Bifunctional Sensors Based on Phosphomolybdates for Detection of Inorganic Hexavalent Chromium and Organic Tetracycline

Exploring effective sensors for detecting possible hazards in a water system are greatly significant. This work proposed a strategy for stable and effective bifunctional sensors via incorporating hourglass-type phosphomolybdates into metal–organic fragments to construct a high-dimensional framework. Two hourglass-type phosphomolybdate-based electrochemical sensors toward heavy metal ion Cr­(VI) and tetracycline (TC) detection were designed with the formula [Co^II₂(H₂O)₄Na^I₂]­[Co^II(Hbpe)]­[Na^I(bpe)_1.5]­{Co^II[P^V₄Mo^V₆O₃₁H₆]₂}·9H₂O (1) and [Co^II(H₂O)₄Na^I₃]­[Co^II(Hbpe)]­[Co^II(bpe)]­{Co^II[P^V₄Mo^V₆O₃₁H₆]₂}·9H₂O (2) [bpe = 1,2-di­(4-pyridyl)­ethylene]. Structural analysis showed that hybrids 1 and 2 possess three-dimensional POM-supported network features with favorable stability and exhibit reversible redox properties. Experiments found that this kind of hybrids as efficient sensors have excellent electrochemical performance toward Cr­(VI) detection with high sensitivities of 0.111 μA·μM^–1 for 1 and 0.141 μA·μM^–1 for 2, fast response time of 1 s, and low detection limits of 30 nM for 1 and 27 nM for 2, which far meet the standard of WHO for drinking water. Moreover, hybrids 1-2 also exhibit fast responses to TC detection with sensitivities of 0.0073 and 0.022 μA·mM^–1 and detection limits of 0.426 and 0.084 mM. This work offers a novel strategy for the purposeful design of efficient POM-based electrochemical sensors for accurate determination of contaminants in a practical water system.