Heterogeneous Electron Transfer Reactions Research Articles

Inverted Region Electron Transfer Demonstrated by Electrogenerated Chemiluminescence at the Liquid/Liquid Interface

We describe several transinterface electron-transfer processes occurring at the benzonitrile (PhCN)/water interface that produce excited states. When an electrode was moved very close to the interface or even through the interface to produce an electrode/thin layer organic phase/aqueous solution configuration, the electrogenerated radical species formed in the PhCN phase (C12−Ru(bpy)33+, DPA+, or TH+) diffused to the liquid/liquid interface to react with (oxidize) a coreactant (C2O42-) soluble only in the aqueous solution, leading to the formation of a strongly reducing radical species (CO2•-). The electron transfer from CO2•- in the aqueous solution to the oxidized species in the organic phase across the liquid/liquid interface produced an electronically excited state which then emitted light. The appearance of an ECL signal in these systems supports the suggestion of Marcus inverted region behavior in very exothermic heterogeneous electron-transfer reactions at the liquid/liquid interface.

Electrochemical Behavior and In Vivo Determination of the Neurotransmitter Dopamine Using Sodium Montmorillonite Modified Electrodes

A modified glassy carbon electrode is used to examine the mass transport characteristics of the neurotransmitter dopamine (DA) in sodium montmorillonite films. The apparent diffusion coefficient (Dapp) of DA in the film is measured by chronocoulometry and cyclic voltammetry to be 2.3×10–9and 5.1×10–9 cm2/s. According to the Nicholson theory, the standard rate constant (ks) of heterogeneous electron transfer reaction for DA is estimated to be 2.7×10–6 cm/s by cyclic voltammetry. Montmorillonite has been modified on carbon fiber electrodes, and voltammetric response of some neurotransmitters and their metabolites has been examined. The variation of DA concentration in a rat brain with decreasing blood supply has been detected when the modified microelectrode is used in in vivo experiments.

Electrochemical Behavior and In Vivo Determination of the Neurotransmitter Dopamine Using Sodium Montmorillonite Modified Electrodes

A modified glassy carbon electrode is used to examine the mass transport characteristics of the neurotransmitter dopamine (DA) in sodium montmorillonite films. The apparent diffusion coefficient (Dapp) of DA in the film is measured by chronocoulometry and cyclic voltammetry to be 2.3×10–9and 5.1×10–9 cm2/s. According to the Nicholson theory, the standard rate constant (ks) of heterogeneous electron transfer reaction for DA is estimated to be 2.7×10–6 cm/s by cyclic voltammetry. Montmorillonite has been modified on carbon fiber electrodes, and voltammetric response of some neurotransmitters and their metabolites has been examined. The variation of DA concentration in a rat brain with decreasing blood supply has been detected when the modified microelectrode is used in in vivo experiments.

Resolution of coupled electron transfer–ion transfer processes at liquid/liquid interfaces by visualisation of interfacial concentration profiles

A powerful approach for investigating heterogeneous electron transfer (ET) reactions at liquid/liquid (oil/water) interfaces is described and illustrated with studies of the reactions between IrCl62– or Fe(CN)63– in an aqueous phase and decamethylferrocene (DMFc) or ferrocene (Fc) in 1,2-dichloroethane (DCE).

Spectroelectrochemical approaches to heterogeneous electron transfer reactions at the polarised water ∣ 1,2-dichloroethane interfaces

Abstract Heterogeneous electron transfer (ET) reactions at the polarised water ∣ 1,2-dichloroethane (DCE) interface are studied by in situ UV-Visible spectroscopy in total internal reflection mode. The reduction of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and the oxidation of 1,1′-dimethylferrocene (DMFc) by the hexacyanoferrate redox couple are considered. The generation of products in the organic phase is monitored spectroscopically and correlated to the simultaneous current response. Both systems exhibit a good correlation between optical and electrochemical responses, highlighting the ideal behaviour of these redox couples for electron transfer studies at liquid ∣ liquid interfaces. The kinetics of ET between hexacyanoferrate and TCNQ is analysed also by chronoabsorptometry and potential modulated reflectance spectroscopy. The potential dependence of the ET rate constant is discussed within the framework of the ET models for ITIES.

