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Zinc oxide nanoparticles: synthesis, characterization, modification, and applications in food and agriculture.

Graphical Abstract

1. Introduction

2. structure, 3. preparation method, 3.1. conventional synthesis methods, 3.2. biological/green synthesis methods, 3.3. physical synthesis methods, 3.4. a non-conventional method, 4. modifications, 5. common tools and techniques for characterization, 5.1. uv-vis spectrophotometer (uv-vis), 5.2. x-ray diffractometer (xrd), 5.3. fourier transform infrared spectroscopy (ft-ir), 5.4. atomic force microscopy (afm), 5.5. scanning electron microscopy (sem), 5.6. transmission electron microscopy (tem), 5.7. x-ray photoelectron spectroscopy (xps), 6. morphological impacts, 7. advantages and possible risk, 7.1. advantages, 7.2. possible risk, 7.3. regulations, 8. applications, 8.1. role in agriculture, 8.2. as antimicrobial agent against food-borne pathogens, 8.3. role in food processing and storage, 8.4. role in food packaging, 8.5. role in food flavor, 9. summary and future perspectives, author contributions, data availability statement, conflicts of interest.

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Zhou, X.-Q.; Hayat, Z.; Zhang, D.-D.; Li, M.-Y.; Hu, S.; Wu, Q.; Cao, Y.-F.; Yuan, Y. Zinc Oxide Nanoparticles: Synthesis, Characterization, Modification, and Applications in Food and Agriculture. Processes 2023 , 11 , 1193. https://doi.org/10.3390/pr11041193

Zhou X-Q, Hayat Z, Zhang D-D, Li M-Y, Hu S, Wu Q, Cao Y-F, Yuan Y. Zinc Oxide Nanoparticles: Synthesis, Characterization, Modification, and Applications in Food and Agriculture. Processes . 2023; 11(4):1193. https://doi.org/10.3390/pr11041193

Zhou, Xian-Qing, Zakir Hayat, Dong-Dong Zhang, Meng-Yao Li, Si Hu, Qiong Wu, Yu-Fei Cao, and Ying Yuan. 2023. "Zinc Oxide Nanoparticles: Synthesis, Characterization, Modification, and Applications in Food and Agriculture" Processes 11, no. 4: 1193. https://doi.org/10.3390/pr11041193

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Green Synthesis and Characterization of ZnO Nanoparticles Using Pelargonium odoratissimum (L.) Aqueous Leaf Extract and Their Antioxidant, Antibacterial and Anti-inflammatory Activities

Ahmed s. abdelbaky.

1 Department of Biochemistry, Faculty of Agriculture, Fayoum University, Fayoum 63514, Egypt

Taia A. Abd El-Mageed

2 Department of Soil and Water, Faculty of Agriculture, Fayoum University, Fayoum 63514, Egypt; ge.ude.muoyaf@00aat

Ahmad O. Babalghith

3 Department of Medical Genetics, College of Medicine, Umm Al-Qura University, P.O. Box 57543, Makkah 21955, Saudi Arabia; as.ude.uqu@htihglababoa

4 Department of Clinical Laboratory Sciences, College of Applied Medical Sciences, Jouf University, Sakaka 72388, Saudi Arabia; as.ude.uj@malasludbas

Abir M. H. A. Mohamed

5 Department of Agricultural Microbiology, Faculty of Agriculture, Fayoum University, Fayoum 63514, Egypt; ge.ude.muoyaf@50hma

Associated Data

The data presented are included within the article.

Nanoparticles (NPs) exhibit distinct features compared to traditional physico-chemical synthesis and they have many applications in a wide range of fields of life sciences such as surface coating agents, catalysts, food packaging, corrosion protection, environmental remediation, electronics, biomedical and antimicrobial. Green-synthesized metal NPs, mainly from plant sources, have gained a lot of attention due to their intrinsic characteristics like eco-friendliness, rapidity and cost-effectiveness. In this study, zinc oxide (ZnO) NPs have been synthesized employing an aqueous leaf extract of Pelargonium odoratissimum (L.) as a reducing agent; subsequently, the biosynthesized ZnO NPs were characterized by ultraviolet-visible spectroscopy (UV-Vis), dynamic light scattering (DLS), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDX), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). Moreover, aqueous plant leaf extract was subjected to both qualitative and quantitative analysis. Antioxidant activity of ZnO NPs was assessed by DPPH assay, with varying concentrations of ZnO NPs, which revealed scavenging activity with IC 50 = 28.11 μg mL −1 . Furthermore, the anti-bacterial efficacy of the green synthesized ZnO NPs against four foodborne pathogenic bacterial strains was examined using the disk diffusion assay, and Staphylococcus aureus (ATCC 8095), Pseudomonas aeruginosa (ATCC10662) and Escherichia coli (ATCC 25922) were found to be the most sensitive against biosynthesized ZnO NPs, whereas the least sensitivity was shown by Bacillus cereus (ATCC 13753). The anti-inflammatory effect was also evaluated for both ZnO NPs and the aqueous leaf extract of P. odoratissimum through the human red blood cells (HRBC) membrane stabilization method (MSM) in vitro models which includes hypotonicity-induced hemolysis. A maximum membrane stabilization of ZnO NPs was found to be 95.6% at a dose of 1000 μg mL −1 compared with the standard indomethacin. The results demonstrated that leaf extract of P. odoratissimum is suitable for synthesizing ZnO NPs, with antioxidant, antibacterial as well as superior anti-inflammatory activity by improving the membrane stability of lysosome cells, which have physiological properties similar to erythrocyte membrane cells and have no hemolytic activity. Overall, this study provides biosynthesized ZnO NPs that can be used as a safe alternative to synthetic substances as well as a potential candidate for antioxidants, antibacterial and anti-inflammatory uses in the biomedical and pharmaceutical industries.

1. Introduction

Nanotechnology is one of the most quickly evolving fields, potentially forming and underpinning a wide range of technological and biotechnological advancements; as a result, it is seen as the century’s oncoming industrial revolution [ 1 ]. Nanotechnology has been used in different industrial and academic areas, including chemistry, agriculture, biology, medicine, electronics, information technology and physics [ 2 , 3 , 4 ]. Nanomaterials possess great potential in various fields of science due to their excellent physico-chemical and biological characteristics over bulk materials [ 5 ]. Nanoparticles (NPs) have the unique property of having a high surface-to-volume ratio [ 6 ], which means that they are more appropriate candidates for application-oriented performance (e.g., photocatalysis, cosmetics, gas sensing, energy reservoirs, electronics, packaging and environmental remediation) and encourages their incorporation into a wide range of commercial products, biotechnology and biomedical applications [ 7 , 8 , 9 , 10 , 11 , 12 , 13 ].

Among the large variety of NPs available, metal oxide (MO) NPs are thought to be the most promising because they have distinctive physical, chemical, and biological properties like solubility, chemical stability, and adhesiveness [ 8 ]. Additionally, the utilization of harmful compounds for reduction and as a capping agent in the nanoparticle synthesis process causes a variety of adverse effects on the flora life as well as the environment and the living system toxicity. As a result, plant extracts (PEs) are therefore a more promising tool for the easy synthesis of MO NPs through the green route, because this approach is eco-friendly, non-toxic, low cost, environmentally compatible and easy to apply. Additionally, the resultant particles are biocompatible and free of toxic stabilizers compared to classical chemicals. Basically, PEs contain a variety of active biomolecules that aid to reduce and stabilize NPs [ 6 , 12 ].

Zinc oxide (ZnO) is one of the very promising inorganic oxides that has recently attracted the attention of many scientists for the biosynthesis of NPs due to its unique properties and multiple applications such as drug delivery, solar cells, photocatalytic degradation and personal care products like sunscreens and cosmetics [ 14 , 15 , 16 , 17 , 18 , 19 ]. Based on earlier reports in the literature, ZnO NPs have been biosynthesized from several plant extracts such as Cassia auriculata [ 20 ], Aloe vera [ 13 , 21 ], Duranta erecta [ 22 ], Cinnamomum verum [ 23 ], Bauhinia tomentosa [ 24 ], Vitex trifolia [ 25 ], Moringa oleifera [ 26 ], Azadirachta indica [ 27 , 28 ], Artocarpus gomezianus [ 29 ] and Olea europaea [ 30 ]. In biological systems, the overproduction of highly reactive radical species (HRRS) causes oxidative stress, which has been observed in several diseases, i.e., cancer, diabetes, cardiovascular disease, and arthritis [ 31 ]. All biosystems depend heavily on antioxidants to function correctly. As a result, there is an urgent need to search for innovative and safe antioxidants produced from natural sources, which are more effective and less toxic. Additionally, the widespread use of antibacterial and anti-inflammatory drugs has caused resistance, the appearance of new pathogenic strains resistant to antibiotics [ 32 ] and chronic and acute toxicities in several human physiological systems, particularly the immune system. As a result, searching for new, effective antibacterial and anti-inflammatory drugs that can effectively combat drug-resistant bacteria is necessary and does not cause immunosuppression. Biosynthesized NPs have been proposed as an alternate potential approach to address these problems [ 33 ]. Pelargonium odoratissimum (L.) aqueous leaf extract (ALE) was utilized in the present study, for the biosynthesis of ZnO NPs as this is the first report on the use of this plant’s leaves for the green synthesis of NPs. Pelargonium odoratissimum (L.) known as “Apple Geranium” is a perennial and relatively flat-growing shrublet that belongs to the family Geraniaceae, very commonly grown locally in Egypt and is widely utilized for its health benefits [ 34 ]. Essential oils of Pelargonium spp. are in considerable demand in the pharmaceutical, perfumery, and cosmetic industries. Additionally, some reports revealed that essential oils obtained from a variety of Pelargonium spp. possess excellent antioxidant, antibacterial and antifungal properties [ 35 , 36 , 37 , 38 ]

The aerial parts of this Pelargonium spp. are used in traditional medicine for the treatment of wound healing, debility, gastrointestinal disorders (i.e., diarrhea and dysentery), hemorrhage, skin complaints, neuralgia and throat infections due to their various phytochemical constituents such as phenolics, flavonoids, terpenes, saponins and essential oils [ 39 ], which can contribute to their biological activities and facilitate the biosynthesis of NPs by employing them as reducing, capping and stabilizing agents.

