Facile Synthesis of Long-Term Stable Silver Nanoparticles by Kaempferol and Their Enhanced Antibacterial Activity Against Escherichia coli and Staphylococcus aureus.

Silver nanoparticles (AgNPs) have been widely used in antibacterial fields due to their excellent antibacterial effects. Here, by using a dietary flavonoid found in edible plants, kaempferol (Kae), as reducing and capping agent, a facile and eco-friendly method was developed to synthesize the uniform and long-term stable silver nanoparticle/kaempferol composites (Kae-AgNPs). Kae-AgNPs were characterized by ultraviolet visible (UV–Vis) spectroscopy, Fourier transform infrared spectrum (FT-IR) spectroscopy, dynamic light scattering (DLS), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Kae-AgNPs with a size about 10 nm were synthesized by 0.1 mmol/L Kae. Lower concentration of Kae was shown with smaller Kae-AgNPs due to a few of Kae molecules expected to reduce a small number of Ag⁺ ions to metallic silver. Aqueous Kae-AgNPs were very stable and could be stably dispersed in water for about two months as evidenced by DLS detection, and the stability was further confirmed by XPS. Antibacterial analysis against Escherichia coli (ATCC 8099) and Staphylococcus aureus (ATCC 6538) strains showed that Kae-AgNPs displayed superior antibacterial effects than AgNPs or Kae alone, and interestingly, much better than that of the analogous nano-silver composites in the previous studies. Moreover, Kae-AgNPs with a low concentration of 2 μg/mL demonstrated highly effective antibacterial activity against E. coli (1 × 10⁶ CFU/mL). The antibacterial mechanisms of Kae-AgNPs included the destruction of the membrane structure of bacteria, leakage of the cell contents, inducing the production of reactive oxygen species (ROS), and eventually resulting in bacteria death. The results suggest that Kae-AgNPs have high efficiency against bacteria, and have the potential to be further developed as promising antibacterial nanocomposites. [ABSTRACT FROM AUTHOR]