The role of the potential distribution at the electrode interface in determining the electron transfer rate constant

Simultaneous electrochemical impedance (EI) and electroreflectance (ER) measurements were performed on ‘bare’ gold, N-acetyl-l-cysteine(NAC) modified gold, and cytochrome c (cyt c) adsorbed on a NAC modified gold electrode. The electrical impedance of the electrode interface was modeled by a series connection of a constant phase element (CPE), a capacitor, and a resistor. The analysis of the combined EI and ER data yielded the heterogeneous electron transfer (ET) rate constant at each frequency of the modulating potential. This is in contrast to previous techniques which used the frequency dependence of the EI or ER response to obtain a value of the rate constant. Assuming that the Faradaic potential was equal to the potential at the nodes of the three element impedance model yielded frequency dependent rate constants. A small modification of the three element impedance model was needed to obtain a frequency independent rate constant of 850±80 s^?1. This suggests that the distribution of the potential at the electrode interface (reflected in the choice of the electrical impedance model) is an important factor in the determination of the ET rate constant.     

Direct electron transfer between the heme of cellobiose dehydrogenase and thiol modified gold electrodes

Cellobiose dehydrogenase (CDH) is an extracellular fungal enzyme with two domains, one containing flavin adenine dinucleotide (FAD) and one containing heme. The electrochemistry of CDH, as well as its cleaved FAD- and heme-subunits, was studied using a membrane electrode, i.e. the enzyme was trapped under a permselective membrane on a cystamine or 3-mercaptopropionic acid modified gold electrode. Direct un-mediated electron transfer (ET) between the heme of CDH and thiol modified gold electrodes was demonstrated using cyclic voltammetry. At low sweep rate (10 mV s^?1) and low pH (pH 4.3) up-hill ET from heme to FAD in CDH was observed. The formal potential of the heme in CDH and in the cleaved heme-subunit was found to be the same and equal to ?41 mV versus Ag ∣ AgCl at pH 5.1. The dependence of the formal potential on the pH (in the pH range 3.6–6.0) indicates the presence of one redox-linked ionisable functional group. Entropy and enthalpy changes were determined in variable temperature experiments as follows, ΔS°′=?194±14 J mol^?1 K^?1 and ΔH°′=?74±6 kJ mol^?1. The electrocatalytic behaviour of the CDH electrodes was demonstrated by addition of the enzyme substrate, cellobiose. The catalytic current was shown to decrease upon increased pH, in accordance with previous kinetic data in solution. The model of electron transport from the substrate (cellobiose) to FAD, and then through the heme domain to the electrode was confirmed in the experiments.     

Direct electron transfer in the system gold electrode–recombinant horseradish peroxidases

The kinetics of the bioelectrocatalytic reduction of hydrogen peroxide has been studied at gold electrodes modified with different forms of horseradish peroxidase (HRP). Native HRP, wild type recombinant HRP (rec-HRP) and its two mutant forms containing a six-histidine tag at the C- or N-terminus, C_Hisrec-HRP and N_Hisrec-HRP, respectively, have been used for an adsorptive modification of the gold electrodes. The histidine sequences, i.e. histidine tags, were introduced into the peroxidase structure by genetic engineering of non-glycosylated rec-HRP using an Escherichia coli expression system. Experiments with a gold rotating disc electrode demonstrated that electrodes with the adsorbed rec-HRP forms exhibited high and stable current response to H₂O₂ due to its bioelectrocatalytic reduction based on direct (mediatorless) ET between gold and the active site of HRP. The heterogeneous ET rate constants were evaluated to be in the order of 20 or 33 s^?1 between rec-HRP or its histidine mutants and gold, respectively, in 0.01 M phosphate buffer (pH 7.4) containing 0.15 M NaCl. The increase in the heterogeneous ET rate found for C_Hisrec-HRP and N_Hisrec-HRP is probably due to the interaction of the histidine tag with the electrode surface. The kinetic data demonstrate that new possibilities for enhancing the catalytic activity of the enzyme at the electrode ∣ solution interface can be achieved by genetic engineering design of the enzyme molecules.     

