Environmentally benign polyoxometalate materials

Nicotinate-Modified Lanthanide-Substituted Selenotungstate and Its Catalase-like Activity for the Detection of H2O2 and Ascorbic Acid

A rigid nicotinate-modified lanthanide-substituted selenotungstate [H2N(CH3)2]6Na3H[La4SeW8(H2O)16(nica)2O28][SeW9O33]2·32H2O (1, Hnica = nicotinic acid) was synthesized and consists of two trivacant Keggin [B-α-SeW9O33]8- fragments and one unusual [SeW4O18]8- fragment bridged by a heterometallic [La4W4(H2O)16(nica)2O28]18- cluster. In the heterometallic cluster, two carboxyl O atoms in two nicotinate ligands directly coordinate with two W atoms in a stable C-O-W-O-W-O six-membered ring fashion. According to its catalase-like activity, 1 was utilized to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H2O2 to produce blue oxidized TMB (ox-TMB), which can be used to establish a colorimetric sensing method for the detection of ascorbic acid. This work not only provides a promising platform for detecting H2O2 and ascorbic acid but also expands the application potential of polyoxometalate-based materials in biological and clinical analyses.

High-Selectivity Tandem Photocatalytic Methanation of CO2 by Lacunary Polyoxometalates-Stabilized *CO Intermediate

Stabilizing specific intermediates to produce CH4 remains a main challenge in solar-driven CO2 reduction. Herein, g-C3N4 is modified with saturated and lacunary phosphotungstates (PWx, x=12, 11, 9) to tailor the CO2 reduction pathway to yield CH4 in high selectivity. Increased lacuna of phosphotungstates leads to higher CH4 yield and selectivity, with a superior CH4 selectivity of 80 % and 40.8 μmol ⋅ g-1 ⋅ h-1 evolution rate for PW9/g-C3N4. Conversely, g-C3N4 and PWx alone show negligible CH4 production. The conversion of CO2 to CH4 follows a tandem catalytic process. CO2 is initially activated on g-C3N4 to form *CO intermediates, meanwhile photogenerated electrons derived from g-C3N4 transfer to PWx. Then the reduced PWx captures *CO, which is subsquently hydrogenated to CH4. With the injection of two photogenerated electrons, PW9 is capable of adsorbing and activating *CO. However, the reduced PW12 and PW11 are incapable of adsorbing *CO due to the small energy of occupied molecular orbitals, which is the reason for the poorer activity of PWx/g-C3N4 (x=12, 11) compared with that of PW9/g-C3N4. This work provides new insights to regulate highly selective CO2 photoreduction to CH4 by utilizing lacuna of polyoxometalates to enhance the interaction of metals in polyoxometalates with key intermediates.

Polyoxometalates and their composites for antimicrobial applications: Advances, mechanisms and future prospects

The overuse of antibiotics can lead to the development of antibiotic-resistant bacteria, which can be even more difficult to treat and pose an even greater threat to public health. In order to address the issue of antibiotic-resistant bacteria, researchers currently are exploring alternative methods of sterilization that are both effective and sustainable. Polyoxometalates (POMs), as emerging transition metal oxide compounds, exhibit significant potential in various applications due to their remarkable tunable physical and chemical performance, especially in antibacterial fields. They constitute a diverse family of inorganic clusters, characterized by a wide array of composition, structures and charges. Presently, several studies indicated that POM-based composites have garnered extensive attention in the realms of the antibacterial field and may become promising materials for future medical applications. Moreover, this review will focus on exploring the antibacterial properties and mechanisms of different kinds of organic-inorganic hybrid POMs, POM-based composites, films and hydrogels with substantial bioactivity, while POM-based composites have the dual advantages of POMs and other materials. Additionally, the potential antimicrobial mechanisms have also been discussed, mainly encompassing cell wall/membrane disruption, intracellular material leakage, heightened intracellular reactive oxygen species (ROS) levels, and depletion of glutathione (GSH). These findings open up exciting possibilities for POMs as exemplary materials in the antibacterial arena and expand their prospective applications.

Laser-Induced Preparation of Anderson-Type Polyoxometalate-Derived Sulfide/Oxide Electrocatalysts for Electrochemical Water Oxidation

Developing cost-effective and high-active electrocatalysts is vital to enhance the electrocatalytic performance for oxygen evolution reaction (OER). However, traditional pyrolysis methods require complicated procedures, exact temperatures, and long reaction times, leading to high costs and low yields of electrocatalysts in potential industrial applications. Herein, a rapid and economic laser-induced preparation strategy is proposed to synthesize three bimetallic sulfide/oxide composites (MMoOS, M=Fe, Co, and Ni) on a nickel foam (NF) substrate. A focused CO2 laser with high energy is applied to decompose Anderson-type polyoxometalate (POM)-based precursors, enabling the creation of abundant heteropore and defective structures in the MMoOS composites that have multi-components of MS/Mo4O11/MoS2. Remarkably, owing to the structural interactions between the active species, FeMoOS shows superior electrocatalytic performance for OER in an alkaline medium, exhibiting a low overpotential of 240 mV at 50 mA cm-2, a small Tafel slope of 79 mV dec-1, and good durability for 80 h. Physical characterizations after OER imply that partially dissolved Mo-based species and new-formed NiO/NiOOH can effectively uncover abundant active sites, fasten charge transfer, and modify defective structures. This work provides a rapid laser-induced irradiation method for the synthesis of POM-derived nanocomposites as promoted electrocatalysts.

