A multi-responsive turn-on flurogenic probe to sense Zn2+, Cd2+ and Pb2+: left-right-center emission signal swing

A versatile new fluorogenic Schiff base probe (L) has been synthesized by the reaction of quinoline-2-carbohydrazide (which acts as the chelating site) and 4-dimethylamino cinnamaldehyde (which acts as the signaling unit). L can sense three of the most biologically and environmentally important metal ions, Zn2+, Cd2+ and Pb2+, among various tested metal ions through selective TURN-ON fluorescence responses in physiological pH. Interestingly, L can not only sense Zn2+, Cd2+ and Pb2+ fluorometrically in physiological conditions but can also distinguish one from another by exhibiting individual intrinsic left-right-center TURN-ON emission signal swings. These selective fluorescence responses were explained by a chelation-enhanced fluorescence (CHEF) mechanism. Theoretical calculations were carried out to ascertain the preferred L–metal ion binding mode.