P,N,S-Co-Doped Red-Emissive Carbon Dots for Microenvironment-Sensitive Organelle Imaging and Monitoring Cellular...

Red-emissive carbon dots (R-CDs) with reversible redox responsiveness and microenvironment-sensitive imaging remain largely unexplored. Herein, phosphorus-, nitrogen-, and sulfur-co-doped red-emissive carbon dots (PNS-R-CDs) were synthesized via a facile hydrothermal strategy using o-phenylenediamine, sulfanilic acid, and phosphoric acid for reversible hypochlorite/glutathione (ClO^-/GSH) sensing and intracellular bioimaging. The multifunctional surface-engineered nanodots exhibited strong red emission at 605 nm with a quantum yield of 24% and pH-dependent switching from red to greenish-yellow emission, originating from heteroatom-induced modulation of surface emissive states within a defect-rich graphitic framework. The PNS-R-CDs exhibited a sequential redox-responsive fluorescence switching behavior, in which fluorescence was quenched by ClO^- and subsequently restored upon GSH addition, with low detection limits (0.182 µm for ClO^- and 0.205 µm for GSH), high selectivity, and excellent performance in both solution- and solid-state platforms. Mechanistic studies revealed a radical-mediated surface oxidation-reduction process governing fluorescence modulation. Importantly, PNS-R-CDs exhibited low cytotoxicity and enabled excitation-dependent dual-channel organelle-specific imaging in HeLa cells, showing distinct fluorescence localization within RNA-rich nucleolar domains and nuclear regions. This work establishes triple heteroatom-doped red-emissive CDs as multifunctional nanoprobes for spatially resolved redox sensing and microenvironment-responsive bioimaging. The developed platform may further provide opportunities for adaptive oxidative stress imaging and intelligent fluorescence-guided bioanalytical applications.