Fabrication of Fc-SWNTs modified glassy carbon electrode for selective and sensitive determination of dopamine in the presence of AA and UA

The electrochemistry of dopamine (DA) was investigated by cyclic voltammetry (CV) and differential pulse voltammograms (DPV) at a glassy carbon electrode modified by the hybridization adducts of Fc-SWNTs. The electro-oxidation of DA could be catalyzed by Fc/Fc⁺ couple as a mediator and had a higher electrochemical response due to the unique carbon surface of carbon nanotubes. The anodic peaks of DA, ascorbic acid (AA) and uric acid (UA) in their mixture can be well separated by the prepared electrode. Under optimum conditions linear calibration graphs were obtained over the DA concentration range 5.0 × 10⁻⁶ to 3.0 × 10⁻⁵ M with a correlation coefficient of 0.9998 and a detection limit of 5.0 × 10⁻⁸ M based on the equation C_m = 3s_b1/m. The modified electrode has been successfully applied for the assay of DA in human blood serum. This work provides a simple and easy approach to selectively detect DA in the presence of AA and UA.     

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.     

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 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.     

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.     

Electrochemical sensor for sulfite determination based on a nanostructured copper-salen film modified electrode

The electrochemical preparation described herein involved the electrocatalytic oxidation of sulfite on a platinum electrode modified with nanostructured copper salen (salen = N,N′-ethylenebis(salicylideneiminato)) polymer films. The complex was prepared and electropolymerized at a platinum electrode in a 0.1 mol L⁻¹ solution of tetrabutylammonium perchlorate in acetonitrile by cyclic voltammetry between 0 and 1.4 V vs. SCE. After cycling the modified electrode in a 0.50 mol L⁻¹ KCl solution, the estimated surface concentration was found to be equal to 2.2 × 10⁻⁹ mol cm⁻². This is a typical behavior of an electrode surface immobilized with a redox couple that can usually be considered as a reversible single-electron reduction/oxidation of the copper(II)/copper(III) couple. The potential peaks of the modified electrode in the electrolyte solution (aqueous) containing the different anions increase with the decrease of the ionic radius, demonstrating that the counter-ions influence the voltammetric behavior of the sensor. The potential peak was found to be linearly dependent upon the ratio [ionic charge]/[ionic radius]. The oxidation of the sulfite anion was performed at the platinum electrode at +0.9 V vs. SCE. However, a significant decrease in the overpotential (+0.45 V) was obtained while using the sensor, which minimized the effect of oxidizable interferences. A plot of the anodic current vs. the sulfite concentration for chronoamperometry (potential fixed = +0.45 V) at the sensor was linear in the 4.0 × 10⁻⁶ to 6.9 × 10⁻⁵ mol L⁻¹ concentration range and the concentration limit was 1.2 × 10⁻⁶ mol L⁻¹. The reaction order with respect to sulfite was determined by the slope of the logarithm of the current vs. the logarithm of the sulfite concentration.     

Development of highly sensitive non-enzymatic sensor for the selective determination of glucose and fabrication of a working model

Copper oxide (CuO)/copper oxalate (CuOx) modified non-enzymatic electrochemical sensor for the detection of glucose in alkaline medium was fabricated by electrochemical anodisation of copper electrodes in potassium oxalate solution. Morphology of the modified copper electrode was studied by Scanning Electron Microscopy (SEM) and its electrochemical behaviour by Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). The formation of CuOx on the copper electrode was confirmed by the Infra-red Reflection Absorption Spectrum (IRRAS). The modified electrodes were found to be microporous and rough. Linear Sweep Voltammetry (LSV) and amperometry were adopted to investigate the direct electrocatalytic oxidation of glucose on CuO/CuOx modified electrode in alkaline medium which showed excellent catalytic activity. The best performance of the sensor was obtained at 0.7 V and in 0.1 M sodium hydroxide (NaOH). At this optimum potential, the sensor was highly selective to glucose in the presence of ascorbic acid (AA) and uric acid (UA) which are common interfering species in biological fluids. The sensitivity was found to be very high (1890 μA mM⁻¹ cm⁻²) with excellent linearity (R = 0.9999) up to 15 mM having a low detection limit of 0.05 μM (S/N = 3). The modified electrode was tested for glucose level in blood serum. Based on the optimised conditions, a working model of the sensor was made and successfully tested for glucose.     

