Self-assembly of and charge transfer at N-docosyl-N′-methyl viologen on electrode surfaces

Self-assembled molecular films were formed on the electrode surfaces of glassy carbon (GC), platinum (Pt), gold, silver and indium–tin oxide in aqueous electrolyte solutions of N-docosyl-N′-methyl viologen (C₂₂VC₁). The temperature dependence of voltammetric responses showed a higher stability with higher surface coverage and with a Pt than a GC surface. Charge transfer reactions of the solution redox species of Ru(NH₃)₆³⁺ and Fe(CN)₆^4? at the irreversibly self-assembled C₂₂VC₁ ∣ GC interface were found to take place by an interplay of direct penetration of solution species through the self-assembled molecular layer of C₂₂VC₁ and of cross reaction between solution and surface bound redox agents.     

An old workhorse of oxide investigations: new features of Co3O4

The electrochemical behaviour of Co₃O₄ from exhaustive cobalt nitrate decomposition (T=260–850°C) is investigated on graphite-supported electrodes, mainly relying on quasi-reversible results. Two well defined 1?e^? redox systems are observed related to Co²⁺/Co³⁺ and Co³⁺/Co⁴⁺ reactions of non-equivalent oxide surface sites. Oxide stoichiometry is directly estimated by voltammetry in ‘wet’ conditions and depends on preparation temperature as in ex situ solid state data. Adsorption of surface intermediates involved in charge transfer is examined. Langmuir and Temkin-type adsorption isotherms are obeyed for the Co²⁺/Co³⁺ and Co³⁺/Co⁴⁺ redox systems, respectively. Unusually negative (attractive) lateral interaction parameters are calculated for the latter system. Separation of oxide and graphite currents is achieved to permit real electrode surface area determination.     

A voltammetric study of 6-mercaptopurine monolayers on polycrystalline gold electrodes

6-Mercaptopurine (6MP) monolayers were formed spontaneously on different gold surfaces by chemisorption from 6MP solutions. The present cyclic voltammetric study reveals that the monolayer formed is stable in a wide potential range up to values where the oxidative desorption occurs. The reductive desorption of 6MP at a polyfaceted gold electrode shows a multi-wave response. This has been analysed by comparing the observed peak potentials with those obtained at low-index single crystal gold electrodes. The film has no apparent effect on the electron transfer rate constant of Ru(NH₃)₆^3+/2+ and Fe(CN)₆^3?/4? at the substrates studied, but modifies the electron transfer rate of the benzoquinone redox couple. This behaviour is typical of thin films where only the slower reactions should display a larger apparent decrease of the rate constant. In contrast, the 6MP-monolayer shows a strong ability to inhibit electroreduction of dioxygen at polyfaceted gold while it remains stable.     

ESR/UV–Vis–NIR cyclovoltammetry of macrocyclic complex [CuI(bite)]BF4 at different temperatures

Using in situ ESR/UV–Vis–NIR cyclovoltammetry at different temperatures, model systems for blue copper proteins like [Cu^I(bite)]BF₄ and [Cu^II(bite)](BF₄)₂ (bite=biphenyldiimino dithioether) were studied with respect to their redox behavior. Chemically reversible processes in the electrode reaction were observed by ESR and UV–Vis–NIR spectroscopy during the repetitive redox cycling of [Cu^I(bite)]BF₄. The complex cyclovoltammetric feature of the compounds in acetonitrile can be explained by adsorption and a slow heterogeneous electron transfer with rate constant k_s=2×10^?4 cm s^?1. Spectroelectrochemical studies of the redox products at 77 K confirmed the formation of a metastable pseudo-tetrahedral [Cu^II(bite)]²⁺ intermediate. This is direct experimental proof of processes in redox reactions of the [Cu^I(bite)]⁺ within a dual-pathway square reaction scheme indicating preferable electron transfer from the pseudo-tetrahedral [Cu^I(bite)]⁺ as the initial step. The DFT optimized geometries of quasi-tetrahedral and quasi-planar [Cu^I(bite)]^+/2+ complexes are in reasonable agreement with experimental data.     

