Cluster of Hexamolybdenum [Mo6Cl14]2- for Sensing Nitroaromatic Compounds

This contribution describes a novel method for the detection of trace amounts of trinitrotoluene (TNT) using a cluster of hexamolybdenum with general formula [Mo₆Cl₁₄]^2-. The molybdenum cluster was characterized by UV-visible, FT-IR, and fluorescence techniques, and the synthesis was efficient and reproducible. The evaluation of the molybdenum cluster by TNT detection was perfomed by fluoresecent measurements, and the results were interpreted by the Stern-Volmer equation, obtaining K _SV values of 2.9 × 10⁵ and 1.6 × 10⁴ M^-1 in different concentration ranges. Further, the results suggest that at TNT concentrations higher than 4 × 10^-5 mM (0.01 mg L^-1) it is possible to measure the quenching of the cluster fluorescence. The DFT calculations indicate that the contribution of the TNT in the active lowest unoccupied molecular orbitals that are involved in the higher intensity transitions in the complex cluster-TNT are significant. This situation differs from all the luminescent [M₆X₈L₆]^2- clusters (M = Mo; X = facial bridging ligand, and L = labile axial ligands), where most of the closely spaced excited states are located in the {M₆X₈} ^q+ core. Thus, the TNT switches off the cluster luminescence. The approach using a [Mo₆Cl₁₄]^2--based fluorescence sensor has the potential to be a sensing technology for the detection of nitroaromatic explosives.

Polyoxometalate-based materials in extraction, and electrochemical and optical detection methods: A review

Polyoxometalates (POMs) as metal-oxide anions have exceptional properties like high negative charges, remarkable redox abilities, unique ligand properties and availability of organic grafting. Moreover, the amenability of POMs to modification with different materials makes them suitable as precursors to further obtain new composites. Due to their unique attributes, POMs and their composites have been utilized as adsorbents, electrodes and catalysts in extraction, and electrochemical and optical detection methods, respectively. A survey of the recent progress and developments of POM-based materials in these methods is therefore desirable, and should be of great interest. In this review article, POM-based materials, their properties as well as their identification methods, and analytical applications as adsorbents, electrodes and catalysts, and corresponding mechanisms of action, where relevant, are reviewed. Some current issues of the utilization of these materials and their future prospects in analytical chemistry are discussed.

Polyoxometalate Functionalized Sensors: A Review

Polyoxometalates (POMs) are a class of metal oxide complexes with a large structural diversity. Effective control of the final chemical and physical properties of POMs could be provided by fine-tuning chemical modifications, such as the inclusion of other metals or non-metal ions. In addition, the nature and type of the counterion can also impact POM properties, like solubility. Besides, POMs may combine with carbon materials as graphene oxide, reduced graphene oxide or carbon nanotubes to enhance electronic conductivity, with noble metal nanoparticles to increase catalytic and functional sites, be introduced into metal-organic frameworks to increase surface area and expose more active sites, and embedded into conducting polymers. The possibility to design POMs to match properties adequate for specific sensing applications turns them into highly desirable chemicals for sensor sensitive layers. This review intends to provide an overview of POM structures used in sensors (electrochemical, optical, and piezoelectric), highlighting their main functional features. Furthermore, this review aims to summarize the reported applications of POMs in sensors for detecting and determining analytes in different matrices, many of them with biochemical and clinical relevance, along with analytical figures of merit and main virtues and problems of such devices. Special emphasis is given to the stability of POMs sensitive layers, detection limits, selectivity, the pH working range and throughput.

Wells-Dawson Arsenotungstate Porous Derivatives for Electrochemical Supercapacitor Electrodes and Electrocatalytically Active Materials

Two Wells-Dawson arsenotungstate coordination polymers, [{Cu^II(bim)₂}₃(As₂W₁₈O₆₂)] (1) and [(Cu^I₁₀pz₁₀Cl₄)(As₂W₁₈O₆₂)] (bim = 2,2'-biimidazole; pz = pyrazine), have been assembled via a hydrothermal method and fully characterized. Compound 1 exhibits a 2,6-connected two-dimensional hybrid layer based on asymmetrically modified {As₂W₁₈} anions and {Cu(bim)₂} linkers, which is extended to a three-dimensional network with a special interlayer structure and a one-dimensional tunnel. Compound 2 is a host-guest framework that consists of a Cu-pz-Cl network with 20-member square rings, 16-member irregular rings, and embedded eight-node {As₂W₁₈} guest molecules. Compounds 1 and 2 show uncommon specific capacitance (834.8 and 960.1 F g^-1, respectively, at a current density of 2.4 A g^-1), enduring cycling stability (capacitance retention rates of 89.3% and 91.9%, respectively, after 5000 cycles), and good electrical conductivity, which are superior to those of the unmodified zero-dimensional Dawson arsenotungstate compound and most reported electrode materials in terms of their stable structure, special layer spacing, and orderly channels. Moreover, the title compounds exhibit excellent electrocatalytic activity for oxidizing ascorbic acid and reducing nitrite.