A sensor of a polyoxometalate and Au–Pd alloy for simultaneously detection of dopamine and ascorbic acid

Engineering Chiral Confinement Environment in Polyoxometalate Intercalated Graphene Oxide Sensor for Electrochemical Enantioselective Recognition

The electrochemistry recognition of enantiomeric chiral molecules holds great significance for the pharmaceutical industry and scientific research. However, enhancing sensitivity and selectivity simultaneously, and elucidating chiral recognition mechanism, are two primary challenges. Here, an electrochemical chiral sensor L-C4-PMoV/GO is developed by confining chiral imidazole cations (L-C4) and [PMo10V2]5- (PMoV) signal anions within the interlayer of graphene oxide (GO). The L-C4-PMoV/GO is highly sensitive to recognition towards the chiral drug Levodopa (L-DOPA), which exhibits 16 times higher than the L-C4/GO. In addition, the enantioselectivity of ΔS = 19.92 is achieved. Mechanism studies suggest that the chiral confinement effect plays a crucial role in the synergism between the signal site PMoV and the enantioselectivity L-C4. In the chiral-confined microenvironment, the chiral induction from L-C4 to PMoV is facilitated, which results in the distortion of Mo (V)─O bonds. The hydrogen-bonding networks among the L-C4, Mo (V)─O, and DOPA generate the adsorption energy difference between the L/D-DOPA, as revealed by the in situ Raman spectroscopy and theoretical calculation. Compared to the conventional techniques, the electrochemical sensor shows comparable enantiomer excess (ee) value determination, low limits of detection (LOD) (6.7 nm for L-DOPA, 50.6 nm for D-DOPA), and portability, enabling practical chiral recognition.

Efficient oxidation of sulfides to sulfoxides catalyzed by heterogeneous Zr-containing polyoxometalate grafted on graphene oxide

In this study, a tri-component composite named Zr/SiW12/GO was meticulously prepared through an ultrasonic-assisted method. This composite incorporates zirconium nanoparticles (Lewis acid), a negatively charged Keggin type polyoxometalate, and graphene oxide, and serves as a remarkable heterogeneous catalyst. The Keggin component plays multiple roles as reducing, encapsulating, and bridging agents, resulting in a cooperative effect among the three components that significantly enhances the catalytic activity. The catalytic performance of Zr/SiW12/GO was thoroughly investigated in the oxidation of sulfides to sulfoxides under mild conditions, employing H2O2 as the oxidant. Remarkably, this composite exhibited exceptional stability and could be effortlessly recovered and reused up to four times without any noticeable loss in its catalytic activity.

Opioid Drug Detection in Hair Samples Using Polyoxometalate-Based Frameworks

A series of two-dimensional (2D) polyoxometalate-based frameworks, [Ln₃(PDA)₃(H₂O)₆(PMo₁₂O₄₀)]·xH₂O (Ln = La (1); Ce (2); Pr (3); Nd (4); PDA = 1,10-phenanthroline-2,9-dicarboxylate), have been synthesized and structurally characterized by various analytical techniques. Single-crystal X-ray diffraction reveals that 1-4 have a unique 2D layer structure in which Keggin anions have coordinated upward and downward the plane, and this feature makes them suitable candidates for surface binding of common drugs via supramolecular and electrostatic interactions. Also, the ability of 1-4 (as the first polyoxomolybdate-containing frameworks) as sorbents for the extraction and quantitative determination of opioid drugs (morphine, methadone, and pethidine) was investigated by using dispersive micro-solid-phase extraction (D-μSPE) and high-performance liquid chromatography (HPLC). The method showed wide linear ranges in the range of 0.3 to 300 ng mg^-1 and low limits of detection (LODs) ranged from 0.1 to 0.2 ng mg^-1 of hair.

