4 CONCLUSIONS
The swift proliferation of antibiotic resistance has necessitated the development of antibacterial materials that afford excellent antibacterial property while enjoying universal recognition akin to antibiotics. In this study, a new type of CDs with photodynamic and synergistic antibacterial effect is synthesized by means of a one-step hydrothermal process, offering a new train of thought for combating antibiotic resistance. Apart from the ROS intrinsically generated by the CDs, evolution of singlet oxygen (1O2) can also be triggered by the CDs under light irradiation, contributing to a photoinduced synergistic bactericidal activity. The CDs possess three advantages over the previously reported antibacterial materials. By using citric acid and PEI as the precursor, first, the tiny (~3 nm) yet uniform CDs can be produced in 2 h in a single-step, one-pot fashion. The procedure may be achievable without the need for a professional background. Second, the CDs afford enhanced antibacterial effect with broad-spectrum effectiveness under photoirradiation, depending upon their quantum yield that is tunable with the molecular weight of PEI. Mechanistic insights into the antibacterial effect reveal the CDs-induced disruption of bacterial membrane and elevation of intracellular ROS level. Third, the CDs exert minimal cytotoxicity even at a concentration of 100 mg/mL, pointing to a high biocompatibility. This makes the CDs particularly useful for applications in the fields of food packing, antibacterial coating of medical devices and pharmaceutics. Given the synthetic accessibility and wide availability of carbon sources, we suggest that the CDs be prepared in a scalable and lower-cost way from waste organic materials. We also hope that our findings will inspire more efforts on creation of promising materials as potent antibacterial agents, which can combat bacterial infections and biofilms while overcoming antibiotic resistance.