"On-Off-On" fluorescence sensor based on g-C₃N₄ nanosheets for selective and sequential detection of Ag⁺ and S²

Detection of silver (Ag⁺) and sulfide (S^2-) ions is important because their presence in large amounts can cause many diseases. In this study, a novel, simple, "on-off-on" fluorescence sensor based on g-C₃N₄ nanosheets for sequential detection of Ag⁺ and S^2- was designed. The fluorescence signal of the g-C₃N₄ nanosheets is quenched because Ag⁺ chelates with the N of the g-C₃N₄ nanosheets, leading to photoinduced electron transfer from the sheets to Ag⁺. After adding S^2-, the fluorescence of the g-C₃N₄ nanosheets is recovered due to formation of Ag₂S, which activates the fluorescence of the g-C₃N₄ nanosheets. The recovery efficiency was found to increase with increasing concentrations of S^2-, with linear calibration ranging from 0nmol/L to 30nmol/L. Other potentially interfering species, such as SO₄^2-, PO₄^3-, HPO₄^2-, H₂PO₄^-, CO₃^2-, NO₃^-, Ac^-, and HCO₃^-, presented negligible effects on S^2- detection. Moreover, the proposed sensor exhibited several advantages, including low cost, easy preparation, rapid detection, excellent biocompatibility, and a switchable fluorescence response. These attributes make this fluorescent sensor a promising tool for environmental applications.