A study of the electrocatalytic oxidation of aspirin on a nickel hydroxide-modified nickel electrode

The electrocatalytic oxidation of aspirin has been investigated on a nickel oxide-modified nickel electrode in alkaline solution. The process of oxidation and its kinetics have been investigated by using cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy techniques and also steady-state polarization measurements. Voltammetric studies have indicated that in the presence of aspirin, the anodic peak current of low-valence nickel species increases, followed by a decrease in the corresponding cathodic current. This indicates that aspirin was oxidized on the redox mediator immobilized on the electrode surface via an electrocatalytic mechanism. The rate constant of the catalytic oxidation of aspirin and the electron transfer coefficient have been found to be 1.15×10⁵ cm³ mol⁻¹s⁻¹ and 0.49, respectively. Impedance measurements show that aspirin is diffused into the bulk of the modifier film, and the oxidation process of aspirin occurs in the bulk of nickel oxide film. It has been shown that by using this modified electrode, aspirin can be determined with a detection limit of 4.8×10⁻⁵ and successfully applied for determination of aspirin in tablet.     

Electrochemical behaviors of metol on ionic-liquid-modified carbon paste electrode and its analytical application

The electrochemical behaviors of metol on an ionic liquid N-butylpyridinium hexafluorophosphate modified carbon paste electrode (IL-CPE) were studied in this paper. The results indicated that a pair of well-defined quasi-reversible redox peaks of metol appeared with the decrease of overpotential and the increase of redox peak current, which was the characteristics of electrocatalytic oxidation. The electrocatalytic mechanism was discussed and the electrochemical parameters were calculated with results of the charge-transfer coefficient (α) as 0.45, the electrode reaction rate constant (k _s) as 4.02 × 10⁻³ s⁻¹, and the diffusion coefficient (D) as 6.35 × 10⁻⁵ cm²/s. Under the optimal conditions, the anodic peak current was linear with the metol concentration in the range of 5.0 × 10⁻⁶ ∼ 1.0 × 10⁻³ mol/L (n = 11, γ = 0.994) and the detection limit was estimated as 2.33 × 10⁻⁶ mol/L (3σ). The proposed method was successfully applied to determination of metol content in synthetic samples and photographic solutions.     

Electrochemical characterization of 2, 2′-[1, 2-ethanediylbis (nitriloethylidyne)]-bis-hydroquinone-carbon nanotube paste electrode and its application to simultaneous voltammetric determination of ascorbic acid and uric acid

The preparation and electrochemical characterization of a carbon nanotube paste electrode modified with 2,2′-[1,2-ethanediylbis (nitriloethylidyne)]-bis-hydroquinone, referred to as EBNBH, was investigated. The EBNBH carbon nanotube paste electrode (EBNBHCNPE) displayed one pair of reversible peaks at E _pa = 0.18 V and E _pc = 0.115 V vs Ag/AgCl. Half wave potential (E _1/2) and ΔE _p were 0.148 and 0.065 V vs Ag/AgCl, respectively. The electrocatalytic oxidation of ascorbic acid (AA) has been studied on EBNBHCNPE, using cyclic voltammetry, differential pulse voltammetry and chronoamperometry techniques. It has been shown that the oxidation of AA occurs at a potential where oxidation is not observed at the unmodified carbon paste electrode. The heterogeneous rate constant for oxidation of AA at the EBNBHCNPE was also determined and found to be about 1.07 × 10⁻³ cm s⁻¹. The diffusion coefficient of AA was also estimated as 5.66 × 10⁻⁶ cm² s⁻¹ for the experimental conditions, using chronoamperometry. Also, this modified electrode presented the property of electrocatalysing the oxidation of AA and uric acid (UA) at 0.18 and 0.35 V vs Ag/AgCl, respectively. The separations of anodic peak potentials of AA and UA reached 0.17 V. Using differential pulse voltammetry, the calibration curves for AA and UA were obtained over the range of 0.1–800 μM and 20–700 μM, respectively. With good selectivity and sensitivity, the present method provides a simple method for selective detection of AA and UA in biological samples.     

