A novel ratiometric AIE-based fluorescent probe for specific detection of Hcy/Cys and imaging of living cells in vivo

Thiol-containing small molecules such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) play a key role in a wide range of physiological processes. But it is a challenging work to distinguish these biothiols since they have a similar structure and reactivity. An “AIE + ICT” ratiometric fluorescent probe is synthesized by incorporating TPE into the coumarin ring through a Schiff-base CN. The sensing mechanism of the probe lies on the nucleophilicity of the sulfhydryl group and the adjacent amino group of Hcy/Cys, breaking the Schiff-base CN in the probe structure and forming a five or six-membered ring thiazolidine adduct. Only Hcy/Cys triggers an obvious ratiometric change from orange-yellow to blue. The ratiometric probe can detect Hcy/Cys quantitatively, and the lower limits of detection for Hcy and Cys are 0.554 μM and 1.816 μM, respectively. Moreover, the probe showed a slight preference for Hcy over Cys. The difference in the sensing behavior of the probe for Hcy/Cys is believed to be the consequence of the more highly efficient ICT and PET of thiazolidine adducts. Preliminary studies show that the probe has low cytotoxicity, is cell-permeable and can be employed for the ratiometric fluorescence imaging of exogenous and endogenous Cys in living cells.