Despite the widespread use of Pelargonium species as therapeutic agents, to date, there have been no data on their use for green synthesis of NPs, antioxidant, antibacterial and anti-inflammatory effects from Pelargonium odoratissimum leaf extract.

The aim of this study was to explore the application of P. odoratissimum ALE as a capping and reducing agent for the biosynthesis of ZnO NPs. The biosynthesized ZnO NPs were characterized and confirmed by various spectroscopic and microscopic techniques, i.e., UV-Vis spectroscopy, FTIR, XRD, DLS, HR-TEM, FE-SEM and EDX, in addition, to evaluate the antioxidant effects, as well as the antibacterial activities against some food-borne pathogens strains beside evaluating the anti-inflammatory activities of both ZnO NPs and the ALE of P. odoratissimum .

2. Materials and Methods

2.1. chemicals.

Gallic acid, rutin, 1,1-diphenyl-2-picrylhydrazyl (DPPH, ≥99%), Folin–Ciocalteu’s reagent, L-ascorbic acid (Sigma-Aldrich, St. Louis, MO 63103, USA), aluminum chloride anhydrous (Fluka, Buchs, Switzerland), sodium carbonate (>99%), zinc acetate dihydrate (Advent Chembio PVT. LTD, Mumbai, India), Luria-Bertani (LB) broth medium (Himedia, Mumbai, India) gentamycin (Tody Laboratories Int., 22nd Vadul Moldovei Street, Bucharest, Romania). All chemicals used in this study were of analytical grade.

2.2. Plant Collection and Processing

Fresh leaves of P. odoratissimum (L.) were collected from the Botanical Garden of Fayoum University, Fayoum, Egypt, in March 2021. The taxonomic identification of the plant was identified by Mrs. Therese Labib, Head of the Taxonomy specialists at El-Orman Botanical Garden, Cairo, Egypt. A voucher specimen with number 126 was deposited in the herbarium of the Biochem. Dept. Fac. Agric., Fym. Univ., Fym., Egypt. The leaves were completely air dried in the shade before being ground into a fine powder in a lab mill and sieved using a 24 mesh sieve. The powdered leaves were maintained in an air-tight container at room temperature (28 ± 2 °C) and kept away from light until use.

2.3. Preparation of P. odoratissimum Leaf Extract

The air-dried powder (20 g) of P. odoratissimum leaves was taken and immersed in 400 mL of deionized water (dH 2 O). The extraction process was performed via the ultrasonic-assisted solvent extraction (UASE) method [ 40 ] by placing the conical flask in a Probe Sonicator homogenizer (Benchmark Scientific, USA, 150 W, 25 kHz) at room temperature (35 ± 2 °C) for 30 min. The solvent (d.H 2 O) and powder layer were filtered using muslin cloth first and then Whatman filter paper No.1. The filtrate solution of P. odoratissimum leaf extract was kept in a refrigerator to be utilized for further use.

2.4. Qualitative Phytochemical Screening

The detection of various phytoconstituents present in the ALE of P. odoratissimum was carried out using the standard phytochemical methods [ 41 , 42 , 43 ].

2.5. HPLC-Analysis

The HPLC analysis was carried out using an Agilent 1260 series. The separation was performed using Eclipse C18 column (4.6 mm × 250 mm i.d., 5 μm). The mobile phase consisted of water (A) and 0.05% trifluoroacetic acid (TFA) in acetonitrile (B) at a flow rate of 0.9 mL/min. The mobile phase was programmed consecutively in a linear gradient as follows: 0 min (82% A); 0–5 min (80% A); 5–8 min (60% A); 8–12 min (60% A); 12–15 min (82% A); 15–16 min (82% A) and 16–20 (82%A). The multi-wavelength detector was monitored at 280 nm. The injection volume was 5 μL for each of the sample solutions. The column temperature was maintained at 40 °C.

2.6. Estimation of Total Phenolic and Flavonoid Contents (TPC and TFC)

The determination of both TPC as mg gallic acid equivalents (GAE) mg GAE/g plant extract) and TFC as mg rutin equivalents (RE)/g plant extract were performed spectrophotometrically by the Folin-Ciocalteu reagent [ 44 ] and aluminum chloride methods [ 45 ] respectively.

2.7. Green Synthesis of ZnO Nanoparticles

After heating twenty milliliters of P. odoratissimum leaf extract at 50 °C for 10 min, fifty milliliters of 0.1 M zinc acetate dihydrate (Zn(CH 3 COO) 2 ·2H 2 O) (1.095 g of zinc acetate dihydrate was dissolved in 50 mL of d.H 2 O) was added drop-by-drop to it under stirring at 800 rpm that resulted in cream-colored zinc hydroxide precipitate formation. For the complete reduction in zinc hydroxide, the reaction mixture was left for 30 min. Then the precipitate was centrifuged (Sigma Laborzentrifugen 2k15, Osterode, Germany) at 16,000 rpm for 10 min at 4 °C by dH 2 O followed by ethanol repeatedly in order to remove the impurities. The precipitate was dried overnight in an oven at 100 °C. The obtained dried powder was calcined in a muffle furnace at 600 °C for 2 h and the white powder of ZnO NPs was obtained after calcination as shown in Figure 1 . The resulted powder was used for characterization.

Represent (pictorial) the synthesis of ZnO NPs via P. odoratissimum ALE.

2.8. Characterization Methods of ZnO NPs

2.8.1. uv-vis spectroscopy.

In order to study the optical characteristics of green synthesized ZnO NPs, a known amount of ZnO NPs (0.05 g) was dispersed in 5 mL of ethanol (96%). The absorption spectrum was recorded by using a UV-Vis (U-2900) double beam spectrophotometer (Hitachi, Tokyo, Japan) in between a wavelength scan of 200–800 nm.

2.8.2. Dynamic Light Scattering (DLS)

A particle size analyzer (Zetasizer V 2.2, Worcestershire, Malvern, UK) was utilized to determine the particle size distribution (PSD) of ZnO NPs obtained using ALE. The zeta potential of ZnO NPs was carried out in the water as a dispersant through a Zeta sizer (V 2.3, Worcestershire, Malvern, UK) to identify the stability of the synthesized NPs.

2.8.3. Fourier Transform Infra-Red Spectroscopy (FTIR)

FTIR analysis (Bruker, Berlin, Germany) was employed to identify the functional groups (FGs) involved in biosynthesized ZnO NPs. At a wavelength of 4000–400 cm −1 , the FTIR spectra were scanned with a resolution of 4.0 cm −1 .

2.8.4. X-ray Diffraction (XRD)

The crystalline structure of ZnO NPs was analyzed by an X-ray diffractometer (Bruker D8 DISCOVER, Bruker, Germany) with Cu-Kα radiation (λ = 1.54060 Angstrom). The relative intensity data were collected over a 2θ range of 5°–80°, 2θ values and relative intensities (I/Io) were determined from the chart, and the minerals of core materials were identified with JCPDS carts.

2.8.5. Field Emission-Scanning Electron Microscopy (FE-SEM)

The topography and surface morphology of the biosynthesized ZnO NPs were examined using FE-SEM (Carl- ZEISS Sigma 500 VP, Sigma, Osterode, Germany) equipped with an energy dispersive X-ray spectrometer (EDX, Bruker, Germany) for the element composition present in the powder of ZnO NPs. A portion of the sample was set on a carbon-coated copper (CCC) grid, and the film on the FE-SEM grid was then dried by fixing it under gold for 5 min.

2.8.6. High-Resolution Transmission Electron Microscopy (HRTEM)

The shape and size distribution of powdered ZnO NPs were studied by using HRTEM (JEM-2100, JEOL, Tokyo, Japan) at an accelerated voltage of 200 kV.