Simultaneous determination of uric acid and ascorbic acid at the film of chitosan incorporating cetylpyridine bromide modified glassy carbon electrode

A glassy carbon electrode (GCE) modified with the film composed of chitosan incorporating cetylpyridine bromide is constructed and used to determine uric acid (UA) and ascorbic acid (AA) by differential pulse voltammetry (DPV). This modified electrode shows efficient electrocatalytic activity and fairly selective separation for oxidation of AA and UA in mixture solution. UA is catalyzed by this modified electrode in phosphate buffer solution (pH 4.0) with a decrease of 80 mV, while AA is catalyzed with a decrease of 200 mV in overpotential compared to GCE, and the peak separation of oxidation between AA and UA is 260 mV, which is large enough to allow the determination of one in presence of the other. Under the optimum conditions, the anodic peak currents (I _pa) of DPV are proportional to the concentration of UA in the range of 2.0 × 10⁻⁶ to 6.0 × 10⁻⁴ M, with the detection limit of 5.0 × 10⁻⁷ M at a signal-to-noise ratio of 3 (S/N = 3) and to that of AA in the range of 4.0 × 10⁻⁶ to 1.0 × 10⁻³ M, with the detection limit of 8.0 × 10⁻⁷ M (S/N = 3).     

Voltammetric investigation and amperometric detection of the bisphosphonate drug sodium alendronate using a copper nanoparticles-modified electrode

The electrochemical behavior of sodium alendronate on copper microparticle- and copper nanoparticle-modified carbon paste electrodes was investigated. In the voltammograms recorded using microparticles, a single anodic oxidation peak appeared, while using nanoparticles, two anodic peaks appeared. The anodic currents were related to the electrocatalytic oxidation of alendronate via the active species of Cu(III). The catalytic rate constant for the electrocatalytic oxidation process and the diffusion coefficient of alendronate were obtained to be 1.57 × 10³ cm³ mol⁻¹ s⁻¹ and 2.44 × 10⁻⁶ cm² s⁻¹, respectively. A sensitive and time-saving detection procedure was developed for the analysis of alendronate, and the corresponding analytical parameters were reported. Alendronate was determined with a limit of detection of 11.26 μmol L⁻¹ with a linear range of 50–6,330 μmol L⁻¹. The proposed amperometric method was applied to the analysis of commercial pharmaceutical tablets, and the results were in good agreement with the declared values.     

Electrocatalytic oxidation of quinine sulfate at a multiwall carbon nanotubes-ionic liquid modified glassy carbon electrode and its electrochemical determination

The electrocatalytic oxidation of quinine sulfate (QS) was investigated at a glassy carbon electrode, modified by a gel containing multiwall carbon nanotubes (MWCNTs) and room-temperature ionic liquid of 1-Butyl-3-methylimidazolium hexafluorophate (BMIMPF₆) in 0.10 M of phosphate buffer solution (PBS, pH 6.8). It was found that an irreversible anodic oxidation peak of QS with E _pa as 0.99 V appeared at MWCNTs-RTIL/glassy carbon electrode (GCE). The electrode reaction process was a diffusion-controlled one and the electrochemical oxidation involved two electrons transferring and two protons participation. Furthermore, the charge-transfer coefficient (α), diffusion coefficient (D), and electrode reaction rate constant (k _f) of QS were found to be 0.87, 7.89 × 10⁻³ cm²⋅s⁻¹ and 3.43 × 10⁻² s⁻¹, respectively. Under optimized conditions, linear calibration curves were obtained over the QS concentration range 3.0 × 10⁻⁶ to 1.0 × 10⁻⁴ M by square wave voltammetry, and the detection limit was found to be 0.44 μM based on the signal-to-noise ratio of 3. In addition, the novel MWCNTs-RTIL/GCE was characterized by the electrochemical impedance spectroscopy and the proposed method has been successfully applied in the electrochemical quantitative determination of quinine content in commercial injection samples and the determination results could meet the requirement.     