Detailed Study on the Interfacial Interaction between Different Polyoxometalates and Tetronic Block Copolymers Exploring the Langmuir-Blodgett Technique

Polyoxometalates (POMs) interact with various biologically relevant entities. A basic understanding of this interaction is very important for various applications in the biological field. In this work, the focus is on the study of the interaction between tetronics and Keggin POMs. T701 and T90R4 are the two tetronics considered here; they have different solubilities in water due to different PPO/PEO ratios. The arrangement of PPO and PEO is also different with respect to the central ethylenediamine groups. Three different Keggin-type POMs, phosphomolybdic acid (PMA), phosphotungstic acid (PTA), and silicotungstic acid (STA), with different charge densities are chosen for an elaborate investigation using Langmuir-Blodgett technique. The observation is analyzed thoroughly, which shows both electrostatic interaction and adsorption of POMs on the PPO blocks of the tetronics due to the chaotropic effect, which is responsible for the binding of POMs (in subphase) with the tetronic monolayer. This interaction results in an expanded yet rigid monolayer for POM-tetronic association on the surface. Surface pressure vs mean molecular area isotherm is the key characterization to reach the conclusion. UV-vis spectroscopy, NMR, ITC, ellipsometric studies, FTIR, and SEM also serve as supportive characterization techniques.

Vanadium-Silsesquioxane Nanocages as Heterogeneous Catalysts for Synthesis of Quinazolinones

The functionalization of polyoxovanadate clusters is promising but of great challenge due to the versatile coordination geometry and oxidation state of vanadium. Here, two unprecedented silsesquioxane ligand-protected "fully reduced" polyoxovanadate clusters were fabricated via a facial solvothermal methodology. The initial mixture of the two polyoxovanadate clusters with different colors and morphologies (green plate V14 and blue block V6) was successfully separated as pure phases by meticulously controlling the assembly conditions. Therein, the V14 cluster is the highest-nuclearity V-silsesquioxane cluster to date. Moreover, the transformation from a dimeric silsesquioxane ligand-protected V14 cluster to a cyclic hexameric silsesquioxane ligand-protected V6 cluster was also achieved, and the possible mechanism termed "ligand-condensation-involved dissociation reassembly" was proposed to explain this intricate conversion process. In addition, the robust V6 cluster was served as a heterogeneous catalyst for the synthesis of important heterocyclic compounds, quinazolinones, starting from 2-aminobenzamide and aldehydes. The V6 cluster exhibits high activity and selectivity to access pure quinazolinones under mild conditions, where the high selectivity was attributed to the confinement effect of the macrocyclic silsesquioxane ligand constraining the molecular freedom of the reaction species. The stability and recyclability as well as the tolerance of a wide scope of aldehyde substrates endow the V6 cluster with a superior performance and appreciable potential in catalytic applications.

Role of Metal-Organic Framework Topology on Thermodynamics of Polyoxometalate Encapsulation

Polyoxometalates (POMs) are discrete anionic clusters whose rich redox properties, strong Bro̷nsted acidity, and high availability of active sites make them potent catalysts for oxidation reactions. Metal-organic frameworks (MOFs) have emerged as tunable, porous platforms to immobilize POMs, thus increasing their solution stability and catalytic activity. While POM@MOF composite materials have been widely used for a variety of applications, little is known about the thermodynamics of the encapsulation process. Here, we utilize an up-and-coming technique in the field of heterogeneous materials, isothermal titration calorimetry (ITC), to obtain full thermodynamic profiles (ΔH, ΔS, ΔG, and Ka) of POM binding. Six different 8-connected hexanuclear Zr-MOFs were investigated to determine the impact of MOF topology (csq, scu, and the) on POM encapsulation thermodynamics.

Selective direct oxidation of 1-butanol into acetal using hydrogen peroxide and Cs5MPW11(H2O)O39 (M=Fe, Co, Cu) catalysts

Direct catalytic oxidation of alcohols to acetals in one step process is very attractive, because the two steps process leads firstly to aldehydes, which are unstable species serving as reactive intermediates to generate several by-products reducing process efficiency and selectivity. In this work, a new selective catalytic acetalization of 1-butanol into 1-1-dibutoxybutane acetal using H2O2 in one step process is investigated using Keggin-type polyoxometalates catalysts. The materials developed consisted of new lacunary phophotungstate salts Cs5MPW11(H2O)O39 (M=Fe, Co and Cu) which were prepared by inorganic solution condensation method and characterized using XRD, IR, SEM and EDX analysis to verify their structure, surface morphology and chemical composition. Cs5CuPW11(H2O)O39 catalyst allowed the highest performance for the oxidation of 1-butanol at 60 °C using H2O2 excess with a yield approaching 92 % and a turnover number of 784. Such activity is related to a bifunctional behavior of the catalyst as superacid and redox operating system and the synergistic effect created between the [PW11O39]7- Keggin framework, Cs+ and Cu2+.