Cyclic voltammetric study of the disproportionation reaction of the nitro radical anion from 4-nitroimidazole in protic media

We have chosen 4-nitroimidazole (4-NIm) as a prototype of nitroimidazolic compounds in order to carry out an exhaustive cyclic voltammetric study in protic media, 0.1 M Britton–Robinson buffer+0.3 M KCl with ethanol as a co-solvent. The one electron reduction of 4-NIm in protic media at alkaline pH produces a stable nitro radical anion on the time scale of the cyclic voltammetric technique. The nitro radical anion decays according to a coupled chemical reaction and we have focused the study to follow this reaction using cyclic voltammetric methodology. The one electron reduction of the 4-NIm to form the nitro radical anion and the subsequent reaction of the radical obey an EC₂ mechanism that follows very well the cyclic voltammetry theory for the disproportionation reactions previously described by Olmstead and Nicholson (Anal. Chem. 41 (1969) 862). We have obtained the disproportionation rate constant k₂ which is strongly dependent on both pH and ethanol content according to the following regression equations: log k₂=?0.932pH+12.771 and log(10^-3k₂)=?1.998log[% EtOH]+3.873. The results obtained from this study in protic media differ substantially from previous studies in aprotic media wherein the nitro radical anion was not stabilized.     

Electrochemistry of alumina pillared clay modified electrodes

A simple method for preparing metal oxide pillared clay films on graphite electrodes, based on the use of preformed pillared structures, is described. The films exhibited very good electroactivity when immersed in solutions containing redox active cationic species, e.g. Fe(bpy)₃²⁺, Os(bpy)₃²⁺, Ru(NH₃)₆³⁺ and methyl viologen. Aging a pillared clay film, loaded with electroactive cations, in de-ionized water or varying the concentration of the supporting electrolyte have pronounced effects on the voltammetric behavior of the pillared clay films. These effects demonstrate clearly the key role of ion-paired electroactive cations for the electroactivity of pillared clay films. The pH dependence of the observed electroactivity for an alumina pillared clay film arises in part from the binding of the electroactive cations at the surface hydroxyl groups of the alumina pillars. This binding favors further formation of electroactive ion pairs. The voltammetric response from anionic redox couples, observed in acidic solutions, is attributed to the amphoteric nature of the alumina pillars. Discrepancies in the voltammetric behavior of silica and alumina pillared montmorillonite are explained in the light of different degrees of attachment of the redox active cations to the surface hydroxyl groups of the two metal oxide pillars. A model describing the charge transport in metal oxide pillared clays is also presented.     

Electrocatalytic oxidation of NADH at graphite electrodes modified with osmium phenanthrolinedione

We report here a detailed study concerning the electrochemical behavior of Os(4,4′-dimethyl, 2,2′-bipyridine)₂(1,10-phenanthroline 5,6-dione) complex, adsorbed on spectrographic graphite, and about its electrocatalytic activity for NADH oxidation. Cyclic voltammetric measurements, performed in aqueous phosphate buffer solutions, at different scan rates and pH values, allowed us: (i) to relate the redox response of the o-quinone ligand (phendione) to that of the Os(II) central ion; (ii) to confirm that, in aqueous solutions, the phendione based redox process globally involves two electrons and two protons; (iii) to estimate the rate constant for the heterogeneous electron transfer corresponding to the phendione redox couple (k_s≈20.1 s^?1). The second order rate constant for electrocatalytic oxidation of NADH (k_1,[NADH]=0=1.9×10³ M^?1 s^?1, at pH 6.1) as well as its pH dependence (from pH 5.5 to 8.1) were evaluated from RDE experiments, using both Koutecky–Levich and Lineweaver–Burk data interpretations.     

