A novel bis-reaction-triggered cascade fluorescent probe for improved specific detection and biological visualization of Cys over Hcy/GSH

The striking similarities in chemical structures and properties between cysteine (Cys), homocysteine (Hcy) and glutathione (GSH) bring huge interferences to achieve specific detection of Cys, limiting the further physiological and pathological researches on Cys. Herein, a bis-reaction-triggered cascade-responsive highly specific Cys fluorescent probe CT-C was constructed, containing coumarin group as fluorophore, alpha,beta-unsaturated ethanoyl as a Michael-addition site and 2-thiophenecarbonyl moiety as a nucleophilic substitution site, which overcame the interference of Hcy and GSH by two successive reaction processes. CT-C presented a two-stage fluorescence response towards Cys, a fast and strong "turn-on" response in the first process and a lengthy "on-off" response in the subsequent second process, while CT-C only displayed a very slow and weak fluorescence enhancement towards Hcy, and a slight fluorescence change towards GSH. To our knowledge, CT-C was the first reported probe utilizing two-stage cascade response for specific single detection of Cys. Furthermore, the novel two-stage response mode was also useful for the specific fluorescence visualization of exogenous Cys in BHK-21 cells.