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 Bixa orellana 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.