An “on-off-on” fluorescent sensor based on TSPP-gallic acid: Visual detection of S2− in actual samples

A novel “on-off-on” switches fluorescence sensor based on TSPP-gallic acid (GA) for S${}^{2-}$-detecting was designed in this article. 5,10,15,20- (4-sulphonatophenyl) porphyrin (TSPP) was synthesized by a modified “one-pot” method. The synthesized TSPP was used as a fluorescence probe for the construction of a fluorescence sensing system. At room temperature, the maximum fluorescence emission wavelength could be obtained at 642 nm when the excitation wavelength was 515 nm when the fluorescence signal was in the “on” mode. The TSPP solution appeared pink in color at this moment. When GA was introduced into the TSPP solution, the fluorescence signal was turned off (fluorescence intensity was quenched) due to the protonation of TSPP, which was caused by the weak acidity of GA. The color of the TSPP solution changed from pink to green. Following that, an acid-base reaction between S${}^{2-}$and GA was initiated when S${}^{2-}$was introduced into the TSPP-GA system, leading to the liberation of TSPP and restoration of fluorescence intensity. The solution color reverted to red, indicating that the fluorescence sensing system had returned to the “on” mode. The TSPP-GA system had good selectivity and sensitivity to S${}^{2-}$, the linear range was 4.52 × 10${}^{-6}$ M $\sim$ 4.34 × 10${}^{-4}$ M, and the detection limit (LOD) was 0.87 $\mu$M. It could be used to detect sulfur ions in actual samples with satisfactory recoveries and RSD values.