Targeted cysteine and glutathione detection in extra/intracellular systems by copper-based fluorescent imaging probes

The three primary biothiols are cysteine (Cys), glutathione (GSH), and homocysteine (Hcy) which plays key functions in a variety of biological processes. Intracellular and extracellular microenvironments are perfectly regulated in the living system, while in case of any fluctuations in the redox, buffer level may lead to several diseases. The unbalance in the extracellular compartment is influenced by the changes in intracellular metabolism and these are correlated with cellular processes such as death, differentiation, and proliferation, in the live system. Therefore, a study was performed to develop copper-based fluorescence probes for the sensitive and selective detection of extracellular and intracellular targets such as cysteine (Cys) and glutathione (GSH) respectively. Conversely, the quenched fluorescence of complex (Probes + Cu) by the coordination of either Cys or GSH, leads to retained Cu2+ ion due to its approximate binding with Cys or GSH. In this review, we have segmented the reported probes according to the fluorophore used, analyzed their photophysical characteristics, and highlighted the advantages and disadvantages of each (limit of detection, mechanism).