Green synthesis of zinc oxide nanoparticles using <i>Silybum marianum</i> leaf extract: Characterization and antimicrobial activity

This work presents a green synthesis approach using the aqueous extract of Silybum marianum leaves—an approach less explored than those using seeds or callus—for the production of zinc oxide nanoparticles ( ZnO-NPs ) with potential biomedical applications. The method offers a potentially advantageous route due to its cost-effectiveness, simplicity, and non-toxicity. The characterization and evaluation of ZnO-NPs were performed using various spectroscopic and microscopic techniques, including X-ray powder diffraction (XRD), fourier-transform infrared (FT-IR), ultraviolet-visible (UV–vis), and transmission electron (TEM) microscopy. FT-IR analysis of the leaf extract by revealed the presence of functional groups that potentially contribute to the formation, stabilization, and capping of the green-synthesized ZnO-NPs . A high-intensity absorption peak at 373 nm, characteristic of ZnO-NPs , appeared in the UV–vis analysis, confirming their successful production. The optical band gap (E g ) was estimated using the Tauc plot, and found to be 2.77 eV. The XRD analysis confirmed the presence of a well-crystalline hexagonal wurtzite structure of ZnO-NPs , with an average crystalline size of 42 nm. TEM analysis revealed that the nanoparticles (NPs) are predominantly uniform, non-agglomerated, spherical, with particle diameters of (19 – 58) nm. ζ- potential analysis indicated good colloidal stability of the ZnO-NPs which are predominantly non-agglomerated, and homogeneously distributed, with an average particle diameter of 60 nm. The antimicrobial activity of the ZnO-NPs was evaluated using the agar diffusion method against a panel of microbial strains: four fungal strains, four Gram-positive, and three Gram-negative bacterial strains. The green-synthesized ZnO-NPs exhibited a measurable inhibitory activity against Staphylococcus hominis, while only a weak antimicrobial effect against the other strains was recorded. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the ZnO-NPs against S. hominis were found to be 12.5 μg/mL and 25 μg/mL, respectively. This work demonstrates the feasibility of employing plant-based extracts for synthesizing metal NPs and contributes to the advancement of green nanotechnology and its promising applications in biomedicine.