of milling. The crystal size, however, was found to increase with temperature with the rise being steep after 600 °C. The activation energies for nanocrystallite growth in different temperature ranges were calculated using the Scott equation. The activation energy was found to be 3.99 for growth in between 400 and 600 °C while it reached 20.75 kJ mol −1 beyond 600 °C. The higher growth rate at higher temperatures was thus attributed to extensive interfacial reactions driven by greater activation energy.

While the XRD analysis substantiated a perfect long-range order and a pure wurtzite structure of the synthesized ZnO powders regardless of the milling time, Raman spectroscopy revealed that lattice defects and impurities were introduced into ZnO powders at the middle-range scale depending on milling duration. Extended milling was found to reduce crystal defects but introduce impurities. The SEM images suggested that the milling duration of the reactant mixture positively regulated the morphology of the particles irrespective of the additional thermal treatment.

ZnO NPs were also prepared through a mechanochemical method by using ZnCl 2 , NaCl and Na 2 CO 3 as starting materials. 34 A solid phase reaction triggered by milling the starting powders led to the isolation of ZnCO 3 in the NaCl matrix. The ZnCO 3 was finally subjected to a thermal treatment at 400 °C which induced its decomposition to ZnO. The anatomization of TEM results indicated a mean particle size of 26.2 nm. The mean nanocrystallite size evaluated from the XRD peak width at 2 θ = 36° using the Scherrer equation was found to be 28.7 nm. Meanwhile, the surface area of the ZnO nanopowder evaluated from BET analysis was 47.3 m 2 g −1 corresponding to a spherical particle size of 27 nm.

Another study on the optical properties of ZnO NPs synthesized through mechanochemical means and using ZnCl 2 , NaCl and Na 2 CO 3 as raw materials was conducted by Moballegh et al. 35 The XRD and TEM results revealed that particle size increased with calcination temperature. The work proposed improved optical properties as a result of the decrease in particle size owing to the enhanced ratio of surface to volume in ZnO NPs. In another study 36 a mixture of starting powders (anhydrous ZnCl 2 , Na 2 CO 3 and NaCl) was milled at 250 rpm and then calcined at 450 °C for 0.5 h to yield ZnO NPs with a crystallite size of 28.5 nm as estimated from subsequent XRD analysis. The particle size that emerged from TEM and SEM analysis ranged in between 20 and 30 nm. The incongruent particle size estimated from BET analysis was ascribed to an agglomeration of nanoparticles in the course of drying.

The foremost shortcoming of the procedure exists in its fundamental difficulty encountered in the homogeneous grinding of the powder and controlled minimization of the particles to the required size. Note that the particle size reduces with increasing time and intensity of milling. However, if the powder is subjected to milling for longer periods of time, the chances of contamination increase. A highly shrunk size of nanoparticles is the prime advantage that can be extracted from the method apart from the benefit of a significantly low cost of generation coupled with diminished agglomeration of particles and pronouncedly homogeneous crystallite morphology and architecture. The mechanochemical process is particularly desirable for large-scale production of ZnO NPs.

2.2 Controlled precipitation

Kumar et al. 38 used zinc acetate (Zn(OAc) 2 ·2H 2 O) and NaOH as reagents, and the settled white powder was separated followed by washing with deionized water thrice and dried overnight under dust-free conditions at room temperature. XRD revealed the formation of hexagonal ZnO nanostructures. SEM and TEM analyses revealed the formation of crystalline ZnO flowers in which a bunch of ZnO nanorods assembled together to form a leaf-like structure followed by flower-shaped ZnO nanostructures. The ZnO nanoflowers were each formed by the combination of 8–10 leaf-like petals as shown. The length of each petal did not exceed 800 nm. The as-synthesized ZnO nanostructures showed good antimicrobial activity towards Gram-positive bacteria Staphylococcus aureus as well as Gram-negative bacteria Escherichia coli with a MIC/MBC of 25 mg L −1 . Zn(CH 3 COO) 2 ·2H 2 O and (NH 4 ) 2 CO 3 were employed as reagents by Hong et al. 39 in their method of synthesizing ZnO NPs. XRD and TEM tests revealed particle sizes of 40 and 30 nm. Heterogeneous azeotropic distillation thoroughly prevents agglomeration and reduces the size of ZnO NPs.

In the precipitation method of synthesizing nanopowders, it is more or less a ritual these days to use surfactants that would enable control over the growth of particles with the simultaneous prevention of coagulation and flocculation of particles thereby preventing an appreciable reduction in the final yield. The surfactants act as chelates encapsulating the metal ions in an aqueous medium. Wang et al. 41 used ZnCl 2 and NH 4 OH and a cationic surfactant, CTAB (cetyltrimethyl-ammonium bromide), for the generation of ZnO NPs. The formation of sharply crystalline ZnO NPs with a wurtzite structure and crystallite size of 40.4 nm was confirmed by XRD data, while TEM examination of the powder bore out the formation of spherical nanoparticles of size 50 nm.

2.3 Sol–gel method

Suwanboon et al. 43 using Zn(CH 3 COO) 2 ·2H 2 O, polyvinyl pyrrolidone (PVP) and NaOH prepared nano-structured ZnO crystallites via the sol–gel method. The XRD characterization revealed a wurtzite structure having an average crystallite size of about 45 nm. The role of PVP at its different concentrations on the morphology was checked. There occurred a shift from a platelet-like to a rod shape with an increase in PVP concentration. TEM images bore out the grain size of platelet-like ZnO to be 150 nm while the diameter of the rod-shaped ZnO was likewise determined to be 100 nm. In another sol–gel method-based synthesis by Benhebal et al. 44 zinc acetate dihydrate and oxalic acid were used to generate ZnO nanopowder with ethanol as a solvent which showed a hexagonal wurtzite structure. The crystallite size obtained from the Scherrer equation was found to be 20 nm. The SEM micrograph confirmed the formation of uniform, spherically shaped ZnO nanoparticles. BET analysis revealed a surface area of 10 m 2 g −1 . This was characteristic of a material with low porosity, or a crystallized material.

Sharma 45 obtained ZnO NPs with outstanding antibacterial properties using the sol–gel method. Zinc acetate, oxalic acid and water were employed as raw materials in this process. A white gel precipitate was first obtained. It was then thermally treated at 87 °C for 5 h, and then at 600 °C for 2 h. The ZnO NPs exhibited high crystallinity as borne out by XRD data. A diameter of 2 μm was obtained for the ZnO nano-aggregates from SEM analysis.

In a study conducted by Ristic et al. 46 nano-structured ZnO crystallites were obtained using the sol–gel route. From XRD examination and using the Scherrer formula, the average value of the basal diameter of the cylinder-shaped crystallites was found to be 25–30 nm, while the height of the crystallites was 35–45 nm. The sol–gel method presents a host of advantages in comparison with the previously mentioned methods. Prime amongst its merits are the low cost of the apparatus and raw materials, reproducibility and flexibility of generating nanoparticles. 47

2.4 Vapour transport method

In water vapour, ZnO nanoflowers were synthesized. The nanoflowers were constructed from tens of ZnO nanosheets with random orientations. In oxygen gas, ZnO hexagonal nanorods were obtained. The size of the nanorods was not uniform. It was argued that the size of the Au catalyst underneath might have influenced the size of the ZnO nanorods. Both the samples, however, exhibited a hexagonal wurtzite structure. Though the samples showed different morphologies and crystal structures, surprisingly, they had almost the same optical properties. The PL spectra revealed only one UV peak close to 389 nm wavelength for both samples, indicating the high quality of the synthesized ZnO samples.

Novel one-dimensional single-crystalline ZnO nanorod and nanoneedle arrays on a Cu catalyst layer-coated glass substrate were investigated by Alsultany et al. 50 via a simple physical vapour deposition method by thermal evaporation of Zn powder in the presence of O 2 gas. The ZnO nanorods and nanoneedles were synthesized along the c -axis growth direction of the hexagonal crystal structure. The diameter and growth rate of the high-quality and well oriented one-dimensional ZnO nanostructures were achieved as a function of varying growth temperature and growth time. At 450 °C, ZnO nanorods were uniformly distributed at a high density on the entire substrate surface and quasi-aligned, and small average diameters were obtained. The diameters and lengths of the obtained nanorods were in the range of 19–27 nm and 2.8 μm, respectively. When the temperature was increased to 550 °C, ZnO nanorods grew perpendicular to the substrate, uniformly throughout their length, and with more consistent shape and dimensions, with approximately 85 nm width and 3.8 μm length. The morphological change and distribution occurred at a growth temperature of 650 °C, and ZnO nanorods with a hexagonal shape at the tips of rods of hexagonal hierarchical structures were formed. These rods possessed a typical hierarchical structure with lengths and diameters of approximately 190–350 nm and 3.9 μm, respectively, whereas short nanorods with a diameter of 95 nm and length of 900 nm were observed on the tip of each rod of hexagonal hierarchical structures. As Cu metal catalysts were used in the study, the growth mechanism of 1D ZnO nanostructures presented therein followed the VLS method. This method could be divided into three stages, as follows: first, the Zn vapor and catalytic Cu formed liquid alloy droplets during the heating process at a certain temperature, representing the initial stage of the nucleation process. Second, crystal nucleation occurred upon gaseous species adsorption until supersaturation was reached, and the formed sites served as nucleation sites on the substrate. Finally, the axial growth of the nanorods began from these sites. Based on this study of the mechanism in the presence of Cu metal catalysts at different growth temperatures and according to the nucleation theory of the VLS growth mechanism, the Cu catalyst nanoclusters formed because of capillarity, which caused beading of the Cu layer at high growth temperature. Consequently, the Cu–Zn alloy process reached a certain solubility depending on the temperature; then, the Zn vapor began to precipitate out at the interface between the surface and droplet. That in turn determined the diameter and size of the nanostructures depending on the size of the liquid alloy droplets. Notably, large-scale ZnO nanorods with a lower diameter were formed at a low growth temperature of 450 °C. The Zn metal powder (melting point of 419 °C) vapor pressure at 450 °C was sufficiently high to investigate the growth of ZnO nanorods on the glass substrate via the VLS method, and the decrease in Zn vapor as a result of the decrease in the growth temperature led to a low lateral growth rate compared with the axial growth rate of the 1D nanostructure. In contrast, the higher growth temperature could also lead to the formation of hierarchical nanostructures. In addition, at high growth temperature along with the consumption of the Zn vapor during growth, the diameter of the nanorods markedly decreased. This condition consequently caused the production of rods with a typical hierarchical structure. At a growth time of 30 min, ZnO nanorods were obtained with a diameter of 19–27 nm and a length of 2.8 μm. When the growth time increased to 45 min, nanoneedles were obtained. The needles exhibited mean diameters of 65–190 nm and length of 3.2 μm. On the other hand, nanoneedles grown at 60 min were approximately 80–250 nm in diameter and 3.8 μm in length.

Diep and Armani 51 designed a flexible light-emitting nanocomposite based on ZnO nanotetrapods (NTPs) which they prepared using a vapour transport technique. The CVT synthesis of the ZnO NTPs was self-catalyzed. In the TEM images, the lattice fringes were clearly visible, indicating the single-crystalline nature of the nanostructures. The lattice spacing was found to be 2.6 Å, indicating growth in the [0001] direction. X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) analysis were also performed to confirm the crystal structure and elemental composition of the NTPs. Based on an analysis of the TEM and SEM images, the ZnO NTP arm lengths ranged from 0.5 μm to 3.5 μm and the diameters varied from 120 nm to 350 nm.

Luo et al. 52 also constructed ZnO tetrapods as potential electrode materials for low-cost and effective electrochemical supercapacitors using an oxidative-metal-vapor-transport method. The SEM images of the ZnO tetrapods collected at different temperatures showed that the products obtained were pure and uniform, and the tetrapods consist of four arms branching from one center, and the angles between the arms were nearly the same, analogous to the spatial structure of the methane molecule. As for the size variation with collected temperatures, it transpired that smaller size tetrapods were obtained with lower evaporation temperature. This demonstrated the power of the technique for controlling the size of the tetrapods. ZnO tetrapods with arms as thin as about 170 nm and shorter than 4000 nm were revealed by SEM analysis. The XRD pattern of the ZnO tetrapods showed that all the diffraction peaks could be indexed to a wurtzite 5 structure with lattice constants of a = 0.324 nm and c = 0.519 nm. The TEM and high resolution TEM (HRTEM) images of the ZnO tetrapods revealed that the arm diameter and length of the tetrapods are, on average, about 22 nm and 90 nm, respectively. The HRTEM image of a single arm revealed clear fringes perpendicular to the arm axis and these fringes were spaced by about 0.25 nm consistent with the interplanar spacing of (0002) suggesting that the nanowire growth direction was along [0001].

2.5 Hydrothermal method

Aneesh et al. 54 carried out an experiment in which they used Zn(CH 3 COO) 2 ·2H 2 O, NaOH and methanol as reagents. The ZnO NPs thus formed had a hexagonal wurtzite structure. XRD analysis demonstrated an enhancement in average grain size with rising temperature and concentration of the substrates. The average grain size of ZnO NPs prepared from 0.3 M NaOH employing a growth time of 6 h was found to increase from 7 to 16 nm with temperature rise from 100 to 200 °C. The average grain size of ZnO synthesized at 200 °C for 12 h revealed an increase from 12 to 24 nm with elevation in concentration of NaOH from 0.2 M to 0.5 M.

This process has many advantages over other methods. Organic solvents do not find use in this process. This coupled with the omission of supplementary processes like grinding and calcination within the ambit of the method endows it with the much sought after eco-friendly character. Low operating temperatures, the diversified morphologies and sizes of the resulting nano-crystals depending on the composition of the starting mixture and the process temperature and pressure, the greatly pronounced crystallinity of the nanoparticles and their high purity are factors that surely make the process more advantageous than others. 54,55

2.6 Solvothermal method

Chen et al. 57 also used a solvothermal route to generate ZnO NPs. They eventually prepared nano-structured ZnO crystals that were devoid of hydroxyl groups. They carried out a reaction of zinc powder with trimethylamine N -oxide (Me 3 N→O) and 4-picoline N -oxide (4-pic→NO). The medium for the reaction was a mixture of organic solvents (toluene, ethylenediamine (EDA) and N , N , N ′, N ′-tetramethylenediamine (TMEDA)) contained in an autoclave which was kept at 180 °C. It was observed that the size and morphology of the ZnO nanoparticles/nanowires were greatly influenced by the oxidants used and the ligating capacities of the solvents. The ramifications of the presence of water in the system were additionally investigated. It emerged that the presence of traces of water catalyzed the zinc/4-picN→O reaction and exerted an effect on the size of the nano-structured ZnO crystallites thus obtained. Depending on the reaction conditions, the ZnO nanostructures had diameters ranging in between 24 and 185 nm. The solvothermal synthesis method has many advantages. Foremost among them is the fact that reactions can be carried out under determined conditions. As a result, nano-structured ZnO with a range of architectures can be generated by exercising due control over the reaction conditions.

2.7 Method using an emulsion or microemulsion environment

SEM and XRD analysis showed that the particle size and phase location were both dependent upon the conditions (ratio of two-phase components, substrates and temperature) employed for the accomplishment of the process. Depending on the process conditions, ZnO NPs with different particle morphologies were obtained. The morphologies that formed during the process included spherical agglomerates, needle shapes, near-hexagonal shapes, near-spherical shapes and irregular agglomerates. These NPs further had a wide range of diameters. Some had diameters ranging in between 2 and 10 μm, while the diameters of others ranged from 90 to 600 nm, some others had diameters in between 100 and 230 nm and yet others were characterized by diameters hovering around 150 nm.

Kołodziejczak-Radzimska et al. 59 used zinc acetate and KOH or NaOH in an emulsion system. For the generation of an emulsion, cyclohexane was utilized. Cyclohexane was held to have furnished a ready organic phase, and also essayed the role of a surfactant that wasn't ionic. In this method for emulsion formation cyclohexane was used as an organic phase, and nonylphenyl polyoxyethylene glycol ethers NP3 and NP6 were used as a mixture of emulsifiers. By tailoring the ZnO precipitation process by way of altering the precipitating agent, substrate ingredients and the tempo of substrate dosing, an amazing variety of ZnO nanostructures were designed. Four samples were obtained, labelled Z1, Z2, Z3, and Z4, composed of particles of different shapes. Morphologies such as solids (Z1), ellipsoids (Z2), rods (Z3) and flakes (Z4) with modal diameters of ∼396 nm, ∼396 nm, ∼1110 nm and ∼615 nm were obtained. They were further characterized by their considerable surface areas. Values of 8 m 2 g −1 , 10.6 m 2 g −1 , 12 m 2 g −1 and 23 m 2 g −1 could be respectively assigned to samples Z1, Z2, Z3, and Z4.

If a surfactant possessing balanced hydrophilic and lipophilic properties is used in the right proportion, a different oil and water system will be produced. The system remains an emulsion, but exhibits some characteristics that are different from emulsions. These new systems are “microemulsions”. The drop size in a microemulsion is significantly smaller than in an emulsion, and lies in the range 0.0015–0.15 μm. 60,61 In contrast to emulsions, microemulsions form spontaneously under appropriate conditions. This synthesis method does not require any complex preparation procedure, sophisticated equipment or rigorous experimental conditions, but still provides possibilities in controlling the size and morphology of the ZnO powders in a size scale approaching nanometers. Even though the product yield is low, the narrow size distribution due to well-dispersed cage-like small reactors (5–100 nm) formed under uniform nucleation conditions is the superior aspect of the ZnO nanoparticles obtained by microemulsion routes. Such low-dimensional uniform ZnO nanostructures offering size and morphology dependent tunable electrical and optical properties are of particular technological interest for applications such as quantum dots, UV-emission optoelectronic and lasing devices, and transparent conducting thin films.

Yildirim and Durucan 63 also synthesized ZnO NPs through the use of microemulsions. They made an endeavour to reshape the microemulsion modus operandi with an eye to generate monodisperse ZnO nanostructures. They subjected the zinc complex precipitate obtained in the course of the microemulsion method to thermal decomposition. Subsequent calcination was adopted. The use of glycerol as the internal phase of a reverse microemulsion imparted the intended modification. The synthesized ZnO NPs had spherical shapes. They were monodisperse and their diameter measured in between 15 and 24 nm.

All the procedures involving chemical synthesis of ZnO NPs generate a few toxic chemicals and their adsorption on the surface increases the likelihood of harmful effects being wielded in medical applications. Further, these approaches include reactions requiring high temperature and intense pressure for their commencement while some reactions require operations in an inert atmosphere or under inert conditions. Toxic materials such as metallic precursors, toxic templates and capping agents and even H 2 S find application in quite a few chemical routes. 64 Very often toxic substances are employed for the generation of nano-structured particles and for their stabilization as well. This in turn produces secondary products and residues that are detrimental to the ecosystem. 65,66

3. Green methods for the synthesis of ZnO nanoparticles

An extract prepared from Ajwain ( Carom-Trachyspermum ammi ) seeds has also been used to synthesize ZnO NPs. 70 The work boasts of its operation under ambient temperature conditions. The ZnO NPs were found to have a wurtzite structure. The synthesized ZnO nanostructures were morphologically characterized by FE-SEM images. The ZnO nanostructure showed uniform hexagonal plates, as well as irregular and highly aggregated nanoparticles with a rough surface. The average diameter of the nano-sized ZnO clusters has been observed to be ∼41 nm. XRD results showed an increase in interplanar spacing with an increase in the extract volume from 0.2474 nm to 0.2765 nm with a simultaneous decrease in crystallite size from 39.51 nm to 28.112 nm. The band gap also fell from 3.592 eV to 3.383 eV as the amount of extract increased. Phytoconstituents in the extract thus evidently played a key role of reductants and furthermore acted as capping agents in the generation and stabilization of ZnO NPs.

Jamdagni et al. 72 used an aqueous flower extract of Nyctanthes arbortristis for making ZnO NPs. The starting materials consisted of zinc acetate dihydrate and sodium hydroxide. XRD results showed an average crystallite size of 16.58 nm while TEM analysis revealed that the individual particle size ranged within 12–32 nm and the nanoparticles were obtained in the form of aggregates. In a very recent study, 73 Ulva lactuca seaweed extract was used to prepare ZnO nanoparticles. XRD analysis revealed strong characteristic peaks of ZnO suggesting high crystallinity of the synthesized material. Further, the average crystallite size thus calculated was found to range in between 5 and 15 nm. TEM micrographs revealed an agglomeration of asymmetrically shaped NPs bearing an average crystallite size of 15 nm.

Muraya koenigii seed extract was also recently reported to have been used as a stabilizer as well as a reductant in the preparation of ZnO NPs. 74 Sharp diffraction peaks in XRD results indicated remarkable crystallinity of the NPs whose average crystallite size was calculated to be 70–100 nm. Both SEM and TEM micrographs revealed nanoparticles with an average size of about 100 nm and bearing a wide range of morphologies – spherical, triangular, radial, hexagonal, rod-like and rectangle-shaped.

One recent experiment used Calotropis procera leaf extract and Zn(NO 3 ) 2 ·6H 2 O to synthesize ZnO NPs. 75 An XRD test confirmed a hexagonal wurtzite structure of the nanoparticles with marked crystallinity. The average crystallite size was calculated using the Scherrer equation and found to be 24 nm. Diffuse Reflectance Spectroscopy (DRS) revealed a band gap of 3.1 eV for the synthesized nanoparticles. In the FT-IR analysis of the synthesized ZnO NPs, a peak attributed to the metal–oxygen bond of ZnO appeared in between 500 and 700 cm −1 . Further, a conspicuous shift and broadening of peaks corresponding to functional groups like hydroxyl, aldehyde, amine, ketone, and carboxylic acid suggests their participation in the stabilization of ZnO by the extract. Surface attachment of groups like aldehyde, amine, phenol and terpenoid enhances stabilization additionally allowing the extract to function as a bio-template thereby preventing aggregation of ZnO NPs. TEM images revealed an average particle size of 15–25 nm, while SAED and HR-TEM further confirmed the high crystallinity of the material prepared.

The effects of Artocarpus heterophyllus leaf extract and varying temperatures on the morphology and properties of the ZnO NPs thus prepared were studied by Vidya C. et al. 76 XRD results show an increase in crystallinity and average crystallite size with temperature, the diffraction peaks being increasingly sharper and narrower with temperature. The particles were all spherical and a grain size of 50 nm was obtained from SEM images. SEM analysis also shows similar trends of size and morphology upon temperature variation. TEM analysis revealed a particle size of ∼10–15 nm at 400 °C, ∼15–25 nm at 600 °C and ∼25–30 nm at 800 °C. This further corroborated the results of XRD and SEM tests. Diffuse Reflectance Spectroscopy (DRS) showed a decrease in the calculated band gaps with increasing calcination temperatures.

