Capped Keggin Type Polyoxometalate-Based Inorganic-Organic Hybrids Involving In Situ Ligand Transformation as Supercapacitors and Efficient Electrochemical Sensors for Detecting Cr(VI)

Advances in polyoxometalate-based electrochemical sensors in the last three years

As a famous subclass of metal-oxide cluster materials, polyoxometalates (POMs) feature variable architectures, reversible multi-electron transport capability, catalytic activity, and redox capacity. These attributes endow POMs with great potential as promising electrode materials in electrochemical sensors (ECSs). Up to now, POM-based ECSs have been passionately studied, and diverse POM-based redox ECSs, aptasensors and immunosensors have emerged. And these POM-based ECSs generally demonstrate fast response, low detection limit, strong selectivity and high antijamming capability. This review mainly focuses on the remarkable advancement of POM-based ECSs in environmental monitoring, food safety and biomedicine from 2021, aiming to furnish theoretical insights that inform the design and development of innovative sensors.

Polyoxometalate Functionalized Cyclic Trinuclear Copper Compounds for Bifunctional Electrochemical Detection and Photocatalytic Reduction of Cr(VI)

The design and intentional construction of crystalline materials containing two clusters with redox properties in one framework still remains challenging. Linking oxidative polyoxometalate (POM) clusters and a reductive cyclic trinuclear copper complex (Cu-CTC) to prepare stable catalysts is rarely reported. Herein, we successfully obtained two new polyoxometalate-based metal-organic compounds (POMOCs) [CuII3(PyCA)3(μ3-OH)(β-Mo8O26)0.5(H2O)2]·5H2O (1), [CuII3(PyCA)3(μ3-OH)]2(CuIIW12O40)[CuII(H2O)6] (2) (PyCA = 1H-pyrazole-4-carbaldehyde) by enabling precursors of Cu-CTC and POM cocrystallization in one pot via hydrothermal method. The [β-Mo8O26]4- cluster in compound 1 combined with Cu-CTC units to form a 1D structure, and the [CuW12O40]6- unit in compound 2 linked two Cu-CTC units to form a sandwich-like 0D structure. Also, Cu-CTC CuI3(PyCA)3·H2O (Cu3) was synthesized for performance comparison. A series of characterizations indicate that compound 1 is more conducive to electron transfer than compound 2. In addition, compounds 1 and 2 can act as bifunctional catalysts for the electrochemical detection and photocatalytic reduction of Cr(VI). Particularly, the photoreduction rates of Cr(VI) by compounds 1 and 2 are 96.7% and 96.3% for only 10 and 14 min under visible light, respectively, and it is better than that of Cu3 and most other reported photocatalysts. Furthermore, the active sites and mechanisms for electrochemical detection and photocatalytic reduction of Cr(VI) were discussed.

Polyoxovanadate-Based Metal-Organic Frameworks with Dual Active Sites for the Synthesis of p-Benzoquinones

p-Benzoquinones are important organic intermediates in the synthesis of biopharmaceuticals and fine chemicals. In this study, two crystalline 3D polyoxovanadate-based metal-organic frameworks, H[Cu(tpi)2]{Cu2V7O21}·H2O (1, tpi = C18N5H13) and [Co(Htpi)2]{V4O12} (2, Htpi = C18N5H14), were synthesized, which as heterogeneous catalysts showed excellent catalytic activities for the synthesis of p-benzoquinones. Both compounds were characterized by IR, UV-vis diffuse reflectance spectroscopy, TG, XPS, X-ray diffraction, etc. In 1, {Cu2V7} clusters are connected together by copper cations and 1D Cu-organic coordination chains to yield a 3D polyoxometalate-based metal-organic framework (POMOF); in 2, adjacent 2D bimetallic oxide layers, constructed from 1D polyoxovanadate chains and cobalt ions, are further connected by 1D Co-organic coordination chains to form a 3D POMOF. Noteworthily, in the synthesis of trimethyl-p-benzoquinone, the key intermediate of vitamin E, using 2,3,6-trimethylphenol as the model substrate, the turnover frequency values for compounds 1 and 2 can, respectively, reach 607 and 380 h-1 in 8 min. Furthermore, both compounds demonstrated excellent recyclability and structural stability, characterized by PXRD and IR. The catalytic mechanism reveals that both the homolytic radical mechanism and heterolytic oxygen atom transfer mechanism are involved.

