Investigations into the electrochemical, surface, and electrocatalytic properties of the surface-immobilized polyoxometalate, TBA3K[SiW10O36(PhPO)2]

Organic-Inorganic Hybrid Films of the Sulfate Dawson Polyoxometalate, [S2W18O62]4-, and Polypyrrole for Iodate Electrocatalysis

The Dawson-type sulfate polyoxometalate (POM) [S₂W₁₈O₆₂]^4- has successfully been entrapped in polypyrrole (PPy) films on glassy carbon electrode (GCE) surfaces through pyrrole electropolymerization. Films of varying POM loadings (i.e., thickness) were grown by chronocoulometry. Film-coated electrodes were then characterized using voltammetry, revealing POM surface coverages ranging from 1.9 to 11.7 × 10^-9 mol·cm^-2, and were stable over 100 redox cycles. Typical film morphology and composition were revealed to be porous using atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy, and the effects of this porosity on POM redox activity were probed using AC impedance. The hybrid organic-inorganic films exhibited a good electrocatalytic response toward the reduction of iodate with a sensitivity of 0.769 μA·cm^-2·μM^-1.

Efficient Preparation of a Nonenzymatic Nanoassembly Based on Cobalt-Substituted Polyoxometalate and Polyethylene Imine-Capped Silver Nanoparticles for the Electrochemical Sensing of Carbofuran

The ever-growing exploitation of pesticides and their lethal effects on living beings have made it a dire need of the day to develop an accurate and reliable approach for their monitoring at trace levels. The designing of an enzyme-free electrocatalyst to electrochemically detect the pesticide residues is currently gaining much importance. In this study, a novel redox-sensing film was constructed successfully based on cobalt-substituted Dawson-type polyoxometalate [P₂W₁₇O₆₁ (Co²⁺·OH₂)]^7- (Co-POM) and polyethylene imine (PEI)-capped silver nanoparticles (AgNPs). A nanohybrid assembly was fabricated on a glassy carbon electrode's surface by alternately depositing Co-POM and PEI-AgNPs using the layer-by-layer self-assembly method. The surface morphology of the immobilized CoPOM/AgNP multilayer nanoassembly was analyzed through scanning electron microscopy along with energy-dispersive spectroscopy for elemental analysis. The redox properties and surface morphologies of fabricated assemblies were evaluated by cyclic voltammetry and electrochemical impedance spectroscopy. The practicability and feasibility of the proposed sensing layer was tested for the detection of a highly toxic insecticide, that is, carbofuran. The fabricated sensor exhibited a limit of detection of 0.1 mM with a sensitivity of 13.11 μA mM^-1 for carbofuran. The results depicted that the fabricated nonenzymatic hybrid film showed excellent electrocatalytic efficiency for the carbofuran oxidation. Furthermore, the obtained value of "apparent Km", that is, 0.4 mM, illustrates a good electro-oxidation activity of the sensor for the detection of carbofuran. The exceptionally stable redox activity of Co-POM, high surface area and greater conductivity of AgNPs, and the synergistic effect of all components of the film resulted in an excellent analytical performance of the proposed sensing assembly. This work provides a new direction to the progress and designing of nonenzymatic electrochemical sensors for pesticide determination in real samples.

Covalent Linkage of BODIPY-Photosensitizers to Anderson-Type Polyoxometalates Using CLICK Chemistry

The covalent attachment of molecular photosensitizers (PS) to polyoxometalates (POMs) opens new pathways to PS‐POM dyads for light‐driven charge‐transfer and charge‐storage. Here, we report a synthetic route for the covalent linkage of BODIPY‐dyes to Anderson‐type polyoxomolybdates by using CLICK chemistry (i. e. copper‐catalyzed azide‐alkyne cycloaddition, CuAAC). Photophysical properties of the dyad were investigated by combined experimental and theoretical methods and highlight the role of both sub‐components for the charge‐separation properties. The study demonstrates how CLICK chemistry can be used for the versatile linkage of organic functional units to molecular metal oxide clusters.

Bio-inspired incorporation of phenylalanine enhances ionic selectivity in layer-by-layer deposited polyelectrolyte films

The addition of a common amino acid, phenylalanine, to a Layer-by-Layer (LbL) deposited polyelectrolyte (PE) film on a nanoporous membrane can increase its ionic selectivity over a PE film without the added amino acid. The addition of phenylalanine is inspired by detailed knowledge of the structure of the channelrhodopsins family of protein ion channels, where phenylalanine plays an instrumental role in facilitating sodium ion transport. The normally deposited and crosslinked PE films increase the cationic selectivity of a support membrane in a controllable manner where higher selectivity is achieved with thicker PE coatings, which in turn also increases the ionic resistance of the membrane. The increased ionic selectivity is desired while the increased resistance is not. We show that through incorporation of phenylalanine during the LbL deposition process, in solutions of NaCl with concentrations ranging from 0.1 to 100 mM, the ionic selectivity can be increased independently of the membrane resistance. Specifically, the addition is shown to increase the cationic transference of the PE films from 81.4% to 86.4%, an increase on par with PE films that are nearly triple the thickness while exhibiting much lower resistance compared to the thicker coatings, where the phenylalanine incorporated PE films display an area specific resistance of 1.81 Ω cm2 in 100 mM NaCl while much thicker PE membranes show a higher resistance of 2.75 Ω cm2 in the same 100 mM NaCl solution.

