Nitrogen-engineered red-emissive carbon dots from anthraquinone precursors: Structure-photoluminescence relationships and promising biomedical potential.

Hydrophobic anthraquinone derivatives hold great promise for preparing red-emitting carbon dots (R-CDs), yet precise control over water solubility and luminescence remains challenging. Herein, we report a one-pot hydrothermal synthesis of water-soluble R-CDs (AM-CDs) using aloe-emodin and urea. By tuning the nitrogen doping level, the carbon core size (1.48-4.11 nm) and emission behavior of AM-CDs are systematically regulated: low N/C ratios (0.15-0.3) yield molecular/carbon-core emission; moderate ratios (0.6-0.9) produce ordered ~4 nm cores with high graphitic N (77.7%) and excitation-independent 625 nm red emission; excess N/C (>1.2) introduces defect states. DFT calculations reveal that graphitic nitrogen regulates sp2 carbon growth via C-N-C crosslinking, synergistically with intrinsic hydroxyls to narrow the bandgap (4.14→3.76 eV) and enhance polarization. Comparative studies of anthraquinone precursors with distinct substituents (alizarin, rhein, chrysophanol) under identical nitrogen doping conditions demonstrate that hydroxyl positioning and reactivity determine nitrogen-guided carbonization efficiency, establishing a "precursor structure-limited and nitrogen-regulated" mechanism. The optimized AM-CDs achieve 36 h tumor imaging and 92.2% tumor inhibition in mice, alongside preferential antibacterial activity against Staphylococcus aureus (MIC 341 μg mL-1). This work reveals the critical role of nitrogen in rational R-CD construction from anthraquinone precursors, providing a viable strategy for advanced theranostic nanomaterials.