Critical Conditions Regulating the Gelation in Macroionic Cluster Solutions

The critical gelation conditions observed in dilute aqueous solutions of multiple nanoscale uranyl peroxide molecular clusters are reported, in the presence of multivalent cations. This gelation is dominantly driven by counterion-mediated attraction. The gelation areas in the corresponding phase diagrams all appear in similar locations, with a characteristic triangle shape outlining three critical boundary conditions, corresponding to the critical cluster concentration, cation/cluster ratio, and the degree of counterion association with increasing cluster concentration. These interesting phrasal observations reveal general conditions for gelation driven by electrostatic interactions in hydrophilic macroionic solutions.

A density functional study of the photocatalytic degradation of polycaprolactone by the decatungstate anion in acetonitrile solution

A recent experimental study has reported that decatungstate [W10O32]4- can degrade various polyesters in the presence of light and molecular oxygen [Li et al., Nanoscale, 2023, 15, 15038]. We apply density functional theory to the photocatalyst-polycaprolactone model complex in acetonitrile solution and elucidate the degradation mechanisms and catalytic cycle. We consider hydrogen atom transfer (HAT) and single electron transfer (SET) mechanisms. The potential energy profiles show that the former proceeds exergonically in a single step but that the latter involves a subsequent proton transfer and finally yields HAT products as well. Oxygenated polymer species can regain the transferred hydrogen and regenerate the reduced photocatalyst. We propose a photocatalytic cycle that realizes both the photocatalyst regeneration and the polymer degradation.

Polyoxometalate-Guanosine Monophosphate Hydrogels with Haloperoxidase-like Activity for Antibacterial Performance

Haloperoxidases represent an important class of enzymes that nature adopts as a defense mechanism to combat the colonial buildup of microorganisms on surfaces, commonly known as biofouling. Subsequently, there has been tremendous focus on the development of artificial haloperoxidase mimics that can catalyze the oxidation of X- (halide ion) in the presence of H2O2 to form HOX. The natural intermediate HOX disrupts the bacterial quorum sensing, thus preventing biofilm formation. Herein, we report a simple method for the formation of supramolecular hydrogels through the self-assembly of Keggin-structured polyoxometalates, phosphotungstic acid, and silicotungstic acid with the small biomolecule guanosine monophosphate (GMP) in an aqueous medium. The polyoxometalate-GMP hydrogels that contained highly entangled nanofibers were mechanically robust and showed thixotropic properties. The gelation of the polyoxometalates with GMP not only rendered manifold enhancement in biocompatibility but also the fibril network in the hydrogel provided high water wettability and the polyoxometalates acted as an efficient haloperoxidase mimic to trigger oxidative iodination, as demonstrated by a haloperoxidase assay. The antifouling activity of the phosphotungstic acid-GMP hydrogel was demonstrated against both Gram-positive and Gram-negative bacteria, which showed enhanced antibacterial performance of the hydrogel as compared to the polyoxometalate alone. We envision that the polyoxometalate-GMP hydrogels may facilitate mechanically robust coatings in a simple pathway that can be useful for antifouling applications.

Fabrication of next-generation multifunctional LBG-s-AgNPs@ g-C3N4 NS hybrid nanostructures for environmental applications

Toxic industrial wastes and microbial pathogens in water pose a continuous threat to aquatic life as well as alarming situations for humans. Developing advanced materials with an environmentally friendly approach is always preferable for heterogeneous visible light photocatalysis. As a green reducing tool, LBG-s-AgNPs@ g-C3N4 NS hybrid nanostructures were anchored onto graphitic carbon nitride (g-C3N4) using an environmentally friendly approach of anchoring/decorating AgNPs onto g-C3N4. With the help of advanced techniques, the fabricated hybrid nanostructures were characterized. Using a sheet like matrix of g-C3N4, nanosized and well-defined uniform AgNPs displayed good antibacterial activity as well as superior photodegradation of hazardous dyes, including methylene blue (MB) and Rhodamine B (RhB). Based on the disc diffusion method, three pathogenic microorganisms of clinical significance can be identified by showing the magnitude of their susceptibility. As a result, the following antimicrobial potency was obtained: E. coli ≥ M. luteus ≥ S. aureus. In this study, green synthesized (biogenic) AgNPs decorated with g-C3N4 were found to be more potent antimicrobials than traditional AgNPs. Under visible light irradiation, LBG-s-AgNPs@g-C3N4 NS (0.01 M) demonstrated superior photocatalytic performance: ∼100% RhB degradation and ∼99% of MB degradation in 160 min. LBG-s-AgNPs@g-C3N4 NS showed the highest kinetic rate, 3.44 × 10-2 min-1, which is 27.74 times for the control activity in case of MB dye. While in case of RhB dye LBG-s-AgNPs@g-C3N4 NS showed the highest kinetic rate, 2.26 × 10-2 min-1, which is 17.51 times for the control activity. Due to the surface plasmon resonance (SPR) and reduction in recombination of the electrons and holes generated during photocatalysis, anchoring AgNPs to g-C3N4 further enhanced the photocatalytic degradation of dyes. Using this photocatalyst, hazardous dyes can be efficiently and rapidly degraded, allowing it to be applied for wastewater treatment contaminated with dyes. It also showed remarkable antimicrobial activity towards Gram-ve/Gram + ve pathogens.

