Green Synthesis and Characterization of Zinc Oxide Nanoparticles Using Mentha aquatica Leaf Extract: Photocatalytic, Antibacterial, and Antioxidant Applications

Abstract

This study reports the green synthesis of zinc oxide nanoparticles (ZnO NPs) using an aqueous extract of Mentha aquatica (water mint) leaves as a bio-reducing and capping agent. The synthesis was performed by the reduction of zinc acetate dihydrate  [Zn(CH3COO)2·2H2O] in the presence of the plant extract under alkaline conditions and thermal treatment at 60 °C for 1 h, followed by calcination at 400 °C for 2 h. The resulting nanoparticles were characterized using UV-Vis spectrophotometry, Fourier Transform Infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and Scanning Electron Microscopy (SEM). UV-Vis analysis revealed a characteristic absorption band at ~380 nm, confirming the formation of ZnO NPs. FTIR analysis identified the functional groups responsible for the bioreduction of zinc ions. XRD patterns confirmed the hexagonal wurtzite crystal structure of the synthesized particles, consistent with JCPDS card No. 36-1451, with principal diffraction peaks at 2θ values of 32.25°, 34.98°, and 36.79°. SEM images revealed spherical to granular morphologies with particle sizes in the range of 16–22 nm. The synthesized ZnO NPs exhibited excellent photocatalytic degradation of methylene blue (MB) dye under natural solar irradiation, achieving near-complete decolorization within 120 minutes. Disk diffusion assays demonstrated strong antibacterial activity against Gram-negative bacteria (E. coli: 35 mm inhibition zone; P. acip: 23 mm) and moderate activity against Gram-positive S. aureus (ATCC 43300: 15 mm). Antioxidant screening via the DPPH radical scavenging assay yielded an IC50 of 9.5 mg/mL for ZnO NPs, indicating moderate antioxidant potential. These results establish Mentha aquatica leaf extract as a promising, eco-friendly bio-template for ZnO NP synthesis with multifunctional applications.