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The crucial role of analyte adsorption in electrode reaction kinetics underscores the need for advanced methodologies to study the extent and nature of adsorbed molecules on electrode surfaces. In this study, we employed the classical yet underutilized technique of double potential step chronocoulometry (DPSC) to investigate the nature of adsorbed molecules on electrodes. We assessed the extent of adsorption of dopamine (DA) and the products of ascorbic acid (AA) and uric acid (UA) at increasing concentrations on carbon nanofiber (CNF) electrodes via DPSC measurements and compared the results with those obtained from the commonly used cyclic voltammetry (CV). DPSC findings indicated significant adsorption of component ions, specifically phosphate and chloride of phosphate buffer saline (PBS) electrolyte, on the electrode surface. The results presented in the manuscript demonstrate that DPSC can be a reliable technique for studying the specific adsorption in DA sensors, thus providing valuable insights for modifying materials to enhance DA sensor performance. [ABSTRACT FROM AUTHOR]

[Display omitted] • GO-ZnO nanocomposite modified electrode was designed as an electrochemical biosensor. • This biosensor was used for the electrochemical determination of uric acid. • To demonstrate applicability of the developed method in real samples, measurements were performed in fetal bovine serum. • The determination of uric acid was tested in the presence of different biomolecules such as ascorbic acid and glucose. Uric acid (UA) is the end product of purine metabolism, which is involved in many physiological processes in humans. This compound is produced daily in the human body and the excess is excreted in urine. For various reasons, uric acid in human serum that is lower or higher than the reference range causes some diseases such as liver diseases, nephritis, multiple sclerosis and diabetes. Hence, determination of UA at trace levels is very crucial. Here, a pencil graphite electrode (PGE) modified with graphene oxide-zinc oxide (GO-ZnO) nanocomposite was used to develop an electrochemical biosensor (GO-ZnO/PGE) for UA detection. The prepared GO-ZnO modified electrodes was characterized by scanning electron microscopy (SEM), Energy-Dispersive X-Ray Spectroscopy (EDX), Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV). It was revealed that the existence of ZnO in the GO-ZnO/PGE largely enhanced the surface area and increased its sensitivity for electrochemical sensing of analyte. The uric acid biosensor exhibited voltammetric peaks in the differential pulse voltammetry (DPV) measurements, allowing electrochemical detection of UA. The detection limit of UA was found to be 2.56 μM in buffer medium in the range of 10–100 µM UA. The detection limit of UA was found to be 4.76 μM in 1:1000 diluted FBS medium in the range of 10–100 µM of UA. The GO-ZnO/PGE provided the results in good reproducubility based on electrochemical measuruments while detecting UA. Furthermore, selectivity tests were also performed in FBS medium containing possible interferents such as ascorbic acid (AA) and Glucose (Glu), demonstrating the applicability of GO-ZnO modified electrodes. Sensitivity was calculated for phosphate-buffered saline (PBS) and fetal bovine serum (FBS) mediums and found to be 0.22 and 0.47 μA μg−1 mL cm−2, respectively, which validates the good sensitivity of the GO-ZnO/PGE electrodes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Pectin was extracted from Arthrocnemum indicum leaves for GCE modification. • A wide linear range between 9.10 −5 mM to 2.10−2 mM is obtained. • A detection limit of 0.09 µM is found. • A good selectivity versus amino acids, uric acid and ascorbic acid is observed. • The detection of l -tryptophan in urine samples was successful. Pectin from Arthrocnemum indicum leaves (PAI) extracted from plants, harvested in the Tunisian Sahel, was used for the first time as a sensitive matrix for the design of an electrochemical sensor of l -tryptophan via differential pulse voltammetry (DPV). A glassy carbon electrode (GCE) was modified by a PAI layer through simple adsorption, leading to a DPV signal of l -tryptophan that was increased by a factor of 15. The PAI-modified electrode exhibited a good analytical performance towards the detection of l -tryptophan with a wide linear range between 9.10−5 mM to 2.10−2 mM, a detection limit of 0.09 µM, and a sensitivity of 90.15 µA. mM−1. Compared to other reported electrochemical sensors, PAI/GCE presents a lower detection limit, even without the use of nanomaterials. In addition, the PAI/GCE showed good reproducibility and good selectivity towards the determination of l -tryptophan versus other amino acids, uric acid, and ascorbic acid, making it suitable for the determination of l -tryptophan in biological fluids. [ABSTRACT FROM AUTHOR]

• A nanocomposite BiVO 4 /MWCNT/MIP was developed for photoelectrochemical determination of dopamine. • A synergic effect of BiVO 4 and MWCNT/MIP was noticed. • The PEC sensor was selective towards the dopamine even in the presence of other compounds. This paper describes the development of a new photoelectrochemical (PEC) sensor for dopamine (DA) determination based on the nanocomposite constituted of a molecularly imprinted poly(acrylic acid- co -TRIM) functionalized onto MWCNT with vinyl groups (MWCNT-MIP) physically mixed with monoclinic bismuth vanadate. Synthesized polymeric materials were characterized by FT-IR, TGA, SEM and TEM. The sensor based on glassy carbon electrode modified with BiVO 4 /MWCNT-MIP provided a limit of detection (LOD) of 31.1 nmol/L and a linear range from 103.6 to 2500 nmol/L. The proposed sensor exhibited selectivity to DA in competitive analysis with epinephrine, norepinephrine, ascorbic acid, citric acid, uric acid, creatine and urea, and it was applied for analysis of DA in simulated urine samples using external calibration curve with accuracy attested by recovery values between 91 and 106 %. [ABSTRACT FROM AUTHOR]

• Successful immobilisation of POMs into polypyrrole films. • Redox accessibility of POMs in films. • Catalytic ability of films towards ascorbic acid. The investigation focused on integrating heteropolyanions (HPAs) into a polymeric matrix of polypyrrole (PPy), aiming to achieve effective immobilisation for potential electrocatalytic applications. Specifically, the successful immobilization of a Dawson-type mixed addenda HPA, α 2 – K 8 [P 2 W 17 V4+O 62 ].10H 2 O, within conductive PPy films was achieved. These [P 2 W 17 V4+O 62 ]-doped PPy films exhibited robust stability at both pH 2 and pH 7 during continuous potential cycling involving the V5+/4+ redox process. However, it was noted that in the negative potential range leading to polymer backbone reduction, the HPA tended to leach out from the film. In terms of catalytic assessments, the [P 2 W 17 V4+O 62 ]-dopped PPy films displayed noteworthy electrocatalytic performance in the oxidation of ascorbic acid. Notably, they demonstrated a sensitivity of 8.18 (+/-0.15) μA mM−1 and a limit of detection (LOD) of 1.65 mM at pH 2.0. Furthermore, under neutral conditions, the films exhibited a sensitivity of 6.97 (+/-0.30) μA mM−1 and a LOD of 4.24 mM. Further, endeavours were made to immobilise alternative variations of HPAs, specifically, the crown HPA, K 28 Li 5 H 7 P 8 W 48 O 184 ·92H 2 O, and Cu2+ substituted Krebs HPA, K x Na (10-x) Bi 2 W 20 Cu 2 O 70 (H 2 O) 6 ·30H 2 O (with × ranging from 1 to 10), into PPy films. Initial findings from these attempts are also briefly discussed in this study. [ABSTRACT FROM AUTHOR]

[Display omitted] Schematic presentation of the proposed multiple electron transfer with intervening chemical reaction-ECE mechanism of dopamine, and its application in selective determination of dopamine in the presence of high concentrations of ascorbic acid through electrochemical reductive approach using reverse voltammetry. • A new insight for the electrochemical analysis of dopamine based on reverse voltammetry preoxidation was presented. • The proposed analytical strategy of reverse voltammetry preoxidation is applicable to monoamine neurotransmitters with a catechol structure. • The dual quantitative analysis of dopamine in the presence of ascorbic acid (1.0 mM) was achieved with the linear range of 2.0 to 80 μM and 60 to 200 μM, respectively. In this paper, the cyclic voltammetric behavior of dopamine (DA) was investigated systematically at different potential windows on electrochemical reduce graphene oxide modified glassy carbon electrode, and we found that the electrochemical redox mechanism of DA is a multi-step ECE process and the redox peak appearing at low potentials (∼–0.30 V) was dependent on the oxidation of DA at high potentials (∼0.20 V). The electrolyte enriched with the DA preoxidation product was then analyzed by LC-MS/MS and its structure was identified as dopaminochrome (DC). So, we proposed a new insight for the non-oxidative electrochemical analysis of DA by reverse voltammetry preoxidation and intramolecular cyclization. Compared with the traditional forward scaning mode, the approach could minimize the interference of ascorbic acid (AA) and enable the dual quantitative analysis of DA in the presence of high concentration of AA with the linear range of 2.0 to 80 μM and 60 to 200 μM, respectively, and a detection limit of 1.0 μM. Furthermore, the voltammetric properties of norepinephrine (NE), epinephrine (EP), 3,4-dihydroxy- l -phenylalanine (l -DOPA), 4-O-methyldopamine (HMP) were also investigated. The similar voltammetric properties to DA of NE, EP, l -DOPA and HMP suggested that the proposed reverse voltammetric preoxidation method is generalizable and equally applicable to monoamine neurotransmitters with a catechol structure. [ABSTRACT FROM AUTHOR]

