Simultaneous voltammetric detection of ascorbic acid and uric acid at a carbon-paste modified electrode incorporating thionine-nafion ion-pair as an electron mediator

The electrochemical behavior of ascorbic acid (AA) and uric acid (UA) at the surface of a carbon-paste electrode modified with incorporate thionine-nafion ion-paired was thoroughly investigated. The results show the presence of nafion inside the matrix of modified electrode, because of the effective ion-pairing and hydrophobic interactions, significantly enhances the stability of thionine as an electron mediator inside the modified electrode. A high reproducibility in voltammetric response to analyte species results because of this enhancement. The cyclic voltammetric studies using the prepared modified electrode show the best electrocatalytic property for the electro-oxidation of AA and noticeable decrease in anodic overpotential. Although the catalytic effect is observed to some extent for UA, the property cannot be seen for other biologically reducing agents such as cysteine. The voltammetric studies using the thionine-nafion modified electrode show two well-resolved anodic peaks for AA and UA, revealing the possibility of the simultaneous electrochemical detection of these compounds in the presence of biological thiols. The detection limits of 5 × 10⁻⁸ and 5 × 10⁻⁷ M were obtained in differential pulse voltammetric (DPV) measurements for UA and AA, respectively. Spectrophotometric investigations were used to confirm the selective catalytic effect of thionine in oxidation of AA and to some extent, UA. The detection system is stable (R.S.D. for the slope of the calibration curves was less than 4% for six measurements in one month) and is of high selectivity for electro-oxidation of AA and UA in complex biological and clinical matrices. The prepared modified electrode is applied for the DPV measurement of AA in pharmaceutical preparations. Also, the electrode is used to determine UA in human urine and serum samples and recovery of the amounts of UA added to these complex samples.     

Electrocatalytic oxidation and simultaneous determination of uric acid and ascorbic acid on the gold nanoparticles-modified glassy carbon electrode

A new gold nanoparticles-modified electrode (GNP/LC/GCE) was fabricated by self-assembling gold nanoparticles to the surface of the l-cysteine-modified glassy carbon electrode. The modified electrode showed an excellent character for electrocatalytic oxidization of uric acid (UA) and ascorbic acid (AA) with a 0.306 V separation of both peaks, while the bare GC electrode only gave an overlapped and broad oxidation peak. The anodic currents of UA and AA on the modified electrode were 6- and 2.5-fold to that of the bare GCE, respectively. Using differential pulse voltammetry (DPV), a highly selective and simultaneous determination of UA and AA has been explored at the modified electrode. DPV peak currents of UA and AA increased linearly with their concentration at the range of 6.0 × 10⁻⁷ to 8.5 × 10⁻⁴ mol L⁻¹ and 8.0 × 10⁻⁶ to 5.5 × 10⁻³ mol L⁻¹, respectively. The proposed method was applied for the detection of UA and AA in human urine with satisfactory result.     

Determination of uric acid in the presence of ascorbic acid with hexacyanoferrate lanthanum film modified electrode

A glassy carbon electrode modified with LaHCF was constructed and was characterized by cyclic voltammetry (CV) and electrochemical impedance spectrum (EIS). The resulting LaHCF modified glassy carbon electrode had a good catalytic character on uric acid (UA) and was used to detect uric acid and ascorbic acid (AA) simultaneously. This modified electrode exhibits potent and persistent electron-mediating behavior followed by well-separated oxidation peaks towards UA and AA with activation overpotential. For UA and AA in mixture, one can well separate from the other with a potential large enough to allow the determination of one in presence of the other. The DPV peak currents obtained increased linearly on the UA in the range of 2.0 × 10⁻⁷ to 1.0 × 10⁻⁴ mol/L with the detection limit (signal-to-noise ratio was 3) for UA 1.0 × 10⁻⁷ mol/L. The proposed method showed excellent selectivity and stability, and the determination of UA and AA simultaneously in urine was satisfactory.     

