Phenothiazine and semi-cyanine based colorimetric and fluorescent probes for detection of sulfites in solutions and in living cells

Four hemicyanine probes for selectively detecting sulfites (HSO 3 − /SO 3 2− ) have been constructed by the condensation reaction of 7-substituted (CN, Br, H and OH) phenothiazine aldehyde with 1-ethyl-2,3,3-trimethylindolium iodide. All four probes show a fast and sensitive response to HSO 3 − /SO 3 2− via a Michael addition, with a detection limit lower than 40 nM based on monitoring their UV/vis absorption changes. Although all four probes display an increase in fluorescence when responding to HSO 3 − /SO 3 2− , the increment is larger for the probe with an electron-withdrawing group than the probe with an electron-donating group, except for Br. Thus, among four probes the 7-cyano probe (PI-CN) possesses the largest fluorescent response to HSO 3 − /SO 3 2− , and the lowest detection limit (7.5 nM). More expediently and easily, a film and a test paper with PI-CN have been prepared to detect HSO 3 − /SO 3 2− in a sample aqueous solution selectively. Finally, the detection of HSO 3 − /SO 3 2− by PI-CN in biological environments has been demonstrated by cell imaging. Four 7-substituted phenothiazine hemicyanines display a substituent effect on the fluorescence response toward sulfites. The CN-substituted probe exhibits the best sensing behavior.