Two Types of Subgroup-Valence Heteroatoms (PIII, TeIV) Synergistically Controlling Octa-CeIII-Encapsulated Heteropolyoxotungstate and Its Electrochemical Recognition Properties

Rapid development of the synthetic chemistry of polyoxometalates (POMs) has greatly driven the generation of structurally variable innovative POM-based materials. Herein, we synthesized a novel P^III and Te^IV synergistically controlling octa-Ce^III-encapsulated heteropolyoxotungstate [H₂N(CH₃)₂]₁₁K₂Na₆H₁₁[Ce₈(CH₃COO)₂(HP^IIIO₃)₂W₈O₂₀(H₂O)₁₂(B-β-TeW₈O₃₀)₂(B-α-TeW₈O₃₁)₄]·64H₂O (1). Its distinctive anion skeleton [Ce₈(CH₃COO)₂(HP^IIIO₃)₂W₈O₂₀(H₂O)₁₂(B-β-TeW₈O₃₀)₂(B-α-TeW₈O₃₁)₄]^30- is built by two tetra-vacancy [B-β-TeW₈O₃₀]^8- and four tetra-vacancy [B-α-TeW₈O₃₁]^10- moieties linked through an inorganic-organic hybrid [Ce₈(CH₃COO)₂(HP^IIIO₃)₂W₈O₂₀(H₂O)₁₂]²⁶⁺ {Ce₈P₂W₈} cluster core. Interestingly, {Ce₈P₂W₈} is assembled from four [W₂O₁₁]^10- groups and two [HP^IIIO₃]^2- anions and eight Ce³⁺ ions. Besides, 1 was further composited with carboxylated multiwalled carbon nanotube (CMCN), resulting in a bi-component 1/CMCN nanocomposite. An electrochemical recognition platform (named as 1/CMCN/GCE) was built by modifying 1/CMCN on a glassy carbon electrode (GCE) for electrochemical detection of dopamine (DPA) at physiological pH (pH = 7.0). The findings have shown that 1/CMCN/GCE exhibits a good detection limit of 4.95 nM for DPA. This work provides considerable inspiration to promote innovative and rational structure designs of POM-based materials and expand their applications to electrochemical and biological detection fields.

Lanthanoid-containing polyoxometalate nanocatalysts in the synthesis of bioactive isatin-based compounds

Lanthanoid-containing polyoxometalates (Ln-POMs) have been developed as effective and robust catalysts due to their Lewis acid–base active sites including the oxygen-enriched surfaces of POM and the unique 4f. electron configuration of Ln. As an extension of our interest in Ln-POMs, a series of as-synthesized nanocatalysts K₁₅[Ln(BW₁₁O₃₉)₂] (Ln-B₂W₂₂, Ln = La, Ce, Nd, Sm, Gd, and Er) synthesized and fully characterized using different techniques. The Ln³⁺ ion with a big ionic radius was chosen as the Lewis acid center which is sandwiched by two mono-lacunary Keggin [BW₁₁O₃₉]⁹⁻ units to form Ln-containing sandwiched type cluster. Consequently, the catalytic activity of nanocatalysts with different Ln was examined in the synthesis of bioactive isatin derivatives and compared under the same optimized reaction conditions in terms of yields of obtained products, indicating the superiority of the nano-Gd-B₂W₂₂ in the aforementioned simple one-pot reaction. The effects of different dosages of nanocatalyst, type of solvent, reaction time, and reaction temperature in this catalytic system were investigated and the best results were obtained in the presence of 10 mol% of nano-Gd-B₂W₂₂ in water for 12 min at the reflux condition.

Facile one-pot method of AuNPs/PEDOT/CNT composites for simultaneous detection of dopamine with a high concentration of ascorbic acid and uric acid

In this research, a facile one-pot method was used to synthesize gold/poly-3,4-ethylene-dioxythiophene/carbon nanotube (AuNPs/PEDOT/CNTs) composite material. The composite material was investigated by Fourier Transform Infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Then the synthesized nanocomposite material was dropped on a bare glassy carbon electrode (GCE) to improve the detection performance of dopamine with a high concentration of ascorbic acid and uric acid. The electrochemical behavior of AuNPs/PEDOT/CNTs/GCE was studied by Cyclic Voltammetry (CV), Differential Pulse Voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). Under optimum conditions, AuNPs/PEDOT/CNTs/GCE showed a good linear response in the concentration range from 9.14 to 29.704 μM with a detection limit (LOD) and sensitivity of 0.283 μM and 1.557 μA μM-1, respectively. This sensor was applied to detect practical samples with good average recovery. It also exhibited good reproducibility and stability.

