Interfacial Electron Transfer Rate Research Articles

Garden compost inoculum leads to microbial bioanodes with potential-independent characteristics

Garden compost leachate was used to form microbial bioanodes under polarization at −0.4, −0.2 and +0.1V/SCE. Current densities were 6.3 and 8.9Am−2 on average at −0.4 and +0.1V/SCE respectively, with acetate 10mM. The catalytic cyclic voltammetry (CV) showed similar electrochemical characteristics for all bioanodes and indicated that the lower currents recorded at −0.4V/SCE were due to the slower interfacial electron transfer rate at this potential, consistently with conventional electrochemical kinetics. RNA- and DNA-based DGGE evidenced that the three dominant bacterial groups Geobacter, Anaerophaga and Pelobacter were identical for all bioanodes and did not depend on the polarization potential. Only non-turnover CVs showed differences in the redox equipment of the biofilms, the highest potential promoting multiple electron transfer pathways. This first description of a potential-independent electroactive microbial community opens up promising prospects for the design of stable bioanodes for microbial fuel cells.

Ultrafast electron and energy transfer in dye-sensitized iron oxide and oxyhydroxide nanoparticles

An emerging area in chemical science is the study of solid-phase redox reactions using ultrafast time-resolved spectroscopy. We have used molecules of the photoactive dye 2',7'-dichlorofluorescein (DCF) anchored to the surface of iron(III) oxide nanoparticles to create iron(II) surface atoms via photo-initiated interfacial electron transfer. This approach enables time-resolved study of the fate and mobility of electrons within the solid phase. However, complete analysis of the ultrafast processes following dye photoexcitation of the sensitized iron(III) oxide nanoparticles has not been reported. We addressed this topic by performing femtosecond transient absorption (TA) measurements of aqueous suspensions of uncoated and DCF-sensitized iron oxide and oxyhydroxide nanoparticles, and an aqueous iron(III)-dye complex. Following light absorption, excited state relaxation times of the dye of 115-310 fs were found for all samples. Comparison between TA dynamics on uncoated and dye-sensitized hematite nanoparticles revealed the dye de-excitation pathway to consist of a competition between electron and energy transfer to the nanoparticles. We analyzed the TA data for hematite nanoparticles using a four-state model of the dye-sensitized system, finding electron and energy transfer to occur on the same ultrafast timescale. The interfacial electron transfer rates for iron oxides are very close to those previously reported for DCF-sensitized titanium dioxide (for which dye-oxide energy transfer is energetically forbidden) even though the acceptor states are different. Comparison of the alignment of the excited states of the dye and the unoccupied states of these oxides showed that the dye injects into acceptor states of different symmetry (Ti t2gvs. Fe eg).

Electrochemical detection of dopamine at poly(solochrome cyanine)/Pd nanoparticles doped modified carbon paste electrode and simultaneous resolution in the presence of ascorbic acid and uric acid: a voltammetric method

A novel chemically modified electrode was constructed based on the electrodeposition of palladium nanoparticles (PdNps) on to a poly(solochrome cyanine) (polySCCy) film coated carbon paste electrode (CPE). Cyclic voltammetry (CV), differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy (EIS) were used to characterize the properties of the modified electrode. The polySCCy/PdNPs/CPE was used for the impressionable, individualized determination and simultaneous resolution of dopamine (DA), ascorbic acid (AA) and uric acid (UA) in 0.1 M acetate buffer solution (ABS) of pH 5.0 by employing CV and DPV. The polySCCy/PdNPs/CPE exhibited strong electrocatalytic activity towards the oxidation of a mixture solution of DA, AA and UA with obvious redox potentials. The overlapping voltammetric response of biomolecules were separated into three well resolved oxidation peaks for DA, AA and UA with lower oxidation potentials and significant increase in the anodic peak currents in the presence of polySCCy/PdNPs/CPE by using CV and DPV. The results showed good sensitivity, selectivity and high reproducibility of the electrosynthesized polymer nanoparticles inclusion electrode. The limit of detection, limit of quantification and correlation coefficient of DA at polySCCy/PdNPs/CPE were 1.87 μM, 6.24 μM and 0.99375, respectively. The developed chemical sensor is ideal for the analysis of DA in pharmaceutical formulations providing satisfactory results. The interfacial electron transfer rate of DA was studied by EIS.

