An On–Off–On Fluorescence Probe Based on Coumarin for Cu2+, Cysteine, and Histidine Detections

A simple coumarin-based Schiff base (probe L) was successfully developed. It showed strong green fluorescence emission at 527 nm with a 70.3% of fluorescence quantum efficiency (Φ F ). However, after the addition of common metal ions, probe L can only combine with Cu 2+ ions and displayed significant fluorescence quenching of > 96.2% (Φ F = 2.7%) due to the paramagnetic quenching action from Cu 2+ . Conversely, by the coordination action of cysteine (Cys) and histidine (His), the quenching fluorescence of the complex (L-Cu 2+ ) between probe L and Cu 2+ ions was recovered mostly because the Cys and His can usurp Cu 2+ of L-Cu 2+ and led to the liberation of probe L. Based on the fluorescence changes of probe L with the actions of Cu 2+ , Cys, and His, an on–off–on reversible fluorescence probe for sensitive and specific monitoring Cu 2+ , Cys, and His has been prepared. More importantly, the probe L and L-Cu 2+ ensemble can be used, respectively, to test Cu 2+ and Cys/His in live cells and human urine samples with great reliability.