Electrochemical behaviour of a lead electrode in phosphoric acid

 A lead electrode was studied in 6 and 12 M H₃PO₄. Oxidation of a freshly polished electrode occurred in the −0.5 to −0.3 V vs. SCE range, and led to PbHPO₄ growth on the electrode surface. The dissolution of this layer by electrochemical reduction occurred between −0.5 and −0.7 V. The influence of temperature (20 °C and 65 °C) was investigated and showed that the anodic and the cathodic peaks were increasing, and more markedly for the 12 M H₃PO₄. The ratio Q _cathodic/Q _anodic (Q=electrical charge flowing through the electrode) was equal or close to the unity at 20 °C and decreased as the temperature was increased. The influence of Cl⁻, Br⁻ and I⁻ ions was also evaluated. The addition of Cl⁻ and Br⁻ predominantly led to Pb₅(PO₄)₃Cl and Pb₅(PO₄)₃Br, respectively, while I⁻ led to a mixture of PbI₂ and PbHPO₄. Received: 18 July 1999 / Accepted: 2 November 1999     

Fabrication of gold nanoparticles/polypyrrole composite-modified electrode for sensitive hydroxylamine sensor design

A highly sensitive hydroxylamine (HA) electrochemical sensor is developed based on electrodeposition of gold nanoparticles with diameter of 8 nm on the pre-synthesized polypyrrole matrix and formed gold nanoparticles/polypyrrole (GNPs/PPy) composite on glassy carbon electrode. The electrochemical behavior and electrocatalytic activity of the composite-modified electrode are investigated. The GNPs/PPy composite exhibits a distinctly higher electrocatalytic activity for the oxidation of HA than GNPs with twofold enhancement of peak current. The enhanced electrocatalytic activity is attributed to the synergic effect of the highly dispersed gold metal particles and PPy matrix. The overall numbers of electrons involved in HA oxidation, the electron transfer coefficient, catalytic rate constant, and diffusion coefficient are investigated by chronoamperometry. The sensor presents two wide linear ranges of 4.5 × 10⁻⁷–1.2 × 10⁻³ M and 1.2 × 10⁻³–19 × 10⁻³ M with the detection limit of 4.5 × 10⁻⁸ M (s/n = 3). In addition, the proposed electrode shows excellent sensitivity, selectivity, reproducibility, and stability properties.     

Covalent Modification of Glassy Carbon Electrode with L-Cysteine for the Determination of Acetaminophen

A novel L-cysteine film modified electrode has been fabricated by means of an electrochemical oxidation procedure, and it was successfully applied to the electrochemical determination of acetaminophen. This method utilizes the electrooxidation of amines to their analogous cation radicals to form a chemically stable covalent linkage between the nitrogen atom of the amine and edge plane sites at the glassy carbon electrode surface. The electrochemical behaviour of acetaminophen at the film electrode was investigated in 0.1 mol L⁻¹ phosphate buffer (pH 6.20). It was found that the redox peak current of acetaminophen was enhanced greatly on the film electrode. Linearity between the oxidation peak current and the acetaminophen concentration was obtained in the range of 1.0 × 10⁻⁴–2.0 × 10⁻⁷ mol L⁻¹ with a detection limit of 5.0 × 10⁻⁸ mol L⁻¹. For seven parallel detections of 1.0 × 10⁻⁵ mol L⁻¹ acetaminophen, the relative standard deviation (RSD) was 1.46%, suggesting that the film electrode has excellent reproducibility. Application to the determination of acetaminophen in drug tablets and human urine demonstrated that the film electrode has good stability and high sensitivity.     

Electrocatalytic determination of reduced glutathione in human erythrocytes

The determination of reduced glutathione (GSH) in human erythrocytes using a simple, fast and sensitive method employing a glassy carbon electrode modified with cobalt tetrasulfonated phthalocyanine (CoTSPc) immobilized in poly(l-lysine) (PLL) film was investigated. This modified electrode showed very efficient electrocatalytic activity for anodic oxidation of GSH, decreasing substantially the anodic overpotentials for 0.2 V versus Ag/AgCl. The modified electrode presented better performance in 0.1 mol l⁻¹ piperazine-N,N′-bis(2-ethanesulfonic acid) buffer at pH 7.4. The other experimental parameters, such as the concentration of CoTSPc and PLL in the membrane preparation, pH, type of buffer solution and applied potential, were optimized. Under optimized operational conditions, a linear response from 50 to 2,160 nmol l⁻¹ was obtained with a high sensitivity of 1.5 nA l nmol⁻¹ cm⁻². The detection limit for GSH determination was 15 nmol l⁻¹. The proposed sensor presented good repeatability, evaluated in terms of the relative standard deviation (1.5%) for n = 10. The modified electrode was applied for determination of GSH in erythrocyte samples and the results were in agreement with those obtained by a comparative method described in the literature The average recovery for these fortified samples was 100 ± 1)%. Applying a paired Student’s-t test to compare these methods, we could observe that, at the 95% confidence level, there was no statistical difference between the reference and the proposed methods.     

