Reduced phosphomolybdate as photoassisted electrochemical crystalline sensor for trace Cr(VI) detection

The development of a sensitive and efficient detection technology for trace toxic hexavalent chromium (Cr(VI)) in water is a pressing concern. In this study, an hourglass-type phosphomolybdate-based metal–organic network with the formula [Na_0.5Cu_5.5(H₂O)₂(btmbp)₄][Mn(H₂O)₃]₂{Mn[H₆P₄Mo₆O₃₁]₂}·10H₂O (1, btmbp = 4,4'-bis((1H-1,2,4-triazol-1-yl)methyl)biphenyl) was hydrothermally synthesized. The crystal network consists of a ladder-like two-dimensional layered structure constructed by vertical connections of one-dimensional (1D) [Na_0.5Cu_5.5(H₂O)₂(btmbp)₄]⁶⁺ metal–organic chains and 1D inorganic polyanionic chains. Compound 1 exhibits excellent electrochemical property and a wide light absorption range including visible light to accelerate the electron transfer in redox processes. When serving as a photoassisted electrochemical (PAEC) sensor for trace Cr(VI) detection, compound 1 exhibits a high sensitivity of 330.5 μA·μM⁻¹ and a low detection limit of 0.95 nM (98.79 ppt) along with high anti-interference ability and excellent PAEC detection stability, outperforming most reported polyoxometalate-based sensors and equaling noble-metal sensors, far satisfying World Health Organization standards for Cr(VI) concentration in drinking water. This work provides a new photoelectrochemical sensor material for monitoring environmental pollutants.