Microgravimetric monitoring of transport of cations during redox reactions of indium(III) hexacyanoferrate(III,II): Radiotracer evidence for the flux of anions in the film

We demonstrate here that, primarily, electrolyte cations but also, to some extent, anions are capable of penetrating indium hexacyanoferrate films during redox reactions. We find from electrochemical quartz crystal microbalance measurements that the electrolyte cation (K⁺) undergoes sorption and desorption during the system's reduction and oxidation, respectively. The formal potential, which has been determined from the system's well-defined voltammetric peaks recorded with the use of an ultramicroelectrode, decreases ～40 mV per decade of decreasing K⁺ concentration. The latter value is lower than the 60 mV change expected for the involvement of a cation in the reaction mechanism according to the ideal Nernstian dependence. We also demonstrate, using 35-S labelled sulfate, that anion penetrates the reduced film and its concentration markedly increases during oxidation. Careful examination of cyclic voltammetric responses of the system shows that, in addition to the well-defined peaks, capacitance-like currents appear during oxidation. During reduction anion is largely expelled from the film. This complex ionic penetration and transport in indium hexacynoferrate may be explained in terms of formation of two forms: ‘soluble’, KIn^III[Fe^II(CN)₆], and ‘insoluble’ (‘normal’), In^III₄[Fe^II(CN)₆]₃, during the electrochemical growth or potential cycling of the films. These forms would require cations and anions, respectively, to provide charge balance during reactions. Regardless of the actual mechanism, penetration of anions cannot be neglected completely in the discussion of charging of indium hexacyanoferrate.     

XPS and electrochemical studies of ferrocene derivatives anchored on n- and p-Si(1 0 0) by Si–O or Si–C bonds

High-coverage functionalization of H-terminated n- and p-Si(1 0 0) with vinylferrocene (VFC) and ferrocenecarboxaldehyde (FCA) has been obtained by wet chemistry methods, via the formation of a covalent bond between silicon atoms and the carbon (VFC) or the oxygen (FCA) termination of the molecules. The resulting functionalized electrodes have been analyzed by XPS, before and after cyclic voltammetry and capacitance–voltage measurements in an electrochemical cell. The very high quality of the hybrid species Si–CH₂–CH₂–C₅H₄–Fe²⁺–C₅H₅, resulting from VFC, has been certified by the negligible presence of silicon oxide and ferrocenium ions, and by the correct carbon/iron atomic ratio, accounting for the molecular species. The hybrid produced from FCA, Si–O–CH₂–C₅H₄–Fe²⁺–C₅H₅, presents a higher amount of silica, carbon and ferrocenium, as a consequence of the higher temperature of the functionalization procedure and of the higher reactivity of FCA. The use of two closely similar redox molecules has allowed to compare the behaviour of the Si–C–Y vs. Si–O–Y bond (Y being the redox moiety), with respect to their electrochemical reactivity in an organic solution. Both hybrids behave similarly on n- and p-Si substrates, in terms of redox potentials, stability to more than 1000 voltammetric cycles, linearity of the current intensity with the scan rate. The vinyl derivative, however, showed a faster and more reversible electron transfer kinetics than the carboxaldehyde derivative and an enhanced stability to long-term electrochemical experiments. VFC/p-Si is the first reported ferrocene derivative anchored to monocrystalline silicon via a C–Si bond. It is also the only large-area hybrid capacitor, to date, to be frequency-independent up to several hundred Hz, having a redox capacitance of 10^?4 F cm^?2, the highest reported so far for a monolayer on a Si(1 0 0) face.     

Mass transport accompanied with electron transfer between the gold electrode modified with 11-ferrocenylundecanethiol monolayer and redox species in solution — an electrochemical quartz crystal microbalance study

An electrochemical quartz crystal microbalance (EQCM) was employed to investigate mass transport during the redox reaction of the ferrocenylundecanethiolate (FcC₁₁S(H)) monolayer modified gold electrode in solution containing other redox species. The FcC₁₁S-monolayer on gold acts as a barrier for the electron transfer between a gold electrode and Fe(CN)₆^4?/3? in solution and as a mediator for the reduction of Fe³⁺ in solution. In both cases, electrochemical current responses were complicated because the observed currents were due to the redox of both the ferrocenyl group immobilized on gold and others in electrolyte solutions. The frequency change, i.e. interfacial mass change on the gold electrode surface, was observed only during the redox of ferrocenyl groups. The complex current response was deconvoluted into the current components of the redox reaction of ferrocene and that of other redox species in solution by comparing cyclic voltammograms with the current calculated from frequency changes.     

