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.     

Poly(o-aminophenol) film prepared in the presence of sodium dodecyl sulfate: Application for nickel ion dispersion and the electrocatalytic oxidation of methanol and ethylene glycol

Poly(o-aminophenol) (POAP) was formed by successive cyclic voltammetry in monomer solution in the presence of sodium dodecyl sulfate (SDS) on the surface of a carbon paste electrode. The electrochemical behavior of the SDS-POAP carbon paste electrode has been investigated by cyclic voltammetry in 0.5 M HClO₄ and 5 mM K₄[Fe(CN)₆]/0.1 M KCl solutions as the supporting electrolyte and model system, respectively. Ni(II) ions were incorporated into the electrode by immersion of the polymeric modified electrode having amine groups in 0.1 M Ni(II) ion solution. Cyclic voltammetric and chronoamperometric experiments were used for the electrochemical study of this modified electrode. A good redox behavior of the Ni(III)/Ni(II) couple at the surface of electrode can be observed. The electrocatalytic oxidations of methanol and ethylene glycol (EG) at the surface of the Ni/SDS-POAP electrode were studied in a 0.1 M NaOH solution. Compared to bare carbon paste and POAP-modified carbon paste electrodes, the SDS-POAP electrode significantly enhanced the catalytic efficiency of Ni ions for methanol oxidation. Finally, using a chronoamperometric method, the catalytic rate constants (k) for methanol and ethylene glycol were found to be 2.04 × 10⁵ and 1.05 × 10⁷ cm³ mol⁻¹ s⁻¹, respectively.     

Electrochemical characterization of cetyltrimethyl ammonium bromide modified carbon paste electrode and the application in the immobilization of DNA

A cetyltrimethyl ammonium bromide modified carbon paste electrode (CTAB/CPE) was developed in this work based on the surface modification method. The improved electrochemical response of K₄Fe(CN)₆ at this electrode indicated that CTAB could change the surface property of carbon paste electrodes (CPEs), which was demonstrated by the electrochemical impedance spectroscopy (EIS). In 0.1 mM [Fe(CN)₆]^3−/4−, a low exchange current (i₀) of 2.72×10⁻⁷ A at bare CPE was observed while that at CTAB/CPE was 6.79×10⁻⁵ A. The effect of CTAB concentration on the electrode quality revealed that CTAB formed a compact monolayer on the electrode surface with high density of positive charges directed outside the electrode. This electrode showed strong accumulation ability toward Fe(CN)₆⁴⁻ and can also accumulate Co(phen)₃²⁺ by the adsorption of the organic ligands in the hydrophobic area of the monolayer. The electrode was applied to the immobilization of DNA, which was characterized by the isotherm adsorption of Co(phen)₃²⁺.     

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.     

Hexacyanoferrate ion adsorbed on propylpyridiniumsilsesquioxane polymer film-coated SiO2/Al2O3: use in an electrochemical oxidation study of cysteine

Hexacyanoferrate ion, [Fe(CN)₆]⁴⁻, was immobilized by an ion-exchange reaction on the propylpyridiniumsilsesquioxane chloride polymer thin-film-coated SiO₂/Al₂O₃ surface. The amount of [Fe(CN)₆]⁴⁻ immobilized was 0.22 mmol g⁻¹ with a surface coverage of 9.6×10⁻⁶ mmol cm⁻². A carbon paste electrode made with this material was prepared and its electrochemical properties studied. The electrode presented two well-defined redox peaks with midpoint potentials, E_m, of 0.152 V vs SCE. This potential was not significantly affected by pH changes between 2 and 9.5. The electrode showed much reproducible responses and was successfully used to study the electrochemical oxidation of cysteine.     

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.     

