Electrochemical Electron Transfer Rate Research Articles

Alternative voltammetry on self-assembled monolayers: An original approach to estimate the electrochemical electron-transfer rate constants when electroactive adsorbed species interact

This work presents original methodologies to estimate the electrochemical electron-transfer rate constants (k0) of immobilized electroactive species when strong intermolecular interactions occur in a redox-responsive material, for which the redox reaction is described by Butler–Volmer theory. Based on Laviron's equations and the initial Creager and Wooster’s work, these methodologies encompass the general lateral interaction model and have been experimentally tested to estimate k0 in order to provide evidence of the relevance and reliability of our approach.

Long-range interfacial electron transfer and electrocatalysis of molecular scale Prussian Blue nanoparticles linked to Au(111)-electrode surfaces by different chemical contacting groups

We have explored interfacial electrochemical electron transfer (ET) and electrocatalysis of 5–6 nm Prussian Blue nanoparticles (PBNPs) immobilized on Au(111)-electrode surfaces via molecular wiring with variable-length, and differently functionalized thiol-based self-assembled molecular monolayers (SAMs). The SAMs contain positively (−NH3 +) or negatively charged (–COO–) terminal group, as well an electrostatically neutral hydrophobic terminal group (–CH3). The surface microscopic structures of the immobilized PBNPs were characterized by high-resolution atomic force microscopy (AFM) directly in aqueous electrolyte solution under the same conditions as for electrochemical measurements. The PBNPs displayed fast and reversible interfacial ET on all the surfaces, notably in multi-ET steps as reflected in narrow voltammetric peaks. The ET kinetics can be controlled by adjusting the length of the SAM forming linker molecules. The interfacial ET rate constants were found to depend exponentially on the ET distance for distances longer than a few methylene groups in the chain, with decay factors (β) of 0.9, 1.1, and 1.3 per CH2, for SAMs terminated by −NH3 +,–COO–, and–CH3, respectively. This feature suggests, first that the interfacial ET processes follow a tunneling mechanism, resembling that of metalloproteins in a similar assembly. Secondly, the electronic contact of the SAM terminal groups that anchor non-covalently the PBNP are crucial as reported for other types of molecular junctions. Highly efficient PBNP electrocatalysis of H2O2 reduction was also observed for the three linker groups, and the electrocatalytic mechanisms analyzed.

The Single-Molecule Conductance and Electrochemical Electron-Transfer Rate Are Related by a Power Law

This study examines quantitative correlations between molecular conductances and standard electrochemical rate constants for alkanes and peptide nucleic acid (PNA) oligomers as a function of the length, structure, and charge transport mechanism. The experimental data show a power-law relationship between conductances and charge transfer rates within a given class of molecules with the same bridge chemistry, and a lack of correlation when a more diverse group of molecules is compared, in contrast with some theoretical predictions. Surprisingly, the PNA duplexes exhibit the lowest charge-transfer rates and the highest molecular conductances. The nonlinear rate-conductance relationships for structures with the same bridging chemistries are attributed to differences in the charge-mediation characteristics of the molecular bridge, energy barrier shifts and electronic dephasing, in the two different experimental settings.

Ordered Assembly and Controlled Electron Transfer of the Blue Copper Protein Azurin at Gold (111) Single-Crystal Substrates

We have shown that Pseudomonas aeruginosa azurin can be immobilized on alkanethiol monolayers self-assembled on Au(111). Immobilization is achieved through hydrophobic interactions between the hydrophobic area around the copper atom in azurin and methyl heads of alkanethiol to form submonolayers or monolayers. In this orientation mode azurin molecules on Au(111) are oriented with the redox center (copper atom) facing the electrode surface. This is opposite to the orientation of azurin on bare gold which is via a surface disulfide group such as recently reported. Scanning tunneling microscopy (STM) with molecular resolution reveals that both well-ordered alkanethiol and protein adlayers are present. Adsorbed azurin molecules exhibit high stability and retain electron transfer (ET) function. Long-range interfacial ET between azurin and Au(111) across variable-length alkanethiol bridges was systematically investigated by different electrochemical techniques. Distance-dependent ET can be controlled by adjusting the length of the alkanethiol chain. The electrochemical ET rate constant is almost independent of the chain length up to ca. 9 methylene units but follows exponential distance decay with a decay factor (β) of 1.03 ± 0.02 per CH2 unit at longer chain lengths. Overvoltage-dependent ET was also examined. The results provide a strategy to ordered molecular assemblies, and controlled orientation and ET of azurin at atomically planar metallic surfaces. This approach can in principle be extended to other redox metalloproteins.

Interaction of horse heart cytochrome c with lipid bilayer membranes: effects on redox potentials.

Cyclic voltammetry has been used to study the effects of interactions between horse cytochrome c and solid-supported planar lipid membranes, comprised of either egg phosphatidylcholine (PC) or PC plus 20 mol.% cardiolipin (CL), on the redox potential and the electrochemical electron transfer rate between the protein and a semiconductor electrode. Experiments were performed over a wide range of cytochrome c concentrations (0-440 microM) at low (20 mM) and medium (160 mM) ionic strengths. Three types of electrochemical behavior were observed, which varied as a function of the experimental conditions. At very low cytochrome c concentration (approximately 0.1 microM), and under conditions where electrostatic forces dominated the protein-lipid membrane interaction (i.e., low ionic strength with membranes containing CL), a redox potential (approximately 265 mV) and an electrochemical electron transfer rate constant (0.09 s[-1])were obtained which compare well with those measured in other laboratories using a variety of different chemical modifications of the working electrode. Two other electrochemical signals (not reported with chemically modified electrodes) were also observed to occur at higher cytochrome c concentrations with this membrane system, as well as with two other systems (membranes containing CL under medium ionic strength conditions, and PC only at low ionic strength). These involved positive shifts of the cytochrome c redox potential (by 40 and 60 mV) and large decreases in the electron transfer rate (to 0.03 and 0.003 s[-1]). The observations can be rationalized in terms of a structural model of the cytochrome c-membrane interaction, in which association involves both electrostatic and hydrophobic forces and results in varying degrees of insertion of the protein into the hydrophobic interior of the membrane.

