Visible-light-induced antibacterial activity of carbon and nitrogen co-doped rutile TiO₂ films

The visible-light-responsive photocatalytic activity of TiO₂ coatings offers a promising route for the antibacterial functionalization of Ti implants. In this study, carbon and nitrogen co-doped single-phase rutile TiO₂ films were fabricated on Ti substrates via a two-step thermal oxidation process involving subsequent treatments with N₂-1%CO and N₂-1%O₂. Under optimized treatment conditions, the single-phase rutile TiO₂ films exhibited antibacterial activity against Escherichia coli under visible-light irradiation (λ > 400 nm) at 15 mW·cm^-2 for 15 min. The TiO₂ films exhibited a double-layered structure comprising an inner layer (containing carbon and nitrogen) on the Ti substrate side and an outer layer (without carbon or nitrogen) on the topmost surface. The excitation of electrons and holes generated by dissolved carbon and nitrogen in the inner layer of the TiO₂ films contributed to visible-light responsiveness. The bonding strength between the TiO₂ film and Ti substrate exceeded 60 MPa. The cytotoxicity against the mouse osteoblast-like cell (MC3T3-E1) of the TiO₂ film was comparable to that of well plates, under visible light and in the dark. This study demonstrates the visible-light-responsive antibacterial activity of a single-phase rutile TiO₂ film on Ti. Further, the TiO₂ film exhibited excellent bonding strength with the Ti substrate, indicating its high potential as an antibacterial active coating in combination with visible-light irradiation for Ti implants.