Cross-linked luminescent films via electropolymerization of multifunctional precursors for highly efficient electroluminescence

To fabricate stable organic light-emitting diodes, a cross-linked network that firmly fixes the position of the chromophore is an ideal structure, because quenching caused by aggregation and/or phase separation effects can be effectively restrained in such robust films. Herein, in situ electropolymerization (EP) of fluorene-based precursors containing multi-electroactive carbazole units (multi-functionality) is utilized to construct highly cross-linked, smooth and compact luminescent films for highly efficient and stable electroluminescent devices. Cyclic voltammetry (CV), UV and atomic force microscopy (AFM) studies of the series of precursors with varied functionality (2, 4 and 8) reveal that increasing the functionality of the EP precursor can significantly increase the deposition rate, cross-linking degree and packing density as well as the luminous efficiency and stability of the fabricated EP films. The electrode kinetic parameters, determined by CV and chronocoulometry (CC), indicate that such improvements in EP film properties could be attributed to the enhanced interfacial electron-transfer rate. These observations could potentially offer a new approach to adjust the microstructures of EP films and provide a simple way of enhancing the performance of organic luminescent devices.

Design of Truxene-Based Organic Dyes for High-Efficiency Dye-Sensitized Solar Cells Employing Cobalt Redox Shuttle

Developing photosensitizers with high extinction coefficients, proper electronic structures, and steric properties is warranted for the dye-sensitized solar cells (DSCs) employing one-electron outer-sphere redox shuttles. DSCs incorporating Co(II/III)tris(1,10-phenanthroline)-based redox electrolyte and three synthesized organic dyes as photosensitizers (M14, M18, and M19) are described. The hexapropyltruxene group on the dyes retards the rate of interfacial back electron transfer from the conduction band of the nanocrystalline titanium dioxide film to the [Co(III)(phenanthroline)3]3+ ions, which enables attainment of high photovoltages approaching 0.9 V. The measurement of photocurrent transients shows that the mass transport limitation of the cobalt redox shuttle has been largely removed by using thin TiO2 films. DSCs sensitized with M14 in combination with the cobalt redox shuttle yield a DSC with an overall power conversion efficiency (PCE) of 7.2% under 100 mW cm–2 AM1.5 G illumination. The influence...

Layer-by-layer assembly of chemical reduced graphene and carbon nanotubes for sensitive electrochemical immunoassay

In this work, uniform and stable multi-walled carbon nanotubes (MWCT) and chemically reduced graphene (GR) composite electrode interface was fabricated by using layer-by-layer assembly method. The performances of these GR–MWCT assembled electrode interfaces were studied by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). It was demonstrated that the assembled composite film significantly improved the interfacial electron transfer rate compared with that of GR or MWCT modified electrode. Based on the GR–MWCT assembled interface, a sandwich-type electrochemical immunosensor was constructed using human IgG as a model target. In this assay, human IgG was fixed as the target antigen, the HRP-conjugated IgG as the probing antibody and hydroquinone as the electron mediator. The detection limit of the immunosensor was 0.2ngmL−1 (signal-to-noise ratio of 3). A good linear relationship between the current signals and the concentrations of Human IgG was achieved from 1ngmL−1 to 500ngmL−1. Moreover, this electrochemical immunosensor exhibited excellent selectivity, stability and reproducibility, and can be used to accurately detect IgG concentration in human serum samples. The results suggest that the electrochemical immunosensor based on GR–MWCT assembled composite will be promising in the point-of-care diagnostics application of clinical screening of multiple diseases.