Archana et al. 77 used Moringa oleifera natural extract and Zn(NO 3 ) 2 ·6H 2 O for the preparation of ZnO NPs. They took different volumes of the extract, viz. 2, 6, 10 and 14 mL, to prepare ZnO NPs which were accordingly labeled ZnO-2, ZnO-6, ZnO-10 and ZnO-14. The PXRD results of all the samples showed great crystallinity. They had a hexagonal wurtzite structure. And the average crystallite size was found to be 21.6 nm. Field Emission Scanning Electron Microscopy (FE-SEM) analysis showed highly crystalline ZnO-10 and ZnO-14 having a spherical shape and average crystallite size of 20–150 nm. HR-TEM micrographs revealed d -spacing of 0.28 and 0.19 nm for the (001) and (101) planes of wurtzite ZnO. The band gaps calculated using the results from Diffuse Reflectance Spectroscopy (DRS) had values of 2.92 eV for ZnO-2, 3.05 eV for ZnO-6, 3.12 eV for ZnO-10 and 3.10 eV for ZnO-14. The increase in band gap with the amount of fuel was attributed to quantum size effects.

In their research work, Rajeswari Rathnasamy et al. 78 used papaya leaf extract for the synthesis of ZnO NPs. Both FESEM and TEM data revealed an average size of ∼50 nm for the individual nanoparticles. The extract of Nephelium lappaceum L. (rambutan) peels (a natural ligation agent) was put into use for the preparation of ZnO NPs in another investigation. 79 The bio-mediated ZnO NPs were found to be spherical in shape. They were characterized by diameters between 20 and 50 nm. Some of the particles were found in agglomerated form. After a day, multi-dimensional chain-like structures formed. In these chains spherical nanoparticles were found intertwined to each other.

An investigation conducted by Matinise et al. 80 used Moringa oleifera extract as a remarkably operative chelating agent to prepare ZnO nanoparticles. The ZnO NPs eventually obtained were characterized by a particle size in between 12.27 and 30.51 nm. The sample obtained just after drying at 100 °C consisted of agglomerates of spherical particles while that obtained after annealing at 500 °C also had nanorods in addition to the clusters of spherical nanostructures.

The biocomponents of leaves of Catharanthus roseus have also been utilized to prepare ZnO NPs with zinc acetate and sodium hydroxide as reagents. 81 SEM micrographs revealed that in addition to the individual ZnO-NPs, aggregates were also formed and they were spherical with diameter ranging from 23 to 57 nm. Sharp and clear XRD peaks confirmed high purity and excellent crystallinity. Shah et al. 82 generated ZnO NPs using the aqueous extract of green tea ( Camellia sinensis ) leaves. The size of the particles was determined using a particle size analyzer. The average diameter of the particles was found to be 853 nm. These nano-sized ZnO particles demonstrated remarkable antimicrobial properties against Gram-positive and Gram-negative bacteria as well as against a fungal strain.

In another experiment, 50 mL of aqueous Citrus aurantifolia extract was boiled to 60–80 °C. 83 It was followed by the addition of a specific amount (5 g) of Zn(NO 3 ) 2 to the solution as its temperature rose to 60 °C. The synthesized nanoparticles were characterized by moderate stability. They had near-spherical shapes with the most probable particle size in the range of 9–10 nm. The extract of Oryza sativa rice 84 was also used to generate ZnO NPs. The extract has been considered a renewable bio-resource. Its abundance adds to its list of merits. The extract has also been cited as a source of bio-template that typically assists the generation of a variety of multifunctional nano-structured materials. ZnO NPs were prepared using the hydrothermal method. The method involved the use of zinc acetate, sodium hydroxide, and uncooked rice flour at several ratios at 120 °C for 18 h. The rice bio-template was found to exert considerable influences upon the size and morphology of ZnO NPs. Fig. 2 shows field emission scanning electron microscopy (FESEM) images of the samples synthesized at different concentrations of uncooked rice (UR). To investigate the effects of raw rice on the resulting ZnO morphology, FESEM was conducted on ZnO synthesized without UR ( Fig. 2a and b ). As seen in Fig. 2c and d , the ZnO structures were mostly flake-like structures assembling together. They were much more ordered in contrast to the one synthesized without UR (as a control) ( Fig. 2a and b ). The diameter of ZnO flakes dramatically decreased after adding 0.25 g UR. This was proposed to have occurred due to the inhibition of lateral growth of ZnO crystals. It was further proposed that the accessibility of the zinc ions to the ZnO crystal seeds was controlled by a bio-template. However, the size of particles seemed to increase when the synthesis was done using 0.25 g UR. Different morphologies of the as-synthesized ZnO were observed with increasing the amount of uncooked rice to 0.5 g. Particles with a very small flower-like shape could be observed ( Fig. 2e and f ). A lower magnification FESEM image indicated that the mentioned structure showed denticulated petals aggregated and form larger flowers of particles. Notably the size of the ZnO particles had been obviously decreased for the sample prepared using 0.5 g UR. In addition, the tooth-like flakes were more dominant for the ZnO sample prepared using 0.5 g UR compared to the one synthesized using 0.25 g UR. Fig. 1g and h indicate the FESEM images of the ZnO sample synthesized using 1 g UR. A very unique star-like structure could be clearly observed at low to high magnification. The star-like structure contained small flakes with denticulated edges which attach to other similar flakes in the center. A closer look showed that the lateral flake acted as a substrate for other flakes to grow on the surface and form a star-like structure. It was therefore argued that the branched pattern for soft templates of starch revealed that the semicrystalline granules of starch were made from concentric rings in which the amylose and amylopectin basic components were aligned perpendicularly to the growth rings and to the granule surface. Fig. 2g and h show that the size of the star-like ZnO particles decreased in comparison with the previous lower amount of uncooked rice. In the case of ZnO crystals synthesized at 2 g UR, increasing the amount of bio-template resulted in different morphologies of ZnO particles being produced. It formed lots of agglomerated toothed-edge flakes which became a secondary unit for larger particles. The star-like shape of the particles could be perceived in some areas but aggregation seemed to be dominant and prevented clearer observation of the particles as they really are. Fig. 2k and l show the FESEM images of the as-synthesized ZnO particles synthesized using 4 g UR. The ZnO morphology changed to flower-like structures, mostly rose-like shapes. A detailed view of the flower-like particles revealed that their flakes had the largest diameter compared to other samples. In the case of ZnO synthesized using 8 g UR, a new morphology, different from other and control samples, was observed. The ZnO crystals appear mostly as rods with around 100 nm size. Moreover, agglomerated without any specific shape, particles coexisted with nanorods in the structure of ZnO synthesized using 8 g UR. Fig. 3 shows the particle size distribution of the ZnO samples synthesized using 0.25, 0.5, 1, 2, 4, and 8 g UR. The particle size distribution of ZnO synthesized without rice is also given for comparison. As shown in Fig. 3 , the range of particle size for ZnO synthesized without UR lies between 200 and 800 nm. When 0.25 g UR was used in the synthesis, the size of particles increased dramatically to 800–2000 nm. Notably the size of ZnO synthesized using 0.5 g UR considerably decreased to the 200–1000 nm range. The decreasing trend continued for the sample synthesized using 1 g UR and with a size range of 250–700 nm. Although this distribution was quite similar to that of ZnO synthesized without a bio-template, it was slightly narrower. On the basis of the particle size distribution for the samples synthesized using 2 and 4 g UR, it could be clearly observed that the size of particles decreased to 200–700 nm and 150–700 nm, respectively. In the case of the ZnO sample synthesized using 8 g UR, the size of particles was within the nano regime, between 40 and 100 nm. As mentioned in the growth mechanism suggested by the study, adding a bio-template, which presumably acts as a flocculant, forces aggregation. Therefore, the surface-active sites of the template might influence the size and state of aggregation during the particle growth process and ultimately the resulting ZnO particle size distribution. Another procedure used the aqueous leaf extract of Passiflora caerulea. L. (Passifloraceae). 85 The SEM analysis revealed that the ZnO NPs had diameters ranging in between 30 and 50 nm.

Sucrose was used in a study as the capping agent to synthesize a ZnO/C nanocomposite adapting the sol–gel method. 86 The presence of carbon in the prepared ZnO/C was confirmed through EDAX. SEM images of the ZnO/C samples indicate a wide distribution of particles ranging from 10 to 100 nm and exhibit only an irregular granular feature. This kind of surface morphology was argued to be more suitable for supercapacitor electrode materials. Electrochemical investigations of the ZnO/C electrode were carried out using cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy. The ZnO/C electrode exhibits a maximum specific capacitance of 820 F g −1 at a constant specific current of 1 A g −1 . The symmetric aqueous supercapacitor device exhibits a specific cell capacitance of 92 F g −1 at a specific current of 2.5 A g −1 . The aqueous symmetric supercapacitor device achieved an energy density of 32.61 W h kg −1 and a power density of approximately 1 kW kg −1 at a discharge current of 1.0 A g −1 . It has been found that the cells have an excellent electrochemical reversibility (92% after 400 continuous cycles) and capacitive characteristics in 1 M Na 2 SO 4 electrolyte.

Zinc oxide (ZnO) nanoparticles were successfully synthesized using a whey-assisted sol–gel method. 87 X-ray diffraction (XRD) and Raman spectroscopy analysis revealed a wurtzite crystalline structure for ZnO nanoparticles with no impurities present. Transmission electron microscopy (TEM), XRD observations, and UV-vis absorption spectroscopy results showed that with an increase in calcination temperature from 400 to 1000 °C, the size of the spherical nanoparticles increased from 18.3 to 88.6 nm, while their optical band gap energy decreased to ∼3.25 eV. The whey-assisted sol–gel method proved to be highly efficient for the synthesis of crystalline ZnO nanoparticles whose applications are of great interest in materials science technology. Eryngium foetidum L. leaf extract was also used for the nontoxic, cost-effective biosynthesis of ZnO nanoparticles (NPs) following the hydrothermal route. 88 The biosynthesized ZnO NPs served as an excellent antibacterial agent against pathogenic bacteria like Escherichia coli , Pseudomonas aeruginosa , Staphylococcus aureus susp. aureus and Streptococcus pneumoniae . The maximum zone of inhibition in ZnO NPs is 32.23 ± 0.62 and 28.77 ± 1.30 mm for P. aeruginosa and E. coli , respectively.

Another report presented an efficient, environmentally friendly, and simple approach for the green synthesis of ZnO nanoparticles (ZnO NPs) using orange fruit peel extract. 89 The approach aimed to both minimize the use of toxic chemicals in nanoparticle fabrication and enhance the antibacterial activity and biomedical applications of ZnO nanoparticles. The sample obtained without annealing exhibited relatively small spherical particles (10–20 nm) which were coagulated in large clusters on a matrix of residual organic material from the reducing agents. In the samples annealed at 400 °C and 700 °C, the particle sizes were randomly distributed and ranged from 35 to 60 nm and 70 to 100 nm, respectively. For an annealing temperature of 900 °C, the particle size increased intensively in the range of 200–230 nm. It was thus found that the morphology and size of the ZnO NPs depended on the annealing temperature. Specifically, with increasing annealing temperature, the particle size tended to increase and shape larger particles due to crystal growth. For pH values of 4.0 and 6.0, the particles were sphere-like in shape, and were distorted with distinct grain boundaries and low coagulation. At pH = 6, the particle size was in the 10–20 nm range and exhibited relative separation. Meanwhile, for a pH of 8.0, the particles had a variable shape and were coagulated in large clusters around 400 nm in size with indistinct grain boundaries. For a pH of 10.0, the particles were coagulated into large blocks with lengths of ∼370 nm and widths of ∼160 nm. The ZnO NPs exhibited strong antibacterial activity toward Escherichia coli ( E. coli ) and Staphylococcus aureus ( S. aureus ) without UV illumination at an NP concentration of 0.025 mg mL −1 after 8 h of incubation. In particular, the bactericidal activity towards S. aureus varied extensively with the synthesis parameters. This study presents an efficient green synthesis route for ZnO NPs with a wide range of potential applications, especially in the biomedical field.

4. Modification of zinc oxide nanoparticles

Cao et al. 90 used silica and trimethyl siloxane (TMS) for modifying ZnO in order to achieve a two-fold benefit: enhancing the compatibility of ZnO and cutting down on its agglomeration in the organic phase. A chemical precipitation method using zinc sulfate heptahydrate (ZnSO 4 ·7H 2 O), ammonium solution (NH 4 OH) and ammonium bicarbonate (NH 4 HCO 3 ) was adopted to first obtain the precursor, zinc carbonate hydroxide (ZCH). The surface of the ZCH was then successively modified by an in situ method using TEOS and hexamethyldisilazane (HMDS) in water. The functionalized ZHC was subjected to calcination, to yield extremely fine nanoparticles of ZnO. Reduced agglomeration was thereby effected through such functionalization of the surfaces of ZnO NPs although a lowered photocatalytic activity of the oxide was observed. Nevertheless, a marked increase in the compatibility of ZnO with the organic matrix lent credence to the method. Further, the greater shielding capacity of UV radiation renders the synthesized nanomaterial an excellent candidate for use in cosmetics. Below is a schematic representation ( Fig. 4 ) of the synthesis of surface-modified ZnO ultrafine particles using an in situ modification method.

The FTIR spectra for the SiO 2 -modified ZnO revealed interphase bonds between ZnO and SiO 2 . A thin film coating of SiO 2 on the ZnO surface resulted in enhanced dispersion and reduced agglomeration of nanoparticles, a fact fairly well corroborated by HR-TEM data. The photocatalytic activity of SiO 2 -modified ZnO however suffered a setback in comparison with that demonstrated by uncoated ZnO. The work further demonstrated that the thorough reduction of the crystallinity of ZnO achieved through heterogeneous azeotropic distillation of the zinc oxide precursor not only precludes aggregation but also brings about a decline in the average particle size.

Yuan et al. 93 modified ZnO using Al 2 O 3 . A basic carbonate of zinc was obtained from the reaction between zinc sulfate and ammonium bicarbonate followed by precipitating aluminum hydroxide over it. The resulting compound-precipitate was then calcined at 400–600 °C to obtain ZnO NPs coated with Al 2 O 3 . It was discovered from TEM analysis that as the Al 2 O 3 -coating content rose from 3 to 5%, agglomeration decreased significantly and correspondingly the particle size decreased from an average value of 100 nm to 30–80 nm. The coating thus designed was 5 nm thick and was highly uniform. The coating-core interphase possibly had the structure of ZnAl 2 O 4 . Zeta potential data clearly confirm modifications on the ZnO surface by Al 2 O 3 deposition. The change in pH at the isoelectric point for ZnO NPs upon coating with Al 2 O 3 from around 10 to a value of 6 might have assisted a greater degree of dispersion of ZnO NPs.

In a study by Hu et al. , 94 nano-sized ZnO rods doped with transition metals such as Mn, Ni, Cu, and Co were designed by a plasma enhanced chemical vapor deposition method. The ZnO thus modified had a greater amount of crystal defects within its structure. This led to its greater sensitivity towards formaldehyde. When the 1.0 mol% Mn doped ZnO nanorods were activated by 10 mol% CdO, a maximum sensing of ∼25 ppm was obtained and the corresponding response and recovery time were found to be appreciably short.

Wysokowski et al. 95 developed a β-chitin/ZnO nanocomposite material. The β-chitin used in the synthesis was derived from Sepia officinalis , a cephalopod mollusk. This nanocomposite was found to exhibit remarkable anti-bacterial activity and was touted as an excellent ingredient for the making of wound-dressing materials.

Ong et al. 96 in their work synthesized a heterogeneous photocatalytic material by loading ZnO on solvent exfoliated graphene sheets. For anchoring ZnO onto the graphene sheet, they used poly(vinyl pyrrolidone) as an inter-linker which was also found to enhance the functionalization of the acid treated graphene sheets. The thermal stability of the decorated ZnO was found to be higher than that of the undecorated oxide. The modified ZnO proved to be an outstanding photocatalyst being able to cause 97% degradation of Reactive Black 5 under visible light. This improvement was attributed to a host of favourable parameters achieved through the modification, namely, an enhancement of light absorption intensity, widening of the light absorption range, suppression of charge carrier recombination, improvement of surface active sites and rise in the chemical stability of the designed photocatalyst.

Tang et al. 97 demonstrated a way to tackle the agglomeration tendency of ZnO NPs. They prepared ZnO/polystyrene nanocomposites via a mini-emulsion polymerization method. For this, a silane coupling agent, namely γ-glycidoxypropyl trimethoxysilane (KH-560, AR), was first allowed to cling to ZnO NPs via reaction between its Si–OCH 3 groups and the hydroxyl groups on the surface of the nanoparticles followed by anchoring of 4,4′-azobis(4-cyanovaleric acid) (ACVA) onto their surface through reaction of its carboxyl groups with the terminal epoxy groups of the aforementioned coupling agent. Subsequently, polymerization of the styrene monomer was initiated using the azo group of ACVA for designing the final nanocomposites. The monomer droplet of the mini-emulsion polymerization system thus obtained contained well dispersed ZnO/polystyrene nanocomposites with a high grafting efficiency of 85% as calculated from TGA. It was evident from scanning electron microscopy (SEM) that while pure ZnO NPs suffered considerable agglomeration in poly(vinyl chloride) (PVC) film, the ZnO/polystyrene nanocomposite particles underwent homogeneous dispersion in the PVC matrix. The scheme depicted in Fig. 5 explains the mechanism of the mini-emulsion polymerization method to construct ZnO/polystyrene nanocomposites adopted by Tang and his research group. From SEM micrographs ( Fig. 5 ), it was observed that functionalized ZnO (f-ZnO) nanoparticles had been well dispersed in the polymer matrix because the f-ZnO nanofiller had outstanding adhesion and strong interfacial bonding to PEA. As was observed, f-ZnO nanoparticles were homogeneously dispersed in the polymer matrix and their sizes were estimated to be between 20 and 50 nm.

Cyclodextrins (CDs) make up a class of cyclic torus-shaped oligosaccharides. CD has a hydrophilic external surface and a hydrophobic internal cavity. CDs have been extensively used as eco-friendly coupling agents. 98,99 Among the derivatives of CDs, monochlorotriazinyl-β-cyclodextrin (MCT-β-CD) with a monochlorotriazinyl group as a reactive anchor was found to possess the ability to form covalent bonds with substituents of the nucleophilic type, viz. , –OH or –NH 2 groups. 100–103 Therefore, MCT-β-CD provides an interesting way of surface modification for inorganic nanomaterials. Abdolmaleki et al. 104 accomplished surface modification of ZnO NPs by covalently grafting MCT-β-CD onto the surfaces of ZnO NPs through a facile and single-step procedure. In the next step, f-ZnO nanoparticles were employed for construction of a new series of poly(ester-amide)/ZnO bionanocomposites (PEA/ZnO BNCs) whose TEM image is shown in Fig. 6 . MCT-β-CD has monochloro-triazinyl groups that react with –OH groups on the surfaces of ZnO NPs through nucleophilic reaction ( Fig. 7 ). After the incorporation of MCT-β-CD on the surfaces of ZnO NPs, polymer/ZnO bionanocomposites (BNCs) were designed using a biodegradable amino acid containing poly(ester-amide) (PEA). ZnO NPs with β-CD functional groups incorporated on their surfaces exhibited a near-complete suppression of their tendencies towards agglomeration while simultaneously displaying enhanced compatibility with the polymer matrix. Scores of functional groups on the surfaces of ZnO NPs enable possible interactions with PEA chains that lead to excellent dispersion and compatibility with the polymer matrix. FE-SEM and TEM results bore out a reduction of agglomeration that can be safely attributed to the steric hindrance induced by the organic chains of MCT-β-CD between the inorganic nanoparticles. The dispersibility, surface morphology and particle dimensions of functionalized ZnO (f-ZnO) with β-CD are shown in Fig. 8 .

5. Potential applications

5.1 concrete and rubber industries.

In their attempt to enhance the interactions between the nano-sized ZnO particles and the polymer, Yuan et al. 110 by incorporating vinyl silane groups on the surfaces of ZnO NPs using vinyl triethoxysilane through a procedure premised on the hydrosilylation reaction during curing carried out their surface modification. The vinyl silane groups on the ZnO surface enabled improved cross-linking with the rubber matrix. In order to solve this problem, surface modification techniques are applied to improve the interaction between the nanoparticles prepared by the sol–gel method and the polymer. In comparison with the nanocomposites of silicone rubber with ZnO, the nanocomposites of silicone rubber with vinyl triethoxysilane modified ZnO possessing extensive cross-linking and a higher degree of dispersion with the rubber matrix exhibited superior mechanical properties and enhanced thermal conductivity.

ZnO NPs have been widely used as an efficient material for the enrichment of cross-linking in elastic polymers. 111,112 The cured polymer produced through incorporation of ZnO NPs exhibited high ultimate tensile strength, tear strength, toughness and hysteresis. The slippage of polymer chains on the surfaces of ionic clusters and the renewal of ionic bonds when the sample gets externally deformed give rise to enhanced capacity of the ionic elastic polymer for stress relaxation which in turn results in its upgraded mechanical properties. Furthermore, the thermoplastic properties of such polymers enable their processing in a fused state in a manner akin to a thermoplastic polymer. 113 Nevertheless, carboxylic elastic polymers with ZnO as a cross-linker suffer from a few drawbacks prominent among which are their tendency to get scorched, feeble flex properties and high value of compression set. The tendency to get scorched is gotten rid of by the incorporation of either zinc peroxide (ZnO 2 ) or ZnO 2 /ZnO cross-linkers. ZnO 2 serves to not only create ionic cross-links but also generate covalent cross-links as a result of peroxide action. However, prolonged curing is needed to obtain elastomers with an ultimate strength and cross-link density comparable to that of ZnO-cross-linked elastomers. The three vital processes that amount to the curing of XNBR by ZnO 2 /ZnO cross-linkers are rapid creation of ionic crosslinks due to the initial ZnO present, covalent links resulting from peroxide cross-links and further ionic cross-linking due to the generation of ZnO from the decomposition of ZnO 2 . Leaving aside the problem of scorching, ZnO NPs make good and therefore widely used cross-linkers in carboxylated nitrile rubbers.

The prime factors affecting the involvement of ZnO in the formation of ionic cross-links with the carboxylic groups of the elastic polymers are its particle size, surface area and morphology. They are also found to govern the dimensions of the interphase between the cross-linkers and elastomer chains. 114 With a view to ascertain the correlation between the characteristics of ZnO NPs and their roles in the curing of elastic polymers, Przybyszewska et al. 115 employed a variety of ZnO NPs with different morphological characteristics (spheres, whiskers, and snowflakes) as cross-linkers in a carboxylated nitrile elastomer. It emerged from their investigation that ZnO NPs as cross-linkers imparted improved mechanical properties to vulcanizates than commercially used ZnO micro-particles. The ultimate tensile strength of vulcanizates with ZnO NPs was found to be four times higher than that of ZnO micro-particles containing vulcanizates. As a result, there is a 40% reduction of the quantity of ZnO that is put to such use. Since ZnO is known to have deleterious effects on aquatic life, an approach that reduces its usage is highly commendable from the point of view of eco-friendliness. However, ZnO cross-linked XNBR undergoes shrinkage on prolonged exposure to heat.

Among all the aforesaid morphologies, it was observed that ZnO snowflakes with a surface of approximately 24 m 2 g −1 had the highest activity. However, surface area and particle size exerted little influence on the activity of ZnO cross-linkers. It was also observed that the ZnO NPs exhibited a minimum tendency to agglomerate in the rubber matrix. There gathered smaller agglomerates with ZnO NPs as cross-linkers upon sample deformation as compared to the large agglomerates observed with ZnO microparticles.