[Display omitted] • A label-free PEC sensor based on BiVO 4 @GO was applied to antioxidants analysis. • Ascorbic acid, caffeic acid, chlorogenic acid, fisetin and quercetin were tested. • This PEC sensor exhibited good sensitivity of above five kinds of antioxidants. • This PEC sensor presented excellent anti-interference and stability. • This PEC sensor was used for antioxidant capacity evaluation of food. Antioxidant can protect body from free radical and dietary intake is a major source of antioxidants for human body. Therefore, it is particularly important to evaluate the antioxidant capacity of food and drink in daily diet. Herein, a label-free photoelectrochemical (PEC) sensor based on BiVO 4 @graphene oxide (GO) hybrid has been developed for antioxidants analysis and applied it to antioxidant capacity detection of real samples. Five representative antioxidants including ascorbic acid (AA), caffeic acid (CA), chlorogenic acid (CHA), fisetin (FT) and quercetin (QT) were determined by this PEC sensor to acquire photocurrent response at different concentrations. This PEC sensor exhibited excellent sensitivity and anti-interference. Furthermore, antioxidant capacity of food samples was assessed utilizing this PEC sensor, indicating the practicability and accuracy of this PEC sensor. This study suggests that BiVO 4 @GO composites constructed PEC sensor can provide a practical and fast method for detecting antioxidant capacity in the field of food. [ABSTRACT FROM AUTHOR]

[Display omitted] • Na 3.75 Mn 0.75 V 1.25 (PO 4) 3 / x VitC composites are fabricated by substitution of Mn for V and vitamin C as carbon source. • Na 3.75 Mn 0.75 V 1.25 (PO 4) 3 /2VitC reveals an outstanding rate capability and long cyclic stability. • Na 3.75 Mn 0.75 V 1.25 (PO 4) 3 /2VitC shows a high initial Coulombic efficiency up to 97.5% at 0.2C and capacity retention of 78% over 4000 cycles at 10C. Owning to non-toxic and high working potential, Mn-substituted NASICON-type Na 3+ x Mn x V 2- x (PO 4) 3 cathodes have promising application in sodium ion batteries (SIBs). However, the poor cycling stability limits their further practical applications. Herein, a succession of carbon coated Na 3.75 Mn 0.75 V 1.25 (PO 4) 3 cathodes are constructed via a sol–gel combining with heating treatment. The effects of carbon content on the structure, electrochemical properties and sodium ion diffusion kinetics are investigated. The optimal composition, Na 3.75 Mn 0.75 V 1.25 (PO 4) 3 /2VitC, displays an outstanding specific capacity and long-cycling stability. It releases a discharge capacity of 105.6 mAh g-1 with a high initial Coulombic efficiency of 97.5% at 0.2C. It can also deliver a high capacity of 92.6 mAh g-1 with a capacity retention of 88.3% at 5C over 2000 cycling, and the reversible capacity can achieve to 79.1 mAh g-1 with 78% capacity retention after 4000 cycles at high rate of 10C. Ex-situ X-ray diffraction and X-ray photoelectron spectroscopy reveal a biphasic reaction process and high structural reversibility during sodium ion insertion/extraction. The good multiplicity performances, excellent long cycle stability and low cost of the optimized Na 3.75 Mn 0.75 V 1.25 (PO 4) 3 composite indicate that it is a promising cathode material for SIBs. [ABSTRACT FROM AUTHOR]

A novel electrochemical sensor was constructed based on a zinc porphyrin dye (YD2-o-C8, designated as YD) functionalized multi-walled carbon nanotubes (MWCNTs) hybrid. YD has a special donor-π-bridge-acceptor planar structure, which is beneficial for the π-π stacking interaction between YD and MWCNTs. The interaction was characterized by UV–vis, fluorescence, Fourier transform infrared (FT-IR) spectroscopy, and electrochemistry. The results showed that YD easily interacted with MWCNTs, which enhanced the electron transfer between YD and the electrode. The obtained YD/MWCNT hybrid was modified on the glassy carbon electrode (GCE) by a surfactant of tetraoctylammonium bromide (TOAB) to fabricate an electrochemical sensor (TOAB/YD/MWCNT/GCE). In the interface material of the sensor, YD acted as an electron intermediate, MWCNTs acted as an electron accelerator, and TOAB was used as an arouser to trigger the inner redox activity of YD. With their synergistic effect, the proposed sensor exhibits excellent electrocatalytic activity for the oxidation of ascorbic acid (AA). At the optimized conditions, the sensor exhibited a linger response to AA in the concentration range from 18.72 μM to 1.85 mM, high sensitivity of 19.16 μA mM−1, and a low limit of determination (0.18 μM). Furthermore, the TOAB/YD/MWCNT/GCE exhibited excellent stability and reproducibility for the response of AA in an aqueous solution. The desirable detecting recoveries for vitamin C tablets were obtained from 97.1% to 105.2%, indicating the reliability and feasibility in determining AA in real samples. Unlabelled Image • A porphyrin dye functionalized multi-walled carbon nanotubes to obtain new hybrid. • An electrochemical sensor based on the hybrid has high sensitivity and stability. • The method broadens the application of the porphyrin dye of YD. • The study paves a new avenue for fabricating non-enzymatic electrochemical sensor. [ABSTRACT FROM AUTHOR]

To eliminate the interference effect of ascorbic acid (AA) and reduced nicotinamide adenine dinucleotide (NADH) on electrochemical detection of uric acid (UA), a non-enzymatic approach for electrochemical detection of UA was developed using the Nafion-modified screen-printed carbon electrode strip (SPCEs). Electrodeposition was utilized to control the thickness of Nafion film. With Nafion-modified SPCEs, the selectivity of UA from AA and NADH was investigated. The Nafion-modified SPCEs exhibits high selectivity of UA, good linearity, and reasonable detection limit in neutral pH environment. The interference effect can be evidently reduced. The advantages are no requirements of enzyme reagents, acid environment, and complicated preparation of the electrodes. Unlabelled Image • Non-enzymatic EC detection of UA with Nafion Film was investigated in neutral pH. • Thickness of Nafion film was controlled at the micron scale by electrodeposition. • SPCEs modified by ~6 μm Nafion film can selectively detect UA from AA and NADH. [ABSTRACT FROM AUTHOR]

Abstract A novel photoelectrochemical (PEC) aptasensor based on the g-C 3 N 4 nanosheets modified WO 3 film with inverse opal photonic crystals (IOPCs) structure was developed. Oxytetracycline (OTC) aptamer was immobilized on the surface of composites through π-π interaction between g-C 3 N 4 and DNA nucleotide. Benefiting from the great optical absorption ability and photo-generated charge separation capacity, the g-C 3 N 4 /WO 3 IOPCs aptasensor exhibited excellent PEC response performance in an electrolyte containing 0.1 M ascorbic Acid (AA). After the recognition and capture of OTC, the configuration change of OTC-aptamer restrained the oxidation of AA by the photogenerated hole on the valance band of g-C 3 N 4 , resulting in photocurrent change. The results showed that the PEC aptasensor possessed a linear detection range from 1 nM to 230 nM and a detection limit of 0.12 nM (≈59.58 ng L−1) for the detection of OTC in water. The satisfactory detection results in real sample further revealed the potential of WO 3 IOPCs and its heterostructure for PEC analytical applications, and provided a novel approach for design of PEC platform. Graphical abstract Unlabelled Image Highlights • The g-C 3 N 4 /WO 3 inverse opal photonic crystals (IOPCs) were designed as photoelectrochemical (PEC) sensing platform. • The g-C 3 N 4 /WO 3 IOPCs possessed great optical absorption and fast charge separation. • The g-C 3 N 4 /WO 3 IOPCs PEC aptasensors were used to detect the OTC concentration in water. • It exhibited good reproducibility, high selectivity and excellent sensitivity. [ABSTRACT FROM AUTHOR]