Efficient mediatorless superoxide sensors using cytochrome c-modified electrodes: surface nano-organization for selectivity and controlled peroxidase activity

Amperometric superoxide anion sensor electrodes were prepared by immobilizing cytochrome c (Cyt c) on COOH-terminated Au–alkanethiolate monolayers. Monolayers and mixed-monolayers of 3-mercaptopropionic acid (MPA) with the coadsorbate, 3-mercaptopropanol (MP), were constructed on the surface of Au electrodes. The electrochemical characteristics and the superoxide anion sensor activities of cytochrome c immobilized on the monolayers and mixed-monolayers of MPA were found to depend largely on the structure of the underlying alkanethiolate monolayer. While cytochrome c on a MPA monolayer, Au/MPA/Cyt c, showed a reversible redox wave at 0.08 V (vs. Ag ∣ AgCl ∣ NaCl (sat.)), cytochrome c on the mixed-monolayer of MPA and MP, Au/MPA+MP/Cyt c, showed a wave at ?0.01 V. Cytochrome c immobilized on the Au–alkanethiolate layer was supposed to acquire different conformations at each of the Au/MPA/Cyt c and Au/MPA+MP/Cyt c electrodes, leading to a difference in the thermodynamic potential of cytochrome c. The apparent heterogeneous electron-transfer rate constant, k_het, is determined by fast-scan cyclic voltammetry to be (2.8±0.4)×10³ s^?1 for the Au/MPA+MP/Cyt c electrode. Both Au/MPA/Cyt c and Au/MPA+MP/Cyt c electrodes show anodic currents to O₂^? anion generated by enzymatic reaction with a response time of ～15 s, and the magnitude of the current response is higher at Au/MPA+MP/Cyt c relative to that at Au/MPA/Cyt c. While H₂O₂ did not give any current response at the Au/MPA electrode at the applied potential of 0.15 V, cytochrome c immobilized on the Au–alkanethiolate layers was found to reduce H₂O₂; the cathodic current recorded for the reduction of H₂O₂ is very much higher at Au/MPA/Cyt c than that at Au/MPA+MP/Cyt c at all the applied potentials. The O₂^? anion sensor activity of the two electrodes was compared with that of Au/HS(CH₂)₁₀COOH/Cyt c. Among the cytochrome c-modified electrodes tested, Au/MPA+MP/Cyt c showed a high anodic current for O₂^? anion and low interferences, 17 and 7%, due to the electrochemical interferents, H₂O₂ and uric acid, respectively.     

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.     

Electron transfer kinetics and morphology of cytochrome c at the biomimetic phospholipid layers

Electrochemistry of cytochrome c (cyt c) at biomimetic phospholipid layers was studied in a phosphate buffer solution, which were formed with dilauroyl phosphatidic acid (DLPA, C12:0), dipalmitoyl phosphatidic acid (DPPA, C16:0), distearoyl phosphatidic acid (DSPA, C18:0), and palmitoyl–oleoyl phosphatidic acid (POPA, C16:0–18:1). The lipid-layers formed firstly at the air/water interface were immediately transferred onto the electrode surface using the Langmuir–Blodgett (LB) technique. The electrochemical properties of cyt c at the lipid covered electrodes depended on the orientation, number of layers of phospholipids, tail (or head) group down, and vice versa. Atomic force microscopy (AFM) images of cyt c adsorbed on the POPA monolayer (showing the head group diameter of POPA to be ca. 0.7 nm) formed on highly oriented pyrolytic graphite (HOPG) displayed uniform surface morphology of lipid layer and clumps of aggregated cyt c molecules with a minimum size corresponding to four cyt c molecules. The heterogeneous electron transfer rate constants, k⁰ values, of cyt c were determined to be 1.02 × 10⁻³, 0.98 × 10⁻³, and 0.67 × 10⁻³ cm/s for the lipid monolayer in the tail down orientation (X-type) of POPA, DLPA, and DPPA, and 0.67 × 10⁻³ and 0.50 × 10⁻³ cm/s for the head down orientation (Z-type) of POPA and DLPA monolayers, respectively.     