Evaluation of Phosphopolyoxometalates with Mixed Addenda (Mo, W, V) as Corrosion Inhibitors for Steels

Nowadays, choosing a corrosion inhibitor is not only based on efficiency, but must also consider the toxicity of the compound, the impact on the environment, and, obviously, the regulations in the field. In the last two decades, a special class of substances has begun to be studied, namely polyoxometalates (POMs). Their electronic properties and redox characteristics make the polyoxometalates potential candidates to be used in many electrochemical processes, and as potential corrosion inhibitors. Electrochemical methods such as a Tafel extrapolation plot, chronopotentiometry, or gravimetry have been used to establish the capacity of corrosion inhibition of S235 and SS304 steels in the presence of phosphovanadomolibdate acid (@PMoV) and phosphovanadotungstate acid (@PWV) in 0.5 M sulphuric acid solution. The inhibition efficiency for S235 steel is about 90.6% for @PMoV, and 69.5% for @PWV after 24 h of immersion. In the case of SS304 steel, polyoxometalates have similar effects: the inhibition degree, as a function of Flade potential, is 4.66 for @PMoV; better than 3.26 for @PWV, with both proving the passivant effect.

Heterogeneous Transformation of Ginsenoside Rb1 with Ethanol Using Heteropolyacid-Loaded Mesoporous Silica and Identification by HPLC-MS

Rare ginsenosides with major pharmacological effects are barely present in natural ginseng and are required to be obtained by transformation. In the current study, ginsenoside Rb1 was chemically transformed with the involvement of ethanol molecules to prepare rare ginsenosides using the synthesized heterogeneous catalyst 12-HPW@MeSi. A total of 16 transformation products were obtained and identified using high-performance liquid chromatography coupled with multistage tandem mass spectrometry and high-resolution mass spectrometry. Ethanol molecules were involved in the production of 6 transformation products by adding to the C-20(21), C-20(22), or C-24(25) double bonds on the aglycone to produce ethoxyl groups at the C-25 and C-20 positions. Transformation pathways of ginsenoside Rb1 are summarized, which involve deglycosylation, elimination, cycloaddition, epimerization, and addition reactions. In addition, 12-HPW@MeSi was recyclable through a simple centrifugation, maintaining an 85.1% conversion rate of Rb1 after 3 cycles. This work opens up an efficient and recycled process for the preparation of rare ginsenosides with the involvement of organic molecules.

Recent advances in oxidative catalytic applications of polyoxovanadate-based inorganic-organic hybrids

Polyoxovanadates (POVs) have received widespread attention in catalytic applications due to their various structures and remarkable redox properties. By introducing a second transition metal, POV-based inorganic-organic hybrid (POVH) catalysts show increasing stability and more catalytic active sites compared with pure POVs. In this perspective article, POVH materials as oxidative catalysts have been classified into two main categories according to the interactions between transition metal-complex units and POV clusters: (i) hybrids with metal-organic units act as isolated cations and (ii) hybrids with an organic ligand coordinate to the second transition metal, which is further linked to a POV cluster via oxygen bridges directly or indirectly to give zero-, one-, two- or three-dimensional supramolecular structures. The oxidative conversion of organic compounds, including thiophene derivatives, thioethers, alkanes, alcohols, and alkenes, and oxidative detoxification of a sulfur mustard simulant or degradation of lignin, along with the oxidative photo/electrocatalytic transformation of organic compounds catalyzed by POVH materials, are discussed in detail. Furthermore, the challenges and prospects toward the development of POVH catalysts are explored briefly from our perspectives.

K6[P2Mo18O62] as DNase-Mimetic Artificial Nucleases to Promote Extracellular Deoxyribonucleic Acid Degradation in Bacterial Biofilms

In the current study, the DNase-like activity of the Dawson-type polyoxometalate K6[P2Mo18O62] was explored. The obtained findings demonstrated that K6[P2Mo18O62] could effectively cleave phosphoester bonds in the DNA model substrate (4-nitrophenyl phosphate) and result in the degradation of plasmid DNA. Moreover, the application potential of this Dawson-type polyoxometalate as a DNase-mimetic artificial enzyme to degrade extracellular DNA (eDNA) in Escherichia coli (E. coli) bacterial biofilm was explored. The results demonstrated that K6[P2Mo18O62] exhibited high cleavage ability toward eDNA secreted by E. coli and thus eradicated the bacterial biofilm. In conclusion, Dawson-type polyoxometalate K6[P2Mo18O62] possessed desirable DNase-like activity, which could serve as a bacterial biofilm eradication agent by cleaving and degrading eDNA molecules.