Abstract Serotonin is a monoamine neurotransmitter produced from tryptophan and abnormal levels of these molecules have been associated with several disorders, notably depression, migraine, bipolar disorder, anxiety and schizophrenia. In the present work, we have developed a novel electrochemical sensor using SnO 2 -SnS 2 nanocomposite for selective and simultaneous determination of depression biomarkers serotonin (SE) and tryptophan (Trp) in the presence of ascorbic acid (AA). The SnO 2 -SnS 2 composite prepared by hydrothermal method was characterized by powder XRD, FE-SEM, FITR, UV–Visible, CV, EIS and SWV techniques. The SnO 2 -SnS 2 modified glassy carbon electrode (SnO 2 -SnS 2 /GCE) exhibited two well defined oxidation peaks at 0.43 and 0.83 V corresponding to SE and Trp respectively in phosphate buffer saline at pH 7. Under optimum conditions, linear relationships between the peak currents and concentrations were obtained in the range of 0.1–700 μM for SE and 0.1–800 μM for Trp with the lowest detection limits (3 s/m) of 45 and 59 nM respectively. Significant peak to peak separations and higher peak currents make this electrode suitable for simultaneous detection of the two important neurochemicals SE and Trp. The sensor was successfully applied for the detection of SE in human blood samples and Trp in spirulina with high recovery percentage. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract By using cyclic voltammetry (CV), sunset yellow film was electrodeposited on the surface of carbon paste electrode by electropolymerization technique to obtain the poly (Sunset yellow) modified carbon paste electrode (CPE). The fabricated poly (Sunset yellow) modified CPE showed excellent electro-catalytic activity towards the oxidation of dopamine, ascorbic acid and uric acid individually and simultaneously. The oxidation peaks were well developed with more enhancement of oxidation peak current in 0.2M PBS of physiological pH (7.4) at the sweep rate of 50 mV/s. The effect of variation of sweep rate (20 to 300 mV/s) and pH (5.8–7.8) on CV of DA were studied. The lower limit of detection of DA was 0.9 μM and simultaneous separation DA, AA and UA was achieved by both CV and DPV techniques. The proposed method was applied to the detection of dopamine in injection samples. Graphical abstract Unlabelled Image Highlights • Electropolymerisation of sunset yellow was carried out by cyclic voltammetry. • The modified electrode shows stability, selectivity and sensitivity. • Simultaneous electrocatalytic activity towards the determination of DA, AA and UA was confirmed. [ABSTRACT FROM AUTHOR]

Abstract Ferrocenyl dendrimers based on octasilsesquioxane cores electrodeposited on the electrode surface form a 3D structured film with an interesting highly effective surface. This property, together with the presence of interacting ferrocenes, make these films useful electrode materials for two-electron exchanges. This work presents the morphologic and kinetic studies of electrodes modified with a second-generation octasilsesquioxane dendrimer with 32 ferrocenyl terminal groups and their application in the determination of l -tryptophan (TR). The modified electrode shows a reversible redox system without kinetic limitations and allows to determine TR in two wide linear ranges (from 0.06 to 100 μM and from 100 to 440 μM), with low detection limit (0.01 μM). In addition, the modified electrode is successfully applied to detect TR in presence of ascorbic acid, dopamine and uric acid and to their simultaneous determination. The new device has been also applied to the direct determination of TR in pharmaceutical and biological samples with satisfactory results. Graphical abstract Unlabelled Image Highlights • 3D electrocatalytic surface based on ferrocenyl silsesquioxane dendrimer. • Morphologic, electrochemical, kinetic and analytic studies of FOD/GCE are included. • The developed FOD/GCE showed absence of kinetic limitation and fast electron transfer. • FOD/GCE show wide linear range and low detection limit toward the TR determination. • The simultaneous determination of AA, DA, UA and TR has been successfully achieved. [ABSTRACT FROM AUTHOR]

Abstract Owing to the challenges faced in simultaneous determination of dopamine(DA), uric acid(UA) and ascorbic acid (AA) in mixtures by electrochemical method, the present work is aimed at developing a simple, sensitive and selective sensor using sudan III as the modifier. The poly (sudan III) modified carbon paste electrode (PS/MCPE) was prepared by electropolymerization of sudan III monomer on the surface of bare carbon paste electrode (BCPE). Electrochemical sensing properties of poly (sudan III) MCPE were evaluated by cyclic voltammetry for individual analytes and findings revealed that it has a good electrocatalytic activity towards the determination of dopamine (DA), uric acid (UA) and ascorbic acid (AA) characterized by a low detection limit of 9.3 × 10−6 M, 1 × 10−5 M and 4.5 × 10−5 M for DA, UA and AA, respectively. The scan rate study reveals that the electrode process is adsorption-controlled. The simultaneous determination of the analytes in a mixture showed that the modified electrode exhibits an outstanding selectivity towards DA, UA and AA with their distinct oxidation potentials occurring at 0.14 V, 0.27 V, and −0.07 V, respectively. The fabricated modified electrode sensor exhibited excellent electrocatalytic properties towards the determination of DA, UA and AA with satisfactory results. Graphical abstract Unlabelled Image Highlights • The poly (sudan III)/MCPE was fabricated through electropolymerization method using sudan III as monomer. • The developed sensor exhibited excellent electrocatalytic activity towards the oxidation of DA, UA and AA. • Good LOD and LOQ were obtained. • Real sample analysis was carried out. [ABSTRACT FROM AUTHOR]

Abstract Sensitive and selective electrochemical sensors based on indium-doped tin oxide electrodes (ITO) modified with differently nitrogen-doped graphene (NGr) films were successfully developed for the individual and simultaneous determination of uric acid (UA) in the presence of ascorbic acid (AA). The influence of the contents of various N-configurations on the detection of UA on the NGr/ITO based sensors is herein reported for the first time. The contents of various N-configurations on the NGr/ITO electrodes displayed varying promotional effects of the electro-oxidation of UA; attributed to the unique properties of the NGr films arising from the N-configurations. In particular, three N-doped materials prepared with varying amount of graphitic-N, pyrrolic-N, and pyridinic-N allowed the individual detection of UA, in the presence of AA, in the concentration range from 2 to 320 μM with the detection limits of 0.043 μM to 0.19 μM. Finally, the NGr/ITO-based sensors exhibited successful simultaneous detection of UA and AA, depicting detection limits of 0.051–0.19 μM and 0.93–5.1 μM within the concentration ranges of 6–280 μM and 8–800 μM, for the detection of UA and AA, respectively. The study provided insight into an understanding as to how the different contents of N-configurations influenced the electrocatalytic activity of NGr/ITO electrodes for the individual and simultaneous detection of uric acid. Graphical abstract Study highlights the correlation between the N-configurations and the electrochemical detection of uric acid in the presence of ascorbic acid. Unlabelled Image Highlights • The use of a surfactant free method , in which films (not particles) of N-doped graphene deposited on indium tin oxide (ITO) have been a used as the electroactive material. • Correlation of the type of N doping with the electrochemical activity for uric acid detection in the presence of high concentrations of ascorbic acid. • Indication that graphitic N hinders with the activity, whereas all the defective N-configurations show correlation to the improved performance. [ABSTRACT FROM AUTHOR]

Abstract In sensor research community, the advances in materials are main strategy for driving developments in the field, with an ultimate objective to enhance three interrelated analytical parameters that are sensor sensitivity, selectivity and response time. Herein, we demonstrate the design and fabrication of highly sensitive and selective amperometric sensor to detect ascorbic acid (AA) with rapid and reproducible response behavior. The sensor based on glassy carbon electrode (GCE) that initially modified with novel TiO 2 /reduced graphene oxide (rGO) nanocomposites synthesized by simple chemical and calcination processes. The as-synthesized nanocomposite was fully characterized by XRD, FTIR-Raman, SEM-EDS and TEM, which revealed wrinkled wave like 2-D graphene nano-sheets with crystalline anatase phase of TiO 2 nanoparticles anchored on the surface. Cyclic voltammetry and electrochemical impedance spectroscopy showed enhanced electrochemical activity of TiO 2 /rGO nanocomposite modified GCE compared to either bare GCE or TiO 2 modified electrode. Sensor analytical parameters derived from amperometric measurements indicated outstanding sensing performance, giving a superior sensitivity 1.061 μA μM−1 cm−2, low limit of detection (LOD) 1.19 μM at (S/N = 3) within a linear range of AA concentration (25 to 725 μM) with a correlation coefficient R 2 = 0.9888 and a rapid response time <5 s. The newly proposed sensor electrode exhibits unique electrochemical stability, repeatability and high selectivity in the presence of common interfering species of glucose, dopamine, citric acid or uric acid. The kinetic study indicated the electrode reaction proceeds via a diffusion-controlled process. The current TiO 2 /rGO nanocomposite modified GCE gives acceptable detection result towards a commercially available vitamin C supplement. Graphical abstract Unlabelled Image Highlights • Novel TiO 2 /rGO nanocomposite as efficient ascorbic acid (AA) amperometric sensor • Remarkable sensitivity 1.061 μA μM−1 cm−2, low LOD 1.19 μM and rapid response time <5 s • A wide linear range over AA concentration from 25 up to 725 μM • The electro-oxidation of AA proceeds via diffusion-controlled kinetics • Sensor electrode accuracy was validated using a vitamin C supplement. [ABSTRACT FROM AUTHOR]