Kinetics of the ferric/ferrous electrode reaction on Nafion®-coated electrodes

Cyclic and convolution voltammetry is used to study the kinetics of the ferric/ferrous electrode reaction on Nafion® coated Au and Pt electrodes in a perchloric acid solution. The reaction is characterized by the standard rate constant k_s⁰≈10^?6 cm s^?1 of the electron transfer across the metal ∣ Nafion® interface and by the diffusion coefficient of the ferric ion in the Nafion film D^(m)=(6.7±1.4)×10^?8 cm² s^?1. The Nafion® film apparently prevents the trace anions present in the perchloric acid solution (e.g. chlorides and sulfates) reaching the electrode surface and, thereby, influencing the ferric/ferrous electron transfer reaction via the inner-sphere catalysis. In addition, the polymer phase introduces a steric factor which causes the rate of the electron transfer reaction to decrease compared to the uncoated electrode, and proportionally the ion diffusion flux toward the metal surface, so that the values of the ratio k_s⁰/D^1/2 for the coated and uncoated electrodes are comparable. It is proposed that the considerably higher rate of the electron transfer on the Nafion® coated electrode in the sulfuric acid solution is due to the catalytic effect of sulfate or hydrogensulfate anions, which are transported through the Nafion® film as the sulfate complexes of the ferric cation, i.e. FeSO₄⁺ and FeHSO₄²⁺. The catalytic effect of oxides on the Nafion® coated electrodes is demonstrated.     

Electrochemical behaviour of 2-(4′-hydroxybenzeneazo)benzoic-acid at pyrolytic graphite electrodes

The electrochemical reduction of 2-(4′-hydroxybenzeneazo)benzoic acid (I) has been studied at pyrolytic graphite electrodes in the pH range 2.0–10.4. The cyclic voltammetric behaviour clearly indicated an ECE mechanism in acidic medium in which the two-electron two-proton reduction of I gives the hydrazo derivative. The acid catalysed disproportionation of the hydrazo intermediate was also studied in the pH range 2.0–6.0 and the value of k′/[H⁺] was found to be 1.4 × 10^?2 1 mol^?1 s^?1 The products of the reduction have been isolated and characterized using IR, melting point, mass and related techniques.     

The electrochemistry and thin-layer luminescence spectroelectrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode

We report on the first direct electrochemistry and fluorescence spectroelectrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode. The value of n determined in spectropotentiostatic experiments at 1.87×10^?6 mol l^?1 of rhodamine 6G in 0.20 mol l^?1 KCl solution is 1.15, and the experimental value obtained for E⁰′ is ?0.787 V versus Ag ∣ AgCl ∣ KCl_sat, which agrees very well with the value (E⁰′=?0.791 V) obtained using cyclic voltammetry at a modified gold electrode. The values of the diffusion coefficients D_O and D_R for the oxidized and reduced forms of rhodamine 6G calculated from results of potential step and in situ fluorescence measurement experiments are 4.0×10^?6 cm² s^?1 and 4.2×10^?6 cm² s^?1, respectively. Cyclic voltammograms of rhodamine 6G show that the peak current I_p is proportional to the square root of the potential scan rate v^1/2, the ratio of the reduction to the oxidation peak height is about unity, and the separation of both reduction and reoxidation peak potentials ΔE_P is essentially constant at 135 mV at low scan rates. These results indicate that electrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode is a quasi-reversible one-electron electrode process.     

Characterization and electrocatalytic properties of cobalt hexacyanoferrate films immobilized on Au-colloid modified gold electrodes

An electroactive cobalt hexacyanoferrate (CoHCF) film was electrodeposited from a solution containing Co²⁺ and Fe(CN)₆^3? ions on the bare gold or the Au-colloid modified electrode. The cation (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) and the anion (F^?, Cl^? and Br^?) effects on the redox peak of the CoHCF film were investigated in detail. On the other hand, the electrocatalytic oxidations of thiosulfate at the CoHCF/gold and CoHCF/Au-colloid/gold electrodes were compared. At the CoHCF/Au-colloid/gold electrode, we obtained a response current larger by a factor of 2 and a three times lower detection limit than those at a CoHCF/gold electrode. The linear ranges were 1.0 × 10^?4 to 2.8 × 10^?3 M for the CoHCF/gold electrode and 7.5 × 10^?5 to 4.8 × 10^?3 M for the CoHCF/Au-colloid/gold electrode. These results showed that the immobilized CoHCF at the Au-colloid modified electrode exhibited a higher catalytic activity and a wider linear range toward thiosulfate. Additionally, the effects of the applied potential and the solution pH were studied.     