Solid state electrochemical of the erbium hexacyanoferrate-modified carbon ceramic electrode and its electrocatalytic oxidation of <Emphasis Type="SmallCaps">l</Emphasis>-cysteine

A kind of erbium hexacyanoferrate (ErHCF)-modified carbon ceramic electrodes (CCEs) fabricated by mechanically attaching ErHCF samples to the surface of CCEs derived from sol–gel technique was proposed. The resulting modified electrodes exhibit well-defined redox responses with the formal potential of +0.215 V [vs saturated calomel electrode (SCE)] at a scan rate of 20 mV s⁻¹ in 0.5 M KCl (pH 7) solution. The voltammetric characteristics of the ErHCF-modified CCEs were investigated by voltammetry. Attractively, the ErHCF-modified CCEs presented good electrocatalytic activity with a marked decrease in the overvoltage about 400 mV for l-cysteine oxidation. The calibration plot for l-cysteine determination was linear at 5.0 × 10⁻⁶–1.3 × 10⁻⁴ M with a linear regression equation of I(A) = 0.558 + 0.148c (μM) (R ² = 0.9989, n = 20), and the detection limit was 2 × 10⁻⁶ M (S/N = 3). At last, the ErHCF-modified CCEs were used for amperometric detection of l-cysteine in real samples.     

Electrochemistry and electrocatalytic activity of polypyrrole/ferrocyanide films on a glassy carbon electrode

Functionalized polypyrrole films were prepared by incorporation of Fe(CN)₆ ³⁻ as doping anion during the electropolymerization of pyrrole at a glassy carbon electrode from aqueous solution. The electrochemical behavior of the Fe(CN)₆ ³⁻/Fe(CN)₆ ⁴⁻ redox couple in polypyrrole was studied by cyclic voltammetry. An obvious surface redox reaction was observed and dependence of this reaction on the solution pH was illustrated. The electrocatalytic ability of polypyrrole film with ferrocyanide incorporated was demonstrated by oxidation of ascorbic acid at the optimized pH of 4 in a glycine buffer. The catalytic effect for mediated oxidation of ascorbic acid was 300 mV and the bimolecular rate constant determined for surface coverage of 4.5 × 10⁻⁸ M cm⁻² using rotating disk electrode voltammetry was 86 M⁻¹ s⁻¹. Furthermore, the catalytic oxidation current was linearly dependent on ascorbic acid concentration in the range 5 × 10⁻⁴–1.6 × 10⁻² M with a correlation coefficient of 0.996. The plot of i _p versus v ^1/2 confirms the diffusion nature of the peak current i _p. Received: 12 April 1999 / Accepted: 25 May 1999     

Voltammetry and amperometric detection of tetracyclines at multi-wall carbon nanotube modified electrodes

The voltammetric behaviour and amperometric detection of tetracycline (TC) antibiotics at multi-wall carbon nanotube modified glassy carbon electrodes (MWCNT-GCE) are reported. Cyclic voltammograms of TCs showed enhanced oxidation responses at the MWCNT-GCE with respect to the bare GCE, attributable to the increased active electrode surface area. Hydrodynamic voltammograms obtained by flow-injection with amperometric detection at the MWCNT-GCE led us to select a potential value E _det = +1.20 V. The repeatability of the amperometric responses was much better than that achieved with bare GCE (RSD ranged from 7 to 12%), with RSD values for i _p of around 3%, thus demonstrating the antifouling capability of MWCNT modified electrodes. An HPLC method with amperometric electrochemical detection (ED) at the MWCNT-GCE was developed for tetracycline, oxytetracycline (OTC), chlortetracycline and doxycycline (DC). A mobile phase consisting of 18:82 acetonitrile/0.05 mol L⁻¹ phosphate buffer of pH 2.5 was selected. The limits of detection ranged from 0.09 μmol L⁻¹ for OTC to 0.44 μmol L⁻¹ for DC. The possibility to carry out multiresidue analysis is demonstrated. The HPLC-ED/MWCNT-GCE method was applied to the analysis of fish farm pool water and underground well water samples spiked with the four TCs at 2.0 × 10⁻⁷ mol L⁻¹. Solid-phase extraction was accomplished for the preconcentration of the analytes and clean-up of the samples. Recoveries ranged from 87 ± 6 to 99 ± 3%. Under preconcentration conditions, limits of detection in the water samples were between 0.50 and 3.10 ng mL⁻¹.     