Abstract Conductive polymer hydrogels (CPHs) have exhibited great potential as advanced material for sensing. Here, a facile approach to 3D networked polypyrrole (PPy) hydrogel is reported. This strategy relies on the doping of the azo dye molecule Tartrazine during the synthesis of nanostructured conductive PPy hydrogel. The prepared PPy hydrogel can be highly dispersible in the mixed solvent of isopropanol/H 2 O (4/1). The unique 3D network nanostructure endow PPy hydrogels with high electric conductivity and effective interface area for enhanced sensing performance towards the simultaneous determination of ascorbic acid, dopamine and uric acid with respective lower detection limits of 1.283, 0.044 and 0.046 μmol L−1. The as-designed electrochemical sensor also exhibited excellent stability and reproducibility. Graphical abstract Unlabelled Image Highlights • A facile approach to highly interconnected PPy hydrogel is reported with azo dye Tartrazine as cross-linker. • The optimized PPy hydrogels exhibit high electric conductivity and abundant pore architecture. • The 3D networked PPy hydrogels realize simultaneous determination of ascorbic acid, dopamine and uric acid. [ABSTRACT FROM AUTHOR]

Abstract Parkinson's disease is the most common neurodegenerative disorders in human mainly due to insufficient norepinephrine supply. Hence, determination of trace level of norepinephrine (NE) in presence of interfering compounds in biological fluids such as paracetamol (APAP), tyrosine (TR), and ascorbic acid (AA) is essential to diagnosis some psychiatric and neurological disorders diseases. A layer-by-layer stacked composite sensor based on crown ether (crown), Nafion® (NF), poly (3-4-ethylene dioxythiophene) (PEDOT) and multi-walled carbon nanotubes (CNT) was fabricated (GC/CNT/PEDOT/NF/Crown). Each component possesses a unique feature that contributes to the remarkable electro-catalytic activity of the sensor. Crown ether allows excellent host-guest recognition and binding ability; NF has good ionic conductivity; PEDOT has high electrical conductivity and stability; and CNT has large surface area and mechanical strength. The proposed sensor was successfully applied for electro-oxidation and separation of quaternary mixture of AA, NE, APAP, and TR into four well-defined peaks with separations in potential (ΔE p) of: 210, 163 and 257 mV for AA-NE, NE-APAP and APAP-TR, respectively. The separation of these compounds in real human blood serum samples was also successful. Detection limits were 8.46 nmol L−1, 0.0862 nmol L−1, 0.0627 nmol L−1 and 0.429 nmol L−1 in linear dynamic ranges of 0.1–30 μmol L−1, 0.007–10 μmol L−1, 0.005–8 μmol L−1 and 0.06–20 μmol L−1, for AA, NE, APAP and TR, respectively. Graphical abstract Unlabelled Image Highlights • Layer-by-layer composite sensor is fabricated from crown-Nafion-PEDOT-MWCNT. • NE was determined in trace level in quaternary mixture containing APAP, TR and AA. • The sensor was applied for NE in human blood serum with excellent stability. • The sensor detection relied on host-guest, ionic/electronic conduction and large surface area. [ABSTRACT FROM AUTHOR]

Abstract Psidium guajava leaves are laden with potential reducing analytes namely quercitin, chlorogenic acid, d ‑glucose, ascorbic acid and capping agents like citric acid. This work reported on the one-pot synthesis of AgNPs by these analytes extracted in an aqueous medium and the synthesis rate was synergized by microwave (MW). The synergy between the MW input powers and disintegration of capping agents leading to particle aggregation was explored. The modified electrode (AgNPs (pH 9.5)/GPE) exhibited a reduction in charge transfer resistance from 3.2 to 0.453 kΩ. A single electron reaction (Tafel slope 103 mV/decade, α = 0.57) of ascorbic acid (AA, 0.380 V vs. Ag/AgCl) was found in a phosphate buffer media (pH 7.2) and, AA oxidation was diffusion control at AA ≤ 150 μM and activation control at 150 ≤ AA ≤ 2000 μM. Dopamine and uric acid couldn't interfere the response current of the chronoamperometric test of AgNPs(pH 9.5)/GPE and, the (peak)separation potential of 0.272 and 1.114 V was found for AA-dopamine and AA-uric acid. The limit of detection and sensitivities of AgNPs(pH 9.5)/GPE were estimated as 14.63 μM and, 0.719 (diffusion control) and 0.390 (activation control) μA/cm2·μM. The concentration of AA in S. edule and Z. mauritiana extract determined using AgNPs(pH 9.5)/GPE well matched with high performance liquid chromatography. Graphical abstract Unlabelled Image Highlights • Microwave irradiation decreased AgNPs synthesis time to 90 s from 10 h at pH 9.5 using Psidium guajava leaves extract • Microwave overexposure led to AgNPs aggregation with 5 times particle size increase • AgNPs electrocatalyzed selective ascorbic acid sensing in phosphate media with a low detection limit (14.63 μM) • AgNPs catalyzed ascorbic acid sensing in biological entities was comparable with HPLC (variation ≤5%) [ABSTRACT FROM AUTHOR]

A fast and sensitive electrochemical sensor for simultaneous determination of ascorbic acid (AA) and acetaminophen (AP) was proposed based on g-C 3 N 4 nanofibers (h-CN) doped poly(3,4-ethylenedioxy-thiophene) (PEDOT) composite. PEDOT/h-CN hybrid film was directly deposited on glass carbon electrode (GCE) by in-situ electropolymerization. The incorporation of h-CN and PEDOT brought out enhanced electro-catalytic activity and additional binding sites for AA and AP, which could be ascribed to the synergetic effect between h-CN and PEDOT. Under the optimal experimental condition, the oxidation peak currents of AA and AP give linear response over the range of 4–20 μM, 20–1800 μM and 1–10 μM, 10–50 μM, respectively. The detection limit of the modified electrode was calculated to be 1.51 μM for AA and 0.49 μM for AP (S/N = 3). The electrochemical experiments also suggested that PEDOT/h-CN modified electrode was feasible practice for quantitative detection of AA and AP. [ABSTRACT FROM AUTHOR]

Based on self-assembly and electrodepositing, a two-dimensional PMo 10 V 2 /GO/PDDA hybrid nanocomposite modified glass carbon electrode (GCE) was successfully fabricated for the first time. Graphene oxide (GO), PDDA, and PMo 10 V 2 have been modified on GCE surface. Analysis by SEM and FTIR, GO has provided larger specific surface area for improving electrocatalysis activity of PMo 10 V 2 . The electrochemical behavior and electrocatalysis of the modified electrode were studied by detailed characterization of cyclic voltammetry (CV) and amperometric (i-t) methods. The experimental results demonstrated that PMo 10 V 2 /GO/PDDA/GCE has high stability, fast response, and good electrocatalytic activity towards the oxidation of nitrite ion (NO 2 − ), ascorbic acid (AA), and dopamine (DA) compared to GCE, PDDA/GCE, and GO/PDDA/GCE respectively. Their concentrations are linearly proportional to the amperometric signals. The correlation coefficients of NO 2 − , AA, and DA are 0.999, 0.998, and 0.998 respectively. The detection limits were estimated to be 0.45 μM, 0.03 μM, and 0.18 μM for NO 2 − , AA, and DA with the signal to noise ratio of 3. Almost no interference effect was observed for sensing of NO 2 − , AA, and DA in the presence of each other. The proposed modified electrode was employed for the determination of the NO 2 − , AA, and DA level in human blood serum with recoveries between 97.35% and 104.2%. [ABSTRACT FROM AUTHOR]

