Protonated Defect-Engineered Carbon Nitride Enables Bio-Interface-Enhanced Photodynamic Antibacterial Activity with Potential Periodontal Application

Periodontitis is a biofilm-associated inflammatory disease that still requires effective local non-antibiotic antibacterial strategies. In this study, we developed a protonated defect-engineered atomic-layered graphitic carbon nitride nano-system (PVCN) for visible light photodynamic antibacterial therapy. Defect engineering was used to improve visible light absorption and photodynamic activity, while protonation introduced a positively biased surface potential to strengthen bacteria–material interactions and enhance interfacial antibacterial efficacy. Under visible light irradiation, PVCN showed increased ROS production, stronger bacterial adhesion, and rapid killing activity against both Staphylococcus aureus and Escherichia coli, with bactericidal efficiency above 95%. PVCN also disrupted S. aureus biofilms and induced membrane damage, intracellular content leakage, and metabolic suppression. Atomic force microscopy and omics analyses further supported enhanced bacterial adsorption as an important contributor to the improved antibacterial efficacy of PVCN. In vitro assays demonstrated preliminary cytocompatibility and hemocompatibility. In a ligature-induced mouse periodontitis model, PVCN reduced bacterial burden, alleviated inflammation, and attenuated alveolar bone loss. These results support PVCN as a promising photodynamic antibacterial material with preliminary therapeutic potential in experimental periodontitis, and highlight bio-interface regulation as a useful strategy for designing efficient carbon nitride-based photodynamic antibacterial materials.