Simultaneous voltammetric determination of ascorbic acid and uric acid using a Nafion/multi-wall carbon nanotubes composite film-modified electrode

A Nafion/multi-wall carbon nanotubes (MWNT) composite film-modified electrode was fabricated. The modified electrode showed excellent electrocatalytic activity toward ascorbic acid (AA) and uric acid (UA) in 0.1-mol L⁻¹ NaCl medium (pH 6.5). Compared to the bare electrode that only displayed a broad and overlapped oxidation peak, the Nafion/MWNT film-modified electrode not only remarkably enhanced the anodic peak currents of AA and UA but also avoided the overlapping of the anodic peaks of AA and UA with a 320-mV separation of both peaks. Under the optimized conditions, the peak currents of AA and UA were proportional to their concentration at the ranges of 8.0 × 10⁻⁵ to 6.0 × 10⁻³ mol L⁻¹ and 6.0 × 10⁻⁷ to 8.0 × 10⁻⁵ mol L⁻¹, respectively. The proposed method was used for the detection of AA and UA in real samples with satisfactory results.     

Electrocatalytic oxidation of pyridoxine (vitamin B<Subscript>6</Subscript>) on aluminum electrode modified by metallic palladium particles/iron (III) hexacyanoferrate (II) film

The electrocatalytic activity of a Prussian blue (PB) film on the aluminum electrode by taking advantage of the metallic palladium characteristic as an electron-transfer bridge (PB/Pd–Al) for electrooxidation of 2-methyl-3-hydroxy-4,5-bis (hydroxyl–methyl) pyridine (pyridoxine) is described. The catalytic activity of PB was explored in terms of Fe^III [Fe^III (CN)₆]/Fe^III [Fe^II (CN)₆]¹⁻ system. The best mediated oxidation of pyridoxine (PN) on the PB/Pd–Al-modified electrode was achieved in 0.5 M KNO₃ + 0.2 M potassium acetate of pH 6 at scan rate of 20 mV s⁻¹. The mechanism and kinetics of the catalytic oxidation reaction of PN were monitored by cyclic voltammetry and chronoamperometry. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. The charge transfer-rate limiting reaction step is found to be a one-electron abstraction, whereas a two-electron charge transfer reaction is the overall oxidation reaction of PN by forming pyridoxal. The value of α, k, and D are 0.5, 1.2 × 10² M⁻¹ s⁻¹, and 1.4 × 10⁻⁵ cm² s⁻¹, respectively. Further examination of the modified electrodes shows that the modifying layers (PB) on the Pd–Al substrate have reproducible behavior and a high level of stability after posing it in the electrolyte or Pyridoxine solutions for a long time.     

Sensitive determination of adenine on poly(amidosulfonic acid)-modified glassy carbon electrode

A novel and reliable direct electrochemical method was established for the detection of adenine, based on the differential pulse anodic stripping response at a poly(amidosulfonic acid) (poly-ASA)-modified glassy carbon electrode (GCE) fabricated by electropolymerization. The characterization of electrochemically synthesized poly-ASA film was investigated by atomic force microscopy, electrochemical impedance spectroscopy, and voltammetric methods. This poly-ASA-modified GCE could greatly enhance the detection sensitivity of adenine. At optimum conditions, the anodic peak exhibits a good linear concentration dependence in the range from 3.0 × 10⁻⁸ to 1.0 × 10⁻⁶ M (r = 0.9994). The detection limit is 8.0 × 10⁻⁹ M (S/N = 3). The proposed method could be used to determinate the adenine in tablets of vitamin B₄ with satisfactory results.     

