Mechanistic investigation of xanthene oxidation by heterogeneous and homogeneous electron transfers

Anodic oxidation of xanthene is investigated in acetonitrile at a platinum electrode by means of cyclic voltammetric and exhaustive potentiostatic electrolysis techniques. On the voltammetric scale time, the process involves two electrons and leads to xanthydrol. The corresponding mechanism is an ECE (electron–deprotonation–electron) type electrode reaction, the rate-determining step being the deprotonation of the cation radical obtained after the first electron transfer. On the other hand the analysis of the oxidation by homogeneous redox catalysis is carried out, using three organic catalysts. This allows the determination of the rate constants of the homogeneous electron transfers between xanthene and catalysts, the xanthene cation radical deprotonation rate constant and the standard potential of the xanthene cation radical/xanthene couple.     

Catalysis by immobilized redox enzymes. Diagnosis of inactivation and reactivation effects through odd cyclic voltammetric responses

Odd cyclic voltammetric responses, with an inverted peak appearing on the reverse scan, have been recently reported for the catalysis of immobilized enzymes involved in a direct electron transfer at the electrode surface and implicated in a chemical inactivation/redox reactivation mechanism. In this work, it is shown that this twisted reverse trace behavior can be related rigorously and quantitatively to such a reaction scheme by means of a minimal number of dimensionless parameters. As a prelude, the requirements for ‘pure catalytic’ conditions to be achieved are established quantitatively. It is also shown that simple irreversible or reversible inactivation does not entail the appearance of twisted reverse traces. The quantitative analysis of the inactivation/reactivation mechanism does not lead to a closed form expression of the current responses, but rather requires the numerical resolution of the pertinent differential equations. This approach may be readily extended to virtually any kind of mechanism, including more complex reactions schemes, distance-dependent electron transfer kinetics, the use of immobilized or free-moving redox cosubstrates, consideration of substrate mass transport limitations, etc.     

Role of proton transfer in the electrochemical reduction mechanism of salicylideneaniline

The electrochemical reduction mechanism of salicylideneaniline has been investigated by cyclic voltammetry, controlled potential electrolysis and coulometry. The main reduction product, characterised by HPLC, IR, ¹H NMR in X-ray diffractometry, is an anionic dimer, present in two diastereoisomeric forms, together with the conjugate base of the substrate. The latter stems from an intermolecular proton transfer from the substrate to a basic reduction intermediate. Kinetic analysis of the voltammetric results has allowed the electrode reaction mechanism to be fully characterised, showing in particular that the rate-determining step is the coupling between two anionic radicals, promoted by intramolecular H-bridging.     

Mechanism and kinetic analysis of the photo-induced electron transfer mediated by a stacked metallotriporphyrin in planar lipid bilayers

The photo-induced transmembrane charge transfer across planar lipid bilayers sensitized by a stacked ZnCuZn triporphyrin and enclosed between two aqueous phases is shown to occur via monomers spanning the membrane. The first step of the proposed mechanism involves the oxidation of the excited state of the porphyrin ring which faces the aqueous oxidizing phase. The positive charge then shifts from the resulting radical cation toward the porphyrin ring facing the reducing phase. This mechanism for the intramolecular electron transfer is preferred to the one based on the formation of a charge-separated state such as P⁺—P^?—P. The reaction scheme selected is supported by fluorescence and UV—visible absorption data. The kinetic equations derived fit the experimental time course of the photocurrents and photopotentials. A numerical simulation provides a lower estimation of the rate constant of the photo-induced intramolecular electron-transfer step.     

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.     

Electrochemistry of some novel hole transport materials

Several charge transfer agents have been investigated using cyclic voltammetry. We report multistep mechanisms involving both chemical and electrochemical steps, for the three compounds in the study. The overall mechanism consists of two quasi-reversible electron transfer events, each followed by a chemical step, an ECEC mechanism. The first cyclic voltammetric wave for each compound was studied separately. After the second quasi-reversible electron transfer, a third oxidation wave is observed under conditions of high concentration and slower scan rates. This is attributed to direct oxidation of the product of the second chemical step, which is identified as oligomer formation. This hypothesis is consistent with the physically observed formation of a film on the surface of the electrode, presumably from electrooxidative oligomerization of the compound.     