A nanocomposite of SiO 2 @Fe 3 O 4 were synthesized and characterized by Fourier-transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy. The poly(dipicolinic acid)/SiO 2 @Fe 3 O 4 based electrochemical based sensor was fabricated by the electropolymerization of dipicolinic acid (DPA) on the surface of SiO 2 @Fe 3 O 4 nanocomposite immobilizing the carbon paste electrode (CPE). The developed electrochemical sensor was abbreviated as poly(DPA)SiO 2 @Fe 3 O 4 /CPE. This electrochemical sensor shows an excellent electrochemical activity towards the detection of uric acid. Under the optimized experimental conditions the linear dynamic range (LDR) for the sensing of uric acid was calculated to be 1.2 to 1.8 μM with a good limit of detection (LOD) and limit of quantification (LOQ) of 0.4 μM and 1.2 μM, respectively. The poly(DPA)SiO 2 @Fe 3 O 4 /CPE has also shown the significant sensitivity and selectivity towards the sensing of uric acid in the presence of ascorbic acid and dopamine without any interference. The developed sensor was also successfully applied for the determination of uric acid in real sample analysis. [ABSTRACT FROM AUTHOR]

A simple and sensitive sensor of poly(3,4-ethylenedioxythiophene)/reduced graphene oxide/manganese dioxide modified glassy carbon electrode (PrGO/MnO 2 ) was fabricated via cyclic voltammetry (CV) for simultaneous detection of uric acid (UA), dopamine (DA) and ascorbic acid (AA). The PrGO/MnO 2 composite film possessed excellent electrocatalytic rate and high selectivity towards the oxidation of UA, DA and AA in 0.1 M PBS (pH 6.0). The peak potential separation ( ∆E p ) of AA-DA, AA-UA and DA-UA were 166, 312 and 146 mV, respectively. The detection limits of 1.00, 0.02 and 0.05 μM with a linear response of 1–800, 0.03–45 and 0.3–80 μM were obtained for AA, DA and UA, respectively. This sensor also showed an excellent stability (reproducibility and repeatability). [ABSTRACT FROM AUTHOR]

[Display omitted] • Creatinine can be quantitated though its reaction with an electrochemical probe molecule: 3,5-dinitrobenzoate (3,5-DNB). • Chemical interference of urinary species can be greatly suppressed by 5 min reaction of creatinine with 3,5-DNB at 0.75 M NaOH. • With the use of real human urine samples, reliability of this method was confirmed by comparing the data obtained from this and conventional method. A simple and reliable method for creatinine quantitation was developed for urine-based diagnosis as creatinine is an internal standard molecule for the calibration of fluctuating urinary concentration. Herein, the quantitation of creatinine in artificial urine was performed in the range of 1–30 mM, covering the normal concentration of human urine, using 3,5-dinitrobenzoate (3,5-DNB) as an electrochemical probe molecule. The chemical interference from various urinary species, including urea, uric acid, ammonia, ascorbic acid, alcohol, albumin, bilirubin, acetaminophen, and glucose, is confirmed to be significantly suppressed when the reaction of creatinine and 3,5-DNB is limited to 5 min with an increased OH– concentration. Additionally, the reliability of this method is confirmed for the use of real human urine samples by comparing the data obtained from this and with that of the conventional method. [ABSTRACT FROM AUTHOR]

[Display omitted] • Bidimensional Spectroelectrochemistry is used for quantitative analysis. • The best spectroelectrochemistry configuration is easily selected. • Interferences can be avoided using the most suitable experimental setup. • Good figures of merit for epinephrine were obtained using parallel configuration. This work demonstrates how the way a chemical system is sampled plays a key role in spectroelectroanalysis, illustrated by the quantification of an analyte in presence of an antioxidant compound. For this purpose, bidimensional spectroelectrochemistry experiments were performed using epinephrine as the model analyte and ascorbic acid as antioxidant and interfering compound, as a proof of concept. This is the first time that three calibration curves are obtained simultaneously on a single spectroelectrochemistry data set, one for the electrochemical signal and two for the optical responses in normal and parallel configurations. The differences between the two optical arrangements, that are related to the diffusion process which is an essential feature for the spectroelectrochemical detection of compounds, have been experimentally demonstrated. As can be observed, the spectral signal in parallel configuration allows us to obtain the best analytical results, since in this configuration only the first micrometers of the solution adjacent to the electrode surface are sampled, thus removing the interfering effect of the antioxidant compound. This fact does not occur with either the electrochemical signal or the spectral response in normal configuration. Furthermore, it has been shown that the parallel configuration provides better results than the normal configuration in terms of sensitivity. In summary, epinephrine is successfully detected in a simple and effective way, even in the presence of a direct antioxidant compound such as ascorbic acid at different concentrations levels, which makes spectroelectrochemistry a good choice for quantitative analysis. [ABSTRACT FROM AUTHOR]

Novel branch-trunk Ag hierarchical nanostructures were fabricated by hydrothermal process combined with microwave-assisted synthesis. Te nanowires prepared by hydrothermal process were used as template. And then the branch-truck Ag hierarchical nanostructures were constructed on Te nanowires via galvanic replacement reaction with the Te nanowires as the sacrifice temple with microwave-assisted synthesis method. We further investigated the potential application of the obtained hierarchical nanostructures as sensors for the electrochemical analysis of ascorbic acid. Results showed that the as-prepared sensors had a wide linear range of 0.17 μM to 1.80 mM (R = 0.999) and a detection limit of approximately 0.06 μM (S/N = 3). These indicated that the branch-trunk Ag hierarchical nanostructures were excellent candidate materials for sensing applications. [ABSTRACT FROM AUTHOR]

In this work, a new electrochemical sensor was fabricated based on Sr 2 PdO 3 nano-perovskite manually mixed with carbon nanotubes (CNTs) and casted on glassy carbon (GC) electrode surface; GC/(CNTs-Sr 2 PdO 3 ). Enhanced sensitivity, selectivity, stability and electron transfer kinetics were realized by the application of nano-perovskite modified carbon nanotubes composite for determination of dobutamine (DB). The proposed sensor showed high electro-catalytic activity in the linear dynamic range of 0.03 μmol L −1 to 50 μmol L −1 with sensitivity of 20.91 μA/μmol L −1 , detection limit of 0.012 nmol L −1 and a quantification limit of 0.04 nmol L −1 . The proposed method was successfully applied for DB detection in blood serum samples with good recovery results. The sensor showed anti-interfering ability in presence of common interferences as ascorbic acid, uric acid, acetaminophen, serotonin and morphine. [ABSTRACT FROM AUTHOR]

In this work, we report a novel, cost effective and environmental-friendly green synthesis method of reduced graphene oxide (rGO) – tin oxide (SnO 2 ) composite using lemon extract by hydrothermal process for selective and ultra-sensitive electrochemical detection of ascorbic acid. The electrochemical behavior of ascorbic acid at the surface of rGO-SnO 2 composite modified glassy carbon electrode (GCE) was investigated by cyclic voltammetry (CV) technique whilst ascorbic acid sensing was performed by using differential pulse voltammetry (DPV) technique. An excellent sensitivity of 1200 μA mM −1  cm −2 (R 2  = 0.990) towards ascorbic acid in the wide dynamic range of 400 μM −1600 μM without any surface fouling effect was observed which is, to the best of our knowledge, ~1.6 folds higher than previously reported ascorbic acid sensors. The as-fabricated sensor exhibited lower limit of detection (38.7 μM), superior reproducibility and selectivity towards interfering species such as glucose, urea, uric acid and Na + ions. This enhanced sensing ability was studied and discussed in terms of forward biased nano-Schottky barriers at the rGO-SnO 2 interface. The sensor was successfully evaluated for the detection of ascorbic acid in the vitamin C tablets and to predict unknown concentrations of ascorbic acid in PBS solution. This ultrasensitive, binder free rGO–SnO 2 composite based sensor can be used for a simple, low cost and voltammetric detection of ascorbic acid in clinical diagnostics, pharmaceutical and food industries. [ABSTRACT FROM AUTHOR]

A novel and simple strategy was proposed to prepare molecularly imprinted polymer (MIPs) nanocomposites consisting of both conducting polypyrrole (PPy) and a new two-dimensional layered graphene-like black phosphorene quantum dots (BPQDs) onto the surface of conducting poly(3,4-ethylenedioxythiophene) nanorods (PEDOTNRs) via the electrochemical polymerization. The negatively charged BPQDs and template molecules vitamin C (VC) were self-assembled on the surface of the positively charged PEDOTNRs, then the functional monomer pyrrole was also self-assembled with template molecules. A novel molecularly imprinted electrochemical sensor based on PPy-BPQDs-MIPs/PEDOTNRs/GCE was successfully fabricated by the electrochemical polymerization of pyrrole in the presence of VC and BPQDs. The structure and morphology of the prepared MIPs electrode were characterized by transmission electron microscope, scanning electron microscopy, energy dispersive X-ray spectroscopy, and its parameters such as electropolymerization cycles, pH buffer solution, concentrations of template molecules and incubation time were optimized. Under the optimized conditions, the peak currents recorded by differential pulse voltammetry (DPV) demonstrated a linear proportional on VC concentrations ranging from 0.01 to 4 mM with a detection limit of 0.0033 mM. Moreover, the prepared imprinted electrode exhibited a good reproducibility, repeatability stability and selectivity for the electrochemical analysis of VC, which was successfully applied to detect VC in commercial drink soft samples. [ABSTRACT FROM AUTHOR]