2.9. Estimation of Antioxidant Activity—DPPH Radical Scavenging Activity

The ability to scavenge the free radical DPPH of the ALE of P. odoratissimum , biosynthesized ZnO NPs and standard L-ascorbic acid at different concentrations ranging from 3125–100 μg mL −1 were performed using the Brand-Williams et al. method [ 46 ]. Briefly, 2 mL of the DPPH solution (Sigma-Aldrich, 3050 Spruce Street, St. Louis, MO 63103, USA) (25 mg L −1 in methanol) was added to 0.1 mL of different concentrations of each sample and standard L-ascorbic acid (3125–100 μg mL −1 ). After shaking vigorously for 1 min, the reaction mixture was maintained in the dark for 30 min at room temperature (35 ± 2 °C) and the absorbance was recorded at 517 nm using the U-2900 UV-Vis double beam spectrophotometer (Hitachi, Tokyo, Japan). Each measurement was taken in three replications. The free radical scavenging activity (FRSA) of each sample was expressed as percent inhibition of DPPH free radical and was calculated as:

% inhibition (% Anti-radical activity) = [(A control − A sample )/A control ] × 100, where A is the absorbance. The IC 50 values were measured from the relationship curve of FRSA versus concentrations of the respective sample curve.

2.10. Estimation of Antibacterial Activity

2.10.1. bacteria strains.

The antibacterial effect of the biosynthesized ZnO NPs with P. odoratissimum ALE was established against two Gram-positive bacteria (GPB), Bacillus cereus (ATCC13753) and Staphylococcus aureus (ATCC8095), and two Gram-negative bacteria (GNB), Escherichia coli (ATCC25922) and Pseudomonas aeruginosa (ATCC10662). These four strains were acquired from the Microbiol. Dept., Fac. Agric., Fym. Univ., Egypt. The bacterial strains used were maintained in the Luria–Bertani (LB) agar at 30 °C for 24 h and then kept at 4 °C in a refrigerator. During this study, LB media was used for all bacterial cultures.

2.10.2. Antibacterial Assay

The antibacterial effect against the examined bacterial strains was determined using the agar disc diffusion method (ADDM) described by Bauer et al. [ 47 ]. In this method, three different ZnO NPs concentrations (10, 20 and 30 μg mL −1 ) and ALE (20 μg mL −1 ) were dissolved in ethanol and then used to fill sterilized Whatman filter paper discs of approximately 40 μL with the proper volume containing the tested ZnO NPs concentrations and ALE and left to totally dry. A disc containing only solvent was used as a negative control and a disc containing zinc acetate dihydrate was employed. A positive control gentamicin (10 μg mL −1 ) was used. Overnight bacterial cultures were prepared in LB broth for obtaining tested bacterial suspensions for the assay. The discs were then placed on the plates having the tested bacterial cultures and diluted to obtain about 1 × 10 −7 colony-forming unit (CFU). The inoculated plates were incubated at 37 °C for 24 h and then the activity was assayed by measuring the inhibition diameter in millimeters (mm). All tests were performed in triplicate.

2.11. Estimation of Anti-inflammatory Activity

The human red blood cells (HRBCs)-membrane stabilization method (HRBCs-MSM) has been performed for the evaluation of in vitro anti-inflammatory activity according to the procedure outlined by Anosike et al. [ 48 ].

2.12. Statistical Analysis

All of the tests (antioxidant, antibacterial, and anti-inflammatory activity) were performed in triplicates, with the results provided as mean ± SD. Using the statistical software SPSS (SPSS version 21, IBM Corporation, Armonk, NY, USA), the statistical data were examined using the two-way ANOVA technique. The difference in significance was calculated at p < 0.05.

3. Results and Discussion

3.1. qualitative phytochemical screening (qps).

The results of the QPS of P. odoratissimum ALE are summarized in ( Table 1 ), which displays the existence of saponins, phenolics and tannins, flavonoids, carbohydrates and/or glycosides and the absence of steroids, triterpenoids and alkaloids. These present compounds could be responsible for the bio-reduction of the metal salts into nanosize particles [ 49 ].

Qualitative phytochemical screening of P. odoratissimum ALE.

(+): present; (−): absent.

3.2. HPLC-Analysis

HPLC analysis of ALE indicates the presence of sixteen phenolic compounds in appropriate amounts: Gallic acid, Chlorogenic acid, Catechin, Methyl gallate, Caffeic acid, Syringic acid, Rutin, Ellagic acid, Coumaric acid, Ferulic acid, Naringenin, Daidzein, Quercetin, Cinnamic acid, Apigenin and Kaempferol ( Table 2 and Figure 2 , respectively), that may be responsible for the bio-reduction of the metal salts into ZnO-nanosize particles. Additionally, Gallic acid, Syringic acid, Chlorogenic acid, Ferulic acid, Naringenin, Ellagic acid, Rutin and Coumaric acid were found to be highly prevalent among several significant phenolic components identified. Both phenolic acids and flavonoids are known to be potent hydrogen donors [ 50 ], which are responsible for a variety of biological activities because of their functional (carboxyl and hydroxyl) groups. The amounts (µg/g) and structures of polyphenols are illustrated in Table 2 and Figure 3 , respectively.

HPLC chromatogram: ( a ) standard polyphenolic compounds; ( b ) ALE of P. odoratissimum .

Chemical structures of polyphenolic compounds present in P. odoratissimum ALE.

Polyphenolic compounds of P. odoratissimum ALE.

3.3. Characterization of ZnO NPs

3.3.1. visual observation.

The first essential indicator that confirms the biosynthesis of ZnO NPs is visual observation. When the Zn(CH 3 COO) 2 ·2H 2 O, as a precursor for ZnO NPs, was added to the P. odoratissimum leaf extract, the color of the P. odoratissimum leaf extract was changed from light red to cream-colored precipitate ( Figure 4 ). Similar color changes of synthesized ZnO NPs employing Hibiscus subdariffa leaf extract, from light red to cream-colored precipitate, were displayed by Bala et al. [ 16 ], confirming the biosynthesis of ZnO NPs.

The visual observation of colour changes at 0 time ( a ) ( P. odoratissimum ALE) and after 30 min ( b ) ( P. odoratissimum ALE and (Zn(CH 3 COO) 2 ·2H 2 O)).

3.3.2. UV-Vis Spectroscopy

To confirm the synthesis of ZnO NPs, UV/Vis spectrophotometry was performed in order to examine the optical characteristics of green synthesized ZnO NPs using P. odoratissimum ALE. The UV-Vis spectrum recorded the maximum absorbance peak at 370 nm as shown in Figure 5 , which verified the synthesis of ZnO NPs via P. odoratissimum ALE, which is consistent with earlier studies by Senthilkumar et al. [ 51 ], who examined the ability of Tecona grandis (L.) ALE to synthesize ZnO NPs with surface plasmon resonance (SPR) at 370 nm. Additionally, there are no other peaks recorded in the spectrum which means that the biosynthesized ZnO NPs are a pure product. Furthermore, the high absorption band seen at 378 nm might be attributed to ZnO’s inherent band-gap absorption caused by electron transitions from the valence band (E V ) to the conduction band (E C ) (O 2p –Zn 3d ) [ 52 , 53 ]. The formula for calculating the energy bandgap (E G ) of ZnO NPs was used as follows:

UV/Vis spectrum of ZnO NPs biosynthesized using P. odoratissimum ALE.

Where h is Planck’s constant (6.626 × 10 − 34 Js), c is the velocity of light (3 × 10 8 m/s) and λ is the wavelength (378 nm). In total, 3.28 eV was found to be the bandgap energy of ZnO. The significant UV absorption of ZnO NPs demonstrates the product’s suitability for a variety of medicinal applications, including sun-screen protectors and antibacterial ointments [ 54 ].

3.3.3. Dynamic Light Scattering (DLS)

The Z-average diameter (nm) and PSD of the biosynthesized ZnO NPs were measured using the DLS technique. As shown in Figure 6 A, the measurements demonstrated that the average size (nm) of the ZnO NPs with P. odoratissimum ALE was about 76 nm. The result obtained from the PSD profile of the ZnO nanoparticles revealed two notable peaks with intensities of 98.7% and 1.3%. Additionally, the ZnO NPs have a polydispersity index (PDI) of 0.241. This indicated that ZnO nanoparticles are very homogeneous and have a uniform size range [ 55 ]. This finding is completely compatible with Badran, Chen et al. and Putri et al. [ 56 , 57 , 58 ] who reported that PDI values of 0.3 and below are considered to be monodisperse. Because of the hydrodynamical shell, the DLS technique is known to produce significantly higher values than HRTEM size analyses. Additionally, the size of the hydrodynamical shell is influenced by particle structure, particle shape, and roughness [ 59 ].

PSD ( A ) and ZP ( B ) of green synthesized P. odoratissimum -ZnO nanoparticles.

The surface charges and stability of biosynthesized ZnO NPs have been assessed through zeta potential (ZP) analysis. The ZP graph of ZnO nanoparticles is presented in ( Figure 6 B). As shown in Figure 6 B, the ZP was found to be −19.3 mV which indicates the potential stability of the examined NPs [ 51 ]. As a result, the reducing agents (i.e., phenolic and flavonoid components) found in the leaf extract (LE) are probably responsible for the negative charge potential of the produced ZnO NPs. It also confirms that the produced substance contains substantial electrostatic forces [ 60 ].