Voltammetric study of the interfacial electron transfer between bis(butylcyclopentadienyl)iron in 1,2-dichloroethane and hexacyanoferrate in water

The electron transfer (ET) reaction between bis(butylcyclopentadienyl)iron(II) ([Fe^II(C₅H₄Bu)₂]) in 1,2-dichloroethane (1,2-DCE) and the hexacyanoferrate redox couple ([Fe^II/III(CN)₆]^4?/3?) in water (W) at the 1,2-DCE∣W interface has been studied by use of normal pulse voltammetry and cyclic voltammetry. In normal pulse voltammetry, S-shaped current vs. potential curves with well-defined limiting currents attributable to the interfacial ET reaction were observed. The voltammetric results can be explained well by the theoretical equations [M. Senda, Rev. Polarogr. (Jpn), 49 (2003) 219; 50 (2004) 60] according to a so-called IT-mechanism, that is, the [Fe^II(C₅H₄Bu)₂] molecule is transferred from the 1,2-DCE to the W phase across the interface, then the ET reaction takes place homogeneously in the W phase between [Fe^III(CN)₆]^3?(W) and [Fe^II(C₅H₄Bu)₂](W) to produce [Fe^II(CN)₆]^4?(W) and [Fe^III(C₅H₄Bu)₂]⁺(W), which is followed by the transfer of the [Fe^III(C₅H₄Bu)₂]⁺ ion from the W to the 1,2-DCE phase across the interface to give the voltammetric current. Experimental results from cyclic voltammetry are also explained well by the IT-mechanism.     

Electrochemistry of GaAs/AlGaAs superlattice electrodes in acetonitrile solutions with metallocene redox couples

The photoelectrochemical properties of novel GaAs/AlGaAs superlattice electrodes were investigated in nonaqueous solutions containing reversible and irreversible redox couples. The superlattice materials consisted of 35 alternating layers of 17 Å Al_0.35Ga_0.65As and 150 Å GaAs grown onto a highly doped p-GaAs substrate. Under illumination, the superlattice electrodes exhibited current/potential curves with hysteresis in decamethylferrocene^+/0 solutions. The hysteresis was due to photoinduced dark current associated with hole injection into the valence band of the semiconductor. More reducing couples such as cobaltocene^+/0 and dicarbomethoxycobaltocene^+/0 afforded stable current/potential curves with the superlattice electrodes. The irreversible homogeneous and electrochemical reduction of a series of vicinal dibromides was investigated in order to develop a redox system capable of detecting hot electron reactions.     

Enhanced electrostatic interactions for selective and controllable permeation of ionic species at a monolayer of self-assembled dicationic nickel complex

Electrodic reactions of solution-dissolved ionic redox probes were carried out at a self-assembled monolayer (SAM) and mixed monolayers of {dinickel(II) (2,2-bis(1,3,5,8,12-pentaazacyclotetradec-3-yl)-diethyl disulfide) perchlorate}, 1, on polycrystalline Au electrodes. Electrostatic repulsive interaction between the dicationic nickel(II) redox center of the SAM of 1 and the solution-dissolved [Ru(NH₃)₆]³⁺ complex inhibits this complex from undergoing electrodic reaction, e.g. the peak-to-peak separation (ΔE_p) of the cyclic voltammogram (CV) for the [Ru(NH₃)₆]^3+/2+ couple in a 20 mM NaNO₃ solution is more than 600 mV, which is much larger than that (60 mV) at the bare Au electrode. On the other hand, the redox reaction of electroactive anions, [W(CN)₈]^3?/4? and [Fe(CN)₆]^3?/4? redox couples, occurs at the SAM of 1 without any inhibition such as that for the [Ru(NH₃)₆]^3+/2+ couple. A reversible redox wave was observed for the [W(CN)₈]^3?/4? redox couple at the SAM of 1 with the ΔE_p value of 69 mV; but at a bare Au electrode, a quasi-reversible redox wave with ΔE_p=270 mV was obtained. Such different electrodic reactions of the highly positively and negatively charged redox probes at the SAM of 1 possessing a short alkyl chain (its carbon number is 2) were explained as being due solely to the electrostatic interactions between the SAM of 1 and the redox probes in solution. The results obtained here illustrate the selective and controllable permeation of electroactive cations and anions at the monolayer electrodes.     