Selective detection of dopamine on poly(indole-5-carboxylic acid)/tyrosinase electrode

We report here on a simple tyrosinase (TYR) modified electrode designed through the covalent bonding of the enzyme with poly (indole-5-carboxylic acid) (PIn5COOH) conducting polymer. This electrode was applied to the amperometric detection of dopamine (DA) in the presence of ascorbic acid (AA), uric acid (UA) and their mixtures, in the concentration range and ratios similar to those found in blood serum. Our experiments demonstrate that the presence of these interferents (AA, UA) does not affect the selectivity of such electrode towards dopamine with linear concentration dependence in the range of 0.5–20 μM, depending on the experimental conditions, however its sensitivity depends on the type and amount of interferent present. The lower limit of detection of DA in the presence of high AA (1000 fold) or UA (500 fold) concentration was found to be 0.1–0.5 μM. The sensitivity for DA detection is 6.2 A/M cm² with UA and 2.3 A/M cm² with AA present as interfering agents. For both interferents present in the ratio 12.5:1 (AA:UA), the sensitivity drops to the value of ca. 1.3 A/M cm². The Michaelis–Menten (K_M) constant and I_max values were evaluated, showing improved electrode sensitivity towards dopamine as judged from the decrease of the Michaelis–Menten constant.     

In situ fabricated iodine-adlayer assisted selective electrooxidation of uric acid in alkaline media

This work presents the electrooxidation of uric acid (UA) at an iodine-adlayer-modified gold, Au (I|Au (poly)) electrode in 0.1 M NaOH solution using cyclic voltammetric, amperometric and open-circuit potential measurement techniques. A tremendous enhancement of the electrode activity towards the electrooxidation of UA was achieved by virtue of the simple modification of the Au (poly) electrode surface with a neutral iodine-adlayer, fabricated in situ through the spontaneous oxidative chemisorption of iodide present in the sample solution. The cyclic voltammetric peak current increases remarkably for the oxidation of UA and the peak potential shifts by 365 mV to the negative direction of potential compared to the bare Au (poly) electrode. Oxidation of ascorbic acid (AA) at the I|Au (poly) electrode takes place at the same potential as that at the bare electrode, but the peak current intensity is almost twice at the bare Au (poly) electrode as compared to the modified one. In the mixture of the AA and UA, the cyclic voltammetric signals corresponding to the oxidations of AA and UA were resolved by 340 mV. The electrode response in the mixture was highly reproducible because of the inhibition of adsorption of oxidation products and UA.     

Simultaneous detection of ascorbic acid, uric acid and homovanillic acid at copper modified electrode

The copper was deposited on glassy carbon (GC) and indium tin oxide (ITO) electrodes by electrochemical method. The copper structures on electrode were characterized by atomic force microscope, X-ray diffractometeric pattern and differential pulse voltammetric studies. Optimal conditions for uniform growth of copper structures on the electrode were established. Voltammetric sensor was fabricated using the copper deposited GC electrode for the simultaneous detection and determination of uric acid (UA) and homovanillic acid (HVA) in the presence of excess concentrations of ascorbic acid (AA). The voltammetric signals due to AA and UA oxidation were well separated with a potential difference of 400 mV and AA did not interfere with the measurement of UA and HVA at the GC/Cu electrode. Linear calibration curves were obtained in the concentration range 1-40 μM for AA and 20-50 μM for UA at physiological pH and a detection limit of 10 nM of UA in the presence of 10-fold excess concentrations of AA was achieved. The simultaneous detection of submicromolar concentrations of AA, UA and HVA was achieved at the GC/Cu electrode. The practical utility of the present GC/Cu modified electrode was demonstrated by measuring the AA content in Vitamin C tablet, UA content in human urine and blood serum samples with satisfactory results.     

A carboxyalkanethiol monolayer modified electrode for blocking of ascorbic acid oxidation and its application to a biosensor

Upon the application of amperometric biosensor to the biological fluid, ascorbic acid interferes the amperometric determination of analytes, because the oxidative potential of ascorbic acid is lower than that of electro active substances such as H₂O₂ produced by the enzymatic reaction. In this study we propose a method to block ascorbic acid based on the electrostatic interaction with self-assembled monolayer (SAM) and its application of the surface modified electrode to biosensor. In order to form SAM on the gold electrode with carboxyl group, 7-carboxy-heptanethiol (7-CHT) was used. The 7-CHT modified electrode did not show anodic response to ascorbic acid, but oxidized phenanthroline cobalt complex [Co(phen)₃²⁺], which can be used as a mediator of biosensor. Thus, the 7CHT-modified electrode was applied to biosensor mediated with Co(phen)₃²⁺. Fructose dehydrogenase (FDH) was immobilized to the 7-CHT modified electrode. Fructose was determined selectively with the FDH/7-CHT modified electrode at the range of 0.2-2 mM.     

