Electrocatalytic oxidation of dopamine at aluminum electrode modified with nickel pentacyanonitrosylferrate films,synthesized by electroless procedure

The electrocatalytic oxidation of dopamine (DA) at a home-made aluminum electrode modified with nickel pentacyanonitrosylferrate (NiPCNF) film, has been studied by electrochemical approaches. The immobilization of NiPCNF film was performed by a simple dip-coating procedure. The cyclic voltammogram of the resulting modified Al electrode prepared under optimum conditions, shows a well-behaved redox couple due to the [Ni^IIFe^III/II(CN)₅NO]^0/?1 system. The NiPCNF films, formed on the Al electrode show excellent electrocatalytic activity toward the oxidation of DA. The effect of the solution pH on the voltammetric response of DA was examined using phosphate buffer solution of different pHs. Under optimum conditions a linear calibration graph was obtained over the DA concentration range 2–33 mM. The kinetics of the catalytic reaction were investigated by cyclic voltammetry and rotating disk electrode voltammetry. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. The rate constant for the catalytic reaction k, the diffusion coefficient of DA in the solution D, the electron diffusion coefficient in the film D_e and transfer coefficient α, were found to be 3.1×10² M^?1 s^?1, 3.4×10^?6 cm² s^?1, 2.2×10^?11 cm² s^?1 and 0.67, respectively. The interference of ascorbic acid was investigated and greatly reduced using a thin film of Nafion^® on the surface-modified electrode. Further examination of the modified electrode shows that the modifying layers (NiPCNF) on the aluminum substrate show reproducible behavior and a high level of stability during electrochemical experiments, making it interesting for analytical applications.     

Electrochemical behavior of iodide at a nickel pentacyanonitrosylferrate film modified aluminum electrode prepared by an electroless procedure

The surface of an aluminum disk electrode was modified by a thin film of nickel pentacyanonitrosylferrate and used for electrocatalytic oxidation of iodide. The cyclic voltammogram of the modified Al electrode showed surface redox behavior due to the [Ni^IIFe^III/II(CN)₅NO]^0/1? redox couple. The modifying layer shows excellent catalytic activity toward the oxidation of iodide. Different supporting electrolytes containing different alkali metal cations affected the apparent formal potential of the redox films and thus, changed the thermodynamic tendency and kinetics of the modifying film toward the catalytic oxidation of iodide. This was explained by including the concept of a surface coverage normalized-catalytic current. The kinetics of the catalytic reaction were investigated by cyclic voltammetry and rotating disk electrode voltammetry in a suitable supporting electrolyte. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. The heterogeneous rate constant for the catalytic reaction, k, diffusion coefficient of iodide in solution, D, and transfer coefficient, α, were found to be 5.8 × 10² M^?1 s^?1, 1.3 × 10^?5 cm² s^?1 and 0.66, respectively. In addition the effect of electrode surface coverage on the dynamic range of a calibration curve was investigated. Under optimum conditions a linear calibration graph was obtained over an iodide concentration range of 2–100 mM.     

Electrochemical characteristics of a cobalt pentacyanonitrosylferrate film on a modified glassy carbon electrode and its catalytic effect on the electrooxidation of hydrazine

Electrochemically prepared thin films of cobalt pentacyanonitrosylferrate (CoPCNF) were used as surface modifiers for glassy carbon electrodes. The electrochemical behavior of a CoPCNF-modified glassy carbon electrode was studied by cyclic voltammetry; the modified electrode shows one pair of peaks with a surface-confined characteristic in 0.5 M KNO₃ as supporting electrolyte. The effect of different alkali metal cations in the supporting electrolyte on the behavior of the modified electrode was studied and the transfer coefficient (α) and charge transfer rate constant (k_s) for the electron transfer between the electrode and modifier layer were calculated. The experimental results show that the peak potential and peak current vary with different alkali metal cations, but anions such as Cl^?, NO₃^?, CH₃COO^?, H₂PO₄^?/HPO₄^2? and SO₄^2? at 0.5 M concentration have no effect on the peak potential and peak current. An extensive study showed that the response of the modified electrode is not affected within a pH range of 2–8. The CoPCNF films on glassy carbon electrodes show excellent electrocatalytic activity toward the oxidation of hydrazine in 0.5 M KNO₃. The kinetics of the catalytic reaction were investigated by using cyclic voltammetry, rotating disk electrode (RDE) voltammetry and chronoamperometry. The average value of the rate constant for the catalytic reaction and the diffusion coefficient were evaluated by different approaches for hydrazine.     