Heterogeneous electron transfer reactions at liquid/liquid interfaces studied by time-resolved absorption spectroscopy

The triplet excited state of water-soluble porphyrin has been found to be quenched by tetracyanoquinodimethane both in homogeneous solutions, in a water/acetonitrile mixture, and at the liquid/liquid interface between water and toluene. Time-resolved transient absorption spectroscopy in total internal reflection excitation mode (in the liquid/liquid interface case) is used to monitor the decay of the excited porphyrin triplet state. In homogeneous solutions, the quenching is efficient and diffusion controlled. Photoproducts are obtained with a high yield only with negatively charged porphyrins. In the case of heterogeneous electron transfer, kinetics data are obtained from the analysis of the relevant mass transfer regime.

Unmediated heterogeneous electron transfer reaction of ascorbate oxidase and laccase at a gold electrode.

The unmediated electrochemistry of two large Cu-containing proteins, ascorbate oxidase and laccase, was investigated by direct-current cyclic voltammetry. Rapid heterogeneous electron transfer was achieved in the absence of promoters or mediators by trapping a small amount of protein within a solid, electrochemically inert, tributylmethyl phosphonium chloride membrane coating a gold electrode. The problems typical of proteins in solution, such as adsorption on the electrode surface, were avoided by this procedure. In anaerobic conditions, the cyclic voltammograms, run at a scan rate of up to 200 mV/s, showed the electron transfer process to be quasi-reversible and diffusion-controlled. The pH-dependent redox potentials (+360 mV and +400 mV against a normal hydrogen electrode at pH7.0 for ascorbate oxidase and laccase respectively and +390 mV and +410 mV at pH5.5) were similar to those of the free proteins. The same electrochemical behaviour was recorded for the type 2 Cu-depleted derivatives, which contain reduced type 3 Cu, whereas the apoproteins were electrochemically inactive. Under aerobic conditions the catalytic current intensity of holoprotein voltammograms increased up to approx. 2-fold at a low scanning rate, with unchanged redox potentials. The voltammograms of type 2 Cu-depleted proteins and of apoproteins were unaffected by the presence of oxygen. This suggests that electron uptake at the electrode surface involves type 1 Cu and that only in the presence of oxygen is the intramolecular electron transfer to other protein sites rapid enough to be observed. The analogy with available kinetic results is discussed.

The effect of magnesium ion on the electrochemistry of cytochrome c and cytochrome b5 at a gold electrode modified with cysteine

Abstract The direct electrochemistry of cytochrome c, cytochrome b 5 and Cyt c/Cyt b 5 complex has been studied by cyclic voltammetry at a cysteine-modified gold electrode. The results indicate that this functional electrode permits selectively a quasi-reversible electrochemical response of cytochrome c only. The near-reversible electrochemical response of cytochrome b 5 was stimulated by Mg 2+ ion. From titration experiments, we observed that either the Mg 2+ ion or cytochrome b 5 could improve the cyclic voltammetric response of cytochrome c in different ways, and proved that the 1:1 protein–protein complex formed in the presence of Mg 2+ ion even at higher ionic strength. Meanwhile, we observed that the Mg 2+ ion and the ionic strength affected the electrochemical behaviour of cytochrome c more strongly than cytochrome b 5 . Thus, we propose that there are different interaction models involving protein–electrolyte–electrode between cytochrome c, cytochrome b 5 and the 1:1 protein complex with the electrode. The mutual interaction between two proteins was favourable for the heterogeneous electron transfer reaction, and the assumed interaction patterns of the 1:1 protein complex with the electrode were also confirmed by potential-step chronocoulometry.