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.

Single-molecule magnets within polyoxometalate-based frameworks

As an extension of our interest in polyoxometalates (POMs) and lanthanoids, we report the design and synthesis of two polyoxometalate-based frameworks under hydrothermal conditions; [Ho₄(PDA)₄(H₂O)₁₁][(SiO₄)@W₁₂O₃₆]·8H₂O (1) and [Tb₄(PDA)₄(H₂O)₁₂][(SiO₄)@W₁₂O₃₆]·4H₂O (2) (H₂PDA = 1,10-phenanthroline-2,9-dicarboxylic acid). Both hybrids have been characterized by elemental analysis, Fourier transform infrared spectroscopy, thermogravimetric analysis, and powder/single-crystal X-ray diffraction. According to the structural analysis, 1 and 2 consist of 2D-cationic coordination polymers based on the respective lanthanoids and PDA^2- as well as Keggin anions that reside in the interspaces between two adjacent layers as discrete counterions connected by extensive hydrogen bonding. Although the overall structures of 1 and 2 are composed of cationic and anionic layers, there are many differences in the cationic layers such as various coordination modes of PDA^2-, different void shapes, and the existence of dinuclear Tb(III) units only in 2. Frameworks 1 and 2 were further characterized by dc and ac magnetic measurements and both exhibit slow relaxation of magnetization at low temperatures under an applied dc field. Their single-molecule magnet (SMM) properties are investigated, where weak field-induced SMM behaviour is observed at low temperatures in dynamic magnetic studies.

Polyoxometalates in Analytical Sciences

Polyoxometalates (POMs) have been used for spectrophotometric determinations of silicon and phosphorus under acidic conditions, referred to as the molybdenum yellow method and molybdenum blue method, respectively. Many POMs are redox active and exhibit fascinating but complicated voltammetric responses. These compounds can reversibly accommodate and release many electrons without exhibiting structural changes, implying that POMs can function as excellent mediators and can be applied to sensitive determination methods based on catalytic electrochemical reactions. In addition, some rare-earth-metal-incorporated POMs exhibit fluorescence, which enables sensitive determination by the enhancement and quenching of fluorescence intensities. In this review, various analytical applications of POMs are introduced, mainly focusing on papers published after 2000, except for the molybdenum yellow method and molybdenum blue method.

Facile and recyclable dopamine sensing by a label-free terbium(III) metal-organic framework

Herein, we present a facile strategy for dopamine (DA) sensing by a water-stable MOF of {[Tb(Cmdcp)(H₂O)₃]₂(NO₃)₂·5H₂O}_n (1, H₃CmdcpBr = N-carboxymethyl-(3,5-dicarboxyl)pyridinium bromide). Without any post-modification, MOF 1 functions as an effective fluorescent sensor for the label-free detection of DA with the detection limit of 0.41 μM (S/N = 3). Under the optimum condition of 80 °C, pH 9 for 80 min in Tris-HCl with natural ambient oxygen, DA polymerizes to give polydopamine (pDA), which adheres to the surface of MOF 1 and quenched its green luminescence thoroughly. The sensing process is visible to naked eyes under 365 nm UV light irradiation due to the partial overlap of its excitation spectrum with the absorption spectrum of pDA. The sensing process is not interfered by coexisting of bio-related organic substances, such as glucose (Glu), 5-hydroxytryptamine (5-HT), homocysteine (Hcy), ascorbic acid (AA), uric acid (UA), cysteine (Cys), glutathione (GSH), as well as the presence of metal ions, including Zn²⁺, Ca²⁺, Mg²⁺, Ni²⁺ and Co²⁺. The sensing process is also adaptable in biological fluids of serum and urine with satisfactory recoveries ranging from 96.14% to 104.32%.

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.