Voltammetric and XPS investigations of nickel hydroxide electrochemically dispersed on gold surface electrodes

A chemically modified electrode composed of mixed hydroxide and oxyhydroxide nickel film (6–8 nmol cm^?2) on the gold substrate (Au ∣ Ni) was characterized by cyclic voltammetry and XPS techniques. The gold substrate electrodes were firstly electrochemically conditioned in 0.2 M NaOH by cycling the potential between ?0.25 and 0.6 V versus SCE, then modified by cathodic electrodeposition of nickel hydroxide films. These nickel films were obtained either by voltage cycling (50 mV s^?1) between 0.0 and ?0.5 V (SCE) or at constant potential of ?0.3 or ?0.5 V using non-deaerated 50 mM Ni(NO₃)₂ solutions. X-ray photoelectron spectroscopy (XPS) characterisation and voltammetric behaviour of Au ∣ Ni electrodes in alkaline solutions are described. Continuous electrochemical cycling of the Au ∣ Ni electrodes induces significant changes of the nickel films in terms of crystallographic structures and chemical composition. Combination of XPS and electrochemical methodologies have demonstrated the ability to follow the morphological and chemical changes in alkaline solutions upon cycling potentials. Angular-dependent XPS measurements have demonstrated that electrochemical treatment induces the formation of a uniform film layer with the following chemical distribution: Au ∣ Ni(OH)₂ ∣ NiOOH. The electrocatalytic activity of the Au ∣ Ni electrodes is investigated in alkaline medium using glucose as a model compound. The favourable combination of active species such as gold and nickel leads to a sensing electrode with strong catalytic activity over a wide range of applied potentials.     

Electrochemical self-assembly of nordihydroguaiaretic acid/Nafion modified electrodes and their electrocatalytic properties for catecholamines and ascorbic acid

Electrochemically active 1,4-bis(3,4-dihydroxyphenyl)-2,3-dimethylbutane (nordihydroguaiaretic acid, NDGA) films can be directly deposited on Nafion modified electrodes from aqueous nordihydroguaiaretic acid. The deposited NDGA/Nafion films are stable, and show obvious electrochemical activity in aqueous solutions with various pH. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ growth of the NDGA/Nafion films. When freshly prepared NDGA/Nafion films were transferred to aqueous solutions at various pH, the formal potential was found to be pH dependent. The NDGA/Nafion films electrocatalytically oxidized dopamine, epinephrine, and norepinephrine in aqueous solutions, with the electrocatalytic oxidation current developing through the oxidized form of the NDGA/Nafion film. NDGA/Nafion films also showed reversible electrocatalytic properties. Such films electrocatalytically reduce the oxidation products of dopamine, epinephrine, and norepinephrine resulting from the electrocatalytic oxidation of these compounds by the NDGA/Nafion film. The NDGA/Nafion modified films also electrocatalytically oxidized a mixture of dopamine and ascorbic acid. The chemical reaction of dopamine quinone with ascorbic acid was investigated using the rotating ring-disk electrode method.     

Electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH) at a chlorogenic acid modified glassy carbon electrode

The redox response of chlorogenic acid solution at an inactivated glassy carbon electrode was investigated and an ECE mechanism was proposed for the electrode process. It has been shown that the oxidation of chlorogenic acid at an activated glassy carbon electrode leads to the formation of a deposited layer of about 4.5×10^?10 mol cm^?2 at the surface of the electrode. Cyclic voltammetry was used for the deposition process and the resulting modified electrode retains the activity of the quinone/hydroquinone group anticipated for a surface-immobilized redox couple. The properties of the electrodeposited films, during preparation under different conditions, and the stability of the deposited film were also examined. The pH dependence of the redox activity of these films was found to be 57 mV per pH unit, which is very close to the anticipated Nernstian dependence of 59 mV per pH unit. The modified electrode exhibits potent and persistent electrocatalysis for NADH oxidation in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of about 430 mV and an increase in peak current. The electrocatalytic current increases linearly with NADH concentration from 0.1 to 1.0 mM. The apparent electron transfer rate constant, k_s, and the heterogeneous rate constant for electrooxidation of NADH, k_h, were also determined using cyclic voltammetry and rotating disk electrode voltammetry, respectively.     

Direct electron transfer between the heme of cellobiose dehydrogenase and thiol modified gold electrodes

Cellobiose dehydrogenase (CDH) is an extracellular fungal enzyme with two domains, one containing flavin adenine dinucleotide (FAD) and one containing heme. The electrochemistry of CDH, as well as its cleaved FAD- and heme-subunits, was studied using a membrane electrode, i.e. the enzyme was trapped under a permselective membrane on a cystamine or 3-mercaptopropionic acid modified gold electrode. Direct un-mediated electron transfer (ET) between the heme of CDH and thiol modified gold electrodes was demonstrated using cyclic voltammetry. At low sweep rate (10 mV s^?1) and low pH (pH 4.3) up-hill ET from heme to FAD in CDH was observed. The formal potential of the heme in CDH and in the cleaved heme-subunit was found to be the same and equal to ?41 mV versus Ag ∣ AgCl at pH 5.1. The dependence of the formal potential on the pH (in the pH range 3.6–6.0) indicates the presence of one redox-linked ionisable functional group. Entropy and enthalpy changes were determined in variable temperature experiments as follows, ΔS°′=?194±14 J mol^?1 K^?1 and ΔH°′=?74±6 kJ mol^?1. The electrocatalytic behaviour of the CDH electrodes was demonstrated by addition of the enzyme substrate, cellobiose. The catalytic current was shown to decrease upon increased pH, in accordance with previous kinetic data in solution. The model of electron transport from the substrate (cellobiose) to FAD, and then through the heme domain to the electrode was confirmed in the experiments.     

Attachment of gold nanoparticles to glassy carbon electrode and its application for the voltammetric resolution of ascorbic acid and dopamine

Gold nanoparticles have been attached on glassy carbon electrode surface through sulfhydryl-terminated monolayer and the gold nanoparticles-immobilized glassy carbon electrodes have been applied to the electrocatalytic oxidation of ascorbic acid, reducing the overpotential by about 200 mV with obviously increased current response. Due to its strong electrocatalytic activity towards ascorbic acid, the gold nanoparticles modified electrode can resolve the overlapped voltammetric waves of ascorbic acid and dopamine into two well-defined voltammetric peaks with peak-to-peak separation in potentials of about 300 mV. This can be used to allow the selective determination of ascorbic acid in the presence of dopamine. The catalytic current obtained from differential pulse voltammetry is linearly dependent on ascorbic acid concentration over the range of 6.5 × 10^?6 to 1.45 × 10^?4 M with correlation coefficient of 0.998 in the presence of dopamine. The detection limit (3σ) for AA was found to be 2.8 × 10^?6 M. The simultaneous determination of ascorbic acid and dopamine in their binary mixture has also been investigated. The modified electrode shows good selectivity, stability and anti-fouling properties. The proposed methods have been used for the selective determination of ascorbic acid in the presence of dopamine and for the simultaneous determination of both them in their mixtures with satisfactory results.     

