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.     

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.     

Monolayer covalent modification of 5-hydroxytryptophan on glassy carbon electrodes for simultaneous determination of uric acid and ascorbic acid

5-Hydroxytryptophan (5-HTP) was covalently grafted on the surface of glassy carbon electrodes (GCEs) using cyclic voltammetric method in a phosphate buffer solution. The prepared electrode, denoded as 5-HTP/GCE, was characterized by X-ray photoelectron spectroscopy, cyclic voltammetry and differential pulse voltammetry (DPV). Tryptophan grafted GCE (TRP/GCE) and 5-hydroxytryptamine grafted GCE (5-HTP/GCE) were also prepared by the same method for comparison. It was found that the electrocatalytic activities toward the oxidation of uric acid (UA) and ascorbic acid (AA) was in the order of 5-HT/GCE > 5-HTP/GCE > TRP/GCE for UA oxidation and 5-HT/GCE = 5-HTP/GCE > TRP/GCE for AA oxidation. However, the CV current sensitivity was estimated as 4:2:1 for 5-HTP/GCE:5-HT/GCE:TRP/GCE. The DPV peaks for UA and AA oxidation appeared at 0.07 V and 0.34 V versus SCE, respectively, allowing simultaneous determination in mixtures. A linearly response in the range of: 5.0 × 10⁻⁷ to 1.1 × 10⁻⁵ M with the detection limit (s/n = 3) of 2.8 × 10⁻⁷ M for UA determination, and a linear response in the range of: 5.0 × 10⁻⁶ to 1.0 × 10⁻⁴ M with the detection limit of 4.2 × 10⁻⁶ M for AA determination were obtained. This electrode was used for UA and AA determinations in human urine samples satisfactorily.     

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.     

Voltammetric determination of ascorbic acid and dopamine in the same sample at the surface of a carbon paste electrode modified with polypyrrole/ferrocyanide films

Functionalized polypyrrole film were prepared by incorporation of [Fe(CN)₆]⁴⁻ as a doping anion, during the electropolymerization of pyrrole onto a carbon paste electrode in an aqueous solution by potentiostatic method. The electrochemical behavior of dopamine (DA) and ascorbic acid (AA) in one solution was studied at the surface of bare and modified carbon paste electrodes using cyclic voltammetry (CV), linear sweep voltammetry (LSV) and differntial pulse voltammetry (DPV) methods. The well separated anodic peaks for oxidation of DA and AA were observed at the surface of the modified carbon paste electrode under optimum condition (pH 6.00), which can be used for determination of these species simultaneously in mixture by LSV and DPV methods. The linear analytical curves were obtained in the ranges of 0.10-1.00 mM and 0.10-0.95 mM for ascorbic acid and 0.10-1.20 mM and 0.20-0.95 mM for dopamine concentrations using LSV and DPV methods, respectively. The detection limits (2σ) were determined as 3.38 × 10⁻⁵ M and 1.34 × 10⁻⁵ M of ascorbic acid and 3.86 × 10⁻⁵ M and 1.51 × 10⁻⁵ M of dopamine by CV and DPV methods.     

The ultrasonic electropolymerization of an 5-[o-(4-bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) film electrode and its electrocatalytic properties to dopamine oxidation in aqueous solution

5-[o-(4-Bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) was electropolymerized on a glassy carbon electrode (GCE), and the electrocatalytic properties of the prepared film electrode response to dopamine (DA) oxidation were investigated. A stable o-BrPETPP film was formed on the GCE under ultrasonic irradiation through a potentiodynamic process in 0.1 M H₂SO₄ between −1.1 V and 2.2 V versus a saturated calomel electrode (SCE) at a scan rate of 0.1 V s⁻¹. The film electrode showed high selectivity for DA in the presence of ascorbic acid (AA) and uric acid (UA), and a 6-fold greater sensitivity to DA than that of the bare GCE. In the 0.05 mol L⁻¹ phosphate buffer (pH 6.0), there was a linear relationship between the oxidation current and the concentration of DA solution in the range of 5 × 10⁻⁷ mol L⁻¹ to 3 × 10⁻⁵ mol L⁻¹. The electrode had a detection limit of 6.0 × 10⁻⁸ mol L⁻¹(S/N = 3) when the differential pulse voltammetric (DPV) method was used. In addition, the charge transfer rate constant k = 0.0703 cm s⁻¹, the transfer coefficient α = 0.709, the electron number involved in the rate determining step n_α = 0.952, and the diffusion coefficient D_o = 3.54  10⁻⁵ cm² s⁻¹ were determined. The o-BrPETPP film electrode provides high stability, sensitivity, and selectivity for DA oxidation.     

