Green-synthesised metal oxide nanoparticles: Phytochemical mechanisms, structure–property relationships, and biofunctional applications

Abstract

Green synthesis of metal oxide nanoparticles (NPs) using plant-derived extracts has emerged as a sustainable alternative to conventional fabrication routes while introducing distinctive surface chemistries that influence nanoparticle structure, redox behaviour, and biological interactions. Although numerous reviews have summarised green synthesis approaches and applications, the mechanistic links between phytochemical-mediated synthesis, nanoparticle physicochemical properties, and biofunctional performance remain insufficiently integrated. This review critically examines how plant secondary metabolites regulate nucleation, growth, stabilisation, and surface functionalisation of metal oxide NPs, with particular emphasis on zinc oxide (ZnO), copper oxide (CuO), and chromium oxide (Cr2O3) systems, which serve as representative models owing to their extensive mechanistic and biofunctional investigation. The relationships between particle size, crystallinity, defect density, surface charge, retained phytochemicals, and biological activities including antioxidant, antimicrobial, enzyme inhibitory, and cytotoxic responses are discussed within a unified structure–property–biofunction framework. Current evidence suggests that the biological performance of green-synthesised metal oxide NPs depends not only on their inorganic composition but also on phytochemical-derived surface chemistry, synthesis conditions, and exposure dose. Key challenges, including extract variability, insufficient surface characterisation, limited standardisation of biological evaluation, and translational safety considerations, are critically discussed together with future perspectives emphasising mechanism-driven synthesis, reproducibility, and harmonised safety assessment. Overall, green-synthesised metal oxide NPs should be regarded as chemically and biologically programmable nanomaterials that support the rational design of sustainable biofunctional materials for biomedical, biochemical, and environmental applications in alignment with the United Nations Sustainable Development Goals.