Electrocatalytic oxidation of nitrite on a carbon paste electrode modified with Co(II) porphyrin adsorbed on SiO2/SnO2/Phosphate prepared by the sol–gel method

This paper describes the immobilization of 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphyrin ion on SiO₂/SnO₂/Phosphate obtained by the sol–gel processing method. The porphyrin was adsorbed on the surface of the modified material and furthermore metallized in situ with Co(II) ion. The porphyrin metallation process was followed using UV–Vis spectroscopy by inspecting the Q bands of the free and metallated porphyrin. A carbon paste electrode modified with material containing metallated porphyrin was used to study the electro-catalytic oxidation of nitrite ions by means of cyclic voltammetry, chronoamperometry and RDE voltammetry. The modified electrode was very stable and exhibited the electro-catalytic oxidation of nitrite ions at 0.72 V vs. SCE by a two electron mechanism producing nitrate ions at pH 5.4. The kinetic parameters of the electrode reaction process were calculated; (1 ? α)n_a was 0.479, D was (5.3 ± 0.11) × 10^?5 cm s^?1, and k⁰ could be determined as (5.4 ± 0.14) × 10^?3 cm s^?1.     

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 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.     

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.     

Electrocatalytic characteristics of ascorbic acid oxidation at nickel plated aluminum electrodes modified with nickel pentacyanonitrosylferrate films

Nickel pentacyanonitrosylferrate (NiPCNF) films have been deposited on the surface of an aluminum electrode by a simple electroless dipping method. The cyclic voltammogram of the resulting surface 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 towards the oxidation of ascorbic acid in phosphate buffer solution of pH 7.2. A linear calibration graph is obtained over the ascorbic acid concentration range 2–50 mM using linear sweep voltammetry. The kinetics of the catalytic reaction were investigated using cyclic voltammetry, rotating disk electrode (RDE) voltammetry and chronoamperometry. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. The rate constants for the catalytic reaction evaluated by three different approaches, are in good agreement and were found to be around 10^?3 cm s^?1. Further examination of the modified electrodes shows that the modifying layers (NiPCNF) on the aluminum substrate have reproducible behavior and a high level of stability.     

Characterization and electrocatalytic properties of cobalt hexacyanoferrate films immobilized on Au-colloid modified gold electrodes

An electroactive cobalt hexacyanoferrate (CoHCF) film was electrodeposited from a solution containing Co²⁺ and Fe(CN)₆^3? ions on the bare gold or the Au-colloid modified electrode. The cation (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) and the anion (F^?, Cl^? and Br^?) effects on the redox peak of the CoHCF film were investigated in detail. On the other hand, the electrocatalytic oxidations of thiosulfate at the CoHCF/gold and CoHCF/Au-colloid/gold electrodes were compared. At the CoHCF/Au-colloid/gold electrode, we obtained a response current larger by a factor of 2 and a three times lower detection limit than those at a CoHCF/gold electrode. The linear ranges were 1.0 × 10^?4 to 2.8 × 10^?3 M for the CoHCF/gold electrode and 7.5 × 10^?5 to 4.8 × 10^?3 M for the CoHCF/Au-colloid/gold electrode. These results showed that the immobilized CoHCF at the Au-colloid modified electrode exhibited a higher catalytic activity and a wider linear range toward thiosulfate. Additionally, the effects of the applied potential and the solution pH were studied.     

Direct simultaneous determination of dihydroxybenzene isomers at C-nanotube-modified electrodes by derivative voltammetry

The voltammetric behavior of dihydroxybenzene isomers was studied with glassy carbon electrodes modified with multi-wall carbon nanotubes. In 0.1 mol L^?1 HAc + NaAc buffer solution (pH 5.5), the modified electrode showed a good electrocatalytic response towards dihydroxybenzenes. The peak currents increased significantly and their oxidation potentials shifted negatively. Through a derivative technique, the three oxidation peaks of dihydroxybenzene isomers can be separated, thus the method can be applied to direct simultaneous determination without previous separation. The linear calibration ranges were 2 × 10^?6–1 × 10^?4 mol L^?1 for hydroquinone and catechol, respectively, and 5 × 10^?6 to 8 × 10^?5 mol L^?1 for resorcinol, with detection limits of 6 × 10^?7, 6 × 10^?7 and 1 × 10^?6 mol L^?1, respectively. This method has been applied to the direct determination of dihydroxybenzene isomers in artificial wastewater, and the recovery was from 92% to 104%.     

