A single colorimetric sensor for multiple targets: the sequential detection of Co²⁺ and cyanide and the selective detection of Cu²⁺ in aqueous solution

A new highly selective and multifunctional chemosensor 1 for the detection of Co²⁺, Cu²⁺ and CN⁻, based on 4-diethylaminosalicyl aldehyde and thiophene-2-carbohydrazide moieties, was designed and synthesized. 1 could simultaneously detect both Co²⁺ and Cu²⁺ by changing its color from colorless to yellow in aqueous solution. The binding modes of 1 to Co²⁺ and Cu²⁺ were determined to be a 2 : 1 complexation stoichiometry through job plot and ESI-mass spectrometry analysis. The detection limits (0.19 μM and 0.13 μM) of 1 for Co²⁺ and Cu²⁺ were lower than the DEP guideline (1.7 μM) of Co²⁺ and the WHO guideline (31.5 μM) of Cu²⁺ for drinking water. Importantly, 1 could detect and quantify Co²⁺ and Cu²⁺ in real water samples. Moreover, the resulting Co²⁺-2·1 complex sensed cyanide through naked-eye, showing recovery from Co²⁺-2·1 to 1. The sensing mechanisms of Cu²⁺ by 1 were explained by theoretical calculations.