[α-AsW9O33]9- bridged hexagonal clusters of Ln(III) showing field induced SMM behavior: experimental and theoretical insight

Polyoxometalates (POM), as inorganic polydentate oxygen donors, provide binding opportunities for oxophilic lanthanide metal centers to construct novel Ln-substituted POM materials with exciting structures and attractive properties. Herein, we have reported four arsenotungstate [α-AsW₉O₃₃]^9- based lanthanide-containing polyoxometalates [Cs_xK_36-x{Ln₆(H₂O)₁₂(α-AsW₉O₃₃)₆}]·yH₂O (Ln = Er (1), Gd (2), Ho (3), and Tb (4)), which are synthesized in an alkaline medium. Complexes 1-3 are the dimeric structures of [Ln₃(H₂O)₆(α-AsW₉O₃₃)₃]^18- polyanions, whereas complex 4 is a hexamer of the polyanion [Tb (H₂O)₂(α-AsW₉O₃₃)]^6- as a building unit. In all the complexes, [α-AsW₉O₃₃]^9- units are staggered up and down and give rise to the chair conformation, where one [α-AsW₉O₃₃]^9- unit bridges two Ln(III) centers through four μ₂-oxygen and two terminal oxygen atoms, resulting in the hexagonal arrangement of lanthanides. The dynamic magnetic measurement indicates that only complex 1 exhibits slow relaxation of magnetization with an applied dc field (1500 Oe). To gain insight into the slow relaxation of magnetization in complex 1, the ligand-field parameters and the splitting of the ground-state multiplet of the Er(III) ions have been estimated. The ab initio calculation results confirm that the ground state wave function of these molecules (1, 3, and 4) is mainly composed of a mixture of m_J states, and the non-axial crystal field (CF) terms are more predominant than the axial CF term. The solid-state fluorescence spectra of 1-4 reveal that the photoexcitation O → M ligand-to-metal charge-transfer (LMCT) of arsenotungstate fragments is effectively quenched due to the spatial coordination environment around the Ln(III) ion.

Novel morphological mono-metallic substituted polyoxometalate immobilized 3-(aminopropyl)-imidazole photocatalysts for visible-light driven degradation: Anti-bacterial activity, membrane bacterial activity applications

A novel keggin-type tetra-metalates substituted polyoxometalate was functionalized by 3-(aminopropyl)-imidazole (3-API) supporting a ligand substitution method. In this paper, polyoxometalate (POMs) (NH₄)₃ [PMo₁₂O₄₀] and transition metal substituted of (NH₄)₃ [{PM^IVMo₁₁O₄₀}.(H₂O)] (M = Mn, V) are used as one of the adsorbents. The 3-API/POMs hybrid have been synthesized and used as adsorbent for the photo-catalysis of azo-dye molecule degradation after visible-light illumination as a simulated organic contaminant in water. The transition metal (M = M^IV, V^IV) substituted keggin-type anions (MPOMs) were synthesized, which reveals the degradation of methyl orange (MO) of about 94.0 % and 88.6 %. Immobilizing high redox ability POMs as an efficient acceptor of photo generated electron, on metal 3-API. In the presence of visible light irradiation result reveals that 3-API/POMs (89.9 %) have incredibly achieved after certain irradiation time and at specific conditions (3)-API/POMs; photo-catalysts dose = 5mg/100 ml, pH = 3 and MO dye concentration = 5 ppm). As the surface of POM catalyst has strong absorption of azo-dye MO molecule engaged as a molecular exploration through photo catalytic reactant. From the SEM images it is clear that the synthesized POMs based materials and POMs conjugated MO have varieties of morphological changes observed such as flakes, rods and spherical like structures. Anti-bacterial study reveals that the process of targeted microorganism occur higher activity against pathogenic bacterium for 180 min of visible-light irradiation is measured in terms of zone of the inhibition. Furthermore, the photo catalytic degradation mechanism of MO using POM, metaled POMs and 3-API/POMs also has been discussed.

Epoxidation of Fatty Acid Methyl Esters with Hydrogen Peroxide Catalyzed by Peroxopolyoxotungstate PW4 Encapsulated in the MIL-100(Cr) Framework

The MIL-100(Cr), PW12@MIL-100(Cr) and PW4@MIL-100(Cr) catalysts were prepared and characterized through XRD, FTIR, BET, SEM, EDS and Raman spectroscopy. A comparison of the catalytic properties of the synthesized materials in the epoxidation of FAMEs with hydrogen peroxide was made. The PW4@MIL-100(Cr) catalyst exhibited the highest catalytic activity and provided a high selectivity for the formation of epoxides. The effects of the reaction temperature, catalyst loading, reaction time and FAME:hydrogen peroxide molar ratio on the reaction performance were investigated, and the optimal process conditions were determined. An epoxide yield of 73% with a selectivity of 77% could be obtained using PW4@MIL-100(Cr) after 4 h at 40 °C. The catalytic stability test showed that PW4@MIL-100(Cr) could be easily separated and reused without any treatment for at least five consecutive cycles without a loss of activity or selectivity.

