A fluorescence on–off sensor for Cu2+ and its resultant complex as an off–on sensor for Cr3+ in aqueous media

A new coumarin–quinoline based sensor 1 has been synthesized. The paramagnetic Cu2+ ion turned off the fluorescence of sensor 1 with a 1 : 1 binding stoichiometry. The resultant 1–Cu2+ complex could act in situ as an efficient “off–on” fluorescence sensor for the paramagnetic Cr3+ ion triggered by the 1 : 1 replacement of Cu2+ with Cr3+. This selective fluorescence “on–off–on” sensor was used to identify Cu2+ and Cr3+ in living breast cancer MCF-7 cells using confocal fluorescence microscopy, indicating that sensor 1 has a potential application for the selective detection of Cu2+ and Cr3+ in living cells.