Electrochemical Reduction-Assisted In Situ Fabrication of a Graphene/Au Nanoparticles@polyoxometalate Nanohybrid Film: High-Performance Electrochemical Detection for Uric Acid

Nanohybrid films had attracted much attention owning to the enhancement of catalytic activity. However, the fabrication time took hours to days, no matter if it was the preparation of nanohybrids or the assembly process. Furthermore, the catalytic efficiency of the nanohybrid film still remained to improve. In this paper, a reduced graphene oxide (rGO)/gold nanoparticles (Au NPs)@polyoxometalate (POM) nanohybrid film was successfully fabricated by combining electrodeposition and electrochemical reduction in situ processes. The assembly process involving no organic or polymer linker molecules [except for a precursor poly(ethylenimine) (PEI) coating for indium tin oxide (ITO)] can be completed within 1 h. The reduced POM K₆[P₂W₁₈O₆₂]·19H₂O (P₂W₁₈) was employed as reducing agents and bridging molecules for rGO and Au nanoparticles and the encapsulating molecules for the Au nanoparticles. The most interesting one is the {rGO/Au@P₂W₁₈} modified electrode loading only the monolayer catalyst of Au@P₂W₁₈ and exhibiting comparable, even better electrochemical detection performance toward uric acid than other sensors with three to eight layers of the catalyst. The amperometric detection displayed a great sensitivity, lower detection limit, wide linear range, good long-time stability, superior selectivity, and reproducibility. The enhanced catalytic property may attribute to the improved conductivity of the film without organic or polymer linker molecules (except for a precursor PEI coating) and the electron transfer in the process of film fabrication.

Graphene-like boron carbide monolayer as an electronic and work function-type sensor for dopamine drug

The electronic response of both pristine and Al-doped BC₃ nanosheets toward 3, 4-dihydroxyphenyl ethylamine, i.e. dopamine (DA) was studied through density functional theory. Based on the adsorption energy the tendency of pristine BC₃ toward DA drug insignificant and also after adsorption of DA drug the electronic properties of BC₃ were changed negligibly. While doping the sheet by Al significantly increases its reactivity and sensitivity toward the DA drug. By adsorption of DA HOMO-LUMO gap dramatically decreased of from 1.34 to 1.12 eV, thereby enhancing the electrical conductivity. It indicates that the doped BC₃ nanosheets may be a suitable candidate as a DA electronic sensor, unlike the pristine BC₃. Furthermore, the work function of doped BC₃ was changed significantly after DA adsorption. Based on these results the doped BC₃ can also act as a work function-type sensor to the sensing of DA was used. Finally, the most important factor of the doped BC₃ sheet is a short recovery time of 7.36 ms for the desorption process of DA.

Employing Conductive Metal-Organic Frameworks for Voltammetric Detection of Neurochemicals

This paper describes the first implementation of an array of two-dimensional (2D) layered conductive metal-organic frameworks (MOFs) as drop-casted film electrodes that facilitate voltammetric detection of redox active neurochemicals in a multianalyte solution. The device configuration comprises a glassy carbon electrode modified with a film of conductive MOF (M₃HXTP₂; M = Ni, Cu; and X = NH, 2,3,6,7,10,11-hexaiminotriphenylene (HITP) or O, 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP)). The utility of 2D MOFs in voltammetric sensing is measured by the detection of ascorbic acid (AA), dopamine (DA), uric acid (UA), and serotonin (5-HT) in 0.1 M PBS (pH = 7.4). In particular, Ni₃HHTP₂ MOFs demonstrated nanomolar detection limits of 63 ± 11 nM for DA and 40 ± 17 nM for 5-HT through a wide concentration range (40 nM-200 μM). The applicability in biologically relevant detection was further demonstrated in simulated urine using Ni₃HHTP₂ MOFs for the detection of 5-HT with a nanomolar detection limit of 63 ± 11 nM for 5-HT through a wide concentration range (63 nM-200 μM) in the presence of a constant background of DA. The implementation of conductive MOFs in voltammetric detection holds promise for further development of highly modular, sensitive, selective, and stable electroanalytical devices.

