Effect of Optimized Precursor Concentration, Temperature, and Doping on Optical Properties of ZnO Nanoparticles Synthesized via a Green Route Using Bush Tea (Athrixia phylicoides DC.) Leaf Extracts

Enhanced third-order nonlinear optical properties of ZnO@C-N composite microspheres

We report the third-order nonlinear optical (NLO) properties of ZnO@C-N composite microspheres and pure ZnO which have been investigated with the Z-scan technique under continuous wave laser. ZnO@C-N composite microspheres have been hydrothermally synthesized at two different precursor concentrations to have structures at different impurity levels. Moreover, pure ZnO is prepared under the annealing process. The nonlinear optical absorption of samples was measured by using the open-aperture Z-scan technique and was evaluated relating to the two-photon absorption (TPA) mechanism. Moreover, both ZnO@C-N and ZnO microstructures exhibited a negative nonlinear refractive index (NLR) referring to the self-defocusing effect. The order of the (NLR) value, is about 10-10(cm2/W) and, the NLA coefficients of specimens are in the order of 10-5(cm/W). The NLA coefficient has a similar behavior as the NLR versus increasing incident intensity of the laser. The results show that the nonlinearity response of ZnO@C-N composites is higher than the pure ZnO and ZnO@C-N at higher precursor concentrations exhibits the maximum amount of NLA and NLR coefficients compared to other samples. This observation which is attributed to the change in optical and structural properties of material due to impurity presence, underscores the presence of impurity for engineering materials to improve the nonlinearity properties.

Biosynthesis of ZnO nanoparticles using aqueous extracts of Eclipta prostrata and Piper longum: characterization and assessment of their antioxidant, antibacterial, and photocatalytic properties

Chemical syntheses of nanoparticles that release toxic substances into the environment rarely meet the strict requirements of green chemistry principles. Instead, green synthesis of nanoparticles using plant extracts brings a safe, rapid, and effective process, contributing to solving environmental pollution problems. Here, we report the green synthesis of multifunctional ZnO nanoparticles (ZnONPs) using aqueous extracts of E. prostrata leaves and P. longum fruits. The XRD results affirmed the existence of hexagonal crystalline ZnONPs with particle sizes of 17-30 nm. The optical analysis revealed bandgap energies of 3.10 eV and 3.16 eV for ZnONPs biosynthesized using E. prostrata and P. longum extracts, respectively. The synthesized ZnONPs showed potential antioxidant activity through DPPH and ABTS methods. Among the antibacterial outcomes against pathogenic bacterial strains (S. aureus, B. cereus, E. coli, and S. typhimurium), ZnONPs exhibited the highest zone of inhibition (18.5 mm) for S. aureus. Moreover, both ZnONPs biosynthesized using E. prostrata and P. longum extracts served as strong photocatalysts in the degradation of crystal violet with degradation efficiencies of 95.64% and 99.90%, respectively. Therefore, biosynthesized ZnONPs hold significant promise as antioxidants, antibacterial agents, and photocatalysts.

Enhanced healing of burn wounds by multifunctional alginate-chitosan hydrogel enclosing silymarin and zinc ‎oxide ‎nanoparticles

Multifunctional wound dressings have been applied for burn injuries to avoid complications and promote tissue regeneration. In the present study, we fabricated a natural alginate-chitosan hydrogel comprising silymarin and green-synthesized zinc ‎oxide ‎nanoparticles (ZnO NPs). Then, the physicochemical attributes of ZnO NPs and loaded hydrogels were analyzed. Afterward, wound healing efficacy was evaluated in a rat model of full-thickness dermal burn wounds. The findings indicated that ZnO NPs were synthesized via reduction with phytochemicals from Elettaria cardamomum seeds extract. The microscopic images exhibited fairly spherical ZnO NPs (35-45 nm), and elemental analysis verified the relevant composition. The hydrogel, containing silymarin and biosynthesized ZnO NPs, displayed a uniform appearance, smooth surfaces, and a porous structure. Moreover, infrared spectroscopy ‎identified functional groups, confirming the successful loading without adverse interactions. The obtained hydrogel exhibited great water absorption, high porosity, sustainable degradation for several days, and enhanced antioxidant capability of the combined loaded component. In vivo studies revealed faster and superior wound healing, achieving nearly complete closure by day 21. Histopathology confirmed improved cell growth, tissue regeneration, collagen deposition, and neovascularization. It is believed that this multifunctional hydrogel-based wound dressing can be applied for effective burn wound treatment.