Electrochemical Characteristics of Self-Assembled Monolayers of a Novel Nickel(II) Pentaazamacrocyclic Complex on a Gold Electrode

Ethyl disulfide possessing two nickel(II) pentaazamacrocyclic redox centers at its end carbon atoms, 1, adsorbs on gold electrodes from methanol solutions and yields stable, electroactive self-assembled monolayers (SAMs). Cyclic voltammetry of SAMs of 1 shows a stable redox wave at 0.55 V in aqueous 0.1 M Na2SO4 (pH 2), corresponding to the Ni2+/3+ redox reaction. For SAMs of 1 with the surface coverage (Γ) of nickel(II) redox centers of (1.41 ± 0.04) × 10-10 mol cm-2, the cyclic voltammograms (CV) show very small peak-to-peak separations (ΔEp < 20 mV) with the ΔEfwhm (full-width at half-maximum of CV peaks) values ranging from 140 to 150 mV, indicating electrostatic repulsive interactions among the surface-immobilized nickel(II) redox centers. The constructed SAM of 1 was defined as a square close-packed monolayer with the polyazamacrocyclic ring of the nickel(II) redox center oriented perpendicular to the electrode surface. Coadsorption of 1 with electroinactive alkyl/aryl disulfides furnishes mixed monolayers, and the Γ of the electroactive nickel(II) redox centers can be varied by the mole ratio of 1 and coadsorbate in the adsorption solution. In mixed monolayers, the ΔEfwhm decreases as the Γ of 1 decreases from the limiting monolayer surface coverage, suggesting a homogeneous distribution of 1 and coadsorbate in the constructed mixed monolayers. The formal potentials (E°‘) of the solution-dissolved 1 and the self-assembly of 1 with different Γ values, equivalent to 0.3−1 monolayer, are nearly equal, evidencing the same microenvironment around the nickel(II) redox centers in both the self-assembly and the solution phase. Exchange between the surface-immobilized gold−alkanethiolates with disulfides in solution has been observed and was found to accompany ordering of the monolayer. The apparent rate constant for the heterogeneous electron-transfer reaction, kapp, has been determined by fast-scan cyclic voltammetry to be 1.3 × 103 s-1 for the SAM of 1. An increase in the kapp with a decrease in the Γ of 1 in mixed monolayers was observed.

Successive electron-transfers in low ionic strength solutions. Migrational flux coupling by homogeneous electron transfer reactions

Analysis of successive heterogeneous electron transfer reactions requires consideration of the coupling of molecular fluxes by homogeneous electron transfer reactions. We demonstrate that bimolecular electron transfer between members of the redox system that differ by +2 in oxidation state (i.e., reproportionation) can strongly influence the magnitude of transport-limited currents when ion migration contributes to molecular transport. A theoretical description of the problem is developed in the limit of infinitely fast homogeneous electron transfer and identical diffusion coefficients. An analytic solution is presented that allows calculation of voltammetric limiting currents at microelectrodes for any stepwise n-electron transfer as a function of the ratio of redox and supporting electrolyte concentrations. The results demonstrate that migrational flux coupling of homogeneous electron transfer reactions must be included in the reaction mechanism to obtain even qualitatively accurate predictions of the current. The theory is found to be in good agreement with values of voltammetric currents measured at an inlaid 11.2-μm radius Pt microdisk for the two- and three-electron reductions of Ru(bpy)32+ in acetonitrile+toluene solutions.

Kinetics of the Electroreduction and Electrooxidation of Tetrakis(dimethylamino)-p- benzoquinone in Polar Aprotic Solvents

(dimethylamino)-p-benzoquinone. This study was conducted at a gold ultramicroelectrode in seven polar aprotic solvents, with varying viscosity, in the presence of 0.1 M tetraethylammonium perchlorate. In addition, the temperature dependence of the standard rate constants was determined in three of these solvents. On the basis of an analysis of the solvent effects, it is shown that the kinetic data show a variation in the degree of reaction adiabaticity and the relative sizes of both the inner- and outer-sphere components of the Gibbs energy of activation. Solvent effects on diffusion coefficients and standard potentials were also investigated. The standard reduction potential is a linear function of the solvent acceptor number, a parameter reflecting molecular solvating properties, whereas the corresponding standard potential for oxidation is a linear function of solvent polarizability, a parameter that reflects nonspecific interactions between the reactant and the solvent. The significance of these results is discussed within the context of the current models for simple heterogeneous electron transfer reactions.