Ligand-Controlled Rates of Photoinduced Electron Transfer in Hybrid CdSe Nanocrystal/Poly(viologen) Films

This paper describes a study of the rates of photoinduced electron transfer (PET) from CdSe quantum dots (QDs) to poly(viologen) within thin films, as a function of the length of the ligands passivating the QDs. Ultrafast (<10 ps), quantitative PET occurs from CdSe QDs coated with HS-(CH(2))(n)-COOH for n = 1, 2, 5, and 7 to viologen units. The observed decrease in the magnitude of the PET rate constant with n is weaker than that expected from the decay of the electron tunneling probability across extended all-trans mercaptocarboxylic acids but well-described by electron tunneling across a collapsed ligand shell. The PET rate constants for films with n = 10 and 15 are much slower than those expected based on the trend for n = 1-7; this deviation is ascribed to the formation of bundles of ligands on the surface of the QD that make the tunneling process prohibitively slow by limiting access of the viologen units to the surfaces of the QDs. This study highlights the importance of molecular-level morphology of donor and acceptor materials in determining the rate and yield of interfacial photoinduced electron transfer in thin films.

Electron transfer dynamics of single quantum dots on the (110) surface of a rutile TiO2 single crystal

The interfacial electron transfer (IET) dynamics of single CdSe core/multilayer shell (CdS2MLZnCdS1MLZnS1ML) quantum dots (QDs) on the (110) surface of a rutile TiO2 single crystal and TiO2 nanoparticles have been compared. The fluorescence decay rates of single QDs on TiO2 are faster than those on glass, an insulating substrate, due to IET from the QDs to TiO2. Whereas the average IET rates are similar for QDs on the single crystal and nanoparticles, the distribution of IET rates is much broader in the latter, indicating a broad distribution of QD adsorption sites on the TiO2 nanoparticles.

Investigation of liquid–liquid interfacial electron transfer kinetics using multicenter ferrocenyl complexes

The redox behavior of two novel multicenter redox molecules (triferrocenylmethane and triferrocenylmethanol) has been studied in a thin film of nitrobenzene (NB) imposed between a graphite electrode and an aqueous electrolyte. The well separated three sets of redox peaks indicate strong intramolecular electronic communications between the three ferrocene centers in each molecule. They were adapted as model compounds for the study of electron transfer kinetics across the liquid/liquid interface with varied overall driving force using only one-type redox couples in the organic and aqueous phase, respectively. It has been shown that in both cases the dependence of interfacial electron transfer rate on the increased overall driving force across the nitrobenzene/water interface is not monotonic.

Voltammetry and in situscanning tunnelling spectroscopy of osmium, iron, and ruthenium complexes of 2,2′:6′,2′′-terpyridine covalently linked to Au(111)-electrodes

We have studied self-assembled molecular monolayers (SAMs) of complexes between Os(II)/(III), Fe(II)/(III), and Ru(II)/(III) and a 2,2',6',2''-terpyridine (terpy) derivative linked to Au(111)-electrode surfaces via a 6-acetylthiohexyloxy substituent at the 4'-position of terpy. The complexes were prepared in situ by first linking the terpy ligand to the surface via the S-atom, followed by addition of suitable metal compounds. The metal-terpy SAMs were studied by cyclic voltammetry (CV), and in situ scanning tunnelling microscopy with full electrochemical potential control of substrate and tip (in situ STM). Sharp CV peaks were observed for the Os- and Fe complexes, with interfacial electrochemical electron transfer rate constants of 6-50 s(-1). Well-defined but significantly broader peaks (up to 300 mV) were observed for the Ru-complex. Addition of 2,2'-bipyridine (bipy) towards completion of the metal coordination spheres induced voltammetric sharpening. In situ STM images of single molecular scale strong structural features were observed for the osmium and iron complexes. As expected from the voltammetric patterns, the surface coverage was by far the highest for the Ru-complex which was therefore selected for scanning tunnelling spectroscopy. These correlations displayed a strong peak around the equilibrium potential with systematic shifts with increasing bias voltage, as expected for a sequential two-step in situ ET mechanism.

The voltammetric responses of nanometer-sized electrodes in weakly supported electrolyte: A theoretical study