A practical sensor based on a glassy carbon electrode (GCE) modified with Ag nanoparticles (AgNPs), graphene oxide (GO) and poly( l -arginine) (P(Arg)) was prepared to determine ascorbic acid (AA), dopamine (DA), uric acid (UA) and l -tryptophan (L-Trp) levels simultaneously via differential pulse voltammetry. The new GCE/AgNPs/P(Arg)-GO electrode exhibited high electrocatalytic activity for AA, DA, UA and L-Trp. Scanning electron microscopy (SEM) was utilized to characterize the surface morphology of the composite electrode. Electrochemical characterizations of the bare and composite electrodes were carried out via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). According to differential pulse voltammetry (DPV) results, there were linear relationships between the peak currents and the concentrations in the ranges of 4.0–2400.0 μmol L − 1 for AA, 0.05–50.0 μmol L − 1 for DA, 0.5–150.0 μmol L − 1 for UA and 1.0–150 μmol L − 1 for L-TRP, with the detection limits (3 s/m) of 0.984, 0.01, 0.142 and 0.122 μmol L − 1 for AA, DA, UA and L-TRP, respectively. The modified electrode could eliminate the interference effects of Na + , K + , l -lysine, glucose, l -cysteine, urea and citric acid. Furthermore, the sensor was successfully applied for the detection of these substances in the human urine samples, and it showed a very high recovery percentage. The comparison of this method with official method also affirmed the accuracy of the data obtained by GCE/AgNPs/P(Arg)-GO. [ABSTRACT FROM AUTHOR]

Chrysanthemum-like titanium nitride (CL-TiN) was fabricated at 800 °C through ammonia nitridation treatment of TiO 2 precursor. An electrochemical biosensor based on the glassy carbon electrode modified with TiN (CL-TiN/GCE) was prepared for the simultaneously detection of ascorbic acid (AA) and uric acid (UA). Excellent results are obtained for the separate quantification of UA and AA by differential pulse voltammetry due to the special structure and large surface area of TiN. The detection limits of the electrochemical sensor are 1.52 μM and 0.28 μM for AA and UA (S/N = 3), and the working linear ranges for AA and UA are 50–1500 μM and 10–300 μM, respectively. This sensor also exhibits outstanding anti-interference ability toward co-existing molecules with good stability, sensitivity and reproducibility. The present work opens a new window for simultaneous detection of AA and UA based on the CL-TiN/GCE. [ABSTRACT FROM AUTHOR]

Optimization of electrochemical methods for total antioxidant capacity (TAC) determination of Bunchosia glandulifera (Jacq.) Kunth (Malpighiaceae) extracts was performed. Other methods for antioxidant studies were also used for comparison with the results obtained by electrochemical assays. The extracts exhibited good solubility in the aprotic solvent dimethylsulfoxide. Hence, two electrochemical methods, namely cyclic (C) and differential pulse (DP) voltammetry (V) were optimized for TAC determination in this medium and by using ascorbic acid. The CV presented better linearity than DPV in the optimum conditions. TAC values determined by ABTS, FRAP and DPPH increased in the order seed < pulp < bark < leaf < root extract, in generally the same order observed by CV, indicating the applicability of the method in these conditions. The phenolic compounds contents were 302 ± 10, 327 ± 11, 1008 ± 71, 4003 ± 13 and 7502 ± 30 mg gallic acid/100 g extract, respectively, increasing in the same order as observed for TAC. On the other hand, such order disagreed with the amounts of major antioxidant vitamins, carotenoids and ascorbic acid, estimated in these extracts, which corresponded, by comparison with the contents of phenolic compounds, to 0.62, 34, 9.9, 0.39 and 0.86%, respectively. Thus, it can be inferred that specially in the case of roots and leaves, the contents of phenolic compounds may have greater contribution for their TAC values. [ABSTRACT FROM AUTHOR]

In this work, we have been successfully developed a new electrochemical sensor for the simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA) using ferrocene hybrid (Fc)/three dimensional graphene hydrogel (3DGH) nanocomposite. The composite materials were characterized using different techniques, such as scanning electron microscopy (SEM), atomic force microscopy (AFM), ultraviolet-visible spectrometer (UV–vis). The electrochemical detection methods using for the individual and simultaneous detection of AA, DA and UA were cyclic voltammetry (CV), differential pulse voltammetry (DPV) and amperometric (i–t). Due to the excellent properties and unique structure of the nanocomposite, this sensor exhibited sharp peaks for the oxidation of AA, DA and UA as compared with the bare glassy carbon electrode (GCE). The linear responses of AA, DA and UA were observed from 20 to 450 μM, 10–180 μM and 8–400 μM with detection limits (S/N = 3) of 0.183 μM (AA), 0.042 μM (DA) and 0.067 μM (UA), respectively. In addition, the developed 3DGH-Fc/GCE electrode showed well anti-interference ability, great stability and excellent reproducibility. Moreover, the analytical application of this electrochemical sensor was successfully conducted for the determination of these species in human serum samples. These features implicate the potential applicability of the developed 3DGH-Fc/GCE biosensor in the development of is promising for the simultaneous determination of DA, AA and UA. [ABSTRACT FROM AUTHOR]

A facile one-pot route was developed to synthesis NiFe bimetallic nanoparticles via a controlled co-reduction of Ni(II) acetylacetonate and Fe(II) acetylacetonate in oleylamine (OAm). Due to its unique properties originating from the synergistic effects of transition metals, NiFe exhibited excellent electrochemical catalytic activities. The electrochemical sensor based on carbon supported NiFe nanoparticles (NiFe/C) was constructed to determine various small biomolecules, and intensively showed high selectivity towards the oxidation of ascorbic acid (AA), dopamine (DA), uric acid (UA) and four DNA bases (guanine (G), adenine (A), thymine (T) and cytosine (C)), with large peak separations of 179 mV (AA-DA), 123 mV (DA-UA), 128 mV (G-A), 274 mV (A-T), 112 mV (T-C) and little mutual interference. The detection limits were AA (1.0 μM), DA (0.1 μM), UA (0.1 μM), G (0.25 μM), A (0.25 μM), T (10 μM) and C (10 μM) at S/N = 3, respectively. The feasibility of the proposed sensor for real sample analysis was also investigated. These results demonstrate that NiFe bimetallic catalyst is a promising candidate of electrode material for determination of various analytes, which make it a fascinating sensor with versatile application. [ABSTRACT FROM AUTHOR]

• A split-type-based cathodic amplification strategy was designed in bipolar ECL. • The anodic ECL responses were regulated by the conductivity of the cathode. • The work effectively avoids the reduced light transmittance or protein activity. • The bipolar ECL sensor exhibits high sensitivity and good selectivity. A bipolar electrochemiluminescence (ECL) detection of myoglobin (Myo) in a split-type mode was designed using a cathodic signal amplification strategy. Specifically, a sandwich immunoreaction was firstly constructed in a 96-well plate with the target Myo as the middle unit to immobilize the phosphatase labeled probes, which could catalyze 2-phosphate to generate ascorbic acid (AA). In ECL detection procedure, the AA was introduced into the cathodic pole of the bipolar ECL system and the in-situ reduction of HAuCl 4 to Au nanoparticles on the cathodic pole occurred. The improvement conductivity of cathodic pole generated an enhanced ECL response of the bipolar system. Because the amount of AA generated depends on the concentration of Myo, the conductivity of cathodic pole is positive correlation with the Myo concentration, resulting in different output of the anode reporting ECL signal. The introduction of split-type mode in the signal amplification strategy and anodic pole without any modification as the signal output end could effectively avoid the weakness caused destruction of the protein activity and the reduced light transmittance in the traditional mode, generating an improved sensitivity and accuracy. The detection limits of Myo by using photomultiplier tube and charge-coupled device imaging are as low as 3.0 × 10−13 and 5.0 × 10−13 g mL−1, respectively. Furthermore, it showed excellent performance in the determination of Myo in human serum samples. The developed cathodic signal amplification strategy based bipolar ECL biosensor could be a potential analytical tool for future trace analysis of biomarkers. [ABSTRACT FROM AUTHOR]

