Nickel(II)-Based Two-Dimensional Coordination Polymer Displaying Superior Capabilities for Selective Sensing of Cr(VI) Ions in Water

Three fluorescent coordination polymers (CPs) including [Zn(ppvppa)(5-NO2-1,3-BDC)(H2O)] (1), [Cd-(ppvppa)(2)(2,5-FDC)]center dot 1.5H(2)O (2), and [Ni(ppvppa)(5-NO2-1,3-BDC)(H2O)]center dot 0.5MeCN (3) (ppvppa = dipyridin-2-yl-[4-(2-pyridin-4-yl-vinyl)-phenyl]-amine; 5-NO2-1,3-H2BDC = 5-nitroisoph-thalic acid and 2,5-H2FDC = 2,5-furandicarboxylic acid) were assembled from solvothermal reactions of Zn(NO3)(2)center dot 6H(2)O, Cd-(NO3)(2)center dot 4H(2)O, or Ni(NO3)(2)center dot 6H(2)O with ppvppa in the presence of 5-NO2-1,3-H2BDC or 2,5-H2FDC. Compound 1 has a one-dimensional (1D) double-stranded chain structure. 2 and 3 have different two-dimensional (2D) grid-like networks formed by either bridging 1D [Cd(ppvppa)(2)](n) chains with 2,5-FDC ligands or linking 1D [Ni-2(ppvppa)(2)](n) chains with 5-NO2-1,3-BDC ligands. The ligand ppvppa and the three CPs could detect Cr2O72- and CrO42- in aqueous solutions by selective fluorescence quenching. The detection limit of the most responsive complex 3 was 1.39 ppb for Cr2O72- (pH = 4) or 0.09 ppb for CrO42- (pH = 12), which are below the allowable limits set by the United States Environmental Protection Agency and European Union and the lowest among the known CPs-based sensors. The mechanism of such an emission quenching process may be ascribed to the overlap between the absorption band of the analyte and those of the excitation and emission bands of 3.