Ca-doped ZnO nanoparticles for MB dye degradation and adsorptive removal of tinidazole

Industrial dye degradation involves several processes by which dyes are broken down, ideally into innocuous products. Methylene blue (MB) is one of the most commonly employed dyes in the textile industry and is released into water in routine industry processes. These discharges lead to creating a nocuous nature for humans and animals. Drugs are also discharged into water bodies from various pharmaceutical industries. In these two contexts, in the present work, the green synthesis of calcium-doped zinc oxide nanoparticles (Ca-doped ZnO NPs) is achieved using the aqueous peel extract of Citrus limetta by the solution combustion technique. The structural, morphological, and optical properties of the synthesized Ca-doped ZnO NPs are investigated using XRD, FTIR, SEM, EDX, and UV-visible spectroscopy. The prepared NPs were subjected to photocatalytic degradation of MB dye under visible-light illumination, which shows ~ 95% dye degradation. The synthesized Ca-doped ZnO NPs were also employed to adsorb tinidazole (TDZ), a nitroimidazole antibiotic, from water samples. An excellent adsorptive capacity of the NPs was observed for selectively adsorbing the TDZ ~ 96.2%. The drug TDZ was found to have pseudo-second-order kinetics. The catalyst recycling proved its repeatability; removal of the dye reached up to 92% after three successive usages. Therefore, using waste Citrus limetta peel extract, the multifunctional Ca-doped ZnO NPs were synthesized, which maintained effective adsorption potential and photocatalytic abilities and could be used as an effective material for environmental remediation.

Retention of ZnO nanoparticles onto polypropylene and polystyrene microplastics: Aging-associated interactions and the role of aqueous chemistry

Microplastics (MPs) are pervasive and undergo environmental aging processes, which alters potential interaction with the co-contaminants. Hence, to assess their contaminant-carrying capacity, mimicking the weathering characteristics of secondary MPs is crucial. To this end, the present study investigated the interaction of Zinc oxide (nZnO) nanoparticles with non-irradiated (NI) and UV-irradiated (UI) forms of the most abundant MPs, such as polypropylene (PP) and polystyrene (PS), in aqueous environments. SEM images revealed mechanical abrasions on the surfaces of NI-MPs and their subsequent photoaging caused the formation of close-ended and open-ended cracks in UI-PP and UI-PS, respectively. Batch-sorption experiments elucidated nZnO uptake kinetics by PP and PS MPs, suggesting a sorption-desorption pathway due to weaker and stronger sorption sites until equilibrium was achieved. UI-PP showed higher nZnO (∼3000 mg/kg) uptake compared to NI-PP, while UI-PS showed similar or slightly decreased nZnO (∼2000 mg/kg) uptake compared to NI-PS. FTIR spectra and zeta potential measurements revealed electrostatic interaction as the dominant interaction mechanism. Higher nZnO uptake by MPs was noted between pH 6.5 and 8.5, whereas it decreased beyond this range. Despite DOM, MPs always retained ∼874 mg/kg nZnO irrespective of MPs type and extent of aging. The experimental results in river water showed higher nZnO uptake on MPs compared to DI water, attributed to mutual effect of ionic competition, DOM, and MP hydrophobicity. In the case of humic acids, complex synthetic and natural water matrices, NI-MPs retained more nZnO than UI-MPs, suggesting that photoaged MPs sorb less nZnO under environmental conditions than non-photoaged MPs. These findings enhance our understanding on interaction of the MPs with co-contaminants in natural environments.

