Effects of Visible-Light Irradiation Conditions on the Antibacterial Activity of Carbon-Doped TiO2 Films on Ti Substrates

Photocatalytic TiO2 coatings can provide antibacterial functions for Ti dental implant surfaces. However, their performance under visible-light irradiation has not been systematically evaluated across a wide range of conditions. In this study, carbon-doped anatase-rich TiO2 films were fabricated on Ti substrates via a two-step thermal oxidation. Their antibacterial activity against Escherichia coli and Staphylococcus aureus was evaluated using a cover glass method under a wide range of conditions, including varying irradiation conditions and specimen temperatures. The radical concentration after visible-light irradiation was measured using electron spin resonance. The cytotoxicity against mouse osteoblast-like cells (MC3T3-E1) was evaluated via the alamarBlue assay. The TiO2 films exhibited antibacterial activity after 25 min of visible-light irradiation at 15 mW·cm–2. The number of viable bacteria decreased as the irradiation intensity, duration, and specimen temperature increased. The radical concentration increased with visible-light irradiation time but decreased slightly with specimen temperature. The enhanced antibacterial activity at higher temperatures was primarily attributed to increased bacterial susceptibility rather than changes in photocatalytic radical formation. The cell viability of the TiO2 film was higher than that of the untreated Ti substrate under dark conditions, and its cytotoxicity remained low even under visible-light irradiation. The antibacterial activity and low cytotoxicity under visible light suggest the potential application of the carbon-doped anatase-rich TiO2 films for antibacterial functionalization of Ti dental implants.