3.3.4. FTIR Analysis of Biosynthesized ZnO NPs and P. odoratissimum ALE

The FTIR technique was used in order to detect possible FGs present in the ALE of P. odoratissimum that contribute to the reduction in and stabilization of ZnO NPs. Figure 7 a,b represents the FTIR spectra of biosynthesized ZnO nanoparticles and P. odoratissimum leaf extract. The peaks of P. odoratissimum ALE and biosynthesized ZnO nanoparticles are displayed in Table 3 . The broad stretch peak at 3409 cm − 1 and 3417 cm − 1 indicates the presence of an O-H stretch band for the extract and ZnO NPs which are corresponded to the O-H stretching of alcohol, phenolic and flavonoid constituents [ 61 , 62 ]. The low-intensity peaks that arise at 2923 cm − 1 and 2920 cm − 1 were assigned to –CH stretching vibration of the hydroxyl compounds [ 63 , 64 ]. The absorption peaks at 2356 cm − 1 and 2356 cm − 1 were ascribed to O=C=O (stretching vibration) [ 65 ]. The peaks observed at 1616 cm − 1 and 1621 cm − 1 indicate the stretching C=C vibration of the aromatic ring system [ 66 , 67 ]. The absorption peaks at 1400 cm − 1 and 1403 cm − 1 correspond to the C-N stretching vibration of amino acids [ 63 ]. The strong intensity peaks at 1068 cm − 1 and 1072 cm − 1 are due to the C-O stretching bond of the aromatic rings [ 67 ] and may also be related to phenols and flavonoids found in the P. odoratissimum ALE in Table 1 . The bands at 852 cm − 1 and 855 cm − 1 are attributed to –CH stretching vibration of aromatics [ 64 ]. The absorption band observed at 435 cm − 1 confirmed the successful formation of Metal-Oxygen (ZnO). The ZnO absorption peak obtained by FTIR analysis of biosynthesized ZnO NPs has been detected at wavelengths 436 cm − 1 [ 51 ], 442 cm − 1 [ 68 ], 450 cm − 1 [ 69 ] and 485 cm − 1 [ 70 ], in the range 400 to 500 cm − 1 [ 71 ], which are consistent with our findings. The similarity of bands in both P. odoratissimum ALE and P. odoratissimum -synthesized ZnO NPs ( Table 3 ) could be attributable to capped biomolecules on the surface of green synthesized ZnO nanoparticles.

FTIR absorption spectra of ( a ) ZnO NPs and ( b ) ALE of P. odoratissimum .

FTIR spectra of biosynthesized ZnO NPs and P. odoratissimum ALE.

3.3.5. X-ray Diffraction (XRD) Analysis of ZnO NPs

The XRD pattern of biosynthesized ZnO NPs using ALE of P. odoratissimum is illustrated in Figure 8 . The XRD diffraction peaks existed at 2θ angles of 31.85°, 34.55°, 36.35°, 47.69°, 56.75°, 63.09°, 66.56°, 68.17°, 69.29°, 72.87° and 77.21° corresponding to lattice planes (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202), respectively [ 72 ]. These peaks are in accordance with those of (JCPDS card No: 36-1451), which is indicating the confirmation of the hexagonal wurtzite structure of ZnO NPs formation [ 73 ]. The average crystalline size (ACS) of biosynthesized ZnO NPs was calculated using Deby-Scherrer’s formula [ 74 ] and the ACS of the ZnO NPs was estimated to be 14 nm, which is derived from the full width at half maximum (FWHM) of the most intense peak corresponding to (101) plane located at 36.35°. Furthermore, the XRD pattern revealed no additional peaks other than the characteristic ZnO peaks, confirming the purity of the produced ZnO NPs. Additionally, the narrow and strong diffraction peak clearly indicates that the ZnO NPs have an optimal crystalline structure [ 75 , 76 ].

XRD pattern of biosynthesized ZnO NPs via P. odoratissimum L. ALE.

3.3.6. FE-SEM of ZnO NPs

The size and the morphology of the biosynthesized ZnO nanoparticles were imaged via FE-SEM ( Figure 9 ), and the chemical composition of the biosynthesized ZnO nanoparticles was determined using EDX ( Figure 10 ). The FE-SEM image demonstrated that the ZnO NPs were spherical and hexagonal in the morphology shape with good distribution. AN FE-SEM examination showed that the average size of ZnO NPs was 21.6 nm.

FE-SEM image of biosynthesized ZnO NPs.

EDX Spectrum of biosynthesized ZnO NPs.

3.3.7. Energy Dispersive X-ray Analysis (EDX) Spectrum of ZnO NPs

The elemental mapping of the EDX ( Figure 10 ) verified that the examined sample displayed the elemental peaks of zinc and oxygen which are summarized in Table 4 . The EDX analysis proved that the examined sample contained the biosynthesized ZnO NPs.

Elemental constituents of ZnO NPs.

3.3.8. HR-TEM of ZnO NPs

The high-resolution TEM analysis ( Figure 11 a–g) was carried out to confirm the formation of the biosynthesized ZnO NPs. Based on the results obtained, it can be concluded that the pure green ZnO NPs display hexagonal shapes with an average size of 34.12 nm ( Figure 11 i) and also clearly reveal lattice fringes without any distortion, indicating that ZnO NPs have high crystallinity. The selected area electron diffraction (SAED) ( Figure 11 h) pattern revealed a series of rings with bright spots, indicating that ZnO nanoparticles are crystalline in nature [ 74 , 76 ]. Additionally, the hexagonal wurtzite crystalline structure of ZnO NPs is also proven by the diffraction rings on the SAED image and the peaks in the XRD pattern.

( a – g ) HR-TEM images of biosynthesized ZnO NPs, ( h ) SAED pattern and ( i ) histogram of particle size distribution.

3.4. Antioxidant Activity

The antioxidant activity of ZnO NPs, the ALE of P. odoratissimum and L-ascorbic acid are shown in Figure 12 . The results obtained show the DPPH scavenging activity of ZnO NPs, ALE and L-ascorbic acid at six different concentrations (3.125 to 100 μg mL − 1 ) ranging from 10.78 to 76.14%, 23.05 to 89.92% and 14.70 to 83.02% respectively. The DPPH assay showed the scavenging effect of ZnO nanoparticles having an IC 50 value of 28.11 ± 0.01 μg mL − 1 when compared with the IC 50 value of L-ascorbic acid (11.50 ± 0.03 μg mL − 1 ) and aqueous extract (04.56 ± 0.02 μg mL − 1 ). Additionally, the aqueous extract revealed a superior antioxidant potential to traditional reference L-ascorbic acid, which could be due to various bioactive constituents and the higher content of phenolics and flavonoids present in the P. odoratissimum ALE. Moreover, the IC 50 value of P. odoratissimum ALE exhibited higher antioxidants than the aqueous extract of P. graveolens , which had an IC 50 value of 16.59 μg mL − 1 [ 77 ].

DPPH FRSA of ZnO NPs, ALE and L-ascorbic acid at different concentrations.

Generally, phenolic and flavonoid compounds are almost present in all plants in varying proportions and have been reported to act as bio-reductants of metallic ions in an aqueous medium and display a wide range of biological activities such as antioxidant and antimicrobial activity [ 78 ]. Many studies have specified that various OH groups’ presence in phenolic and flavonoids are responsible for the formation and stabilization of metal and metal oxide nanoparticles [ 79 , 80 , 81 ].

As presented in Table 5 , the total phenolic content (TPC) of P. odoratissimum ALE was found to be 21.93 ± 0.01 mg GAE/g of dried leaf extract, while the total flavonoid content (TFC) was recorded to be 17.11 ± 0.001 mg RE/g of dried leaf extract. From the above results, the ALE of P. odoratissimum possesses phytoconstituents that can be used in the formation, capping, stabilization and reduction of zinc acetate salt into ZnO NPs via the green route.

IC 50 , total phenolic (TP), and total flavonoid (TF) contents of P. odoratissimum ALE.

n.d. not determined; values expressed as mean of triplicates ± SD ( p < 0.05). The means of each column with the letters (a–c) differ significantly ( p < 0.05).

3.5. Antibacterial Activity

The antibacterial effect of the biosynthesized ZnO NPs was evaluated by disc diffusion assay against S. aureus (ATCC 8095), B. cereus (ATCC 13753) as GPB, and E. coli (ATCC 25922) and P. aeruginosa (ATCC10662) as GNB. The results are represented in Table 6 and Figure 13 . Generally, the results revealed that the biosynthesized ZnO NPs using P. odoratissimum ALE possessed a significant antibacterial effect against all tested bacterial strains. The significant antibacterial zone of inhibition was recorded in S. aureus (28 ± 0.35 mm) followed by B. cereus (24 ± 0.14 mm), P. aeruginosa (21 ± 0.28 mm) and E. coli (16 ± 0.21 mm). ALE does not observe any zone of inhibition in the tested bacterial strains. Furthermore, compared to gentamycin as a positive control and ALE of P. odoratissimum , biosynthesized ZnO NPs displayed higher antibacterial activity. The antibacterial activities of ZnO NPs differ depending on the cell wall nature of GPB or GNB [ 82 , 83 ]. In the present study, the biosynthesized ZnO NPs showed higher antibacterial activity against GPB ( S. aureus and B. cereus ) compared to GNB ( P. aeruginosa and E. coli ). A similar trend was obtained by Vijayakumar et al. [ 10 ] who stated that ZnO NPs synthesized from Laurus nobilis leaf extract displayed greater antibacterial activity against GPB ( S. aureus ) than GNB ( P. aeruginosa ). This is maybe owing to the structure and the components of GPB (i.e., peptidoglycan layer) and may improve the ZnO NPs’ attachment to the cell wall, while the components of GNP avoid this attachment [ 84 ].

