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Numerous studies have been reported to improve the selectivity of uric acid (UA) by eliminating the interference from other electroactive species that coexist in biological fluids. However, two main challenges associated with the nonenzymatic electrochemical detection of UA need to be overcome to achieve practical applications in biological samples. Those are the chemical fouling of electrodes caused by the oxidation product of UA and biofouling due to the non-specific absorption of biological macromolecules. It was found that the residual oxo-functional groups and defects on graphene played a crucial part in both electrocatalysis and anti-biofouling. Here, graphene oxide (GO) was tuned by electro-oxidation and electro-reduction and was investigated in antifouling and electrocatalytic performances for the electrochemical sensing of UA by using pristine GO, BSA bound GO, electro-reduction-treated GO and electro-oxidation-treated GO. The electro-oxidation-treated GO was explored in electrochemical sensing for the first time and exhibited the highest sensitivity and low fouling properties. Holey GO might be formed on the electrode surface by the electrochemical oxidation method in a mild and green solution without the use of an acid. The different electrode interfaces as well as the interaction with BSA were investigated by Raman spectroscopy, X-ray photoelectron spectroscopy, contact angle measurements, scanning electron microscopy, electrochemistry, and electrochemical impedance spectroscopy.

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Electrochemical sensors have attracted enormous attention for their precision, high sensitivity, rapid response, and ease-of-use for analysis [...].

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Covalent organic frameworks (COFs) are a new type of metal-free porous architecture with a well-designed pore structure and high stability. Here an efficient electrochemical sensing platform was demonstrated based on COFs TpPa-1 constructed by 1,3,5-triformylphloroglucinol (Tp) with p-phenylenediamine (Pa-1), which possesses abundant nitrogen and oxo-functionalities. COFs TpPa-1 exhibited good water dispersibility and strong adsorption affinities for Pd2+ and thus was used as loading support to modify Pd2+. The Pd2+-modified COFs TpPa-1 electrode (Pd2+/COFs) showed high electrocatalytic activity for both hydrazine oxidation reaction and nitrophenol reduction reaction. In addition, TpPa-1-derived nitrogen-doped carbon presented high activity for the electro-oxidation of reduced glutathione (GSH), and sensitive electrochemical detection of GSH was achieved. The presented COFs TpPa-1 can be utilized as a precursor as well as support for anchoring electro-active molecules and nanoparticles, which will be useful for electrochemical sensing and electrocatalysis.

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Electrode interfaces with both antibiofouling properties and electrocatalytic activity can promote the practical application of nonenzymatic electrochemical sensors in biological fluids. Compared with graphene, graphene oxide (GO) possesses unique properties such as superior solubility (hydrophilicity) in water, negative charge, and abundant oxygenated groups (oxo functionalities) in the plane and edge sites, which play an essential role in electrocatalysis and functionalization. In this work, a micro electrochemical sensor consisting of GO-modified screen-printed electrode and PDMS micro-cell was designed to achieve multi-analyte detection with excellent selectivity and anti-biofouling properties by electrochemically tuning the oxygen-containing functional species, hydrophilicity/hydrophobicity, and electrical conductivity. In particular, the presented electrodes demonstrated the potential in the analysis of biological samples in which electrodes often suffer from serious biofouling. The interaction of proteins with electrodes as well as uric acid was investigated and discussed.

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Electrochemical detection can be used to achieve intracellular or in vivo analysis of reduced glutathione (GSH) in tissues such as brain by using a microelectrode, which can help to better understand the complex biochemical processes of this molecule in the human body. The main challenges associated with electrochemical GSH detection are the chemical fouling of electrodes, caused by the oxidation product of GSSG, and biofouling due to the non-specific absorption of biological macromolecules. Oxo-functionalized graphene was generated in situ on the surface of a glassy carbon electrode using a green electrochemical method without using any other modifiers or materials in a mild water solution. The fabricated oxo-functionalized graphene interface was characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, electrochemistry, electrochemical impedance spectroscopy, and contact angle measurements. The interface showed high electrocatalytic activity towards the oxidation of GSH, and a simple and efficient GSH sensor was developed. Interestingly, the electrode is reusable and could be recovered from the chemical fouling via electrochemical oxidation and reduction treatment. The electrode also exhibited good antibiofouling properties. The presented method could be a promising method used to treat carbon materials, especially carbon-based microelectrodes for electrochemical monitoring of intracellular glutathione or in vivo analysis.