The usage of ZnO as a cross-linker in rubber has an adverse impact on the environment, particularly when it is discharged into the surroundings upon degradation of rubber. 116 Zinc is known to cause great harm to aquatic species 117 and efforts to cut down on the content of ZnO in rubber are hence being made. 118 Bringing down the ZnO level in rubber, therefore, may follow any of the following three fundamental procedures:

(i) substituting the commonly used micro-dimensional ZnO of surface area 4–10 m 2 g −1 with nano-structured ZnO with surface area of up to 40 m 2 g;

(ii) carrying out surface modifications of ZnO with carboxylic acids ( viz. , stearic acid, maleic acid and the like);

(iii) using additional activators. 119

In order to get over the eco-toxicity associated with the usage of ZnO in large quantities, Thomas et al. 120 designed a few unique accelerators, namely, N -benzylimine aminothioformamide (BIAT)-capped-stearic acid-coated nano-ZnO (ZOBS), BIAT-capped ZnO (ZOB), and stearic acid-coated nano-zinc phosphate (ZPS), to probe their effects on the curing of natural rubber (NR) and thereby its mechanical properties. ZnO NPs prepared by the sol–gel route were surface-decorated using accelerators such as BIAT and fatty acids such as stearic acid. The capping agents functioned to reduce the size of agglomerates leading to an improvement of vulcanization and physicochemical properties of NR. Capping of ZnO further ensured a decline in the time and energy required for dispersion in the rubber matrix. As a result, there happened a further enhancement of the acceleration of vulcanization and a remarkable upgrade of the mechanical properties of the emerging vulcanizates. The rubber vulcanized with an optimal dose of BIAT-capped-stearic acid-coated zinc oxide (ZOBS) was found to possess superlative curing and mechanical properties in comparison with other countertypes and the reference polymer containing pristine ZnO NPs. The rigidity of vulcanizates containing ZPS was found to increase as a result of an enhanced cross-link density. The vulcanizates exhibited reduced tendency to get scorched as a result of incorporation of capped ZnO NPs and this was attributed to the delayed release of BIAT from the capped ZnO into the rubber matrix for interaction with CBS (conventional accelerator). Sabura et al. 121 adopted a solid-phase pyrolytic procedure to synthesize ZnO NPs of particle size in between 15 and 30 nm and surface area in the range 12–30 m 2 g −1 for use in neoprene rubber as cross-linkers. Two findings emerged from this study. One, the optimal content of ZnO required was found to be low in comparison with commercially used ZnO. Two, the cure characteristic and mechanical properties of the rubber showed a marked improvement when compared with those containing conventional ZnO.

5.2 Opto-electronic industry

The last decade has seen an upsurge in the fabrication of ZnO-based perovskite solar cells (PSCs). Although the conventional choice for an electron transport layer has been TiO 2 , ZnO with higher electron mobility is increasingly replacing it as an efficient and low-cost material for electron transport in PSCs. Additionally, the power conversion efficiency of PSCs at large has exceeded 20% of late giving the necessary impetus to delve deep into the fabrication of ZnO electron transport layers (ETLs) for yet more brilliant perovskite solar devices. Bi et al. 134 fabricated a PSC with ZnO nanorods aligned vertically over the substrate. With the length of nanorods, the J sc (short-circuit current density), FF (fill factor) and PCE of solar cells were found to increase. They however reported a decrease in V oc with nanorod length. They reasoned that nanorod length has a bearing on the electron transport time and lifetime that in turn influence the performance of the solar cell. They achieved a maximum overall cell efficiency of 5%. Son et al. 135 substituted the single-step method used by Bi et al. by a two-step coating procedure. Such a treatment generated a fully filled perovskite film that covered all ZnO nanorods of varying lengths without voids and formed an overlayer on the surface of nanorods. As a further consideration, the two-step coating treatment induced optimization of the cuboid size of MAPbI 3 and reduced the series resistance of the solar cell. 136 As a result, a maximum PCE of 11.13% was obtained. Tang et al. designed ZnO nanowall ETLs. 137 The best performance PSC based on ZnO nanowalls produced a J sc of 18.9 mA cm −2 , V oc of 1.0 V, FF of 72.1%, and PCE of 13.6%. Meanwhile, the control device shows a J sc of 18.6 mA cm −2 , V oc of 0.98 V, FF of 62%, and PCE of 11.3%. The introduction of ZnO nanowalls led to an evident boost in the FF and PCE of the PSCs and this can be ascribed to the greater contact area between ZnO and perovskite offered by the ZnO nanowalls in comparison with the planar ZnO film which improves not only the electron collection but also transportation efficiency at the interface of the ZnO nanowalls and perovskite. Moreover, the decomposition of ZnO by perovskite triggered by the alkaline nature of the ZnO surface leads to the formation of PbI 2 on the perovskite/ZnO interface. The presence of PbI 2 can suppress the surface recombination and improve the FF. 138

5.3 Gas-sensing

The electrons injected into the conduction band lower the resistance of the Al-doped ZnO gas sensors. The response and recovery times observed for all Al-loaded ZnO samples were 6–8 s sand 16–30 s, respectively. The unloaded ZnO sample was marked by longer response and recovery times of 30 s and 70 s, respectively. The sensing films exhibited excellent thermo-mechanical and electrical stability.

Therefore, the ZnO/SnO 2 nanocomposite gas sensor demonstrated a sharper response to ethanol gas than the pristine SnO 2 sensor. Moreover, a possible increase in the effective barrier height of the n–n heterojunction enabled better engagement with adsorbed oxygen causing greater depletion of electrons from the conduction band eventually leading to an enhanced gas sensing response by the system. Additionally, remarkable detection at a lower (ppb) limit was shown by the heterostructured sensor.

5.4 Cosmetic industry

In a study by Reinosa et al. , 149 it was brought to light that a nano/micro-composite comprising nanosized TiO 2 dispersed on ZnO micro-particles showed a higher sun protection factor (SPF) than individual TiO 2 and ZnO particles. The SPF of the synthesized nano-sized TiO 2 was found to be higher than that of its micro-sized counterpart with the former showing maximum absorption at 319 nm while the latter showed maximum absorption at 360 nm. The synthesized micro-sized ZnO had a higher SPF than its nano equivalent. Both exhibited maximum absorption at 368 nm. These data suggested that ZnO has a higher critical wavelength because it covers the entire UV range and has a higher UVA/UVB ratio since the maximum of the SPF curve lies in the UVA region ( Fig. 16a ). Additionally, it was observed that TiO 2 , with a lower UVA/UVB ratio owing to the presence of the SPF maximum in the lower wavelength region, has a lower critical wavelength ( Fig. 16b ). Therefore, to boost the SPF output, a suitable combination of the two oxides was thought out. A dry dispersion procedure was adopted to prepare the composite consisting of 15 wt% TiO 2 NPs and 85 wt% ZnO micro-structured particles. The results obtained from this composite were compared with those obtained by the standard procedure. Raman spectroscopy revealed a superior dispersion of the NPs and their anchoring with higher quantum confinement resulting from dry dispersion by using ZnO micro-structures as host particles. The SPF output was found to be higher for the sunscreen with the filter prepared by the dry dispersion method than the one with the filter synthesized following the standard method ( Fig. 17 ). This observation was chiefly attributed by the authors to the correct dispersion of TiO 2 NPs over the host ZnO micro-sized particles.

5.5 Textile industry

It has been shown in many research investigations that the use of ZnO in the processing of fabrics promotes their anti-bacterial and self-cleaning properties apart from upgrading their UV absorption capacity. 158 Moreover, in textile applications, coatings of ZnO in the nano-dimensions aside from being bio-compatible are found to exhibit air-permeability and UV-blocking ability far greater than their bulk equivalents. 159 Therefore, ZnO nanostructures have become very attractive as UV-protective textile coatings. Different methods have been reported for the production of UV-protective textiles utilizing ZnO nanostructures. For instance, hydrothermally grown ZnO nanoparticles in SiO 2 -coated cotton fabric showed excellent UV-blocking properties. 160 Synthesis of ZnO nanoparticles elsewhere through a homogeneous phase reaction at high temperatures followed by their deposition on cotton and wool fabrics resulted in a significant improvement in UV-absorbing activity. 161 Similarly, ZnO nanorod arrays that were grown onto a fibrous substrate by a low-temperature growth technique provided excellent UV protection. 162

Zinc oxide nanowires were grown on cotton fabric by Ates et al. 163 to impart self-cleaning, superhydrophobicity and ultraviolet (UV) blocking properties. The ZnO nanowires were grown by a microwave-assisted hydrothermal method and subsequently functionalized with stearic acid to obtain a water contact angle of 150°, demonstrating their superhydrophobic nature, which is found to be stable for up to four washings. The UV protection offered by the resulting cotton fabric was also examined, and a significant decrease in transmission of radiation in the UV range was observed. The self-cleaning activity of the ZnO nanowire-coated cotton fabric was also studied, and this showed considerable degradation of methylene blue under UV irradiation. These results suggest that ZnO nanowires could serve as ideal multifunctional coatings for textiles.

Research on the use of zinc oxide in polyester fibres has also been carried out at Poznan University of Technology and the Textile Institute in Lodz. 164 Zinc oxide was obtained by an emulsion method, with particles measuring approximately 350 nm and with a surface area of 8.6 m 2 g −1 . These results indicate the product's favourable dispersive/morphological and adsorption properties. Analysis of the microstructure and properties of unmodified textile products and those modified with zinc oxide showed that the modified product could be classed as providing protection against UV radiation and bacteria.

5.6 Antibacterial activity

Epidemic disease cholera mainly affects populations in developing countries. 169,180 It is a serious diarrheal disease caused by the intestinal infection of Gram-negative bacterium V. cholerae. The effective antibacterial activity of ZnO NPs and their mechanism of toxicity were explored against Vibrio cholerae (two biotypes of cholera bacteria (classical and El Tor)) by Sarwar et al. 176 Strong arguments and detailed justifications of the toxicity mechanism emerged as a result of this rigorous investigation. The bacterial membrane bears an overall negative charge that can be ascribed to the acidic phospholipids and lipopolysaccharides in it while ZnO NPs possess a positive charge in water suspension. An initial NP–membrane interaction via electrostatic attraction may result from this charge difference following which membrane disruption occurs. As the membrane plays an essential role by maintaining the vital function of the cell, such damage induces depolarization of the membrane, increased membrane permeabilization – loss in membrane potential and protein leakage and denaturation upon subsequent contact with ZnO NPs. Besides, ZnO NPs also have the ability of interacting with DNA as well as forming abrasions on it. Significant oxidative stress was also noticed inside the bacteria cells. They thus arrived at a conclusion that binding of ZnO NPs with the bacterial cell surface induces membrane damage followed by internalization of NPs into the cells, leakage of cytoplasmic content, DNA damage and cell death. Disruption of the membrane by ZnO NPs would additionally give easy access of antibiotics into the cell. Their findings further corroborated a synergic effect produced by the actions of ZnO NPs and antibiotics. They also encountered the antibacterial activity of the ZnO NPs in cholera toxin (CT) mouse models. It emerged that ZnO NPs could induce the CT secondary structure collapse gradually and interact with CT by interrupting CT binding with the GM1 ganglioside receptor. 181

In bacteria treated with NPs of ZnO, it was observed that the damage to cell membranes was an inevitable phenomenon. The pathways of the antibacterial activity of ZnO NPs were investigated using Escherichia coli ( E. coli ) as a prototype organism. 182 As was evident from the SEM images of E. coli obtained after treatment with ZnO NPs, a greater number of cell damage sites were noted at higher doses of ZnO NPs. This cell damage has been ascribed to pathways involving both the presence and absence of ROS. In the absence of ROS, the interaction of ZnO NPs with bacterial membranes would lead to damage to the molecular structure of phospholipids culminating in cell membrane damage.

Jiang et al. 183 studied the potential antibacterial mechanisms of ZnO NPs against E. coli . They reported that ZnO NPs with an average size of about 30 nm caused cell death by coming into direct contact with the phospholipid bilayer of the membrane and destroying the membrane integrity. The significant role of ROS production in the antibacterial properties of ZnO NPs surfaced when it emerged that the addition of radical scavengers such as mannitol, vitamin E, and glutathione could block the bactericidal action of ZnO NPs. However, the antibacterial effect triggered by Zn 2+ released from ZnO NP suspensions was not apparent. Reddy synthesized ZnO NPs with sizes of ∼13 nm and investigated their antibacterial ( E. coli and S. aureus ) activities. 168 It was discovered that ZnO NPs effected complete cessation of the growth of E. coli at concentrations of about 3.4 mM but induced growth inhibition of S. aureus at much lower concentrations (≥1 mM). Besides, Ohira and Yamamoto 184 also discovered that the antibacterial ( E. coli and S. aureus ) activity of ZnO NPs with small crystallite sizes was far more pronounced than for those with large crystallite sizes. From ICP-AES measurement, it emerged that the amount of Zn 2+ released from the small ZnO NPs was much higher than from the large ZnO powder sample and E. coli was more sensitive to Zn 2+ than S. aureus . This is a further confirmation that eluted Zn 2+ ions from ZnO NPs also play a key role in antibacterial action.

Iswarya et al. , 185 having extracted crustacean immune molecule β-1,3-glucan binding protein (Phβ-GBP) from the haemolymph of Paratelphusa hydrodromus , successfully designed Phβ-GBP-coated ZnO NPs. The Phβ-GBP-ZnO NPs were spherical shaped having a particle size of 20–50 nm and halted the growth of S. aureus and P. vulgaris . S. aureus was found to be more prone to the bactericidal action of Phβ-GBP-ZnO NPs than P. vulgaris . In addition, Phβ-GBP-ZnO NPs could induce drastic modification in cell membrane permeability and set off outrageous levels of ROS formation both in S. aureus and P. vulgaris . This work was thus pivotal in bringing to the forefront the immensely great antibacterial hallmark of Phβ-GBP-ZnO NPs.

The mechanism of breaking into bacterial cells by membrane disruption and then inducing oxidative stress in bacterial cells, thereby stalling cell growth and eventually causing cell death has been reported in many recent research studies. 186–191 Important bacterial biomolecules can also adsorb on ZnO NPs. Bacterial toxicity, in the recent past, has been heavily reported to have resulted from structural changes in proteins and molecular damage to phospholipids. 192 The antibacterial activity of ZnO NPs thus finds its apt application in the discipline of food preservation. As a formidable sanitizing agent, it can be used for disinfecting and sterilizing food industry equipment and containers against attack and contamination by food-borne pathogenic bacteria. ZnO NPs showed both toxicity on pathogenic bacteria ( e.g. , Escherichia coli and Staphylococcus aureus ) and beneficial effects on microbes, such as Pseudomonas putida , which has bioremediation potential and is a strong root colonizer. 193

Investigations into the antibacterial activities of ZnO micro-sized particles, ZnO NPs, and ZnO NPs capped with oxalic acid against S. aureus were carried out in the presence and absence of light. 194 It was observed that the efficiency of ZnO NPs was just 17% in the dark. However, their antibacterial properties saw a surge up to 80% upon application of light. The antibacterial behaviour was greatest for ZnO NPs while it was minimum for ZnO micro-sized particles, suggesting a higher release of Zn 2+ ions from ZnO NPs than ZnO micro-sized particles. The examination revealed that surface defects of the ZnO NPs boosted ROS production in the presence as well as absence of light. Additionally, it was also found that capping lowers the amount of superoxide radicals generated because capping blocks the oxygen vacancies that are chiefly accountable for the generation of superoxide radicals. In another investigation, the influence of NP size on bacterial growth inhibition by ZnO NPs and the mechanistic routes of their action were demonstrated. 195 ZnO NPs with diameters ranging from 12 nm to 307 nm were first generated. Thereafter, they were administered to Gram-positive and Gram-negative microorganisms ( Fig. 18 ). The results clearly illustrated the greater bactericidal efficacy of smaller ZnO NPs under dark conditions. The use of UV light resulted in an enhanced antibacterial behaviour of ZnO NPs owing to the enhanced formation of ROS from them. The antibacterial properties were rooted in the generation of ROS and the build-up of ZnO nano-sized particles in the cytoplasm and on the external membranes.

In another intriguing investigation, the toxicity induced in antibiotic resistant nosocomial pathogens such as Acinetobacter baumannii ( A. baumannii ) and Klebsiella pneumoniae ( K. pneumoniae ) by photocatalytic ZnO NPs was studied. 196 It was seen that A. baumannii and K. pneumoniae were significantly destroyed by 0.1 mg mL −1 of ZnO nano-structures with 10.8 J cm −2 of blue light. Further, the mechanistic pathway of the antibacterial activity of photocatalytic ZnO NPs against antibiotic defiant A. baumannii was investigated. While cytoplasm leakage and membrane disruption of A. baumannii were evident after treatment with ZnO NPs under blue light exposure, there was no sign of plasmid DNA fragmentation. Therefore, membrane disruption could be associated with the mechanistic route via which the photocatalytic ZnO NPs demonstrated antibacterial activity. The possibility of the role of DNA damage therein was categorically ruled out.

A novel approach comprising a combined application of ultrasonication and light irradiation to ZnO NPs has been developed to boost their antibacterial properties. 197 The sono-photocatalytic activity of ZnO nanofluids against E. coli was tested. The results revealed a 20% rise in the antibacterial efficacy of ZnO nanofluids. Further, ROS generation by ZnO nanofluids played a crucial role in bacterial elimination. The sono-photocatalysis of ZnO nanofluids also enhanced the permeability of bacterial membranes, inducing more efficacious penetration of ZnO NPs into the bacteria.

Although ZnO NPs make a promising antibacterial agent owing to their wide-ranging activities against Gram-positive as well as Gram-negative bacteria, the exact antibacterial pathway of ZnO NPs has not been adequately established. Hence, deep investigations into it hold a lot of important theoretical and practical value. In the future, ZnO NPs can be explored as antibacterial agents, such as ointments, lotions, and mouthwashes. Additionally, they can be overlayed on various substrates to prevent bacteria from adhering, spreading, and breeding in medical devices.

5.7 Drug delivery

Reports bearing evidence of the applications of ZnO NPs in the delivery of chemotherapeutic agents to treat cancers have emerged prolifically in the last few years. For instance, a porous ZnO nanorod based DDS (ZnO-FA-DOX), enclosing folic acid (FA) as a targeting agent and doxorubicin (DOX) as a chemotherapeutic drug, was fabricated by Mitra et al. 204 The ZnO-FA-DOX nano-apparatus was found to exhibit pH-triggered release of DOX and potent cytotoxicity in MDA-MD-231 breast cancer cells. The biocompatible nature of the ZnO-FA material, as observed from the acute toxicity study in a murine model also emerged from the investigation. In another research study by Zeng et al. , 205 a lymphatic-targeted DDS with lipid-coated ZnO-NPs (L-ZnO-NPs) enclosing 6-mercaptopurine (6-MP) as an anticancer agent was designed. The L-ZnO-NP apparatus demonstrated pH-susceptive drug release and remarkable cytotoxicity to cancer cells as a result of the generation of intracellular reactive oxygen species (ROS). Liu et al. 206 also reported the fabrication of DOX-loaded ZnO-NPs. The researchers encountered a pH-susceptive drug release from the DDS and diminished drug efflux with enhanced cytotoxicity in drug defiant breast cancer cells (MCF-7R). Likewise, Li et al. 207 fabricated a novel DDS enclosing hollow silica nanoparticles (HSNPs) embedded with ZnO quantum dots to co-deliver DOX and camptothecin. The nano-apparatus evinced pH-susceptive drug release and cytotoxicity to drug defiant cancer cells. A research study used ZnO NPs as caps to cover the pores of mesoporous silica NPs (MSNs), and when the designed drug delivery apparatus came into contact with acids, there took place a decomposition of ZnO NPs followed by a release of doxorubicin (DOX) molecules from the MSN nanostructures. 208 One major drawback of such an apparatus was that it had difficulty in degradation thereby resulting in an incomplete release of drugs. 209,210 Another scheme employed the technique of loading drugs onto the ZnO NPs directly. 211 Upon contact with acids, the drug molecules are released following the complete decomposition of ZnO NPs. In another investigation, a liposome-incorporated ZnO-NP based DDS (ZNPs-liposome-DNR) enclosing anticancer drug daunorubicin (DNR) was designed by Tripathy et al. 212 The incorporation of ZnO-NPs in the DDS was observed to prevent the premature release of DNR, which could be prompted only in acidic medium, thereby efficiently exerting an anticancer effect on A549 cells. The study of intracellular release in cancer cells with confocal laser scanning microscopy (CLSM) revealed that treatment with ZNPs-liposome-DNR induced a marked DNR release, causing greater cytotoxicity to cancer cells, compared to pure DNR and DNR-conjugated liposomes (liposome-DNR), as evidenced by the green fluorescence intensity. For the treatment of lung cancer, Cai et al. accomplished the construction of ZnO quantum dot-based drug delivery apparatus that was conjugated with a targeting agent (hyaluronic acid) and an anticancer agent (DOX). 213 The nano-apparatus demonstrated CD44 receptor-specific uptake and pH-driven drug release in lysosomal compartments of the cancer cells. Kumar et al. 214 also designed sub-micron sized self-assembled spherical capsules of ZnO nanorods that successfully effected the delivery of anticancer agent DOX to K562 cancer cells. Furthermore, Han et al. 215 also synthesized ZnO NPs conjugated with an aptamer as a functionalization agent and DOX as an anticancer agent and demonstrated the effect of combined chemo- and radiation therapy in MCF-7 breast cancer cells employing the nano-apparatus. Recently, Zhang et al. 216 as a part of their investigation devised a new scheme to restrain the proliferation of human hepatocarcinoma cells (SMMC-7721) via a combined application of ZnO nanorod based DNR in photodynamic therapy (PDT), where ROS generation had the possibility to play a key role in the net anticancer behaviour of the hybrid nano-apparatus ( Fig. 19 ). The researchers further discovered that ZnO NPs were able to transport a larger quantity of DNR via internalization into SMMC-7721 cells, thereby inducing outstanding restraint on the multiplication of these cancerous cells. Besides, UV irradiation on this drug delivery nano-apparatus further reinforced the arrest of cell proliferation through photocatalysis of ZnO nanorods. To look into the signaling pathway of anticancer activity of the DDS in PDT, the researchers monitored the caspase-3 activity, which is a hallmark of apoptosis. The results of immunocytochemistry study confirmed that upon treatment with a DNR–ZnO nanocomposite under UV irradiation, the cells demonstrated far more pronounced activation of caspase-3 molecules in cancer-afflicted cells. It was consequently proposed that ZnO nanorods could raise the drug's targeting efficiency and minimize the associated toxicity. Therefore, the DNR–ZnO hybrid nano-apparatus with UV irradiation was claimed to have the potential of a fruitful scheme for the treatment of cancers ( Fig. 20 and 21 ). In another study, Ye et al. 217 using a copolymerization process also prepared water soluble ZnO–polymer core–shell quantum dots, and designed a drug delivery apparatus based on these quantum dots containing Gd 3+ ions and anticancer drug DOX. The ZnO-Gd-DOX nano-system was found to be biocompatible, pH-responsive and led to a marked release of DOX into the acidic environment of cancer-afflicted cells and tumors. When administered to human pancreatic cancer (BxPC-3) tumor containing nude mice, this polymer-modified drug delivery nano-apparatus was found to display higher therapeutic efficiency compared to the FDA-approved liposomal DOX formulation DOXIL at 2 mg kg −1 DOX concentration. The histopathology study and ICP-AES analysis of the vital organs further confirmed that this ZnO-Gd-DOX nano-apparatus could substantially bring about growth-inhibition of tumors without exerting any toxic effects 36 days post administration. Additionally, the histopathology study of tumor sections also demonstrated severe damage to the tumor cells caused by the administration of the DDS, compared to the control, DOX and DOXIL groups.