Acid-Induced, Oxygen-Atom Defect Formation in Reduced Polyoxovanadate-Alkoxide Clusters

Here, we present the first example of acid-induced, oxygen-atom abstraction from the surface of a polyoxometalate cluster. Generation of the oxygen-deficient vanadium oxide, [V₆O₆(OC₂H₅)₁₂]^1-, was confirmed via independent synthesis. Spectroscopic analysis using infrared and electronic absorption spectroscopies affords resolution of the electronic structure of the oxygen-deficient cluster (oxidation state distribution = [V^IIIV^IV₅]). This work has direct implications toward the elucidation of possible mechanisms of acid-assisted vacancy formation in bulk transition metal oxides, in particular electron-proton codoping that has recently been described for vanadium oxide (VO₂). Ultimately, these molecular models deepen our understanding of proton-dependent redox chemistry of transition metal oxide surfaces.

Polyoxometalate-based composite materials in electrochemistry: state-of-the-art progress and future outlook

Polyoxometalates (POMs) have been developed as a class of promising smart material candidates not only due to their multitudinous architectures but also their good redox activities and outstanding electron and proton transport capacities. Recently, abundant studies on POMs composited with metal nanoparticles (NPs), carbon materials (e.g., carbon nanotubes (CNTs), carbon quantum dots (CQDs), graphene), and conducting polymers or highly-porous framework materials (e.g., MOFs, ZIFs) have been performed and POM-based composite materials (PCMs) undoubtedly show enhanced stability and improved electrochemical performances. Therefore, POMs and PCMs are of increasing interest in electrocatalysis, electrochemical detection and energy-related fields (such as fuel cells, redox flow batteries and so on), thus, developing novel PCMs has long been the key research topic in POM chemistry. This review mainly summarizes some representative advances in PCMs with electrochemical applications in the past ten years, expecting to provide some useful guidance for future research.

From Simplicity to Complexity in Grafting Dawson-Type Polyoxometalates on Porphyrin, Leading to the Formation of New Organic-Inorganic Hybrids for the Investigation of Third-Order Optical Nonlinearities

Four new organic-inorganic hybrids, [N(C₄H₉)₄]₅H[{HNC₂O(CH₂O)₃P₂V₃W₁₅O₅₉}C₄₄N₄H₂₉O₃] (TPPOH-1POM), [N(C₄H₉)₄]₁₀H₂[{HNC₂O(CH₂O)₃P₂V₃W₁₅O₅₉}₂C₄₄N₄H₂₈O₂] (TPPOH-2POM trans), [N(C₄H₉)₄]₁₀H₂[{HNC₂O(CH₂O)₃P₂V₃W₁₅O₅₉}₂C₄₄N₄H₂₈O₂] (TPPOH-2POM cis), and [N(C₄H₉)₄]₁₅H₃[{HNC₂O(CH₂O)₃P₂W₁₅V₃O₅₉}₃C₄₄N₄H₂₇O] (TPPOH-3POM), have been synthesized by covalently grafting the Dawson-type polyoxometalate (POM) [N(C₄H₉)₄]₅[H₄P₂W₁₅V₃O₆₂] onto new porphyrins N-[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]-4-[10,15,20-tris(4-hydroxyphenyl)porphyrin-5-yl]benzamide (TPPOH-1Tris), 4,4'-[5,15-bis(4-hydroxyphenyl)porphyrin-10,20-diyl]bis[ N-[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]benzamide] (TPPOH-2Tris trans), 4,4'-[10,15-bis(4-hydroxyphenyl)porphyrin-5,20-diyl]bis[ N-[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]benzamide] (TPPOH-2Tris cis). and 4,4',4″-[20-(4-hydroxyphenyl)porphyrin-5,10,15-triyl]tris[ N-[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]benzamide] (TPPOH-3Tris), respectively, in N, N-dimethylacetamide under nitrogen. The hybrid compounds were thoroughly characterized by elemental analysis, FT-IR, NMR (¹H, 2D NOESY, ³¹P, and ⁵¹V), ESI-MS, MALDI-TOF-MS, UV-vis, and fluorescence spectroscopy. All porphyrins and their corresponding hybrids exhibited remarkable third-order nonlinear-optical responses under laser irradiation, with the intensity of light at focus E₀ being 4.2 μJ at wavelength 532 nm, pulse duration τ = 6 ns, and repetition rate 10 Hz. The molecular second hyperpolarizability (γ) values of the hybrids (TPPOH-1POM, TPPOH-2POM trans, TPPOH-2POM cis, TPPOH-3POM) were calculated to be 8.48 × 10^-28 esu, 8.94 × 10^-28 esu, 10.16 × 10^-28 esu, and 8.93 × 10^-28 esu, while the molecular second hyperpolarizability (γ) of their corresponding porphyrin precursors (TPPOH-1Tris, TPPOH-2Tris trans, TPPOH-2Tris cis, and TPPOH-3Tris) were 7.19 × 10^-28 esu, 8.09 × 10^-28 esu, 9.07 × 10^-28 esu, and 7.24 × 10^-28 esu, respectively. The γ values of the cis forms were found to be superior to those of all other compounds, indicating the significant role of the greater dipole moment and low symmetry of the cis form compared to other compounds. It was also found that the fluorescence intensity was decreased by 81.52%, 90.37%, 93.93%, and 84.62% for the hybrids TPPOH-1POM, TPPOH-2POM trans, TPPOH-2POM cis, and TPPOH-3POM with respect to their corresponding precursors, respectively, which reveals the Dawson-type POM as a strong quencher. This quenching phenomenon not only indicates again unequivocally the formation of hybrid compounds but also may imply a photoinduced electron/energy-transfer process favored from a porphyrin moiety to a Dawson-type POM moiety.