Comparative Effect of Antioxidant and Antibacterial Potential of Zinc Oxide Nanoparticles from Aqueous Extract of Nepeta nepetella through Different Precursor Concentrations

Antibiotic resistance is a global health crisis caused by the overuse and misuse of antibiotics. Accordingly, bacteria have developed mechanisms to resist antibiotics. This crisis endangers public health systems and medical procedures, underscoring the urgent need for novel antimicrobial agents. This study focuses on the green synthesis of ZnO nanoparticles (NPs) using aqueous extracts from Nepeta nepetella subps. amethystine leaves and stems, employing different zinc sulfate concentrations (0.5, 1, and 2 M). NP characterization included transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD), along with Fourier transform infrared spectroscopy (FTIR) analysis. This study aimed to assess the efficacy of ZnO NPs, prepared at varying concentrations of zinc sulfate, for their capacity to inhibit both Gram-positive and Gram-negative bacteria, as well as their antioxidant potential using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) method. SEM and TEM results showed predominantly spherical NPs. The smallest size (18.5 ± 1.3 nm for leaves and 18.1 ± 1.3 nm for stems) occurred with the 0.5 M precursor concentration. These NPs also exhibited remarkable antibacterial activity against both Gram-positive and Gram-negative bacteria at 10 µg/mL, as well as the highest antioxidant activity, with an IC50 (the concentration of NPs that scavenge 50% of the initial DPPH radicals) of 62 ± 0.8 (µg/mL) for the leaves and 35 ± 0.6 (µg/mL) for the stems. NPs and precursor concentrations were modeled to assess their impact on bacteria using a 2D polynomial equation. Response surface plots identified optimal concentration conditions for antibacterial effectiveness against each species, promising in combating antibiotic resistance.

Investigating the effect of acidic and basic precipitation on the antibacterial activity of ZnO nanoparticles against Gram-negative and Gram-positive bacteria

In the present work, acidic (direct) and basic precipitation (indirect) methods were used to demonstrate the influence of the mode of precipitation on the structural properties of ZnO nanoparticles (NPs). Four samples of ZnO nanoparticles were prepared, two samples via each mode of precipitation. DZOa and IZOa were the aged samples prepared via acidic and basic precipitation methods, and DZOwa and IZOwa were processed without aging. Both precipitation processes were carried out without using any surfactant reagents. Zinc hydroxide precipitate, which was formed during the basic precipitation method, could be critical in deciding the properties of ZnO NPs, unlike zinc hydroxide formed during acidic precipitation. Aging of zinc hydroxide, synthesised by basic precipitation method for 48 hours was found to be an added advantage in controlling the properties of ZnO NPs. The influence of the mode of precipitation on the structural properties and antibacterial activity of ZnO NPs against Gram-positive and Gram-negative bacterial strains was tested. The antibacterial activity of all four ZnO NPs was analysed via zone of inhibition measurements at a concentration dose of 200 μg ml-1. IZOa nanoparticles prepared using the basic precipitation method showed a higher antibacterial activity against three Gram-negative and one Gram-positive strains, namely, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, and Escherichia coli. DZOa nanoparticles synthesized through acidic precipitation showed relatively high antibacterial activity against Salmonella typhimurium, a Gram-negative strain. ZnO NPs prepared without aging, IZOwa and DZOwa, showed a higher antibacterial activity against E. coli and Bacillus sp. strains, respectively. All ZnO NPs were characterized via UV-visible, FTIR, XRD, and HRSEM techniques.