Bifunctional 3D POM-based coordination polymers for improved pseudocapacitance and catalytic oxidation performance

Developing novel high-efficiency supercapacitors as energy storage devices to solve the energy crisis is of vital significance. Meanwhile, designing highly active and selective oxidation catalysts for various sulfides is desirable but still a big challenge. To work out these problems, three novel 3D POM-based coordination polymers (POMCPs), formulated as [{Ag6(pytz)4}{SiMo12O40}] (1), [{Cu3(pytz)4}{SiMo12O40}]·5.5H2O (2) and [{Cu6(pytz)6}{SiMo12O40}]·2H2O (3) (pytz = 4-(5-(4-pyridyl)-1H-tetrazole)), are successfully prepared via a one-step synthetic strategy by changing different temperatures under hydrothermal or solvothermal conditions. In compounds 1 and 2, {SiMo12}, as 9-capped and 2-capped polyoxoanions, are engaged among the 2D Ag/Cu-organic sheets to generate the novel 3D POM-based coordination polymers. In addition, 1D Cu-organic chains are combined with 3-capped {SiMo12} polyoxoanions to construct 2D POM-based coordination polymers in 3. To our delight, as electrode materials for supercapacitors, the three compounds exhibit excellent specific capacitances of 261.76 F g-1, 248.82 F g-1 and 156.47 F g-1 at 0.5 A g-1, respectively. Besides, they can effectively and selectively catalyze the oxidation of various sulfides to sulfoxides.

One-Step Synthesis of Hollow CoS2 Spheres Derived from Polyoxometalate-Based Metal-Organic Frameworks with Peroxidase-like Activity

In this work, hollow CoS2 particles were prepared by a one-step sulfurization strategy using polyoxometalate-based metal-organic frameworks as the precursor. The morphology and structure of CoS2 have been monitored by scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray powder diffraction. The mechanism for the formation of CoS2 is discussed. The reaction time and sulfur content are found to be important factors that affect the morphology and pure phase formation of CoS2, and a hollow semioctahedral morphology of CoS2 with open voids was obtained when the sulfur source was twice as large as the precursor and the reaction time was 24 h. The CoS2 (24 h) particles show an excellent peroxidase-like activity for the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxidized (oxTMB) by hydrogen peroxide. The polyoxometalate used as a precursor helps to stabilize oxTMB during catalytic oxidation, forming a stable curve platform for at least 8 min. Additionally, the colorimetric detection of hydroquinone is developed with a low detection limit of 0.42 μM. This research provides a new strategy to design hollow materials with high peroxidase-mimicking activity.

Organo-Functionalized Lacunary Double Cubane-Type Oxometallates: Synthesis, Structure, and Properties of [(MII Cl)2 (VIV O)2 {((HOCH2 CH2 )(H)N(CH2 CH2 O))(HN(CH2 CH2 O)2 )}2 ] (M=Co, Zn)

Organofunctionalized tetranuclear clusters [(MII Cl)2 (VIV O)2 {((HOCH2 CH2 )(H)N(CH2 CH2 O))(HN(CH2 CH2 O)2 )}2 ] (1, M=Co, 2: M=Zn) containing an unprecedented oxometallacyclic {M2 V2 Cl2 N4 O8 } (M=Co, Zn) framework have been prepared by solvothermal reactions. The new oxo-alkoxide compounds were fully characterized by spectroscopic methods, magnetic susceptibility measurement, DFT and ab initio computational methods, and complete single-crystal X-ray diffraction structure analysis. The isostructural clusters are formed of edge-sharing octahedral {VO5 N} and trigonal bipyramidal {MO3 NCl} units. Diethanolamine ligates the bimetallic lacunary double cubane core of 1 and 2 in an unusual two-mode fashion, unobserved previously. In the crystalline state, the clusters of 1 and 2 are joined by hydrogen bonds to form a three-dimensional network structure. Magnetic susceptibility data indicate weakly antiferromagnetic interactions between the vanadium centers [Jiso (VIV -VIV )=-5.4(1); -3.9(2) cm-1 ], and inequivalent antiferromagnetic interactions between the cobalt and vanadium centers [Jiso (VIV -CoII )=-12.6 and -7.5 cm-1 ] contained in 1.

