Ratiometric displacement approach to Cu(II) sensing by fluorescence.

A chelation-enhanced fluorescence method for the detection of paramagnetic copper(II) ions is developed. Two dyes with unequal metal ion binding constants are used, each giving strong fluorescence enhancement in the presence of a diamagnetic reporter ion such as cadmium(II). Upon presentation of copper(II) to a 1:1:1 mixture of the two dyes and cadmium(II), the Cd(II) is displaced from one dye to the other, resulting in quenching of one dye by the Cu(II) and enhancement of the weaker binding dye by complexation of the Cd(II). Although several criteria must be met, this method holds promise for analysis of a wide range of analytes.