Electrochemical characterization of poly(eriochrome black T) modified glassy carbon electrode and its application to simultaneous determination of dopamine, ascorbic acid and uric acid

A polymerized film of eriochrome black T (EBT) was prepared on the surface of a glassy carbon (GC) electrode in alkaline solution by cyclic voltammetry (CV). The redox response of the poly(EBT) film at the GC electrode appeared in a couple of redox peak in 0.1 M hydrochloride and the pH dependent peak potential was −55.1 mV/pH which was close to the Nernst behavior. The poly(EBT) film-coated GC electrode exhibited excellent electrocatalytic activity towards the oxidations of dopamine (DA), ascorbic acid (AA) and uric acid (UA) in 0.05 mM phosphate buffer solution (pH 4.0) and lowered the overpotential for oxidation of DA. The polymer film modified GC electrode conspicuously enhanced the redox currents of DA, AA and UA, and could sensitively and separately determine DA at its low concentration (0.1 μM) in the presence of 4000 and 700 times higher concentrations of AA and UA, respectively. The separations of anodic peak potentials of DA-AA and UA-DA reached 210 mV and 170 mV, respectively, by cyclic voltammetry. Using differential pulse voltammetry, the calibration curves for DA, AA and UA were obtained over the range of 0.1-200 μM, 0.15-1 mM and 10-130 μM, respectively. With good selectivity and sensitivity, the present method provides a simple method for selective detection of DA, AA and UA in biological samples.     

Simultaneous voltammetric determination of ascorbic acid, dopamine and uric acid by methylene blue adsorbed on a phosphorylated zirconia-silica composite electrode

This paper describes the preparation, characterization and application of a composite electrode based on methylene blue adsorption to phosphorylated zirconia-silica mixed oxide particles prepared by a sol-gel process. This electrode electrocatalytically oxidizes ascorbic acid (AA), dopamine (DA) and uric acid (UA), allowing their simultaneous voltammetric detection. Well-defined and -separated oxidation peaks were observed by differential pulse voltammetry in a 0.35 mol l⁻¹ Tris-HCl buffer solution (pH 7.4) containing 0.5 mol l⁻¹ KCl. The anodic peak currents observed at −74, 94 and 181 mV increased with increasing concentrations of AA, DA and UA, respectively. Linear calibration plots were obtained over the range of 100-1600 μmol l⁻¹ for ascorbic acid, 6-100 μmol l⁻¹ for dopamine and 22-350 μmol l⁻¹ for uric acid with detection limits of 8.3 ± 0.1, 1.7 ± 0.1 and 3.7 ± 0.2 μmol l⁻¹, respectively. DA and UA concentrations could also be determined under conditions of excess AA (1 mmol l⁻¹).     

Electrochemical study on cobalt film modified glassy carbon electrode and its application

A novel electroactive material for ascorbic acid (AA) determination was successfully prepared by plating/potential cycling method. The cobalt film was first deposited on the surface of glassy carbon electrode (GCE) in CoSO₄ solution by potential cycling, and then a cobalt film on the surface of GCE was activated by potential cycling in 0.1 mol L⁻¹ NaOH. The electrochemical performance of the resulted film (Co/GCE) and factors affecting its electrochemical activity were investigated by cyclic voltammetry and amperometry. This film electrode exhibited good electrocatalytic activity to the oxidation of AA. This biosensor had a fast response of AA less than 3 s and excellent linear relationships were obtained in the concentration range of 3 × 10⁻⁷ to 1 × 10⁻⁴ mol L⁻¹ with a detection limit of 2 × 10⁻⁷ mol L⁻¹ (S/N = 3) under the optimum conditions. Moreover, the selectivity, stability and reproducibility of this biosensor were evaluated with satisfactory results.     

