We have synthesized a styryl boron-dipyrromethene (BODIPY)/2,4-dinitrobenzenesulfonyl (DNBS) dyad based red-emitting molecular probe for specific detection of cysteine among the biological thiols. The probe shows intensive absorption at 556 nm and the probe is non-fluorescent. The DNBS moiety can be cleaved off by thiols, the red emission of the BODIPY fluorophore at 590 nm is switched on, with an emission enhancement of 46-fold. The probe shows good specificity toward cysteine over other biological molecules, such as glutathione and amino acids. The emission of the probe is pH-independent in the physiological pH range. The probe is used for fluorescent imaging of cellular thiols. Theoretical calculations based on density functional theory (DFT) were used to elucidate the fluorescence sensing mechanism of the probe, which indicate a dark excited state (S(1)) for the probe but an emissive excited state (S(1)) for the cleaved product (i.e. the fluorophore).
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