Electrochemical behavior of quercetin: Experimental and theoretical studies

Electrochemical oxidation of quercetin, as important biological molecule, has been studied in 0.1 M phosphate buffer solution, using cyclic voltammetry, chronoamperometry, rotating disk electrode voltammetry as well as quantum mechanical calculations. The heterogeneous charge transfer rate constant, k′, transfer coefficient, α, and exchange current density, j₀, for oxidation of quercetin at the glassy carbon electrode are determined as 4.84 × 10^?2 cm s^?1, 0.65 ± 0.01 and (1.17 ± 0.39) × 10^?7 A cm^?2, respectively. The formal potential, E^0′, of quercetin is pH dependent with a slope of ?60.1 mV per unit of pH which is close to the anticipated Nernstian value of ?59 mV for a two electrons and two protons process. The standard formal potential, E⁰, of quercetin was found to be equal with 558 mV versus saturated calomel electrode (SCE). The mechanism of oxidation was deduced from voltammetric data in various pHs and also in different concentrations of quercetin. The diffusion coefficient of quercetin was calculated as 3.18 × 10^?6 cm² s^?1 for the experimental condition, using chronoamperometric results. The results of density functional theory (DFT) calculations for the oxidation of quercetin in aqueous solution, are also presented. The theoretical standard electrode potential of quercetin is obtained to be 568 mV versus SCE, which is in good agreement with the experimental value. The discrepancy between theoretical and experimental values is only 10 mV. The agreement verifies the accuracy of experimental method and the validity of mathematical model.     

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.     

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.     

The electrochemistry and thin-layer luminescence spectroelectrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode

We report on the first direct electrochemistry and fluorescence spectroelectrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode. The value of n determined in spectropotentiostatic experiments at 1.87×10^?6 mol l^?1 of rhodamine 6G in 0.20 mol l^?1 KCl solution is 1.15, and the experimental value obtained for E⁰′ is ?0.787 V versus Ag ∣ AgCl ∣ KCl_sat, which agrees very well with the value (E⁰′=?0.791 V) obtained using cyclic voltammetry at a modified gold electrode. The values of the diffusion coefficients D_O and D_R for the oxidized and reduced forms of rhodamine 6G calculated from results of potential step and in situ fluorescence measurement experiments are 4.0×10^?6 cm² s^?1 and 4.2×10^?6 cm² s^?1, respectively. Cyclic voltammograms of rhodamine 6G show that the peak current I_p is proportional to the square root of the potential scan rate v^1/2, the ratio of the reduction to the oxidation peak height is about unity, and the separation of both reduction and reoxidation peak potentials ΔE_P is essentially constant at 135 mV at low scan rates. These results indicate that electrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode is a quasi-reversible one-electron electrode process.     

Square-wave voltammetry of an aqueous solution of indigo

Indigo is dissolved in water in the concentration of 5×10^?5 M and measured by highly sensitive adsorptive stripping square-wave voltammetry in order to determine the parameters of its electrochemical reduction. The formal potential of the indigo/leucoindigo redox couple dissolved in the aqueous 0.7 M KNO₃, pH 12 (NaOH) is ?0.566±0.003 V versus Ag ∣ AgCl ∣ 3 M KCl. Both indigo and leucoindigo are adsorbed on the surface of the mercury electrode, and the standard rate constant of this redox reaction is 920±130 s^?1. The ratio of adsorption constants of indigo and leucoindigo is 0.75.     

Catechin antioxidant action at various pH studied by cyclic voltammetry and PM3 semi-empirical calculations

The mechanism of catechin electro-oxidation at various pH was studied using cyclic voltammetry (CV) on the glassy carbon (GC) electrode and PM3 semi-empirical calculations. The influence of activation of the surface of the GC electrode on CV results has been discussed. Mixed adsorption–diffusion control has been observed by applying mechanistic criteria of CV to the results obtained at the activated electrode. The calculated catechin diffusion coefficient D = 2.78 × 10^?6 cm² s^?1. A linear increase of the current peak has been observed with the increase of substrate concentration up to 40 μmol dm^?3 (surface coverage Γ ～ 10^?11 mol cm^?2). In the whole investigated pH range, the dE/dpH value is very close to the anticipated Nernstian dependence of ?59 mV/pH indicating that the slope is not affected by the different sequences of e^? and H⁺ transfer. Molecular modeling results show a decrease of ≈5 kcal mol^?1 in ΔHoF (between radical and parent molecule) and a decrease of ≈6 eV in IP (of the parent molecule) when the parent molecule is changed from neutral to monoanionic form of catechin showing that both processes – hydrogen and electron abstraction are facilitated by deprotonation. Electrochemical oxidation of catechin is known to proceed as a two step one-electron oxidation of the B-ring of o-phenolic groups. Upon an increase in the pH, the mechanistic pathway of catechin electro-oxidation in both oxidation steps changes from an eH to the He process. In the reaction with a free radical, this may induce the change from hydrogen to electron donation.     