Electrochemical properties of a tetrabromo-p-benzoquinone modified carbon paste electrode. Application to the simultaneous determination of ascorbic acid,dopamine and uric acid

The electrochemical study of a tetrabromo-p-benzoquinone modified carbon paste electrode (TBQ-MCPE), as well as its efficiency for electrocatalytic oxidation of ascorbic acid, dopamine and uric acid, is described. Cyclic voltammetry was used to investigate the redox properties of this modified electrode at various solution pH values and at various scan rates. Three linear segments were found with slope values of ?58.4 mV/pH, ?28.1 mV/pH and 0.0 mV/pH in the pH range 2.0–7.1, pH 7.1–9.0 and pH 9.0–11.0, respectively. The apparent charge transfer rate constant, k_s, and transfer coefficient, α, for electron transfer between TBQ and CPE were calculated as 3.79 ± 0.10 s^?1 and 0.55, respectively. The electrode was also employed to study the electrocatalytic oxidation of AA, using cyclic voltammetry, chronoamperometry and differential pulse voltammetry as diagnostic techniques. It has been found that the oxidation of AA at the surface of TBQ-MCPE occurs at a potential of about 430 mV less positive than that of an unmodified CPE. The diffusion coefficient of AA was also estimated using chronoamperometry. The kinetic parameters such as the electron transfer coefficient, α, and heterogeneous rate constant, $k_{\mathrm{h}}^{\prime }$, for oxidation of AA at the TBQ-MCPE surface was determined using cyclic voltammetry. Differential pulse voltammetry (DPV) exhibits two linear dynamic ranges and a detection limit of 0.62 μM for AA. In DPV, the TBQ-MCPE could separate the oxidation peak potentials of AA, DA and UA present in the same solution, though at the unmodified CPE the peak potentials were indistinguishable. This modified electrode was quite effective not only to detect AA, DA and UA, but also in simultaneous determination of each component concentration in the mixture.     

Selective determination of dopamine in the presence of ascorbic acid at a multi-wall carbon nanotube-poly(3,5-dihydroxy benzoic acid) film modified electrode

A sensitive and selective method for determination of dopamine (DA) using multi-wall carbon nanotube (MWCNT)-poly(3,5-dihydroxy benzoic acid) [poly(DBA)] modified electrode is developed. The modified electrode shows excellent electrocatalytic activity toward the oxidation of dopamine in phosphate buffer solutions at pH 7.4. Using cyclic voltammetry, the linear range of 1 × 10⁻⁷–7.0 × 10⁻⁵ M in the interference of 500 μM ascorbic acid (AA) and the detection limit of 1.0 × 10⁻⁸ M were estimated for the measurement of DA in pH 7.4 phosphate buffer solutions. The value of DA current retained 98.36% of the initial response current after the modified electrode exposed to the air for one week. The interference studies showed that the modified electrode excludes effectively large excess of AA. The kinetic characteristics of the transfer of DA demonstrated that the electron propagation between DA and electrode was accelerated at MWCNT-poly(DBA) modified electrode. The work provided a valid and simple approach to selectively detect dopamine in the presence of AA in physiological environment.     

A differential pulse voltammetric method for simultaneous determination of ascorbic acid,dopamine, and uric acid using poly (3-(5-chloro-2-hydroxyphenylazo)-4,5-dihydroxynaphthalene-2,7-disulfonic acid) film modified glassy carbon electrode

A stable modified glassy carbon electrode based on the poly 3-(5-chloro-2-hydroxyphenylazo)-4,5-dihydroxynaphthalene-2,7-disulfonic acid (CDDA) film was prepared by electrochemical polymerization technique to investigate its electrochemical behavior by cyclic voltammetry. The properties of the electrodeposited films, during preparation under different conditions, and their stability were examined. The homogeneous rate constant, k_s, for the electron transfer between CDDA and glassy carbon electrode was calculated as 5.25(±0.20) × 10² cm s⁻¹. The modified electrode showed electrocatalytic activity toward ascorbic acid (AA), dopamine (DA), and uric acid (UA) oxidation in a buffer solution (pH 4.0) with a diminution of their overpotential of about 0.12, 0.35, and 0.50 V for AA, DA, and UA, respectively. An increase could also be observed in their peak currents. The modified glassy carbon electrode was applied to the electrocatalytic oxidation of DA, AA, and UA, which resolved the overlapping of the anodic peaks of DA, AA, and UA into three well-defined voltammetric peaks in differential pulse voltammetry (DPV). This modified electrode was quite effective not only for detecting DA, AA, and UA, but also for simultaneous determination of these species in a mixture. The separation of the oxidation peak potentials for ascorbic acid–dopamine and dopamine–uric acid were about 0.16 V and 0.17 V, respectively. The final DPV peaks potential of AA, DA and UA were 0.28, 0.44, and 0.61 V, respectively. The calibration curves for DA, AA, and UA were linear for a wide range of concentrations of each species including 5.0–240 μmol L⁻¹ AA, 5.0–280 μmol L⁻¹ DA, and 0.1–18.0 μmol L⁻¹ UA. Detection limits of 1.43 μmol L⁻¹ AA, 0.29 μmol L⁻¹ DA and 0.016 μmol L⁻¹ UA were observed at pH 4. Interference studies showed that the modified electrode exhibits excellent selectivity toward AA, DA, and UA.     