Correlation of the Structural Decomposition and Performance of Pyridinethiolate Surface Modifiers at Gold Electrodes for the Facilitation of Cytochrome c Heterogeneous Electron-Transfer Reactions

This paper describes the results of an electrochemical and spectroscopic (infrared reflection and X-ray photoelectron spectroscopies) investigation of the modified gold electrode surfaces prepared from dilute ethanolic solutions of 4-mercaptopyridine (PySH) and 4,4‘-dipyridyl disulfide (PySS). Both precursors have been used extensively as facilitators for the electron transfer of redox proteins like cytochrome c (cyt c). During the course of an investigation of the interfacial architectures formed from the two different precursors, a previously unreported structural instability in the adlayers was discovered. This instability manifests itself as a decrease in the ability of the modified surfaces to facilitate the electron transfer of cyt c that correlates with an increase of the immersion time in the precursor solutions. Results are presented that delineate the decrease in facilitator performance and probe the structural changes resulting in the decrease in performance. Together, the electrochemical and surface spectroscopic findings reveal that the modified surfaces spontaneously decompose to yield an adlayer composed largely of adsorbed atomic and oligomeric sulfur, an adlayer that we found to be ineffective in the facilitation of the electron transfer reaction of cyt c. The implications of these findings on the use of this type of modifier to studies of electron transfer reactions of redox proteins and to issues of the general structural stability of organosulfur-based monolayers are briefly discussed.

Heterogeneous electron transfer at the polarised water/1,2-dichloroethane interface studied by in situ UV–VIS spectroscopy and differential cyclic voltabsorptometry

The heterogeneous electron transfer reaction between the redox couple ferricyanide–ferrocyanide in water and 7,7,8,8-tetracyanoquinodimethane (TCNQ) in 1,2-dichloroethane (DCE) is studied by combining conventional electrochemical measurements and UV–VIS absorption spectroscopy in total internal reflection (TIR) geometry.

Imaging of microstructured biochemically active surfaces by means of scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) has been used to create and to image thin film structures of functionalized polypyrrole, to which glucose oxidase has been covalently immobilized. In addition, the SECM was applied to optimize the quality of all steps of this microfabrication process. For this purpose, imaging of the obtained structures was performed in the conventional feedback mode of the SECM with ferrocene derivatives or [Os (bpy) 2 fpy] (bpy = 2,2′-bipyridine, fpy = 4-formylpyridine). The same working mode is also used for the enzyme-modified microstructures exploiting the ability of glucose oxidase to accept the oxidized forms of these inorganic transition metal complexes as artificial cofactors. Successive images in the absence and presence of the enzymes's substrate glucose added to the mediator solution allowed us to distinguish between the mediator regeneration caused by a heterogeneous electrontransfer reaction at the underlying gold support and the enzyme-mediated feedback. Prospects for the development of miniaturized biosensors are discussed.

The role of the metal and the solvent in simple heterogeneous electron transfer reactions

Solvent effects in simple electron transfer reactions are reviewed. It is shown that estimation of the outer sphere Gibbs activation energy is often difficult. Data for the electroreduction of Fe(H 2O) 6 3+ at single crystal gold electrodes are also discussed. This system shows a very large double layer effect.

Electrochemical measurement of second-order electron transfer rate constants for the reaction between cytochrome b5 and cytochrome c.