An electrochemical biosensor based on Nafion-ionic liquid and a myoglobin-modified carbon paste electrode

An electrochemical biosensor was constructed based on the immobilization of myoglobin (Mb) in a composite film of Nafion and hydrophobic ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆) for a modified carbon paste electrode (CPE). Direct electrochemistry of Mb in the Nafion-BMIMPF₆/CPE was achieved, confirmed by the appearance of a pair of well-defined redox peaks. The results indicate that Nafion-BMIMPF₆ composite film provided a suitable microenvironment to realize direct electron transfer between Mb and the electrode. The cathodic and anodic peak potentials were located at −0.351 V and −0.263 V (vs. SCE), with the apparent formal potential (Ep) of −0.307 V, which was characteristic of Mb Fe(III)/Fe(II) redox couples. The electrochemical behavior of Mb in the composite film was a surface-controlled quasi-reversible electrode process with one electron transfer and one proton transportation when the scan rate was smaller than 200 mV/s. Mb-modified electrode showed excellent electrocatalytic activity towards the reduction of trichloroacetic acid (TCA) in a linear concentration range from 2.0 × 10⁻⁴ mol/L to 1.1 × 10⁻² mol/L and with a detection limit of 1.6 × 10⁻⁵ mol/L (3σ). The proposed method would be valuable for the construction of a third-generation biosensor with cheap reagents and a simple procedure.     

An electrochemical DNA biosensor developed on novel multinuclear nickel(II) salicylaldimine metallodendrimer platform

An electrochemical DNA biosensor (EDB) was prepared using an oligonucleotide of 21 bases with sequence NH₂-5′-GAGGAGTTGGGGGAGCACATT-3′ (probe DNA) immobilized on a novel multinuclear nickel(II) salicylaldimine metallodendrimer on glassy carbon electrode (GCE). The metallodendrimer was synthesized from amino functionalized polypropylene imine dendrimer, DAB-(NH₂)₈. The EDB was prepared by depositing probe DNA on a dendrimer-modified GCE surface and left to immobilize for 1 h. Voltammetric and electrochemical impedance spectroscopic (EIS) studies were carried out to characterize the novel metallodendrimer, the EDB and its hybridization response in PBS using [Fe(CN)₆]^3−/4− as a redox probe at pH 7.2. The metallodendrimer was electroactive in PBS with two reversible redox couples at E°′ = +200 mV and E°′ = +434 mV; catalytic by reducing the E_pa of [Fe(CN)₆]^3−/4− by 22 mV; conducting and has diffusion coefficient of 8.597 × 10⁻⁸ cm² s⁻¹. From the EIS circuit fitting results, the EDB responded to 5 nM target DNA by exhibiting a decrease in charge transfer resistance (R_ct) in PBS and increase in R_ct in [Fe(CN)₆]^3−/4− redox probe; while in voltammetry, increase in peak anodic current was observed in PBS after hybridization, thus giving the EDB a dual probe advantage.     

A study of ion exchange at the poly(butyl viologen)-electrolyte interface by SECM

In this work, the ion exchange characteristics of poly(butyl viologen) (PBV) thin films on a platinum electrode has been investigated by cyclic voltammetric (CV) scans. Since ferrocyanide anions (Fe(CN)₆⁴⁻) were added during the polymerization of the PBV thin-film for its stability, Fe(CN)₆⁴⁻ could form charge transfer complex with monomer and co-deposited with polymer. Scanning electrochemical microscopy (SECM) was used to probe the released Fe(CN)₆⁴⁻ ions from PBV film with Os(bpy)₃Cl₂ as a mediator for the approaching process in 0.5 M KCl medium. Mass changes during the redox process of the film were also monitored in-situ by electrochemical quartz crystal microbalance (EQCM). The ion exchange and transport behavior was observed during CV cycling of the film of the SECM and EQCM. The insertion and extraction of anions were found to be potential-dependence. Moreover, the decrease in tip current of released Fe(CN)₆⁴⁻ with increasing cycle number accounted for the ion exchange between Fe(CN)₆⁴⁻ and Cl⁻ in the KCl electrolyte. However, the Fe(CN)₆⁴⁻/Fe(CN)₆³⁻ redox couple was found to be highly stable between 0.0 and 0.5 V (vs. Ag/AgCl/saturated KCl) in the phosphate buffer solution. Therefore, the electrochemical property of Fe(CN)₆⁴⁻/Fe(CN)₆³⁻ redox couple was studied at different scan rates using CV technique. The peak currents were directly proportional to the scan rate as predicted for a surface confined diffusionless system. The surface coverage (Γ) and the concentration of Fe(CN)₆⁴⁻ were determined to be 1.88 × 10⁻⁸ mol/cm² and 0.641 mol/dm³, respectively. By neglecting cations incorporation during redox reaction of the PBV film and also based on the results obtained from energy-dispersive X-ray spectroscopy for the films of as-deposited, reduced and oxidized states, an ion exchange mechanism was proposed.     