The effect of the supporting electrolyte concentration on the interfacial profiles and voltammetric responses of nanometer-sized disk electrodes have been investigated theoretically by combining the Poisson–Nernst–Planck (PNP) theory and Butler–Volmer (BV) equation. The PNP-theory is used to treat the nonlinear couplings of electric field, concentration field and dielectric field at electrochemical interface without the electroneutrality assumption that has been long adopted in various voltammetric theories for macro/microelectrodes. The BV equation is modified by using the Frumkin correction to account for the effect of the diffuse double layer potential on interfacial electron-transfer (ET) rate and by including a distance-dependent ET probability in the expression of rate constant to describe the radial heterogeneity of the ET rate constant at nanometer-sized disk electrodes. The computed voltammetric responses for disk electrodes larger than 200 nm in radii in the absence of the excess of the supporting electrolyte using the present theoretical scheme show reasonable agreements with the predications of the conventional microelectrode voltammetric theory which uses the combined Nernst–Planck equation and electroneutrality equation to describe the mixed electromigration-diffusion mass transport without including the possible effects of the diffuse double layer (Amatore et al. [25]). For electrodes smaller than 200 nm, however, the voltammetric responses predicated by the present theory exhibit significant deviation from the microelectrode theory. It is shown that the deviations are mainly resulted from the overlap between the diffuse double layer and the concentration depletion layer (CDL) at nanoscale electrochemical interfaces in weakly supported media, which will result in the invalidation of the electroneutrality condition in CDL, and from the radial inhomogeneity of ET probability at nanometer-sized disk electrodes.

Fabrication and characterization of polymer insulated carbon nanotube modified electrochemical nanoprobes

Electrochemical nanoprobes were fabricated from polymer insulated multiwalled carbon nanotube modified tapping mode atomic force microscope probes. An electrochemically active length of carbon nanotube was exposed by laser ablation of the insulating polymer. Characterization of these probes is done by cyclic voltammetry of ferrocenemethanol in an aqueous solution and by finite element analysis. The fabricated nanoelectrodes were found to be stable and yielded an interfacial electron transfer rate constant (k(0)) of 1.073 +/- 0.36 cm s(-1) for ferrocenemethanol.

Photocatalytic Degradation of RhB by Fluorinated Bi2WO6 and Distributions of the Intermediate Products

Fluorinated Bi2WOs catalyst was synthesized by a simple hydrothermal process. The effects of fluorine doping on crystal structure, optical property, photoinduced hydrophilicity, surface acidity, and photocatalytic activity of the as-prepared sample were observed in detail. Fluorinated Bi2WOs presented the enhanced photoactivity for the RhB degradation under the simulative sunlight (lamda > 290 nm), which could be a synergetic effect of the surface fluorination and the doping of crystal lattice. To get a better handle on the mechanistic details of this photocatalytic system, the photodegradation process of RhB was examined. In the fluorinated Bi2WO6 system, five intermediates, namely, N,N-diethyl-N'-ethylrhodamine, N,N-diethylrohodamine, N-ethyl-N'-ethylrhodamine, N-ethylrhodamine, and rhodamine were thus identified, whereas the first three intermediates could only be identified in the case of the Bi2WO6 system. This result indicated that more RhB molecules were degraded via the deethylation process in the fluorinated Bi2WO6 system. It was proposed that the (F-)-containing function on the catalyst surface could serve as an electron-trapping site and enhance interfacial electron-transfer rates by tightly holding trapped electrons. On the basis of the experimental results, a photocatalytic mechanism was discussed in detail.

PH effects on the rate of heterogeneous electron transfer across a fluorine doped tin oxide/monolayer interface

Spontaneously adsorbed monolayers of [Ru(bpy) 2PIC](PF 6) 2 have been formed on fluorine doped tin oxide macro- and microelectrodes, bpy is 2,2′-bipyridyl and PIC is 2-(4-carboxyphenyl)imidazo[4,5-f][1,10]phenanthroline. These monolayers exhibit well-defined, almost ideal electrochemical responses over a wide range of voltammetric scan rates. The formal potential of the Ru 2+/3+ process shifts by less than 30 mV upon immobilization suggesting that the monolayers are well solvated. Significantly, chronoamperometry, conducted on a microsecond timescale, reveals that protonation of the PIC bridging ligand modulates the rate of interfacial electron transfer. The heterogeneous electron transfer rate constant, measured at an overpotential of +50 mV, decreases from 7.0 ± 1.1 × 10 5 to 0.7 ± 0.1 × 10 5 s −1 as the pH of the supporting electrolyte is increased from 1.7 to 9.3. These observations are consistent with the redox mechanism occurring via a heterogeneous electron transfer process, the rate of PIC which depends on the energy difference between the metal dπ-orbitals and the lowest unoccupied molecular orbital (LUMO) of the bridge. Protonation of the bridging ligand decreases this energy gap, resulting in an overall increase in the rate of the redox reaction. Significantly, despite the close proximity of the luminophores to a conducting surface, the monolayers remain luminescent suggesting that the electronically excited state is only weakly coupled to the electrode surface. This is consistent with bipyridyl as the site of the excited state in the metal complex.