Potentiometric stripping analysis of bismuth based on carbon paste electrode modified with cryptand [2.2.1] and multiwalled carbon nanotubes

An electrochemical method based on potentiometric stripping analysis (PSA) employing a cryptand [2.2.1] (CRY) and carbon nanotube (CNT) modified paste electrode (CRY-CNT-PE) has been proposed for the subnanomolar determination of bismuth. The characterization of the electrode surface has been carried out by means of scanning electron microscopy (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronocoulometry (CC). It was observed that by employing CRY-CNT-PE, a 9-fold enhancement in the PSA signal (dt/dE) was observed as compared to plain carbon paste electrode (PCPE). Under the optimized conditions, dt/dE (s/V) was proportional to the Bi(III) concentration in the range of 5.55 × 10⁻⁸ to 9.79 × 10⁻¹¹ M (r = 0.9990) with the detection limit (S/N = 3) of 3.17 × 10⁻¹¹ M. The practical analytical utilities of the modified electrode were demonstrated by the determination of bismuth in pharmaceutical formulations, human hair, sea water, urine and blood serum samples. The prepared modified electrode showed several advantages, such as a simple preparation method, high sensitivity, very low detection limits and excellent reproducibility. Moreover, the results obtained for bismuth analysis in commercial and real samples using CRY-CNT-PE and those obtained by atomic absorption spectroscopy (AAS) are in agreement at the 95% confidence level.     

Enhancement of electrochemiluminesence of lucigenin by ascorbic acid at single-wall carbon nanotube film-modified glassy carbon electrode

The electrochemiluminescent behavior of lucigenin on a single-wall carbon nanotube/DMF film-modified glassy carbon electrode was studied in this paper. Comparing with the bare glassy carbon electrode, the electrochemiluminescent of lucigenin at modified electrode is more stable and without tedious procedure for clean-up the surface of modified electrode. It has been found that ascorbic acid could enhance the electrochemiluminescent intensity of lucigenin greatly at this modified electrode. Based on which, a new sensitive and simple electrochemiluminescent method for determination of ascorbic acid could be developed. The condition for the determination of ascorbic acid was optimized. Under the optimized condition, the enhanced electrochemiluminescent intensity versus ascorbic acid concentration was linear in the range of 1.0 × 10⁻⁸ to 4.0 × 10⁻⁶ mol/L with a detection limit of 2.0 × 10⁻¹⁰ mol/L, and the relative standard derivation for 1.0 × 10⁻⁷ mol/L ascorbic acid was 3.8% (n = 8). The possible mechanism was also discussed.     

Development of a biomimetic chitosan film-coated gold electrode for determination of dopamine in the presence of ascorbic acid and uric acid

A gold electrode surface was modified using a dinuclear copper complex [Cu^II₂ (Ldtb)(μ-OCH₃)](BPh₄) and then coated with a chitosan film. This biomimetic polymer film-coated electrode was employed to eliminate the interference from ascorbic acid and uric acid in the sensitive and selective determination of dopamine. The optimized conditions obtained for the biomimetic electrode were 0.1 M phosphate buffer solution (pH 8.0), complex concentration of 2.0 × 10⁻⁴ M, 0.1% of chitosan and 0.25% of glyoxal. Under the optimum conditions, the calibration curve was linear in the concentration range of 4.99 × 10⁻⁷ to 1.92 × 10⁻⁵ M, and detection and quantification limits were 3.57 × 10⁻⁷ M and 1.07 × 10⁻⁶ M, respectively. The recovery study gave values of 95.2-102.6%. The lifetime of this biomimetic sensor showed apparent loss of activity after 70 determinations. The results obtained with the modified electrode for dopamine quantification in the injection solution matrix were in good agreement with those of the pharmacopoeia method.     

Development of a voltammetric sensor based on a molecularly imprinted polymer (MIP) for caffeine measurement

A new voltammetric sensor for caffeine measurement is introduced. A caffeine-selective molecularly imprinted polymer (MIP) and a non-imprinted polymer (NIP) were synthesized and then used for carbon paste (CP) electrode preparation. The MIP, embedded in the carbon paste electrode, functioned as a selective recognition element and a pre-concentrator agent for caffeine determination. The prepared electrode was used for caffeine measurement via a three-step procedure including analyte extraction in the electrode, electrode washing and electrochemical measurement of caffeine. The MIP-CP electrode showed very high recognition ability in comparison to NIP-CP. It was shown that electrode washing after caffeine extraction led to enhanced selectivity. Differential pulse voltammetry for caffeine determination was more effective than square wave voltammetry. Some parameters affecting sensor response were optimized, and a calibration curve was then plotted. A linear range of 6 × 10⁻⁸ to 2.5 × 10⁻⁵ mol L⁻¹ was obtained. The detection limit of the sensor was calculated to be equal to 1.5 × 10⁻⁸ mol L⁻¹. This sensor was used successfully for caffeine determination in spiked beverage and tea samples.     

