Hourglass-type M{P4Mo6}2 as photo-assisted electrochemical sensors for ultrasensitive detection of trace-level Cr(VI).

The development of highly sensitive photo-assisted electrochemical (PAEC) sensors for trace detection of heavy metal ions (such as chromium (VI) [Cr(VI)]) is crucial. Herein, three isomorphic compounds, namely {[M(H2O)2]2[M(HTetp)(H2O)2]2[M(P4Mo6O31H6)2]}·nH2O (M = Ni/Co/Fe, n = 5 for Ni-P4Mo6, n = 1 for Co-P4Mo6 and Fe-P4Mo6, Tetp = 4-[4-(2-Thiophen-2-yl-ethyl)-4H-[1,2,4]triazole-3-yl]-pyridine) are synthesized and directly used as electrode materials for PAEC sensors to detect Cr(VI). The excellent photo-electrochemical properties of the M{P4Mo6} based compounds originate from two key factors: (1) the synergistic combination of polyoxometalates (POMs) clusters and transition metal-organic components (TMCs), which promotes interfacial charge transfer. (2) the unique hourglass-shaped structure of the M{P4Mo6} clusters, providing abundant active sites for Cr(VI) adsorption and reduction. In particular, Ni-P4Mo6 exhibits a high sensitivity of 584.81 μA·μM-1 and an ultra-low LOD of 2.34 nM under 40 W white light, surpassing most reported Cr(VI) sensors. The detection of Cr(VI) in real water samples highlights the potential of the sensor in environmental monitoring.