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 of ascorbic acid by [Fe(CN)6] redox couple electrostatically trapped in cationic N,N-dimethylaniline polymer film electropolymerized on diamond electrode

Multinegatively charged metal complex, hexacyanoferrate ([Fe(CN)₆]⁴⁻), was electrostatically trapped in the cationic polymer film of N,N-dimethylaniline (PDMA) which was electrochemically deposited on the boron-doped diamond (BDD) electrode by controlled-potential electro-oxidation of the monomer. This ferrocyanide-trapped PDMA film was used to catalyze the oxidation of ascorbic acid (AA). Increase in the oxidation current response with a negative shift of the anodic peak potential was observed at the cationic PDMA film-coated BDD (PDMA|BDD) electrode, compared with that at the bare BDD electrode. A more drastic enhancement in the oxidation peak current as well as more negative shift of oxidation potential was found at the ferrocyanide-trapped PDMA film-coated BDD ([Fe(CN)₆]^3−/4−|PDMA|BDD) electrode. This [Fe(CN)₆]^3−/4−|PDMA|BDD electrode can be used as an amperometric sensor of AA. Ferrocyanide, electrostatically trapped in the polymer film shows more electrocatalytic activity than that coordinatively attached to the polymer film or dissolved in the solution phase. The electrocatalytic current depends on the surface coverage of ferricyanide, Γ_Fe, within the polymer film. Diffusion coefficient (D) of AA in the solution was estimated by rotating disk electrode voltammetry: D = (5.8 ± 0.3) × 10⁻⁶ cm² s⁻¹. The second-order rate constant for the catalytic oxidation of AA by ferricyanide was also estimated to be 9.0 × 10⁴ M⁻¹ s⁻¹. In the hydrodynamic amperometry using the [Fe(CN)₆]^3−/4−|PDMA|BDD electrode, a successive addition of 1 μM AA caused the successive increase in current response with equal amplitude and the sensitivity was calculated as 0.233 μA cm⁻² μM⁻¹.     

A glassy carbon supported bilayer lipid-like membrane of 5,5-ditetradecyl-2-(2-trimethyl-ammonioethyl)-1,3-dioxane bromide for electrochemical sensing of epinephrine

A self-assembled bilayer lipid-like membrane (BLM) supported on glassy carbon electrode (GCE) was fabricated using 5,5-ditetradecyl-2-(2-trimethyl-ammonioethyl)-1,3-dioxane bromide (DTDB) for epinephrine (EP) determination in the presence of ascorbic acid (AA). This modified electrode (DTDB/GCE) has strong membrane adsorption accumulation and electrocatalytic ability toward EP and AA. The oxidation of EP was controlled by double step adsorption accumulation process of the DTDB-BLM. The parameters of fitted Langmuir isotherm Γ_max, B_ADS, and ΔG_ADS values were determined as 1.0×10⁻¹¹ mol cm⁻², 2.04×10⁶ dm³ mol⁻¹, and −45.17 kJ mol⁻¹ for the fist step for EP concentration less than 1 mM, and 4.92×10⁻¹¹ mol cm⁻², 7.35×10⁴ dm³ mol⁻¹, and −37.1 kJ mol⁻¹ for the second step for EP concentration higher than 1 μM. The DPV peaks for EP and AA oxidations were appeared at 0.220 and 0.085 V versus SCE, respectively, allowing the determination of EP in the presence of high concentration of AA. The advantage of DTDB-BLM was demonstrated experimentally in comparison with other three BLMs, and attributed to the dioxane group as well as the suitable length of the carbon chain of DTDB molecule. The current response of the DTDB/GCE was fast and reproducible, suitable for the electrochemical sensing in flow-injection systems. A linear range of 1×10⁻⁸ to 1×10⁻⁴ M EP was preliminary obtained using a simple setup.     