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The developments of alternative signal readout strategies for the ion-selective electrodes (ISEs) are necessary in order to break through the limitation of the Nernst equation. In this work, a simple, convenient and easily operated strategy based on the non-enzymatic amperometric measurement of H2O2 is proposed to read out the potentiometric responses for the ISEs. The proposed amperometric signal readout based on H2O2 is carried out in a two compartment electrochemical cell configuration containing a detection cell and a sample cell, physically connected by a salt bridge. A glassy carbon (GC) electrode is placed in the detection cell to monitor the oxidation current of H2O2, and an ISE is placed in the sample cell to act as both the reference electrode and the potentiometric sensor for obtaining the ion activities. The oxidation of H2O2 is induced by a constant potential applied between the GC electrode and the ISE, and subsequently modulated by the potential change of the ISE in the presence of the primary ion. The proposed amperometric signal readout based on H2O2 shows the satisfied slope sensitivity and detection limit, which are better than or compared to those for the potentiometric responses for the ISEs. This work provides a general strategy for transforming the potential response of the ISEs into the amperometric readout, and is promising for detection of cations (eg., Ca2+) and anions (eg., NO3-) with high sensitivity and excellent selectivity.

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A novel sandwich-type electrochemical immunosensor has been fabricated for simultaneous determination of three kinds of tumor markers (TMs). The signal-amplified platform for sensor was assisted by graphene oxide (GO) loaded with Poly(styrene-alt-maleic anhydride) (PSMA). Three TMs of prostate specific antigen (PSA), human a-fetoprotein (AFP) and carbohydrate antigen 125 (CA 125) were employed as model test molecules. This sandwich-type immunoassay could detect the three antigens in differential pulse voltammetry (DPV) scan well at the same time. Under the optimized conditions, the multiplexed immunosensor displayed a splendid linear response in the range of 1.13 pg mL-1 - 113 ng mL-1 for PSA, 0.35 pg mL-1 - 35 ng mL-1 for AFP and 0.025 U mL-1 - 250 U mL-1 for CA 125. The detection limit was 86 fg mL-1, 14 fg mL-1 and 0.0019 U mL-1 for PSA, AFP and CA 125, respectively. This strategy provides a simple and sensitive method for immunoassay for the identification and validation of specific early cancers.

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The metal-free cousins of metal-organic frameworks, covalent organic frameworks (COFs), are a class of pre-designable crystalline polymers composed of light elements and connected by strong covalent bonds. COFs are being given more and more attention in the electrochemical sensor field due to their fascinating properties, such as highly tunable porosity, intrinsic chemical and thermal stability, structural diversity, large specific surface area, and unique adsorption characteristics. However, there are still some key issues regarding COFs that need to be urgently resolved before they can be effectively applied in electrochemical sensing. In this review, we summarized recent achievements in developing novel electrochemical sensors based on COFs, and discussed the key fundamental and challenging issues that need to be addressed, including the mechanisms underlying charge transport, methods to improve electrical conductivity, immobilization methods on different substrates, synthesis strategies for nanoscale COFs, and the application of COFs in different fields. Finally, the challenges and outlooks in this promising field are tentatively proposed.

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Traditional potentiometric NO3--selective electrodes suffer from a fundamental limitation of the Nernst slope (59.1 mV/dec at 25 °C) due to the relationship between the potential and the logarithmic of ionic activity. Herein, a coulometric signal readout is proposed instead of the potentiometric response for detection of NO3- based on an ordered mesoporous carbon (OMC)-based solid-contact ion-selective electrode (ISE). The mechanism for obtaining the coulometric signal is based on the electrical double layer capacitance of OMC compensating the potential change at the ion-selective membrane/solution interface during the measurements under the control of a constant applied potential. Under the optimized conditions, the coulometric signal for the OMC-based solid-contact NO3--ISE shows two linear responses in the activity range of 1.0 × 10-6-8.0 × 10-6 M and 8.0 × 10-6-8.0 × 10-4 M, and the detection limit is 4.0 × 10-7 M (3σ/s). The proposed coulometric response also shows excellent reproducibility and stability in the presence of O2 and CO2 and light on/off. Additionally, the coulometric response shows acceptable and reliable results for detection of NO3- in mineral water as compared to the traditional potentiometric response and the ion chromatography. This work provides a promising alternative signal readout for detection of ions by using solid-contact ion-selective electrodes.

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A sensitive electrochemical sensor based on the synergistic effect of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and graphene oxide (GO) for low-potential amperometric detection of reduced glutathione (GSH) in pH 7.2 phosphate buffer solution (PBS) has been reported. This is the first time that the combination of GO and TCNQ have been successfully employed to construct an electrochemical sensor for the detection of glutathione. The surface of the glassy carbon electrode (GCE) was modified by a drop casting using TCNQ and GO. Cyclic voltammetric measurements showed that TCNQ and GO triggered a synergistic effect and exhibited an unexpected electrocatalytic activity towards GSH oxidation, compared to GCE modified with only GO, TCNQ or TCNQ/electrochemically reduced GO. Three oxidation waves for GSH were found at -0.05, 0.1 and 0.5V, respectively. Amperometric techniques were employed to detect GSH sensitively using a GCE modified with TCNQ/GO at -0.05V. The electrochemical sensor showed a wide linear range from 0.25 to 124.3µM and 124.3µM to 1.67mM with a limit of detection of 0.15µM. The electroanalytical sensor was successfully applied towards the detection of GSH in an eye drop solution.

