Colorimetric sensing of anions by Cu(II), Co(II), Ni(II) and Zn(II) complexes of naphthoquinone-imidazole hybrid—Influence of complex formation on selectivity and sensing medium

Abstract

The effect of formation of Cu(II), Co(II), Ni(II) and Zn(II) complexes of a napthoquinone-imidazole hybrid anion receptor on selectivity and medium of sensing is investigated and reported. The ligand and its transition metal complexes showed a color change from yellow to red with both fluoride and cyanide ions in neat DMF. The Ni(II) complex displayed an obvious color change from pale yellow to red upon selective binding with fluoride ion, while the Zn(II) complex exhibited similar color change with cyanide ion in 50% aq. DMF, thus increasing the selectivity and solubility in aqueous solution. The UV–vis, fluorescence and 1H NMR spectral studies indicate that the mechanism of anion sensing is thorough formation of H-bond between imidazole NH and the corresponding anion. Electrochemical and theoretical studies are also carried out to substantiate the proposed mechanism. The detection limits by Ni(II) complex (for fluoride) and Zn(II) complex (for cyanide) are determined to be 0.54 and 30nM, which are far lower than the WHO guidelines. Moreover, Ni(II)/Zn(II) complex based the test strips could successfully detect the corresponding anion in real water solutions.