Two 3D Two-Fold Interpenetrated Dia-Like Polyoxometalate-Based Metal-Organic Frameworks: Synthesis and Sulfide Selective Oxidation Activity

Two new three-dimensional (3D) polyoxometalate-based metal-organic frameworks (POMOFs), [M2(btap)4(H2O)4(HPMo10VI Mo2VO40)] (M = Co (1) and Cd (2); btap = 3, 5-bis(1', 2', 4'-triazol-1'-yl)pyridine), have been synthesized under mild hydrothermal conditions and characterized in detail. Single-crystal X-ray diffraction (SXRD) analysis indicates that 1 and 2 are isostructural. In complexes 1 and 2, the metal ion is coordinated with the ligand to form two different left and right helical one-dimensional chains, which are alternately connected in a twisted form to build a two-fold interpenetrated three-dimensional structure, and the polyoxometalate is encapsulated into in the pores generated by the interpenetrating structure. It is noteworthy that 1 and 2, as recyclable catalysts, possess favorable heterogeneous catalytic activity and excellent sulfoxide selectivity in sulfide oxidation reactions, with H2O2 as an oxidant. By reason of the high dispersion of polyoxometalate with good intrinsic activity in the skeleton structure, the title complex has high activity. In addition, no obvious decrease of sulfoxide yield is observed after at least five cycles. These results indicate the excellent catalytic activity and sustainability of 1 and 2.

Nanomaterial with Core-Shell Structure Composed of {P2W18O62} and Cobalt Homobenzotrizoate for Supercapacitors and H2O2-Sensing Applications

Designing and preparing dual-functional Dawson-type polyoxometalate-based metal-organic framework (POMOF) energy storage materials is challenging. Here, the Dawson-type POMOF nanomaterial with the molecular formula CoK₄[P₂W₁₈O₆₂]@Co₃(btc)₂ (abbreviated as {P₂W₁₈}@Co-BTC, H₃btc = 1,3,5-benzylcarboxylic acid) was prepared using a solid-phase grinding method. XRD, SEM, TEM et al. analyses prove that this nanomaterial has a core-shell structure of Co-BTC wrapping around the {P₂W₁₈}. In the three-electrode system, it was found that {P₂W₁₈}@Co-BTC has the best supercapacitance performance, with a specific capacitance of 490.7 F g^-1 (1 A g^-1) and good stability, compared to nanomaterials synthesized with different feedstock ratios and two precursors. In the symmetrical double-electrode system, both the power density (800.00 W kg^-1) and the energy density (11.36 Wh kg^-1) are greater. In addition, as the electrode material for the H₂O₂ sensor, {P₂W₁₈}@Co-BTC also exhibits a better H₂O₂-sensing performance, such as a wide linear range (1.9 μM-1.67 mM), low detection limit (0.633 μM), high selectivity, stability (92.4%) and high recovery for the detection of H₂O₂ in human serum samples. This study provides a new strategy for the development of Dawson-type POMOF nanomaterial compounds.

"Tree"-like Multidentate Ligand-Assisted Synthesis of Polymolybdate-Based Architectures with Multinuclear Metal Clusters: Supercapacitor and Electrochemical Sensing Performances

In this work, aiming for constructing multinuclear metal cluster-modified polymolybdate-based architectures with novel conformation, the "tree"-like multidentate ligand 5-(3-pyridyl)-1H-tetrazole) (3-ptzH) is introduced into the polymolybdate reaction system. Three new polymolybdate-based architectures with various multinuclear metal clusters, H₄[Cu₆(μ₃-OH)₂(3-ptz)₆(γ-Mo₈O₂₈) (H₂O)₂]·2H₂O (BOHU-1), H₂[Ag₄(3-ptz)₂(Mo₈O₂₆)] (BOHU-2), and H₄[Cu₅(3-ptzH)₂(3-ptz)₂(MnMo₉O₃₂)₂(H₂O)₄] (BOHU-3) (BOHU = Bohai University), have been prepared via the hydrothermal method and structurally characterized. In BOHU-1, a kind of pentanuclear copper cluster unit: [Cu₅(μ₃-OH)₂(3-ptz)₆]²⁺ is formed, which connects to construct a one-dimensional (1D) cluster-based chain. The 1D chains are extended to a two-dimensional (2D) layer via the Cu ions, which are further linked by the 4-connected [γ-Mo₈O₂₈]^8- anions to build a three-dimensional (3D) framework. In BOHU-2, when a Ag^I ion was used as the central metal, the 3-ptz adopts different coordination modes to link the Ag ions, forming hexanuclear [Ag₆(3-ptz)₄]²⁺ cluster and finally 1D chains. These 1D cluster-based chains are connected by the 6-connected [γ-Mo₈O₂₆]^4- anions to establish a 2D layer, which is further extended by [Mo₈O₂₆]_n^4n- 1D chains to a 3D framework. For BOHU-3, the chiral [MnMo₉O₃₂]^6- anions are introduced and coordinated with the Cu ions to build left- and right-handed 1D chains, which are connected via the [Cu₃(3-ptz)₄]²⁺ cluster to form a 1D ladder-like chain. The effects of 3-ptz on the formation of multinuclear clusters, as well as the metals and polymolybdates on the multinuclear clusters and final structures of BOHU-1∼3, are discussed. The electrochemical performances of BOHU-1∼3 as electrode materials for supercapacitors and electrochemical sensors are investigated.