Fabrication of CeO2 nanoparticles modified glassy carbon electrode and its application for electrochemical determination of UA and AA simultaneously

A glassy carbon electrode modified with CeO₂ nanoparticles was constructed and was characterized by electrochemical impedance spectrum (EIS) and cyclic voltammetry (CV). The resulting CeO₂ nanoparticles modified glassy carbon electrode (CeO₂ NP/GC electrode) was used to detect uric acid (UA) and ascorbic acid (AA) simultaneously in mixture. This modified electrode exhibits potent and persistent electron-mediating behavior followed by well-separated oxidation peaks towards UA and AA with activation overpotential. For UA and AA in mixture, one can well separate from the other with a potential difference of 273 mV, which was large enough to allow the determination of one in presence of the other. The DPV peak currents obtained in mixture increased linearly on the UA and AA in the range of 5.0 × 10⁻⁶ to 1.0 × 10⁻³ mol/L and 1.0 × 10⁻⁶ to 5.0 × 10⁻⁴ mol/L, with the detection limit (signal-to-noise ratio was 3) for UA and AA were 2.0 × 10⁻⁷ and 5.0 × 10⁻⁶ mol/L, respectively. The proposed method showed excellent selectivity and stability, and the determination of UA and AA simultaneously in serum was satisfactory.     

Development of an amperometric sensor for simultaneous determination of uric acid and ascorbic acid using 2-[bis(2-aminoethyl)amino]ethanol, 4,4′-bipyridine bridged dicopper(II) complex

Binuclear copper complex (2-[bis(2-aminoethyl)amino]ethanol, 4,4′-bipyridine bridged dicopper(II) complex) was grafted onto the surface of a glassy carbon electrode (GCE) using the cyclic voltammetric method in a phosphate buffer solution (PBS). The modified electrode resulted in efficient electrocatalytic activity for anodic oxidation of uric acid (UA) and ascorbic acid (AA) via a substantial decrease in anodic over-potentials for both compounds. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using this modified electrode result in two well-resolved anodic waves for the oxidation of UA and AA in mixed solution, making possible the simultaneous determination of both compounds. Linear analytical curves were obtained in the ranges 5.0–300.0 μM and 5.0–160.0 μM for UA and AA concentrations through DPV methods, respectively. The detection limits were 2.0 μM of UA and AA. This electrode was used for UA and AA determinations in urine samples with satisfactory results.     

Formic acid oxidation on Pd-modified Pt(100) and Pt(111) electrodes: A DEMS study

Formic acid oxidation on palladium submonolayers on well-defined Pt(100) and Pt(111) electrodes has been studied using voltammetry and Differential Electrochemical Mass Spectrometry (DEMS). A combination of the two techniques allows a better understanding of the reaction taking place on the electrode surface. Thus, an exact correlation between the CO₂ mass signal and the current density in the voltammogram corresponding to the formic acid oxidation has been obtained. On palladium modified Pt(100) electrodes and in the potential region below 0.3 V, the currents in the positive scan are higher than those recorded in the negative scan. This diminution on the signal in the negative scan has been associated with CO₂ reduction to CO on the palladium adlayer. In addition, the CO₂ reduction reaction seems to take place on the border of the palladium islands. Finally, the adsorption of (bi)sulfate anions has an inhibiting behavior on the formic acid oxidation reaction.     