Tyrosinase/Chitosan/Reduced Graphene Oxide Modified Screen-Printed Carbon Electrode for Sensitive and Interference-Free Detection of Dopamine

Tyrosinase, chitosan, and reduced graphene oxide (rGO) are sequentially used to modify a screen-printed carbon electrode (SPCE) for the detection of dopamine (DA), without interference from uric acid (UA) or ascorbic acid (AA). The use of tyrosinase significantly improves the detection’s specificity. Cyclic voltammetry (CV) measurements demonstrate the high sensitivity and selectivity of the proposed electrochemical sensors, with detection limits of 22 nM and broad linear ranges of 0.4–8 μM and 40–500 μM. The fabricated tyrosinase/chitosan/rGO/SPCE electrodes achieve satisfactory results when applied to human urine samples, thereby demonstrating their feasibility for analyzing DA in physiological samples.

On the preferences of five-membered chelate rings in coordination chemistry: insights from the Cambridge Structural Database and theoretical calculations

The purpose of this review is to give an overview of three important N-bidentate ligands: 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), and ethylenediamine (en). We have not attempted to be comprehensive because of the huge amount of activity being done in coordination chemistry using these ligands. Instead we present a full structural and geometrical study by using the Cambridge Structural Database (CSD) combined with theoretical calculations that allow us to parameterize their coordinating properties and ability to coordinate to transition and non-transition metals. More importantly, we illustrate that upon coordination and formation of the five-membered chelate ring, these ligands are able to adapt themselves to the requirements of the different metals by changing the MN distances and NMN angles. Therefore, a redefinition of the preferences of these ligands to metals with large ionic radii is needed. Finally, we will present some facts about the participation of these ligands in inorganic-organic hybrids (IOHs) based on Keggin polyoxometalates (POMs).

One-pot facile simultaneous in situ synthesis of conductive Ag–polyaniline composites using Keggin and Preyssler-type phosphotungstates

Two heteropolytungstate structures, Keggin (H₃PW₁₂O₄₀) and Preyssler (H₁₄[NaP₅W₃₀O₁₁₀]), were used to synthesize conductive silver nanoparticle–polyaniline–heteropolytungstate (AgNPs–PAni–HPW) nanocomposites. During the oxidative polymerization of aniline, heteropolyblue was generated and served as the reducing agent to stabilize and distribute AgNPs within “PAni–Keggin” and “PAni–Preyssler” matrixes as well as on their surfaces. The prepared nanocomposites and AgNPs were characterized using UV-visible (UV-Vis) and Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), pore size distribution BET, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). UV-Vis results showed different stages of the formation of metal NPs embedded in the polymer–HPW composites, and FT-IR spectra presented characteristic bands of PAni, Keggin and Preyssler anions in the composites confirming no changes in their structures. The presence of AgNPs and an intensely crystalline matrix were confirmed by the XRD pattern. The BET surface areas were found to be 38.426 m² g⁻¹ for “AgNPs–PAni–Keggin” and 29.977 m² g⁻¹ for “AgNPs–PAni–Preyssler” nanocomposites with broad distributions of meso-porous structure for both nanocomposites. TEM and SEM images confirmed that the type of heteropolyacids affected the size of AgNPs. This is the first report that uses Keggin and Preyssler-type heteropolytungstate to synthesize “AgNPs–PAni–HPW” nanocomposites in an ambient condition through a low-cost, facile, one-pot, environmentally friendly and simultaneous in situ oxidative polymerization protocol.

