SOYBEAN-MEDIATED GREEN SYNTHESIS OF RGO/SIO₂/ZNO NANOCOMPOSITES FOR ANTIMICROBIAL PROTECTION AND SEED NANO-PRIMING ENHANCEMENT

Abstract

Developing multifunctional nanomaterials remains critical for sustainable crop protection and the mitigation of biotic stress. This study describes the green synthesis of a ternary reduced graphene oxide/silicon dioxide/zinc oxide (rGO/SiO₂/ZnO) nanocomposite, utilizing an aqueous extract of soybean (Glycine max) as a sustainable reducing and stabilizing agent. FTIR spectroscopy confirmed the reduction of graphene oxide (GO) to rGO. Transmission electron microscopy (TEM) revealed the successful anchoring of crystalline ZnO and SiO₂ nanoparticles, with an average diameter of 55 nm, onto the rGO matrix. The stability of the synthesized architecture was validated by a zeta potential of -30.9 mV, indicating high colloidal stability. Biological assays demonstrated significant broad-spectrum antimicrobial efficacy. The nanocomposite achieved a 3.5-fold increase in the inhibition zone against Escherichia coli compared to monometallic ZnO NPs and exhibited significant inhibitory efficacy against Fusarium oxysporum, Penicillium digitatum, Aspergillus niger, and Candida albicans. Time-kill kinetics demonstrated rapid, concentration-dependent microbicidal efficacy, while TEM ultrastructural analysis of treated F. oxysporum revealed cellular degradation, characterized by expanded vacuoles and lipid droplet accumulation. In seed nano-priming experiments, soybean seeds treated with rGO/SiO₂/ZnO (50–500 mg/L) exhibited improved germination percentages and reduced disease incidence. The 150 mg/L concentration was identified as the optimal treatment, yielding a 57% increase in fungicidal action against F. oxysporum relative to ZnO NPs alone. Furthermore, this treatment mitigated pathogen-induced damage, resulting in a 400% increase in germination rate over infected controls and a 197% increase in shoot fresh mass compared to healthy, unprimed controls. These findings indicate that the rGO/SiO2/ZnO nanocomposite exhibits a multifunctional profile, acting concurrently as a biostimulant and a protective agent. This suggests its potential as an integrated approach for managing fungal-induced biotic stress in soybean cultivation. This study provides a foundational step toward developing sustainable nanomaterials that may contribute to long-term crop resilience in agricultural systems.