Dynamic GelMA/Oxidized Hyaluronic Acid Hydrogels Enable Acid-Accelerated Localized Delivery of Photothermal Melatonin-Derived Carbon Dots for Infected Bone Repair

Abstract Infected bone defects remain difficult to reconstruct because bacterial persistence, acidic stress, excessive reactive oxygen species (ROS), inflammatory dysregulation, and impaired osteogenesis reinforce one another. Here, sulfur-doped melatonin carbon dots (DM-CDs) were synthesized from melatonin and d-cysteine and immobilized within a gelatin methacryloyl (GelMA)/oxidized hyaluronic acid hydrogel (GH-DM) through dynamic Schiff-base interactions. Sulfur-containing surface states endowed DM-CDs with strong 808 nm absorption, an approximately 50% photothermal conversion efficiency, stable heating cycles, and NIR-amplified antibacterial activity. The melatonin-derived surface simultaneously scavenged ROS, preserved mitochondrial membrane potential, and was associated with increased Nrf2 and Sirt1 transcript levels together with recovery of CAT and SOD activities in mouse bone marrow mesenchymal stem cells (BMSCs). Under acidic conditions, dynamic imine interactions were associated with modestly accelerated DM-CD release, whereas NIR irradiation provided an external means to amplify local heating and antibacterial performance. GH-DM reduced planktonic bacterial viability and biofilm biomass, decreased cell-associated Staphylococcus aureus signals, and enhanced BMSC osteogenic differentiation, as shown by ALP activity, matrix mineralization, and increased RUNX2, ALP, COL1, and OCN expression. Mouse BMSC RNA sequencing revealed enrichment of collagen-containing extracellular-matrix programs and attenuation of NF-κB, IL-1β, IL-6, and osteoclast-differentiation-related signatures. Separate LPS-stimulated RAW264.7 experiments supported attenuation of a pro-inflammatory state and increased IL-10 expression. In infected femoral defects, GH-DM plus NIR markedly reduced bacterial burden and improved trabecular regeneration, with no detectable major-organ histopathology or abnormalities in the measured hematological, hepatic, and renal indices. These findings support a locally delivered carbon-dot hydrogel with acid-accelerated release that coordinates infection control, redox protection, inflammatory regulation, and bone regeneration.