Merocyanine-based turn-on fluorescent probe for the sensitive and selective determination of thiophenols via a pKa shift mechanism

Abstract

The development of fluorescent probes for the sensitive and selective determination of highly toxic thiophenols is considerably important in the fields of biological and environmental sciences. Herein, a turn-on fluorescent probe for thiophenol, named MCSH, was constructed based on a pKa shift mechanism, employing merocyanine dye as the fluorophore and 2,-4-dinitrobenzenesulfonamide (DNBS) group as the recognition unit. The imine nitrogen of MCSH has a pKa value of 4.12, which renders its non-fluorescent Schiff base form exclusively under neutral conditions. However, after reacting with thiophenols, its DNBS group was removed to afford a merocyanine dye as the final product, whose pKa value upshifts to 8.11, and was present mainly as the fluorescent protonated Schiff base form under neutral media. Such drastic change in pKa values leads to a significant fluorescence enhancement and can be utilized for the detection of thiophenols. The fluorescence intensity at 627 nm increases linearly with thiophenol concentration in the range of 0.2–3 μM with a detection limit of 15 nM (S/N = 3). MCSH displays high selectivity for the detection of thiophenols over a wide range of other analytes, including aliphatic thiols. Furthermore, the preliminary applications of MCSH for monitoring thiophenols in living cells and environmental have been carried out.