An IFE-based lysosome-targeted probe for the differential detection and bioimaging of biothiols

Biothiols (Cys, Hcy, and GSH) are generated by reducing disulfide bonds with the help of proteolysis in lysosomes, and these three biothiols play different roles in preserving the optimal activity of lysosomes. It is well-known that different biothiols have different physiological functions, and abnormal levels of different biothiols can trigger different diseases; thus, discrimination of these three biothiols in lysosomes is essential. Herein, nitrobenzoxadiazole (NBD) is employed and an inner filter effect (IFE) is designed to overcome the inadequate discrimination of biothiols by NBD-based probes. This new probe (Lyso-II-NBD) exhibits almost no fluorescence; upon addition of biothiols, Lyso-II-NBD first splits into Lyso-II-OH with green fluorescence and non-fluorescent S-substituted NBD compounds (NBD-S-Cys/Hcy/GSH) by nucleophilic substitution reaction, then only NBD-S-Cys/Hcy rapidly undergoes intramolecular rearrangement, forming N-substituted compounds (NBD-N-Cys/Hcy) with yellow fluorescence. At this moment, the IFE process occurs between Lyso-II-OH and NBD-N-Cys/Hcy, which makes the green fluorescence of Lyso-II-OH disappear and enhances the yellow fluorescence by 7.4 times for Cys and 37 times for Hcy, while NBD-S-GSH remains unchanged and only the split Lyso-II-OH presents green fluorescence. The co-localization study reveals that Lyso-II-NBD can target lysosomes and discriminate Cys/Hcy and GSH by dual fluorescence channels without interference from each other, indicating that Lyso-II-NBD has good potential in biomedical applications.