Flow injection amperometric determination of pyridoxine at a Prussian blue nanoparticle-modified carbon ceramic electrode

This work describes the electrocatalytic properties of a carbon composite electrode (CCE) modified with Prussian blue (PB) nanoparticles (NPs) for the electrocatalytic oxidation of pyridoxine (PN). The morphology of the PBNP-modified CCE was characterized by scanning electron microscopy (SEM). The mechanism and kinetics of the catalytic oxidation reaction of PN were monitored by cyclic voltammetry and chronoamperometry. The rate-limiting step of the charge transfer reaction was found to be a one-electron abstraction. The value of α, k, and D were calculated as 0.66, 6.7 × 10⁴ M⁻¹ s⁻¹, and 1.88 × 10⁻⁵ cm² s⁻¹, respectively. The modified electrode showed electrocatalytic activity toward the oxidation of PN and was used as an amperometric sensor. The sensor exhibited good linear response for PN over the concentration ranges 5-69 and 1-80 μM with detection limits of 0.51 and 0.87 μM, and sensitivities of 0.97 and 0.673 A M⁻¹ cm⁻² in batch and flow conditions, respectively. Some important advantages such as simple preparation, fast response, good stability, and reproducibility of the sensor for the amperometric determination of PN were achieved.     

Selective and sensitive detection of dopamine in the presence of ascorbic acid by molecular sieve/ionic liquids composite electrode

A simple, sensitive, and reliable method based on a molecular sieve/ionic liquids composite electrode has been successfully developed for selective determination of dopamine (DA). The electrochemical behavior of dopamine (DA) at the modified electrode was investigated by cyclic voltammetry (CV) and square wave voltammetry (SWV). The influence of experimental parameters including pH of solution, amount of modifier, accumulation potential and time on the response of DA was investigated. At the optimum conditions, the peak current of DA was linear with the concentration of DA in the wide range of 5.0 × 10⁻⁸ mol L⁻¹ to 8 × 10⁻⁴ mol L⁻¹, with the correlation coefficient of 0.9982. The detection limit was 1.0 × 10⁻⁸ mol L⁻¹ (S/N = 3) in the presence of 0.2 mM ascorbic acid (AA). The interference studies showed that the modified electrode had excellent selectivity. What's more, the modified electrode also exhibited good reproducibility and stability for determination of DA, and could be applied to determine human serum samples.     

An electrochemical methanol sensor based on a Pd-Ni/SiNWs catalytic electrode

A novel electrochemical methanol sensor based on a catalytic electrode of palladium-nickel/silicon nanowires (Pd-Ni/SiNWs) is presented in this paper. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and electrochemical methods are employed to investigate the Pd-Ni/SiNWs electrode materials. These nanocomposite materials exhibit a highly ordered, wire-like structure with a wire length of ∼50 μm and a wire diameter ranging from 100 to 300 nm. The substrate has good electrocatalytic activity towards the oxidation of methanol in alkaline solutions. The performances of the prototype sensor are characterized by cyclic voltammetry and fixed potential amperometry techniques. In a 1 mol L⁻¹ KOH solution containing different methanol concentrations, the sensor exhibits a good sensitivity of 1.96 mA mmol⁻¹ L cm⁻² with R² = 0.99 and the corresponding detection limit of 18 μmol L⁻¹ (signal-to-noise ratio = 3, S/N = 3) for cyclic voltammetry. Meanwhile, the electrode also displays a sensitivity of 0.48 mA mmol⁻¹ L cm⁻² with R² = 0.98 and the corresponding detection limit of 25 μmol L⁻¹ (S/N = 3) for a fixed potential amperometry at −0.3 V versus an Ag/AgCl reference electrode. The results demonstrate that the Pd-Ni/SiNWs catalytic electrode has potential as an efficient and integrated sensor for methanol detection.     