Electrochemical dissolution of tin in methanesulphonic acid solutions

High-rate electroplating of tin on a moving steel strip is generally carried out in cells with dimensionally stable anodes. To obtain a matt tin deposit a concentrated acidic tin methanesulphonate solution containing a small concentration of sulphuric acid is used. The concentrated tin methanesulphonate solution is prepared by dissolution of tin particles with oxygen in a special column. To describe this dissolution process electrode reactions (namely, reduction of oxygen, hydrogen peroxide and hydrogen ions on a tin electrode and oxidation of tin) were studied using electrochemical techniques. It was concluded that on tin, oxygen is almost entirely reduced to water and that H₂O₂ cannot corrode tin directly, but its decomposition products, for instance oxygen, can. The exchange current density and the charge transfer coefficient for the investigated electrode reactions are estimated. The dissolution of tin by oxygen is determined by the kinetic parameters of the oxygen reduction reaction and by the mass transfer of (i) dissolved oxygen to and (ii) Sn²⁺ ions from the tin electrode surface. Hydrogen evolution can be neglected during the dissolution of tin in the presence of oxygen. Moreover, it was found that the rate of tin corrosion increases with (i) increasing H⁺ concentration, (ii) oxygen concentration, (iii) convection intensity and (iv) temperature. It is likely that the tin surface is not covered with oxygen during corrosion in pure methanesulphonic acid solutions, but an oxide layer may be present on the tin surface during oxygen corrosion in pure sulphuric acid solutions. This oxide layer may hinder the oxygen corrosion of tin.     

Influence of underpotentially deposited Sb onto Pt anode surface on the performance of direct formic acid fuel cells

We first reported on electrocatalytic activity and stability of antimony modified platinum (PtSb_upd) as anode catalyst in direct formic acid fuel cells. Sb modified Pt (PtSb_upd) was prepared by underpotential deposition technique applying constant potential of 0.2 V (vs. Ag/AgCl, 3M KCl) and its modified surface was characterized by XRD and XPS. The electrocatalytic oxidation activity by cyclic voltammograms and the single cell power performance of Sb modified Pt were measured and their results were compared with the data of unmodified Pt electrode. PtSb_upd induced lower onset potential of formic acid oxidation and twice higher power density of 250 mW cm⁻² was observed.     

Preparation of polypyrrole/ferrocyanide films modified carbon paste electrode and its application on the electrocatalytic determination of ascorbic acid

Functionalized polypyrrole film were prepared by incorporation of (Fe(CN)₆)⁴⁻ as doping anion, during the electropolymerization of pyrrole onto a carbon paste electrode (CPE) in aqueous solution by using potentiostatic method. The electrochemical behavior of the (Fe(CN)₆)³⁻/(Fe(CN)₆)⁴⁻ redox couple in polypyrrole was studied by cyclic voltammetry and double step potential chronoamperometry methods. In this study, an obvious surface redox reaction was observed and dependence of this reaction on the solution pH was illustrated. The electrocatalytic ability of polypyrrole/ferrocyanide films modified carbon paste electrode (Ppy/FCNMCPEs) was demonstrated by oxidation of ascorbic acid. It has been found that under optimum condition (pH 7.00), the oxidation of ascorbic acid at the surface of such electrode occurs at a potential about 540 mV less positive than unmodified carbon paste electrode. The kinetic parameters such as electron transfer coefficient, α and catalytic reaction rate constant, k_h′, were also determined by using various electrochemical approaches.The catalytic oxidation peak current showed a linear dependent on the ascorbic acid concentration and a linear calibration curve was obtained in the range of 4.5×10⁻⁴ to 9.62×10⁻³ M of ascorbic acid with a correlation coefficient of 0.9999. The detection limit (2σ) was determined as 5.82×10⁻⁵ M.     

Photoelectrocatalytic activity of mesoporous TiO<Subscript>2</Subscript> films prepared using the sol–gel method with tri-block copolymer as structure directing agent

Mesoporous TiO₂ films were synthesized using a sol–gel process with a tri-block copolymer (Pluronic F127) as a structure directing agent. The films were dip-coated onto conductive glass substrates followed by thermal treatment to remove the polymeric surfactant. The specific surface area, the pore size and morphology, and the crystallinity of the films were characterized. The analysis showed the structural properties of the films could be tailored by varying the surfactant concentration as well as the annealing temperature. The photocurrent responses of the prepared films were measured using a three-electrode photoelectrochemical cell in the presence of oxalic acid. When annealed at 450 °C, the dense electrode (control sample) had the strongest photoelectrocatalytic oxidation aptitude toward oxalic acid. The lower photocurrent response of the mesoporous electrodes compared to the dense electrode can be explained by poor inter-particle connectivity within the mesoporous TiO₂ network, leading to high electron transport resistance. The mesoporous electrode annealed at 550 °C, with improved connectivity, outperformed the dense electrode at a high oxalic concentration, as the lower surface area of the dense electrode restricted photoelectrocatalysis reactions on the surface. A further increase in annealing temperature to 650 °C resulted in a poor photocurrent response as the significant decrease in surface area outweighed the beneficial effect of improved connectivity.     