Severe inhibition of the diffusion and electroactivity of Cu(bpy)2+2 complexes incorporated in Nafion® coatings on electrodes

The Cu(bpy)²⁺₂ complex (bpy = 2,2′-bipyridine) is readily incorporated by Nafion^® coatings in amounts that can exceed the normal cation exchange capacity of the Nafion^®. The voltammetric responses of the incorporated complex are unusual in that, after the usual initial increases, the currents for reduction of Cu(bpy)²⁺₂ to Cu(bpy)²⁺₂ diminish as the quantity of complex incorporated in the Nafion^® grows. Quartz crystal microbalance measurements indicate that ca. 25 H₂O molecules are expelled from Nafion^® coatings for every Cu(bpy)²⁺₂ complex that is incorporated. The inhibition of the voltammetric response is attributed to large decreases in the physical diffusion rates of Cu(bpy)²⁺₂ in the resulting highly loaded, dehydrated Nafion^® coatings. The contrasting behavior of Os(bpy)²⁺₃, Ru(bpy)²⁺₃ and Co(bpy)²⁺₃ complexes are noted and possible reasons for the behavioral differences are suggested.     

Carbon paste electrodes of the mixed oxide SiO2/Nb2O5 prepared by the sol–gel method: dissolved dioxygen sensor

A carbon paste of SiO₂/Nb₂O₅ material was used as the electrode in the development of a dissolved dioxygen sensor in 1.0 mol l^?1 KCl solution at pH 6.2. The material was prepared by the sol–gel method. In the investigation of its electrochemical properties, linear and cyclic voltammetric and chronoamperometric techniques were employed. Dioxygen reduction, which was diffusion controlled, occurred at ?280 mV vs. SCE by a two electron mechanism, producing peroxide. A linear response between the cathodic peak current intensity and the dissolved O₂ concentration was obtained for the region between 1.0 and 13.6 mg l^?1. The stability proved to be very good over successive voltammetric cycles. The electrode response time was about 5 s. The electron transfer reactions were explained as being to an n-type semiconductor of niobia dispersed in the silica surface.     

Preparation of a novel clay/metal complex hybrid film and its catalytic oxidation to chiral 1,1′-binaphthol

An ITO electrode modified with a hybrid film of chiral metal complex (Λ-[Os(phen)₃]²⁺) and a clay (montmorillonite) has been prepared for the purpose of chiral sensing. As a first step, a floating monolayer of amphiphilic Os(II) complex, [Os(phen)₂(dC18bpy)](ClO₄)₂ (phen=1,10-phenanthroline, dC18bpy=4,4^′-dioctadecyl-2,2^′-bipyridyl), was formed on an aqueous dispersion of sodium montmorillonite. The monolayer acted as an organic part for the hybridization of clay particles in an aqueous phase. The hybrid film of clay and amphiphilic metal complex was transferred onto an indium tin oxide (ITO) substrate by the vertical dipping method. The next step was to immerse the electrode in chloroform, during which the amphiphilic Os(II) complex was removed from the clay surface. Thereafter the electrode was immersed in an aqueous solution of 0.5 mM Λ-[Os(phen)₃](ClO₄)₂ and rinsed with water. Cyclic voltammetric measurements were performed at each step of the above procedures. When the observed curves were simulated on the basis of a double-layered modified electrode, the electron transfer rate constant (k₁) for Λ-[Os(phen)₃]²⁺/Λ-[Os(phen)₃]³⁺ was determined to be 0.25 s^?1. This Os^II/Os^III redox couple was found to mediate the electrochemical oxidation of chiral 1,1^′-2-binaphthol in a stereoselective way: i.e., the S-isomer was oxidized at a 1.4 times higher rate than the R-isomer.     

