A Thermosensitive Hydrogel Embedded with Metronidazole-Derived Carbon Dots for Antibacterial Treatment of Periodontitis

Introduction: Microorganisms are the primary initiators of periodontitis, and local antibiotic therapy remains an important adjunctive strategy for controlling periodontal pathogenic bacteria. However, conventional antimicrobial agents often exhibit limited local retention and may induce cytotoxicity or bacterial resistance. This study aimed to develop a thermosensitive hydrogel system incorporating metronidazole-derived carbon dots (Met-CDs) for localized antibacterial treatment of periodontitis. Methods: Met-CDs were synthesized via a one-step hydrothermal method at different temperatures, and their physicochemical characteristics were compared. Human gingival fibroblasts (HGFs) were used to evaluate cytocompatibility through CCK-8 and live/dead staining assays. Antibacterial activity of Met-CDs against Streptococcus sanguinis, Porphyromonas gingivalis, and Fusobacterium nucleatum was assessed using metabolic activity assays, live/dead bacterial staining, scanning electron microscopy, and Western blot analysis. A Poloxamer 407 (P407)-based thermosensitive hydrogel was further fabricated and characterized by rheological, swelling, and in vitro release analyses, while its antibacterial activity was evaluated using agar well antibacterial assays. In vivo biosafety was evaluated in an SD rat model through hematological analysis, inflammatory cytokine ELISA, and histological staining, while therapeutic efficacy was assessed by bacterial load analysis, immunohistochemical analysis, and micro-CT assessment. Results: Met-CDs synthesized at 180 °C retained relatively enriched polar and oxidized surface groups compared with those synthesized at 250 °C, which was associated with enhanced antibacterial activity, and were therefore selected for subsequent studies. Compared with metronidazole (Met), Met-CDs demonstrated enhanced antibacterial activity and lower cytotoxicity toward HGFs. Western blot analysis suggested that the antibacterial effect of Met-CDs may be associated with bacterial membrane disruption. The Met-CDs@P407 hydrogel exhibited favorable thermosensitive behavior, swelling properties, in vitro release characteristics, and retained in vitro antibacterial activity. In vivo experiments demonstrated acceptable biosafety of the hydrogel system, with no obvious systemic inflammatory response or organ toxicity observed under the tested conditions. In addition, the hydrogel reduced periodontal bacterial burden and alleviated alveolar bone resorption in SD rats with experimental periodontitis. Although immunohistochemical staining revealed no obvious enhancement of VEGF expression, indicating a limited effect on VEGF-mediated angiogenic activity, decreased TNF-α expression was observed in periodontal tissues, suggesting that Met-CDs@P407 may alleviate local inflammatory responses through effective control of periodontal pathogenic bacteria. Conclusion: The Met-CDs@P407 thermosensitive hydrogel demonstrated favorable antibacterial activity, cytocompatibility, localized retention potential, and in vivo therapeutic efficacy, suggesting its potential as a localized antimicrobial biomaterial for periodontitis treatment. Plain Language Summary: 1. Metronidazole-derived carbon dots synthesized at 180 °C retain richer surface functionalities.2. Metronidazole-derived carbon dots exhibit lower cytotoxicity than metronidazole.3. The metronidazole-derived carbon dot-embedded thermosensitive hydrogel demonstrates effective localized antibacterial activity against periodontal pathogens.