Sustainable design of multifunctional zinc oxide nanoparticles using Cleistocalyx operculatus for high-efficiency UV photocatalysis and potent antibacterial applications

Abstract

A sustainable, comprehensive green synthesis route for zinc oxide nanoparticles (ZnO NPs) was developed using Cleistocalyx operculatus leaf extract as both a reducing and a capping agent. The extraction of bioactive compounds was systematically optimized via Response surface methodology (RSM), with extraction time, temperature, ethanol concentration, and leaf-to-solvent ratio as key parameters. Optimized conditions (76°C, 87 min, 85% v/v ethanol, and a 3:8 g mL⁻¹ ratio) were used to obtain a polyphenol-rich extract with high total phenolic content, antioxidant capacity, and reducing power. The resulting extract facilitated the synthesis of high-purity, spherical ZnO NPs exhibiting a hexagonal wurtzite structure. Physicochemical analyses confirmed high crystallinity (79–80%) and tunable crystallite sizes ranging from 8.8 to 16.3 nm. The precursor-to-extract ratio emerged as a critical parameter; higher extract volumes resulted in more uniform, well-dispersed particles with enhanced bio-derived surface functionalities. The synthesized ZnO NPs exhibited high photocatalytic efficiency, degrading 99.5% of methylene blue under UV irradiation within 150 min. Kinetic studies indicated the process followed a pseudo-first-order model (with a rate constant k = 1.84 × 10⁻² min⁻¹). Scavenging experiments confirmed that photogenerated hydroxyl radicals (•OH) were the primary active species responsible for degradation. The catalyst maintained near-complete efficiency over five reuse cycles, demonstrating robust stability. Furthermore, the NPs showed potent, dose-dependent antibacterial activity against Escherichia coli (MBC/MIC = 0.625/0.313 mg mL⁻¹) and Staphylococcus aureus (MBC/MIC = 2.50/0.625 mg mL⁻¹). This efficacy is attributed to the synergistic action of Reactive oxygen species (ROS) generation, membrane disruption, and Zn²⁺ ion release. The results validate C. operculatus–assisted ZnO NPs as efficient and multifunctional candidates for advanced environmental cleanup and antimicrobial applications, highlighting the broad utility of polyphenol-mediated green synthesis for designing metal oxide nanomaterials.