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

Sonoelectrochemical exfoliation of defective black phosphorus nanosheet with black phosphorus quantum dots as a uric acid sensor

To maintain human health, the development of rapid uric acid (UA) sensing is crucial. In this study, defective black phosphorus nanosheets with black phosphorus quantum dots (dBPN/BPQDs) were successfully and rapidly prepared by sonoelectrochemical exfoliation. In this process, the intercalation of phosphate ions into the black phosphorus working electrode was improved by coupling ultrasonic radiation with a high intercalating potential (8 V vs. Ag/AgCl/3M). The dBPN/BPQDs with various vacancies (5-9 defects, 5-7-7-5 defects, and 5-8-5 defect vacancies) exhibited a remarkable mass activity (jm, 1.22 × 10-3 mA μg-1) for uric acid oxidation, which was 5.92 times greater than that of reduced graphene oxide (rGO) (2.06 × 10-4 mA μg-1). In addition, the sensitivity of the dBPN/BPQD UA sensor was 474.2 μA mM-1 cm-2 in the linear analysis range of 0.1-1.3 mM. The sensitivity of the sensor was apparently higher than 67.7 μA mM-1cm-2 for rGO. The data from real sample experiments using serum showed that the dBPN/BPQD catalyst had high recoveries (97.3 %-100.2 %) and low related standard deviation (0.44 %-1.52 %). The dBPN/BPQDs exhibited the potential as an amperometric sensor to detect UA without needing enzymes.

Target-Induced Electrochemical Sensor Based on Foldable Aptamer and MoS2@MWCNTs-PEI for Enhanced Detection of AFB1 in Peanuts

Herein, a sensitive and selective electrochemical sensor based on aptamer folding was constructed to detect aflatoxin B1 (AFB1) in peanuts. Specifically, polyethylenimine-functionalized multiwalled carbon nanotubes modified with molybdenum disulfide (MoS2@MWCNTs-PEI) were used as the electrode matrix to enable a large specific surface area, which were characterized by the Randles-Sevcik equation. Additionally, AuNPs were used to immobilize the aptamer via the Au-S covalent bond and provide a favorable microenvironment for signal enhancement. Methylene blue (MB) was modified at the proximal 3' termini of the aptamer as the capture probe, while the signal transduction of the sensor was obtained through changes in conformation and position of MB induced by the binding between AFB1 and the probe. Changes in spatial conformation could be recorded by electrochemical methods more readily. This electrochemical aptasensor demonstrated remarkable sensitivity to AFB1 with an extensive detection range (1 pg/mL to 100 ng/mL) and a lower limit detection (1.0 × 10-3 ng/mL). Moreover, using the constructed aptasensor, AFB1 was identified successfully in peanut samples, with recoveries ranging from 95.83 to 107.53%, illustrating its potential use in determining AFB1 in food.

Sonoelectrochemical nitrided graphene nanosheets with vacancies and their applications for catalysis and sensing of uric acid oxidation

A sonoelectrochemical method for preparing N-doped defective graphene nanosheets (N/O-dGNs) with point defects and 5-9 or 5-8-5 vacancies and oxygen-containing groups was successfully demonstrated. In this one-pot approach, the N-bonding configuration and N content of N/O-dGNs were finely tuned by the ultrasonic power (192, 320, and 640 W). The N content in atomic percentage (at%) for N/O-dGN (N/O-dGN320W) with point defects and 5-8-5 vacancy prepared at 320 W power was 5.6 at%, greater than 3.0 at% and 2.6 at% for N/O-dGN with point defects and 5-9 vacancies at 192 W and 640 W power (N/O-dGN192W and N/O-dGN640W), respectively. N-bonding sites on N/O-dGN320W were dominantly amine N (2.1 at%) and pyrrolic N (2.4 at%). Additionally, the electrocatalytic activity of N/O-dGN192W, N/O-dGN320W, and N/O-dGN640W was successfully demonstrated for the sequential uric acid (UA) oxidation reaction (UOR), in which N/O-dGN320W displayed a significant mass activity (2.51 A/g). As in the transient catalysis of UOR, N/O-dGN320W with amine N showed 400.8 μA mM-1 cm-2 in sensitivity within a wide linear analysis range (1.5 × 10-2-6 mM) for amperometrically sensing UA. The results of real sample experiments using serum samples further demonstrated the potential of N/O-dGN320W as a non-enzymatic UA sensor.

Co-assembly of graphene/polyoxometalate films for highly electrocatalytic and sensing hydroperoxide

Graphene oxide (GO) films mixed with polyethylenimine (PEI) were prepared by a layer-by-layer assembly (LBL) method, in which the GO component is then converted to reduced GO (rGO) in situ through an electron transfer interaction with a polyoxometalate (POM) that is assembled on the outer surface. With this, devices were manufactured by spreading composite films of (PEI/rGO)_n-POM with different numbers of PEI/rGO layers on ITO substrates. Cyclic voltammetry (CV) reveals that the catalytic activity for H₂O₂ of (PEI/rGO)_n-POM films was significantly higher than that of similar films of (PEI/GO)_n/PEI/POM manufactured LBL with the same number of layers, although the catalyst POM content of (PEI/rGO)_n-POM was only half that of (PEI/GO)_n/PEI/POM. The catalytic activity of (PEI/rGO)_n-POM films first increases and then decreases as the number of PEI/rGO layers increases. The result shows that (PEI/rGO)₃-POM films with three PEI/rGO layers exhibit the highest efficiency. Amperometric measurements of the (PEI/rGO)₃-POM films showed improved current response, high sensitivity, wide linear range, low detection limit, and fast response for H₂O₂ detection. The enhanced catalytic property of (PEI/rGO)_n-POM films is attributed to the electron transfer interaction and electrostatic interaction between POM and rGO.

