Solid-state fabrication of red-emissive carbon dots for reversible multi-channel sensing and logic-gate operations of ferric and fluoride ions.

The development of reversible and multimodal optical platforms for simultaneous multi-analyte detection remains a significant challenge. Herein, we synthesized red-emissive nitrogen/sulfur co-doped carbon dots (N,S-CDs) via a solvent-free solid-phase strategy and employed as multifunctional sensing substrates for ferric (Fe) and fluoride (F) ions. The N,S-CDs exhibited excellent biocompatibility and optical properties, enabling a reversible Fe/F-regulated optical response. Mechanistically, Fe significantly enhances absorbance and quenches fluorescence through static complex formation and inner filter effect, while subsequent addition of F restores the optical signals via competitive chelation. Based on this mechanism, a dual-mode colorimetric and fluorescence sensing platform was established, achieving limits of detection (LOD) of 90.0 nM and 150.0 nM for Fe, and 6.0 μM and 3.0 μM for F, respectively. Furthermore, a smartphone-assisted scanometric method was developed for rapid, high-throughput on-site analysis. The system was successfully applied to determine Fe/F in real water samples and enabled visual tracking in living cells. Additionally, the reversible optical response was encoded into molecular "AND" and "INH" logic gates, highlighting the platform's potential for integrated sensing and molecular information processing.