A new oxadiazole-based dual-mode chemosensor: Colorimetric detection of Co2+ and fluorometric detection of Cu2+ with high selectivity and sensitivity

An asymmetrical oxadiazole derivative (Sensor 1 ) containing one pyridine and one formyl connecting to two benzene rings on both sides of oxadiazole unit was designed and synthesized. Sensor 1 displayed selective colorimetric sensing for Co 2+ and Cu 2+ ions via color changes to blue and yellow from colorless in CH 3 CN solutions, respectively. Sensor 1 not only exhibited an instantaneous naked-eye color change and but also could be used as a convenient test kit for recognition and detection of Co 2+ ions. Sensor 1 presented high colorimetric sensitivity for Co 2+ with low detection limit (3.92 × 10 −6  mol/L). Sensor 1 also displayed “On-Off” fluorescent sensing for Cu 2+ over a wide range of competitive metal ions and could accurately detect the existence of Cu 2+ in water samples via the fluorescent quenching and color change. The fluorescence intensity of sensor 1 in CH 3 CN solution showed a linear response to Cu 2+ in the wide concentration range of 0–40 μM which is beneficial for quantitative analysis. The studies on absorbance and fluorescence titration, 1 H NMR spectrum and ESI-MS spectra confirmed that colorimetric sensing for Co 2+ and fluorescent sensing for Cu 2+ are ascribed to the complex formation with the stoichiometry of 1:1 for Co 2+ and 1:2 for Cu 2+ .