A ratiometric fluorescent chemosensor for the detection of cysteine in aqueous solution at neutral pH

A ratiometric fluorescent chemosensor 2‐(2′‐hydroxy‐3′‐formyl‐5′‐methoxyphenyl)benzothiazole ( 1 ) was developed for the detection of cysteine (Cys). In aqueous solution at neutral pH, 1 exhibited a ratiometric fluorescent response to Cys with a remarkable red‐to‐green shift in the emission wavelength. This fluorescence change was attributed to the cyclization reaction between the formyl group in 1 and the amino and sulfhydryl group in Cys in a stoichiometry of 1: 1 according to the proposed mechanism. At neutral pH, 1 displayed a significant fluorescence ratio signal enhancement with the addition of Cys. Furthermore, 1 showed good selectivity toward Cys. The detection limit and linear range were 5.6 and 0–100 μmol/L, respectively, which demonstrated that 1 could recognize relatively low concentrations of Cys and is a good candidate for applications in detecting Cys.