A mitochondria-targeted colorimetric and NIR ratiometric fluorescent probe for biothiols with large Stokes shift based on thiol-chromene click reaction.

In this study, a carbazole-based mitochondria-targeted colorimetric and NIR ratiometric fluorescent probe 1 for biothiols based on the thiol-chromene click reaction was subtly designed and synthesized. Upon interaction with biothiols (Cys, Hcy and GSH), the absorption of 1 shifted from 496 nm to 388 nm, while its fluorescence spectrum shifted from 650 nm to 530 nm. These transformations were accompanied by a visible color change from pink to colorless under visible light and from red to green when observed under a 365 nm UV lamp, which can be attributed to the click reaction of biothiols with the α,β-unsaturated ketone of the chromene moiety, subsequent pyran ring-opening and phenol formation as well as 1,6-elimination of a p-hydroxybenzyl moiety yielding 2. These advancements in 1 have allowed us to ratiometrically detect biothiols with high sensitivity (LODs of 97 nM, 94 nM and 93 nM for Cys, GSH and Hcy, respectively), a large Stokes shift (154 nm) and excellent selectivity. In addition, 1 can target mitochondria and image the fluctuation of intracellular biothiols through fluorescence ratiometry. Furthermore, the novel design strategy of modifying chromene to the N atom of quinoline was proposed for the first time.