Antibacterial effects (zone of inhibition (mm)) at different concentrations of ZnO NPs (A: 10 μg mL − 1 ; B: 20 μg mL − 1 ; C: 30 μg mL − 1 and D: standard) towards various pathogens.

Evaluation of the antibacterial activity toward pathogenic bacteria.

Values are means ( n = 3). According to LSD (as a post hoc test (PHT) at p ≤ 0.05), the means of ZnO NPs concentrations sharing different capital letters are significantly different. Interactions between each concentration and bacterial strains are indicated with different superscripted small letters and significantly differ according to LSD as a PHT at p ≤ 0.05.

Additionally, the results indicated that the inhibitory effect of biosynthesized ZnO NPs using P. odoratissimum leaf extract increased when the concentration of ZnO NPs was increased. This was in agreement with Gunalan et al. [ 85 ], who reported that increasing the concentration of ZnO NPs in discs and wells consistently increased the growth inhibition due to optimal NPs diffusion in the agar medium.

For the effect of ZnO NPs, there are some proposed bactericidal mechanisms ( Figure 14 ) that have been suggested by scientists. Some suggested that the released Zn from ZnO NPs possess toxic properties that are leading to inhibiting a lot of bacterial cell activities such as bacterial metabolism, and enzyme activity resulting in cell bacterial death [ 86 , 87 ]. The other suggested mechanism is the formation of reactive oxygen species (ROS) that activates oxidative stress which subsequently leads to cell death [ 88 , 89 ]. Another proposed mechanism is the lethal activity of the ZnO NPs due to the attachment of the NPs to the bacterial cell membranes, and the accumulation inside the cytoplasm resulting in damaging the cell membrane integrity and loss of cell contents because of the leakage ending up with cell death [ 90 ].

Various proposed mechanisms of ZnO NPs toxicity against bacteria [ 17 ].

3.6. Anti-inflammatory Activity

During times of inflammation, lysosomes lyse and release their component enzymes, resulting in a variety of disorders. Nonsteroidal anti-inflammatory drugs (NSAIDs) work by either blocking lysosomal enzyme release or stabilizing lysosomal membranes [ 91 ]. When RBCs are exposed to harmful substances such as hypotonic medium, heat, methyl salicylate (MeS) or phenylhydrazine (PhNHNH 2 ), the membranes lyse, resulting in hemolysis and hemoglobin oxidation [ 92 ]. Because the membranes of HRBCs are similar to those of lysosomes [ 91 ], the inhibition of hypotonicity-induced RBCs membrane lysis was used as a measure of the mechanism of the anti-inflammatory effect of ZnO NPs and P. odoratissimum ALE.

From the results obtained in Table 7 , the ZnO NPs and P. odoratissimum ALE have an anti-inflammatory effect that is concentration-dependent, with the percentage of protection increasing as the concentration of the samples increases. At the concentration of 1000 μg mL − 1 , the ZnO NPs significantly ( p ≤ 0.05) produced 95.60% inhibition of RBC hemolysis, and it was comparable to the results achieved with standard indomethacin ( Table 7 ). The hemolytic effect of the hypotonic solution is due to an excessive accumulation of fluid within the cell, which causes the cell membrane to rupture. Damage to the red cell membrane (RCM) increases the cell’s vulnerability to subsequent damage caused by free radical-induced lipid peroxidation [ 93 ]. During a time of increased permeability produced by inflammatory mediators, membrane stability prevents leaking the flow of serum protein and fluids into the tissues [ 94 ]. The ZnO NPs and ALE of P. odoratissimum maybe stabilized the RBC membrane by preventing the release of active mediators of inflammation and lytic enzymes. Furthermore, many studies have revealed that plant flavonoids have anti-inflammatory and antioxidant activity [ 95 , 96 , 97 ]. Their anti-inflammatory properties are thought to be owing to an inhibitory action on enzymes involved in the synthesis of the chemical mediators of inflammation and arachidonic acid metabolism [ 98 , 99 ].

Effect of the biosynthesized ZnO NPs and ALE of P. odoratissimum on hypotonicity-induced hemolysis of HRBCs.

Values are expressed as the mean of triplicates ± SD. Different superscripted small letters significantly differ based on LSD as a post hoc test at p ≤ 0.05.

4. Conclusions

This study presents the biosynthesized ZnO NPs for the first time using an ALE of P. odoratissimum via a simple green route. The biosynthesized ZnO NPs showed a characteristic Uv-Vis absorption peak at 370 nm. The XRD pattern also indicated the hexagonal pure Wurtzite structure. FE-SEM coupled with EDX, HR-TEM, FTIR and DLS, confirmed the formation of NPs with an average size of 34.12 nm as obtained from HR-TEM analysis. The DPPH assay revealed that ZnO NPs possess antioxidant activity with an IC 50 value of 28.11 μg mL − 1 . Furthermore, ZnO NPs showed excellent antibacterial effects against both GNB and GPB. In addition, ZnO NPs were found to be more effective as anti-inflammatory via stabilizing the RBCs’ membrane in in vitro models. Our findings suggest the possibility of using the aqueous leaf extract of P. odoratissimum for synthesizing stable ZnO NPs. The biosynthesized ZnO NPs possess a significant antioxidant, antibacterial against foodborne pathogenic bacteria and anti-inflammatory activities that can be used as a safe and stable alternative to synthetic substances in the fields of pharmaceutical and biomedical research.

Funding Statement

This research received no external funding.

Author Contributions

Conceptualization, A.S.A. and A.M.H.A.M.; methodology, A.S.A. and A.M.H.A.M.; biosynthesis and all characterization, A.S.A. and A.M.H.A.M.; validation, A.S.A. and A.M.H.A.M.; data analyzation, A.S.A. and A.M.H.A.M., software.; A.S.A. and A.M.H.A.M., resources., A.S.A. and A.M.H.A.M., data interpretation., A.S.A. and A.M.H.A.M., writing—original draft preparation., A.S.A. and A.M.H.A.M., writing—review and editing., A.S.A., T.A.A.E.-M., A.O.B., S.S. and A.M.H.A.M., visualization, A.S.A. and A.M.H.A.M. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Data availability statement, conflicts of interest.

The authors declare that they have no conflict of interest.

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Chemical-based synthesis of ZnO nanoparticles and their applications in agriculture

• Original Paper
• Published: 10 February 2022
• Volume 7 , pages 269–275, ( 2022 )

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• S. Srujana   ORCID: orcid.org/0000-0002-5413-8473 1 , 2 &
• Deepa Bhagat 2  

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ZnO nanoparticles (ZnO-NPs) were effectively synthesized using mechanical stirring in this work. In-house produced nanoparticles were tested using XRD, SEM, TEM, FTIR and UV visible spectroscopy to determine their structure and composition. The sol–gel process was used to synthesize zinc oxide nanoparticles. In this method, 15 ml of distilled water was measured using measuring cylinder and transferred into a beaker. UV peaks at 342 nm, and the existence of pure ZnO NPs was verified by XRD patterns matching those of the JCPDS ZnO card and an XRD pattern matching those of the UV–Vis spectrophotometer. Antioxidant functional groups in ZnO nanoparticles were discovered by the use of FTIR synthesis. The SEM results indicated that the NPs were from 39.8 nm, 42.5 nm and 43 nmin size or shape. Another approach for chemically synthesizing ZnO nanoparticles is Sol gel, according to this research.

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Srujana, S., Bhagat, D. Chemical-based synthesis of ZnO nanoparticles and their applications in agriculture. Nanotechnol. Environ. Eng. 7 , 269–275 (2022). https://doi.org/10.1007/s41204-022-00224-6

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DOI : https://doi.org/10.1007/s41204-022-00224-6

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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: 04 February 2023

Efficacy of ZnO nanoparticles in Zn fortification and partitioning of wheat and rice grains under salt stress

• Zuhra Mazhar 1 , 2 ,
• Javaid Akhtar 1 , 2 ,
• Aiyeshah Alhodaib 3 ,
• Tayyaba Naz 1 , 2 ,
• Mazhar Iqbal Zafar 4 ,
• Muhammad Mazhar Iqbal 1 , 5 ,
• Humaria Fatima 6 &
• Iffat Naz 7  

Scientific Reports volume  13 , Article number:  2022 ( 2023 ) Cite this article

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• Environmental sciences
• Materials science
• Nanoscience and technology
• Plant sciences

Zinc (Zn) deficiency is a major health concern in developing countries due to dependency on cereal based diet. Cereals are inherently low in Zn and inevitable use of stressed land has further elevated the problem. The aim of current research was to improve wheat and rice grains grain Zn concentration grown in saline soils through zinc oxide nanoparticles (ZnO-NPs) due to their perspective high availability. The ZnO-NPs were prepared by co-precipitation method and characterized through X-ray diffraction (XRD) and Scanning Electron Microscope (SEM). Two separate pot experiments for wheat and rice were conducted to check the relative effectiveness of ZnO-NPs compared to other bulk Zn sources i.e., zinc sulphate heptahydrate (ZnSO 4 ·7H 2 O) and ZnO. Results showed that salt stress negatively impacted the tested parameters. There was a significant (p ≤ 0.05) improvement in growth, salt tolerance, plant Zn uptake and grain Zn concentrations by Zn application through Zn sources. The ZnO-NPs showed maximum improvement in crops parameters as compared to other sources due to their higher uptake and translocation in plants under both normal and stressed soil conditions. Thus, ZnO nanoparticles proved to be more effective for grain Zn fortification in both tested wheat and rice crops under normal and saline conditions.