Iron(III) octaethylporphyrin chloride supported on glassy carbon as an electrocatalyst for oxygen reduction

Oxygen reduction was investigated at iron(III) octaethylporphyrin chloride adsorbed on a glassy carbon electrode. The title porphyrin was adsorbed irreversibly and strongly on the surface of a glassy carbon electrode. The electrochemical behavior and stability of the modified electrode were investigated using cyclic voltammetry, chronoamperometry and rotating disk electrode methods. The modified electrode showed clear but modest electrocatalytic activity for the reduction of oxygen to a mixture of water and hydrogen peroxide in buffered solutions on both the acid and basic sides of neutral with the domination of an overpotential of about 690 mV and an increase in peak current. The heterogeneous rate constant for the reduction of O₂ at the surface of the modified electrode and the diffusion coefficient of oxygen were determined by rotation disk electrode voltammetry using the Koutecký–Levich plots. In addition, iron(III) octaethylporphyrin chloride exhibited strong catalytic activity toward the reduction of H₂O₂.     

Surface copper immobilization by chelation of alizarin complexone and electrodeposition on graphite electrodes,and related hydrogen sulfide electrochemistry; matrix isolation of atomic copper and molecular copper sulfides on a graphite electrode

The irreversibly adsorbed alizarin complexone (AC) was employed to immobilize and maintain Cu^II ions on the graphite electrode. The coordination chemistry between the adsorbed alizarin complexone ligand and the Cu^II ion on the surface was examined by surface cyclic voltammetry. Upon reduction of the Cu^II center to a Cu⁰ atom, a submonolayer of individual atoms of Cu⁰ rather than a continuous layer is formed on the electrode surface. The immobilized surface displays electrocatalytic activity towards the oxidation of sulfide ion from [S^2?] ion to S⁰. The electrocatalytic activity for the sulfide oxidation on a [Cu^II(AC)]^? adsorbed electrode is shown to be essentially identical with that of a electrode that contains an electrodeposited submonolayer of Cu⁰. The active catalyst in both cases is identified to be a submonolayer of cupric sulfide. The electrochemistry of the Cu⁰ submonolayer-coated electrode in aqueous solution containing hydrogen sulfide was also examined. When the modified electrode was polarized from ?1.1 V to 0.2 V, three electrode processes were observed. The first, near ?0.7 V, is a surface reaction between surface Cu⁰ and adsorbed [S^2?] ion to form a submonolayer of cuprous sulfide. The second appeared near ?0.23 V and is another surface process between Cu₂S and adsorbed sulfide ion to form a submonolayer of cupric sulfide. The third reaction is the electrochemical oxidation of sulfide ion catalyzed by CuS to form sulphur which deposits on the electrode surface when the potential is positive of ?0.2 V.     

