Reduced phosphomolybdate as photo-assisted electrochemical crystalline sensor for trace Cr(VI) detection

Sensitive and efficient detection technology for trace toxic hexavalent chromium (Cr(VI)) in water is urgent. Herein, an hourglass-type phosphomolybdate-based metal-organic network was hydrothermally synthesized with formula of [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). Crystal network consists of ladder-like two-dimensional layer-like structure constructed by the vertical connections of 1-D [Na_0.5Cu_5.5(H₂O)₂(btmbp)₄]⁶⁺ metal-organic chains and 1-D inorganic polyanionic chains. Compound 1 exhibits good excellent electrochemical property and wide light absorption that can absorb visible light to accelerate the electron transfer in redox process. When serving as a photo-assisted electrochemical (PAEC) sensor for trace Cr(VI) detection, compound 1 exhibits high sensitivity of 330.5 μA·μM^-1 and low detection limit of 0.95 nM (98.79 ppt) along with high anti-interference ability and excellent PAEC detection stability. These results are superior to most reported POM-based sensors, and even comparable to noble-metal sensors, far satisfying WHO standards for Cr(VI) concentration in drinking water. This work provides a new promising photoelectrochemical sensor material for actual environmental pollutant monitoring.