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Binary metal oxides have recently attracted extensive attention from researchers in the energy storage and conversion field due to their high energy densities and multiple oxidation states. Novel 3D Co3V2O8 porous rose-like structures and 2D NiCo2V2O8 nanoplates were facilely synthesized via a solvothermal method, and the morphologies, Ni/Co ratios, and surface area of these samples can be easily tuned in the same procedure. The as-prepared Co3V2O8 porous rose-like structure exhibited good electrocatalytic oxygen evolution performance with excellent activity and stability. In addition, 2D NiCo2V2O8 nanoplates delivered a high specific capacitance of 1098.9 F g-1 at 4 A g-1 and good cycling stability (remaining 68% after 7000 cycles) in aqueous KOH electrolyte. The NiCo2V2O8 nanoplates inherit the pseudocapacitive benefits of both Ni3V2O8 and Co3V2O8, showing a higher specific capacitance than pure Co3V2O8 porous rose-like structures.

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The synthesis of chiral zeolites remains a significant challenge because the primary tetrahedral building units are achiral and weak interactions exist between the guest and host frameworks. Here, we present the syntheses and characterization of three new chiral zeolitic halides, [H3(Dabco)2]Ag3X6 (X = Br (1) or I (2), Dabco = 1,4-diazabicyclo[2.2.2]octane) and [H2(Dabco)][(Dabco)Ag4I6] (3). Compounds 1 and 2 are isostructural, containing a 4-connected zeolitic framework built up from 3-ring units, with high-charge [H3(Dabco)2]3+ located in chiral cages. Compound 3 contains a similar zeolitic [Ag3I6]3- framework to that of 2, but a [Ag(Dabco)]+ unit is incorporated in each 3-ring, with [H2(Dabco)]2+ located in channels. These frameworks are chiral, representing the first examples of chiral zeolitic halides. The chirality transference of the frameworks for 1 and 2 was attributed to the template effect of the chiral [H3(Dabco)2]3+ through strong electrostatic interactions and multiple hydrogen-bond interactions. For compound 3, direct coordination interactions play important roles in the chirality transference from the chiral Dabco ligand to the framework.

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Novel electroactive materials with high surface area and stability have great potential for electrochemical sensor. Herein, we demonstrate the exploitation of a porous Cu-based metal-organic framework (Cu-MOF) with large pore size as nonenzymatic sensors for the electrochemical determination of hydrogen peroxide (H2O2) and glucose. The Cu-MOF shows high stability even in NaOH solution. The as-prepared Cu-MOF modified carbon paste electrode (CPE) presents a well-behaved redox event from electroactive metal centers in the MOF at the physiological pH which can be utilized to catalyze the electroreduction of H2O2. It also exhibited excellent electrocatalytic activity towards the oxidation of glucose in alkaline solution. The results showed that the nonenzymatic sensors based on the Cu-MOF display excellent analytical performances, which make it a promising candidate in electrochemical sensor.

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Glycoproteins play important roles in a wide variety of biological processes. The change in the concentration levels has been associated with many cancers, as well as other diseases. Thus, rapid, sensitive and selective determination of glycoproteins is much preferred. In this work, we reported a sandwich-type electrochemical biosensor based on dual-amplification of 4-mercaptophenylboronic acid (MBA)-capped gold nanoparticles (MBA-AuNPs) and dopamine (DA)-capped AuNPs (DA-AuNPs). Biological recognition elements such as synthetic receptor and aptamer immobilized onto gold electrodes were used to capture glycoproteins. The captured glycoproteins were then derivatized with MBA-AuNPs through the formation of tight covalent bonds between the boronic acids of MBA-AuNPs and diols of glycoproteins. Electroactive DA-AuNPs were attached by the anchored MBA-AuNPs via the interaction of boronic acids with DA tags, which facilities the amplified voltammetric detection of glycoproteins. With avidin and prostate specific antigen (PSA) as model analytes, we demonstrated the feasibility and sensitivity of the proposed method. The results indicated that sub-picomolar avidin/PSA can be readily measured. We believe that this strategy will be valuable for the electrochemical detection of other glycoproteins.