Characterization of copper hexabromoplatinate films and their electrocatalytic properties with dopamine,NADH, and S2O32−

Copper(II) hexabromoplatinate (CuPtBr₆) films have been prepared by mixing Cu²⁺ and PtBr₆^2? ions in an aqueous KBr solution. An electrochemical quartz crystal microbalance (EQCM), a rotating ring-disk electrode, UV–visible absorption spectroscopy and cyclic voltammetry were used to study the deposition and growth mechanism of the copper(II) hexabromoplatinate films. The electrochemical and EQCM properties of the films indicate that a single redox process was confined to the immobilized copper(II) hexabromoplatinate films. The deposition of a copper hexabromoplatinate film occurs when Cu²⁺ is reduced to Cu⁺. In the aqueous KBr solution, Pt^IVBr₆^2? is reduced electrochemically to Pt^IIBr₆^4?, and the Cu⁺ reacts with the Pt^IIBr₆^4? and Pt^IVBr₆^2?species. The electrocatalytic oxidation properties of dopamine, NADH, and S₂O₃^2? were determined using the copper(II) hexabromoplatinate films. The electrocatalytic reaction of dopamine with a copper(II) hexabromoplatinate film was investigated using the rotating ring-disk electrode method.     

Electrochemical study of mefexamide at glassy-carbon electrodes and its determination in urine by differential pulse voltammetry

The electrochemical oxidation of mefexamide N-[2-(diethylamino)ethyl]-2-(4-methoxyphenoxy)acetamide was investigated using cyclic, linear scan and rotating disk voltammetry at glassy-carbon electrodes. The value of pK_a (9.01) was determined by the potentiometric method. In cyclic voltammetry, in neutral media, the compound shows two electrochemical irreversible oxidation peaks (both 2e^?), Ox1 and Ox2. A new redox couple Red3/Ox3, formed as a result of the oxidation Ox1 peak, followed for an irreversible chemical reaction, appears on the reverse negative sweep. In acidic media, only the Ox1 peak was observed. The most defined peaks were obtained in 0.040 M Britton–Robinson buffer (pH 6.0) and 0.010 M sulfuric acid with 0.10 M sodium sulfate. The Ox1 and Ox2 peak currents were diffusion-controlled, showing an adsorption effect for low mefexamide concentrations (1.0×10^?4 M) and calibration plots at 20 mV s^?1, being linear in the range 5.0×10^?5–5.0×10^?4 M. The limiting currents in a rotating disk electrode were mass transport controlled for rotation speeds lower than 3000 rpm. The anodic charge transfer coefficient, the mass-transport rate constant, the diffusion coefficient and the charge-transfer conditional constant were determined. Also, a method for the electrochemical determination of mefexamide in human urine was developed using differential pulse voltammetry, in 0.040 M Britton–Robinson buffer (pH 6.0), being extracted with dichloromethane. The standard addition method was applied. The detection limit was 0.8 μg of mefexamide per milliliter of urine. The statistical validation reveals that the method is free from significant systematic errors.     

Electropolymerization of polymerized fuchsin acid films enhanced by Nafion® and their electrocatalytic properties with melatonin and 3,4-dihydroxyphenylalanine

Electropolymerization of the acid dye of fuchsin acid (or acid fuchsin) can produce electrochemically active films in strong acidic aqueous solutions. The electropolymerization of fuchsin acid can be enhanced by Nafion^®, and polymerized fuchsin acid films are stable and show obvious electrochemical activity in acidic aqueous solutions. These films can be produced on glassy carbon, platinum, gold, and transparent semiconductor tin oxide electrodes. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ growth of poly(fuchsin acid) films and the growth enhancement by Nafion^®. Fuchsin acid is a molecule that has three branched monomers, and on polymerization, it forms a dendrimer. The polymer films exhibited one redox couple, and when transferred to acidic aqueous solutions at various pH, the formal potential was found to be pH dependent. The electrocatalytic reduction of epinephrine, melatonin, and 3,4-dihydroxyphenylalanine by poly(fuchsin acid) films in acidic aqueous solutions showed obvious electrocatalytic activity.     

Kinetic study of the catalytic oxidation of benzyl alcohols by phthalimide-N-oxyl radical electrogenerated in acetonitrile using rotating disk electrode voltammetry

The catalytic oxidation of benzyl alcohols by the phthalimide-N-oxyl radical electrogenerated from N-hydroxyphthalimide was examined by cyclic voltammetry and rotating disk electrode voltammetry. The quantitative analysis of the electrocatalytic reaction was conducted and the rate constants for the oxidation of benzyl alcohols were estimated.     