Growth and electrochemical activity of the poly-l-lysine|poly-l-glutamic acid thin layer films: an EQCM and electrochemical study

Quartz crystal microbalance (QCM) measurement and cyclic voltammetry were used to study the layer-by-layer growth of poly-l-lysine (p-Lys) and poly-l-glutamic acid (p-Glu) thin films and their ability to act as support for redox relays between metal electrodes and redox species in organic electrolytes. The mass of the film grown from buffered solution at constant pH depends on the composition of the buffer used. The growth proceeds in several stages with different mass increments for each poly-l-lysine and poly-l-glutamic acid layer. Ferro/ferricyanide confined into the polypeptide films can play the role of a redox relay between the electrode surface and the organic electrolyte. This was demonstrated on the impregnated polypeptide film|1,2-dichloroethane interface with decamethylferrocene dissolved as redox species in the organic phase. The transport of the ferro/ferricyanide in the polypeptide film is the rate limiting step of the electron transfer process. The diffusion coefficient of the ferro/ferricyanide determined from voltammetric experiments was of the order of 10^?11–10^?12 cm² s^?1. The bimolecular rate constant of the electron transfer reaction was found to be 0.2 cm s^?1 M^?1.     

Electrochemical reduction of phenyl-substituted cyclopentadienes: a case of an ‘indirect father–son’ self-protonation process

The electrochemical reduction of 1,2,3,4-tetraphenyl-1,3-cyclopentadiene and 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene has been investigated in DMF. At low temperatures (≤−30°C) and under anhydrous conditions cyclic voltammetry experiments indicate that both substrates are reducible in two successive, chemically reversible, one-electron steps, affording the corresponding radical anions and dianions. More complex voltammetric behaviour is detected at higher temperatures, when the radical anions are protonated by the substrate itself, giving rise, via the so-called ‘father–son’ self-protonation process, to further reducible compounds generating additional basic intermediates. In exhaustive electrolyses, 1/3 of the starting substrate is converted into dihydroreduction products and 2/3 into its conjugate bases. On the other hand, the quantitative formation of the dihydroreduction products is observed when exhaustive electrolyses are carried out in the presence of phenol, which acts as the proton donor instead of the substrate. Under voltammetric conditions, however, evidence of deprotonation of the substrate by the phenoxide anions thus generated is obtained, indicating the occurrence of an ‘indirect father–son’ self-protonation process, which takes place since phenol is kinetically more acidic, but thermodynamically less acidic, than the substrate itself. The mechanisms of decay of the radical anions are proposed on the basis of the comparison of experimental and simulated voltammetric data; this approach allowed also the determination of the characteristic kinetic rate constants, some of which are strongly dependent on the structure of the substrate, and of the importance of the homoconjugation reaction involving phenol and phenoxide anion. The stereochemistry of the dihydroreduction process and the voltammetric behaviour of the corresponding products are considered also, even in relation to the voltammetric behaviour of the starting substrates.     

Electrocatalytic activity of a cobalt hexacyanoferrate modified glassy carbon electrode toward ascorbic acid oxidation

A cobalt hexacyanoferrate (CoHCF) modified glassy carbon (CoHCF/GC) electrode was prepared electrochemically. The voltammetric responses of CoHCF are stable and the electrochemical behaviour is related to the concentrations of supporting electrolyte and counterions. The CoHCF/GC electrode shows electrocatalytic activity toward the oxidation of ascorbic acid in phosphate buffer solution. The electrocatalytic rate constant of the CoHCF/GC electrode for the oxidation of ascorbic acid is determined using rotating disk electrode measurements.     

Electrochemical study of self-assembled monolayers of a β-cyclodextrin methyl sulfide covalently linked to anthraquinone

Self-assembled monolayers of a β-cyclodextrin with methyl sulfides on the primary sites of the molecule and mono-functionalized with anthraquinone on a secondary site have been formed on gold electrodes. The surface coverages obtained by electrochemistry are consistent with values reported previously or calculated for similar non-electroactive β-CD derivatives. The electron transfer rate constants vary markedly with pH, increasing rapidly between pH 7 and 9 to values of approximately 700–900 s^?1. These values are faster than reported previously for thiol derivatives of anthraquinone dispersed in an alkylthiol SAM matrix. Near pH 6, slow electron transfer is observed with a splitting into individual one-electron steps. Lowering the pH further results in very sluggish kinetics and indistinguishable peaks. Inclusion of naphthalene into the β-CD cavity appears to affect the anodic rate constant, and a small potential shift occurs. The efficient electron transfer of the anthraquinone spaced from the gold surface by a β-CD unit suggests promise for use of such molecules as ‘immobilized artificial enzymes’.     

