Fully Reduced Cu^I{P₄Mo₆}₂ Supramolecular Architectures for Ultraselective Photoelectrochemical Detection of Cr(VI)

The sensitive and selective detection of toxic Cr(VI) depends on the development of high-performance photoelectrochemical sensing materials. Herein, two supramolecular architectures of Cu^I{P₄Mo₆}₂-based phosphomolybdates, with formula of (H₂bib)₃Pb{Cu^I[P₄Mo₆O₃₁]₂H₁₅}·10H₂O (1), [Cu^I(bib)]₄[Na₂(H₂O)₇]{Cu^I[P₄Mo₆O₃₁]₂H₁₇}·26H₂O (2) (bib = 4,4'-bis(imidazolyl)bipheny), were synthesized as photoelectrochemical sensors for determining Cr(VI) ion. Compound 1 consisted of one-dimensional Pb²⁺-bridged fully reduced Cu^I{P₄Mo₆}₂ clusters and protonated [H₂bib]²⁺ cations. Compound 2 was composed of Na⁺-capped Cu^I{P₄Mo₆}₂ clusters and [Cu^I(bib)]⁺ chains. Both compounds 1 and 2 displayed excellent electrochemical redox properties, broad spectral absorption capabilities, and rapid photoelectrochemical response. Using as photoelectrochemical sensors, compound 1 showed a high sensitivity of 255.41 μA·μM^-1 and an extremely low detection limit of 0.54 nM (0.056 ppb) toward Cr(VI) detection, and compound 2 presented a sensitivity of 62.53 μA·μM^-1 and detection limit of 3.64 nM (0.38 ppb). Structure-function relationship analysis showed that the one-dimensional framework of Cu^I{P₄Mo₆}₂ clusters effectively promotes the separation and transport of photogenerated charge carriers. Compound 1 also exhibited high anti-interference capability and stability in the photoelectrochemical detection process. This work provides some insights for designing advanced polyoxometalate-based sensor materials for highly efficient monitoring of environmental pollutants.