A novel fluoro-chromogenic Cu 2+ probe for living-cell imaging based on rhodamine 6G-pyridine conjugation.

A novel fluoro-chromogenic rhodamine spirolactam probe (RP) has been prepared through the condensation of rhodamine hydrazine and 2-acetylpyridine, which displayed the detection of Cu2+ with high selectivity over a large number of other common metal ions. It shows a "turn-on" response to paramagnetic Cu2+ with an about 12-fold enhancement, and a color change from colorless to red that is observable by the naked eye. These changes are ascribed to the ring-opening of the spirolactam in RP, and subsequent host-guest coordination. The 2:1 binding stoichiometry of RP to Cu2+ was confirmed by Job's and B-H plots. The resulting fluorescence enhancement can be used to detect Cu2+ at concentrations from 2.0 to 20.0 μM with a limit of detection of 0.21 μM, which was lower than the maximum allowable Cu2+ level set by the WHO. Finally, RP has been utilized to monitor Cu2+ in living cells and natural water. Graphical abstract.