5.8 Anti-cancer activity

Several studies have thus suggested the cytotoxic effects of ZnO NPs on cancer cells. The cancer cell viability percentage on the MCF7 cell line, A549 cell line, HL60 cell line and VERO cell line has been studied at various concentrations of ZnO. Results show that the cell viability of the above cell lines exhibits a marked decrease with a rise in ZnO concentration 221,222 with minimal damage to healthy cells.

The mitochondrial electron transport chain is known to be closely linked to intracellular ROS generation, and anticancer agents accessing cancer cells could impair the electron transport chain and release huge amounts of ROS. 223,224 However, an inordinate amount of ROS brings about mitochondrial damage thereby resulting in the loss of protein activity balance that eventually induces cell apoptosis. 225 ZnO NPs introduce certain cytotoxicity in cancer cells chiefly by a mechanism that involves a higher intracellular release of dissolved Zn 2+ ions, followed by enhanced ROS induction and induced cancer cell death by way of the apoptosis signaling pathway. The effects of ZnO NPs on human liver cancer HepG2 cells and their possible pharmacological mechanism were investigated by Sharma et al. 226 They observed that ZnO NP-exposed HepG2 cells exhibited higher cytotoxicity and genotoxicity, which were related to cell apoptosis conciliated by the ROS triggered mitochondrial route. The loss of the mitochondrial membrane potential led to the opening of outer membrane pores following which some related apoptotic proteins including cytochrome c were released into the cytosol thereby activating the caspase in due course. Mechanistic studies had proved that the loss of mitochondrial membrane potential-mediated HepG2 cell apoptosis was mainly due to the decrease in mitochondrial membrane potential and Bcl-2/Bax ratios as well as accompanying the activation of caspase-9. Besides, ZnO NPs could noticeably activate p38 and JNK and induce and attract p53 ser15 phosphorylation but this was not dependent on JNK and p38 pathways ( Fig. 21 ). These results afforded valuable insights into the mechanism of ZnO NP-induced apoptosis in human liver HepG2 cells. Moghaddam et al. 227 took recourse to biogenic synthesis and successfully generated ZnO NPs using a new strain of yeast ( Pichia kudriavzevii GY1) and examined their anticancer activity in breast cancer MCF-7 cells. ZnO NPs have been observed to exhibit powerful cytotoxicity against MCF-7 cells. This cytotoxicity is affected more likely via apoptosis than cell cycle arrest. The apoptosis induced by ZnO NPs was largely by way of both extrinsic and intrinsic apoptotic pathways. A few antiapoptotic genes of Bcl-2, AKT1, and JERK/2 were subjected to downregulation, while upregulation of some proapoptotic genes of p21, p53, JNK, and Bax was prompted. ZnO NPs have been widely employed in cancer therapy and reported to promote a selective cytotoxic effect on cancer cell proliferation. Chandrasekaran and Pandurangan evaluated the cytotoxicity of ZnO nanoparticles against cocultured C2C12 myoblastoma cancer cells and 3T3-L1 adipocytes. The study revealed that ZnO NPs could be more cytotoxic to C2C12 myoblastoma cancer cells than 3T3-L1 cells. Compared to 3T3-L1 cells, it emerged that ZnO NPs stalled C2C12 cell proliferation and brought about a more pronounced apoptosis by way of a ROS-conciliated mitochondrial intrinsic apoptotic route, an upregulation of p53, tempered Bax/Bcl-2 ratio, and caspase-3 routes. 228

In a study, biogenic zinc oxide nanoparticles (ZnO NPs) were developed from aqueous Pandanus odorifer leaf extract (POLE) with spherical morphology and approximately 90 nm size. 229 The anticancer activity of the ZnO NPs was evaluated by MTT assay and neutral red uptake (NRU) assays in MCF-7, HepG2 and A-549 cells at different doses (1, 2, 5, 10, 25, 50, and 100 μg mL −1 ). Moreover, the morphology of the treated cancer cells was examined by phase contrast microscopy. The results suggest that the synthesized ZnO NPs inhibited the growth of the cells when applying a concentration from 50–100 μg mL −1 . Overall, the study demonstrated that POLE derived biogenic ZnO NPs could serve as a significant anticancer agent. Phytomediated synthesis of metal oxide nanoparticles have become a key research area in nanotechnology due to its wide applicability in various biomedical fields. The work by Kanagamani et al. 230 explored the biosynthesis of zinc oxide nanoparticles (ZnO-NPs) using Leucaena leucocephala leaf extract. Biosynthesized ZnO-NPs were found to have a wurtzite hexagonal structure with particles distributed in the range of 50–200 nm as confirmed by TEM studies. The anticancer activity of ZnO-NPs against MCF-7 (breast cancer) and PC-3 (human prostate cancer) cell lines was evaluated using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. From the assay, biosynthesized ZnO-NPs were found to have better cytotoxic activity on PC-3 cell lines than MCF-7 cell lines. The in vitro cytotoxicity studies of biosynthesized ZnO-NPs against Dalton lymphoma ascites (DLA) cells revealed better antitumor activity with 92% inhibition at a ZnO-NP concentration of 200 μg mL −1 , and as the concentration increased, the anticancer efficiency also increased. These results suggested that ZnO NPs could selectively induce cancer cell apoptosis making them a bright candidate for cancer therapy.

Photodynamic therapy requires the administration of a photosensitizing agent that is subjected to activation by light of a specific wavelength thereby generating ROS. The application of ZnO NPs as effective photosensitizers can be ascribed to their capability to generate ROS in response to visible light or UV light. Recent studies exhibited that photo-triggered toxicity of ZnO NPs renders them aptly suitable for targeted PDT in a spatiotemporal manner, providing a surer way to selectively terminate cancerous cells. 231–234 An attempt was made to utilize the synergic effects of anticancer drugs with ZnO NPs in PDT to induce cell-death in cancer cells. 231 The cytotoxic effects of daunorubicin (DNR), an anti-cancer drug, on drug defiant leukemia K562/A02 cancer cells were put to the test in combination with ZnO NPs. The combination of DNR and ZnO NPs under UV irradiation could appreciably check the proliferation of drug-defiant cancer cells in a dose-dependent manner. Additionally, ZnO NPs were found to induce an enhanced cellular uptake of DNR.

An anticancer treatment using DNR-conjugated ZnO nanorods in PDT was investigated with human hepatocarcinoma cells (SMMC-7721) ( Fig. 19 ). 216 The fabrication of photo-excited ZnO nanorods with DNR displayed an outstanding boost in the anticancer properties of the ZnO nanorods ( Fig. 20 ). The ZnO nanorods raised the intracellular concentration of DNR and augmented the anticancer efficiency. This is further evidence of the drug carrying capacity of ZnO nanorods into target cancer cells. UV irradiation additionally reinforced the growth inhibition of cancerous cells via photocatalytic activity of ZnO nanorods. In this study, the promoted mortality of cancer cells indicates that ZnO nanorods under UV irradiation could efficiently induce the formation of ROS and further attack the cell membrane (mainly by lipid peroxidation), nucleic acids, and proteins (such as enzyme deactivation). The mechanism of ROS generation of ZnO nanorods under UV irradiation is displayed in Fig. 21 . ZnO is a direct band gap semiconductor with a band gap energy of 3.36 eV at room temperature, high exciton binding energy of 60 meV and high dielectric constant, which under UV irradiation will produce a hole (h + ) in the valence band and an electron (e − ) in the conduction band, namely electron/hole pairs. These electron/hole pairs will induce a series of photochemical reactions in an aqueous suspension of colloidal ZnO nanorods, generating ROS. Generally, at the surface of the excited ZnO nanorods, the valence band holes abstract electrons from water and/or hydroxyl ions, generating hydroxyl radicals (˙OH). Electrons reduce O 2 to produce the superoxide anion O 2 − ˙. ZnO nanorods can be one of the promising nanomaterials for PDT in cancer.

The size of ZnO NPs has been reported to have a strong association with their anticancer activities. The UV light-activated anti-cancer effects of various ZnO NPs with different sizes have been examined against human hepatocarcinoma cells (SMMC-7721). 232 To achieve synergetic cytotoxicity, a combination of ZnO NPs and an anticancer agent, DNR, was subjected to investigation. A schematic illustration of the anticancer effect of DNR-conjugated ZnO NPs under UV irradiation is shown in Fig. 22 . The outcome showed higher cytotoxicity of smaller NPs. UV irradiation greatly boosted the cytotoxic effect on SMMC-7721 cells treated with ZnO NPs via generation of ROS and a consequent cell apoptosis. Additionally, when the ZnO NPs were conjugated with DNR, their cytotoxicity against the cancer cells further increased by leaps and bounds.

To secure concomitant intracellular drug delivery and PDT for cancer treatment, poly(ethylene glycol) (PEG)-capped ZnO NPs enclosing DOX were fabricated. 233 It was found that DOX-loaded PEG-ZnO NPs on exposure to UV irradiation achieved significantly enhanced cell cytotoxicity through light-driven ROS production from the NPs. The synergistic anticancer activity of a combined treatment with PEG-ZnO NPs and DOX under UV irradiation came to the fore as a result of this investigation.

Likewise, poly(vinylpyrrolidone) (PVP)-capped ZnO nanorods (PVP-ZnO nanorods) were designed as a drug carrying nano-apparatus for the delivery of daunorubicin (DNR), as well as a photosensitizer for PDT. 234 The DNR-loaded PVP-ZnO nanorods (DNR-PVP-ZnO) encouraged an exceptional upswing in the anticancer activity of DNR due to elevated cellular uptake of the DNR delivered by the nanorods. The DNR-PVP-ZnO nanorods also demonstrated efficient PDT under UV light irradiation. It has been demonstrated that NPs can furnish solutions to confront the acute demerits of conventional photosensitizers. 235 By a dramatic enhancement of the solubility of photosensitizers, NPs can facilitate their increased cellular internalization. They also upgrade the target-specificity of photosensitizers by way of passive targeting to tumor tissues through the enhanced permeability and retention (EPR) effect. Further, cell-specificity of photosensitizers can be remarkably increased by surface modification of the NPs to bind active targeting components. Complexation of ZnO NPs with other photosensitizers has been widely researched to increase the efficacy of ZnO NPs in PDT by synergistically enhancing the ROS generation. 234,235 Meso -tetra( o -aminophenyl)porphyrin (MTAP)-conjugated ZnO nanocomposites were fabricated and examined for synergistic PDT against ovarian cancer cells. 236 The MTAP-ZnO NPs induced generation of ROS upon UV irradiation, the controlling parameters being concentration and light intensity. It emerged that 30 μM MTAP-ZnO NPs wielded high light-induced toxic effects in cancer-afflicted ovarian cells under UV illumination, while they remained inactive in the dark. The cytotoxic activity of MTAP-ZnO NPs under UV illumination was markedly boosted weighed against that of porphyrin alone. 235 This study elucidated the targeted and synergistic PDT by nanoparticles of ZnO loaded with photosensitizing substances. ZnO NPs were combined with protoporphyrin IX (PpIX) as a drug delivery nano-apparatus for photosensitizers. 237 Simple ZnO NPs and PEG-capped ZnO NPs were synthesized and examined for their cancer-eliminating effect against human muscle carcinoma cells. In the absence of laser light, ZnO NPs at 1 mM concentration were found to exert very low cytotoxicity (98% viability). In the presence of 630 nm laser light, PEG-capped ZnO NPs loaded with PpIX exhibited outstanding cytotoxicity owing to the increased ROS generation. Additionally, a high build-up of PpIX in the tumor area was observed when it was delivered by ZnO NPs, exhibiting the potency of ZnO NPs as a tumor-selective drug delivery system for photosensitizers.

5.9 Anti-diabetic activity

A natural extract of red sandalwood (RSW) as an effective anti-diabetic agent in conjugation with ZnO NPs has been tested by Kitture et al. 246 The anti-diabetic activity was evaluated with the help of α-amylase and α-glucosidase inhibition assay with murine pancreatic and small intestinal extracts. Results revealed that the ZnO–RSW conjugate effected a moderately higher percentage of inhibition (20%) against porcine pancreatic α-amylase and proved more effective against the crude murine pancreatic glucosidase than either of the two components alone (RSW and ZnO NPs). The conjugated ZnO–RSW induced 61.93% inhibition in glucosidase while the bare ZnO NPs and RSW exhibited 21.48 and 5.90% inhibition, respectively.

In an investigation conducted to compare the anti-diabetic activity and oxidative stress of ZnO NPs and ZnSO 4 in diabetic rats it was observed that ZnO NPs with small dimensions at higher doses (3 and 10 mg kg −1 ) had a much greater antidiabetic effect compared to ZnSO 4 (30 mg kg −1 ). The observation was backed up by a marvelous decline in the blood glucose level, a steep rise in the insulin level and a refinement of the serum zinc status in a time- and dose-dependent manner. However, it was finally inferred in the study that ZnO nanoparticles severely elicited oxidative stress particularly at higher doses corroborated by the altered erythrocyte antioxidant enzyme activity, enhancement in malondialdehyde (MDA) production, and remarkable drop in serum total antioxidant capacity. 240 Hyperglycemia can squarely trigger off an inflammatory state via activation of C-reactive protein (CRP) and cytokines, such as interleukins, eventually resulting in the development of cardiovascular diseases. Hussein et al. designed ZnO NPs using hydroxyl ethyl cellulose as a stabilizing agent to alleviate diabetic complications. 247 The study demonstrated that ZnO NPs could significantly decrease malondialdehyde (MDA), fast blood sugar and asymmetric dimethylarginine (ADMA) levels. The inflammatory markers, interleukin-1 (IL-1α) and CRP, were also notably lowered after ZnO NP treatment, concomitant with a rise in nitric oxide (NO) and serum antioxidant enzyme (PON-1) levels in diabetic rats.

An investigation was conducted in 2014 into the anti-diabetic potential of ZnO NPs in streptozotocin-induced diabetic albino (Sprague-Dawley) rats. 243 The researchers inferred that the administration of ZnO NPs in diabetic rats brought about a marked lowering of the blood glucose level, boosted the serum insulin level, and elicited the expression of insulin receptor and GLUT-2 proteins, suggesting the inherent capacity of ZnO NPs for diabetic remedy. The anti-diabetic activity of ZnO NPs in streptozotocin-induced diabetic (types 1 and 2) Wistar rats was also demonstrated by Umrani et al. in their research work. 248 The research revealed that ZnO NPs raised the levels of parameters like glucose, insulin, and lipid in rats attesting to the efficient anti-diabetic activity of ZnO NPs. The same research group recently undertook an enquiry into the mechanistic pathway behind the anti-diabetic properties of ZnO NPs in vitro . 249 They demonstrated that ZnO NPs led to protein kinase B (PKB) activation, enhanced glucose transporter 4 (GLUT-4) translocation and uptake of glucose, reduced glucose 6 phosphatase expression, proliferation of pancreatic beta cells, etc. , which were critically responsible for the anti-diabetic behaviour of ZnO NPs. The antidiabetic effectiveness of ZnO nanoparticles prepared using U. diocia leaf extract for treating alloxan-caused diabetic rats was evaluated. 250 From the characterization of the samples, the envelopment of extract over the ZnO-extract sample resulted in individual particles with enhanced properties compared to bulk ZnO. The occurrence of the nettle phytochemicals linked to the ZnO-extract sample was verified by various techniques, especially using TGA, FT-IR, and GC-MS analyses. Among all the employed treatments, the ZnO-extract performed the best for controlling the common complications accompanying diabetes. This biologically produced sample significantly lowered the levels of Fasting Blood Sugar (FBS), Total Triglycerides (TG), and Total Cholesterol (TC) and enhanced the high-density lipoprotein cholesterol (HDLC) and insulin levels in the diabetic rats when compared to the rest of the remedies. The results confirmed the synergistic relationship between ZnO and U. diocia leaf extract where ZnO-extract performed the best compared with the only extract and ZnO. From the results, the as-prepared ZnO-extract sample can be introduced as a non-toxic, applicable, and active phyto-nanotherapeutic agent for controlling diabetes complications.

ZnO nanoparticles were synthesized using a microwave-assisted method in the presence of Vaccinium arctostaphylos L. fruit extract. 251 A decrease in crystallite size was observed for the biologically synthesized ZnO compared to the chemically synthesized sample. Furthermore, the existence of organic moieties over the biologically synthesized ZnO NPs was approved using characterizing methods. Then, the alloxan-induced diabetic rats were divided into an untreated diabetic control group and a normal healthy control group, and the groups received insulin, chemically synthesized ZnO, plant extract, and biologically synthesized ZnO. After treatment, fasting blood glucose (FBS), high-density lipoprotein (HDL), total triglyceride (TG), total cholesterol (TC) and insulin were measured. Analysis showed a significant decrease in FBS and increase in HDL levels in all groups under treatment. However, the results for cholesterol reduction were only significant for the group treated with biologically synthesized ZnO. Despite the changes in the triglyceride and insulin levels, the results were not significant. For all the studied parameters, bio-mediated ZnO NPs were found to be the most effective in treating the alloxan-diabetic rats compared to the other studied treatment agents. All reports of ZnO NPs for diabetes treatment indicated that ZnO NPs could be employed as a promising agent in treating diabetes as well as attenuating its complications.

5.10 Anti-inflammatory activity

5.11 immunotherapy, 5.12 wound healing.

The ensuing results revealed that, two weeks after administration, the synthesized nanocomposite induced a 90% reduction of wound area, while mere 70% wound repair was noted in the control experiment thereby bearing evidence of its commanding wound-fixing capacity. Augustine et al. 270 also fabricated ZnO NP decorated polycaprolactone (PCL) scaffolds and demonstrated that their implantation was able to boost faster wound-fixing by elevating the proliferation and migration of fibroblasts in an in vivo model (wound-healing model of American satin guinea pigs), without showing any marked signs of inflammation. Similarly, Bellare et al. 271 designed biocompatible ZnO NP based scaffolds of gelatin and poly(methyl vinyl ether)/maleic anhydride (PMVE/MA) with remarkable antibacterial effects. Their report threw light on the ability of the scaffolds for endothelial progenitor cell (EPC) adhesion and proliferation. Further, the topical application of the scaffolds on wounds of Swiss/alb mice displayed the potential to expedite the process of wound-fixing. Modern wound care materials suffer from several serious shortcomings that include inadequate porosity, inferior mechanical strength, lessened flexibility, lack of antibacterial properties, etc. Given this backdrop, a CS hydrogel/nanoparticulate ZnO-based bandage which exerted antibacterial effects against both Gram-negative ( E. coli ) and Gram-positive ( S. aureus ) bacteria was fabricated by Kumar et al. 272 The nanocomposite bandage characterized by biodegradability, microporosity and biocompatibility produced elevated wound healing in Sprague-Dawley rats and boosted re-epithelialization and collagen deposition at a remarkable pace. Taking into account the crucial factors of biocompatibility, antibacterial effects, and wound-fixing capacity, the researchers held that the hybrid nanomaterial-based bandage could be valuable for the healing of chronic wounds, burn wounds, diabetic foot ulcers, etc. Likewise, a porous bandage consisting of ZnO NPs conjugated with alginate hydrogel and exhibiting blood clotting capacity and bactericidal effects against E. coli , S. aureus , Candida albicans , and methicillin resistant S. aureus was fabricated by Mohandas et al. 261 The bandage made from the nanocomposite was observed to exhibit biocompatibility at a lower concentration of ZnO-NPs. Further, an ex vivo re-epithelialization investigation with porcine ear skin demonstrated that faster wound-fixing was effected by the hybrid nanomaterial-based bandage than only alginate control bandage. This was ascribed to the release of zinc ions that would enhance the proliferation and migration of keratinocyte cells to the wound area. Nair et al. 273 also developed a bandage consisting of a biocompatible nanocomposite of ZnO NPs conjugated with β-chitin hydrogel. The bandage showed efficient antibacterial activity (against S. aureus and E. coli ) and had the ability of blood clotting and activation of platelets. It was elucidated that the application of the bandage on wounds of Sprague Dawley rats led to faster healing, with enhanced collagen deposition and a reduced number of bacterial colonies than in the control experiment, indicating the remarkable wound repairing capacity of the hybrid nanomaterial-based bandage. A novel, biocompatible ZnO QDs@GO-CS hydrogel was constructed by Liang et al. 274 through the simple assembly of ZnO quantum dots (QDs) with GO sheets and via a simple electrostatic interaction with the loaded CS hydrogel. The antibacterial efficacy could reach 98.90% and 99.50% against S. aureus and E. coli bacteria, respectively, with a low-cost, rapid, and effective treatment. ZnO QDs in antibacterial nanoplatforms could immediately produce ROS and Zn 2+ under acidic intracellular conditions. In parallel, when exposed to 808 nm laser irradiation, hyperthermia from GO sheets could simultaneously kill bacteria. Thus, the excellent performance of the material stems from the combined effects of hyperthermia produced under the near-infrared irradiation of GO sheets, reactive oxygen species, the release of Zn 2+ from ZnO QDs under an acidic environment, and the antibacterial activity of the hydrogel. This work demonstrated that the synergy of antibacterial nanoplatforms could be used for wound anti-inflammatory activity in vivo indicated by the wound healing results. The hybrid hydrogel caused no evident side effects on major organs in mice during wound healing. Therefore, the biocompatible multimodal therapeutic nanoplatforms were proposed to possess great potential for antibacterial activity and wound healing. In a study by Dodero et al. , 275 the possibility of using for biomedical purposes alginate-based membranes embedding ZnO nanoparticles that were prepared via an electrospinning technique was extensively evaluated. The morphological investigation showed that the prepared mats were characterized by thin and homogeneous nanofibers (diameter of 100 ± 30 nm), creating a highly porous structure; moreover, EDX spectroscopy proved ZnO-NPs to be well dispersed within the samples, confirming the efficiency of the electrospinning technique to prepare nanocomposite membranes. Mouse fibroblast and human keratinocyte cell lines were used to assess the biological response of the prepared mats; cytotoxicity tests evidenced the safety of all the samples, which overall showed very promising outcomes in terms of keratinocyte adhesion and proliferation. In particular, the strontium- and barium-cross-linked mats were characterized by similar cell viability results to those obtained with a commercial porcine collagen membrane used as a control; moreover, except for the calcium-cross-linked sample, the prepared mats exhibited a good stability over a period of 10 days under physiological conditions. Antibacterial assays confirmed the proficiency of using ZnO nanoparticles against E. coli without compromising the biocompatibility of the membranes. The mechanical properties of the strontium cross-linked mats were similar to those of human skin ( i.e. , Young's modulus and tensile strength in the range 280–470 MPa and 15–21 MPa for the samples with and without nanoparticles, respectively), as well as the water vapor permeability ( i.e. , 3.8–4.7 × 10 −12 g m −1 Pa −1 s −1 ), which was held to be extremely important in order to ensure gas exchange and exudate removal; furthermore, due to the low moisture content ( i.e. , 11%), the prepared mats could be easily and safely stored for quite a long period without any negative effect on their properties. Consequently, the achieved results demonstrated that the prepared mats could be successfully employed for the preparation of surgical patches and wound healing products by using alginate as an economic and safer alternative to the commonly employed commercial animal collagen-derived membranes.

