Bisquinoline-based fluorescent cadmium sensors

Rational molecular design afforded fluorescent Cd²⁺ sensors based on bisquinoline derivatives. Introduction of three methoxy groups at the 5,6,7-positions of the quinoline rings of BQDMEN (N,N'-bis(2-quinolylmethyl)-N,N'-dimethylethylenediamine) resulted in the reversal of metal ion selectivity in fluorescence enhancement from zinc to cadmium. Introduction of bulky alkyl groups and an N,N-bis(2-quinolylmethyl)amine structure, as well as replacement of one of the two tertiary amine binding sites with an oxygen atom and the use of a 1,2-phenylene backbone significantly improved the Cd²⁺ specificity. The fluorescent cadmium ion selectivity could be explained by the differential binding with Cd²⁺ and Zn²⁺, and the formation of a bis(μ-chloro) dinuclear cadmium complex in contrast to the mononuclear zinc complex.