Fluorescent detection of Zn(II) and In(III) and colorimetric detection of Cu(II) and Co(II) by a versatile chemosensor

A new versatile chemosensor 1 ((Z)-N′-(quinolin-2-ylmethylene)furan-2-carbohydrazide), based on quinoline and furan, has been synthesized. Sensor 1 showed immediate responses toward Zn²⁺ and In³⁺ ions through selective fluorescence enhancement in different solvent conditions, respectively. In addition, sensor 1 could detect Cu²⁺ and Co²⁺ by “naked eye” color changes from colorless to pale yellow in a near-perfect aqueous solution. This observation was the first case that a single sensor can detect the four metal ions. The binding modes of 1 to Zn²⁺, In³⁺, Cu²⁺ and Co²⁺ were verified through Job plots and ESI-mass spectrometry analyses. The limits of detection (0.33 μM, 0.23 μM and 0.06 μM) for Zn²⁺, Cu²⁺ and Co²⁺ were far lower than the World Health Organization (WHO) guidelines (76 μM for Zn²⁺ and 31.5 μM for Cu²⁺) and Department of Environmental Protection (DEP) guideline (1.7 μM for Co²⁺) for drinking water. Moreover, sensor 1 could quantify Zn²⁺ and Co²⁺ in water samples. The sensing mechanisms of Zn²⁺, In³⁺ and Cu²⁺ by sensor 1 were explained by theoretical calculations.