Effect of alkaline precipitation chemistry on morphology, colloidal stability, and antibacterial activity of ZnO nanoparticles synthesized using plant extracts

This is the authors' abstract. We don't add key points for this paper.

The antibacterial performance of plant-mediated ZnO nanoparticles frequently varies despite their similar crystal structure, suggesting that additional synthesis parameters influence their nano–bio interactions. In particular, the influence of alkaline precipitation chemistry on colloidal organization and biological activity remains poorly resolved. In this study, ZnO nanoparticles were synthesized using aqueous leaf extracts of Piper auritum and Mimosa pudica with NaOH or NH 4 OH as alkaline precipitating agents to evaluate their role in controlling physicochemical and antibacterial properties. Four materials were obtained, designated ZnO-PA, ZnO-MA, ZnO-PS, and ZnO-MS. FTIR analysis confirmed Zn–O formation and the presence of residual phytochemical functional groups on the nanoparticle surface. XRD confirmed the formation of the hexagonal wurtzite phase with crystallite sizes ranging from 15.4 to 25.5 nm. The precipitating agent strongly influenced nanoparticle morphology and colloidal behavior. NH 4 OH produced rod-like particles, whereas NaOH generated quasi-spherical structures. Consistently, ZnO-PS and ZnO-MS exhibited markedly smaller hydrodynamic diameters (214.8–271.5 nm) than ZnO-PA and ZnO-MA (1448–1763 nm), together with more negative zeta potentials and lower polydispersity indices. Antibacterial activity, evaluated by broth microdilution against Staphylococcus aureus , Listeria innocua , Escherichia coli , and Salmonella typhimurium , showed strain-dependent activity, with minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values ranging from 156 to 5,000 µg·mL −1 and from 625 to more than 20,000 µg·mL −1 , respectively. ZnO-PA exhibited bactericidal activity (MBC/MIC ≤ 4) against all tested strains. Overall, these results indicate that alkaline precipitation chemistry governs the colloidal organization and morphology of plant-mediated ZnO nanoparticles, thereby contributing substantially to the antibacterial performance of the resulting nanoparticles.