Ce-doped ZnO nanostructures: A promising platform for NO2 gas sensing

In this comprehensive study, Ce-doped ZnO nanostructures were hydrothermally synthesized with varying Ce concentrations (0.5%, 1.0%, 1.5%, and 2.0%) to explore their gas-sensing capabilities, particularly towards NO2. Structural characterization revealed that as Ce doping increased, crystal size exhibited a slight increment while band gap energies decreased. Notably, the 0.5% Ce-doped ZnO nanostructure demonstrated the highest NO2 gas response of 8.6, underscoring the significance of a delicate balance between crystal size and band gap energy for optimal sensing performance. The selectivity of the 0.5% Ce-doped ZnO nanostructures to NO2 over other gases like H2, acetone, NH3, and CO at a concentration of 100 ppm and an optimized temperature of 250 °C was exceptional, highlighting its discriminatory prowess even in the presence of potential interfering gases. Furthermore, the sensor displayed reliability and reversibility during five consecutive tests, showcasing consistent performance. Long-term stability testing over 30 days revealed that the gas response remained almost constant, indicating the sensor's remarkable durability. In addition to its robustness against humidity variations, maintaining effectiveness even at 41% humidity, the sensor exhibited impressive response and recovery times. While the response time was swift at 11.8 s, the recovery time was slightly prolonged at 56.3 s due to the strong adsorption of NO2 molecules onto the sensing material hindering the desorption process. The study revealed the intricate connection between Ce-doping levels, structure, and gas-sensing. It highlighted the 0.5% Ce-doped ZnO nanostructure as a highly selective, reliable, and durable NO2 gas sensor, with implications for future environmental monitoring and safety.

Photocatalytic and Antimicrobial Properties of ZnO and Mg-Doped ZnO Nanoparticles Synthesized Using Lupinus albus Leaf Extract

Dye effluents discharged from various industries contribute to environmental contamination, making their treatment highly necessary. Infectious diseases also pose a threat to public health worldwide. Nanomaterials have promising features and are potential candidates for overcoming the problems of drug resistance in microbes and environmental pollution. Therefore, this study aimed to synthesize zinc oxide (ZnO) and magnesium-doped zinc oxide (Mg-doped ZnO) nanoparticles (NPs) using the plant extract of Lupinus albus for applications in photocatalysis and antimicrobial activity. A sample of Lupinus albus leaves was collected from Motta, in the eastern Gojjam zone of Ethiopia. The leaves were air-dried and then ground into a powder. The powdered plant material was extracted using distilled water. The ZnO and Mg-doped ZnO NPs were synthesized using 0.1 M Zn(NO3)2·6H2O, 7.5% 0.1 M Mg(NO3)2.6H2O, and 10 mL of the leaf extract. The nanoparticles (NPs) were characterized using UV-vis, FT-IR, XRD, and SEM. The average crystallite sizes of ZnO and Mg-doped ZnO NPs were determined using the Debye-Scherrer formula and were found to be 28.1 and 34.4 nm, respectively. The antimicrobial activity of the synthesized NPs was evaluated against four bacterial strains (Escherichia coli, Bacillus cereus, Salmonella typhi, and Staphylococcus aureus) and one fungal strain (Candida albicans) by using the agar disk diffusion method. The Mg-doped ZnO NPs exhibited significant antimicrobial activity, with a maximum zone of inhibition measuring 24 and 22 mm against Escherichia coli and Salmonella typhi, respectively. The photocatalytic activity of ZnO and Mg-doped ZnO NPs was investigated by studying the degradation of methylene blue (MB) dye under sunlight irradiation for 120 min. The results showed that Mg-doped ZnO NPs exhibited higher photocatalytic activity (99.6%) than ZnO NPs (94.1%). In conclusion, the synthesized NPs could serve as viable alternatives for antimicrobial drugs and photocatalysts to mitigate the pollution of the environment caused by organic dyes.

