Green microwave-assisted Ag/Zn nanoparticle production: mechanistic assessment of antibacterial and antioxidant properties and structural analysis

This study reports a rapid microwave-assisted green synthesis of monometallic and bimetallic Ag/Zn nanoparticles (NPs) using <i>Bixa orellana</i> L. leaf extract as a reducing agent. Monometallic B-AgNPs and B-ZnONPs were synthesized from AgNO₃ and Zn(CH₃COO)₂ · H₂O, while bimetallic B-Ag-ZnONPs were prepared by microwave-assisted co-reduction at 2450 MHz and 800 W for 10 min. NP formation was confirmed by color changes and UV-Vis absorption peaks at 407 nm (B-AgNPs), 347 nm (B-ZnONPs), and 359 nm (B-Ag-ZnONPs). Dynamic light scattering revealed average hydrodynamic sizes of 72, 84, and 121 nm, respectively. Scanning electron microscopy showed particle size ranges of 31.25-88.61nm, 33.8-96.6nm, and 20.9-58.27nm, while transmission electron microscopy indicated mean sizes of 55.29, 99.76, and 49 nm. Fourier transform infrared spectroscopy and X-ray diffraction confirmed stabilization and crystallinity. Bimetallic B-Ag-ZnONPs exhibited superior antibacterial activity, with lower minimum inhibitory concentration and minimum bactericidal concentration values and larger inhibition zones against Gram-positive and Gram-negative bacteria, particularly Gram-negative strains, whereas the plant extract showed negligible activity. Enhanced bactericidal effects were attributed to membrane oxidation and biomolecular leakage. Antioxidant assays demonstrated lower IC₅₀ values for B-Ag-ZnONPs in 2,2-diphenyl-1-picrylhydrazyl (16.63 µg/mL) and 2,2-azino-bis-3-ethylbenzothiazoline-6-sulphonic acid (13.78 µg/mL) compared to monometallic NPs and plant extract, indicating synergistically enhanced bioactivity.