Electron storage mediated dark antibacterial action of bound silver nanoparticles: smaller is not always better

Size tunable silver nanoparticles (Ag NPs) are synthesized and incorporated into titanium oxide coatings (TOCs) by manipulating the atomic-scale heating effect of silver plasma immersion ion implantation (Ag PIII). The resulting Ag NPs/TOC composite coatings possess electron storage capability that gives rise to both controlled antibacterial activity and excellent compatibility with mammalian cells. The precipitation behavior of these Ag NPs is qualitatively constrained by the classical nucleation theory. Both photoluminescence (PL) spectra and fluorescence microscopy results demonstrate that larger Ag NPs (5-25 nm) are better at reserving electrons than smaller ones (∼4 nm). The antibacterial activities of the as-sprayed and Ag PIII treated TOCs show that Ag NPs with a different size act distinctively to bacteria: large particles induce serious cytosolic content leakage and lysis of both Staphylococcus aureus and Escherichia coli cells while small ones do not. The excellent activity of larger Ag NPs against bacteria is highly related to their stronger electron storage capability, which can induce accumulation of adequate valence-band holes (h⁺) at the titanium oxide side, arousing oxidation reactions to bacterial cells in the dark. Moreover, the in vitro cell culture assay (using both MG63 and MC3T3 cells) reveals no significant cytotoxicity and even good cytocompatibility on the Ag PIII treated samples. Our results show that, by taking advantage of the boundary property between Ag NP and titanium oxide, the antibacterial activity of Ag NPs can be accurately controlled. This study provides a distinct criterion for the design of nanostructured surfaces such that their osteoblast functions and antibacterial activity are perfectly balanced.