Ratiometric sensing of Zn²⁺ with a new benzothiazole-based fluorescent sensor and living cell imaging

A new fluorescent probe, 3-(benzo[d]thiazol-2-yl)-5-bromosalicylaldehyde-⁴N-phenyl thiosemicarbazone (BTT), for ratiometric sensing of Zn²⁺ ions in methanol/HEPES buffer solution (3 : 2, pH = 7.4) is reported in this paper. The presence of Zn²⁺ ions yields a significant blue shift in the maximum emission of BTT from 570 nm to 488 nm, accompanied by a clear color change from orange to green. This emission change of BTT upon binding to Zn²⁺ in a 1 : 1 ratio may be due to the block of excited state intramolecular proton transfer (ESIPT) as well as chelation enhanced fluorescence (CHEF) on complex formation. The limit of detection (LOD) determined for Zn²⁺ quantitation was down to 37.7 nM. In addition, the probe BTT displays the ability to image both exogenous Zn²⁺ ions loaded into HeLa cells and endogenous Zn²⁺ distribution in living SH-SY5Y neuroblastoma cells.