Colorimetric and fluorescent probes for real-time naked eye sensing of copper ion in solution and on paper substrate

In this paper, BT (( E )-2-(4-(4-(bis(pyridin-2-ylmethyl)amino)styryl)-3-cyano-5,5-dimethylfuran-2(5H)-ylidene)malononitrile) with strong donor–π-acceptor structure was synthesized, which showed both colorimetric and fluorescent sensing ability toward Cu 2+ with high selectivity and sensitivity. Job plot and mass spectra measurement revealed a 1 : 1 coordination mode between Cu 2+ and probe BT in ethanol/HEPES (1 : 4 v/v) buffer (pH 7.2) solution, and the binding constant was calculated to be 3.6 × 10 4  M –1 . The colour of BT solution (10 µM) immediately turned from purple red to yellow and the red fluorescence was quenched obviously when a certain amount of Cu 2+ was added, which enabled a dual-channel detection of Cu 2+ . A paper strip pre-stained with BT solution was further fabricated and it also showed excellent sensing ability toward Cu 2+ with a detection limit as low as 10 −6  M with the naked eye, which represents better portability and operation simplicity that is favourable for on-site analysis of Cu 2+ in water.