Investigation of photoelectrocatalytic activity of Cu2O nanoparticles for p-nitrophenol using rotating ring-disk electrode and application for electrocatalytic determination

A cuprous oxide (Cu₂O) nanoparticles modified Pt rotating ring-disk electrode (RRDE) was successfully fabricated, and the electrocatalytic determination of p-nitrophenol (PNP) using this electrode was developed. Cu₂O nanoparticles were obtained by reducing the copper-citrate complex with hydrazine hydrate (N₂H₄·H₂O) in a template-free process. The hydrodynamic differential pulse voltammetry (HDPV) technique was applied for in situ monitor the photoelectrochemical behavior of PNP under visible light using nano-Cu₂O modified Pt RRDE as working electrode. PNP undergoes photoelectrocatalytic degradation on nano-Cu₂O modified disk to give electroactive p-hydroxylamino phenol species which is compulsive transported and can only be detected at ring electrode at around 0.05 V with oxidation signal. The effects of illumination time, applied bias potential, rotation rates and pH of the reaction medium have been discussed. Under optimized conditions for electrocatalytic determination, the anodic current is linear with PNP concentration in the range of 1.0 × 10⁻⁵ to 1.0 × 10⁻³ M, with a detection limit of 1.0 × 10⁻⁷ M and good precision (RSD = 2.8%, n = 10). The detection limit could be improved to 1.0 × 10⁻⁸ M by given illumination time. The proposed nano-Cu₂O modified RRDE can be potentially applied for electrochemical detection of p-nitrophenol. And it also indicated that modified RRDE technique is a promising way for photoelecrocatalytic degradation and mechanism analysis of organic pollutants.     

Voltammetric studies of a cobalt(II)-4-methylsalophen modified carbon-paste electrode and its application for the simultaneous determination of cysteine and ascorbic acid

A carbon-paste electrode (CPE) chemically modified with the cobalt(II)-4-methylsalophen (CoMSal) as a Schiff base complex was used as a highly sensitive and fairly selective electrochemical sensor for simultaneous determination of minor mounts of ascorbic acid (AA) and cysteine. This modified electrode shows very efficient electrocatalytic activity for anodic oxidation of both AA and cysteine via substantially decreasing of anodic overpotentials for both compounds. The mechanism of electrochemical oxidation of both AA and cysteine using CoMSal-modified electrode was thoroughly investigated by cyclic voltammetry and polarization studies. Results of cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using this modified electrode show two well-resolved anodic waves for the oxidation of AA and cysteine, which makes it possible for simultaneous determination of both compounds. A linear range of 1 × 10⁻⁴ to 5 × 10⁻⁷ M for cysteine in a constant concentration of 1 × 10⁻⁴ M AA in buffered solution (as a background electrolyte) was obtained from DPV measurements using this electrode. The linear range, which is obtained for AA in the presence of 1 × 10⁻⁴ M cysteine, was in the range of 1 × 10⁻⁴ to 1 × 10⁻⁶ M. The modified electrode has good reproducibility (RSD ≤ 2.5%), low detection limit (sub-micromolar) and high sensitivity for the detection of both AA and cysteine with a very high stability in its voltammetric response. Differential pulse voltammetry using the modified electrode exhibited a reasonable recovery for a relatively wide concentration range of cysteine spiked to a synthetic human serum sample.     

Electrodeposition of Pt-Fe(III) nanoparticle on glassy carbon electrode for electrochemical nitric oxide sensor

An electrochemical sensor for the detection of nitric oxide (NO) was developed by electrodeposition of Pt-Fe(III) nanoparticle on a glassy carbon electrode. This sensor exhibits excellent electrocatalytic activity for the oxidation of NO. A Nafion membrane coating was used to avoid the interference of nitrite and other potential interferences which may co-exist with NO in the biological systems. The effect of scan number in the electrodeposition process and the behavior of the sensor with respect to bulk pH have been studied. The catalytic peak current is found to be linear with the NO concentration over a wider range of 8.4 × 10⁻⁸ to 7.8 × 10⁻⁴ M, with a detection limit of 1.8 × 10⁻⁸ M (s/n = 3). In addition, the sensor has also good stability and anti-interference ability.     