Construction of a ZnO Heterogeneous Structure Using Co3O4 as a Co-Catalyst to Enhance Photoelectrochemical Performance

Recently, heterostructured photocatalysts have gained significant attention in the field of photocatalysis due to their superior properties compared to single photocatalysts. One of the key advantages of heterostructured photocatalysts is their ability to enhance charge separation and broaden the absorption spectrum, thereby improving photocatalytic efficiency. Zinc oxide is a widely used n-type semiconductor with a proper photoelectrochemical activity. In this study, zinc oxide nanorod arrays were synthesized, and then the surfaces of ZnO nanorods were modified with the p-type semiconductor Co3O4 to create a p-n junction heterostructure. A significant increase in the photocurrent for the ZnO/Co3O4 composite, of 4.3 times, was found compared to pure ZnO. The dependence of the photocurrent on the morphology of the ZnO/Co3O4 composite allows for optimization of the morphology of the ZnO nanorod array to achieve improved photoelectrochemical performance. The results showed that the ZnO/Co3O4 heterostructure exhibited a photocurrent density of 3.46 mA/cm2, while bare ZnO demonstrated a photocurrent density of 0.8 mA/cm2 at 1.23 V. The results of this study provide a better understanding of the mechanism of charge separation and transfer in the heterostructural ZnO/Co3O4 photocatalytic system. Furthermore, the results will be useful for the design and optimization of photocatalytic systems for water splitting and other applications.

Antibacterial Activity of Zinc Oxide Nanoparticles Loaded with Essential Oils

One major problem with the overuse of antibiotics is that the microorganisms acquire resistance; thus the dose must be increased unsustainably. To overcome this problem, researchers from around the world are actively investigating new types of antimicrobials. Zinc oxide (ZnO) nanoparticles (NPs) have been proven to exhibit strong antimicrobial effects; moreover, the Food and Drugs Administration (FDA) considers ZnO as GRAS (generally recognized as safe). Many essential oils have antimicrobial activity and their components do not generate resistance over time. One of the drawbacks is the high volatility of some components, which diminishes the antimicrobial action as they are eliminated. The combination of ZnO NPs and essential oils can synergistically produce a stronger antimicrobial effect, and some of the volatile compounds can be retained on the nanoparticles' surface, ensuring a better-lasting antimicrobial effect. The samples were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis), and thermal analysis (TG-DSC) coupled with analysis of evolved gases using FTIR. The ZnO NPs, with a size of ~35 nm, exhibited a loading between 1.44% and 15.62%-the lower values were specific for limonene-containing oils (e.g., orange, grapefruit, bergamot, or limette), while high values were obtained from cinnamon, minzol, thyme, citronella, and lavender oils-highlighting differences among non-polar terpenes and alcohol or aldehyde derivatives. The antibacterial assay indicated the existence of a synergic action among components and a high dependency on the percentage of loaded oil. Loaded nanoparticles offer immense potential for the development of materials with specific applications, such as wound dressings or food packaging. These nanoparticles can be utilized in scenarios where burst delivery is desired or when prolonged antibacterial activity is sought.

Biosynthesis of magnesium oxide and calcium carbonate nanoparticles using Moringa oleifera extract and their effectiveness on the growth, yield and photosynthetic performance of groundnut (Arachis hypogaea L.) genotypes

Small-scale crop production has been significantly impacted by the heavy price, limited supply, and frequent shortage of inorganic fertilisers, which is partly attributable to the Covid-19 pandemic outbreak and led to rising oil and food prices. Thus, integrating environmentally friendly agricultural practices that can improve crop productivity and advance the sustainability of agricultural cropping systems is critical. This study synthesized and characterised MgO and CaCO3Moringa oleifera nanoparticles and assessed their effects on groundnut genotypes. The effect of biosynthesized MgO and CaCO3 nanoparticles using Moringa oleifera extract on the growth and yield of groundnut genotypes exposed to different concentrations of 50, 100 and 200 mg/L was examined. The experiment was carried laid out in a 3 × 8 factorial completely randomized design (CRD) with eight replicates per treatment. Each plant was sprayed with 5 ml of the solution crystalline size of the MgO and CaCO3 nanoparticles 2.48 nm and 10.30 nm, respectively. Foliar application of nanoparticle treatments was applied weekly except for the negative control. The collected data were subjected to a two-way analysis of variance (ANOVA). Mean separations were done using Tukey's Honest Significant Difference (HSD) at P < 0.05. The findings demonstrated that foliar application of MgO and CaCO3 nanoparticles positively affected groundnut biomass production. The results further revealed that the concentration of 50 mg/L of MgO and 100 mg/L of CaCO3 considerably improved groundnut plant growth, yield, and nodulation in comparison with other treatments. There is a great deal of evidence signifying that foliar applications of 50 mg/L of MgO 100 mg/L CaCO3 contributed greatly to plant growth and crop production. Therefore, 50 mg/L of MgO and 100 mg/L CaCO3 nanoparticles foliar application could be recommended as nano-fertilisers application rate for groundnut production.