[Display omitted] Schematic diagram of the fabrication process of NiO@LaMnO 3 /MWCNTs/GCE and oxidation mechanisms of AA. • A novel NiO@LaMnO 3 /MWCNTs core–shell p-p homotype heterojunction has been designed. • NiO@LaMnO 3 /MWCNTs/GCE presented satisfying selectivity, repeatability, anti-interference ability. • The electrochemical signal of AA is amplified by the NiO@LaMnO 3 /MWCNTs core–shell heterojunction. A novel core–shell p-p homotype heterojunction based on NiO@LaMnO 3 and multi-walled carbon nanotubes (NiO@LaMnO 3 /MWCNTs) has been designed and synthesized via electrostatic self-assembly method. The phase compositions and morphology were verified by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM). A series of electrochemical tests showed that NiO@LaMnO 3 /MWCNTs modified electrode had a magnifying effect on the electrochemical response signal of ascorbic acid (AA) detection. The current response is linearly increased in the range of 2.60–36.44 μmol/L with the linear regression equation being I (μA) = 0.5902 C (μmol/L) + 1.4407 (R2 = 0.9909), and the detection limit (S/N = 3) being 2.41 μmol/L. Moreover, NiO@LaMnO 3 /MWCNTs/GCE electrochemical sensor presented satisfying selectivity, stability, repeatability, and can be used to analyze the AA concentration in real daily drinks. The relatively excellent electrocatalytic activity should be attributed to high electroactive surface area, well-matched core–shell structure and synergetic effect of p-p homotype heterojunction. This work offer applicable guidelines for the design of core–shell p-p heterojunction electrode materials for the application of electrochemical sensor. [ABSTRACT FROM AUTHOR]

• Sensitive DA and UA sensors in the coexistence of high AA levels were developed. • 3D-GFs with unique hierarchical pore structures showed good sensing performances. • Successful detections of DA and UA in dopamine injections and urine samples were achieved. Three-dimensional graphene foams (3D-GFs), composed of two hierarchical pore morphologies, were prepared by a facile soft foam templating strategy, and served as building blocks to construct a metal-free multi-component electrochemical sensing platform for sensitive discrimination of dopamine (DA) and uric acid (UA) in the coexistence of high concentration of ascorbic acid (AA). In virtue of the effective foam template and directional freezing, the formed quasi-spherical macropores and irregular minor pores between adjacent macropores in 3D-GFs could provide multiple electron transfer pathways and more exposed electroactive sites for the evident distinction of three analytes. When 500 μM AA coexisted in the ternary mixture, well-resolved current responses and desirable linear relationships of DA and UA were obtained after the synchronous change of their levels, and the corresponding concentration ranges and detection limits were determined to be 1–80 μM and 0.21 μM for DA and 5–500 μM and 1.27 μM for UA, respectively. The 3D-GFs-based metal-free electrochemical sensor was further applied to monitor DA and UA in medical dopamine injections and human urine specimens with satisfactory results, which demonstrates that 3D-GFs is expected to become a promising electrode material candidate in multi-component sensitive discriminations. [ABSTRACT FROM AUTHOR]

A sensitive and selective method was developed for the electrochemical determination of dopamine by using a sodium dodecyl benzene sulfonate modified expanded graphite paste electrode (SDBS-EGPE). The SDBS-EGPE was prepared by mixing expanded graphite (EG) self-assembled with sodium dodecyl benzene sulfonate (SDBS) with solid paraffin. SDBS formed a monolayer on EG surface with a high density of negatively charged end. Compared with EGPE, SDBS-EGPE showed improved electrochemical response of dopamine (DA) due to the enhanced accumulation of protonated DA via electrostatic interactions between positively charged DA and negatively charged SDBS. Under optimal experimental conditions, the designed electrode exhibited wide linear responses to DA ranging from 0.5 μmol/L and 500 μmol/L with a low detection limit of 0.027 μmol/L at a signal-to-noise ratio of 3. The SDBS-EGPE showed a good reproducibility, stability and reusability. The SDBS-EGPE showed a high selectivity for DA in the presence of ascorbic acid (UA) and uric acid (AA) due to the electrostatic repulsions between negatively charged UA, AA and SDBS. The proposed electrode was successfully applied in the determination of DA in the commercial dopamine hydrochloride injection sample. [ABSTRACT FROM AUTHOR]

A chiral electrochemical sensor was developed via an electro-generated molecularly imprinted polymer-based ultrathin film using l -ascorbic acid as a template and its biological proficiency was analyzed. The performance of the chiral selective sensor in serum was evaluated by conducting a comparative study of electrochemical response of l -ascorbic acid imprinted polyaniline ferrocene sulfonic acid pencil graphite electrode in aqueous and serum samples employing cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The sensor shows similar electrochemical response in aqueous and serum solutions of l -ascorbic acid of equal concentrations even at considerably high serum proportion in solutions. This shows its excellent ability to counter multifold interference in extraordinary complex biological medium (serum) and hence validates its worth in clinical applications. Surface morphologies are characterized through scanning electron microscopy of the chiral sensor (before and after de-doping of l -ascorbic acid) and compared with bare and non-imprinted electrode. Electrochemical impedance spectroscopy technique is used for the molecularly imprinted electrodes to quantitatively measure l -ascorbic acid with a detection limit of 1 μM. Finally, the proposed sensor offers excellent prospects in biomedical fields for simple, fast and cost effective chiral quantification of ascorbic acid in serum samples. [ABSTRACT FROM AUTHOR]

We developed a graphene nanoplatelet-modified fluorine-doped tin oxide electrode (GNP/FTO) for the simultaneous detection of dopamine (DA) and uric acid (UA) in the presence of ascorbic acid (AA) and investigated the interaction mechanisms of DA, UA, and AA with GNPs considering their charging states at different pH values. Owing to the unique structure and properties originating from the oxygen and nitrogen functional groups at the edges, GNPs showed high electrocatalytic activity for the electrochemical oxidations of AA, DA, and UA with peak-to-peak potential separations (Δ E P ) between AA-DA and DA-UA of ca . 0.23 and 0.17 V, respectively. These values are sufficiently high to allow the simultaneous detection of DA and UA without interference from AA. The highly sensitive and stable GNP/FTO sensor showed sensitivities of ca . 0.15 ± 0.004 and 0.14 ± 0.007 μA/μM, respectively, with detection limits of ca . 0.22 ± 0.009 and 0.28 ± 0.009 μM, respectively, for DA and UA. The sensor could detect DA and UA concentrations in human serum samples with excellent recoveries. [ABSTRACT FROM AUTHOR]

[(Co(bdmpzm) 2 (NCS) 2 ] (bdmpzm = bis(3,5-dimethylpyrazol-1-yl)methane) was synthesized and combined with single walled carbon nanotubes (SWCNT) and nafion on a screen printed carbon electrode (SPCE). The electrocatalytic activity of modified electrode (Nafion-SWCNT-[Co(bdmpzm) 2 (NCS) 2 ]/SPCE) was evaluated for dopamine (DA) oxidation. Anodic and cathodic peak were seen to 0.42 V.and0.29 V respectively, showed a much more quasi-reversible process, (∆ V 0.128) compared with the electrode without cobalt complex. The detection limit (3σ/b) for dopamine was 0.095 μmol L − 1 . The studies of reproducibility determined as relative standard deviation (RSD) was 2.5%.( n = 5) with four different electrodes. The repeatability were 1.5% ( n = 5) The new sensor was used in the quantify DA in human urine with an average recovery between 83.0 and 93.0%. [ABSTRACT FROM AUTHOR]

We report the preparation of reduced graphene oxide (RGO)/cobalt oxide (Co 3 O 4 ) nanocomposite by a simple in situ electrochemical approach and its application towards the trace level selective determination of serotonin in the presence of ascorbic acid and dopamine. Simultaneous electrochemical reduction of graphene oxide (GO) to RGO and electrodeposition of Co 3 O 4 has been achieved onto the glassy carbon electrode (GCE) surface. Raman and XRD studies confirmed the presence of RGO/Co 3 O 4 nanocomposite formation. Scanning electron microscope (SEM) image shows that the presence of Co 3 O 4 nanoflakes over the entire surface of wrinkled thin graphene nanosheets. The electro-oxidation of serotonin has been assessed by cyclic voltammetry and differential pulse voltammetry (DPV) in physiological pH conditions. Interestingly, the oxidation peak current ( I p ) of serotonin at GCE/RGO/Co 3 O 4 is 4.1 fold higher than that observed at GCE/RGO and 2.4 fold higher than that observed at GCE/Co 3 O 4 modified electrode. A DPV serotonin sensor with a very low detection limit of 48.7 nM and a high sensitivity of 2.2 mA mM − 1 cm − 2 has been demonstrated using the as prepared GCE/RGO/Co 3 O 4 modified electrode. A comparison of the sensor performance with previous literature clearly shows the superior performance of the fabricated sensor. In addition, the fabricated sensors detect serotonin in vitro directly from diluted blood serum samples. [ABSTRACT FROM AUTHOR]

