Green-Synthesized Zinc Oxide Nanoparticles using Stevia rebaudiana Leaves: In Vitro Antioxidant and Antimicrobial Evaluation

Background: Green synthesis of metal oxide nanoparticles using medicinal plants offers a sustainable route to bioactive nanomaterials. Objectives: This study aimed to synthesize zinc oxide nanoparticles (ZnO NPs) using Stevia rebaudiana aqueous leaf extract and evaluate their in vitro antioxidant, antibacterial, and antifungal activities. Materials and Methods: Aqueous S. rebaudiana leaf extract served as both a reducing and stabilizing agent to produce SRZnO NPs. The nanoparticles were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), and dynamic light scattering (DLS). Antioxidant activity was assessed using DPPH free radical scavenging, ferric reducing antioxidant power (FRAP), and phosphomolybdenum (PM) assays. Antimicrobial activity against Escherichia coli, Staphylococcus aureus, Aspergillus niger, and Candida albicans was evaluated by the agar well diffusion method. Results: TEM revealed predominantly round to quasi-spherical nanoparticles with slight aggregation; DLS indicated a hydrodynamic size of ~107–130 nm. XRD confirmed a crystalline hexagonal wurtzite ZnO structure, and EDX verified zinc as the major element. SRZnO NPs exhibited markedly higher antioxidant activity than the crude extract, with a DPPH IC₅₀ of 12.04 µg/mL compared with 83.30 µg/mL. FRAP and PM assays further demonstrated superior reducing power and total antioxidant capacity at all tested concentrations. The nanoparticles exhibited concentration-dependent antimicrobial effects, producing inhibition zones of 14–17 mm at 100 µL, whereas the leaf extract showed minimal activity. Conclusion: Stevia rebaudiana–mediated ZnO nanoparticles display enhanced antioxidant and broadspectrum antimicrobial properties compared with the parent leaf extract, supporting their potential as promising bioactive nanomaterials for pharmaceutical and biomedical applications.