Prediction of Standard Interfacial Electron-Transfer Rate Constants for the Ru(NH3)63+/2+ Couple through ω-Hydroxyalkanethiol Self-Assembled Monolayers on Gold Electrodes

Two relatively simple approaches are developed and used to calculate (predict) the standard interfacial electron-transfer (ET) rate constants (k degrees) of the Ru(NH3)6(3+/2+) couple dissolved in aqueous electrolyte solutions in contact with Au electrodes coated with self-assembled monolayers (SAMs) composed of HS(CH2)nOH as functions of both n and temperature. These approaches are suggested by the conclusion reached by Smalley et al. (J. Electroanal. Chem. 2006, 589, 1-6) that the interfacial ET rate of a solution-dissolved redox couple in contact with a SAM is, within 1 order of magnitude, the same as the (normalized) interfacial ET rate of a similar attached (as a constituent of a similar SAM) couple. The calculations, therefore, employ the measured electronic coupling of the attached (to Au electrodes through alkanethiolate bridges) -PyRu(NH3)5(3+/2+) couple. The two approaches also both include dynamic solvent effects on the ET kinetics and the influence of electronic coupling on the activation barrier for the ET reaction. At T=298 K and n=3, 11, and 14, the predicted rate constants are in very good agreement with the existing measurements of k degrees. However, for n<3 at 298 K, the predicted rate constants are extremely large (i.e., >4.5 cm s(-1)) and do not tend toward a limiting value. Additionally, even if the electronic coupling between a Au electrode and a Ru(NH3)6(3+/2+) moiety located at the surface of the SAM is >0.1 eV, the calculated standard rate constant is not directly proportional to the inverse of the longitudinal dielectric time of the solvent. A primary reason for both the absence of a limiting value for the predicted k degrees's at 298 K and the attenuated influence of dynamic solvent effects is the activation energy barrier suppression caused by large values of the electronic coupling.

Direct Electrochemistry of Tetraheme Cytochrome c554 from Nitrosomonas europaea: Redox Cooperativity and Gating

The tetraheme cytochrome c554 is the redox partner of hydroxylamine oxidoreductase (HAO) of Nitrosomonas europaea, responsible for accepting the four electrons generated in HAO turnover. Here, we have used protein film voltammetry (PFV) to provide a new, highly resolved picture of the redox properties of cyt c554. The four heme groups of the cytochrome are found to have potentials of +32, +50, −183, and −283 mV (versus hydrogen). While the two lowest potential hemes (III and IV) behave as simple napp = 1 redox cofactors in all experiments, the high potential hemes appear to act as a pair that engages in cooperative electron transfer reactions (napp > 1.0). By studying the cytochrome on variable electrode surfaces, gating of the reduction of the two high potential hemes can also be observed. When the interfacial electron transfer rate for cyt c554 is on the time scale of HAO catalysis (on a mercaptoundecanoic acid-modified gold electrode), gating is found, indicating that oxidation of the two-electron reduced form of cyt c554 is prevented, while reduction of the totally oxidized form is facile.

Interfacial bridge-mediated electron transfer: mechanistic analysis based on electrochemical kinetics and theoretical modelling

