An isothiocyanate-functionalized self-immolative near-infrared fluorescence probe for cysteine sensing and bioimaging in living systems

Biothiols play vital roles in human pathophysiology, with cysteine (Cys) a promising clinical biomarker for many diseases. Therefore, new tools to sensitively and specifically monitor Cys under physiological conditions and in living systems are urgently required. Consisting of a hemicyanine dye and isothiocyanate as the recognition group and self-immolative trigger, here we constructed a near-infrared (NIR) fluorescence probe named CyOH-NCS to specifically sense Cys. The hydroxyl-protected molecule maintained inactive fluorescence until specific attack of the isothiocyanate group by Cys to induce self-immolation and release of the native fluorophore as a NIR fluorescence output. Moreover, the CyOH-NCS probe was used to successfully visualize Cys in living cells, zebrafish and mice. Finally, hydrogen sulfide (H₂S) release experiments demonstrated that the CyOH-NCS probe was a responsive H₂S donor, indicating potential dual use in pathophysiological applications.