Ahmed et al. 276 fabricated chitosan/PVA/ZnO nanofiber membranes by using the electrospinning technique. The samples of chitosan/PVA and chitosan/PVA/ZnO tested for antibacterial efficacy and antioxidant potential demonstrated very encouraging results in diabetic wound healing. The nanofiber mats displayed outstanding antibacterial properties against various strains of bacteria. The samples of chitosan/PVA and chitosan/PVA/ZnO nanofiber membranes also manifested higher antioxidant properties which made them promising candidates for applications in diabetic wounds. In experiments involving diabetic rabbits, chitosan/PVA and chitosan/PVA/ZnO nanofiber mats exhibited increased performance of wound contractions in a time interval of 12 days. It was thus concluded in the study that the chitosan/PVA/ZnO nanofibrous membranes could serve as useful dressing materials for diabetic wounds, a major problem for type-2 diabetic patients worldwide.

5.13 Agriculture

5.14 photodegradation.

Ishwarya et al. 73 reported the degradation of methylene blue dye in the presence of ZnO NPs prepared using Ulva lactuca seaweed extract and solar irradiation in their study. With an optimum initial dye concentration of 25 ppm and an optimum catalyst loading of 200 mg, the dye present in 100 mL of water got degraded to 90.4% in 120 min.

Gawade et al. 75 carried out photocatalytic degradation of methyl rrange dye using green fabricated ZnO NPs. 81% of the dye (20 ppm) was degraded after 100 min exposure to UV light. This they carried out after the dye solution was stirred with the catalyst for 30 min in the dark for complete equilibrium of the adsorption–desorption phenomenon when 2% of the dye was found to be adsorbed. The optimum catalyst dose was observed to be 1.5 g dm −3 after the dose had been varied in between 0.1 and 2.0 g dm −3 . The increase in degradation efficiency is ascribed to two favourable factors: (a) an increase in the number of active sites and (b) an increase in the number of photons absorbed by the catalyst. Beyond the optimal quantity of the catalyst, aggregation of the catalyst results in the active sites on the catalyst surface becoming unavailable for light absorption. The turbidity of the suspension leading to the inhibition of photon absorption on the catalytic surface of ZnO NPs because of the scattering effect was cited as an additional cause for the lowered degradation efficiency after the optimal catalyst dose.

Enhanced photocatalytic activity of the Mg doped ZnO/reduced graphene oxide nanocomposite has been recently reported by Nithiyadevi et al. 286 They investigated photodegradation of cationic dyes Methylene Blue (MB) and Malachite Green (MG) under visible light irradiation. They achieved a 94.41% degradation of MB and a 99.56% degradation of MG after exposure to visible light for 75 min in each case. Both the photocatalytic degradations showed a marked increase in efficiency in comparison with that effected by bare ZnO NPs. They obeyed pseudo-first order kinetics and the rate constant assumed values of 0.0391 and 0.0493 min −1 respectively in the case of MB and MG. They cited the following reasons for the enhanced photocatalytic ability of the nanocomposite: (a) the introduction of reduced graphene oxide (RGO) enabled better adsorption of dye molecules through π–π conjugation between the dye and aromatic compounds of RGO, (b) the ability of RGO to facilitate the growth of the ZnO particles on RGO sheets, (c) the availability of a large reactive surface area, (d) the greater interfacial contact between ZnO and RGO, (e) increase in the lifetime of charge carriers most probably attributed to RGO, (f) narrowing of the energy bands of ZnO due to Mg 2+ substitution, (g) presence of oxygen vacancies and (h) the reduction of particle size.

Photodegradation of Methylene Blue (MB) was performed by Debasmita Sardar et al. 287 with an Ag-doped-ZnO nanocatalyst. On increasing the percentage of loaded Ag the rate of photocatalytic decomposition gradually increased and reached the maximum for 20% Ag loading on ZnO. The rate constant was found to be 0.0087 min −1 with a fairly high degradation efficiency of 55.87%. However, a sharp fall in the value of rate constant was observed for 25% Ag loading which remained almost the same on further increasing the Ag content, i.e. for 30 and 35% loading. It was also observed from TEM analysis that too much loading of Ag led to agglomeration and thus covered up the surface of ZnO preventing light absorption. Moreover, there were large numbers of unattached Ag nanoparticles which could be oxidized in the presence of reactive oxygen species. Oxidized silver would not initiate any charge separation in the system. It was thus assumed that silver up to this optimum amount could act as an electron–hole separation centre. Beyond the optimum amount, it could help in charge carrier recombination. In fact, a large number of negatively charged Ag particles (which had already accumulated electrons) on ZnO could capture holes and thus would start acting as a recombination site itself essentially by forming a bridge between an electron and a hole. Thus, the efficiency of charge separation and hence the photocatalytic capability declined to an appreciably large extent.

Very recently, Vaianoa et al. 288 too tried photo-catalytically favourable modification of ZnO by Ag. They too achieved similar results with regard to removal of phenol from water. The loading of Ag responded favourably in the range of 0.14–0.88 wt% but backfired beyond 1.28 wt%. Similar reasons as mentioned above were cited for the trends observed. A photocatalytic test was thus performed by using 0.15 g of the optimized catalyst (1% Ag/ZnO) to treat drinking water containing phenol with an optimized initial concentration of 50 mg L −1 in 100 mL aqueous solution. Near-complete mineralization was accomplished within 180 min of exposure to UV irradiation. Photoreaction was found to fit in the pseudo-first order kinetic model. Another investigation 289 reported a facile microwave assisted synthesis of two-dimensional ZnO nano-triangles with a band gap of around 3.33 eV. The as-synthesized ZnO nano-triangles were applied for the reduction of noxious p -nitroaniline within 50 min. They were further used for the effective elimination of Rose Bengal dye within 150 min.

Likewise, ZnO-nanorods were synthesized 290 by adopting a facile microwave assisted green route of synthesis for the complete reduction of nitro compounds. Lauric acid was used as a complexing and capping agent in the ethanol phase. The nanorods had an average diameter of 5.5–10.0 nm with a hexagonal crystal structure and further demonstrated unusual luminescence properties wherein high intensity UV and yellow emission bands were observed along with negligible blue and green emission bands. Toxic nitro-compounds p -nitrophenol, p -nitroaniline and 2,4,6-trinitrophenol were completely reduced into amino derivatives by NaBH 4 in the presence of these nanorods within 120, 45, and 18 min, respectively.

Chidambaram et al. 291 effectively constructed a ZnO/g-C 3 N 4 heterojunction using a facile, economically viable pyrolysis synthetic route for the photodegradation of methylene blue under visible light illumination. The nanocomposites prepared using 0.1, 0.2 and 0.3 molar ratios of zinc nitrate precursor are labeled 0.1ZnO/GCN, 0.2ZnO/GCN and 0.3ZnO/GCN, respectively. The nanocomposites are found to exhibit a fall in charge recombination corroborated by their photoluminescence spectra that showed a fall in the intensity of the concerned emission peak ( Fig. 25 ). A maximum photodegradation of 86% was achieved with 0.2ZnO/GCN in 60 min following a pseudo-first order kinetic rate constant of 0.032 min −1 while graphitic carbon nitride, 0.1ZnO/GCN and 0.3ZnO/GCN attained 44%, 73% and 76% degradation of methylene blue dye in the same time with lower rate constants. The loading of ZnO over g-C 3 N 4 sheets created a heterojunction ( Fig. 26 ). The excitation of electrons by visible light occurs from the valence band to the conduction band of g-C 3 N 4 . The excited electrons are transferred to the conduction band of ZnO while there occurs a simultaneous movement of holes from the valence band of ZnO to the valence band of g-C 3 N 4 via the smooth interface of the heterostructure. This enabled the generation of the superoxide anion radical and hydroxyl radicals that effected improved mineralization of the dye. An excess of Zn was deemed to cause recombination of photo-induced charges that led to decreased photocatalytic efficiency. In a recent investigation by the authors of the current work, 292 a destructive photocatalyst made up of ZnO nanorods/Fe 3 O 4 nanoparticles anchored onto g-C 3 N 4 sheets was synthesized using hydrothermal synthesis and ultrasonication techniques. HRTEM micrographs shed light on the coupling of Fe 3 O 4 nanoparticles with ZnO nanorods and the successful formation of the intended ternary heterojunction. The g-C 3 N 4 sheets fostered close contact between ZnO nanorods and Fe 3 O 4 nanoparticles thereby inducing a mellowed agglomeration of nanostructured ZnO/Fe 3 O 4 particles. The Tauc plot derived from UV-visible absorbance data showed that the ZnO/Fe 3 O 4 /g-C 3 N 4 nano-hybrid had a band gap of 2.48 eV. PL studies further confirmed the successful development of a staggered type II heterojunction with wide separation between light-induced charge carriers ( Fig. 27 ). The hybrid catalyst showed remarkable photocatalytic activity under visible light, as evident from the efficient degradation of pantoprazole, a pharmaceutical drug widely known as a recalcitrant organic water pollutant. This could be attributed to the synergistic interactions between ZnO, Fe 3 O 4 and g-C 3 N 4 . A degradation efficiency of 97.09% was achieved within 90 min with a remarkable pseudo-first order rate constant of 0.0433 min −1 . The incorporation of Fe 3 O 4 expectedly facilitated the ready recovery of the catalyst and the degradation efficiency displayed fair consistency up to 4 cycles. The work thus offered a cost-efficient strategy for tackling organic water pollutants.

In another study, 293 a facile generation of a quaternary nano-structured hybrid photocatalyst, g-C 3 N 4 /NiO/ZnO/Fe 3 O 4 , was proposed for photodegradation of an ecotoxic pharmaceutical drug, esomeprazole, in aqueous solution. The photocatalytic annihilation of esomeprazole as a prototypical organic contaminant was executed under LED irradiation. By itself the designed ternary heterojunction accomplished a maximum 95.05% photodegradation of esomeprazole and a TOC removal of 81.66% and COD reduction up to 70.68% under optimum conditions of catalyst dose, esomeprazole concentration and pH within 70 min at a superior pseudo-first order kinetic rate constant of 0.06616 min −1 . This actually implied an improvement of degradation over NiO/ZnO, g-C 3 N 4 /NiO and g-C 3 N 4 /ZnO up to ∼74, ∼57, and ∼42%, respectively. The specific reaction rate also went up remarkably by almost ∼3.8, ∼3.18, and ∼2.85 times in comparison with the values obtained for NiO/ZnO, g-C 3 N 4 /NiO and g-C 3 N 4 /ZnO, respectively. The remarkable photocatalytic potential of the heterostructured photocatalyst in practical applications was evident from its reconcilable performances under varying initial concentrations of esomeprazole and initial pH of the solution. The effect of the addition of H 2 O 2 was also put under scrutiny and it was found that the photocatalytic degradation, TOC removal and COD reduction increased to 98.43, 84.72, and 73.86%, respectively, upon addition of an optimum quantity of H 2 O 2 over the same time span. The impacts made by inorganic and organic species on photodegradation and the associated reaction kinetics were investigated and the results were reported. The inhibiting influence of water matrices on esomeprazole degradation was also evaluated for better assessment of the performance of the designed photocatalyst in a real aqueous environment.

CdS/ZnO photocatalysts were prepared by two steps via hydrothermal and photochemical methods for the photodegradation of rhodamine B (RhB) dye. 294 The UV/Vis absorption spectra revealed that the absorption performance of the heterostructure is extended toward the visible light region. The photocatalytic activities of both ZnO nanorod and CdS/ZnO heterostructures were investigated for the photodegradation of RhB dye. It was found that the CdS/ZnO heterostructure prepared with 30 min light illumination shows the best photocatalytic efficiency compared to the one at 15 min and pure ZnO nanorods. The better and enhanced photocatalytic efficiency of the CdS/ZnO heterostructure was ascribed to the high charge separation efficiency. The maximum photocatalytic efficiency of 85% was achieved within 8 h with the CdS/ZnO-30 min photocatalyst.

The photocatalytic degradation of rhodamine B (RhB) over chlorophyll-Cu co-modified ZnO catalysts (Chl-Cu/ZnO) was studied under visible-light irradiation by Worathitanon et al. 295 It was found that chlorophyll as an electron donor and copper in Cu 2+ form help inhibit the recombination of electron–hole pairs and improve the photoactivity of the catalyst. The synergistic effect between chlorophyll and Cu was found to improve the visible-light response of ZnO nanoparticles, resulting in excellent performance in photodegradation of RhB. The appropriate ratio of chlorophyll and Cu loadings over ZnO was 0.5Chl-0.10Cu/ZnO. At this ratio, under visible-light irradiation for 2 h, the degradation efficiency was approximately 99% (60 mg L −1 of RhB solution), of which 18% of RhB adsorption occurred under dark conditions. Moreover, outstanding reusability of Chl-Cu/ZnO, for up to six cycles, was found, with more than 80% degradation efficiency.

In yet another investigation, 296 ZnO nanowires (NWs) were successfully synthesized onto commercially available civil engineering materials using a hydrothermal synthesis method. This easy and low-cost method allowed obtaining an almost homogeneous repartition of nanostructures on the entirety of the surface of the substrates. The measured gap values were similar to those of the ZnO NWs grown on typical substrates, i.e. , ∼3.18 eV and 3.20 eV for concrete and tiling, respectively. The excellent photocatalytic efficiency of our samples was demonstrated on three commonly used dyes, namely, Methyl Orange (MO), Methylene Blue (MB) and Acid Red 14 (AR 14). All of the dyes were fully degraded in less than 2 h for MB and AR 14, and less than 3 h for the more difficult to degrade MO. Investigating the durability of the samples so prepared, very promising results were found, as they showed no loss of efficiency after four experiment cycles. The ability of implementing ZnO NWs on civil engineering materials, their good photocatalytic properties, and the possibility to re-use samples with minimal efficiency losses, even after several months, were found very promising for the use of the nanostructures as road surfaces for air or water depollution.

6. Toxic impacts and mechanisms of ZnO NPs

The toxicity mechanism of ZnO-NPs in zebrafish was investigated by Yu et al. 315 The toxicity caused by ZnO is primarily because of the release of Zn 2+ ions and through mechanical damage in zebrafish. ZnO-NPs induced elevation of intracellular Zn 2+ concentration, leading to over-generation of intracellular reactive oxygen species, leakage of plasma membrane, dysfunction of mitochondria, and ultimately cell death. 316 Therefore, it is demonstrated that cell uptake, intracellular dissolution and release of Zn 2+ are the inherent causes for high toxicity of ZnO-NPs. However, there are some disagreements regarding the role of dissolved Zn 2+ in the toxicity mechanisms of ZnO-NPs. Several researchers suggested that dissolved Zn 2+ from ZnO-NPs played a minor role in the toxicity of ZnO-NPs, 317,318 while other investigations indicated that most of the toxicity of ZnO-NPs is due to the dissolved Zn 2+ . 315,316 This discrepancy may be ascribed to the sensitivities of different organisms to dissolved Zn 2+ , such as single tissue cells, bacteria, zebrafish and so on. In the study of Stella et al. , 319 dissolved Zn 2+ from nZnO was considered to play the vital role in the toxicological mechanisms, which was inferred from the levels of the biomarkers of metallothionein (MT) and heat shock protein 70 (HSP70) in the body of O. melastigma larvae, but this dissolved Zn 2+ was obtained by filtering the ZnO-NP suspensions with a 0.1 μm sterile syringe filter and it might include ZnO-NPs whose diameters were smaller than 100 nm.

The dissolution of Zn 2+ ions from ZnO was also suggested to be the main mechanism for the toxicity of ZnO-NPs as claimed recently. 320,321 Li et al. 322 also reported the same mechanism for the toxicity of ZnO-NPs. They have studied the toxicity of ZnO-NPs with various initial concentrations to E. coli in ultrapure water, NaCl and PBS solutions. For higher concentrations of ZnO-NPs, although a few ZnO particles may attach to the bacterial cells, it was difficult to determine the contribution of nano-ZnO itself considering the high toxicity of co-existing Zn 2+ ions. In addition, bacteria could also release the solutes in response to osmotic down-shock in ultrapure water, resulting in damage to the normal physiological functions and the decrease of tolerance of bacteria to toxicants. 323 Therefore, the toxicity of nano-ZnO at 1 mg L −1 in ultrapure water was much higher than that in 0.85% NaCl solution. To confirm the toxicity mechanism of ZnO-NPs, the ultrastructural characteristics of normal E. coli cells and those treated with ultrapure water, ZnO-NPs, and Zn 2+ ions were investigated with TEM by Li and his research group. The morphologies of E. coli cells treated with ZnO-NPs or Zn 2+ ions were significantly different from those of normal E. coli cells. The cytoplasmic membranes were deformed, wherein some cells swelled and the intracellular substances leaked out under both Zn stress and osmotic stress. Combined with the toxicity results of nano-ZnO, bulk-ZnO, and Zn 2+ ions in ultrapure water, Li and co-workers concluded that the toxicity of nano-ZnO to E. coli was mainly attributed to the released Zn 2+ ions.

7. Challenges and prospects

With higher electron diffusivity than TiO 2 , high electron mobility, exceptionally large exciton binding energy, low cost and considerable stability against photo-corrosion, ZnO has been widely considered a perfect substitute for TiO 2 as the electron transport material in DSSCs and PSCs. However, ineffective surface passivation, interfacial charge recombination and long-term stability have collectively yielded poor electron injection efficiency and thereby low current density and efficiency of the ZnO based photovoltaic device. Probable remedies involve incorporation of organic and inorganic dopants for effective surface passivation and effecting surface modification for marked electronic contact. Poor control of the properties of individual building blocks and low device-to-device reproducibility are further areas that require investigative attention. As a yet further consideration, adequate studies devoted to the impact of facet selectivity, structure and morphology of ZnO nano-structures on the overall efficiency of solar cells and the associated mechanism have to be conducted.

ZnO nanostructured particles have revolutionized the field of photocatalysis. And their efficacy in water splitting and degradation of recalcitrant organic water pollutants has been widely investigated and taken advantage of. However, a few concerning aspects about their photocatalytic activity still need to be dealt with through possible corrective measures. First, the photodegrading ability of a prepared ZnO nano-catalyst needs to be checked by taking the pollutant of interest in lieu of a representative substance which in usual cases is a dye. This is because dye-degradation is relatively plain sailing while removal of pharmaceutical wastes, pesticides, insecticides or other endocrine disruptors offers greater challenges and complicacies. Moreover, the archives of scientific literature are brimming with thorough investigative reports concerning degradation of dyes. Furthermore, waste water contains a mix of different contaminants with varying ranges of pH and ionic strength. Few photodegradation studies have been conducted on organic pollutants in such a simulated sample of water while taking into account the effect of the presence of other contaminants, varying pH and ionic strength on the degradation kinetics. Second, a detailed insight into the mechanistic routes of the degradation of these compounds and their interaction with ZnO based nano-catalysts is elusive as of now and its development remains imperative and will unfold approaches to tackle other emerging contaminants. Third, many improvements in the very architecture of ZnO nanostructures are due specifically in areas such as surface area, particle size, separation and lifespan of charge carriers and so forth. Fourth, since band positions and band gaps are dependent on particle size, it becomes difficult to create heterojunctions able enough to achieve effective charge separation and thereby efficient photocatalytic activity. Systemic studies with a focus on discovering specific synthesis protocols for the achievement of ZnO based nanostructures with desired band positions and band gaps have to be embarked on. Also, there are a few difficulties associated with the operating procedures, such as loss and recovery of nano-structured photocatalysts in the course of post-synthesis treatment and photocatalytic activity. Furthermore, more sweeping research investigations are required to develop and verify the mathematical models for photocatalytic operations/systems for water/wastewater treatment in order to predict the quantum yield, kinetics and optimum conditions of the process.

ZnO nanomaterials may be outstanding candidates as biocompatible and biodegradable nanoplatforms for cancer targeted imaging and therapy. For in vivo imaging and therapy applications, the future of nanomedicine lies in multifunctional nanoplatforms combining both therapeutic components and multimodality imaging. Biocompatibility is also a concern for the applications of nanomaterials in biomedicine. Surface modification of nanomaterials plays a vital role in this context. Biocompatibility of ZnO nanomaterials might be enhanced by slowing down the dissolution rate through Fe doping 324 or surface capping. 325 Therefore, surface coating of ZnO NPs with biocompatible macromolecules, such as poly(lactic) acid, PEG, PEI and chitosan, was attempted to increase their suitability for further clinical usage. Another idea is the synthesis of ZnO nanoplatforms using the biodegradable and biocompatible materials already proven clinically. Some biocompatible polymers, such as liposomes and dendrimers, have been clinically approved for various pharmaceutical applications. Hence, the modification or conjugation of already approved therapeutic formulations or materials with functional ligands which will improve their diagnostic index could be essential. Much effort is needed for long-term in vivo toxicology studies to pave the way for future biomedical applications of these intriguing nanomaterials. Facile conjugation of various biocompatible polymers, imaging labels, and drugs to ZnO nanomaterials can be achieved because of the versatile surface chemistry.

Some other issues of ZnO NPs concerning their biomedical application and their impact on biological systems still need further meticulous inspection. Following are a few such concerns: (a) lack of comparative analysis of the biological advantages of ZnO NPs to other metal nanoparticles, (b) the limitations imposed by the toxicity of ZnO NPs toward biological systems continue to remain a hot potato in recent research, (c) limitations of biocompatible/biodegradable ZnO nanoplatforms for tumor targeted drug/gene delivery, (d) lack of evidence-based research carrying out as its focal point a thorough survey of the therapeutic roles of ZnO NPs in improving anticancer, antibacterial, anti-inflammatory, and anti-diabetic activities, and (e) lack of extensive in vivo investigations into the anticancer, antibacterial, anti-inflammatory, and anti-diabetic activities of ZnO. Fresh studies focused on the abovementioned issues would bring forth further elucidation and comprehension of the potential use of ZnO nanoparticles in biomedical diagnostic and therapeutic fields.

8. Conclusion

Author contributions, conflicts of interest, acknowledgements.