Development of an electrochemical ascorbic acid sensor based on the incorporation of a ferricyanide mediator with a polyelectrolyte-calcium carbonate microsphere

A novel electro-active material was successfully prepared with Fe(CN)₆³⁻ ions loaded by electrostatic interaction onto the layer of poly(allylamine) hydrochloride (PAH), which was first assembled on prepared poly(sodium 4-styrenesulfonate) (PSS)-doped porous calcium carbonate (CaCO₃) microspheres. Further, an electrochemical sensor for use in ascorbic acid (AA) detection was constructed with the use of the above electro-active materials embedded into a chitosan (CS) sol-gel matrix as an electron mediator. The electrocatalytic oxidation of AA by ferricyanide was observed at the potential of 0.27 V, which was negative-shifted compared with that by direct electrochemical oxidation of AA on a glassy carbon electrode. The experimental parameters, including the pH value of testing solution and the applied potential for detection of AA, were optimized. The current electrochemical sensor not only exhibited a good reproducibility and storage stability, but also showed a fast amperometric response to AA in a linear range (1.0 × 10⁻⁶ to 2.143 × 10⁻³ M), a low detection limit (7.0 × 10⁻⁷ M), a fast response time (<6 s), and a high sensitivity (−4.5127 μA mM⁻¹).     

Simultaneous voltammetric determination of ascorbic acid, acetaminophen and isoniazid using thionine immobilized multi-walled carbon nanotube modified carbon paste electrode

A carbon paste electrode (CPE) modified with thionine immobilized on multi-walled carbon nanotube (MWCNT), was prepared for simultaneous determination of ascorbic acid (AA) and acetaminophen (AC) in the presence of isoniazid (INZ). The electrochemical response characteristics of the modified electrode toward AA, AC and INZ were investigated by cyclic and differential pulse voltammetry (CV and DPV). The results showed an efficient catalytic role for the electro-oxidation of AA and AC, leading to a remarkable peak resolution (∼303 mV) for two compounds. On the other hand, the presence of INZ, which is considered as important drug interference for AC, does not affect the voltammetric responses of these pharmaceuticals. The mechanism of the modified electrode was analyzed by monitoring the CVs at various potential sweep rates and pHs of the buffer solutions. Under the optimum conditions, the calibration curves for AA, AC and INZ were obtained in the range of 1 × 10⁻⁶ to 1 × 10⁻⁴ M, 1 × 10⁻⁷ to 1 × 10⁻⁴ M and 1 × 10⁻⁶ to 1 × 10⁻⁴ M, respectively. The prepared modified electrode shows several advantages such as simple preparation method, high sensitivity, long-time stability, ease of preparation and regeneration of the electrode surface by simple polishing and excellent reproducibility. The proposed method was applied to determination of AA, AC and INZ in commercial drugs and in plasma samples and the obtained results were satisfactory.     

Electrochemiluminescent behavior of luminol on the glassy carbon electrode modified with CoTPP/MWNT composite film

A glassy carbon electrode (GCE) modified with cobalt(II) meso-tetraphenylporphrine/multiwall-carbon nanotube (CoTPP/MWNT) was applied to investigate the electrochemiluminescent (ECL) behavior of luminol. The ECL intensity of luminol was found to be increased greatly on this modified electrode. The presence of cobalt(II) meso-tetraphenylporphrine (CoTPP) can catalyze the reduction of oxygen on the electrode surface to produce HOO⁻, which can increase the ECL intensity of luminol. Moreover, MWNT can provide the more effective area of the electrode, and can act as a promoter to enhance the electrochemical reaction. The proposed method enables a detection limit for luminol of 1.0 × 10⁻⁸ mol/L in the neutral solution. Under the optimum condition, the enhanced ECL intensity of luminol by H₂O₂ had a linear relationship with the concentration of H₂O₂ in the range of 1.0 × 10⁻⁷ to 8.0 × 10⁻⁸ mol/L with the detection limit of 5.0 × 10⁻⁹ mol/L.     

