"On-off-on" fluorescent probe based on g-C₃N₄ quantum dots for sequential detection of Ag⁺ and glutathione

This study reports the successful synthesis of graphitic carbon nitride quantum dots (g-C₃N₄ QDs) that demonstrate remarkable fluorescence properties and excellent dispersion through a hydrothermal method. Leveraging the specific fluorescence quenching effect of Ag⁺ on g-C₃N₄ QDs together with the strong chelating affinity of glutathione (GSH) for Ag⁺, a novel "on-off-on" fluorescence sensing platform was developed to enable sequential and highly sensitive detection of both Ag⁺ and GSH. This probe exhibited outstanding sensitivity, anti-interference ability, and selectivity with detection limits of 0.0114 μM for Ag⁺ and 0.3916 μM for GSH. Mechanistic studies revealed that the fluorescence "off" state was mainly governed by a photoinduced electron-transfer (PET) effect, with additional contributions from the internal filtering effect (IFE). Conversely, the fluorescence "on" state was restored when GSH coordinated with Ag⁺, thereby blocking the PET pathway and recovering the intrinsic fluorescence of g-C₃N₄ QDs. Furthermore, spiked-recovery experiments in real water and serum samples yielded satisfactory results, demonstrating the sensor's strong practical applicability and providing a reliable analytical approach for environmental monitoring and biomedical analysis.