Variation of the electrochemical transfer coefficient with potential

The electrochemical electron-transfer rate constant has been determined as a function of the electrode potential for a series of simple electron-transfer processes to organic molecules in media containing acetonitrile or dimethylformamide and a quaternary ammonium salt as supporting electrolyte and using mercury as the electrode material. The reactions and the experimental conditions were selected so as to deal with outer-sphere processes and to minimize the magnitude of double-layer corrections. Convolution potential-sweep voltammetry and the impedance method were used for obtaining the kinetic data. Under these conditions, the electrochemical transfer coefficient was observed, in all cases, to vary, beyond experimental error, with the electrode potential. The magnitude of the variation is of the same order of magnitude as that predicted by the Marcus theory of outer-sphere electron transfer. A more complex reaction, the reduction of benzaldehyde in ethanol, involving dimerization steps following the initial electron transfer was also investigated. A definite variation of the transfer coefficient was again observed. This behaviour, observed for various solvents and functional groups, appears as a general phenomenon in the reduction of organic molecules in the case where charge transfer is fast and mainly governed by solvent reorganization.

Temperature dependent current-reversal chronopotentiometric reduction of nickel(IV), molybdenum(V) and tungsten(V) dithiocarbamates

The electrochemical electron-transfer rate constants for the reduction of Ni(IV), Mo(V) and W(V) dithiocarbamates were measured at a platinum electrode in acetone with the current-reversal chronopotentiometric method in the temperature range −20 to +2°. From these measurements the activation energies were determined, ΔG≠ at 293 K, being 26, 25 and 24 kJ mol−1 respectively for these compounds. The electronic device used is also described and consists of a galvanostat, a comparator and measuring circuit and a digital programmer.

Temperature dependent current-reversal chronopotentiometric reduction of nickel(IV), molybdenum(V) and tungsten(V) dithiocarbamates

The electrochemical electron-transfer rate constants for the reduction of Ni(IV), Mo(V) and W(V) dithiocarbamates were measured at a platinum electrode in acetone with the current-reversal chronopotentiometric method in the temperature range −20 to +2°. From these measurements the activation energies were determined, Δ G ≠ at 293 K, being 26, 25 and 24 kJ mol −1 respectively for these compounds. The electronic device used is also described and consists of a galvanostat, a comparator and measuring circuit and a digital programmer.

A correlation between spin state and electrochemical electron-transfer rate for tris(N,N-disubstituted dithiocarbamato)iron(III) complexes

A correlation between spin state and electrochemical electron-transfer rate for tris(N,N-disubstituted dithiocarbamato)iron(III) complexes

A correlation between spin state and electrochemical electron-transfer rate for tris(N,N-disubtituted dithiocarbamato) iron(III) complexes

The electrochemical electron-transfer rate constants for the redox systems Fe(IV)L 3 +/Fe(III)L 3 (L=N,N-disubstituted dithicarbamate ion) and Fe(III)L 3/Fe(II)L 3 − with a variety of substituents were measured at a platinum electrode in acetonitrile with the galvanostatic double-pulse method. It is known that each of the Fe(III) complexes exists both in a highspin state 6A 1 and a low-spin state 2T 2 in equilibirium of which position is widely changed by a subtle change in substituent. The standard rate constants for Fe(IV)L 3 +/Fe(III)L 3 were larger or smaller than those for Fe(III)L 3/Fe(II)L 3 − according as the Fe(III)L 3 complexes are predominantly low- or high-spin complexes. Since the Fe(IV) and Fe(II) complexes are low-and high-spin complexes respectively, these findings suggest that electrochemical electron-transfer reactions accompanied by a spin-state change are slower than those without it. Such spin-state effect on electrode reactions has rarely been discussed so far.

Electron-transfer Kinetics of Transition-metal Complexes in Lower Oxidation States. Part II. The Electrochemical Electron-transfer Rate of the Cr(bipy)3+/Cr(bipy)3 System as Measured by the Galvanostatic Double-pulse Method

Abstract The kinetic parameters of the electron-transfer reaction, Cr(bipy)3++e\ightleftharpoonsCr(bipy)3, were measured by the galvanostatic double-pulse method at a dropping mercury electrode in N,N-dimethylformamide. The apparent and the corrected standard rate constants were 1.0±0.2 and 0.3 cm s−1 respectively, and the transfer coefficient was 0.6±0.1 at 25 °C. These rate constants were discussed in comparison with the corresponding homogeneous rate constant on the basis of the theoretical equation of Marcus. They were quite inconsistent with the value, 103 cm s−1, predicted from the corresponding homogeneous rate constant. The polarograms of Cr(bipy)3+ and Cr(bipy)3 exhibited three waves with half-wave potentials of −0.21, −0.65 and −1.19 V vs. a cationic glass electrode. The diffusion coefficients of these complexes, as determined polarographically, were identical: 3.2×10−6 cm2 s−1 at 25 °C.