Electrochemical characteristics of dopamine oxidation at palladium hexacyanoferrate film, electroless plated on aluminum electrode

A novel electrode material was obtained at an aluminum electrode (Al) by a simple electroless method including two consecutive procedures: (i) the electroless deposition of metallic palladium on the Al electrode surface from PdCl₂ + 25% ammonia solution and (ii) the chemical transformation of deposited palladium to the palladium hexacyanoferrate (PdHCF) films in a solution containing 0.5 M K₃[Fe(CN)₆]. The modified Al electrode demonstrated a well-behaved redox couple due to the redox reaction of the PdHCF film. The PdHCF film showed an excellent electrocatalytic activity toward the oxidation of dopamine (DA). The effect of solution pH on the voltammetric response of DA has been investigated. A linear calibration graph was obtained over the DA concentration range 2-51 mM. The rate constant k and transfer coefficient α for the catalytic reaction and the diffusion coefficient of DA in the solution D, were found to be 4.67 × 10² M⁻¹ s⁻¹, 0.63 and 2.5 × 10⁻⁶ cm² s⁻¹, respectively. The interference of ascorbic acid was investigated and greatly reduced using a thin film of Nafion on the modified electrode. The modified electrode indicated reproducible behavior and a high level stability during electrochemical experiments, making it particularly suitable for the analytical purposes.     

Separation and concentration effect of f-MWCNTs on electrocatalytic responses of ascorbic acid, dopamine and uric acid at f-MWCNTs incorporated with poly (neutral red) composite films

A novel conductive composite film containing functionalized multi-walled carbon nanotubes (f-MWCNTs) with poly (neutral red) (PNR) was synthesized on glassy carbon electrodes (GC) by potentiostatic method. The composite film exhibited promising electrocatalytic oxidation of mixture of biochemical compounds such as ascorbic acid (AA), dopamine (DA) and uric acid (UA) in pH 4.0 aqueous solutions. It was also produced on gold electrodes by using electrochemical quartz crystal microbalance technique, which revealed that the functional properties of composite film were enhanced because of the presence of both f-MWCNTs and PNR. The surface morphology of the polymer and composite film deposited on transparent semiconductor tin oxide electrodes were studied using scanning electron microscopy and atomic force microscopy. These two techniques showed that the PNR was fibrous and incorporated on f-MWCNTs. The electrocatalytic responses of neurotransmitters at composite films were measured using both cyclic voltammetry (CV) and differential pulse voltammetry (DPV). These experiments revealed that the difference in f-MWCNTs loading present in the composite film affected the electrocatalysis in such a way, that higher the loading showed an enhanced electrocatalytic activity. From further electrocatalysis studies, well separated voltammetric peaks were obtained at the composite film modified GC for AA, DA and UA with the peak separation of 0.17 V between AA-DA and 0.15 V between DA-UA. The sensitivity of the composite film towards AA, DA and UA in DPV technique was found to be 0.028, 0.146 and 0.084 μA μM⁻¹, respectively.     

Direct electrochemistry and electrocatalysis of cytochrome c based on chitosan-room temperature ionic liquid-carbon nanotubes composite

A robust and effective composite film combined the benefits of room temperature ionic liquid (RTIL), chitosan (Chi) and multi-wall carbon nanotubes (MWNTs) was prepared. Cytochrome c (Cyt c) was successfully immobilized on glassy carbon electrode (GCE) surface by entrapping in the composite film. Direct electrochemistry and electrocatalysis of immobilized Cyt c were investigated in detail. A pair of well-defined and quasi-reversible redox peaks of Cyt c was obtained in 0.1 mol L⁻¹ pH 7.0 phosphate buffer solution (PBS), indicating the Chi-RTIL-MWNTs film showed an obvious promotion for the direct electron transfer between Cyt c and the underlying electrode. The immobilized Cyt c exhibited an excellent electrocatalytic activity towards the reduction of H₂O₂. The catalysis current was linear to H₂O₂ concentration in the range of 2.0 × 10⁻⁶ to 2.6 × 10⁻⁴ mol L⁻¹, with a detection limit of 8.0 × 10⁻⁷ mol L⁻¹ (S/N = 3). The apparent Michaelis-Menten constant (K_m) was calculated to be 0.45 ± 0.02 mmol L⁻¹. Moreover, the modified electrode displayed a rapid response (5 s) to H₂O₂, and possessed good stability and reproducibility. Based on the composite film, a third-generation reagentless biosensor could be constructed for the determination of H₂O₂.     

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.     

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.