Voltammetric and XPS investigations of nickel hydroxide electrochemically dispersed on gold surface electrodes

A chemically modified electrode composed of mixed hydroxide and oxyhydroxide nickel film (6–8 nmol cm^?2) on the gold substrate (Au ∣ Ni) was characterized by cyclic voltammetry and XPS techniques. The gold substrate electrodes were firstly electrochemically conditioned in 0.2 M NaOH by cycling the potential between ?0.25 and 0.6 V versus SCE, then modified by cathodic electrodeposition of nickel hydroxide films. These nickel films were obtained either by voltage cycling (50 mV s^?1) between 0.0 and ?0.5 V (SCE) or at constant potential of ?0.3 or ?0.5 V using non-deaerated 50 mM Ni(NO₃)₂ solutions. X-ray photoelectron spectroscopy (XPS) characterisation and voltammetric behaviour of Au ∣ Ni electrodes in alkaline solutions are described. Continuous electrochemical cycling of the Au ∣ Ni electrodes induces significant changes of the nickel films in terms of crystallographic structures and chemical composition. Combination of XPS and electrochemical methodologies have demonstrated the ability to follow the morphological and chemical changes in alkaline solutions upon cycling potentials. Angular-dependent XPS measurements have demonstrated that electrochemical treatment induces the formation of a uniform film layer with the following chemical distribution: Au ∣ Ni(OH)₂ ∣ NiOOH. The electrocatalytic activity of the Au ∣ Ni electrodes is investigated in alkaline medium using glucose as a model compound. The favourable combination of active species such as gold and nickel leads to a sensing electrode with strong catalytic activity over a wide range of applied potentials.     

The high speed channel electrode applied to heterogeneous kinetics: the oxidation of 1,4-phenylenediamines and related species in acetonitrile

The application of the high-speed microband channel electrode to the study of the heterogeneous electron transfer kinetics of the oxidation of some N-substituted phenylenediamines is described. Experiments to investigate the standard electrochemical rate constant, k₀, of the oxidation of 1,4-phenylenediamine (PPD), N,N-dimethyl-1,4-phenylenediamine (DMPD), and N,N-diethyl-1,4-phenylene-diamine (DEPD) in acetonitrile solutions containing 0.10 M tetrabutylammonium perchlorate (TBAP) are reported for 2.5 and 5 μm platinum microband electrodes using a range of centre-line velocities from 12 to 25 m s^?1. The measured values of k₀ for PPD, DMPD, and DEPD are 0.84±0.16, 3.15±0.30 and 1.64±0.25 cm s^?1, respectively. The respective formal oxidation potentials are also found to be 0.287±0.002, 0.245±0.001, and 0.208±0.003 V (all measured vs. Ag). Experiments are also presented using “fast scan” cyclic voltammetry to obtain measurements of the heterogeneous rate constants for PPD, DMPD and DEPD to compare between the steady-state channel electrode and the ‘established’ transient methodologies. Scan rates in the range 10²–10⁴ V s^?1 were used to measure peak separations with the resulting k₀ values of 0.51±0.05, 1.89±0.10, and 1.28±0.20 cm s^?1, respectively. The use of steady-state voltammetry obviates the need for capacitative corrections, perhaps suggesting a greater reliability in the resulting data.     

Improvement of alternariol monomethyl ether detection at gold electrodes modified with a dodecanethiol self-assembled monolayer

The electro-oxidation of alternariol monomethyl ether (AME), one of the main metabolites of the Alternaria genus mycotoxins, is studied at 1-dodecanethiol (DDT)-modified gold electrodes, in acetonitrile (ACN) – aqueous phosphate buffer solutions of different pH values, by using cyclic (CV) and square-wave (SWV) voltammetries. The AME voltammetric response at the bare electrode suffers from two drawbacks: it appears at potentials close to the onset of gold oxide formation, and it is hampered by a fouling of the electrode surface due to the accumulation of oxidized products. These shortcomings are circumvented by the use of DDT-coated electrodes, since the intervening monolayer inhibits gold oxide formation and surface passivation by the electrochemical products, without affecting the oxidation kinetics of AME significantly. Diagnostic criteria based on the voltammetric peak parameters show that the electrochemical behavior of AME at the modified electrode is mainly controlled by reactant diffusion from solution, with a weak adsorption of both the mycotoxin and its oxidation products at monolayer defects. Calibration curves were constructed from the AME square-wave voltammetric response and a detection limit of 9.1 × 10^?8 mol dm^?3 was determined, which is about three times smaller than a previous estimate at platinum and glassy carbon electrodes, and about fifty times smaller than the limit derived from measurements carried out at a polyphenol oxidase-modified carbon paste electrode.     