Understanding the physical and chemical factors that control the kinetics of interfacial electron-transfer (ET) reactions is important for a large number of technological applications. The present article describes electrochemical kinetic studies of these factors, in which standard interfacial ET rate constants (k(0)(l)) have been measured for ET between substrate Au electrodes and various redox couples attached to the electrode surfaces by variable lengths (l) of oligomethylene (OM), oligophenylenevinylene (OPV) and oligophenyleneethynylene (OPE) bridges, which were constituents of mixed self-assembled monolayers (SAMs). The k(0)(l) measurements employed the indirect laser-induced temperature jump (ILIT) technique, which permits the measurement of interfacial ET rates that are orders of magnitude faster than those measurable by conventional techniques using the macroelectrodes that are the most convenient substrates for the mixed SAMs. The robustness of the measured rate constants (k(0)(l)), together with the Arrhenius activation energies (E(a)(l)) and preexponential factors (A(l)), is demonstrated by their invariance with respect to several experimental system parameters (including the chemical nature and length of the diluent component of the mixed SAM). Analysis of the kinetic results demonstrates that all of the observed interfacial ET processes proceed through a common type of transition state (predominantly associated with solvent reorganization around the redox moiety) and that the actual ET step involves direct electronic tunnelling between the Au electrode and the redox moiety. However, for the full range of l investigated, a global exponential decay of A(l) is not found for any of the three types of bridges. Possible reasons for this behavior, including the role of rate determining steps associated with adiabatic mechanisms within or beyond the transition state theory framework, are discussed, and comparisons with related conductance measurements are presented.

Comparison of the Self-Exchange and Interfacial Charge-Transfer Rate Constants for Methyl- versus tert-Butyl-Substituted Os(III) Polypyridyl Complexes

Differences in the self-exchange and interfacial electron-transfer rate constants have been evaluated for a relatively unhindered Os(III/II) redox system, osmium(III/II) tris(4,4'-di-methyl-2,2'-bipyridyl), [Os(Me2bpy)3]3+/2+, relative to those of a relatively hindered system, osmium(III/II) tris(4,4'-di-tert-butyl-2,2'-bipyridyl), [Os(t-Bu2bpy)3]3+/2+. In contrast to the predicted increase in rate constant by a factor of 2-3 due to the difference in reorganization energy of the two complexes, introduction of the tert-butyl functionality decreased the self-exchange rate constant, as measured by NMR line-broadening techniques, by a factor of approximately 50 as compared to that of the analogous methyl-substituted osmium complex. Steady-state current density versus potential measurements, in conjunction with differential capacitance versus potential measurements, were used to compare the interfacial electron-transfer rate constants at n-type ZnO electrodes of [Os(t-Bu2bpy)3]3+/2+ and [Os(Me2bpy)3]3+/2+. The interfacial electron-transfer rate constant for the reduction of [Os(t-Bu2bpy)3]3+ was 100 times smaller than that for [Os(Me2bpy)3]3+. The results indicate that the tert-butyl group can act as a spacer on an outer-sphere redox couple and significantly decrease the electronic coupling of the electron-transfer reaction in both self-exchange and interfacial electron-transfer processes.

A Comparison between Interfacial Electron-Transfer Rate Constants at Metallic and Graphite Electrodes

The Fermi golden rule formalism has been used to derive the rate constant for interfacial electron transfer from a semimetallic electrode, such as highly ordered pyrolytic graphite (HOPG), to a redox couple in solution. A simple expression is presented that semiquantitatively relates the electron-transfer rate constant at a semimetallic electrode to that at a metallic electrode. The approach allows for the estimation of the value of the rate constant for interfacial charge transfer to nonadsorbing outer-sphere redox species at semimetallic electrodes. Rate constants for interfacial electron transfer for a variety of one-electron redox couples at semimetallic electrodes have been calculated relative to the rate constant of the ferrocenium/ferrocene redox couple at a gold electrode. Good agreement is found, in general, between the calculated and observed rate constants.

A simple comparison of interfacial electron-transfer rates for surface-attached and bulk solution-dissolved redox moieties

A relationship is developed that facilitates a comparison between the standard electron-transfer rate constant (k0) of a redox couple covalently attached (as a constituent of a self-assembled monolayer (SAM)) to an electrode and the k0 of a similar redox couple dissolved in an electrolyte solution in contact with a similar (but not electroactive) electrode/SAM assembly. Such comparisons performed for ferrocene and ruthenium redox couples demonstrate that the (normalized) rate of electron-transfer (and, also, the electronic coupling) for a solution-dissolved couple through an alkanethiol SAM/aqueous electrolyte solution interface is, within one order of magnitude, equal to that for a covalently attached redox species. A related comparison involving the Marcus theory limiting rate constants for solution-dissolved couples and the Arrhenius preexponential factors for attached couples supplies additional evidence for this surprising result.