Silver-Cluster and Polyoxometalate-Incorporated Metal-Organic Frameworks for Electrochemical Nitrite Sensing and Capacitive Energy Storage

In this work, four new polyoxometalate-based metal-organic frameworks (POMOFs), namely [Ag10HCl2(dtb)8(PMo12O40)3]·2H2O (JOU-42-Mo), [Ag10HCl2(dtb)8(PW12O40)3]·2H2O (JOU-42-W), [Ag4Cl(dtb)2PMo12O40]·2H2O (JOU-43-Mo), and [Ag4(OH)(H2O)2(dtb)2PW12O40] (JOU-43-W) (dtb = 4,4'-(1,4-phenylene)bis(4H-1,2,4-triazole)), constructed from silver clusters and Keggin-type polyoxometalates (POMs), were synthesized via a solvothermal strategy. Single-crystal X-ray diffraction analyses reveal that these compounds consist of two structural families. The JOU-42 series features a three-dimensional (3D) fcu-type framework assembled from decanuclear Ag clusters, generating tetrahedral and octahedral cages for the confinement of POM guests, whereas the JOU-43 series adopts a 2D + 2D → 3D interpenetrated architecture containing one-dimensional channels for POM encapsulation. The combination of redox-active POM clusters and accessible Ag sites endows these POMOFs with enhanced electrochemical sensing performance toward nitrite (NO2-). Among them, JOU-42-Mo exhibits the best NO2- sensing performance, achieving a wide linear detection range of 20-30,000 μmol L-1 and a low detection limit of 3.88 μmol L-1, together with excellent selectivity, reproducibility, and stability. Furthermore, the POMOF electrodes display promising capacitive energy storage characteristics, demonstrating the multifunctional potential of Ag-cluster-based POMOFs.