Green synthesis of ZnO nanoparticles using Citrus sinensis peel: optimizing volume ratios for enhanced antibacterial and banana preservation activities

This study explores the green synthesis of zinc oxide nanoparticles (ZnO NPs) using aqueous extract of Citrus sinensis peel. C. sinensis peel is bioactive, phytochemical rich and readily available agro-waste. The aim of this study was to investigate the effects of varying the volume ratio of zinc nitrate precursor to peel extract (1:1, 1:2, and 2:1) on the physicochemical properties, antibacterial activity, and preservative efficacy of biosynthesized ZnO NPs. Structural analysis by X-ray diffraction (XRD) confirmed the formation of crystalline hexagonal wurtzite ZnO NPs across all synthesis ratios. The 2:1 ratio exhibiting superior crystallinity and larger crystallite size (24.2 nm). Fourier transform infrared (FTIR) spectra identified zinc-oxygen bonds and confirmed the role of peel-derived biomolecules in NP capping and stabilization. Ultraviolet-visible (UV-vis) spectroscopy revealed strong absorbance peaks at approximately 370 nm, characteristic of ZnO NPs, with the 2:1 sample exhibiting the highest optical absorption. Scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) and transmission electron microscopy (TEM) revealed morphological transitions from aggregated particles to elongated rod-like structures with increasing zinc nitrate ratio, confirming elemental composition of zinc and oxygen. Antibacterial activity testing using disc diffusion methods against Gram-positive and Gram-negative bacteria revealed concentration-dependent inhibition, with ZnO synthesized at a 2:1 ratio providing the most potent antibacterial activity. Furthermore, chitosan-based nano-coating incorporated with ZnO NPs significantly reduced weight loss in banana fruit preservation trials, indicating enhanced shelf life. The 2:1 ratio coating is showing the best preservation performance. This study underscores the critical role of precursor ratio in tailoring NP size, morphology, crystallinity, and bioactivity. The findings demonstrate that green-synthesized ZnO NPs using C. sinensis peel extract represent promising, sustainable nanomaterials for antibacterial applications and food preservation, with minimized environmental impact.