Impact of Diverse Parameters on the Physicochemical Characteristics of Green-Synthesized Zinc Oxide-Copper Oxide Nanocomposites Derived from an Aqueous Extract of Garcinia mangostana L. Leaf

Compared to conventional metal oxide nanoparticles, metal oxide nanocomposites have demonstrated significantly enhanced efficiency in various applications. In this study, we aimed to synthesize zinc oxide-copper oxide nanocomposites (ZnO-CuO NCs) using a green synthesis approach. The synthesis involved mixing 4 g of Zn(NO3)2·6H2O with different concentrations of mangosteen (G. mangostana) leaf extract (0.02, 0.03, 0.04 and 0.05 g/mL) and 2 or 4 g of Cu(NO3)2·3H2O, followed by calcination at temperatures of 300, 400 and 500 °C. The synthesized ZnO-CuO NCs were characterized using various techniques, including a UV-Visible spectrometer (UV-Vis), photoluminescence (PL) spectroscopy, Fourier Transform Infrared (FTIR) spectroscopy, X-ray powder diffraction (XRD) analysis and Field Emission Scanning Electron Microscope (FE-SEM) with an Energy Dispersive X-ray (EDX) analyzer. Based on the results of this study, the optical, structural and morphological properties of ZnO-CuO NCs were found to be influenced by the concentration of the mangosteen leaf extract, the calcination temperature and the amount of Cu(NO3)2·3H2O used. Among the tested conditions, ZnO-CuO NCs derived from 0.05 g/mL of mangosteen leaf extract, 4 g of Zn(NO3)2·6H2O and 2 g of Cu(NO3)2·3H2O, calcinated at 500 °C exhibited the following characteristics: the lowest energy bandgap (2.57 eV), well-defined Zn-O and Cu-O bands, the smallest particle size of 39.10 nm with highest surface area-to-volume ratio and crystalline size of 18.17 nm. In conclusion, we successfully synthesized ZnO-CuO NCs using a green synthesis approach with mangosteen leaf extract. The properties of the nanocomposites were significantly influenced by the concentration of the plant extract, the calcination temperature and the amount of precursor used. These findings provide valuable insights for researchers seeking innovative methods for the production and utilization of nanocomposite materials.

Biodiesel Production from Waste Cooking Oil via β-Zeolite-Supported Sulfated Metal Oxide Catalyst Systems

Waste cooking oil (WCO) is a readily available and cheap feedstock for biodiesel production. However, WCO contains high levels of free fatty acids (FFAs), which negatively impact the biodiesel yield if homogeneous catalysts are used. Heterogeneous solid acid catalysts are preferred for low-cost feedstocks because the catalysts are highly insensitive to high levels of FFA in the feedstock. Therefore, in the present study, we synthesized and evaluated different solid catalysts, pure β-zeolite, ZnO-β-zeolite, and SO₄^2-/ZnO-β-zeolite for the production of biodiesel using WCO as feedstock. The synthesized catalysts were characterized by Fourier transform infrared spectroscopy (FTIR), pyridine-FTIR, N₂ adsorption-desorption, X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy, while the biodiesel product was analyzed using nuclear magnetic resonance (¹H and ¹³C NMR) and gas chromatography-mass spectroscopy. The results revealed that the SO₄^2-/ZnO-β-zeolite catalyst showed excellent catalytic performance for simultaneous transesterification and esterification of WCO, with a higher percentage conversion than the ZnO-β-zeolite and pure β-zeolite catalyst, due to the large pore size and high acidity. The SO₄^2-/ZnO-β-zeolite catalyst exhibits 6.5 nm pore size, a total pore volume of 0.17 cm³/g, and high surface area of 250.26 m²/g. Experimental parameters such as catalyst loading, methanol:oil molar ratio, temperature, and reaction time were varied in order to establish the optimal parameters. The highest WCO conversion of 96.9% was obtained using the SO₄^2-/ZnO-β-zeolite catalyst under an optimum reaction condition of 3.0 wt % catalyst loading, 200 °C reaction temperature, and 15:1 molar ratio of methanol to oil in 8 h reaction time. The WCO-derived biodiesel properties conform to the ASTM6751 standard specification. Our investigation of its kinetics revealed that the reaction follows a pseudo first-order kinetic model, with an activation energy (E_a) of 38.58 kJ/mol. Moreover, the stability and reusability of the catalysts were evaluated, and it was found that the SO₄^2-/ZnO-β-zeolite catalyst exhibited good stability, giving a biodiesel conversion of over 80% after three synthesis cycles.