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Introduction.

Salinity has been a major concern to global agriculture and has become the most threatening abiotic environmental stress resulting in loss of fertility and crop productivity 1 . Increased concentrations of sodium (Na + ) and chloride (Cl − ) and osmotic stress leads to reduced absorption of essential nutrients, reduced water availability and functional disorders of several physiological processes of plants 2 , 3 . Hence crop yield and quality is greatly compromised. Ionic imbalance and high concentration of Na + in plants results in retarded growth and poor nutrient content of cereals 4 including wheat and rice. Occurrence of few or all of the factors like arid climate, high regional temperature resulting in net uphill water movement and surface salt accumulation, imbalanced and insufficient fertilizer usage, low organic matter (OM), high pH, calcareous soil and high carbonate irrigation water etc. leads to soil salinity and deficiency of most nutrients including Zn 5 .

Zinc has been assessed as the most commonly deficient micronutrient in most calcareous soils after N and P 6 . Commonly grown cereal crops, wheat and rice are most likely to suffer from Zn deficiency in developing countries 7 . The Zn is a vital component of several enzymes and acts as a cofactor of more than 300 enzymes. In plants Zn is needed for translocation, transcription and regulation of most enzymatic activities and is vital for structural stability of several proteins 8 and structural component of ribosomes 7 .

About half of the Asian and African countries population is at risk of Zn deficiency and the rate is increasing at an alarming scale 9 . The Zn deficiency is more common in women and children due to severe malnutrition. Dependence on the cereal-based diet with very low Zn concentration and low bioavailability is the main reason behind this scenario. Increasingly deteriorating soils further worsen the issue and it cannot meet the human need for sufficient Zn uptake. Saline and/or sodic soils have reduced solubility of micronutrients, so plants grown on such soils have to face the deficiency of micronutrients especially Zn 10 . It is reported that Zn has an important role in stress alleviation and helps in reduced plant Na + and higher plant K + accumulation under saline conditions 11 .

Zinc deficiency and salt stress are usually discussed as two separate growth limiting factors while, their interaction effect is not studied in details and not well reported. However, few researchers have documented the effect of salt stress on Zn uptake. Soils with high SAR and pH have very low solubility of micronutrients 12 . High ionic strength of growth medium has a high negative affect on plant Zn uptake.

Wheat and rice both are most important staple food especially in developing countries of South-East Asia and people of these countries rely on these two cereals for major part of their daily calorie intake. Both of these cereals are considered as poor source of Zn in terms of bioavailability and total Zn contents. Most commonly used cereals like wheat and rice were reported to suffer from Zn deficiency in calcareous soils 5 , 6 . The situation becomes worse when there is a problem of salt stress.

In view of current scenario, we need to maintain adequately large soil pool of plant available Zn. For that we strongly need development and application of new fertilizer technologies to provide nutritious crops to this rapidly increasing global population. The present study is focused on the agronomic fortification of wheat and rice with Zn. Agronomic fortification has been proved very effective for cereals especially wheat and rice. Given the fact that a higher concentration of Zn is required to achieve a computable impact on human health and also to avoid any yield loss in plants due to deficiency of Zn. Hence, for crop Zn biofortification, providing sufficient Zn through fertilizers by different means is critically important.

In these regards, use of nanotechnology can be an effective way to deal with the situation. Due to the smaller size and higher surface area 13 , NPs have many potential applications in agriculture including nano-fertilizers 14 . Among engineered nano-materials, zinc oxide nanoparticles (ZnO-NPs) are a commonly used metal oxide nano particles. Nano-ZnO is also one of the Zn compounds listed as “generally recognised as safe” (GRAS) by USFDA (United States Food and Drug Administration) 15 . The ZnO nanoparticles normally appear as white powder. It is sparingly soluble in water. Due to their smaller size and large surface area, ZnO nanoparticles are expected to be the ideal replacement for conventional Zn fertilizers for plants 16 .

Keeping in view these current scenarios and issues, present study was set up to understand reactions of ZnO nanoparticles in soil plant system so that an evaluation can be made for its possible use as a more efficient fertilization option as compared to available bulk resources of Zn. Mainly evaluating the effectiveness of ZnO-NPs in salt stressed conditions due to perspective unavoidable use of stressed lands for cereals growth.

Materials and methods

Synthesis of zno nanoparticles.

The ZnO nanoparticles were prepared by co-precipitation method. A proposed procedure was followed with slight modifications 17 . Briefly, freshly prepared NaOH solution was slowly added to the solution of ZnSO 4 ·7H 2 O in drop wise manner at 2:1 ratio respectively. Resulting milky white mixture was stirred for 12 h on magnetic stirrer. Prepared ZnO precipitates were filtered (Whatman No. 42) and then washed thoroughly with deionized water. Washing and filtration was done at least thrice to completely wash the precipitates. Afterwards, precipitates were dried at 105 °C in a forced air oven. Dried precipitates were ground in a pestle and mortar and calcined at 550 °C for 2 h. Step wise method is presented in Fig.  1 . Balanced reaction equation is as follows:

Flow chart of ZnO nanoparticles synthesis.

X-ray powder diffraction (XRD), Zeta sizer and Scanning electron microscopy (SEM) analysis were used to characterize the prepared nanoparticles 18 .

Characterization of synthesized nanoparticles

X-ray powder diffraction (xrd).

X-ray diffractometer analysis was done to determine the crystalline- phase structure and the size of ZnO-NPs. The ZnO NPs crystal size was calculated by the Debye–Scherrer equation 19 :

where, D = the mean crystalline size, k  = Scherer constant (0.89), λ = X-ray wavelength, β = full width of half peak maximum (FWHM) intensity (in radians) denoted as \(\Delta \left(2\uptheta \right)\) and θ = Bragg’s diffraction angle.

ZnO nanoparticles suspension preparation

For each application of nanoparticles, already weighed amount of required ZnO nanoparticles for wheat and rice (Table 2 ) was suspended directly in deionised water in a flask and then particles were dispersed through ultrasonic vibration in a water bath sonicator for 30 min just before the application of treatment. Each replication and treatment was sonicated separately.

Pot experiments

Wheat cultivar FSD-2008 was obtained from Wheat Research Institute Faisalabad and Rice cultivar IR-6 was used and got from Rice Research Institute Kala Shah Kaku. Both are approved varieties and permission was granted to use them for experimental purposes from respective research stations. Two separate pot experiments for wheat and rice crops were arranged in wire house of Institute of Soil and Environmental Sciences, University of Agriculture Faisalabad.

Growth conditions and crops husbandry practices

Normal surface soil (0–20 cm) was collected from agricultural fields of land Utilization Farm, University of Agriculture Faisalabad (UAF) Pakistan. Pre sowing analyses of soil were done following standard methods (Table 1 ). Pots filling were done at rate of 12 kg soil per pot for wheat and 8 kg per pot for rice. Salinity was developed by mixing calculated amount of NaCl in soil of each pot prior to pot filling. The Zn was applied using three sources (ZnSO 4 ·7H 2 O, ZnO and ZnO nanoparticles) for each crop. Treatment plan for wheat and rice experiment is described in Table 2 .

Seeds were sown directly in case of wheat, while nursery was raised for rice in field and then 30 days old seedlings were transplanted in treated pots. Pots were arranged in completely randomized design and each treatment was replicated thrice. Urea (46% N), di-ammonium phosphate (DAP, 46% P 2 O 5 , 18% N) and potassium sulphate (SOP, 50% K 2 O) were used as fertilizer sources of NPK respectively. Complete dose of P and K was applied at sowing (transplanting in case of rice) while, half of the N was applied at sowing/transplanting and other half was applied at early flowering stage. For wheat, 0.52 g urea, 2.68 g DAP and 0.86 g SOP were used for each 12 kg pot while, for rice, 0.313 g urea, 1.20 g DAP and 0.58 g SOP were used for each 8 kg pot.

Total chlorophyll content (TCC) index value in terms of Special Products Analysis Division (SPAD, a division of Minolta) were determined at flag leaf stage using a hand-held SPAD-502m (Minolta, Osaka, Japan). Young fully expanded leaves were selected from apex to get readings. Three readings were taken from leaf tip to leaf base and the average was taken 20 .

Crops were harvested at maturity and growth parameters like plant height, straw yield, grain yield, No. of tillers, No. of spikes and spikelet were recorded. Grain and straw samples were collected and dried in a fan forced oven at 65 ± 5 °C for 72 h or unless the constant weight is achieved for further chemical analysis.

Ionic parameter measurements

Dried plant samples were ground in a mechanical grinder to powder form and stored in zip locked plastic bags. Finely ground and dried plant samples were digested according to modified wet digestion procedure 21 for high recovery of Zn.

Na + , K + and Zn determination

The Na + and K + were determined via flame photometer (Jenway PFP-7, Loughborough, Leicestershire, UK), whereas Zn was determined via flame atomic absorption spectrophotometer (FAAS; Model Thermo S-Series, Thermo Electron Corporation, Cambridge, UK) following the procedures mentioned in ICARDA (International Centre for Agricultural Research in the Dry Areas) manual by Estefan et al. 22 . For Zn determination of rice, husk from the paddy was removed to get results for rice grain.