Towards the conception of an amperometric sensor of l-tyrosine based on Hemin/PAMAM/MWCNT modified glassy carbon electrode

A novel amperometric sensor was fabricated based on the immobilization of hemin onto the poly (amidoamine)/multi-walled carbon nanotube (PAMAM/MWCNT) nanocomposite film modified glassy carbon electrode (GCE). Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and ultraviolet visible (UV-vis) adsorption spectroscopy were used to investigate the possible state and electrochemical activity of the immobilized hemin. In the Hemin/PAMAM/MWCNT nanocomposite film, MWCNT layer possessed excellent inherent conductivity to enhance the electron transfer rate, while the layer of PAMAM greatly enlarged the surface average concentration of hemin (Γ) on the modified electrode. Therefore, the nanocomposite film showed enhanced electrocatalytical activity towards the oxidation of l-tyrosine. The kinetic parameters of the modified electrode were investigated. In pH 7.0 phosphate buffer solution (PBS), the sensor exhibits a wide linear range from 0.1 μM to 28.8 μM l-tyrosine with a detection limit of 0.01 μM and a high sensitivity of 0.31 μA μM⁻¹ cm⁻². In addition, the response time of the l-tyrosine sensor is less than 5 s. The excellent performance of the sensor is largely attributed to the electro-generated high reactive oxoiron (IV) porphyrin (O = Fe^IV-P) which effectively catalyzed the oxidation of l-tyrosine. A mechanism was herein proposed for the catalytic oxidation of l-tyrosine by oxoiron (IV) porphyrin complexes.     

Electrochemical preparation and electrocatalytic properties of PEDOT/ferricyanide film-modified electrodes

The poly(3,4-ethylenedioxy thiophene) (PEDOT)/ferricyanide (FCN) film was synthesized by a potentiostatic and also using potentiodynamic methods namely cyclic voltammetric and chronoamperometric techniques. The EQCM technique was used to study the mechanism of the incorporation of ferricyanide ions on the PEDOT film. The UV-vis absorption results too confirmed the presence of ferricyanide with the PEDOT film. The electrocatalytic oxidation of ascorbic acid was carried out on a glassy carbon electrode modified with the PEDOT/FCN film through cyclic voltammetry, chronoamperometry and rotating disk electrode (RDE) voltammetry as diagnostic techniques. It was found that the catalytic current depended on the concentration of ascorbic acid. The number of electron transfer involved in the rate-determining step was found to be 1 and transfer coefficient (α) equal to 0.476. The diffusion coefficient of ascorbic acid was also estimated through the chrono amperometric and rotating disk electrode methods. The D values of ascorbic acid obtained by through the cyclic and chronoamperometric methods were found to be 4.4103 × 10⁻⁶ and 4.9595 × 10⁻⁶ cm² s⁻¹, respectively. This modified electrode was also used for the simultaneous determination of ascorbic acid and dopamine.     

Electrochemical quartz crystal impedance study on immobilization of glucose oxidase in a polymer grown from dopamine oxidation at an Au electrode for glucose sensing

Glucose oxidase (GOD) was codeposited into a polymer grown from oxidation of dopamine (DA) at an Au electrode in a neutral phosphate aqueous solution for the first time. The electrochemical quartz crystal impedance analysis (EQCIA) method was used to monitor the GOD-immobilization process. Effects of concentrations of phosphate buffer, DA and GOD were investigated, and the optimal concentrations were found to be 20.0 mM phosphate buffer (pH 7.0), 30.0 mM DA and 5.00 mg ml⁻¹ GOD. A glucose biosensor was thus constructed, and effects of various experimental parameters on the sensor performance, including applied potential, solution pH and electroactive interferents, were examined. At an optimal potential of 0.6 V versus the KCl-saturated calomel electrode (SCE), the current response of the biosensor in the selected phosphate buffer (pH 7.0) was linear with the concentration of glucose from 0.05 to 9 mM, with a lower detection limit of 3 μM (S/N = 3), short response time (within 15 s) and good anti-interferent ability. The Michaelis constant () was estimated to be 9.6 mM. The biosensor exhibited good storage stability, i.e. 96% of its initial response was retained after 7-day storage in the selected phosphate buffer at 4 °C, and even after another 3 weeks the biosensor retained 86% of its initial response. In addition, the enzymatic specific activity and enzymatic relative activity of the GOD immobilized in the polymer from dopamine oxidation (PFDO) were estimated from the EQCIA method to be 1.43 kU g⁻¹ and 3.7%, respectively, which were larger than the relevant values obtained experimentally using poly(o-aminophenol) and poly(N-methylpyrrole) matrices, suggesting that the PFDO is a better matrix to immobilize GOD.