Modulation of <i>Staphylococcus aureus</i> antibacterial activity based on silver concentration in ZnO/Ag system

The use and development of green synthesis methods have emerged as a sustainable strategy for producing nanomaterials. These involve using plant extracts as natural reducing and stabilizing agents. In this study, an antibacterial investigation was conducted on zinc oxide (ZnO) and zinc oxide/silver (ZnO/Ag) nanostructures at various silver (Ag) concentrations (0.5, 1, 2, 3, and 8 wt%), synthesized using an infusion of Origanum vulgare combined with the solid-state method at 500°C. The synthesized products were characterized using X-ray diffraction (XRD), identifying the hexagonal wurtzite phase (ZnO) with a preferential orientation on the [101] direction and a crystallite size of 30.63 nm, and the corresponding Ag phase with a preferential orientation in [111] direction, which exhibited an increase in crystallite size from 29.66 to 72.26 nm depending on the Ag concentration. Fourier transform infrared spectroscopy (FTIR) confirmed the presence of functional groups from the extract in the initial samples and their elimination after thermal treatment. Transmission electron microscopy (TEM) analysis showed that the synthesized nanoparticles (NPs) are mostly spherical and semi-spherical, with uniform sizes ranging from 19 to 26 nm, depending on the Ag concentration. High-resolution TEM (HRTEM) and fast fourier transform (FFT) allowed for the identification of interplanar distances and crystal planes, confirming the crystalline nature and growth direction of the samples. Finally, the nanomaterials were subjected to antibacterial tests against Staphylococcus aureus and Escherichia coli . The results showed that the samples containing 2% and 3 wt% Ag exhibited the optimal antibacterial activity against S. aureus. In contrast, the inhibition observed against E. coli was lower and more variable, suggesting that the antibacterial performance is bacterium-dependent, likely due to structural differences in their cell walls.