Boosting Simultaneous Uranium Decorporation and Reactive Oxygen Species Scavenging Efficiency by Lacunary Polyoxometalates

The chemical toxicity and the oxidative stress induced by the internal exposure of uranium is responsible for the long-term adverse effect of in vivo contamination of uranium. An agent with simultaneous removal capability of uranium and excess reactive oxygen species (ROS) is highly desired. Herein, the lacunary Keggin-type polyoxometalate (POM) is demonstrated to selectively bind with uranyl ions in the presence of excess essential divalent ions and exhibits a compelling ROS scavenging efficiency of 78.8%. In vivo uranium decorporation assays illustrate the uranium sequestration efficiencies of 74.0%, 49.4%, and 37.1% from kidneys by prophylactic, prompt, and delayed administration of lacunary POM solution, respectively. The superior ROS quenching and uranium removal performance in comparison with all reported bifunctional agents endow lacunary polyoxometalates as novel agents to effectively protect people from injuries caused by the internal exposure of actinides.

Synthesis and Characterization of Silver and Graphene Nanocomposites and Their Antimicrobial and Photocatalytic Potentials

Microbial pathogens and bulk amounts of industrial toxic wastes in water are an alarming situation to humans and a continuous threat to aquatic life. In this study, multifunctional silver and graphene nanocomposites (Ag)_1−x(GNPs)_x [25% (x = 0.25), 50% (x = 0.50) and 75% (x = 0.75) of GNPs] were synthesized via ex situ approach. Further, the synthesized nanocomposites were explored for their physicochemical characteristics, such as vibrational modes (Raman spectroscopic analysis), optical properties (UV visible spectroscopic analysis), antibacterial and photocatalytic applications. We investigated the antimicrobial activity of silver and graphene nanocomposites (Ag-GNPs), and the results showed that Ag-GNPs nanocomposites exhibit remarkably improved antimicrobial activity (28.78% (E. coli), 31.34% (S. aureus) and 30.31% (P. aeruginosa) growth inhibition, which might be due to increase in surface area of silver nanoparticles (AgNPs)). Furthermore, we investigated the photocatalytic activity of silver (AgNPs) and graphene (GNPs) nanocomposites in varying ratios. Interestingly, the Ag-GNPs nanocomposites show improved photocatalytic activity (78.55% degradation) as compared to AgNPs (54.35%), which can be an effective candidate for removing the toxicity of dyes. Hence, it is emphatically concluded that Ag-GNPs hold very active behavior towards the decolorization of dyes and could be a potential candidate for the treatment of wastewater and possible pathogenic control over microbes. In the future, we also recommend different other in vitro biological and environmental applications of silver and graphene nanocomposites.

Recent Advances in Polyoxometalates with Enzyme-like Characteristics for Analytical Applications

Artificial enzymes based on inorganic solids with both enzyme-mimetic activities and the special material features has been a promising candidate to overcome many deleterious effects of native enzymes in analytical applications. Polyoxometalates (POMs) are an importance class of molecular metal-oxygen anionic clusters. Their outstanding physicochemical properties, versatility and potential applications in energy conversion, magnetism, catalysis, molecular electronics and biomedicine have long been studied. However, the analytical applications of them is limited. Recently, the intrinsic enzymatic activities of POMs have also been found and become an area of growing interest. In this review, along with other reports, we aimed to classify the enzymatic activity of POMs, summarize the construction of POMs-based enzymes, and survey their recent advances in analytical fields. Finally, the current challenges and trends of the polyoxometalates with enzymatic activity in future chemo-/bio-sensing applications are briefly discussed.

Immobilization of Polyoxometalates on Carbon Nanotubes: Tuning Catalyst Activity, Selectivity and Stability in H2O2-Based Oxidations

In recent years, carbon nanotubes (CNTs), including N-doped ones (N-CNTs), have received significant attention as supports for the construction of heterogeneous catalysts. In this work, we summarize our progress in the application of (N)-CNTs for immobilization of anionic metal-oxygen clusters or polyoxometalates (POMs) and use of (N)-CNTs-supported POM as catalysts for liquid-phase selective oxidation of organic compounds with the green oxidant–aqueous hydrogen peroxide. We discuss here the main factors, which favor adsorption of POMs on (N)-CNTs and ensure a quasi-molecular dispersion of POM on the surface and their strong attachment to the support. The effects of the POM nature, N-doping of CNTs, acid additives, and other factors on the POM immobilization process and catalytic activity/selectivity of the (N)-CNTs-immobilized POMs are analyzed. Particular attention is drawn to the critical issue of the catalyst stability and reusability. The scope and limitations of the POM/(N)-CNTs catalysts in H2O2-based selective oxidations are discussed.

Double Trigonal Pyramidal {SeO3} Groups Bridged 2-Picolinic Acid Modified Cerium-Inlaid Polyoxometalate Including Mixed Selenotungstate Subunits for Electrochemically Sensing Ochratoxin A