A Review on Recent Progress in Organic Fluorimetric and Colorimetric Chemosensors for the Detection of Cr3+/6+Ions

Chromium occurs in the environment primarily in two valence states, trivalent Cr3+ and hexavalent Cr6+, which have different physicochemical and biochemical properties. However, the higher concentration of Cr3+/6+ can cause various adverse effects on human health. Therefore, detecting Cr3+/6+ ions is important in various samples. Colorimetric and fluorescent chemosensors are the most powerful tools for the detection of Cr3+/6+ ions. These chemosensors have excellent bioimaging capability and significant sensitivity and selectivity. In this article, different colorimetric and fluorescent chemosensors based on organic compounds, including Schiff base, antipyrine, diarylethene, pyrene, crown ether, dansyl, pyridine, thiazole, coumarin, boradiazaindacene, rhodamine, imidazole, hydrazone, and other functional groups for detection of Cr3+/6+ ions have been reviewed, classified them according to different fluorophore and recognition mode. I hope this article will help the readers for the future design of highly effective, sensitive, and selective chemosensors for the detection and determination of Cr3+/6+ ions.

In situ ball-milling gram-scale preparation of polyoxoniobate-intercalated MgAl-layered double hydroxides for selective aldol and Michael addition cascade reactions

A facile one-step ball-milling strategy to prepare gram-scale Mg3Al-LDH-Nb6 has been demonstrated and the thus-obtained catalyst exhibited efficient selective catalytic activities in the synthesis of biologically active organic molecules in water.

Carbazole-based bis-imidazole ligand-involved synthesis of inorganic–organic hybrid polyoxometalates as electrochemical sensors for detecting bromate and efficient catalysts for selective oxidation of thioether

Considering the potential application on preparing electrode and catalyst materials of inorganic–organic hybrid polyoxometalates, a bis-imidazole ligand with carbazole as a connector, 3,6-di(1H-imidazol-1-yl)-9H-carbazole (L), was used for preparing inorganic–organic hybrid polyoxometalates. As a result, three complexes formulated by [NiL₂(Mo₂O₇)] (1), [Cu(H₂O)₂(HL)₂ (β-Mo₈O₂₆)]·H₂O (2) and [Ni₂(H₂O)₄L₂ (CrMo₆(OH)₅O₁₉)]·6H₂O (3) were obtained successfully. Structural analysis indicated that the different polyoxoanions and metal ions showed important influences on the formation of structures. In the presence of Ni²⁺ ions and heptamolybdate, a 2D network constructed from Ni²⁺ ions and L ligands was formed in complex 1, in which the [Mo₄O₁₄]⁴⁻ polyoxoanions were encapsulated. But the use of Cu²⁺ ions led to a 1D chain of complex 2, which was composed of [β-Mo₈O₂₆]⁴⁻ polyoxoanions and mononuclear {CuL₂} units. By utilizing [CrMo₆(OH)₅O₁₉]⁴⁻ as the inorganic building block, complex 3 showed a 2D (4, 4)-connected layer. Complexes 1–3 could be employed as electrode materials for sensing bromate with the limits of detection of 0.315 μM for 1, 0.098 μM for 2 and 0.551 μM for 3. Moreover, these complexes showed efficient catalytic activity for the selective oxidation of thioethers.

Three inorganic–organic hybrid polyoxometalates were prepared using a bis-imidazole ligand featuring carbazole as a connector, exhibiting not only diverse structures, but also good electrochemical sensing activities for bromate, as well as efficient catalytic performances for oxidation of thioether.