"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 (S2-) 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 S2- 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 S2-, 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 S2-, with linear calibration ranging from 0nmol/L to 30nmol/L. Other potentially interfering species, such as SO42-, PO43-, HPO42-, H2PO4-, CO32-, NO3-, Ac-, and HCO3-, presented negligible effects on S2- 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.