An organic-inorganic hybrid trigonal pyramidal {SeO3} group, bridged cerium-inlaid polyoxometalate (POM) Na16[Se2Ce4(H2O)8W4(HPIC)4O10][B-β-SeW8O30]2[Se2W12O46]2·60H2O (1) (HPIC = 2-picolinic acid), containing two disparate selenotungstate (ST) building blocks was synthesized by a one-step assembly strategy, which is established by two asymmetric sandwich-type {[Ce2(H2O)4W2(HPIC)2O4][B-β-SeW8O30][Se2W12O46]}10- moieties joined by double trigonal pyramidal {SeO3} groups. Its outstanding structural trait is that it contains two types of ST building blocks, Keggin-type [B-β-SeW8O30]8- and Dawson-like [Se2W12O46]12-, which are extremely rare in ST chemistry. Remarkably, [Se2W12O46]12- is first obtained in lanthanide-inlaid STs. Furthermore, 1@PPy conductive film (PPy = polypyrrole) was prepared by electrochemical polymerization and served as the electrode material, and then nano-gold particles (NGPs) were deposited on the surface of 1@PPy conductive film by an electrochemical deposition method in order to immobilize the aptamer of ochratoxin A. With the help of exonuclease I (EN I), the oxidation peak of the metalized Ag works as the detection signal to achieve the detection of ochratoxin A (OTXA). This study offers an available approach for creating organic-inorganic hybrid heteroatom-bridged lanthanide-inlaid POMs and reveals the likelihood of extending heteroatom-bridged lanthanide-inlaid POMs into electrochemical biosensing applications.

Poly(ionic liquid)–polyoxometalate/graphene oxide composites as catalysts for deep desulfurization

A series of supported catalysts were synthesized by loading polyoxometalate salts on the graphene oxide surface. Under the optimum conditions, the maximum desulfurization rates of T, BT and DBT can reach 98.32, 99.98 and 99.28%, respectively.

Organic–inorganic hybrid phosphite-participating S-shaped penta-CeIII incorporated tellurotungstate as electrochemical enzymatic hydrogen peroxide for β-D-glucose detection

Polyoxometalate chemistry has made rapid advances in innovative structural chemistry. The lower valence state and lone electron pair effect of subgroup-valence heteroatom Te(IV) can be introduced into the tungsten-oxygen system...

Oxidation of 1-propanol to propionic acid with hydrogen peroxide catalysed by heteropolyoxometalates

BACKGROUND

Propionic acid as a very valuable chemical is in high demand, and it is industrially produced via the oxo-synthesis of ethylene or ethyl alcohol and via the oxidation of propionaldehyde with oxygen. It is urgent to discover a new preparation method for propionic acid via a green route. Recyclable amino-acid-based organic-inorganic heteropolyoxometalates were first used to high-efficiently catalyse the selective oxidation of 1-propanol to propionic acid with H2O2 as an oxidant.

RESULT

A series of amino-acid-based heteropoly catalysts using different types of amino acids and heteropoly acids were synthesized, and the experimental results showed proline-based heteropolyphosphatotungstate (ProH)3[PW12O40] exhibited excellent catalytic activity for the selective catalytic oxidation of 1-propanol to propionic acid owing to its high capacity as an oxygen transfer agent and suitable acidity. Under optimized reaction conditions, the conversion of 1-propanol and the selectivity of propionic acid reached 88% and 75%, respectively. Over four cycles, the conversion remained at >80%, and the selectivity was >60%. (ProH)3[PW12O40] was also used to catalyse the oxidations of 1-butanol, 1-pentanol, 1-hexanol, and benzyl alcohol. All the reactions had high conversions, with the corresponding acids being the primary oxidation product.

CONCLUSIONS

Proline-based heteropolyoxometalate (ProH)3[PW12O40] has been successfully used to catalyse the selective oxidation of primary alcohols to the corresponding carboxylic acids with H2O2 as the oxidant. The new developed catalytic oxidation system is mild, high-efficient, and reliable. This study provides a potential green route for the preparation propionic acid.

Precise Molecular-Level Modification of Nafion with Bismuth Oxide Clusters for High-performance Proton-Exchange Membranes

Fabricating proton exchange membranes (PEMs) with high ionic conductivity and ideal mechanical robustness through regulation of the membrane microstructures achieved by molecular-level hybridization remains essential but challenging for the further development of high-performance PEM fuel cells. In this work, by precisely hybridizing nano-scaled bismuth oxide clusters into Nafion, we have fabricated the high-performance hybrid membrane, Nafion-Bi₁₂ -3 %, which showed a proton conductivity of 386 mS cm^-1 at 80 °C in aqueous solution with low methanol permeability, and conserved the ideal mechanical and chemical stabilities as PEMs. Moreover, molecular dynamics (MD) simulation was employed to clarify the structural properties and the assembly mechanisms of the hybrid membrane on the molecular level. The maximum current density and power density of Nafion-Bi₁₂ -3 % for direct methanol fuel cells reached to 432.7 mA cm^-2 and 110.2 mW cm^-2 , respectively. This work provides new insights into the design of versatile functional polymer electrolyte membranes through polyoxometalate hybridization.

Deep desulfurization of fuels using supported ionic liquid-polyoxometalate hybrid as catalyst: A comparison of different types of ionic liquids

In this work, polyoxometalates-based monomer ionic liquid, dimer ionic liquid and polyionic liquid were designed and prepared. Then supported catalysts were synthesized by loading polyoxometalate derivatives on the surface of graphene oxide (GO). The catalysts before and after loading were characterized via many tests such as scanning electron microscope (SEM), infrared spectroscopy (IR), X-ray powder diffraction (XRD) and so on. The influences of ionic liquid type and carrier on desulfurization activity were carried out. The result shows that supported catalyst-based polyionic liquid (P[Vim]POM/GO) performed high activity and excellent recyclability in extraction-oxidation desulfurization (EODS) due to unique state of polyoxometalate and the support of graphene oxide. In addition, the possible mechanism of oxidation dibenzothiophene (DBT) with H₂O₂ was proposed according to the kinetic study and gas chromatography-mass spectrometer (GC-MS) result.