The second-order electron transfer reaction between cytochrome b5 and cytochrome c has been studied by cyclic voltammetry utilizing a gold electrode modified with beta-mercaptopropionate. When cyclic voltammetry is performed on a solution containing a mixture of cytochrome b5, cytochrome c and polylysine, cytochrome b5 undergoes reversible electrochemistry at the electrode surface while cytochrome c discriminates against the electrode surface. The selectivity of the modified electrode for negatively charged proteins makes it possible to selectively reduce a protein possessing a net negative charge and a relatively low reduction potential (outer mitochondrial membrane cytochrome b5, Eo = -102 mV; microsomal cytochrome b5, Eo = 3 mV) in the presence of another protein possessing a net positive charge and a relatively high reduction potential (cytochrome c, Eo = 265 mV). The electrochemical reduction of ferricytochrome b5 at the electrode surface is followed by a second-order electron transfer reaction between ferrocytochrome b5 and ferricytochrome c that yields ferricytochrome b5 and ferrocytochrome c. This fast homogeneous electron transfer reaction which is preceded by a heterogeneous electron transfer reaction results in a characteristic cyclic voltammogram containing a pre-peak to the reduction current. The second-order rate constant for the homogeneous reaction was obtained by invoking the above reaction scheme for digital simulation of a cyclic voltammogram which was subsequently fitted to the experimental data. Second-order rate constants obtained with this method are 2.9 x 10(8) and 8.9 x 10(8) for the homogeneous electron transfer reactions between rat liver outer mitochondrial membrane (OM) ferrocytochrome b5 and beef liver microsomal ferrocytochrome b5, with horse heart ferricytochrome c, respectively. These values are in good agreement with second-order rate constants obtained for the same protein systems by flash photolysis. [Meyer, T. E., Rivera, M., Walker, F. A., Mauk, M. R., Mauk, A. G., Cusanovich, M. A., & Tollin, G. (1993) Biochemistry 32, 622-627].

Silver reduction in a thiosulphate + nitrate solution: study of a CE reaction scheme by means of polarization curves and ac impedance

Silver electrodeposition from an electrolyte consisting of 0.1 M NaNo 3 + 0.025 M Na 2S 2O 3 + 1 × 10 −3 M AgNO 3 (pH 12) was investigated by means of polarization curves and ac impedance. In order to work under quasi-stationary conditions, measurements were performed on a rotating disk electrode, the reactive surface of which was covered with a fine silver pre-plating. It is shown that for cathodic overvoltages higher than 30 mV the reduction mechanism followed a CE reaction scheme. The rate constants of both the preceding homogeneous decomplexation reaction of Ag(S 2O 3) 3− 2 to Ag(S 2O 3) − and the heterogeneous electron transfer reaction were determined. The low-frequency limit of electrohydrodynamic impedance measurements was used to check the calculated chemical reaction parameters. In earlier publications in which higher nitrate/thiosulphate ratios were used, the influence of this decomplexation reaction could not be detected. Also, it was found that the lower NO − 3 S 2 O 2− 3 ratio used here caused a substantial difference from the current-overpotential equation at cathodic overvoltages lower than 30 mV. In this potential region, the low-frequency limit of the amplitude of the ac impedance increased with the overvoltage.

Solvent reorganization energy for heterogeneous electron-transfer reactions at liquid | liquid interfaces

Two different expressions for the solvent reorganization energy for the heterogeneous electron-transfer reactions at an ITIES have hitherto been derived by Kharkats (Sov. Electrochem., 12 (1976) 1257) and by Marcus (J. Phys. Chem., 94 (1990) 1050). To settle the controversy, a detailed calculation of this energy is presented. The result obtained corroborates that published by Marcus. A generalization for different geometries is also included.

ChemInform Abstract: The Kinetics of Heterogeneous Electron Transfer Reactions in Polar Solvents

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Calculation of solvent free energies for heterogeneous electron transfer at the water–metal interface: Classical versus quantum behavior

A computer simulation method is developed for the study of the adiabatic heterogeneous electron transfer reactions between an ion in solution and a metal electrode. The particular system studied is the Fe2+/Fe3+ electron transfer reaction with a Pt(111) electrode. The adiabatic classical free energy curve for the reaction is computed using umbrella sampling and molecular dynamics generated by the adiabatic solution to an Anderson–Newns-like Hamiltonian. Reactive flux calculations are then performed to determine the effect of transition state recrossings on the classical adiabatic rate constant. These effects are not found to be large (κ∼0.6). The water solvent model is next quantized using Feynman path integral techniques and the quantum adiabatic free energy curve for electronic transfer is calculated. The latter calculation shows that the solvent activation free energy barrier and thermodynamic driving force for the electron transfer process can be significantly affected by the water quantization. These results suggest that classical models for water may not be adequate, or at least need to be modified, for the accurate computer simulation of many heterogeneous electron transfer reactions.