An Evaluation of the Biocatalyst for the Synthesis and Application of Zinc Oxide Nanoparticles for Water Remediation—A Review

Global water scarcity is threatening the lives of humans, and it is exacerbated by the contamination of water, which occurs because of increased industrialization and soaring population density. The available conventional physical and chemical water treatment techniques are hazardous to living organisms and are not environmentally friendly, as toxic chemical elements are used during these processes. Nanotechnology has presented a possible way in which to solve these issues by using unique materials with desirable properties. Zinc oxide nanoparticles (ZnO NPs) can be used effectively and efficiently for water treatment, along with other nanotechnologies. Owing to rising concerns regarding the environmental unfriendliness and toxicity of nanomaterials, ZnO NPs have recently been synthesized through biologically available and replenishable sources using a green chemistry or green synthesis protocol. The green-synthesized ZnO NPs are less toxic, more eco-friendly, and more biocompatible than other chemically and physically synthesized materials. In this article, the biogenic synthesis and characterization techniques of ZnO NPs using plants, bacteria, fungi, algae, and biological derivatives are reviewed and discussed. The applications of the biologically prepared ZnO NPs, when used for water treatment, are outlined. Additionally, their mechanisms of action, such as the photocatalytic degradation of dyes, the production of reactive oxygen species (ROS), the generation of compounds such as hydrogen peroxide and superoxide, Zn2+ release to degrade microbes, as well as their adsorbent properties with regard to heavy metals and other contaminants in water bodies, are explained. Furthermore, challenges facing the green synthesis of these nanomaterials are outlined. Future research should focus on how nanomaterials should reach the commercialization stage, and suggestions as to how this ought to be achieved are presented.

A Review of the Green Synthesis of ZnO Nanoparticles Utilising Southern African Indigenous Medicinal Plants

Metal oxide nanoparticles (NPs), such as zinc oxide (ZnO), have been researched extensively for applications in biotechnology, photovoltaics, photocatalysis, sensors, cosmetics, and pharmaceuticals due to their unique properties at the nanoscale. ZnO NPs have been fabricated using conventional physical and chemical processes, but these techniques are limited due to the use of hazardous chemicals that are bad for the environment and high energy consumption. Plant-mediated synthesis of ZnO NPs has piqued the interest of researchers owing to secondary metabolites found in plants that can reduce Zn precursors and stabilise ZnO NPs. Thus, plant-mediated synthesis of ZnO NPs has become one of the alternative green synthesis routes for the fabrication of ZnO NPs. This is attributable to its environmental friendliness, simplicity, and the potential for industrial-scale expansion. Southern Africa is home to a large and diverse indigenous medicinal plant population. However, the use of these indigenous medicinal plants for the preparation of ZnO NPs is understudied. This review looks at the indigenous medicinal plants of southern Africa that have been used to synthesise ZnO NPs for a variety of applications. In conclusion, there is a need for more exploration of southern African indigenous plants for green synthesis of ZnO NPs.