Electrochemical investigation and amperometry determination iodate based on ionic liquid/polyoxotungstate/P-doped electrochemically reduced graphene oxide multi-component nanocomposite modified glassy carbon electrode

A novel modified glassy carbon electrode (GCE) was successfully fabricated with a tetra-component nanocomposite consisting of (1,1′-(1,4-butanediyl)dipyridinium) ionic liquid (bdpy), SiW₁₁O₃₉Ni(H₂O) (SiW₁₁Ni) Keggin-type polyoxometalate (POM), and phosphorus-doped electrochemically reduced graphene oxide (P-ERGO) by electrodeposition technique. The (bdpy)SiW₁₁Ni/GO hybrid nanocomposite was synthesized by a one-pot hydrothermal method and characterized by UV-vis absorption, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) analysis, thermogravimetric-differential thermal analysis (TGA/DTA), and transmission electron microscopy (TEM). The morphology, electrochemical performance, and electrocatalysis activity of the nanocomposite modified glassy carbon electrode ((bdpy)SiW₁₁Ni/P-ERGO/GCE) were analyzed by field emission scanning electron microscopy (FE-SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), cyclic voltammetry (CV), square wave voltammetry (SWV), and amperometry, respectively. Under the optimum experimental conditions, the as-prepared sensor showed high sensitivity of 28.1 μA mM⁻¹ and good selectivity for iodate (IO₃⁻) reduction, enabling the detection of IO₃⁻ within a linear range of 10–1600 μmol L⁻¹ (R² = 0.9999) with a limit of detection (LOD) of 0.47 nmol L⁻¹ (S/N = 3). The proposed electrochemical sensor exhibited good reproducibility, and repeatability, high stability, and excellent anti-interference ability, as well as analytical performance in mineral water, tap water, and commercial edible iodized salt which might provide a capable platform for the determination of IO₃⁻.

Constructing a sensitive electrochemical sensor based on (bdpy)SiW₁₁Ni/P-ERGO/GCE for IO₃⁻ detection at the nanomolar level with noticeable selectivity.

Photocatalytic remediation of organic waste over Keggin-based polyoxometalate materials: A review

Photocatalytic remediation of industrial water pollution has courted intense attention lately due to its touted green approach. In this respect, Keggin-based polyoxometalates (POMs) as green solid acids in photocatalytic reaction possess superior qualities, viz. unique photoinduced charge-transfer properties, strong photooxidative-photoreductive ability, high chemical and thermal stability, and so forth. Unfortunately, it suffers from a large bandgap energy, low specific surface area, low recoverability, and scarce utilization in narrow absorption range. Therefore, the pollutant degradation performance is not satisfactory. Consequently, multifarious research to enhance the photocatalytic performance of Keggin-based POMs were reported, viz. via novel modifications and functionalizations through a variety of materials, inclusive of, inter alia, metal oxides, transition metals, noble metals, and others. In order to advocate this emerging technology, current review work provides a systematic overview on recent advancement, initiated from the strategized synthetic methods, followed by hierarchical enhancement and intensification process, at the same time emphasizes on the fundamental working principles of Keggin-based POM nanocomposites. By reviewing and summarizing the efforts adopted global-wide, this review is ended with providing useful outlooks for future studies. It is also anticipated to shed light on producing Keggin-based POM nanocomposites with breakthrough visible- and solar-light-driven photocatalytic performance against recalcitrant organic waste.

Stable monovalent aluminum(i) in a reduced phosphomolybdate cluster as an active acid catalyst

Low-valent aluminum Al(i) chemistry has attracted extensive research interest due to its unique chemical and catalytic properties but is limited by its low stability. Herein, a hourglass phosphomolybdate cluster with a metal-center sandwiched by two benzene-like planar subunits and large steric-hindrance is used as a scaffold to stabilize low-valent Al(i) species. Two hybrid structures, (H₃O)₂(H₂bpe)₁₁[Al^III(H₂O)₂]₃{[Al^I(P₄Mo^V₆O₃₁H₆)₂]₃·7H₂O (abbr. Al₆{P₄Mo₆}₆) and (H₃O)₃(H₂bpe)₃[Al^I(P₄Mo^V₆O₃₁H₇)₂]·3.5H₂O (abbr. Al{P₄Mo₆}₂) (bpe = trans-1,2-di-(4-pyridyl)-ethylene) were successfully synthesized with Al(i)-sandwiched polyoxoanionic clusters as the first inorganic-ferrocene analogues of a monovalent group 13 element with dual Lewis and Brønsted acid sites. As dual-acid catalysts, these hourglass structures efficiently catalyze a solvent-free four-component domino reaction to synthesize 1,5-benzodiazepines. This work provides a new strategy to stabilize low-valent Al(i) species using a polyoxometalate scaffold.

Monovalent aluminum(i) species was successfully stabilized using a reduced phosphomolybdate scaffold as a dual-acid catalyst for a four-component domino reaction.