Design of a NIR fluorescent chemosensor for the detection and imaging of hydrogen sulfide and cysteine in living cells and blood serum

Cysteine (Cys) and hydrogen sulfide (H<inf>2</inf>S) are biologically important thiols involved in cellular redox regulation, signaling, and disease progression. Herein, we report a novel near-infrared (NIR) fluorescent probe, DN-D, based on a naphthalene-modified BODIPY fluorophore conjugated with a 2,4-dinitrobenzenesulfonyl group, for the relatively selective and sensitive detection of Cys and H<inf>2</inf>S. The 2,4-dinitrobenzenesulfonyl group moiety acts as a thiol-responsive trigger, undergoing nucleophilic aromatic substitution upon interaction with Cys or H<inf>2</inf>S, thereby releasing the active fluorophore (DN - OH) and yielding a strong fluorescence signal at 695 nm (λ<inf>ex</inf> = 650). The probe exhibits a large Stokes shift (40 nm), excellent selectivity against other amino acids and biologically relevant analytes, and rapid response times, <1 min for H<inf>2</inf>S and 15 min for Cys. Importantly, the probe enables highly sensitive detection with low limits of detection (LoD): 119 nM for Cys and 94 nM for H<inf>2</inf>S. DN-D performs reliably under physiological pH conditions (pH 6 - 8), shows low cytotoxicity, and strong stability. Its practical utility is validated by the successful detection of thiols in human blood serum (up to 70-fold dilution) and live HeLa cell imaging.