Zn uptake and percentage

Zn uptake of root, shoot and grain was calculated by using following formula:

Zn percentage in each plant part was calculated by following formula:

where ZnX is Zn uptake in specific plant part (root, shoot and grain) mg plant −1 and ZnY is sum of Zn uptake in plant root, shoot and grain (mg plant −1 ).

Characterization of ZnO nanoparticles

The ZnO-NPs with an average particle size of about 20–60 nm were calculated using Scherrer equation. Characterization of synthesized particles was done by XRD and SEM. The XRD analysis was done to determine the purity and crystalline size of synthesized ZnO-NPs. Pattern of ZnO-NPs X-ray diffraction is represented in Figs.  2 and 3 . All the peaks represented in diffraction pattern matched well with the crystal planes of the hexagonal wurtzite ZnO structure (JCPDS card No. 36-1451) as the location of diffraction peaks can be seen at diffraction angles (2°Th.) 31.8°, 34.5°, 36.3°, 47.6°, 56.6°, 62.9°, 66.4°, 67.9°, 69.1° and 77° that correspond well to it 23 , 24 .

X-ray diffraction pattern of ZnO-NPs.

All the diffraction peaks showed sharp peak intensities that indicate that prepared material has good crystalline nature and consists of particles in nano range. It also confirmed the purity of synthesized ZnO-NPs as there were no traces of peaks recorded other than ZnO ones. The prepared ZnO-NPs diameter was calculated by Scherrer equation 19 was found to be 22.96 nm where β is the FWHM of diffraction peak corresponding to plane (101) located at 36.3°. The SEM images at 20 kX also showed a size of 51 nm particles with spherical shape.

Growth and yield responses of wheat

Data for growth and yield parameters e.g. Plant height (PH), No. of tillers (T), grain yield (GY) and total chlorophyll contents (TCC) of wheat are listed in Table 3 .

Analysis of variance for data of all these parameters showed a significant difference (p ≤ 0.05) among sources and salt treatments, while interaction effects were not significant for all these parameters. That means although there is a difference in behaviour of all three sources in both growth conditions (normal and saline), the respective behaviour of every source was almost the same with respect to growth conditions (either normal or saline). Plant growth was greatly affected under salt stress but significantly improved through the application of Zn. The response of plants to the Zn application through each source was different. The maximum increase in growth concerning respective control was showed by ZnO-NPs followed by ZnSO 4 ·7H 2 O and least percentage increase was given by ZnO application in bulk form in both saline and normal growth conditions.

Na + and K + contents of wheat

Data presented in Table 4 regarding shoot Na + and K + concentration showed that Na + concentration was highly increased under saline conditions for all applied treatments, whereas K + concentration was decreased. Zinc application through all sources significantly improved the situation. Maximum decrease in shoot Na + (30%) was showed by ZnSO 4 ·7H 2 O under normal conditions. While, under saline conditions maximum decrease (27%) in shoot Na + was recorded by ZnO-NPs. A significant effect (p ≤ 0.05) of different Zn source and salt treatment on Na + and K + concentration was evident in variance analysis.

Interaction effect of salt imposition and Zn application were also significant in all these parameters. Similarly, maximum increase in shoot K + (28 and 22%) was recorded under ZnO-NPs treatment under normal and saline conditions respectively. Minimum increase in shoot K + was recorded where ZnO in bulk form was applied. Normally there was a substantial difference (p ≤ 0.05) between response of ZnO bulk and other two sources. While, difference in responses of ZnSO 4 ·7H 2 O and ZnO-NPs was less evident in few parameters. K + /Na + ratio was also increased in a positive manner due to Zn application in both normal and saline conditions.

Plant Zn 2+ concentration and translocation in wheat

Analysis of Variance for Zn concentration showed that response of wheat to Zn application was highly significant (p ≤ 0.05) and a substantial difference existed among Zn uptake through different sources of applied Zn. In normal soil, the Zn concentration in grain was 25.6 mg kg −1 without Zn application (Table 4 ). The maximum Zn concentration in grain (52.9 mg kg −1 ) was observed in treatment where ZnO-NPs were applied. Under saline conditions, without Zn application, the Zn concentration in grains was 18.5 mg kg −1 , which was increased to 45.7 mg kg −1 with the application of ZnO-NPs. Similar trend was observed in shoot and root Zn concentrations. The interaction of salinity × Zn source was highly significant for Zn uptake per plant in all three components of wheat (shoot, root and grain). As for Zn partitioning in each plant part more Zn was translocated to grain where ZnO-NPs were applied under both normal and saline conditions. The Zn translocation in each plant part of wheat is depicted in Fig.  4 , which represents the percentage of total Zn translocated in each plant part.

Percentage distribution of Zn translocated in different plant parts of wheat at maturity (Each value is a mean, n = 3 statistically significant at p ≤ 0.05, T bars represent ± standard error of means).

Growth, yield and physiological responses of rice

Data for responses of different growth parameters like, shoot length, No. of tillers, paddy yield and total chlorophyll contents (TCC) of rice is presented in Table 5 . Results for growth responses are almost in accordance to the responses we got for wheat plants. Responses of all the parameters were significantly positive towards Zn application in both normal and saline conditions. But the interaction effects were only significant for paddy yield.

In general growth, maximum tillers were produced by ZnSO 4 ·7H 2 O under saline conditions while all other parameters showed the maximum increase under ZnO-NPs (T 8 ) followed by the application of ZnSO 4 ·7H 2 O. While, the minimum percentage increase was observed under application of ZnO bulk application. In normal soil conditions the difference between T 2 and T 4 is generally non-significant.

In case of yield responses maximum paddy yield was recorded in T 4 (26 g pot −1 ) followed by T 2 (23 g pot −1 ) in normal conditions and T 8 (12 g pot −1 ) followed by T 6 (10 g pot −1 ) in saline conditions. Chlorophyll contents in terms of SPAD value were significantly increased when Zn treatments were applied in both normal and saline conditions. But the difference among the sources was not significant.

Na + and K + concentration of rice

In present pot culture study plant Na + concentration, K + concentration, and K + /Na + ratio (Table 6 ) were significantly (p ≤ 0.05) affected by salt imposition. Na + concentration was highly increased under salt stress while K + concentration and K + /Na + ratio was decreased substantially under saline conditions. Zn application improved the negative aspects of salt stress by increasing K + concentration and vice versa for Na + concentration in all plant parts. Variance analysis of data showed that there exists a significant difference (p ≤ 0.05) among sources towards shoot and root K + and Na + concentration in both normal and saline soil conditions. But interaction effects (salinity × source) for both shoot K + and Na + concentrations were not significant. The ZnO-NPs gives better results under both normal and saline soil conditions as compared to other sources (ZnSO 4 ·7H 2 O and ZnO).

Plant Zn concentration and translocation in rice

There was a significant difference (p ≤ 0.05) among different Zn sources in plant Zn concentration. A profound increase in the shoot, grain, and root Zn concentration (Table 6 ) of rice plant with Zn application under both (normal and saline) growth conditions was recorded. Shoot Zn was increased from 22 mg kg −1 in control to 34 mg kg −1 in T 4 under normal growth conditions and from 15 mg kg −1 in saline control to 25 mg kg −1 in T 8 under saline conditions. Maximum grain Zn was recorded where ZnO-NPs were applied while ZnO in bulk form showed a minimum increase from respective control.

Zn uptake per plant also showed the similar trend as Zn concentration. Zn translocation in rice plant in the form of percentage of total plant Zn that is present in each plant part is presented in Fig.  5 . Zn translocation from shoot to grain was decreased under saline conditions. But application of Zn significantly improved this translocation from shoot to grain. Although in case of rice there was not a significant difference between different sources in improving Zn translocation from shoot to grain under saline conditions.

Percentage distribution of Zn translocated in different plant parts of rice at maturity (Each value is a mean, n = 3 statistically significant at p ≤ 0.05, T bars represent ± standard error of means).

Salt stress decreases plant growth and poses physiological depression due to osmotic stress and nutritional imbalance 25 . Zinc is essential for plant growth and grain yield. The Zn is necessary for the fundamental growth processes like cell division and elongation processes 26 . So, adequate Zn supply improved plant height and number of tillers. It also improved chlorophyll concentration in many rice genotypes showing Zn involvement in chlorophyll synthesis 27 , 28 and application of Zn positively improved the growth of wheat and rice under the salt stress.