Characterization of a 1,4-dithiane gold self-assembled monolayer: an electrochemical sensor for the cyt-c redox process

Electrochemical desorption and spectroscopic investigations of the gold electrode surface modified with 1,4-dithiane (1,4-dt) organothiol species were performed. The wave observed at ?0.87 V versus Ag ∣ AgCl in the LSV (linear sweep voltammetry) reductive curve of the 1,4-dt compared to that for a similar 4-mercaptopyridine (pyS) system (?0.56 V) is indicative of a most effectively chemisorbed monolayer. The evaluation of the capability of the 1,4-dt self-assembled monolayer (SAM) in assessing the direct electron transfer (ET) of cytochrome c (cyt c) metalloprotein was investigated by cyclic voltammetry. The electrochemical response of the cyt c (E_1/2 ≈0.0 V vs. Ag ∣ AgCl, ΔE_p ≈50 mV) showed the characteristics of a reversible redox process. The cyt c voltammetric parameters acquired with the 24-h air exposure modified electrode, and after 100 cycles suggest a considerable improvement of the 1,4-dt electrode performance. The surface enhanced Raman spectroscopy (SERS) spectra revealed that 1,4-dt species is in a mixed gauche and trans orientation on the gold surface. The shift for higher wavenumbers observed for the C–S stretching modes in the SERS spectra, comparatively to the normal Raman spectrum, is assigned to the 1,4-dt coordination to surface gold atoms via a π interaction with the sulfur p-orbitals. The data collected suggest that this π interaction plays an important role on the stability of the 1,4-dt adlayer, improving the assessment of the cyt c heterogeneous electron transfer reaction.     

Electrostatic modification of ferritin onto polypeptide-functionalized indium oxide electrode surfaces: Electrochemical and AFM studies

We demonstrated the direct electron transfer reaction of ferritin immobilized onto polypeptide-functionalized indium oxide electrodes using electrostatic interactions between polypeptides and ferritin. Polypeptides such as poly(l-lysine) and poly(l-arginine) were strongly adsorbed onto indium oxide electrode surfaces by electrostatic interactions. The modification of electrode surfaces with poly(l-lysine) was achieved by immersing indium oxide electrode into 1 mg ml^?1 poly(l-lysine) (molecular weight: 80,000 and 84,000 Da) for approximating 10 min. Ferritin molecules were fully immobilized onto poly(l-lysine)-functionalized electrodes at immersion times approximating 30 min. After accounting for the roughness for the electrode surface, the surface coverage of ferritin on the functionalized-indium oxide electrode was evaluated to be 9–13 × 10¹¹ molecules cm^?2, which indicates that ferritin molecules were densely packed like a full monolayer. Ferritin immobilized onto functionalized-electrodes showed the direct electron transfer reaction with the electrode. Potential value dependence of redox peaks on ferritin immersion times was not observed. Poly(l-arginine) (molecular weight: 94,000 Da) also acted as a modifier for immobilization of ferritin by electrostatic interactions. The electrochemical behavior of ferritin immobilized onto poly(l-ariginine)-functionalized electrodes was similar to that observed in poly(l-lysine)-functionalized systems. We obtained direct evidence for electrostatic interactions between ferritin molecules and poly(l-lysine) by tapping-mode AFM measurements; molecular ferritin binding to poly(l-lysine) molecular wires was observed.     

Surface structures at the initial stages in passive film formation on Ni(1 1 1) electrodes in acidic electrolytes

The adsorption of sulfate or OH species and subsequent Ni(OH)₂ film growth on Ni(1 1 1) single crystal electrodes has been investigated using in situ infrared reflection absorption spectroscopy (IRAS) as well as scanning tunneling microscopy (STM). In a pH 3 sulfuric acid solution, STM images show that a well-defined Ni(1 1 1) surface with a (1 × 1) lattice is exposed at ?300 mV, while hexagonal close-packed images with an atomic spacing of 0.32 nm are grown on this electrode at 300 mV. On the other hand, IRAS results in a sulfuric acid solution (pH 3) reveal that an absorption band at 1116 cm^?1, which can be ascribed to ν(S–O) symmetric stretching of sulfate anion on Ni(1 1 1) surface, starts to appear at ?400 mV and develops its intensity with an electrode potential increase, while an absorption band at 930 cm^?1 begins to develop at 0 mV on Ni(1 1 1), Ni(1 0 0) and Ni(1 1 0) electrodes, which can be assigned to an in-plane δ(Ni–OH) bending vibration in Ni(OH)₂ passive film.     