Regulating the catalytic activity of multi-Ru-bridged polyoxometalates based on differential active site environments with six-coordinate geometry and five-coordinate geometry transitions

Five-coordinate geometry around ruthenium with highly exposed active sites has attracted intensive scientific interest due to its superior properties and extensive applications. Herein, we report a series of structurally controllable multi-Ru-bridged polyoxometalates, K5NaH10[{Ru4(H2O)n}(WO2)4(AsW9O33)4]·mH2O {1, 1-dehyd-373K, 1-dehyd-473K, 1-dehyd-573K; n = 4, m = 36; n = 4, m = 6; n = 4, m = 0; n = 0, m = 0} fabricated through a feasible assembly strategy using arsenotungstate {2, KNa12H17Cl2(As4W40O140)·29H2O} as a structure-directing unit. Systematic characterization methods identified that the six-coordinate geometry can successfully transform into five-coordinate geometry about active sites (Ru) by removing aqua ligands under high reaction temperatures. All the multi-Ru-bridged polyoxometalates demonstrated strong stability and catalytic effectiveness in the transformation of 1-(4-chlorophenyl)ethanol to 4'-chloroacetophenone under very mild conditions. 1-dehyd-573K, specifically, achieves the best catalytic effectiveness with a turnover frequency (TOF) = 25 100·h-1 owing to its unique five-coordinate geometry on the Ru sites. To our knowledge, 1-dehyd-573K outperforms other POM-based catalysts in the oxidative catalysis of 1-(4-chlorophenyl)ethanol. The heterogeneous polyoxometalates were also proven to be strongly reusable, with their structural integrities well maintained after multiple-cycle catalytic reactions.

An enhanced antibacterial nanoflowers AgPW@PDA@Nisin constructed from polyoxometalate and nisin

A new composite, AgPW@PDA@Nisin, with shell-core structure was successfully synthesized by a polydopamine (PDA) surfaced conjugated nisin (an antibacterial 34 amino acid polycyclic peptide) as shell and polyoxometalates (Ag₃PW₁₂O₄₀ = AgPW) as core. The composite was characterized by the zeta potential, scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray diffraction analysis (XRD), fourier transform infrared (FT-IR). The AgPW@PDA@Nisin showed flower hierarchical structure and potential antibacterial activity against S. aureus ATCC29213. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of it were 4 and 32 μg/mL. AgPW@PDA@Nisin nanoflowers-induced bacterial death bears the characteristic of cell morphology, membrane integrity and permeability changing, nucleotide leakage. It indicated that the AgPW@PDA@Nisin interfere with the cell membrane, resulting in antibacterial activity against S. aureus. The cytotoxicity of the nanoflowers was low on HDF-a (human dermal fibroblasts) cells. A new class of hybrid inorganic-organic nanoflowers based on polyoxometalates and nisin with enhanced antibacterial properties can be developed for food preservation.

Polyoxometalates on Functional Substrates: Concepts, Synergies, and Future Perspectives

Polyoxometalates (POMs) are molecular metal oxide clusters that feature a broad range of structures and functionalities, making them one of the most versatile classes of inorganic molecular materials. They have attracted widespread attention in homogeneous catalysis. Due to the challenges associated with their aggregation, precipitation, and degradation under operational conditions and to extend their scope of applications, various strategies of depositing POMs on heterogeneous substrates have been developed. Recent ground-breaking developments in the materials chemistry of supported POM composites are summarized and links between molecular-level understanding of POM-support interactions and macroscopic effects including new or optimized reactivities, improved stability, and novel function are established. Current limitations and future challenges in studying these complex composite materials are highlighted, and cutting-edge experimental and theoretical methods that will lead to an improved understanding of synergisms between POM and support material from the molecular through to the nano- and micrometer level are discussed. Future development in this fast-moving field is explored and emerging fields of research in POM heterogenization are identified.