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• Published: 08 November 2017

Visible and UV photo-detection in ZnO nanostructured thin films via simple tuning of solution method

• Richa Khokhra 1 ,
• Bandna Bharti 1 ,
• Heung-No Lee 2 &
• Rajesh Kumar 1 , 2  

Scientific Reports volume  7 , Article number:  15032 ( 2017 ) Cite this article

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• Materials for devices
• Synthesis and processing

This study demonstrates significant visible light photo-detection capability of pristine ZnO nanostructure thin films possessing substantially high percentage of oxygen vacancies \(({V}_{o}{\rm{s}})\) and zinc interstitials \((Z{n}_{i}{\rm{s}})\) , introduced by simple tuning of economical solution method. The demonstrated visible light photo-detection capability, in addition to the inherent UV light detection ability of ZnO, shows great dependency of \({V}_{o}{\rm{s}}\) and \(Z{n}_{i}{\rm{s}}\) with the nanostructure morphology. The dependency was evaluated by analyzing the presence/percentage of \({V}_{o}{\rm{s}}\) and \(Z{n}_{i}{\rm{s}}\) using photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) measurements. Morphologies of ZnO viz. nanoparticles (NPs), nanosheets (NSs) and nanoflowers (NFs), as a result of tuning of synthesis method contended different concentrations of defects, demonstrated different photo-detection capabilities in the form of a thin film photodetector. The photo-detection capability was investigated under different light excitations (UV; 380~420 nm, white ; λ > 420 nm and green; 490~570 nm). The as fabricated NSs photodetector possessing comparatively intermediate percentage of \({V}_{o}{\rm{s}}\) ~ 47.7% and \(Z{n}_{i}{\rm{s}}\) ~ 13.8% exhibited superior performance than that of NPs and NFs photodetectors, and ever reported photodetectors fabricated by using pristine ZnO nanostructures in thin film architecture. The adopted low cost and simplest approach makes the pristine ZnO-NSs applicable for wide-wavelength applications in optoelectronic devices.

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Introduction.

Photodetectors have a wide range of applications in many important areas such as; space communication, air quality monitoring, flame monitoring, industrial quality control, optical imaging, optoelectronic circuits, military surveillances etc. 1 . Conventional photodetectors employ crystalline semiconductor materials such as; silicon, germanium, gallium arsenide etc. However, in these materials certain issues still need to be addressed; for instance, requirement of high temperature conditions for device fabrication, possibility of blurring, cross talk of optical signals between neighboring pixels 2 and limited freedom in material design. To overcome these problems, studies on inorganic semiconductor nanostructures 3 , 4 such as; ZnS, InSe, CdS, CdSe etc. and metal-oxide semiconductor 5 such as; ZnO 6 , CeO 2 7 ,V 2 O 5 3 etc. could pave the way of fabricating a suitable photodetector. Nonetheless, these materials in nanostructure form provide a higher degree of freedom for material’s properties tuning as well as the reduced dimensionality of the active device 8 , 9 . However, as a key issue, these materials in their pristine form work only for ultra violet (UV) photo-detection applications, as allowed by their wide bandgap structure. Most of the studies correlate their wide bandgap with UV applications; however, in addition to the UV applications there are many areas that urgently require photodetector’s sensitivity for visible-light region, and thus there is a great need to achieve a wide spectral response of the proposed nanostructured semiconductor materials. In other words, the widening of photodetector’s spectral response (extended wavelength photo-detection) would enhance their application area. In this view, studies on the detection of visible spectrum by achieving a broadband photo-detection capability of nanostructured semiconductor materials, specifically metal-oxide semiconductors, have attracted a great attention in the last few years 7 , 10 .

The wide spectral applications of metal-oxide semiconductor materials require tuning of optical properties (bandgap) of semiconductor nanostructures; therefore, normally they are doped with metals 11 , non-metals 12 , combined with other materials/functional groups 13 , 14 , 15 , and formed as composites with another semiconductor materials 1 , 7 , 16 . However, it is noteworthy that most of these processes applied for tuning of optical properties, require complicated and expensive equipments, and a complex device structure to achieve visible-light detection response. Moreover, in these approaches, the requirement of high temperature and pressure conditions is an another issue. In this concern of application, when considering morphology, the one dimensional (1D) nanostructures owing to their large surface to volume ratio and Debye length (that influence electronic/optical properties and thus exhibiting superior photosensitivity) show better performance in the photo-detection applicaion 3 , 5 .

While talking about metal-oxide semiconducting materials, investigated for photo-detection applications, the ZnO is found most promising candidate due to its many peculiar properties such as; high efficiency, low cost, non-toxicity, stability, high temperature operation capability, and environmental compatibility. Looking to the superior photosensitivity of morphologically 1D nanostructure, the ZnO itself is also studied mostly in 1D form with various modifications such as; decorating with gold nanoparticles 17 and CdS 18 , doping with Cu 19 , Mn 20 and making heterostructures 21 , 22 to show multi-spectral visible and UV light photo-detection capability. In principle, in the modifications, mid-gap electronic levels of dopant/s are introduced which generate charge carriers upon visible-light irradiation and thus make the material sensitive to visible light.

In a further and recent advancement 10 , undoped ZnO nanowires with a vertical alignment have been presented to exhibit an extended-range of visible light photo-detection capability upon annealing in hydrogen gas. The hydrogen annealing creates porosity on the surface of the nanowires that makes the assembled nanowire photodetector as a visible-light sensor by the phenomenon comprising antireflection, multiple scattering and defect state excitation induced mechanism. Similarly, the undoped ZnO structures have demonstrated visible-light activity upon vacuum deoxidation 23 , where oxygen energy levels are introduced in the energy band of ZnO. The introduction of oxygen levels in the band gap enables ZnO as an active material for visible and UV-light photo-detection. Despite of many efforts, in the case of undoped ZnO nanostructures, it is still a challenging issue to make a simple and economical photodetector that could use an easily fabricated nanostructure to work in broad spectrum region (ultraviolet and visible), except the lD nanostructures, and avoids the require sophisticated instrumentation. From the reported studies, it is ensured that for an undoped/pristine ZnO photodetector, it is only the multiple scattering or defects ( \({V}_{o}{\rm{s}}\) , \(Z{n}_{i}{\rm{s}}\) and antsites) that enables the visible-light/broadband response. Therefore, the tuning of morphology of undoped ZnO nanostructures, capable of broadband spectral-response through the combined effect of multiple scattering and formation of \({V}_{o}{\rm{s}}\) and \(Z{n}_{i}{\rm{s}}\) , synthesized by the simple solution method would be highly valuable for making an economical photodetector.

In this work, different morphologies of undoped ZnO nanostructures ; NSs, NFs and NPs were formed simply by low temperature chemical route engineering. The \({V}_{o}{\rm{s}}\) and \(Z{n}_{i}{\rm{s}}\) were introduced intentionally during the formation, so as to avoid the cost effective approaches 23 which involve high temperature and vacuum conditions for inclusion of \({V}_{o}{\rm{s}}\) and \(Z{n}_{i}{\rm{s}}\) . Thus generated \({V}_{o}{\rm{s}}\) and \(Z{n}_{i}{\rm{s}}\) levels in the energy band of ZnO exhibited significant photo-response in the visible as well as UV spectral region. The photo-response of these nanostructures was investigated in terms of generated photovoltages by wide range of spectral illumination i.e. λ ≈ 380~420 nm (UV), λ ≈ 490~560 nm (green), and λ > 420 nm (white light). We found that from the fabricated nanostructures, the NSs photodetector in a thin film form shows a faster rise and decay time both in the UV and visible spectrum region than that of the co-fabricated NPs and NFs photodetectors, and the ever reported sophisticated single ZnO nanowire based photodetector working only in UV region 24 , 25 , 26 . Based upon the observations, simply fabricated NSs, a low-cost photodetector could be a highly competent candidate for the applications requiring detection of wide range spectrum.

Results and Discussion

Morphological and x-ray analysis.

Surface morphology of the samples prepared by varying the synthesis parameters such as; variation in the concentration of precursor solutions, ratio, reaction time and solvent, was investigated using FE-SEM (Supplementary Figures  S2 and S3 for C 2 H 5 OH medium and S4 , S5 for H 2 O medium). The large number of synthesis reaction performed using the precursor ‘ZnCl 2 ’ in solvent media C 2 H 5 OH and H 2 O resulted mainly in three types of nanostructures i.e. NPs (for 15 minutes of reaction time in both the reaction media), NSs and NFs that were formed as the only distinguishable forms of the product as shown in Fig.  1 . The solvent media C 2 H 5 OH and H 2 O play a significant role in the different aggregations of initially formed nanoparticles that resulted in NSs and NFs after 4 hours of reaction time. The FE-SEM images show that an abrupt addition of ZnCl 2 solution in alkali solution resulted preliminary in the formation of NPs (Fig.  1a ) for both the solvent media, which then aggregated differently as NSs (Fig.  1b ) and NFs (Fig.  1c ), respectively. The XRD patterns are shown in the right of Fig.  1 . There are five prominent diffraction peaks, in all the cases, at diffraction angles 2θ = 32.7°, 34.5°, 36.42°, 47.44°and 56.58°, which are indexed as lattice planes (100), (002), (101), (102), and (110) with the lattice constants (a = 0.325 nm and c = 0.5211 nm), corresponding to Wurtzite crystal structure of ZnO. The size of nanocrystals estimated for maximum intensity peak, using Debye-Scherrer formula D = 0.9λ/ β cos θ is 17.41 nm, 21.57 nm and 23.29 nm for NP, NS and NF, respectively; with an average crystallite sized 20.16 nm, 24.44 nm and 20.44 nm for NP, NS and NF, respectively, indicates a successive growth mechanism of nanostructures evolving from the nanoparticles.

FE-SEM images and XRD results of ZnO nanostructures. ( a ) NPs formed in 15 minutes by using C 2 H 5 OH solvent, ( b ) ZnO-NSs formed in 4 hours using C 2 H 5 OH solvent and ( c ) ZnO-NFs formed in 4 hours using H 2 O solvent. In the right side, there are XRD plots indicating similar crystallographic structures for all the morphologies.

Optical absorbance of the samples obtained by UV-Visible spectroscopy is shown in Fig.  2 . These absorption spectra appear to be extended from UV to visible-region, with a sharp UV excitonic peak at wavelength, 315 nm for NPs (Fig.  2a ), 355 nm for NSs (Fig.  2b ) and 365 nm for NFs (Fig.  2c ). The presence of excitonic peak in UV region along with an extended absorption region reveals their UV as well as visible-light activity 10 . The shift in the excitonic peak from NPs (315 nm) to NSs (355 nm) and then to NFs (365 nm), corresponds to a red shift in the spectrum. With the shift in the excitonic peaks, the overall absorbance also increases from NPs to NSs and then to NFs (Fig.  2d ). This result is analogous to the observations of enhanced absorbance in the porosity induced antireflections leading to visible-light photo-activity 10 . A rough estimation of the porosity order as NFs > NSs > NPs, can be made from the FE-SEM images of ZnO films (Fig.  1 ). Among these nanostructures, the NFs are expected to have comparatively more multiple reflections of light rays once they enter the film, and these large multiple reflections may lead to higher absorption 10 in the NFs film in comparison with NSs and NPs films as shown in Fig.  2d .

UV-Vis spectroscopy absorbance spectra. ( a ) NPs having excitonic peak at 315 nm and extended absorbance in the visible spectrum region, ( b ) NSs having excitonic peak at 355 nm and showing extended visible region, ( c ) NFs having excitonic peak at 365 nm with comparatively higher absorbance, and ( d ) shows increasing absorbance from NPs to NSs and then to NFs.

PL and XPS studies

Emission/absorption in the UV and visible-region by ZnO nanostructures were investigated via analyzing defects states (vacancies, interstitials and antisite) in the studies conducted by PL and XPS experiments. Figure  3 shows a room temperature PL spectrum of NPs, NSs and NFs nanostructures. For excitation energy ~3.36 eV (corresponding to wavelength ~370 nm, equivalent to the typical band gap of ZnO), the PL spectra of NPs, NSs and NFs samples show near-band emission; respectively, at 386 nm (3.21 eV) (Fig.  3a ), 393 nm (3.15 eV) (Fig.  3b ) and 396 nm (3.12 eV) (Fig.  3c ) which is attributed to the transitions from excitonic levels and/or zinc interstitials \((Z{n}_{i})\) to the conduction band ( CB ) 27 and is analogous to the previous studies 28 , 29 . Along with these near-band emissions, all three samples exhibit visible emissions in the spectrum region 420–569 nm (Fig.  3d ) consisting distinct peaks at 420 nm (2.95 eV),456 nm (2.71 eV),484 nm (2.56 eV), 511 nm (2.42 eV),530 nm (2.34 eV) and 568 nm (2.43 eV). Basically, transition from VB to CB and from VB to shallow levels occurs upon photoexcitation in the PL, which then give the subsequent transitions; CB  → deep levels, shallow levels →  VB , shallow level → deep levels and hole capture at deep levels gives violet, blue and green emissions according to energy levels difference. Since the exciting energy (3.36 eV) is equivalent to ZnO energy gap; therefore, the electrons excited from the valance band ( VB ) can jump to the CB as well as shallow defect levels.

Photoluminescence spectra of nanostructures. ( a) NPs show violet emissions at 386 nm, (b) NSs show violet emissions at 393 nm and (c) NFs show violet emission at 398 nm. Blue emissions at wavelengths positions 420, 456 and 848 nm and green emissions at wavelengths 511, 530 and low intensity peak at 568 nm are common in all nanostructures.

Generally, oxygen vacancies ( \({V}_{o}^{* }\) , \(\,{V}_{o}^{+}{\rm{and}}\,{V}_{o}^{++}\) ) are considered as green emission centers, where \({V}_{o}^{* }\) is neutralized oxygen vacancy that lies 0.86 eV below the CB , and the \({V}_{o}^{++}\) a double ionized vacancy lies ~2.18 eV below the CB . The single ionized oxygen vacancy \(\,{V}_{o}^{+}\) is reported to have two energy locations; 0.9 and 2.47 eV above the VB in the band gap 30 , 31 . Besides the oxygen vacancies, oxygen antisite \(({O}_{Zn})\) located 2.33 eV below the CB do corresponds to green emission of shorter wavelength 30 . Other defects such as; zinc interstitials \(Z{n}_{i}{\rm{s}}\) , natural \((Z{n}_{i})\) , singly and doubly ionized interstitials ( \(Z{n}_{i}^{+}\) , \(Z{n}_{i}^{++}\) ) are responsible for blue emission 27 , 32 , 33 . \(\,Z{n}_{i}\) being located 0.22 eV below the CB edge 34 gives violet emission around 390 nm in the PL. However, some studies show more deeper location of \(\,Z{n}_{i}\) (~0.37 eV below CB ) to explain violet-blue emission 27 . Other two Zn defects; \(Z{n}_{i}^{+}\) and \(Z{n}_{i}^{++}\) lie 0.56 and 0.63 eV below the CB minima 27 , respectively; whereby the transitions to zinc vacancy \(({V}_{Zn})\) and/or VB results in blue emissions of higher wavelengths 35 . Except \(Z{n}_{i}{\rm{s}}\) and \({V}_{Zn},\,\) the oxygen interstitial \(({O}_{i})\) generally located in the band gap at position 0.4 eV above the VB , also participates in the blue emission 35 , 36 .

PL spectra in Fig.  3 show co-existence of violet emission peak in the range 386–398 nm (corresponding to excitonic emissions) and visible emission peak in the range 420–568 nm. All three types of nanostructures; NPs (3a), NSs (3b) and NFs (3c) show common visible light emission peaks at 420 nm (2.95 eV), 456 nm (2.71 eV), 484 nm (2.56 eV), 511 nm (2.42 eV), 530 nm (2.34 eV) and a low intensity peak at 568 nm (2.43 eV), whereas UV region emission peaks are located at different wavelengths. The UV emission peak shows a red shift in a order NPs → NSs → NFs as can be seen in Fig.  3d . This red shift in PL is analogues to the observed red shift in UV-Vis spectrum (Fig.  2 ), indicating slightly different excitonic energy levels in the band gap of synthesized nanostructures. As reported earlier ZnO possesses stable excitonic states just below its CB 33 minima, whereby transition to VB gives near band violet emissions. Thus in our case, the observed peak in PL of NPs at position 386 nm (3.21 eV) can be assigned to the transition from excitonic states to its VB . In other words, the decay of self-trapped exciton to the CB, causes near band violet emission as shown in Fig.  4 . Similarly, the UV emissions in NSs and NFs at 393 nm (3.15 eV) and 398 nm (3.12 eV) are close to the electronic transition from a slightly lower energy excitonic state or Zn interstitial \(\,Z{n}_{i}\) (lying ~0.22 eV below the conduction band) to the VB . The possible transition scheme, corresponds to all the peaks in the PL shown in Fig.  4b , is given in Fig.  4a .

Emission scheme with PL spectra. ( a ) Schematic representation of emission scheme in the prepared samples of NPs, NSs and NFs nanostructures, ( b ) PL emission showing different peak position in violet region 386–398 nm, and common emission peaks in the visible region 420–568 nm in all samples.

In the visible region, the observed blue emission peak at 420 nm (2.95 eV) corresponds to the transition from \(\,Z{n}_{i}\,\) to VB , considering that \(\,Z{n}_{i}\,\) lies ~0.41 eV deeper to the CB edge, this is alike to the previous reports 24 , wherein energetic location of \(\,Z{n}_{i}\,\) is considered 0.38 eV deeper in the energy band. The another possibility is the transition CB  → oxygen interstitial \({O}_{i}\) (located 0.4 eV above VB as proposed earlier 35 ) that also gives blue emission at 420 nm. In fact, the second one is rather in a good agreement as the energy level difference (2.96 eV) between CB edge and \({O}_{i}\,\) is close to the obtained emission energy (2.95 eV). Nonetheless, the excitation energy (3.36 eV) is quite enough to pump the electrons from VB to CB that make a transition CB →  \({O}_{i}\) and emit blue radiation of 420 nm, indicating the latter transition more plausible. The another blue emission at ~456 nm (2.71 eV) is assigned to the transition from extended \(\,Z{n}_{i}\,\,\) states →  VB . The extended \(\,Z{n}_{i}\) states are generally; localized \(\,Z{n}_{i}\) states, \(\,Z{n}_{i}^{++}\) and complex defects, whose energetic locations deep in the band gap depend upon the fabrication methods 27 , 35 . In our case, the blue emission (2.71 eV) is in close agreement to the energy of transition \(Z{n}_{i}^{++}\)  →  CB (2.73 eV), indicating the presence of \(\,Z{n}_{i}^{++}\) states in the band gap. Third blue emission peak located at 484 nm (2.56 eV) corresponds to the transition from \(Z{n}_{i}^{+}\)  →  \({V}_{Zn}\) , as the energy difference between these levels (2.52 eV) is in close agreement to the observed emission energy.

Green emissions falling in the spectral region 511–568 nm, possesses three emissions peaks at 511 nm (2.42 eV), 530 nm (2.34 eV) and 568 nm (2.18 eV). When the electrons from CB , recombine with doubly ionized \({V}_{o}^{++}\,\) located at an energy level 1.12 eV above the VB , generate green emission of wavelength 568 nm 37 . The second green emissions peak centered at 530 nm may come from the transition \(\,{V}_{o}^{+\,}\)  →  VB or CB  →  \({O}_{Zn}\) . The first transition ( \(\,{V}_{o}^{+\,}\)  →  VB ) occurs due to the formation of unstable \({V}_{o}^{+}\) state of \({V}_{o}^{+}\) by capturing electrons form CB 38 . This unstable state, when recombine with photoexcited hole in the VB , would generate green emission around 530 nm 39 . The second transition CB  →  \({O}_{Zn}\) also has a strong possibility as the exciting energy is enough to pump the electrons to CB that after falling to \({O}_{Zn}\) will give rise a green emission at 530 nm. Coming back to the \({V}_{o}^{+}\) states, in the energy band, there may be occurrence of complex \({V}_{o}^{+}\,\) states along with isolated \({V}_{o}^{+}\,\) centers. The complex states lying deeper in the band gap also give a possible explanation of the green emission around 530 nm, whereas the isolated \({V}_{o}^{+}\,\) states suitably explain the emission around 511 nm 38 through the transition \({V}_{o}^{+}\)  →  VB . As mentioned previously, two possible energetic locations of isolated \({V}_{o}^{+}\) states are estimated theoretically at 0.9 and ~ 2.47 eV above the VB . The transition \({V}_{o}^{+}\)  →  VB (corresponding to 2.47 eV energy level position of \(\,{V}_{o}^{+}\) ) gives 511 nm emission as shown in the scheme of Fig.  4a , and the transition CB  →  \({V}_{o}^{+}\) (corresponding to 0.9 eV energy level position of \({V}_{o}^{+}\) ) will give 510 nm emission as shown in the scheme of Fig.  4a by dashed line. Both of the transitions have equal possibility to generate emission around 510 nm in the PL. All these observations in the PL, indicate that all of the samples possess defects states such as; \({V}_{o}{\rm{s}}\) , \(\,Z{n}_{i}\) , \({O}_{i}\) , \({V}_{Zn}\) and \({O}_{Zn}\) .

XPS studies were performed to get information about chemical bonding and defects states present in the ZnO samples. Figure  S6(a–c) shows XPS survey spectra recorded at room temperature for NPs, NSs and NFs samples. The overview of survey spectra reveals the presence of O1s and Zn2p (Zn2p 3/2 and Zn2p 1/2 ) peaks in all the samples. In order to further examination, the high-resolution peaks were deconvoluted in satellite components at different binding energies. Figure  5(a–c) illustrates high-resolution XPS spectra for all the samples corresponding to O1s core level. These spectra are fitted with three Gaussian peaks. In the NPs case (Fig.  5a ), deconvoluted peaks are located at binding energies 530.8, 530.93 and 532.5 eV. Here, the lower binding energy peak is attributed to lattice oxygen ( \({O}_{L}\) ) which contributes to the perfect hexagonal structure of ZnO lattice, the presence of middle peak at binding energy 530.93 eV is ascribed to vacancies ( \({V}_{o}{\rm{s}}\) ) 40 in ZnO lattice. The observation of \({V}_{o}{\rm{s}}\) supports the presence of green emission line in PL. The higher binding energy peak at 532.5 eV corresponds to chemisorbed oxygen ( \({{\rm{OH}}}^{-},\) −CO 3 , adsorbed H 2 O, and O 2 (O C )) 41 , 42 . Similarly, the peaks in high-resolution deconvoluted XPS spectrum of NSs and NFs, when analyzed do correspond to \(\,{O}_{L}\) , \({V}_{o}{\rm{s}}\) and chemisorbed oxygen. However, there is shift in the binding energy values of the corresponding deconvoluted peaks for NSs and NFs with respect to that of NPs, which is ascribed to the difference in their morphologies and synthesis approaches 43 , 44 . Further, the change in the percentage of oxygen content related to each deconvoluted peak was estimated by the change in percentage area of the peaks. From the calculations of percentage area, we found that the lattice oxygen in NPs is about 17%, whereas in case of NSs and NFs it reduces to ~ 12%. At the same time, the percentage area of oxygen vacancies ( \({V}_{o}^{\ast }\) , \(\,{V}_{o}^{+}\,{\rm{and}}\,{V}_{o}^{++}\) ) 45 , 46 , increases from 21.8% (in NPs) to 47.7% (in NSs) and 54.5% (in NFs), and that of the chemisorbed oxygen decreases from NPs (60.3%) to NSs (40.9%) and then to NFs (37.6%).

High-resolution XPS spectra for O1s. ( a) ZnO-NPs different percentages of lattice oxygen, oxygen vacancies and chemisorbed oxygen, (b) ZnO-NSs and (c) ZnO-NFs show variations in the peak positions, and constituent percentages are given in respective tables.

