Multi-functional photoelectric sensors based on a series of isopolymolybdate-based compounds for detecting different ions

Abstract

Five new polyoxometalate(POM)-based compounds, namely [Cu^II₄(btmc)(ctcm)₄(β-Mo₈O₂₆)]·[β-Mo₈O₂₆]·H₂O (1), [Cu^II₄(mct)₂(ctcm)₂(H₂O)₆(β-Mo₈O₂₆)]·[β-Mo₈O₂₆] (2), [Cu^II(dm4bt)(Mo₃O₁₀)] (3), [Co^II(dm4bt)(Mo₂O₇)] (4) and [Co^II(H₂bdpm)(Mo₂O₇)] (5) (btmc = 1,4-bis(1,2,4-triazol-1-methyl)cyclohexane, mct = 4-(4-methyl-cyclohexylmethyl)-4H-[1,2,4]triazole, ctcm = C-(4-[1,2,4]triazol-4-ylmethyl-cyclohexyl)-methylamine, dm4bt = 2,2′-dimethyl-4,4′-bithiazole, and H₂bdpm = 1,1′-bis(3,5-dimethyl-1H-pyrazolate)methane), were obtained through traditional hydrothermal methods and fully characterized by single-crystal X-ray diffraction analysis and IR spectroscopy. Compounds 1 and 2 show one-dimensional (1D) and two-dimensional (2D) structures, respectively, containing metal–organic chains. Compounds 3–5 are 1D, 1D and 2D structures, respectively, including infinite Mo–O chains. The title compounds can act as bi-functional electrochemical sensors to detect not only NO₂⁻ anions but also Cr(VI) cations, and especially compound 3 shows an excellent detection range and sensitivity for Cr(VI) and NO₂⁻. Compound 3 also can act as a fluorescent sensor for the recognition of Hg²⁺. The fluorescence intensities can be reduced 65% by adding the solution of Hg²⁺ ions. So this series of POM-based compounds can be used as multi-functional photoelectric sensors for electrochemical sensing of NO₂⁻ and Cr(VI) and fluorescence sensing of Hg²⁺. Furthermore, the photocatalytic performances of compounds 1–5 were also studied.