Glassy carbon electrode modified with Nafion-Au colloids for clenbuterol electroanalysis

Stable Nafion-Au colloids were immobilized on a glassy carbon electrode (GCE) for detection of β-agonist clenbuterol by electroanalysis. The Au colloids were prepared by a one-step electrodeposition onto GCE, with obvious electrocatalytic activity present. The negatively charged Nafion film was an efficient barrier to negatively charged interfering compounds, resulting in accumulation of positively charged clenbuterol at the Nafion film. The electrochemical characters of the electrode during various modified steps in a redox probe system of K₄[Fe(CN)₆]/K₃[Fe(CN)₆] were confirmed by cyclic voltammetry (CV) and AC-impedance. In Britton-Robinson (B-R) buffer solution (pH = 2.0) and the potential range of −0.2 to 1.2 V, the Nafion-Au colloid modified electrode, compared to a bare GCE, exhibits obvious electrocatalytic activity towards the redox of clenbuterol by greatly enhancing the peak current with a linear calibration curve from 8.0 × 10⁻⁷ to 1.0 × 10⁻⁵ mol/L and a detection limit of (1.0 × 10⁻⁷ mol/L) (R = 0.996). The modified electrode shows high sensitivity, selectivity and reproducibility. The recovery for detecting clenbuterol (∼10⁻⁶ mol/L) in human serum is up to 98.19%.     

Electropolymerization of aniline in acid media on the bare and chemically pre-treated aluminum electrodes: A comparative characterization of the polyaniline deposited electrodes

The electrosynthesis of polyaniline on the bare aluminum and pre-treated aluminum surface achieved in aqueous H₂PtCl₆ solution saturated with NaF for few seconds is described. The effect of some factors such as pre-treatment time, aniline and sulfuric acid concentrations on the electropolymerization process was investigated and optimum conditions were obtained. The stability of polyaniline film on the pre-treated aluminum electrode (Al-Pt) was studied as function of the potential imposed on the electrode. For applied electrode potentials of 0.1-0.7 V, the first-order degradation rate constant, k, of polyaniline film varies between 1 × 10⁻⁶ and 2 × 10⁻⁵ s⁻¹, and a relatively low slope (i.e. 2.1) was obtained for the plot of log k versus E. The coatings were characterized by scanning electron microscopy (SEM), and cyclic voltammetric behavior of the polyaniline-deposited Al electrode (Al/PANI) and polyaniline-deposited Al-Pt electrode (Al-Pt/PANI) in 0.1 H₂SO₄ solutions is described. The electrocatalytic activity of the Al-Pt/PANI electrode against para-benzoquinone/hydroquinone (Q/H₂Q) and Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox systems was investigated and the obtained results are compared with those obtained on Al/PANI and bulk Pt electrodes.     

Preparation and electrocatalytic oxidation properties of a nickel pentacyanonitrosylferrate modified carbon composite electrode by two-step sol-gel technique: improvement of the catalytic activity