Figure  6(a–c) represents high-resolution XPS spectra for Zn peaks in the NPs, NSs and NFs samples, respectively. In these spectra, the energy separation between two Zn components; Zn2p 3/2 and Zn2p 1/2 for NPs is 23.11 eV, NSs is 23.09 eV and NFs is 23.13 eV, which are in agreement with the reported values of ZnO 47 , 48 . The difference of binding energies of these deconvoluted peaks for NPs, NSs and NFs (given in Table  S2 ) is ascribed to the difference in their morphologies and synthesis approaches 43 , 44 . The peak Zn2p 3/2 in NPs sample is deconvoluted in three peaks at binding energies 1022.02 eV (lower side), 1022.41 eV (middle) and 1022.35 eV (higher), also the peak Zn2P 1/2 is deconvoluted in three peaks at binding energies 1044.92 eV (lower), 1045.51 eV (middle) and 1045.64 eV (higher) as shown in Fig.  6a . The middle peaks (1022.41 and 1045.51 eV) have an energy difference of 23.1 eV, which is in good agreement with the spin-orbit splitting value of divalent Zn bounded in ZnO structure 49 , 50 , and thus suggesting that the middle peak corresponds to lattice Zn. The lower energy peaks centered at 1022.02 and 1044.92 eV correspond to metallic Zn in the sample 51 . And, third peaks centered at higher energies 1022.35 and 1045.64 eV, correspond to +2 oxidation state of Zn due to the presence of Zn(OH) 2 or/and ionized \(Z{n}_{i}\) interstitials. It is found 37 , 52 that the binding energy location of satellite peak of 2p 3/2 lying between 1022.70 and 1021.80 eV corresponds to Zn(OH) 2 , and therefore the +2 oxidation state is due to the presence of Zn(OH) 2 . Whereas in our case for NPs sample, the satellite peak 2p 3/2 (at 1022.35 eV) lies between the referred energy limits, which suggests the presence of Zn(OH) 2 or +2 oxidation state by hydroxide form of Zn. This can be further correlated with the corresponding peak in the O1s spectrum of NPs (shown in Fig.  6a ), which is assigned as chemisorbed oxygen (generally in OH − and/or water molecules). In the O1s spectrum, the obtained higher percentage (60.39%) of chemisorbed oxygen can be assigned to \({{\rm{OH}}}^{-}\) group as confirmed form Zn2p 3/2 spectrum; however, there should co-exist a small percentage of interstitial Zn as well, as to produce emission in the corresponding PL spectrum (as the PL of nanoparticles shows the presence of interstitials). Moreover, the lower PL emission intensity of NPs than that of NSs and NFs indicates comparatively smaller interstitial percentage in NPs. Next, in case of NSs, each of the peaks Zn2p 3/2 and Zn2P 1/2 (Fig.  6b ) is deconvoluted in three satellite peaks at energies 1022.81, 1023.62, 1024.16 eV and 1045.37, 1046.6, 1048.66 eV, respectively. The middle peaks centered at binding energies 1023.62 eV and 1046.6 eV have energy difference of 22.98 (approximately 23.0 eV), that again corresponds to lattice Zn with two valance state in ZnO. The satellite peak at lower energy side is assigned to metallic Zn alike to that of the NPs sample. However, in this sample the higher energy satellite peaks 1024.16 and 1048.66 eV located at significantly higher energies do not correspond to Zn(OH) 2 ; instead it indicates that the Zn atom is surrounded by more than one oxygen atoms and is occupied at interstitial positions 37 . These interstitial Zn could be neutral \(\,Z{n}_{i}\) , extended states of \(\,Z{n}_{i};\) single and double ionized zinc interstitials ( \(Z{n}_{i}^{+}\) and \(Z{n}_{i}^{++}\) ). Similarly, in the samples of NFs, the Zn2p spectrum is deconvoluted in three peaks as shown in Fig.  6c . Here also, the higher energy satellite peaks are located at much higher energies as 1024.34 and 1074.04 eV should correspond to the interstitial Zn similar to that of NSs sample. However, the area percentage in NFs is more than that of NSs, suggesting a higher content of interstitials in NFs, which is in good agreement with PL observation (Fig.  4b ) showing higher intensity, revealing the higher Zn interstitials in NFs sample.

High-resolution XPS spectra of Zn2p. ( a) ZnO-NPs possessing dominating Zn peak at higher energy side is due to the hydroxide form as mentioned in table, (b) ZnO-NSs and (c) ZnO-NFs spectra have dominating interstitial Zn peak at higher energy. The position of peaks and their area percentages are mentioned in the corresponding tables.

Performance of ZnO photodetectors

To study the performance of ZnO photodetectors, photo-response was measured in terms of photovoltage by applying a bias voltage ‘ V b  = 5 V’ as shown in Figure  S1 . Figure  7 shows photovoltage versus time plots for all the three types of nanostructures; NPs (Fig.  7a ), NSs (Fig.  7b ) and NFs (Fig.  7c ) thin film photodetectors under the illuminations; violet, white and green. In the photovoltage measurements, rise time is the time required in 90% rise of photovoltage form its initial value (after switching ON the illumination), and fall time is the time taken in the falling of maximum photovoltage to 10% (after switching ‘OFF’ the illumination) 53 . For each photodetector, the period of ON and OFF time was taken different by considering their response time as different, and also to eliminate the heating effects on the sample surface 39 , 54 . In the experiments, the ON and OFF time was controlled using a camera shutter. The photo-response of all ZnO nanostructure photodetectors (in the region ultraviolet to visible) is given in Table  1 .

Response of nanostructured thin film photodetectors. ( a – c) NPs photodetector, (d – f) NSs photodetector and (g – i) NSs photodetector for UV, white and green illuminations, dashed lines are plotted for clear observation of photo-generated ON state saturation voltages and OFF state dark voltages by different illuminations. In each photodetector, the illumination time is taken as short as required to avoid the heating effects. The applied bias voltage in all three cases is 5 V.

The mechanism of photodetector response with light illumination can be explained by desorption and adsorption of O 2 and/or H 2 O on its surface 26 , 55 , 56 , 57 , 58 . In a dark condition, O 2 and H 2 O molecules present in the air get adsorbed on ZnO surface, which by capturing electrons from the conduction band of ZnO becomes negatively charged ions (chemisorbed) that results in the increased depletion barrier height. When the illumination is turned on, the chemisorbed O 2 gets H 2 O is/are desorbed by two ways; (i) capturing a photo-generated hole and/or (ii) direct photo-excitation of captured electron to the conduction band of ZnO 24 . The desorption mechanism occurs in accordance to the energy level of illuminating radiation (above-band or below-band energy). While using above-band illumination ‘UV radiation’, electron and hole are generated directly since the illumination energy is higher than the band gap of ZnO. Thus the produced photo-holes migrate to the chemisorbed ions sites, and release the electron by neutralizing the ions. The desorption of ions decreases barrier height and releases electrons in the conduction band of ZnO. These released electrons along with the photo-electrons would enhance the concentration of carriers in the CB of ZnO, and thus give rise to its photoconductivity. In case of below-band illumination, desorption of O 2 /H 2 O occurs by direct photo-excitation of the captured electrons to the conduction band of ZnO that also increases photoconductivity. In the present study, we used above-band (UV) as well as below-band (white and green) illuminations which resulted in the generation of photovoltage. Thus the observed photovoltage might involve both of the desorption mechanisms explained above, according to the illumination conditions.

First of all, we compare photo-response curves corresponding with different illuminations in the NPs photodetector. In the NPs photodetector, the photovoltage initially at ~0 V (dark voltage) reaches the saturation voltage 3.35 V within 2 sec upon UV exposure (Fig.  7a ), and at 3.12 V and 2.85 V within 4 sec after white and green illuminations (Fig.  7(b,c) ), respectively. Here, the illumination ON and OFF time were taken as 50 sec and 200 sec, respectively. While looking at the photo-response curve of NPs photodetector, the rise time corresponding to different illuminations have small variations, whereas their decay time have large difference, and are much longer in comparison with the corresponding rise time as 93 sec for green, 121 sec for UV and greater than 200 sec for white illumination. After turning OFF the illuminations, the photodetector does not achieve its original dark voltage; instead it remains at a minimum voltage such as; 0.635 V for UV, 0.241 V for green illuminations, whereas for white illumination, it remains unsaturated even after 200 sec as can be seen in Fig.  7a .

The observed difference in the photovoltage rise time with UV, white and green illuminations can be understood by the difference in their illumination energies. UV illumination being an above-band energy, generates carriers both by desorption of adsorbed ions as well as direct excitation of electrons from VB to CB , whereas in case of white and green illuminations the charge carriers are generated only by desorption of the adsorbed ions. Thus for UV illumination, the reduction in depletion region height should be larger, enhancing charge carriers mobility 59 , and hence the fast rise in the photovoltage (Table  1 ). In the response curves, the ON state saturation voltage is also a reflection of different illumination energies. Based upon the energy levels of the illuminations, different concentrations of charge carriers are generated that give rise to the different values of saturation photovoltages as shown in Fig.  7 . The decay of photovoltage after turning OFF the illuminations is due to re-adsorption of O 2 /H 2 O. The re-adsorption/decay curve shows two regions, fast decaying and slows decay regions. The fast decay is due to an instantaneous electron capture by adsorbed chemisorbed O 2 /H 2 O on the surface \({V}_{o}{\rm{s}}\) , and slower decay is due to the adsorption of O 2 /H 2 O deep into the surface between nano-crystallites 60 . The latter involves rate-limited diffusion and rearrangement of O 2 /H 2 O in a closed packed structure for adsorption on the surface that makes the process slower 60 . Just after turning OFF the illuminations, the decrease in photovoltage is faster and almost similar for all three illuminations (UV, white and green), which successively becomes slower on the latter stage and acquires different decreasing rate for each illumination.

A model governing the rate-limited diffusion controlled adsorption process can be represented as 60 ;

where N i represents density of charged ions (after capturing electrons), N s is saturation density of ionized species which prevents further ionization and τ is adsorption rate.

Since for the green illumination, the photo-generated voltage is comparatively lower due to small number of generated charger carriers. As soon as the illumination is turned OFF, the produced smaller number of electrons will be captured by adsorbed O 2 /H 2 O at a faster rate than that for UV illumination 59 . Nevertheless, by UV illumination, electrons and holes are separated away in space that also increases their recombination life time 25 . However, the slow decay response in case of white illumination is unclear. When looking to the minimum value of dark voltage in the OFF state, there exists a shift in the dark voltage for each illumination. This may be ascribed to the presence of neutralized oxygen on the surface of ZnO-NPs. The neutralized oxygen residing on the surface would occupies a part of the surface, and thus will not allow the newly coming oxygen for electron capture, and thus resulting in a shift of photovoltage 60 . The observed shift in the dark voltage is in accordance with the energy leaves of illuminating radiation. The UV radiation, being a high energy, would neutralize more ions as compared with white and green illuminations, and therefore shows a larger shift in dark voltage (Fig.  8(a–c) ).

Comparison of photovoltage vs time curves of ZnO photodetectors thin films for UV, white and green illuminations. (a) NPs photodetector response showing high photo-generated voltage, (b) NSs photodetector shows fast response for both rise and fall time, and (c) NFs photodetector is slow for rise time and intermediate for fall time, also in this case the photo-generated voltage is smaller than that of NPs and NSs photodetectors.

Now let’s compare the photo-response of NPs, NSs and NFs photodetectors with respect to different illuminations (UV, white and green). The NSs and NPs photodetectors show similarity in their photovoltage rise time as 2 sec for UV, 4 sec for white and 5 sec for green; however, their ON state saturation voltages are different as shown in Fig.  8(a–c) . NSs photodetector has smaller value of ON state saturation voltage than that of NPs photodetector. The NFs photodetector shows further smaller value of ON state saturation photovoltages as1.62 V for UV, 1.53 V for white and 0.72 V for green, whereas the photovoltage rise time is longer than that of NPs and NSs. The difference in the saturation photovoltages can be correlated with the different densities of illumination centers/defects in the nanostructures as detected by PL spectra (Fig.  4b ). The observed PL intensities in the order, I NF  > I NS  > I NP are representative of densities of illumination centers/defects in the respective nanostructure. These intensities/densities are adverse to the ON state saturation photovoltages. Indubitably, not all the peaks observed in PL do correspond to the generated photovoltage 61 , rather it indicates the possibility that a corresponding photovoltage may be generated upon illumination when any/all of the defect states participate in the photovoltage generation mechanism. The observed adverse effect of defect states over the ON state saturation voltage indicates that there should be capturing/scattering of photo-generated charge carriers during transportation 62 , 63 . In other words, the higher is defect density, lower is photo-generated voltage or vice versa. In whole of the process; however, unfortunately we could not obtained variation of defect states within a single morphology (either of NPs, NSs or NFs), as that could have been a further better examination of the effect of defects states variation over the ON state saturation voltage.

For decay/off state recovery time of photovoltages, it is known that the decay time depends mainly upon three factors; the available defect states ( V o s and Zn i s in our case), surface area and adsorbate (O 2 or H 2 O). The XPS results (Figs  5a and 6a ) demonstrate that NPs possess smaller content of \({V}_{o}{\rm{s}}\) (21.8%) and smaller content of \(Z{n}_{i}{\rm{s}}\) (as the main contribution being from hydroxide formation), whereas larger content of adsorbed O 2 /H 2 O (60.39%) along with a large surface area (20.5 m 2 /g) in comparison to that of NSs and NFs. The smaller percentage of V o s and Zn i s spreading over a larger surface area would initially promote fast chemisorption process of O 2 /H 2 O after turning OFF the illumination. The fast-initial decay can also be seen in NSs and NFs photodetectors, which latter on becomes slower and different in all the photodetectors. The fast decay, after turning OFF the illumination, indicates that initially the decay is essentially through a chemisorption process that is considered as a fast process. The surface states are then saturated by initial chemisorption process, and O 2 /H 2 O diffuse through the inter-crystallite deep into the surface, where they can find sites for adsorption; this corresponds to the slower decay 64 of photovoltages. In NPs, the content of adsorbed O 2 /H 2 O (60.39%) over the 21.8% of \({V}_{o}{\rm{s}}\) and a smaller percentage of \(Z{n}_{i}{\rm{s}}\) is larger, this shows that during the slower decay step, a large amount of O 2 /H 2 O, in comparison to the chemisorption, is physisorbed on the surface of NPs. Therefore, physisorption process is dominating in the case of NPs photodetector. Since the physisorption process involves rearrangement of adsorbate on the surface, so it will result in an increasing decay time of NPs photodetector (Fig.  8a ). On the other hand, NSs photodetector, in comparison with NPs photodetector, has higher content of \({V}_{o}{\rm{s}}\) (47.7%) and \(Z{n}_{i}{\rm{s}}\) (~13.3%), and smaller content of adsorbed (O 2 /H 2 O ~40.95%) along with smaller surface area (11.8 m 2 /g) would decay prominently through the chemisorption process due to the abundance of defect states to facilitate chemisorption process. Thus the larger content of V o s and Zn i s than the adsorbed O 2 /H 2 O leads to dominating fast chemisorption process that results in faster decay of NSs photodetector (Fig.  8b ). Next, the NFs photodetector decay time shows totally different behavior, which is adverse to the trend obtained from NPs to NSs photodetectors. In this case, an increased decay time despite of higher content of \({V}_{o}{\rm{s}}\) (54.5%) and \(Z{n}_{i}{\rm{s}}(28.6 \% )\) , and smaller content of adsorbate (37.6%), surface area (10 m 2 /g) is observed. Here, the excess of defect states ( V o s and Zn i s) appears to creates adverse effect on photoconductivity, similar to the reduction in the ON state saturation photovoltage (peak voltage in the Fig.  8(a,c) ), and thus increases photovoltage decay time in NFs photodetector (Fig.  8c ). The photovoltage rise time of NFs photodetector (15 sec for UV, 18 sec for white and 10 sec for green illuminations) is also longer than that of both the NSs and NPs photodetectors (Table  1 ). Here also, the deterministic factor appears to be the different content of V o s and Zn i s in nanostructures 64 ; as the higher content of V o s and Zn i s in nanostructure photodetector results in longer photovoltage rise time. The NPs have lower percentage of V o s and Zn i s that would desorb rapidly and thus fast release of charge carriers upon illumination.

Form the above discussed observations, the UV and visible photo-response of thin film NSs is found better in comparison with NPs and NFs photodetectors (as compared in Table  1 ). Another important feature of NSs photodetector is its similarity in the rates of decay curves for both UV and visible illuminations despite of having different saturation voltages. The photo-response of the NSs photodetector in film architecture is even better than that of a sophisticated architectures consisting a single ZnO nanowire that performs only in UV illumination 24 , 25 , 26 , 57 .

As a conclusion, a significant visible light photo-detection along with UV photodetection is achieved in pristine ZnO nanostructure based thin films. The defect states V o s and Zn i s along with morphology giving rise to the visible-light photo response are generated simply by tuning of solution method. The results of PL, XPS and photovoltages show that although the defect states V o s and Zn i s are responsible for exhibiting photo response in the visible-region of spectrum but at the same time their excess reduces the performance of photodetector as observed ~54% of \({V}_{o}{\rm{s}}\) and ~28% of \(Z{n}_{i}{\rm{s}}\) in NFs photodetector, which is probable by scattering/capturing of charge carriers during their transportation. The ZnO-sheet based photodetector possessing moderate amount of \({V}_{o}{\rm{s}}( \sim \,47 \% )\,{\rm{and}}\,Z{n}_{i}s( \sim 13 \% )\) in comparison with ZnO-particle and ZnO-flower photodetectors, shows faster response to photovoltage growth time and decay time under the UV as well as visible-light illumination. The ZnO nanosheet based photodetector can be used as an efficient material for photodetector applications in broad-band spectral applications.

Materials and Methods

Synthesis of zno-nps, zno-nss, zno-nfs, and fabrication of photodetectors.

The simple chemical route was tuned for the formation of surfactant free ZnO nanostructures. Detailed synthesis experiments listed in the Tables  S1 and S2 of the supplementary information were performed with the variation of synthesis time, precursors, solvents and their molar ratio that resulted in the different ZnO morphologies. In the detailed experiments, a new approach was adopted to introduce \({V}_{o}{\rm{s}}\) and \(Z{n}_{i}{\rm{s}}\) in the lattice of ZnO which enabled the ZnO nanostructures active for the visible light photo-detection. In the approach, an abrupt/fast mixing of precursor solution with alkali solution in the reaction chamber resulted in the formation of \({V}_{o}{\rm{s}}\) and \(Z{n}_{i}{\rm{s}}\) as identified by XPS and PL studies. From the detailed synthesis experiments, we found that surfactant free ZnO-NSs and ZnO-NFs resulted only for specific conditions of precursor concentration (0.5 M), precursor’s molar ratio (1:1) and reaction time (4 hours). For NSs synthesis, 0.5 M solution of zinc chloride (ZnCl 2 ) (purity 99.99%, Sigma-Aldrich, USA)) and 0.5 M solution of sodium hydroxide (NaOH) (purity 98%, Merck India Ltd.) were dissolved in ethyl alcohol (C 2 H 5 OH) separately. Then the prepared precursor solution of ZnCl 2 was added abruptly in the solution of NaOH under vigorous stirring conditions at room temperature (~30 °C). After 4 hours of reaction time, the formed precipitate was collected, filtered, and washed with deionized water and C 2 H 5 OH to remove Cl − and Na + ions, which was finally dried at 60 °C. In the second set of experiments for the synthesis of NFs, all the conditions were kept same as that in case of NSs formation, the only solvent C 2 H 5 OH was replaced with deionized water. In this case, ZnO-NFs rather than NSs were obtained for the specific condition of precursor concentrations 0.5 M, precursor’s molar ratio 1:1 and reaction time 4 hours. The NPs were formed in both of the synthesis cases for precursor concentration 0.1 M, molar ratio of precursors 1:4 (NaOH: ZnCl 2 ) and reaction time 15 min. In order to fabricate the photodetector of as synthesized ZnO nanostructures, they were dispersed in C 2 H 5 OH solutions and sonicated for 30 minutes. Then these dispersed nanostructures were spray coated on ultrasonically cleaned glass substrates to make uniform films of thickness about 4 µm. For spray coating, N 2 was used as a carrier gas in the nozzle at spray rate of 1 ml/min. After drying, silver (Ag) interdigitated fingers were printed on the surface of films to make electrical contacts. The width of Ag interdigital electrodes was taken about 1 mm with fringe separation of 2 mm as shown in Figure  S1 .

Characterizations of materials and photodetectors

The morphology of ZnO nanostructures was investigated by field-emission electron microscopy (FE-SEM, Hitachi S-4700, Tokyo, Japan), optical properties (absorbance and bandgap) were investigated by using UV-Vis spectrophotometer (Perkin-Elmer Lambda 750) and structural study was done by X-ray diffractometer (XRD) (Rigaku, radiation Cu kα, λ = 1.5406 Å). Photoluminescence (PL) (LS-55 Luminescence, Perkin Elmer, Germany), and X-ray photoelectron spectroscopy (XPS) were used to investigate defect states in the nanostructures. To estimate photovoltage in the visible region, a mercury lamp (power 100 watt) was used as a light source, whose intensity on the surface of photodetectors was adjusted 1 mW cm −2 . The photovoltage of fabricated photo-detectors was recorded at room temperature using digital multimeter. In the photo-detection experiments, bias voltage of 5 V was applied. UV diode and optical filters with transmittance wavelength in the range 380~420 nm, 490~560 nm and λ > 420 nm (white light) were used to select different spectrum regions, respectively. The ‘ON’ and ‘OFF’ state of incident radiations were controlled by using a camera shutter. Schematic layout of the experimental set-up for the measurement of photo-generated voltage is shown in Figure  S1 .

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Acknowledgements

This work was supported by National Research Foundation of South Korea (NRF) grant funded by the Korean government (MSIP) (NRF-2015R1A2A1A05001826), and research grant for Nanotechnology Lab of Jaypee University of Information Technology, Waknaghat, India.

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Richa Khokhra, Bandna Bharti & Rajesh Kumar

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R. Khokhra fabricated the samples and did their characterizations. B. Bharti helped in XPS studies of the samples. R. Kumar wrote the manuscript and supervised the work. H.N. Lee co-supervised the work and helped in characterizations.

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Khokhra, R., Bharti, B., Lee, HN. et al. Visible and UV photo-detection in ZnO nanostructured thin films via simple tuning of solution method. Sci Rep 7 , 15032 (2017). https://doi.org/10.1038/s41598-017-15125-x

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Mini review article, a mini review of antibacterial properties of zno nanoparticles.

• 1 Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow, Russia
• 2 V. M. Gorbatov Federal Research Center for Food Systems of Russian Academy of Sciences, Moscow, Russia
• 3 Institute of Cell Biophysics of the Russian Academy of Sciences, PSCBR RAS, Moscow, Russia

The development of antibiotic resistance of bacteria is one of the most pressing problems in world health care. One of the promising ways to overcome microbial resistance to antibiotics is the use of metal nanoparticles and their oxides. In particular, numerous studies have shown the high antibacterial potential of zinc oxide nanoparticles (ZnO-NP) in relation to gram-positive and gram-negative bacteria. This mini-review includes an analysis of the results of studies in recent years aimed at studying the antibacterial activity of nanoparticles based on zinc oxide. The dependence of the antibacterial effect on the size of the applied nanoparticles in relation to E. coli and S. aureus is given. The influence of various ways of synthesis of zinc oxide nanoparticles and the main types of modifications of NP-ZnO to increase the antibacterial efficiency are also considered.