Electroless deposition of Pt at a Pd electrode by reaction with sorbed H in Pd/H

In the course of cycling a Pt electrode into the platinum surface oxide formation and reduction region in an aqueous Cl^? ion solution, using a Pd ∣ H reference electrode, it was found that the Pd surface becomes coated with Pt. It is shown that this phenomenon arises from electroless deposition of Pt from Pt anodically dissolved from the Pt working electrode; the adsorbed H at Pd ∣ H is the reducing agent. These observations provide a warning that care must be exercised in the use of Pd ∣ H as a reference electrode.     

Kinetics of the ferric/ferrous electrode reaction on Nafion®-coated electrodes

Cyclic and convolution voltammetry is used to study the kinetics of the ferric/ferrous electrode reaction on Nafion® coated Au and Pt electrodes in a perchloric acid solution. The reaction is characterized by the standard rate constant k_s⁰≈10^?6 cm s^?1 of the electron transfer across the metal ∣ Nafion® interface and by the diffusion coefficient of the ferric ion in the Nafion film D^(m)=(6.7±1.4)×10^?8 cm² s^?1. The Nafion® film apparently prevents the trace anions present in the perchloric acid solution (e.g. chlorides and sulfates) reaching the electrode surface and, thereby, influencing the ferric/ferrous electron transfer reaction via the inner-sphere catalysis. In addition, the polymer phase introduces a steric factor which causes the rate of the electron transfer reaction to decrease compared to the uncoated electrode, and proportionally the ion diffusion flux toward the metal surface, so that the values of the ratio k_s⁰/D^1/2 for the coated and uncoated electrodes are comparable. It is proposed that the considerably higher rate of the electron transfer on the Nafion® coated electrode in the sulfuric acid solution is due to the catalytic effect of sulfate or hydrogensulfate anions, which are transported through the Nafion® film as the sulfate complexes of the ferric cation, i.e. FeSO₄⁺ and FeHSO₄²⁺. The catalytic effect of oxides on the Nafion® coated electrodes is demonstrated.     

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.     

Remarkably high activity of electrodeposited IrO2 film for electrocatalytic water oxidation

A homogenous and transparent IrO₂ film was prepared on an ITO electrode by anodic electrodeposition under galvanostatic conditions from an aqueous solution containing 2 mM K₂IrCl₆ and 40 mM oxalic acid that is aged at 37 °C and pH 10 for ca. 10 days. The absorption spectral change of the solution suggested that an IrO₂ colloid is formed in the solution during ca. 10 day-aging. The scanning electron microscopic (SEM) measurement displayed homogeneous deposition of IrO₂ particles with 100–250 nm of a diameter on the surface of the film. The X-ray diffraction (XRD) measurement indicated that IrO₂ in the film is amorphous. The cyclic voltammogram (CV) of the IrO₂-coated ITO electrode dipped in a 0.1 M KNO₃ aqueous solution exhibited a steep rise of an anodic current at 1.0 V vs SCE for catalytic water oxidation, as well as an anodic wave at 0.3 V and a corresponding cathodic wave at ?0.1 V that are assigned as an Ir^IV/Ir^V redox. The anodic current at 1.3 V on the CV was 660 times higher than that for a blank bare ITO electrode. Ir electrodeposited on the ITO electrode was also shown to be electrocatalytically active for water oxidation. However, the anodic current at 1.3 V on the CV for the Ir-coated ITO electrode was 14 times lower than that for an IrO₂-coated electrode in spite of the 34 times higher coverage of Ir. The potential static electrochemical water oxidation using the IrO₂-coated ITO electrode produced a significant amount of O₂ above 1.1 V vs Ag/AgCl, in contrast to no O₂ detected even at 1.3 V using a bare ITO electrode. The maximum turnover frequency (TOF) of the IrO₂ catalyst was provided as 16,400 ± 450 h^?1 at 1.3 V vs Ag/AgCl from the slope of the linear plots of the amount of O₂ vs coverage of IrO₂ in the range of ～1.5 × 10^?9 mol. The TOF was 450 times higher than that (36.4 ± 1.4 h^?1 at 1.3 V) for electrodeposited Ir showing the very high catalytic activity of the IrO₂ film.