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.     

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.     

Consequences of a potential-dependent transfer coefficient in ac voltammetry and in coupled electron–proton transfer for attached redox couples

The Marcus density-of-states model for simple electron transfer predicts that the transfer coefficient is dependent on overpotential. The nature of the potential dependence is a function of the reorganization energies associated with oxidation and reduction processes. A fifth-order polynomial expression accurately yields the potential dependence of the transfer coefficient and the resulting curved Tafel plots. With this polynomial expression, the effects of the potential-dependent transfer coefficient are examined for two cases, ac voltammetry of an attached redox molecule with simple electron transfer and the kinetic behavior of the 1-electron/1-proton redox system. Simulations of ac voltammograms indicate that the effects are minor and that ac voltammetry is poorly suited for determination of the reorganization energy of the redox molecule. In the coupled electron–proton redox case, the effects are marked. As expected, the apparent standard rate constant decreases dramatically at pH values between the pK_a values of the two oxidation states. More surprisingly, the simulated Tafel plots exhibit asymmetry between the anodic and cathodic branches depending on the pH. The path of electron transfer from the oxidized to the reduced species (electron–proton or proton–electron) at a fixed pH depends on the electrode potential.     

Impedance voltammetry of electro-dimerization mechanisms: Application to the reduction of the methyl viologen di-cation at mercury electrodes and aqueous solutions

The faradaic impedance for an electrode mechanism with a reversible homogeneous dimerization reaction following the electron transfer step is derived. The chemical reaction shows up in the frequency dependence of the faradaic impedance and admittance in a similar way as deduced by Sluyters-Rehbach and Sluyters (J. Electroanal. Chem. 23 (1989) 457; J. Electroanal. Chem. 26 (1990) 237) for a homogeneous first-order chemical reaction. Two limiting cases can be distinguished in which the general expression reduces to the simpler Randles or pseudo-Randles expression. Under those conditions, the presence of the dimerization reaction can be inferred from the potential dependence of the impedance parameters. The theory is applied to the reduction of the methyl viologen di-cation at mercury electrodes in aqueous solution. The rate and the equilibrium constants for the dimerization reaction and the standard potential for the electron transfer step are obtained from the Warburg coefficient, while the potential dependence of the irreversibility coefficient allows the calculation of the standard rate constant and the transfer coefficient for the electron transfer step.     

Mechanistic study of the autoxidation of reduced flavin and quinone compounds

The mechanism of the autoxidation of reduced flavin and quinone compounds was investigated through semiquantitative analysis of voltammetric waves of the reduction of dioxygen (O₂) catalyzed by flavin and quinone adsorbed on electrode surfaces, where the redox equilibrium among the oxidized, (flavo)semiquinone and fully reduced states is easily controlled by the electrochemical method. The analysis provided evidence that the semiquinone radical plays a predominant role in the autoxidation. The generation of a superoxide anion radical as a product of the one-electron transfer from the semiquinone to O₂ was confirmed by the reduction of ferricytochrome c. The fact that the catalytic reduction wave of O₂ increases with pH was ascribed to the increase in the semiquinone formation constant. The mechanism of the reoxidation of reduced flavoprotein glucose oxidase with O₂ was also examined. The result supports the semiquinone-dependent one-electron transfer as the mechanism.     

Scanning electrochemical microscopy (SECM): Study of the formation and reduction of oxides on platinum electrode surfaces in Na2SO4 solution (pH = 7)

The formation and reduction of oxides on polycrystalline platinum were studied in a neutral solution with a scanning electrochemical microscope (SECM). Experiments were carried out with tip—substrate voltammetry where the faradaic current flowing to the tip is recorded while cycling the potential of the substrate, and with tip—substrate chronoamperometry where the faradaic tip current is recorded against time following the application of a potential step to the substrate. The tip current was made pH sensitive by holding the tip potential in a region where a pH dependent reaction occurs. Hydrogen evolution was used to probe pH decreases, oxygen evolution was used for the detection of pH increases and Pt oxide formation was used to detect both pH increases and decreases. The results showed that oxide formation occurs in two stages, each involving the transfer of electrons and the release of protons into the solution. During the first stage the release of H⁺ precedes the transfer of electrons. while in the second stage H⁺ release and electron transfer proceed simultaneously. Results are analysed in terms of the formation of PtOH during stage I and PtO during stage 2. However stage 2 behaves differently under slow potential changes and the release of protons lags behind the transfer of electrons. This is interpreted as the result of a place-exchange mechanism from PtOH to HOPt prior to stage 2, followed by the oxidation of HOPt to OPt during stage 2. Similarly, Oxide reduction was found to occur in two stages, each involving the transfer of electrons and the consumption of protons. During the first stage, the consumption of H⁺ precedes the transfer of electrons. The results suggest that during the transfer of electrons, protons diffuse from the outer layer of the oxide (OPt) into the inner layer to form HOPt. For the second stage results are analysed in terms of a place exchange mechanism from HOPt to PtOH running in parallel with the consumption of H⁺ followed by a surface reduction from PtOH₂⁺ to Pt metal.     

