How green and simple can nontoxic antimicrobial nanoparticles be obtained? Deposition of AgNPs on the TiO₂ surface in a neutral aqueous media by using the flavonoid quercetin

Although silver nanoparticles (AgNPs) are excellent antimicrobial agents, they are considered hazardous materials due to their cytotoxic effects. In this research, AgNPs were deposited on TiO₂ using a natural flavonoid under neutral conditions (pH 7) to obtain antibacterial nanomaterials without cytotoxicity. First, AgNPs were synthesized using two types of quercetin as a reducing agent, reactive grade and dietary supplement, to confirm the formation of AgNPs under this green approach. Later, the deposition of AgNPs on TiO₂ was carried out again at pH 7 using reactive grade quercetin (sTiO₂-Ag(R)) and dietary supplement (sTiO₂-Ag(S)). They were analyzed by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Z potential, and colloidal stability. The results showed that the deposition with dietary supplement is more efficient and safer, while the process carried out with reactive grade is inadequate since it causes nitrates bound to the TiO₂ surface. The antipathogenic activity was studied against the main bacteria in raw food (Escherichia coli) and a major human pathogen (Staphylococcus aureus). The disk diffusion method confirmed higher inhibition when the deposition used the dietary supplement. Furthermore, sTiO₂-Ag(S) was analyzed by single-plate serial dilution method to obtain minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). For S. aureus, the MIC was 31.2 μg/ml, and MBC was 1 mg/ml; for E. coli, the MIC was 7.8 μg/ml, and MBC was 1 mg/ml. Finally, cytotoxicity was evaluated by seeding peripheral blood mononuclear cells, showing that the sTiO₂-Ag(S) is nontoxic up to 100 ppm. These findings open the possibility of using these nanomaterials in numerous applications.