Characterization of oxidized reticulated vitreous carbon electrode for oxygen reduction reaction in acid solutions

The oxidation of reticulated vitreous carbon (RVC) and its impact on the oxygen reduction reaction (ORR) in H₂SO₄ solutions has been studied. The results are compared with that of a planar glassy carbon (GC) electrode. The oxidation process was characterized by using different electrode configurations, GC (planar) and RVC electrodes both with flooded (batch process) and flow-through assembly. Cyclic voltammetry, potentiodynamic and rotating ring-disk electrode voltammetry were used for the characterization of the ORR. Anodically oxidized GC and flooded RVC are similar in that the ORR on both electrodes gave a more defined limiting current plateau. For the flow-through porous electrode, the oxidation process caused a distribution of the oxidation extent within the bed thickness, as evident from the SEM images, and only about half of the porous electrode was utilized in the oxidation process. X-ray photoelectron spectroscopy (XPS) measurements confirmed the above distribution and a gradient of the oxygen-to-carbon ratio was obtained within the porous bed. Oxidation of RVC led to an enhancement of its electrocatalytic properties towards ORR. H₂O₂ production was tested at the oxidized RVC from flowing acid solutions. The oxidation of RVC resulted in higher current efficiencies and higher outlet concentrations of the H₂O₂ acid solutions.     

Capability of a carbon-polyvinylchloride composite electrode for the detection of dopamine, ascorbic acid and uric acid

In this work, the composite carbon-polyvinylchloride (C-PVC) was used as an electrode for the detection of dopamine, ascorbic acid, uric acid and their mixtures by differential pulse voltammetry (DPV). The results showed that the untreated C-PVC electrode was selective and stable for the oxidation of dopamine in a mixture containing uric acid and an excess of ascorbic acid in acidic medium. The pre-treated C-PVC electrode in a neutral medium exhibited good resolution of the mixture components in the micro molar concentration range of DA. The ageing of the C-PVC electrode during longer time periods did not affect the peak potential and the detection of dopamine, uric acid and ascorbic acid in 0.1 M H₂SO₄. The practical analytical utility of the C-PVC electrode was demonstrated by the measurement of uric acid in human urine and serum samples without any preliminary pre-treatment.     

Improved sensitivity and selectivity of uric acid voltammetric sensing with mechanically grinded carbon/graphite electrodes

Determination of uric acid (UA) levels in body fluids is important for diagnostics and prevention of severe metabolic disorders. Electrochemical determination of the UA relies on an oxidation signal measurable at different carbon-based electrodes. Improvement of the UA electrochemical sensing has usually been attained via various modifications of the electrode surfaces. In this paper we show that a strong enhancement of the UA oxidation signal can be reached by a simple mechanical grinding of the surfaces of glassy carbon or edge plane-oriented pyrolytic graphite electrodes with SiC particles of an optimum size 15 μm. In contrast to fine polished electrodes (finally with 1-μm particles), the grinded ones exhibited an excellent separation of oxidation signals of ascorbic acid, dopamine (representing most important natural interferents in UA determination), xanthine and hypoxanthine (precursors of UA in purine catabolism), making it possible to detect these substances in a mixture. Enhancement of UA and dopamine (DA) oxidation signals at the grinded electrodes allowed their easy detection at nanomolar levels in up to 10⁴-fold excesses of ascorbic acid. Due to a strong adsorption at the electrode surface, nanomolar concentrations of UA and DA can be determined by ex situ voltammetry. Similarly strong enhancement of oxidation signals was observed for purine nucleobases, guanine and adenine. The grinded electrodes have been tested in analysis of real clinical samples of human serum or urine. An excellent agreement between electrochemical and routine biochemical determination of UA in the biological samples is demonstrated.     