Two heteropolytungstate structures, (a) Keggin (H₃PW₁₂O₄₀) and (b) Preyssler (H₁₄(NaP₅W₃₀O₁₁₀]), have been used to synthesize conductive silver nanoparticle–polyaniline–heteropolytungstate, (AgNPs–PAni–HPW) nanocomposites.

Fabrication of Metal-Substituted Polyoxometalates for Colorimetric Detection of Dopamine and Ractopamine

A novel colorimetric detection method based on the peroxidase-like activity of metal-substituted polyoxometalates (POMs) of SiW₉M₃ (M = Co²⁺, Fe³⁺, Cu²⁺, Mn²⁺) has been established. POMs can catalyze oxidation of dopamine (DA) and ractopamine (RAC) by H₂O₂ in aqueous solutions. SiW₉Co₃-based POMs detect DA at concentrations as low as 5.38 × 10⁻⁶ mol·L⁻¹ simply by observation of the color change from colorless to orange using the naked eye. RAC is detected by observing the change from colorless to slight red by SiW₉Cu₃ with a detection limit of 7.94 × 10⁻⁵ mol·L⁻¹. This study shows that colorimetric DA and RAC detection using SiW₉Co₃ and SiW₉Cu₃ is highly selective and sensitive as well as visually observable.

Dawson-type polyoxometalate nanoclusters confined in a carbon nanotube matrix as efficient redox mediators for enzymatic glucose biofuel cell anodes and glucose biosensors

Two new inorganic-organic hybrid materials based on heteropolyoxometalates (POMs): (C₄H₁₀N)₆[P₂Mo₁₈O₆₂]·4H₂O (P₂Mo₁₈) and (C₆H₈NO)₄[H₂P₂W₁₈O₆₂]·6H₂O (P₂W₁₈) are reported as mediators for electron transfer between FAD-dependent glucose dehydrogenase (FAD-GDH) and a multiwalled carbon nanotube (MWCNT) matrix for glucose biofuel cell and biosensor applications. These polyoxometalates were chosen due to their promising redox behavior in a potential range for mediated electron transfer with the glucose oxidizing enzyme, FAD-GDH. P₂Mo₁₈ and P₂W₁₈ were immobilized on 1-pyrenemethylamine (PMA) functionalized MWCNT deposits. After immobilization of FAD-GDH, the P₂W₁₈-modified MWCNT electrode demonstrated mediated electron transfer and provided a catalytic current density of 0.34 mA cm^-2 at 0.2 V vs SCE with an open circuit potential (OCP) of -0.08 V vs SCE. A 10-fold increase in catalytic current to 4.7 mA cm^-2 at 0.2 V vs SCE and a slightly lower OCP of -0.10 V vs SCE was observed for an equivalent electrode modified with P₂Mo₁₈.The apparent superiority of P₂Mo₁₈ is related, at least in part, to its improved incorporation in the MWCNT matrix compared to P₂W₁₈. Both POM-modified bioanodes showed exceptional stabilities with 45% of their initial performances remaining after 15 days. The mediated electron transfer capacities of the POMs were also evaluated in a glucose sensor setup and showed very satisfying performances for glucose detection, including a sensitivity of 0.198 mA mol L^-1 cm^-2, a satisfying linear range between 1 mmol L^-1 and 20 mmol L^-1, and good reproducibility for the P₂Mo₁₈ electrode.

A novel sandwich-tungstoantimonate cluster with FeII ions: synthesis, magnetic property and electrochemical sensing of dopamine

A novel sandwich-type tungstoantimonate with ferrous ions was synthesized and its functional application in electrochemical sensing of dopamine was demonstrated.