An NIR ESIPT-based fluorescent probe with large stokes shift for specific detection of Cys and its bioimaging in cells and mice

ESIPT-based (Excited-State Intramolecular Proton Transfer) fluorescent probes have been popular because of their large stokes shift and environmental sensitivity. In this work, by coupling dicyanoisophorone and benzothiazole with toluene, we built a cysteine (Cys) fluorescent probe containing a near-infrared ESIPT fluorophore, and an acryloyl group as a nucleophilic addition and leaving site. It was found that only Cys enabled nucleophilic addition and release of hydroxy toluene in conjugated system, with a turn-on NIR emission based on ESIPT effect of benzothiazole. The probe SYP exhibited an excellent selectivity for Cys without any interference from H2S and other thiols (glutathione and homocysteine), attributed to the formation of the seven-membered heterocyclic compound favored kinetically. More, the sensing mechanism was proved to be the ESIPT process by 1H-NMR and MS titration experiments. Of course, the probe displayed its practical application by imaging endogenous Cys in cells and mice. This work innovatively proposed a method for designing a near-infrared fluorescent probe specifically for Cys based on the ESIPT mechanism.