Plant growth parameters have fairly high response with crop salt resilience and Zn supplementation responsiveness under saline conditions. The Zn supply and adequate Zn availability to plant under salt stress improves grain yield of both wheat and rice 29 , 30 due to improved water relations, maintenance of higher RWC, turgor and photosynthetic pigments 11 , 31 . Zinc application in soil through different sources significantly improved plant growth parameters and plant chlorophyll contents. Soil application of ZnSO 4 ·7H 2 O improved total chlorophyll content of rice that can be due to Zn involvement in chlorophyll synthesis 27 . Photosynthetic apparatus is one of the main action sites of Zn in plants that can be the reason that chlorophyll contents of plant were improved under Zn application 32 , 33 . The ZnO-NPs and ZnSO 4 ·7H 2 O proved to improve growth characteristics of both wheat and rice under normal and saline conditions. That can be because soil diffusion of ZnSO 4 ·7H 2 O is more effective and ZnSO 4 .7H 2 O also cause a bit acidification of the soil zone due to presence of acidic factor SO 4 2− 34 . The ZnSO 4 ·7H 2 O fertilizer has been reported to promote better Zn diffusion than ZnO based fertilizers 35 however, in case of ZnO-NPs they have increased colloidal stability and partitioning in soil pore water especially at soil pH of 8 and there is a higher total Zn concentration in soil solution through ZnO-NPs as compared to ZnSO 4 36 .

Many researchers have reported that plant responds differently to Zn application through different Zn sources, soil Zn status and plant genotype 37 . The ZnO-NPs increased SDW and other growth parameters of cotton plant in saline conditions 38 . Effectiveness of ZnO-NPs over normal size ZnO and ZnSO 4 ·7H 2 O has also been reported in chickpea 39 .

High response of ZnO-NPs with respect to other sources can be attributed to lower activity of SOD and peroxidase and thus lower ROS level and lipid peroxidation in ZnO-NPs treated plants 39 . Chemical composition (Na + , K + and Zn 2+ ) of different plant components of wheat (Table 4 ) and rice (Table 6 ) showed a highly significant effect of salt stress on plants. In term of Na + and K + contents rice seems to be more affected by salt stress. Affinity of rice to uptake Na + (2.93 g kg −1 ) was more as compared to wheat (1.42 g kg −1 ); and percentage decrease in rice growth and yield from respective control was also more as compared to wheat due to salt stress. Rice is reported to be less stress tolerant as compared to wheat 25 . However, the response pattern of both crops to Zn application was almost similar in both normal and saline conditions. There was a notable difference in response pattern of different Zn sources. Response of ZnO-NPs was more pronounced in alleviating salt stress and increasing Zn concentration in root, shoot and grain of both crops (wheat and rice) in comparison to other two sources (ZnSO 4 ·7H 2 O and ZnO bulk).

In present study, Na + concentration of plant roots, grain and shoot was significantly increased up to two times with respect to respective control in both wheat and rice crops under salt stress conditions. Similarly increase in Na + concentration up to two folds and a decrease in K + concentration and K + /Na + ratio under Na 2 SO 4 salt stress was also reported. That can be due to high Na + uptake by plants resulted in lower K + uptake and secondly cytosolic K + efflux 25 , 40 , 41 . High Na + concentration also results in oxidative stress, imbalance in cellular homeostasis, nutrient deficiency, retarded growth and even plant death. Results of current study also showed that with imposition of salinity, Zn concentration in different plant parts was also decreased in both wheat and rice crops which can be due to competition of transport channels to enter into the plant. Under saline conditions, pH of the soil increases due to presence of sodium bicarbonate and availability of Zn decreases 42 . It is also reported that salt stress results in reduced phytosiderophores production and reduced rhizosphere acidification 43 that results in less nutrient availability to plants. High salt contents in growth medium inhibit the ZnO NP uptake by wheat plant 44 .

There was an increase in K + concentration and K + /Na + ratio of shoot root and grain with application of different Zn sources. That can be due to role of Zn in maintaining biomembrane integrity 45 . Preferable binding of Zn to –SH group of membrane protein moiety either direct or close to a –SH group site, reported to protects proteins and phospholipids from disulphide formation and thiol oxidation 46 , 47 . It was reported that with increasing applied Zn in saline and non-saline conditions, shoot and grain Zn concentration is increased 48 , 49 . Zinc application helps in maintaining low shoot Na + and hence cytosolic K + /Na + ratio is increased. Maintaining a higher K + /Na + is a key trait in salt tolerance 50 . Combined effect of salt stress and Zn application showed that salt stress reduces Zn uptake in plants, but progressive application of Zn alleviated the negative impacts of salt stress in wheat 51 . An increased K + /Na + ratio is observed through Zn application in wheat and rice respectively under saline conditions 42 , 43 . The Zn application at any salinity level enhanced Zn concentration in rice shoot. Increase in Zn concentration in plant due to Zn application in soil was the main characteristic that ultimately enhances grain Zn concentration. The Zn fertilizers application has a significant role in enhancing grain Zn concentration of rice 44 .

Zinc concentration in grain can be ranged from 08 to 47 mg kg −1 under different Zn application treatments and soil– Zn status 37 . Normally Zn concentration more than 50 mg kg −1 in cereal grain is considered enviable to get an optimal beneficial impact on human health to combat malnutrition.

In present study, there was a significant difference among Zn sources towards Zn availability to crop plants. The ZnO-NPs improved Zn contents of shoot and grain at higher rate than other sources. That can be due to difference in chemical reactions that each source goes through in different soil conditions. There exists a difference in diffusion rate of different Zn sources in different soil conditions. The ZnSO 4 ·7H 2 O showed more diffusion rate than ZnO due to ionic interactions of Zn 2+ and SO 4 2− 34 . Similar results were reported that zinc sulphate promotes higher Zn diffusion in soil than ZnO based fertilizers 35 . It was also reported that ZnO based fertilizer can be dissolved better at high soil pH and if dispersed well 34 . This can justify the higher response through ZnO-NPs in plant Zn uptake. The ZnO-NPs better responded than ZnSO 4 ·7H 2 O at almost 15-time lower dose in peanut growth 52 . Nano-fertilizers or nano-coated fertilizers have increased utilization of delivered nutrients and more site-specific delivery 53 . It is suggested that due to greater dissolution in the rhizosphere, better Zn contents and uptake is induced by ZnO-NPs coated fertilizers as compared to bulk form coating. Also ZnO NPs coatings pose same Ecotoxicological threat as of bulk form 54 . Many researchers reported better performance of ZnO-NPs in different plants such as in maize grains 55 , groundnut 52 , rice 56 and mung bean 57 .

Nano fertilizers can be a better option in fortifying cereal crop with Zn but in case of soil application there is a need of better understanding of nanoparticles interactions with different soil properties and components. Currently it is critical to develop a thorough understanding of behaviour and fate of ZnO nanoparticles in different soil environments and soil Zn regimes and their possible impact on plant Zn uptake.

It can be summarized that co-precipitation method for ZnO-NPs preparation can be regarded as a better and somewhat economic option for successful nano synthesis. The ZnO-NPs with size range of 22–60 nm can be synthesized through this method and with better set of conditions size can further be improved.

All used Zn sources effectively alleviated the negative effects of salt stress on plant growth, yield and Zn concentration. Maximum improvement were recorded where ZnO-NPs were applied. It can be concluded that nano-fertilizers when used appropriately with improved set of soil and plant conditions can be a better option in fortifying cereal crop with Zn. A thorough understanding of Zn nanoparticles and soil interactions and their retention and availability need further detailed research under field conditions.

Data availability

All obtained data is enclosed with this manuscript.

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Acknowledgements

The researchers would like to thank the Deanship of Scientific Research, Qassim University for funding the publication of this project. The researchers are also grateful to SARC-ISES, University of Agriculture Faisalabad and HEC-Pakistan for assistance in the provision of chemicals and glassware for conducting the present research work.

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Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, 38040, Pakistan

Zuhra Mazhar, Javaid Akhtar, Tayyaba Naz & Muhammad Mazhar Iqbal

Saline Agriculture Research Centre, University of Agriculture, Faisalabad, 38040, Pakistan

Zuhra Mazhar, Javaid Akhtar & Tayyaba Naz

Department of Physics, College of Science, Qassim University, Buraydah, 51452, Saudi Arabia

Aiyeshah Alhodaib

Department of Environmental Sciences, Faculty of Biological Sciences, Quaid-i-Azam University, Islamabad, 45320, Pakistan

Mazhar Iqbal Zafar

Soil and Water Testing Laboratory, Department of Agriculture, Ayub Agricultural Research Institute, Government of Punjab, Chiniot, 35400, Pakistan

Muhammad Mazhar Iqbal

Department of Pharmacy, Faculty of Biological Sciences, Quaid-i-Azam University, Islamabad, 45320, Pakistan

Humaria Fatima

Department of Biology, Science Unit, Deanship of Educational Services, Qassim University, Buraydah, 51425, Saudi Arabia

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Conceptualization: J.A., T.N., Z.M.; Data curation: Z.M., T.N.; Formal analysis: Z.M.; Investigation: Z.M.; Methodology: Z.M., T.N.; resources: J.A., T.N.; Writing—original draft: M.M.I., Z.M.; Writing—review and editing: M.M.I., M.I.Z. T.N.; funding acquisition, A.A., I.N., H.F. All authors have read and agreed to the published version of the manuscript.

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Correspondence to Aiyeshah Alhodaib or Muhammad Mazhar Iqbal .

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Mazhar, Z., Akhtar, J., Alhodaib, A. et al. Efficacy of ZnO nanoparticles in Zn fortification and partitioning of wheat and rice grains under salt stress. Sci Rep 13 , 2022 (2023). https://doi.org/10.1038/s41598-022-26039-8

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Received : 12 February 2022

Accepted : 08 December 2022

Published : 04 February 2023

DOI : https://doi.org/10.1038/s41598-022-26039-8

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