Assembly,electrochemical characterisation and electrocatalytic ability of multilayer films based on [Fe(bpy)3]2+, and the Dawson heteropolyanion, [P2W18O62]6−

Successive adsorption onto a glassy carbon electrode of the Dawson heteropolyanion, [P₂W₁₈O₆₂]^6?, and the multiply charged cation [Fe(bpy)₃]²⁺, resulted in the formation of stable multilayer assemblies on the electrode surface. Surface coverages were found to be typical of monolayer coverages for multilayer systems. Cyclic voltammetric studies of the assembly in aqueous 0.5 M NaHSO₄, gave a range of redox couples associated with the Fe^3+/2+ redox system, of the cationic [Fe(bpy)₃]²⁺ moiety and the tungsten-oxo framework of the Dawson parent heteropolyanion, [P₂W₁₈O₆₂]^6?. It was possible to immobilise up to thirty monolayers, with the system exhibiting well-behaved redox behaviour. The stability of the assembly towards redox switching was investigated, with it being found to be extremely stable once the outer layer is anionic in nature. The immobilised film was also tested for electrocatalytic activity for the reduction of nitrite, hydrogen peroxide and bromate, and was shown to be an efficient electrocatalyst.     

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.     

Electrocatalytic oxidation of some amino acids on a cobalt hydroxide nanoparticles modified glassy carbon electrode

In this study we investigated the electrocatalytic oxidation of cysteine, cystine, N-acetyl cysteine, and methionine on cobalt hydroxide nanoparticles modified glassy carbon electrode in alkaline solution. Different electrochemical techniques such as cyclic voltammetry, chronoamperometry and steady-state polarization were used to track the oxidation process and its kinetics. From voltammetric studies we concluded that in the presence of amino acids the anodic peak current of Co(IV) species increased, followed by a decrease in the corresponding cathodic current peak. This indicates that amino acids were oxidized on the redox mediator which was immobilized on the electrode surface via an electrocatalytic mechanism. Using Laviron’s equation, the values of α_s and k_s for the immobilized redox species were determined as α_s,a = 0.63, α_s,c = 0.38 and k_s = 0.28 s⁻¹, respectively. The catalytic rate constants, the electron transfer coefficients and the diffusion coefficients involved in the electrocatalytic oxidation of amino acids were determined.     

Diamond-like carbon electrodes in electrochemical microgravimetry

Conducting diamond-like carbon films on quartz crystal electrodes were prepared by laser ablation of graphite. They were stable in many solvents including water, dichloromethane, and acetonitrile. Their potential window is much wider than those of the metal electrodes used usually in quartz oscillators and the electron transfer rate at the electrode surface is fast enough to perform electrochemistry. In dichloromethane with 0.1 M (n-Bu)₄NClO₄, DLC electrodes could scan the potential range between +2.0 and ?2.0 V to record cyclic voltammetric (CV) curves of [Ru(bpy)₃]^3+/2+. We also showed that the DLC works satisfactorily as a useful electrode material for electrochemical microgravimetry by examining the deposition–dissolution of [Ru(bpy)₃]³⁺ in dichloromethane solutions. It was found that the oxidized [Ru(bpy)₃]³⁺ have two different fates, that is, some of them make deposits on the DLC electrode and the rest diffuse out of the electrode surface instead of depositing. Based on the frequency data representing the amount of deposit only and the oxidation charge data reflecting the amount of oxidized species, deposits were found to have the average composition of [Ru(bpy)₃](ClO₄)₃?·?1.7(n-Bu)₄NClO₄.