Characterization of a 1,4-dithiane gold self-assembled monolayer: an electrochemical sensor for the cyt-c redox process

Electrochemical desorption and spectroscopic investigations of the gold electrode surface modified with 1,4-dithiane (1,4-dt) organothiol species were performed. The wave observed at ?0.87 V versus Ag ∣ AgCl in the LSV (linear sweep voltammetry) reductive curve of the 1,4-dt compared to that for a similar 4-mercaptopyridine (pyS) system (?0.56 V) is indicative of a most effectively chemisorbed monolayer. The evaluation of the capability of the 1,4-dt self-assembled monolayer (SAM) in assessing the direct electron transfer (ET) of cytochrome c (cyt c) metalloprotein was investigated by cyclic voltammetry. The electrochemical response of the cyt c (E_1/2 ≈0.0 V vs. Ag ∣ AgCl, ΔE_p ≈50 mV) showed the characteristics of a reversible redox process. The cyt c voltammetric parameters acquired with the 24-h air exposure modified electrode, and after 100 cycles suggest a considerable improvement of the 1,4-dt electrode performance. The surface enhanced Raman spectroscopy (SERS) spectra revealed that 1,4-dt species is in a mixed gauche and trans orientation on the gold surface. The shift for higher wavenumbers observed for the C–S stretching modes in the SERS spectra, comparatively to the normal Raman spectrum, is assigned to the 1,4-dt coordination to surface gold atoms via a π interaction with the sulfur p-orbitals. The data collected suggest that this π interaction plays an important role on the stability of the 1,4-dt adlayer, improving the assessment of the cyt c heterogeneous electron transfer reaction.     

Application of multi-walled carbon nanotubes functionalized with hemin for oxygen detection in neutral solution

Functionalization of multi-walled carbon nanotubes (MWNTs) is of paramount importance for developing new sensors. In this study, a hemin-modified MWNT electrode was successfully constructed. Upon saturation, the amount of adsorbed hemin is estimated as 2.7 × 10^?9 mol cm^?2, which is 39 times larger than the value of the hemin monolayer. The electrochemical behavior of the hemin-modified MWNT electrode has been characterized by cyclic voltammetry (CV). The electron-transfer coefficient (α) is found to be 0.38 with a heterogeneous electron transfer rate (k) of 2.9 s^?1 for the adsorbed hemin. Both MWNT and hemin-modified MWNT electrodes show ideal reversibility in 5 mM K₃[Fe(CN)₆] in the range of 0.02–1.00 V s^?1, indicating fast electron-transfer kinetics. CV of the hemin-modified MWNT electrode in pH 7.4 phosphate buffer solution (PBS) clearly shows the dioxygen reduction peaks close to 0 V (vs. Ag|AgCl). These results are useful in the development of a novel oxygen sensor for working at a relatively low potential.     

A correlation study between the conformation of the 1,4-dithiane SAM on gold and its performance to assess the heterogeneous electron-transfer reactions

A gold electrode spontaneously modified by 1,4-dithiane (1,4-dt) organosulfur species has been characterized by cyclic voltammetry of [Fe(CN)₆]^3?/4? and cytochrome c (cyt c) probe molecules, SERS (surface-enhanced Raman scattering), and EIS (electrochemical impedance spectroscopy), using [Fe(CN)₆]^3?/4? as the redox-active molecule in solution. The cyclic voltammograms showed a decrease in the faradaic current response with an increase of the immersion time of the gold substrate into the 1,4-dt modifier solution, suggesting that the heterogeneous electron-transfer (hET) process of the [Fe(CN)₆]^3?/4? probe molecule occurs through the surface pinholes. This observation is an indication that the 1,4-dt chemisorption mechanism must involve a slow step that is responsible for the final configuration of the modifier molecules on the surface. The SERS spectra acquired for the surface as a function of immersion time of the gold electrode in the modifier solution presented an intensity enhancement of the bands assigned to the trans and gauche conformation of the 1,4-dt on the surface. The apparent rate constant, k_app, estimated by the EIS results, decreases with increase of the immersion time indicating that the pinholes on the surface act as a microarray. This result is consistent with those observed by cyclic voltammetry of the probe molecules, i.e., the pinhole density decreases with increase of the immersion time of the gold electrode in the 1,4-dt modifier solution, and a strong attenuation of the probe molecules interfacial electron-transfer response is observed.     