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.

A sensing platform based on Cu-MOF encapsulated Dawson-type polyoxometalate crystal material for electrochemical detection of xanthine

A promising sensing platform based on polyoxometalate-based metal-organic framework (POMOF) was established for sensitive electrochemical detection of xanthine (XA). In the unique structure of POMOF, the Dawson polyoxoanions P₂W₁₈ were encapsulated into 3D open copper-mixed ligand nanotube framework Cu-MOF, in which the cavity of the metal-organic framework provides a specific shelter to prevent the aggregation and loss of polyoxometalate in electrocatalytic reactions; meanwhile, unsaturated Cu(II) active sites of Cu-MOF can also serve as electrocatalytic active center. The POMOF-based sensor (CuMOFP₂W₁₈/XC-72R) was fabricated by using acetylene black (XC-72R) as a support material to enhance the conductivity of POMOF. The performances of the POMOF-based sensor were studied by using different electrochemical testing methods. The composite displayed remarkable electrocatalytic activity for the oxidation of XA due to the synergistic effect of polyoxometalate (POM) and metal-organic framework (MOF). The electrochemical sensor demonstrated a wide linear range (0.5 μM-240 μM), low detection limit (0.26 μM), and excellent selectivity for detecting XA. Furthermore, the composite further demonstrated excellent reproducibility and great stability. More importantly, the proposed sensor was utilized to detect XA in real samples, which may provide a new way for early disease diagnosis.

"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.

Electrochemical DNA sensors for drug determination

In this review, recent achievements in the development of the DNA biosensors developed for the drug determination have been presented with particular emphasis to the main principles of their assembling and signal measurement approaches. The design of the DNA sensors is considered with characterization of auxiliary components and their necessity for the biosensor operation. Carbon nanomaterials, metals and their complexes as well as electropolymerized polymers are briefly described in the assembly of DNA sensors. The performance of the DNA sensors is summarized within 2017-2022 for various drugs and factors influencing the sensitivity and selectivity of the response are discussed. Special attention is paid to the mechanism of the signal generation and possible drawbacks in the analysis of real samples.

Efficient Point-of-Care Detection of Uric Acid in the Human Blood Sample with an Enhanced Electrocatalytic Response Using Nanocomposites of Cobalt and Mixed-Valent Molybdenum Sulfide

This work efficiently detects uric acid (UA) in a human blood sample using cobalt nanoparticle-immobilized mixed-valent molybdenum sulfide on the copper substrate in a point-of-care (PoC) device. The sensor electrode was fabricated by micromachining of Cu clad boards employing an engraver to generate a three-electrode system consisting of working electrode (WE), reference electrode (RE), and counter electrode (CE). The WE was subjected to physical vapor deposition of mixed-valent MoSx layers by a reaction between Mo(CO)6 and H2S at ∼200 °C using a simple setup following which CoNPs were electrochemically deposited. The RE and CE were covered with Ag/AgCl and Ag paste, respectively. A plasma separation membrane acted as the medium of UA/blood serum delivery to the electrodes. The material and electrochemical characterization confirmed that CoNPs over MoSx provided an enlarged electroactive surface for the direct electron transfer to achieve an enhanced electrocatalytic response. The binary combination of CoNPs and MoSx layers over the Cu electrode reduced the charge-transfer resistance by two times, enhanced the surface adsorption by more than two times, and yielded a high diffusion coefficient of 3.46 × 10-3 cm2/s. These interfacial effects facilitated the UA oxidation, leading to unprecedented mA range current density for UA sensing for the PoC device. The electrochemical detection tests in the PoC device revealed a sensitivity of 64.7 μA/μM cm-2, which is ∼50 times higher compared to the latest reported value (1.23 μA/μM cm-2), a high limit of detection of 5 nM, and shelf life of 6 months, confirming the synergistic effect-mediated high sensitivity under PoC settings. Interference tests confirmed no intervention of similar analytes. Tests on blood samples demonstrated a recovery percentage close to 100% in human serum UA, signifying the suitability of the nanocomposite-based sensor and the PoC device for clinical sensing applications.

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

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

Polyoxometalate Functionalized Sensors: A Review

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

Synthesis of a clustered carbon aerogel interconnected by carbon balls from the biomass of taros for construction of a multi-functional electrochemical sensor

In this study, a clustered carbon aerogel interconnected by carbon balls (CCAI-CB) was prepared as an electrode material to construct a multi-functional electrochemical sensor. CCAI-CB derived from taros (Colocasia esculenta (L). Schott) possesses meso-macroporous structure and plenty of defective sites, and shows notable activity in electrocatalysis as an electrode material. We investigated the application of CCAI-CB modified glassy carbon electrode (CCAI-CB/GCE) for determination of ascorbic acid (AA) and hydrogen peroxide (H₂O₂). Compared with carbon nanotubes (CNTs) modified GCE (CNTs/GCE) and bare GCE, CCAI-CB/GCE shows lower detection limit (0.23 μM for AA and 1.31 μM, S/N = 3), higher sensitivities (220.53, 148.86 or 94.39 μA mM^-1 cm^-2 for AA and 83.06 or 49.07 μA mM^-1 cm^-2 for H₂O₂). Concentrations of AA and H₂O₂ in real samples were determined at CCAI-CB/GCE with satisfactory detection results obtained. In addition, when the CCAI-CB/GCE was used for electrocatalysis of other biomolecules, it also exhibits high electrochemical activity. Thus, CCAI-CB could be a promising electrode material for the construction of multi-functional electrochemical sensors.