An innovative study on the “on–off–on” detection of sulfur ions based on a TSPP–riboflavin fluorescent probe

In this paper, 5,10,15,20-(4-sulphonatophenyl) porphyrin (TSPP) was synthesized by a facile route and used as a fluorescent probe to construct a sensor system based on the high water solubility and high quantum yield. It was found that when riboflavin (RF) was introduced into the TSPP solution, the fluorescence intensity of TSPP decreased for the peaks at 645 nm and 700 nm based on the principle of the electrostatic attractions and hydrophobic interactions between TSPP and riboflavin. When the fluorescence emission peak of riboflavin appeared at 550 nm, the fluorescence sensor system changed from the "on" state to the "off" state. When sulfur ions (S2-) were further introduced into the TSPP-riboflavin system, the fluorescence intensity of riboflavin was further decreased based on the specific reaction between S2- and riboflavin. However, the fluorescence signal of TSPP was restored and the fluorescence sensing system changed from the "off" state to the "on" state. Therefore, TSPP was used as a fluorescent probe to construct an "on-off-on" fluorescent sensing system, the linear range of S2- detected by this system is 5.0 × 10-9 to 3.6 × 10-5 M, and the detection limit (LOD) is 1.1 × 10-9 M. The sensing system has higher accuracy and sensitivity, and it can be successfully used in the sensing of S2- in real samples.