Poly(N-methylaniline)/nickel modified carbon paste electrode as an efficient and cheep electrode for electrocatalytic oxidation of formaldehyde in alkaline medium

This research in finding a cheap and efficient catalyst for electrooxidation of formaldehyde give us an attempt to make and examine the behavior of poly(N-methylaniline)/nickel modified carbon paste electrode (Ni/P(NMA)/MCPE) in absence and presence of formaldehyde. This involves in situ electropolymerization of N-methylaniline at carbon paste electrode, which is following to the incorporation of Ni(II) to polymeric layer by immersion of modified electrode in 1.0 M nickel sulphate solution. The electrocatalytic oxidation of formaldehyde was studied by cyclic voltammetry and chronoamperometry methods. The effects of scan rate and formaldehyde concentration on the electrocatalytic oxidation of formaldehyde were also investigated at the surface of Ni/P(NMA)/MCPE. The diffusion coefficient (D = 14.1 × 10⁻⁵ cm² s⁻¹), and some kinetic parameters such as the transfer coefficient (α = 0.45) and also second-order rate constant (k = 8.96 × 10⁻⁴ cm³ mol⁻¹ s⁻¹) of formaldehyde were calculated.     

Nitrite reduction in acidic solution at a GC/Eastman-AQ-Os(bpy)32+-PVP composite modified electrode

The negatively charged polymer polyester sulfonic acid (Eastman-AQ, abbreviated to AQ), positively charged polymer polyvinyl pyridine (PVP) and mediator Os(bpy)₃²⁺ were used to construct composite modified glassy carbon (GC) electrodes (abbreviated to GC/AQ-Os(bpy)₃²⁺-PVP). The reduction of NO₂^? in acidic solution was taken as a model reaction to explore the properties of the modified electrode. On the steady state polarization curve of NO₂^? reduction there were two well-developed waves with much enhanced plateau current densities and positively shifted half-wave potentials compared with bare GC electrodes. In 0.05 mol/l H₂SO₄ + 5 mmol/l NO₂^? the modified electrode exhibited plateau current densities of 723 and 1153 μA cm^?2 and half-wave potential shifts of 0.29 and 0.56 V for the first and second current wave, respectively, showing promising potential for nitrite detection. The catalytic activity for NO₂^? reduction did not decrease appreciably over 4 months. A number of relevant kinetic and thermodynamic parameters are estimated experimentally. NO₂^? reduction at the composite modified electrodes is suggested to follow the SR mechanism (pure kinetic conditions involving mutual compensation between a catalytic reaction and substrate diffusion in the film in addition to diffusion in the solution phase) according to the Savéant–Andrieux theory.     

Reaction mechanism and kinetics of alcohol oxidation at nitroxyl radical modified electrodes

Nitrosonium cations generated by the one-electron oxidation of nitroxyl radicals are known to serve as selective oxidizing agents for alcohols to aldehydes or ketones. We prepared glassy carbon rotating disk electrodes (RDE) modified with 2,2,6,6-tetramethyl-piperidinyl-1-oxyl (TEMPO) via a very thin layer of poly(acrylic acid) and studied the electrode processes of 4-methoxybenzyl alcohol oxidation taking place at the modified electrodes in acetonitrile solution containing 0.2 mol dm^?3 sodium perchlorate using cyclic voltammetry and RDE voltammetry. It was found that though the presence of a base such as 2,6-lutidine is essential for the alcohol oxidation to proceed at a reasonable rate, the limiting current values corresponding to the rate of the alcohol oxidation at the RDE scarcely depend on the lutidine concentration. On the basis of these findings and electrochemical characteristics of TEMPO modified electrodes, we considered three possible reaction mechanisms and derived corresponding equations which relate the limiting current to rotation frequency of the RDE and concentrations of the alcohol and the base. We analyzed the experimental data by using the derived equations and proposed as the most probable reaction mechanism the one giving the most reasonable kinetic parameters and in which the rate-determining step is the decomposition of an intermediate formed from the nitrosonium cation, the alcohol and the base.