Membrane electrodes can modulate the electrochemical response of redox proteins — direct electrochemistry of cytochrome c

Membrane (carbon or gold) electrodes constructed from dialysis membranes varying in cutoff and charge are used to investigate the electrochemical behavior of c-type cytochromes, especially cytochrome c. It is shown from cyclic voltammetry experiments that cytochrome c exhibits direct electrochemical responses at negatively charged membrane (graphite-, glassy carbon-, mercury film glassy carbon-, and gold) electrodes. Different factors (pre-treatment of the membrane, effect of positively charged species, ionic strength, pH, effect of the entrapped layer thickness) have been examined. The electrochemical response of multiheme cytochromes c₃ at the membrane electrode is also investigated. It is demonstrated that the electrochemistry of cytochrome c at the membrane electrodes is essentially governed by favorable electrostatic interactions, and that other factors (especially adsorption and the presence of denatured forms) do not play a dominant role. A discussion on the electrochemistry of c-type cytochromes is given.     

Effect of pH on direct electron transfer between graphite and horseradish peroxidase

The effect of pH on the kinetics of the electroreduction of H₂O₂ catalysed by horseradish peroxidase (HRP) has been studied with LSV in the potential range from 700 to ?50 mV versus SCE (under steady-state conditions and with an RDE system) and at ?50 mV versus Ag/AgCl on HRP-modified graphite electrodes placed in a wall-jet flow-through electrochemical cell. Increasing [H₃O⁺] was shown to enhance significantly the current of the bioelectroreduction of H₂O₂ due to direct electron transfer (ET) between graphite and the enzyme over the potential range involved. It is demonstrated that at high overvoltages (E<0.2 V) H₃O⁺ does not affect the rate of the enzymatic reduction of H₂O₂, but it increases the rate of direct ET between graphite and HRP. The values of the apparent rate constant of heterogeneous ET between HRP and graphite, k_s, changed from a value of 0.54±0.05 s^?1 in phosphate buffer solution (PBS) at pH 7.9, to a value of 11.0±1.7 s^?1 in PBS at pH 6.0. Analysing the pH rate profile and the variation of the k_s with increasing [H₃O⁺] made it possible to consider the reaction mechanism as implying the participation of a proton in the limiting step of charge transfer.     

Electrochemical,AFM and QCM studies on ferritin immobilized onto a self-assembled monolayer-modified gold electrode

A ferritin-immobilized electrode based on self-assembled monolayer (SAM)-modified gold electrodes was developed. Various alkane thiols containing functional groups as the terminal group were used for the SAMs. The ferritin-immobilized SAM-modified electrodes were characterized by electrochemical, atomic force microscopy (AFM) and quartz crystal microbalance (QCM) techniques. The results indicated that ferritin was selectively immobilized onto amino terminal alkane thiol-modified electrodes by the electrostatic interaction between ferritin and the terminal functional groups of the SAMs. The ferritin-immobilized modified electrode showed a direct electron transfer reaction between ferritin and the electrode. The electrochemically regulated uptake and release of iron ions for ferritin immobilized on the SAMs was demonstrated. The AFM and QCM results showed that ferritin molecules covered the whole electrode surface at almost a full monolayer level. The results obtained in this work indicate that ferritin has potential for a biomaterial in nanomaterial synthesis for potential magnetic, catalytic and biomedical sensing applications.     

Kinetic studies on the electron transfer between bacterial c-type cytochromes and metal oxides

Cyclic voltammetry was used to investigate the kinetics of the electron transfer between various soluble or solid metal oxides, and polyheme c-type cytochromes from Desulfuromonas acetoxidans and Desulfovibrio. The second order rate constant for the catalytic reduction of soluble chromate ions by Desulfuromonas acetoxidans cytochrome c₇ was found to be 6×10⁵ M^?1 s^?1. By using the membrane electrode technology, it has been shown that the catalytic process for Cr(VI) reduction is efficient even when the cytochrome is entrapped in the close vicinity of the electrode surface. Moreover, this proceeding allowed the catalytic reduction of solid metal oxides such as manganese(IV), vanadium(V) and iron(III) oxides to be performed. Results suggest that the metal reductase activity of a microorganism is governed by its c-type cytochrome content. Furthermore, only cytochromes with bishistidinyl heme iron coordination act as metal reducers whereas mitochondrial c-type cytochromes do not. This approach opens new pathways for the use of sulfur or sulfate bacteria in the bioremediation of metal contaminated waters and waste streams. Processes involving the use of entrapped enzymes reactors could be developed according to the metal reducing activity of their polyheme c-type cytochromes.