Electrochemical studies of single-wall carbon nanotubes in aqueous solutions

Studies on the voltammetric responses, capacitance, and charge storage capability of single-wall carbon nanotube sheets or papers are described. Broad redox responses have been observed probably due to the presence of oxygen-containing functional groups attached to the surface of the nanotubes or to the impurities produced during nanotube purification in nitric acid. Annealing the material at 900°C eliminates these responses. The voltammetry and capacitance of the nanotube paper varies little with either anion or cation molar mass, ion charge, or ion hydrophobicity. Acids as a group tend to produce higher capacitance but similarity between the electrochemical responses in various acids is also observed. The small variation of the capacitance within a wide range of scan rates suggests that in the time scale investigated the double layer charging process is controlled by the RC time constant and not by electrolyte ion diffusion into the pores of the nanotube paper.     

Novel studies on the electrochemical oxidation of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) in acetonitrile at platinum electrodes

The electrochemical oxidation mechanism of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) is studied in a 0.1 M N(C₄H₉)₄ClO₄ + acetonitrile (ACN) reaction medium by cyclic (CV) and square wave voltammetries (SWV) as well as by controlled potential bulk electrolysis at platinum electrodes. The primary radical cation formed by the one electron oxidation of DPMB undergoes a deprotonation process, which is the rate-determining step, followed by a radical–radical coupling. On the other hand, an initial quasi-reversible monoelectronic charge transfer mechanism is inferred from cyclic and square wave voltammograms recorded at scan rates and frequencies higher than 0.4 V s^?1 and 40 Hz, respectively. Diffusion coefficients of DPMB at different temperatures were calculated from the quasi-reversible convoluted cyclic voltammograms. DigiSim® and COOL software were used to fit the quasi-reversible cyclic and square wave voltammetric responses, respectively. Formal potentials, formal rate constants and positive transfer coefficients at different temperatures were evaluated from the fitting of cyclic voltammograms. The experimental activation parameters were also determined. The effects of the analytical concentration of the reagent and the temperature, as well as the addition of trifluoracetic acid and a strong base such as lutidine on the electrochemical responses are discussed. A general reaction mechanism as well as probable structures for dimeric products are proposed.Besides, the presence of an acid–base equilibrium in DPMB solutions is also studied by employing UV–Vis spectroscopic measurements at different trifluoracetic acid concentrations. An apparent value of (1.5 ± 0.2) × 10³ M^?1 was estimated for the DPMB basic constant at 20.0 °C     

Sinusoidal voltammetry:: a frequency based electrochemical detection technique

Sinusoidal voltammetry is an electrochemical technique, which uses a large amplitude sinusoid as the potential waveform and performs data analysis in the frequency domain. When the amplitude of the applied potential waveform is large (i.e. >50 mV) the current–potential behavior of the electrochemical interface is extremely non-linear. As a result the faradaic response exhibits signal intensity at higher order harmonics of the fundamental excitation frequency. In contrast, the major source of noise, due to the capacitive charging current, is primarily linear and the vast majority of its intensity remains at the fundamental frequency. The dramatic difference in the frequency response between these signals can be exploited in many ways to enhance both the signal-to-noise ratio and selectivity of an electrochemical measurement. The extent of faradaic signal distribution to the harmonics of the fundamental excitation frequency is dependant on many standard voltammetric parameters. In addition to enhanced sensitivity at the higher harmonics, redox species with different electrochemical properties (e.g. E°, number of electrons, electron transfer rate constant, etc.) can be detected selectively based on their unique ‘fingerprint’ frequency response. The experimental parameters (e.g. E_switch, scan rate, excitation potential window amplitude, etc.) can be optimized, as well as the frequency and phase angle to achieve the best selectivity for the analyte of interest.