Bridging Component Strategy in Phosphomolybdate-Based Sensors for Electrochemical Determination of Trace Cr(VI)

Rapid and sensitive electrochemical determination of trace carcinogenic Cr(VI) pollutants remains an urgent and important task, which requires the development of active sensing materials. Herein, four cases of reduced phosphomolybdates with formulas of the (H₂bib)₃[Zn(H₂PO₄)]₂{Mn[P₄Mo₆O₃₁H₇]₂}·6H₂O (1), (H₂bib)₂[Na(H₂O)]₂[Mn(H₂O)]₂{Mn[P₄Mo₆O₃₁H₆]₂}·5H₂O (2), (H₂bib)₃[Mo₂(μ₂-O)₂(H₂O)₄]₂{Ni[P₄Mo₆O₃₁H₂]₂}·4H₂O (3), and (H₂bib)₂{Ni[P₄Mo₆O₃₁H₉]₂}·9H₂O (4) (bib = 4,4'-bis(1-imidazolyl)-biphenyl) were hydrothermally synthesized under the guidance of a bridging component strategy, which function as effective electrochemical sensors to detect trace Cr(VI). The difference of hybrids 1-4 is in the inorganic moiety, in which the reduced phosphomolybdates {M[P₄Mo^V₆O₃₁]₂} (M{P₄Mo₆}₂) exhibited different arrangements bridged by different cationic components ({Zn(H₂PO₄)} subunit for 1, [Mn₂(H₂O)₂]⁴⁺ dimer for 2, and [Mo^V₂(μ₂-O)₂(H₂O)₄]⁶⁺ for 3). As a result, hybrids 1 and 3 display noticeable Cr(VI) detection activity with low detection limits of 14.3 nM (1.48 ppb) for 1 and 6.61 nM (0.69 ppb) for 3 and high sensitivities of 97.3 and 95.3 μA·mM^-1, respectively, which are much beyond the World Health Organization's detection threshold (0.05 ppm) and superior to those of the contrast samples (inorganic Mn{P₄Mo₆}₂ salt and hybrid 4), even the most reported noble-metal catalysts. This work supplies a prospective pathway to build effective electrochemical sensors based on phosphomolybdates for environmental pollutant treatment.