Introduction

Today, antibiotics are the “gold standard” in treatment of many bacterial infections [ 1 , 2 ]. However, microorganisms can develop antibiotic resistance. The majority of pathogenic microorganisms have an ability to develop resistance to at least some antimicrobial agents [ 3 ]. Antibiotic resistance in bacteria is achieved by several mechanisms: prevention of drug penetration into a cell [ 4 , 5 ], changes in an antibiotic target [ 6 , 7 ], enzymatic inactivation of antibiotics [ 8 ], active excretion of an antibiotic from a cell [ 4 ] and so on.

According to the data of the World Health Organization (WHO), lower respiratory infections and gastrointestinal infections are among the top ten factors of morbidity and mortality [ 9 ]. Appearance of antibiotic resistant strains significantly increased the number of deaths and severity of bacterial infections. Deaths of patients due to antibiotic resistant bacterial strains exceed the total number of global deaths due to cancer and diabetes mellitus [ 10 , 11 ]. Despite the significant quantity of available antibiotics, resistance to almost all of them was confirmed. Antibiotic resistance emerges shortly after a new drug is approved for use [ 3 , 12 ]. The indicated events urged WHO to endorse the Global action plan on antimicrobial resistance in 2015 [ 13 ]. Secondary bacterial infections can be a cause of increased lethality among patients in intensive care; in particular, bacterial co-infection and secondary infection are found in patients with COVID-19 [ 14 , 15 ]. All above mentioned make a search for new antimicrobial preparations a high priority task of public health in the world.

The number of scientific publications devoted to a search for new antimicrobial compounds is about 99000 only in 2018–2020; 5900 of them are devoted to a search for antibacterial compounds based on metal compounds [ 16 ].

Humans have been used antimicrobial properties of several metals and their ions since ancient times. For example, utensils from Cu and Ag were used in ancient Persia, Rome and Egypt [ 17 ]. It is known today that a wide range of metals has the antimicrobial activity: Ag, Al, As, Cd, Co, Cr, Cu, Fe, Ga, Hg, Mo, Mn, Ni, Pb, Sb, Te, Zn [ 18 – 20 ].

The basis of the antimicrobial activity of metals is an ability of metal ions to inhibit enzymes [ 21 , 22 ], facilitate generation of reactive oxygen species (the Fenton reaction) [ 23 ], cause the damage of cell membranes [ 24 ], prevent uptake of vitally important microelements by microbes [ 25 ]; moreover, several metals can exert the direct genotoxic activity [ 26 – 28 ].

The use of nanoparticles based on metals and their oxides is of great interest. One of the well-studied metals affecting biological objects is zinc (Zn) and its oxide (ZnO). Zinc is an active element and exhibits strong reduction properties. It can easily oxidize to form zinc oxide. Zinc plays an important role in the human body, since it is one of the most important trace elements [ 29 ]. Zinc is found in all tissues of the human body, with the highest concentration found in myocytes (85% of the total zinc content in the body) [ 30 ]. Zinc has been shown to be critical for the proper functioning of a large number of macromolecules and enzymes, where it plays both a catalytic (coenzyme) and structural role. In turn, structures called Zincfinger provide a unique scaffold that allows protein subdomains to interact with either DNA or other proteins [ 31 ].

Zinc is also essential for the functioning of metalloproteins. Although zinc is considered relatively non-toxic, there is growing evidence that free zinc ions can cause negative effects on cells. To assess the toxicity of a test substance in vitro , animal cell cultures are usually used. It is known that nerve cells are the most sensitive to exogenous influences [ 32 – 34 ]. It has been reported that exposure to zinc ions leads to neuronal degradation [ 35 ]. To eliminate the cytotoxic effect, zinc cations are bound with bioactive ligands (for example, proteins) and zinc oxide nanoparticles are synthesized. Nanostructured ZnO can have various morphological forms and properties.

At present, there is a growing interest to nanoparticles of metals and metal oxides as compounds with antibacterial potential: Ag [ 36 , 37 ], Au [ 38 ], ZnO [10], TiO 2 [ 39 , 40 ], CuO [ 41 , 42 ], Fe 2 O 3 [ 43 , 44 ]. ZnO has many applications in engineering and medicine. In engineering, ZnO nanoparticles are used in solar cells [ 45 , 46 ], gas sensors, in particular, sensors for Liquefied petroleum gas (LPG) and EtOH [ 47 ], chemical sensors and biosensors, in LEDs, photodetectors [ 48 , 49 ]. In biology and medicine, the cytostatic activity of ZnO nanoparticles (ZnO-NPs) against cancer cells [ 50 ], antimicrobial and fungicidal activities [ 51 , 52 ], anti-inflammatory activity [ 53 , 54 ], ability to accelerate wound healing [ 55 ], a possibility to use in bioimaging due to chemiluminescent properties of nanoparticles [ 56 , 57 ], antidiabetic properties [ 58 , 59 ] are of great interest.

ZnO nanoparticles have several advantages: high antibacterial effectiveness at low concentrations (0.16–5.00 mmol/L), activity against a wide range of strains, relatively low cost [ 43 , 51 , 60 ]. ZnO nanoparticles are synthesized by the physio-chemical sol-gel method from zinc salts [ 43 , 61 ], sol-gel combustion method [ 62 ], solochemical method [ 63 ], chemical synthesis at low temperatures [ 64 ] and mechanical method [ 65 ]. In several cases, stabilizing agents, for example, chitosan are added [ 66 , 67 ].

The mechanisms of action of zinc oxide nanoparticles can be reduced to the following: disruption of the cell membrane [ 68 , 69 ], binding to proteins and DNA, generation of reactive oxygen species (ROS) [ 10 , 70 , 71 ], disturbance of the processes of bacterial DNA amplification, alteration (more often, down-regulation) of expression in a wide range of genes [ 72 ]. The direct bactericidal action of ZnO nanoparticles against both gram-negative and gram-positive bacteria and fungi was shown [ 73 , 66 , 74 ].

Nanoparticles of a number of metal oxides lead to the production of ROS upon interaction with bacteria [ 75 ]. The metal ions released by the nanoparticles affect the respiratory chain and inhibit some enzymes. This leads to the formation and accumulation of singlet oxygen, hydroxyl radical, hydrogen peroxide, superoxide anions, and other ROS. ROS can cause damage to the internal components of bacteria, such as proteins and DNA [ 76 ].

It has been shown that exposure to sublethal ROS concentrations can stimulate the manifestation of defense reactions. This process is called hormesis [ 77 ]. Hormesis induces defense mechanisms on two levels. The first level is enzymatic (short-term reaction). At this level, antioxidant enzymes are activated. The second level is long-term adaptation. Long-term adaptation consists of two sublevels: transcriptional and genomic. At the level of transcription, ROS induces adaptation due to the activation of antioxidant mechanisms within a few hours or days [ 78 ]. At the genomic level, ROS can cause damage to the DNA structure, which activates the mechanisms for repairing DNA damage. These mechanisms include homologous recombination and excisional repair. In these mechanisms, two of the DNA polymerases responsible for DNA synthesis have poor validation activity and may include abnormal bases in DNA strands, which leads to a high frequency of spontaneous mutations and genome plasticity under adverse influences [ 79 ]. Such plasticity of the genome can lead to the development of resistance to metals and metal oxide nanoparticles [ 80 ].

The adaptation mechanisms of bacteria in relation to nanoparticles also include overexpression of extracellular substances by bacterial cells, such as flagellin, which form an extracellular matrix that promotes agglomeration and deactivation of nanoparticles [ 81 ]. Despite the existing mechanisms of adaptation of bacteria to the impact, numerous studies have noted the high antibacterial potential of ZnO nanoparticles.

Literature Review

Despite the apparent wide range of strains, against which nanoparticles exert the antimicrobial activity, their effectiveness against particular strains can be significantly different. As a rule, gram-negative bacteria are less sensitive to ZnO nanoparticles than gram-positive bacteria [ 62 , 66 , 82 ]. Somewhat higher resistance of gram-negative bacteria can be explained by the peculiarities of their cell wall structure. In contrast to gram-positive bacteria, the cell wall of gram-negative bacteria includes the additional outer membrane containing lipopolysaccharides (LPS) [ 83 ]. It is shown that LPS can improve the barrier properties of the outer membrane and, therefore, increase bacterial resistance, in particular, to antibiotics [ 84 ]. Epidemiologically significant microorganisms deserve a special attention, for example, Mycobacterium tuberculosis , against which ZnO nanoparticles exert the bacteriostatic effect but not bactericidal [ 85 ].

On the contrary, several microorganisms (for instance, Campylobacter jejuni) have an increased sensitivity to ZnO nanoparticles, which make them a convenient model for studying molecular 126 mechanisms of the antimicrobial effect of nanoparticles [ 24 ]. ZnO nanoparticles (ZnONPs) disturb the processes of bacterial DNA amplification, reduce expression of a wide range of genes of C. jejuni that are responsible for virulence, significantly alter expression of genes of oxidative and general stress [ 24 ]. An important feature of ZnO nanoparticles used in one of the studies is the antibacterial activity against resistant bacterial strains, for example, carbapenem-resistant Acinetobacter baumannii (RS-307 and RS-6694) [ 86 ]. The dependence of effectiveness on a bacterial growth phase was shown for ZnO nanoparticles. In particular, ZnO nanoparticles are effective against gram- negative and gram-positive bacteria at the exponential growth phase; however, the antibacterial properties of nanoparticles are significantly decreased at the lag and stationary phases [ 52 ]. A range of bactericidal concentrations of ZnO nanoparticles is usually significantly less than a range of 4 [ 62 ]. At present, an active search for methods to increase the antimicrobial action of nanoparticles is carried out. Below we present the literature search. Nanoparticles are classified by the method for synthesis, size, structure, form, absence or presence of the envelope or nucleus. The objects, on which nanoparticles influenced, are classified by types, biological effect of nanoparticles, concentration of nanoparticles, duration of exposure, temperature and environment. The data are presented in table 1 .

TABLE 1 . Main characteristics, physicochemical and biological parameters of ZnO nanoparticles presented in the review.

Let us consider proposed methods for increasing antibacterial properties of ZnO nanoparticles. The first method for increasing antibacterial properties of ZnO nanoparticles is to use a combination of different metal compounds [ 52 , 90 ]. For example, the CuO and ZnO have comparable effectiveness against gram-negative Escherichia coli and gram-positive Staphylococcus aureus at the exponential growth phase. ZnO nanoparticles were practically inactive at the lag and stationary phases, while CuO nanoparticles retained the significant activity [ 52 ]. Ag and ZnО nanoparticles in different ratios inhibit the growth of antibiotic resistant Mycobacterium tuberculosis strains but did not lead to bacterial death [ 85 ]. ZrO 2 -ZnO nanoparticles have the pronounced antimicrobial action in contrast to ZrO 2 nanoparticles, but the antimicrobial effect of ZrO 2 -ZnO nanoparticles does not exceed that of ZnO nanoparticles [ 94 ]. However, combinations of metal oxides not always give the synergetic effect. In particular, CdO-ZnO nanoparticles have the antimicrobial action comparable with that of CdO nanoparticles [ 90 ]. Doping of ZnO nanoparticles with the Fe ions enables achieving a significant antibacterial effect against E. coli , Pseudomonas aeruginosa [ 117 ]. TiO 2 /ZnO nanoparticles have more pronounced bactericidal effect against E. coli compared to ZnO nanoparticles. Ag/TiO 2 /ZnO nanoparticles are more effective than TiO 2 /ZnO nanoparticles [ 115 ]. Compared to ZnO nanoparticles, ZnO-Mn nanoparticles have higher antimicrobial activity against K. pneumoniae , Shigella dysenteriae , S. enterica Typhimurium , P. aeruginosa and other bacteria [ 109 ].

The second method for increasing antimicrobial effectiveness is to use combinations of ZnO nanoparticles and carbon nanoparticles, in particular, spindle-shaped graphene oxide (GO) nanoparticles [ 68 , 108 , 109 ]. It is shown that GO-ZnO nanoparticles effectively inhibit the growth of gram-negative ( E. coli , S. typhimurium ) and gram-positive ( Bacillus subtilis , Enterococcus faecalis ) bacteria [ 68 ]. With that, the antibacterial effectiveness of the mixture of GO-ZnO nanoparticles turned to be nearly twice as high as that of ZnO nanoparticles and almost four times higher than that of GO nanoparticles [ 82 ].

The third method is coating ZnO nanoparticles with modifying agents. Gelatin-coated ZnO nanoparticles showed higher inhibition of the growth of gram-negative bacteria compared to gram-positive bacteria [ 91 ]. As was mentioned above, overcoming antibiotic resistance in gram-negative bacteria is a more difficult task. Gelatin-coated ZnO nanoparticles inhibit biofilm formation of C. albicans (an additional resistance factor) [ 91 ]. These nanoparticles also inhibit angiogenesis in chick embryos, which makes them candidates for the development of preparations preventing undesirable angiogenesis [ 91 ]. The chemical surface modification of nanoparticles using (3-glycidyloxypropyl) trimethoxysilane (GPTMS) and decrease in a size up to 5 nm lead to an increase in antimicrobial effectiveness of nanoparticles against S. aureus [ 62 ]. Treatment with polystyrene increased the bacteriostatic effect of ZnO nanoparticles against E. coli and Listeria monocytogenes ; with that, uncoated ZnO nanoparticles did not have the bacteriostatic effect against L. monocytogenes [ 100 ]. Modification of ZnO nanoparticles with polyethylene glycol or starch also alters properties of nanoparticles [ 121 ]. Modification with polyethylene glycol increased the bacteriostatic effect of ZnO nanoparticles against E. coli и S. aureus ; with that, effectiveness against gram-negative bacteria was higher. Polyethylene enhanced cytotoxicity of ZnO nanoparticles toward the cancer cell line (MG-63) by induction of apoptosis. Modification with starch allowed retention of antibacterial properties of ZnO nanoparticles and reduction of cytotoxicity compared to modification with polyethylene glycol [ 121 ]. Treatment with thioglycerol, contrary to the expectations, did not increase the bacteriostatic and bactericidal activity of ZnO nanoparticles [ 71 ]. Polymer films from sodium alginate/polyvinyl alcohol gained bacteriostatic properties after incorporation of ZnO nanoparticles, which can be used in the development of more durable materials [ 116 ].

The fourth method is modification of the synthesis method leading to changes in the geometrical characteristics of nanoparticles. ZnO nanoparticles synthesized by the sonochemical method have more pronounced inhibitory properties against Bacillus cereus , S. aureus , S. Typhimurium and Pseudomonas aeruginosa than ZnO nanoparticles synthesized by the classical physio-chemical methods [ 63 ]. Nanoparticles synthesized at comparatively low temperatures are flower-shaped and have the comparable antimicrobial activity against gram-positive ( S. aureus ) and gram-negative ( E. coli ) bacteria and, to a lesser extent, fungi ( C. albicans ) [ 64 ]. When using ROS photocatalytic generation and release of Zn 2+ , flower-shaped ZnO nanoparticles show more pronounced antimicrobial activity against E. coli than more lacunary hexagon-shaped ZnO-NPs [ 70 ].

Antibacterial properties of nanoparticles depend on their size [ 122 - 124 ]. For several nanoparticles, the highest antibacterial activity is achieved at the smallest size [ 103 , 107 , 125 ]; however, we have not found in the literature a clear dependence of antibacterial effectiveness on a nanoparticle size. We had to analyze literature by ourselves and build a graph reflecting a dependence of an inhibition zone size on a size of ZnO nanoparticles ( Figure 1 ). Analysis of literature allows stating that the highest potential antimicrobial effectiveness of nanoparticles against both E. coli , and S. aureus is observed at a nanoparticle size of about 100 nm. It is necessary to note that “green chemistry” not always leads to synthesis of effective nanoparticles. For example, in studies on S. aureus , only two types of nanoparticles out of six (33%) had the antibacterial activity at a level higher than average. When studying on E. coli , only one of five (20%) types of nanoparticles generated using “green chemistry” exerted the antibacterial activity at a level higher than average. Therefore, it can be suggested that nanoparticles generated by “green chemistry” still have insufficient effectiveness.

FIGURE 1 . Dependence of the minimum inhibitory capacity on the size of NP-ZnO, as well as the dependence of the size of the zone of inhibition on the size of NP-ZnO against gram-negative bacteria by the example of E. coli (A and B) and gram-positive bacteria by the example of S. aureus (C and D) . Gray dots are samples synthesized by methods without using “green synthesis”, green dots are samples obtained using “green synthesis”.

As can be seen in Figures 1B,D , quite high dispersion of effectiveness is seen in the region of small sizes of nanoparticles (1–50 nm). Therefore, we studied the dependence of the minimum inhibitory concentration (MIC) on sizes of ZnO nanoparticles ( Figures 1A,C ). It is shown that the use of nanoparticles with sizes of up to 10 nm is not effective. Usually, at these average sizes of nanoparticles, distribution of nanoparticles by sizes is rather complex and not always narrow. Nanoparticles with small sizes are quite prone to aggregation. Apparently, high dispersion of antibacterial activity at small sizes of nanoparticles can be explained by this fact.

Flower-shaped ZnO nanoparticles can reach large sizes (up to 3 µm) and demonstrate the antimicrobial activity against both gram-positive ( S. aureus ) and gram-negative ( E. coli ) bacteria [ 99 ]. For spherical ZnO nanoparticles, the antimicrobial activity practically does not depend on the type of a targeted organism. Hexagonal ZnO nanoparticles have higher bactericidal activity against antibiotic resistant Staphylococcus epidermidis , B. subtilis , Klebsiella pneumoniae and P. aeruginosa strains compared to ZnO-NPs with the triangular shape [ 111 ]. Thorn-like ZnO nanoparticles cause significant reduction in the growth of B. subtilis , E. coli and C. albicans colonies demonstrating the antibacterial and antifungal activities [ 110 ].

The fifth method is modification by physio-chemical methods, for example, by annealing in the Ar environment at high temperatures, or plasma oxidation. With that, the effects of modification can be different: Ar annealing decreases the antibacterial activity of ZnO nanoparticles, while plasma oxidation improves antibacterial properties of ZnO nanoparticles against E. coli and S. aureus [ 120 ]. The sixth method is the use of additives causing photocatalysis of reactive oxygen species (ROS). This modification enables a significant increase in antibacterial properties of ZnO nanoparticles [ 70 , 93 ]. The seventh method is the so-called “green synthesis” [ 126 – 128 ]. ZnO nanoparticles generated by “green synthesis” have the antimicrobial activity against gram-negative and gram-positive bacteria, as well as several fungi of the genus Candida [ 88 ]. In turn, nanoparticles synthesized using the Tabernaemontana divaricata extract demonstrated the antibacterial activity against S. aureus , E. coli and lower activity against S. enterica Paratyphoid [ 98 ]. The eight method is a change in the environment conditions. At acidic pH levels, ZnO nanoparticles had higher bacteriostatic action against S. aureus and E. coli than at neutral pH [ 101 ]. The combination of all approaches described above can be most promising, for example, the use of Ag-ZnO nanoparticles synthesized in the Cannabis sativa extract. The generated nanoparticles can be used in combination with photocatalysis and have the antibacterial and antifungal activities.

Zinc oxide nanoparticles have significant antibacterial potential. The use of various methods of synthesis, chemical modification, as well as joint use with other nanomaterials affects the physical and morphological characteristics of nanoparticles, which, in turn, leads to a change in their antibacterial properties. As a result, nanoparticles based on zinc oxide are increasingly used not only in nanoelectronics and optics, but also in such industrial areas as cosmetic, food, rubber, pharmaceutical, household chemicals, etc. The use of packaging with incorporated zinc oxide nanoparticles is possible will allow in the future to prevent the growth of microorganisms and spoilage of food. In turn, the use of medical dressing materials containing ZnO nanoparticles will allow avoiding microbial contamination of the wound and promotes its early healing. Thus, zinc oxide nanoparticles can be considered as a promising new generation antimicrobial agent.

Author Contributions

SG designed this topic. DB, MR contributed to collecting related references. DB made a table. DS, SG, MR wrote most of the manuscript. AS and AL were involved in discussing the manuscript and translating it into English.

This work was supported by the Ministry of Science and Education of the Russian Federation (Grant Agreement 075-15-2020-775).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

Authors acknowledge the immense help received from the scholars whose articles are cited and included in references to this manuscript. The authors are also grateful to authors/editors/publishers of all those articles, journals and books from where the literature for this article has been reviewed and discussed.

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Keywords: nanoparticles, zinc oxide, antibiotics, antibacterial, bacteriostatic, bactericidal, fungicidal, green synthesis

Citation: Gudkov SV, Burmistrov DE, Serov DA, Rebezov MB, Semenova AA and Lisitsyn AB (2021) A Mini Review of Antibacterial Properties of ZnO Nanoparticles. Front. Phys. 9 :641481. doi: 10.3389/fphy.2021.641481

Received: 15 December 2020; Accepted: 19 January 2021; Published: 11 March 2021.

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Copyright © 2021 Gudkov, Burmistrov, Serov, Rebezov, Semenova and Lisitsyn. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Sergey V. Gudkov, [email protected]

This article is part of the Research Topic

Gas, Liquid and Solid Nanoparticles in Aqueous Media and their Possible Applications in Medicine and Biology

Optical, morphological and structural characteristics of zinc oxide nanoparticles fabricated by laser ablation in deionized water

• Research Article
• Published: 14 May 2024

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• Mohammed S. Hadi 1 ,
• Hiba Basim Abbas Kadhim 2 &
• Mohammed H. Abbas 3  

The fabrication of nanomaterials under pulsed laser removal treatments is an efficient approach that could help pave the way for achieving tunable properties. Here, a pulsed laser ablation (Nd:YAG laser, 1064 nm) in deionized water is employed to prepare zinc oxide nanoparticles (ZnO NPs) by targeting a zinc metal coin. Optical, morphological, and structural characteristics are studied by observing the impact of laser energy and the number of pulses on the resulting NPs in the solution at two different states. At the first state, different numbers of pulses (200, 400, and 600) are applied at a constant laser energy of 120 mJ. At the second state, the laser energy is changed between 140 and 180 mJ at a constant number (200) of pulses. UV–visible spectroscopy shows the maximum absorbance of NPs in the UV area at the wavelength of 328 nm and laser energy of 140 mJ. Based on atomic force microscopy results, a direct correlation is established between the laser energy and the average root mean square of surface of NPs, indicating that the enhanced laser energy induces an increase in the average roughness. A direct correlation is also found between the number of pulses and the intensity of X-ray diffraction patterns. The grain size of the ZnO NPs decreases when reducing the laser energy. Moreover, increasing the number of pulses causes the concentration of the NPs to be increased in the solution.

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Abbreviations

Nanoparticles

Atomic force microscopy

X-ray diffraction

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Mohammed S. Hadi

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Hadi, M.S., Kadhim, H.B.A. & Abbas, M.H. Optical, morphological and structural characteristics of zinc oxide nanoparticles fabricated by laser ablation in deionized water. J Opt (2024). https://doi.org/10.1007/s12596-024-01872-4

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DOI : https://doi.org/10.1007/s12596-024-01872-4

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