Charge transport through redox active [H7P8W48O184]33- polyoxometalates self-assembled onto gold surfaces and gold nanodots

Polyoxometalates (POMs) are redox-active molecular oxides, which attract growing interest for their integration into nano-devices, such as high-density data storage non-volatile memories. In this work, we investigated the electrostatic deposition of the negatively charged [H7P8W48O184]33- POM onto positively charged 8-amino-1-octanethiol self-assembled monolayers (SAMs) preformed onto gold substrates or onto an array of gold nanodots. The ring-shaped [H7P8W48O184]33- POM was selected as an example of large POMs with high charge storage capacity. To avoid the formation of POM aggregates onto the substrates, which would introduce variability in the local electrical properties, special attention has to be paid to the preformed SAM seeding layer, which should itself be deprived of aggregates. Where necessary, rinsing steps were found to be crucial to eliminate these aggregates and to provide uniformly covered substrates for subsequent POM deposition and electrical characterizations. This especially holds for commercially available gold/glass substrates while these rinsing steps were not essential in the case of template stripped gold of very low roughness. Charge transport through the related molecular junctions and nanodot molecule junctions (NMJs) has been probed by conducting-AFM. We analyzed the current-voltage curves with different models: electron tunneling though the SAMs (Simmons model), transition voltage spectroscopy (TVS) method or molecular single energy level mediated transport (Landauer equation) and we discussed the energetics of the molecular junctions. We concluded to an energy level alignment of the alkyl spacer and POM lowest occupied molecular orbitals (LUMOs), probably due to dipolar effects.

CO oxidation over the polyoxometalate-supported single-atom catalysts M1/POM (Fe, Co, Mn, Ru, Rh, Os, Ir, and Pt; POM = [PW12O40]3–): a computational study on the activation of surface oxygen species

Density functional theory calculations have been carried out to explore the catalytic performance of a series of the M₁/POM (M = Fe, Co, Mn, Ru, Rh, Os, Ir, and Pt; POM = [PW₁₂O₄₀]³⁻) single-atom catalysts for CO oxidation.

Polyelectrolyte layer-by-layer deposition on nanoporous supports for ion selective membranes

This work demonstrates that the ionic selectivity and ionic conductivity of nanoporous membranes can be controlled independently via layer-by-layer (LbL) deposition of polyelectrolytes and subsequent selective cross-linking of these polymer layers. LbL deposition offers a scalable, inexpensive method to tune the ion transport properties of nanoporous membranes by sequentially dip coating layers of cationic polyethyleneimine and anionic poly(acrylic acid) onto polycarbonate membranes. The cationic and anionic polymers are self-assembled through electrostatic and hydrogen bonding interactions and are chemically crosslinked to both change the charge distribution and improve the intermolecular integrity of the deposited films. Both the thickness of the deposited coating and the use of chemical cross-linking agents influence charge transport properties significantly. Increased polyelectrolyte thickness increases the selectivity for cationic transport through the membranes while adding polyelectrolyte films decreases the ionic conductivity compared to an uncoated membrane. Once the nanopores are filled, no additional decrease in conductivity is observed with increasing film thickness and, upon cross-linking, a portion of the lost conductivity is recovered. The cross-linking agent also influences the ionic selectivity of the resulting polyelectrolyte membranes. Increased selectivity for cationic transport occurs when using glutaraldehyde as the cross-linking agent, as expected due to the selective cross-linking of primary amines that decreases the net positive charge. Together, these results inform deposition of chemically robust, highly conductive, ion-selective membranes onto inexpensive porous supports for applications ranging from energy storage to water purification.

This work demonstrates that the ionic selectivity and ionic conductivity of nanoporous membranes can be controlled independently via layer-by-layer (LbL) deposition of polyelectrolytes and subsequent selective cross-linking of these polymer layers.

Polyoxotungstate incorporating organotriphosphonate ligands and lanthanide ions: syntheses, characterization, magnetism and photoluminescence properties

Four structurally intriguing lanthanide-containing organophosphonate-functionalized POTs have been established. The magnetic properties and photoluminescence properties have been researched.

Two New Armtype Polyoxometalates Grafted on Titanium Dioxide Films: Towards Enhanced Photoelectrochemical Performance

Two new carboxyethyltin-functionalized polyoxometalates (POMs) were successfully obtained and confirmed with physicochemical and spectroscopic methods including X-ray crystallography. The lowest unoccupied molecular orbitals of both compounds are higher in energy than that of TiO2 , and the optical band gaps of these compounds are smaller than that of TiO2 . Grafting them onto a TiO2 film created two kinds of novel photoanode materials that showed significantly enhanced photovoltaic and photocurrent responses, as well as improved photoelectrooxidation activities for methanol relative to that shown by a single TiO2 film. Further, P2 W15 -Co-SnR produced the largest photocurrent by exploring the photoelectric activities of a series of carboxyethyltin POM derivatives. This work provides new insight into the photoelectrochemical functionalization of POM-based organic-inorganic hybrids.