The sol-gel technique was used to construct nickel pentacyanonitrosylferrate (NiPCNF) modified composite ceramic carbon electrodes (CCEs). This involves two steps: forming a CCE containing Ni powder and then immersing the electrode into a sodium pentacyanonitrosyl-ferrate solution (electroless deposition). The cyclic voltammograms of the resulting surface modified CCE under optimum conditions show a well-defined redox couple due to the [Ni^IIFe^III/II(CN)₅NO]^0/−1 system. The electrochemical properties and stability of the modified electrode were investigated by cyclic voltammetry. The apparent electron transfer rate constant (k_s) and transfer coefficient (α) were determined by cyclic voltammetry being about 1.1 s⁻¹ and 0.55, respectively. Sulfite has been chosen as a model to elucidate the electrocatalytic ability of NiPCNF-modified CCE prepared by one- or two-step sol-gel technique. The modified electrode showed excellent electrocatalytic activity toward the SO₃²⁻ electro oxidation in pH range 3-9 in comparison with CCE modified by homogeneous mixture of graphite powder, Ni(NO₃)₂ and Na₂[Fe(CN)₅NO] (one-step sol-gel technique). Sulfite was determined amperometrically at the surface of this modified electrode in pH 7. Under the optimized conditions the calibration curve is linear in the concentration range 2 μM to 2.0 mM. The detection limit (signal-to-noise is 3) and sensitivity are 0.5 μM and 13.5 nA/μM. The modified carbon ceramic electrode containing nickel pentacyanonitrosylferrate shows good repeatability, short response time, t (90%) <2 s, long-term stability (3 months), and it is renewed by simple mechanical polishing and its immersing in Na₂[Fe(CN)₅NO] solution. The advantages of the SO₃²⁻ amperometrically detector based on the nickel pentacyanonitrosylferrate-doped CCE is high sensitivity, inherent stability at wide pH range, excellent catalytic activity and less expense and simplicity of preparation. This sensor can be used as amperometric detector in chromatographic instruments.     

Paired electrochemical synthesis of new organosulfone derivatives

Electrochemical oxidation of “cathodically generated 4-aminocatechol (2)” has been studied in the presence of 4-toluenesulfinic acid (4a) and benzenesulfinic acid (4b) as nucleophiles in aqueous solutions, using cyclic voltammetry and controlled-potential coulometry. The results indicate that the o-benzoquinone derived from 4-aminocatechol (2) participates in Michael addition reaction with 4a or 4b to form the corresponding new organosulfone derivatives (5a and 5b). In this work we have proposed a mechanism for the electrode process. A Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox mediator was used for the anodic oxidation of 4-aminocatechol (2) to the corresponding o-quinone 3. The indirect electrochemical process consists of a multi-step such as (a) cathodic reduction of 4-nitrocatechol (1) to 4-aminocatechol (2), (b) chemical oxidation of 4-aminocatechol (2) to 4-aminoquinone (3) with the resulting Fe(CN)₆³⁻, (c) the chemical reaction of 4-aminoquinone (3) with 4-toluenesulfinic acid (4a) or benzenesulfinic acid (4b), and (d) the anodic regeneration of Fe(CN)₆³⁻. The paired electrochemical synthesis of organosulfone derivatives (5a and 5b) has been successfully performed in an one-pot process at carbon rod electrode as a working and platinum as a counter electrode in an undivided cell.     

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 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).     

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.     

Preparation and electrochemistry of cadmium pentacyanonitrosylferrate film modified glassy carbon electrode

A glassy carbon (GC) electrode surface was modified with a cadmium pentacyanonitrosylferrate (CdPCNF) film as a novel electrode material. The modification procedure of the GC surface includes two consecutive procedures: (i) the electrodeposition of metallic cadmium on the GC electrode surface from a CdCl₂ solution and (ii) the chemical transformation of the deposited cadmium to the CdPCNF films in 0.05 M Na₂[Fe(CN)₅NO] + 0.5 M KNO₃ solution. The modified GC electrode showed a well-defined redox couple due to [Cd^IIFe^III/II(CN)₅NO]^0/−1 system. The effects of supporting electrolytes and solution pH were studied on the electrochemical behavior of the modified electrode. The diffusion coefficients of alkali-metal cations in the film (D), the transfer coefficient (α) and the charge transfer rate constant at the modifying film | electrode interface (k_s), were calculated in the presence of various alkali-metal cations. The stability of the modified electrode was investigated under various experimental conditions.     

