This review explores the use of nickel oxide nanoparticle (NiO NPs) for their use in antibacterial and photocatalytic applications. The interest in this area is driven by the need to combat antibiotic resistance and environmental pollution. The focus of this article is the "green" biosynthesis of NiO NPs; as opposed to traditional chemical and/or physical methods of synthesis. Green biosynthesis is defined as the use of biological entities (i.e., plants, microbes, algae), which utilize natural chemicals found within these organisms to synthesize metal oxide nanoparticles. They act as reducing/capping agents, which affect the morphology, crystallinity and particle size of the NiO NPs produced. Biosynthetic NiO NPs demonstrated significant bactericidal action against a variety of bacterial species; specifically, both gram-positive and gram-negative bacteria. Biosynthetic NiO NPs also showed effectiveness against drug-resistant strains of bacteria such as Staphylococcus aureus and Escherichia coli. These results were attributed to the generation of reactive oxygen species (ROS) and disruption of bacterial cell membranes. The reviewed literature further demonstrates that biosynthesized NiO NPs show significant potential for environmental cleanup. Various investigations report effective degradation of organic dyes and pollutants such as methylene blue under visible light exposure, highlighting the viability of green synthesis routes for producing photocatalytically.
Future research efforts should be focused on standardizing the synthesis protocols developed to date, conducting in vivo toxicity testing on biosynthetic NiO NPs, developing heterojunctions using biosynthetic NiO NPs for improved sunlight-driven photocatalytic activity, and exploring various continuous-flow systems for large-scale industrial production of biosynthetic NiO NPs.