A sensor for acetaminophen in a blood medium using a Cu(II)-conducting polymer complex modified electrode

Complexation of Cu ions in a terthiophene carboxylic acid (TTCA) polymer film resulted an enhanced anodic current for acetaminophen oxidation when compared to polymer coated and bare glassy carbon electrodes in human blood and buffer media. Scanning electron microscopy (SEM) and ESCA experiments indicate the involvement of copper in the electrocatalytic oxidation of acetaminophen. No interference was observed from other biologically important and phenolic compounds used with this modified electrode. Especially, the non-interference from N-acetylcysteine, an antidote for the treatment of acetaminophen poisoning, reveals the proposed method’s superiority in medicinal applications. In addition, the present modified electrode avoids surface fouling at higher concentrations of acetaminophen. The calibration range obtained with CV was based between 2.0×10⁻⁵ and 5.0×10⁻³ M [r²=0.997 (n=5, R.S.D.=2.5%); DL=5.0×10⁻⁶ M (S/N=3)]. The analytical utility of the modified electrode was achieved by analyzing the content of acetaminophen in different drugs without pretreatment using CV and amperometric techniques.     

Chemometrics to evaluate the quality of water sources for human consumption

Multilayer films of multiwalled carbon nanotubes (MWNTs) were homogeneously and stably assembled on a glassy carbon (GC) electrode using the layer-by-layer (LBL) method based on electrostatic interaction between MWNTs (negatively charged) and a biopolymer chitosan (CHIT) (positively charged). Scanning electron microscopy (SEM) image of the resulting {CHIT/MWNTs}₉ film indicated that the substrate was mostly covered with MWNTs in the form of small bundles or single nanotubes. The multilayer film was used to study the electrocatalytic oxidation of NADH. The assembled {CHIT/MWNTs}₉/GC electrode could decrease the oxidation overpotential of NADH by more than 350 mV. The {CHIT/MWNTs}₉/GC electrode exhibited a wide linear response range of 8 × 10⁻⁷ to 1.6 × 10⁻³ mol · L⁻¹ with a correlation coefficient of 0.997 for the detection of NADH. The response time and detection limit (S/N = 3) were determined to be 3 s and 0.3 × 10⁻⁶ mol · L⁻¹, respectively. Another attractive characteristic was that the method was simple and the assembled {CHIT/MWNTs}₉/GC electrode was highly stable.     

Electrochemistry of electropolymerized tetra (p-aminophenyl)porphyrin nickel film electrode and catalytic oxidation of acetaminophen

A polymer film of tetra(p-aminophenyl) porphyrin nickel was obtained at a glassy carbon electrode by a cyclic voltammetric method. Cyclic voltammograms of the film electrode exhibited two stable redox waves with anodic peak potential at 0.43V and cathodic peak potential at 0.30 V in 0.5M NaOH aqueous solution. The electrocatalytic characteristics of the film electrode were studied by cyclic voltammetry, a. c. impedance analysis and other methods. The oxidation peak current increased linearly with the addition of acetaminophen to the aqueous NaOH medium in the range 1 × 10^–6–2 × 10^–4 M acetaminophen. The performance of the electrode was verified by the determination of acetaminophen in a paracetamol preparation.     

Oxygen reduction on a Ru x S y (CO) n cluster electrocatalyst in 0.5 M H2SO4

A ruthenium-sulfur carbonyl cluster electrocatalyst, Ru _x S _y (CO) _n , was synthesized by pyrolysis of Ru₃(CO)₁₂ and elemental sulfur in a sealed ampoule at 300 °C. The pyrolyzed compound was characterized by DSC, FT-IR, XRD and SEM (EDX) techniques. The electrocatalytic activity and kinetic parameters for the molecular oxygen reduction were determined by a rotating ring-disk electrode (RRDE) in a 0.5 M H₂SO₄ solution at 25 °C. The cathodic polarization indicates two Tafel slopes: −0.124 ± 0.002 V dec⁻¹ at low and −0.254 ± 0.003 V dec⁻¹ at high overpotentials, and first-order kinetics with respect to O₂ concentration. From the analysis of Levich plots and RRDE results, the oxygen reduction on Ru _x S _y (CO) _n was determined to proceed mostly via a multielectron transfer path (4e⁻) to water formation ( >94%). Received: 4 March 1999 / Accepted: 26 May 1999