A novel carbon electrode for simultaneous determination of ascorbic acid (AA), dopamine (DA), uric acid (UA), tryptophan (Trp), and nitrite (NO 2 − ) was fabricated by chemical vapor deposition of carbon layers on a Ta wire. The carbon electrodes were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), cyclic voltammetry (CV), and differential pulse voltammetry (DPV). The carbon electrodes are composed of stacked graphene nanosheets. The carbon electrode exhibited excellent electro-catalytic activities, high selectivity and sensitivity toward the oxidation of AA, DA, UA, Trp, and NO 2 − in pH 7.0 phosphate buffer solution. For AA, DA, UA, Trp, and NO 2 − , the corresponding DPV current signals have appeared as five well resolved oxidation peaks with peak potential differences of 204 mV (AA-DA), 152 mV (DA-UA), 312 mV (UA-Trp), and 128 mV (Trp-NO 2 − ). For simultaneous determination, the linear responses for AA, DA, UA, Trp, and NO 2 − were obtained in concentration ranges of 6 to 1500, 0.4 to 100, 1 to 250, 0.4 to 100, and 6 to 1500 μM with detection limits of 5 μM, 0.2 μM, 0.8 μM, 0.4 μM, and 3 μM, respectively. The carbon electrode was proved to be applicable for the determination of the above analytes in serum samples. [ABSTRACT FROM AUTHOR]

Gold interdigitated microelectrodes (Au-IDA) modified with either graphene flakes exfoliated using flavin mononucleotide (FMN) (Gr-FMN) or graphene flakes and platinum nanoparticles (Pt-Gr-FMN) have been studied in the oxidation of uric acid (UA). An electrochemical method for the detection and quantification of UA in phosphate buffer solution at physiological pH (PBS, 0.25 M, pH 7) in the absence and presence of ascorbic acid (AA) has been studied by cyclic voltammetry. The quantification of UA was investigated by cyclic voltammetry, presenting an oxidation peak at 0.99 V with both modified electrodes. Linearity range of 60–578 μM and 60–345 μM has been found for Gr-FMN/Au-IDA and Pt-Gr-FMN/Au-IDA electrodes, respectively. Limits of detection of 18 μM were obtained for both electrodes, and the repeatability was studied at 177 μM providing 4% and 8% for Gr-FMN/Au-IDA and Pt-Gr-FMN/Au-IDA, respectively. AA interference has been studied by cyclic voltammetry, showing two clearly separated oxidation peaks, at 0.99 V for UA oxidation and at 0.74 V for Gr-FMN/Au-IDA and 0.70 V for Pt-Gr-FMN/Au-IDA for AA oxidation. Linearity range has been studied in presence of 250 μM AA obtaining a working range of 60–578 μM for Gr-FMN/Au-IDA electrode and of 60–288 μM with Pt-Gr-FMN/Au-IDA electrode. Limits of detection remain at 18 μM for both electrodes and the repeatability was studied at 177 μM providing 8% and 14% for Gr-FMN/Au-IDA and Pt-Gr-FMN/Au-IDA electrodes respectively. [ABSTRACT FROM AUTHOR]

An electrochemical sensor has been developed for simultaneous detection of dopamine (DA), uric acid (UA) in the presence of ascorbic acid (AA) based on three-dimensional porous graphene (PGE) modified glassy carbon electrode (GCE). The scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) were used to characterize the synthesis of the PGE. Differential pulse voltammetry (DPV) results showed that DA, UA could be detected selectively and sensitively in the presence of AA at PGE modified GCE(PGE/GCE), which exhibited a good linearity of DA, UA in the ranges of 0.2–8 μM, 1–60 μM with remarkable sensitivity 2.104 μA/μM, 0.292 μA/μM, respectively. [ABSTRACT FROM AUTHOR]

This work introduces an electrochemical sensor fabricated by electropolymerization of a conductive β-cyclodextrin (β-CD) polymer onto a reduced graphene oxide (rGO)-decorated screen-printed electrode (SPE). The coated β-CD polymer had a high supramolecular recognition capability and superior electrical conductivity, which can improve the electrochemical properties of the β-CD/rGO/SPE electrode. Electrochemical behaviors of ascorbic acid (AA), dopamine (DA), and uric acid (UA) on the β-CD/rGO/SPE electrode were investigated by cyclic voltammetry and differential pulse voltammetry. The anodic peaks of these three molecules were separated perfectly at the sensor, indicating that the β-CD/rGO/SPE electrode can simultaneously detect these three molecules. The linear ranges for AA, DA and UA were 0.2–2 mM, 0.05–50 μM and 0.08–150 μM, respectively. The corresponding detection limits were 0.067 mM, 0.017 μM and 0.026 μM, respectively. As a practical application, the proposed sensor was successfully applied to quantify the concentrations of AA, DA and UA in real biological samples. These results demonstrate that this β-CD/rGO/SPE electrode can be a promising electrochemical sensor for electrocatalytic applications. [ABSTRACT FROM AUTHOR]

The poly (3,4-ethylenedioxythiophene) (PEDOT) nanomaterial was synthesised by a facile electrodeposition on a glassy carbon electrode (GCE) modified by carboxylated multi-wall carbon nanotubes (CA-MWCNTs). The nanocomposite was characterized using transmission electron microscopy, scanning electron microscope, energy-dispersive X-ray spectroscopy and fourier transform infrared spectroscopy. Moreover, a highly sensitive ascorbic acid (AA) sensor was developed based on the nanocomposite. Owing to the combination of the advantages of CA-MWCNTs and PEDOT, the modified electrode exhibited excellent electro-catalytic activity for AA detection which was investigated in detail by cyclic voltammetry and chronoamperometry. The cationic PEDOT film on CA-MWCNTs could interact with anionic AA in neutral PBS at a low potential and distinguish the oxidation peaks of common interfering species. The effects of electrodeposition time and pH value on the current responses of the modified electrode towards AA were optimized to obtain the maximal sensitivity. The prepared AA sensor demonstrated high sensitivity (1699.36 μA mM − 1 cm − 2 ), wide linear range detection (100 μM to 20 mM), low detection limit (4.2 μM), rapid response time (less than 4 s), good selectivity and long-term stability. This work presented a facile approach for future research in AA amperometric sensors. [ABSTRACT FROM AUTHOR]

A new electrochemical sensor made from nanocomposite of manganese dioxide (MnO 2 )-antimony trioxide (Sb 2 O 3 ) was fabricated on polyaniline (PANI) - patterned fluorine doped tin oxide (FTO) electrode (PANI/MnO 2 -Sb 2 O 3 nanocomposite/FTO) through a simple potentiostatic deposition method. The properties of the nanocomposite were characterized by field emission scanning electron microscopy, X-ray diffraction, electrochemical impedance spectroscopy and other electrochemical techniques. Such nanostructure combines the advantages of PANI (high conductivity and stability) with that of electrocatalytic species (good electrochemical activity). The sensor was applied for simultaneous determination of ascorbic acid (AA) and acetylsalicylic acid (ASA). The linear relationships between their current intensity and concentration were in the range of 6–265.42 nmol L − 1 and 1.2–228.68 nmol L − 1 with detection limit (S/N = 3) of 1.05 nmol L − 1 and 0.20 nmol L − 1 , respectively. Experimental results demonstrated that the sensor possesses high selectivity and suffers no interference from competing species. Moreover, it successfully detected AA and ASA in human urine samples with highly satisfactory results. [ABSTRACT FROM AUTHOR]

Selective and reliable method is of great importance for in vivo analysis in the complicated brain. In this work, we developed a new approach for detection of ascorbic acid (AA) based on a new nitrogen-doped nanotube fiber (NCNF) microelectrode. After electrochemical pre-treatment, the NCNF microelectrode (e-NCNF) demonstrated high electrocatalytic capability for AA oxidation with a low working potential, 0 V, resulting in high selectivity for detection of AA level against other biological species in brain, as well as producing high sensitivity. Moreover, the e-NCNF was directly assembled by carbon nanotube bundles, free from the post-modification, thus showing high reproducibility, in which the deviation of anodic current intensity of six electrodes prepared with same method did not exceed 4% (RSD, n = 6). Based on the remarkable analytical performance, the e-NCNF with good biocompatibility was successfully applied to determine the AA level as 134 ± 7 mM, in rat brain microdialysates. [ABSTRACT FROM AUTHOR]