Influence of Drying Methods on Phytochemical Retention of Moringa oleifera Leaves and Green Synthesis of Zinc Oxide Nanoparticles Using the Optimal Moringa oleifera Leaf Extract

ABSTRACT Green synthesis of zinc oxide nanoparticles (ZnO‐NPs) using plant extracts is an environmentally friendly approach; however, the influence of leaf drying pretreatment on nanoparticle synthesis remains underexplored. This study first evaluated the effects of air‐, sun‐, oven‐, and freeze‐drying of Moringa oleifera leaves on phytochemical retention, and then used the extract obtained from the best‐performing drying treatment for the green synthesis of ZnO‐NPs. Freeze‐dried leaves retained the highest levels of phenolics, flavonoids, vitamin C, and β‐carotene and were therefore selected as the sole precursor for nanoparticle synthesis. GC–MS and HPLC confirmed the presence of bioactive compounds responsible for Zn 2+ complexation and nanoparticle stabilization. The synthesized ZnO‐NPs exhibited a characteristic UV–Vis absorption peak at 365 nm with a band gap of 3.40 eV. FT‐IR, XRD, SEM, EDS, dynamic light scattering, and zeta potential analyses confirmed the formation of highly crystalline, flower‐like ZnO‐NPs with particle sizes of 45–82 nm (hydrodynamic diameter ~67 nm) and excellent colloidal stability (−32 mV). The biosynthesized ZnO‐NPs showed significantly higher antibacterial and antioxidant activities, faster radical scavenging kinetics, and excellent storage stability compared with analytical‐grade ZnO. These findings demonstrate that freeze‐drying is an effective pretreatment for producing stable and bioactive ZnO nanoparticles for food and biomedical applications.