Spontaneous adsorption of 3,5-bis(3,5-dinitrobenzoylamino) benzoic acid onto carbon

Dendritic molecules contain multifunctional groups that can be used to efficiently control the properties of an electrode surface. We are developing strategies to generate a highly functionalized surface using multifunctional and rigid dendrons immobilized onto different substrates. In the present work, we explore the immobilization of a dendritic molecule: 3,5-bis(3,5-dinitrobenzoylamino) benzoic acid (D-NO₂) onto carbon surfaces showing a simple and rapid way to produce conductive surfaces with electroactive chemical functions. The immobilized D-NO₂ layer has been characterized using atomic force microscopy and cyclic voltammetry. D-NO₂ adsorbs onto carbon surfaces spontaneously by dipping the electrode in dendron solutions. Reduction of this layer generates the hydroxylamine product. The resulting redox-active layer exhibits a well-behaved redox response for the adsorbed nitroso/hydroxylamine couple. The film permeability of the derivatized surface has been analyzed employing the electrochemical response of redox probes: Ru(NH₃)₆³⁺/Ru(NH₃)₆²⁺ and Fe(CN)₆³⁻/Fe(CN)₆⁴⁻. Electrocatalytic oxidation of nicotinamide adenine dinucleotide onto a modified carbon surface was also observed.     

Modification of glassy carbon electrode with multi-walled carbon nanotubes and iron(III)-porphyrin film: Application to chlorate, bromate and iodate detection

In this study, multi-wall carbon nanotubes (MWCTs) is evaluated as a transducer, stabilizer and immobilization matrix for the construction of amperometric sensor based on iron-porphyrin. 5,10,15,20-Tetraphenyl-21H,23H-porphine iron(III) chloride (Fe(III)P) adsorbed on MWCNTs immobilized on the surface of glassy carbon electrode. Cyclic voltammograms of the Fe(III)P-incorporated-MWCNTs indicate a pair of well-defined and nearly reversible redox couple with surface confined characteristics at wide pH range (2-12). The surface coverage (Γ) and charge transfer rate constant (k_s) of Fe(III)P immobilized on MWCNTs were 7.68 × 10⁻⁹ mol cm⁻² and 1.8 s⁻¹, respectively, indicating high loading ability of MWCNTs for Fe(III)P and great facilitation of the electron transfer between Fe(III)P and carbon nanotubes immobilized on the electrode surface. Modified electrodes exhibit excellent electrocatalytic activity toward reduction of ClO₃⁻, IO₃⁻ and BrO₃⁻ in acidic solutions. The catalytic rate constants for catalytic reduction of bromate, chlorate and iodate were 6.8 × 10³, 7.4 × 10³ and 4.8 × 10² M⁻¹ s⁻¹, respectively. The hydrodynamic amperometry of rotating-modified electrode at constant potential versus reference electrode was used for detection of bromate, chlorate and iodate. The detection limit, linear calibration range and sensitivity for chlorate, bromate and iodate detections were 0.5 μM, 2 μM to 1 mM, 8.4 nA/μM, 0.6 μM, 2 μM to 0.15 mM, 11 nA/μM, and 2.5 μM, 10 μM to 4 mM and 1.5 nA/μM, respectively. Excellent electrochemical reversibility of the redox couple, good reproducibility, high stability, low detection limit, long life time, fast amperometric response time, wide linear concentration range, technical simplicity and possibility of rapid preparation are great advantages of this sensor. The obtained results show promising practical application of the Fe(III)P-MWCNTs-modified electrode as an amperometric sensor for chlorate, iodate and bromate detections.     

Faradaic impedance titration and control of electron transfer of 1-(12-mercaptododecyl)imidazole monolayer on a gold electrode

In this work, we studied interfacial proton transfer of the self-assembled monolayer (SAM) of 1-(12-mercaptododecyl)imidazole on a gold electrode by faradaic impedance titration method with Fe(CN)₆³⁻ as an anionic redox probe molecule. The surface pK_1/2 was found to be 7.3, which was nearly the same as that of 1-alkylimidazole in solution. We also investigated the electrochemical properties of the SAM-modified electrode by cyclic voltammetry. Cyclic voltammetry was performed (1) in the solution containing Fe(CN)₆³⁻ with repeated alternation of pH values to investigate the electrostatic interaction of the protonated or deprotonated imidazole with Fe(CN)₆³⁻ and (2) in the acidic or basic electrolyte containing Ru(NH₃)₆³⁺ as a cationic redox probe to verify the effect of the polarity of a redox probe. We observed the reversible adsorption/desorption of Fe(CN)₆³⁻ and concluded that the adsorbed Fe(CN)₆³⁻ catalyzed the electron transfer of both Fe(CN)₆³⁻ itself and cationic Ru(NH₃)₆³⁺.