Water contamination by industrial waste containing toxic chemicals, particularly dyes, poses a significant threat to aquatic life. The development of efficient catalysts with enhanced photocatalytic and antibacterial properties is crucial for wastewater treatment, especially considering the increasing prevalence of multidrug-resistant pathogens. Metal oxide-based nanocomposites are promising candidates due to their unique physicochemical properties, such as shape, particle size, crystallinity, and porosity. Zinc oxide (ZnO), a widely used metal oxide, exhibits applications in various fields, including photocatalysis, antibacterial agents, cosmeceuticals, gas sensing, and drug delivery. ZnO nanoparticles (NPs) demonstrate antibacterial action against numerous pathogens. However, the release of industrial effluents, such as organic dyes, remains a serious environmental concern. Dyes are considered major organic pollutants, negatively impacting human health and the environment. Nanomaterials are being explored for efficient degradation of these hazardous dyes. Doped ZnO nanomaterials generally exhibit higher photocatalytic activity than pure ZnO NPs due to reduced electron-hole recombination. Hybrid nanomaterials, composed of two or more functional components, offer synergistic properties and enhanced capabilities, making them attractive for various applications. This study focuses on the synthesis of Ni/ZnO nanocomposites through an ammonia evaporation and co-precipitation method, aiming to enhance their photocatalytic and antibacterial properties.

The literature review section extensively cites previous research on ZnO nanoparticles and their applications in photocatalysis and antimicrobial activity. It highlights the advantages of doping ZnO with other metals to enhance its performance. Several studies on different metal-doped ZnO nanocomposites and their efficacy in degrading organic dyes and inhibiting bacterial growth are referenced. The review underscores the importance of controlling the size, shape, and morphology of the nanomaterials to optimize their properties. Additionally, the literature review examines the mechanisms of photocatalysis and the role of reactive oxygen species (ROS) in the antibacterial activity of ZnO-based nanomaterials.

The synthesis of ZnO involved dispersing zinc sulfate in distilled water, followed by dropwise addition of a sodium hydroxide solution to form zinc hydroxide precipitate. The precipitate was washed, dried, and calcined at 400°C to obtain ZnO. The Ni/ZnO nanocomposite was synthesized by dissolving nickel(II) acetylacetonate in distilled water and adding it dropwise to a solution of the synthesized ZnO nanoparticles in water. Ammonia solution was added to maintain a basic pH, and the mixture was stirred for 2 hours. The resulting nanocomposite was filtered, washed, and dried at 60°C. Characterization techniques included UV-Vis spectroscopy to determine optical properties, XRD for crystal structure analysis, FT-IR to identify functional groups, HRTEM for particle size and morphology analysis, SEM for surface morphology, and EDX for elemental composition analysis. Photocatalytic activity was assessed using methylene blue (MB) as a model pollutant under visible light irradiation, measuring the degradation of MB at different time intervals. Antibacterial activity was evaluated against E. coli, S. aureus, and P. aeruginosa using the agar well diffusion method, both in light and dark conditions. Minimum inhibitory concentration (MIC) was determined using a serial dilution approach. ROS production was evaluated using 2,7-dichlorodihydrofluorescein diacetate dye, and hemolytic activity was determined using mouse red blood cells. Antioxidant activity was assessed using the DPPH radical scavenging assay.

UV-Vis spectroscopy showed absorption peaks consistent with Ni/ZnO nanocomposite formation. XRD analysis confirmed the high crystallinity and calculated the crystalline size of the nanocomposite to be approximately 21 nm. FT-IR analysis revealed the presence of various functional groups. HRTEM images showed that Ni nanoparticles were well-dispersed and had a smaller size (13 nm) in the nanocomposite compared to individual Ni nanoparticles (>30 nm). SEM and EDX confirmed the successful synthesis of Ni-coated ZnO nanomaterials. The Ni/ZnO nanocomposite exhibited remarkable photocatalytic degradation of MB (100% in 60 min under visible light irradiation), significantly outperforming individual ZnO. Optimal photodegradation was observed at a specific catalyst dosage and pH. The nanocomposite showed superior antibacterial activity against E. coli, S. aureus, and P. aeruginosa under visible light irradiation compared to dark conditions. The MIC values against E. coli and P. aeruginosa were 30 µg/mL, and 20 µg/mL against S. aureus. The ROS test indicated ROS generation, contributing to the antibacterial activity. The nanocomposite showed negligible hemolytic activity, indicating good biocompatibility. The DPPH radical scavenging assay showed a high antioxidant activity of up to 82% inhibition at a concentration of 1 mg/mL.

The enhanced photocatalytic and antibacterial activities of the Ni/ZnO nanocomposite can be attributed to several factors: the high dispersion of Ni nanoparticles on the ZnO surface, increasing the surface area and active sites; efficient light absorption; reduced electron-hole recombination; and the generation of ROS. The significant difference in antibacterial activity under light and dark conditions confirms the role of light in generating ROS and enhancing the efficacy of the nanocomposite. The negligible hemolytic activity suggests good biocompatibility for biomedical applications. The high antioxidant activity further broadens the potential applications of this nanocomposite. The results of this study align with and expand upon previous research on metal-doped ZnO nanomaterials, demonstrating the potential for designing multifunctional nanomaterials for various applications.

This study successfully synthesized a Ni/ZnO nanocomposite using an eco-friendly method, demonstrating excellent photocatalytic and antibacterial properties. The nanocomposite's high efficacy, good biocompatibility, and remarkable antioxidant capabilities suggest significant potential for environmental remediation and biomedical applications. Future research could explore the scalability of the synthesis method, investigate the long-term stability of the nanocomposite, and evaluate its performance in real-world applications.

The study primarily focused on the laboratory-scale synthesis and characterization of the Ni/ZnO nanocomposite. Further research is needed to assess the scalability and cost-effectiveness of the synthesis method for large-scale production. The evaluation of photocatalytic and antibacterial activities was conducted using model pollutants and bacterial strains. Future studies should investigate the nanocomposite's performance with a wider range of pollutants and bacteria under more realistic conditions. The long-term stability of the nanocomposite under various environmental conditions also warrants further investigation.