A highly selective ICT-based fluorescent probe for cysteine sensing and its application in living cell imaging

Two novel fluorescent probes, viz. BPQ1 and BPQ2, were designed, synthesized and characterized based on a new design strategy that relies upon the specific position of the reactive acrylate group to explore selective recognition of cysteine (Cys) in dual reporter mode. Among these two probes, BPQ1 was shown to be selective and sensitive for cysteine over other biothiols e.g. homocysteine and glutathione through a tandem conjugate addition and an intramolecular cyclization reaction. In the presence of cysteine, the reaction of cysteine with BPQ1 results in the cleavage of the 7-position acrylate moiety from BPQ1, thereby opening up the ICT process and leading to about 992-fold enhancement of fluorescence with a detection limit of similar to 47 ppb. In contrast, for probe BPQ2, the acrylate group at the 2-position via an attachment of an additional pi-conjugated phenyl ring of the quinoline does not show any significant fluorescence change under identical conditions. Finally, the probe BPQ1 was successfully applied for cysteine recognition in live cells by using osteosarcoma (MG-63) cells.