Simultaneous determination of paracetamol and ascorbic acid using tetraoctylammonium bromide capped gold nanoparticles immobilized on 1,6-hexanedithiol modified Au electrode

Tetraoctylammonium bromide stabilized gold nanoparticles (TOAB-AuNPs) attached to 1,6-hexanedithiol (HDT) modified Au electrode was used for the simultaneous determination of paracetamol (PA) and ascorbic acid (AA) at physiological pH. The attachment of TOAB-AuNPs on HDT modified Au surface was confirmed by attenuated total reflectance (ATR)-FT-IR spectroscopy and atomic force microscope (AFM). The ATR-FT-IR spectrum of TOAB-AuNPs attached to the HDT monolayer showed a characteristic stretching modes corresponding to -CH₂ and -CH₃ of TOAB, confirming the immobilization of AuNPs with surface-protecting TOAB ions on the surface of the AuNPs after being attached to HDT modified Au electrode. AFM image showed that the immobilized AuNPs were spherical in shape and densely packed to a film of ca. 7 nm thickness. Interestingly, TOAB-AuNPs modified electrode shifted the oxidation potential of PA towards less positive potential by 70 mV and enhanced its oxidation current twice when compared to bare Au electrode. In addition, the AuNPs modified electrode separated the oxidation potentials of AA and PA by 210 mV, whereas bare Au electrode failed to resolve them. The amperometry current of PA was increased linearly from 1.50 × 10⁻⁷ to 1.34 × 10⁻⁵ M with a correlation coefficient of 0.9981 and the lowest detection limit was found to be 2.6 nM (S/N = 3). The present method was successfully used to determine the concentration of PA in human blood plasma and commercial drugs.     

Electrochemical behavior of phenol in alkaline media at hydrotalcite-like clay/anionic surfactants/glassy carbon modified electrode

The electrochemical behavior of phenol, using glassy carbon (GC) modified electrodes containing a hydrotalcite (HT)-like clay and anionic surfactants such as sodium octyl sulfate (SOS), sodium dodecyl sulfate (SDS), or sodium dodecylbenzenesulfonate (SDBS) in alkaline media, has been examined. Phenol oxidation at the modified electrodes, after a time accumulation under open circuit conditions, promotes increments of the current and shifts the oxidation potential to less positive values, compared to phenol oxidation at HT-GC or GC electrodes. The phenol oxidation is favored by the presence of surfactants in the films. The results suggest that the surfactant molecules intercalate between the HT layers, yielding a hydrophobic clay capable of preconcentrating phenol molecules. X-ray diffraction analyses showed a larger spacing of the HT layers when the surfactant intercalates between them. Cyclic voltammograms have shown that the SOS-HT-GC modified electrode exhibits short-lived activity for phenol oxidation as a consequence of surface fouling, while the SDS-HT-GC and SDBS-HT-GC modified electrodes showed a more stable behavior. The SDBS-HT-GC modified electrode was the most effective adsorbing phenol, since the charge (Q), obtained from the integration of the anodic peak current of the phenol, is higher at this modified electrode. This is probably because the adsolubilization capacity of phenol on the SDBS-HT-GC electrode is higher than on SDS-HT-GC electrode.     

Electrocatalytic oxidation of methanol on the nickel–cobalt modified glassy carbon electrode in alkaline medium

In this work, Ni–Co alloy coating on the surface of glassy carbon (GC) electrode was performed by cyclic voltammetry. The results showed that the deposition of Ni–Co is an anomalous process. The deposition bath was prepared according to the metal ion Ni/Co ratio of 4:1 using NiSO₄·7H₂O and CoSO₄·8H₂O, and the total concentration of all solutions was 40.0 mM. The pH of the bath solution was adjusted at 2.0 using boric acid at room temperature. The modified electrode was conditioned by potential recycling in a potential range of 100–700 mV (vs. Ag/AgCl) by cyclic voltammetric method in an alkaline solution. The Ni–Co modified electrode showed a higher activity towards methanol oxidation in the Ni (III) and Co (IV) oxidation states. Cyclic voltammetry was used for the electrochemical characterization of the Ni–Co modified electrode and the mechanism of methanol oxidation is proposed. The result of double steps chronoamperometry shows that the methanol electrooxidation is an irreversible reaction. Moreover, the effects of various parameters such as mole ratio of Ni–Co in the alloy in modification step, potential scan rate, methanol concentration and solution temperature on the electro-oxidation of methanol have also been investigated.