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Dopamine (DA) is one of the most important neurotransmitters present in brain tissues and body fluids of mammals. The change in the concentration levels has been associated with various diseases and disorders. Thus, sensitive and selective determination of DA is much preferred. In this work, sandwich-type electrochemical biosensor was developed, in which phenylboronic acid immobilized onto gold electrodes was used to capture DA. The anchored DA was then derivatized with biotin for the attachment of ferrocene-capped gold nanoparticle/streptavidin conjugates. The voltammetric responses were found to be proportional to the concentrations of DA ranging from 0.5 to 50 nM. A detection limit of 0.2 nM was achieved, which is 1~2 orders of magnitude lower than those achievable at various chemically modified electrodes. Analytical merits (e.g., dynamic range, reproducibility, detection level, selectivity and interference) were evaluated. The feasibility of the method for analysis of DA in artificial cerebrospinal fluid and dopamine hydrochloride injection has been demonstrated.

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A fully automated chiral capillary electrophoresis-tandem mass spectrometry (CE-MS/MS) method was developed for enantiomeric quantification of 3,4-dihydroxyphenylalanine (DOPA) and its precursors, phenylalanine (Phe) and tyrosine (Tyr). To avoid MS source contamination, a negatively charged chiral selector, sulfated ß-cyclodextrin (sulfated ß-CD), that migrated away from the detector was used in combination with the partial filling technique. The six stereoisomers were simultaneously quantified in less than 12 min. Detection limits were 0.48 and 0.51 µM for l- and d-DOPA enantiomers, respectively. Assay reproducibility values (relative standard deviations [RSDs], n=6) were 4.43, 3.15, 4.91, 5.16, 3.96, and 3.25% for l- and d-DOPA, l- and d-Tyr, and l- and d-Phe at 10 µM, respectively. Thanks to the high enantioseparation efficiency, detection of trace d-DOPA in l-/d-DOPA mixtures could be achieved. The assay was employed to study the metabolism of DOPA, a well-known therapeutic drug for treating Parkinson's disease. It was found that l-DOPA was metabolized effectively in PC-12 cells. Approximately 88% of l-DOPA disappeared after incubation at a cell density of 2×10(6)cells/ml for 3 h. However, d-DOPA coexisting with l-DOPA in the incubation solution remained intact. The enantiospecific metabolism of DOPA in this neuronal model was demonstrated.

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Previous studies have shown that certain 1,2,3,4-tetrahydroisoquinoline derivatives (TIQs) are neurotoxins inducing Parkinsonism. Further, individual enantiomers of these toxins such as (R/S)-N-methylsalsolinol ((R/S)-NMSal) possess distinct neurotoxicological properties. In this work, a chiral capillary electrophoresis (CE) method with electrospray ionization-tandem mass spectrometric (ESI-MS/MS) detection was developed for the quantification of TIQ enantiomers. Enantioseparation was achieved with sulfated ß-cyclodextrin (sulfated ß-CD) as chiral selector. To avoid any potential contamination of MS ionization source by the non-volatile chiral selector, partial filling technique was deployed in the CE separation. TIQ derivatives, including (R/S)-6,7-dihydroxy-1-methy-TIQ (salsolinol, Sal), (R/S)-1-benzyl-TIQ (BTIQ), and (R/S)-NMSal, were base-line resolved with resolution values (R) ranging from 3 (for Sal) to 4.5 (for BTIQ), which were much better than those reported previously by HPLC methods. ESI-MS/MS detection of the resolved TIQ enantiomers was specific and sensitive (LOD=1.2 µM for Sal enantiomers). The proposed chiral CE-MS/MS method was used to study in vitro formation of (R/S)-NMSal. It was found that NMSal was formed from the incubation of epinine (a dopamine metabolite) with acetaldehyde (a metabolite of alcohol). More interestingly, four isomers of NMSal were separated and detected in the incubation solution. They were identified as (R)-e.e-NMSal, (R)-e.a-NMSal, (S)-e.e-NMSal, and (S)-e.a-NMSal. This was the first lab evidence that this Parkinsonian neurotoxin exists in multiple isomeric forms.

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A rapid and simple method was demonstrated for the analysis of atropine, anisodamine, and scopolamine by nonaqueous capillary electrophoresis (NACE) coupled with electrochemiluminescence (ECL) and electrochemistry (EC) dual detection. The mixture of acetonitrile (ACN) and 2-propanol containing 1M acetic acid (HAc), 20mM sodium acetate (NaAc), and 2.5mM tetrabutylammonium perchlorate (TBAP) was used as the electrophoretic buffer. Although a short capillary of 18cm was used, the decoupler was not needed and the separation efficiency was good. The linear ranges of atropine, anisodamine, and scopolamine were 0.5-50, 5-2000, and 50-2000microM, respectively. For six replicate measurements of 100microM scopolamine, 15microM atropine, and 200microM anisodamine, the RSDs of ECL intensity, EC current, and migration time were less than 3.6%, 4.5%, and 0.3%, respectively. In addition, because the organic buffer was used, the working electrode (Pt) was not easily fouled and did not need reactivation. The method was also applied for the determination of these three alkaloids in Flos daturae extract.

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