Mediated Electron Transfer Processes Research Articles

Electrochemistry, AFM, and PM‐IRRA Spectroscopy of Immobilized Hydrogenase: Role of a Hydrophobic Helix in Enzyme Orientation for Efficient H2 Oxidation

A transmembrane helix surrounded by detergent molecules close to the surface electron relay is shown, by electrochemical, AFM, and PM-IRRAS studies, to control the orientation of a membrane-bound [NiFe] hydrogenase on electrochemical interfaces. Hence, H2 oxidation proceeds as a mixture of direct (DET) and mediated electron transfer (MET) on hydrophilic interfaces, but by a MET process on hydrophobic interfaces (see picture).

How far can hydroxyl radicals travel? An electrochemical study based on a DNA mediated electron transfer process

Using photocatalytic reactive nanoparticles and DNA composite modified gold electrodes, exploiting the sensitivity of DNA-mediated electron transfer to base pair stacking, we examined the effects of DNA oxidation damage by photocatalytically generated hydroxyl radicals (˙OH) on charge transfer efficiency and proposed an electrochemical technique to read the effective diffusing distance of ˙OH.

Stabilization Role of a Phenothiazine Derivative on the Electrocatalytic Oxidation of Hydrogen via Aquifex aeolicus Hydrogenase at Graphite Membrane Electrodes

The [NiFe] membrane-bound hydrogenase from the microaerophilic, hyperthermophilic Aquifex aeolicus bacterium (Aa Hase) presents oxygen, carbon monoxide, and temperature resistances. Since it oxidizes hydrogen with high turnover, this enzyme is thus of particular interest for biotechnological applications, such as biofuel cells. Efficient immobilization of the enzyme onto electrodes is however a mandatory step. To gain further insight into the parameters governing the interfacial electron process, cyclic voltammetry was performed combining the use of a phenothiazine dye with a membrane electrode design where the enzyme is entrapped in a thin layer. In the absence of the phenothiazine dye, direct electron transfer (DET) for H(2) oxidation is observed due to Aa Hase adsorbed onto the PG electrode. An unexpected loss of the catalytic current with time is however observed. The effect of toluidine blue O (TBO) on the catalytic process is first studied with TBO in solution. In addition to the expected mediated electron transfer process (MET), TBO is demonstrated to reconnect directly some Aa Hase molecules possibly released from the electrode but still entrapped in the thin layer. On adsorbed TBO the two same processes occur demonstrating the ability of the TBO film to connect Aa Hase via a DET process. Loss of activity is however observed due to the poor stability of adsorbed TBO at high temperatures. Aa Hase immobilization is then studied on electropolymerized TBO (pTBO). The effect of film thickness, temperature, presence of inhibitors and pH is evaluated. Given a film thickness less than 20 nm, H(2) oxidation proceeds via a mixed DET/MET process through the pTBO film. A high and very stable H(2) oxidation activity is reached, showing the potential applicability of the bioelectrode for biotechnologies. Finally, the multifunctional roles of TBO-based matrix are underlined, including redox mediator, Aa Hase anchor, but also buffering and ROS scavenger capabilities to drive pH local changes and avoid oxidative damage.

Influence of Substrate on Electron Transfer Mechanisms in Chambered Benthic Microbial Fuel Cells

This research investigated whether the addition of an exogenous electron donor would affect power production in laboratory-scale benthic microbial fuel cells (BMFC) by differentially influencing microbially mediated electron transfer processes. Six BMFCs were operated for over one year in a temperature-controlled laboratory. Three BMFCs relied on endogenous electron donors, and three were supplemented with lactate. The supplemented BMFCs generated more cumulative charge, but did not generate higher average current between periods of lactate enrichment. Coulombic efficiencies during the lactate treatments ranged from 25 to 65% suggesting that lactate utilization was variably coupled to power production. Cumulative electron flux resulting from lactate additions and chemical changes within the anode chamber, as well as a difference in the anode-hosted microbial communities indicated that lactate supplementation promoted sulfate reduction. After the addition of molybdate to suppress sulfate reduction and sulfur disproportionation, all BMFCs continued to produce current, but no longer responded to lactate additions. Chemical data support a two-step cycle in which endogenous organic carbon and/or supplemented lactate fuel sulfate reduction resulting in sulfide and simple organic molecules (such as acetate) that can act as the electron donors for the BMFC.

Modeling of the Interaction of Pu(VI) with the Mixture of Microorganism and Clay

A model analysis of the transformation of Pu(VI) in a mixture of a common soil bacterium, Bacillus subtilis, and kaolinite clay was carried out. A simplified kinetic model, where the adsorptions of Pu(VI) and Pu(V) are assumed to be in instantaneous equilibrium and the two-step reduction of Pu(VI) to Pu(V) and then to Pu(IV) is described by a first-order rate law, was proposed. When we assumed in the model analysis that the reduction rate of Pu(V) to Pu(IV) was higher for B. subtilis than for kaolinite, the estimated fractions of Pu in the solution and in the mixture, and the oxidation states of Pu in the solution and in the mixture were in good agreement with the measured ones. On the contrary, the assumption that the reduction rate of Pu(V) to Pu(IV) for kaolinite was the same as that for B. subtilis gave a wrong prediction of Pu association with the mixture. These results strongly suggest that Pu(V) on the bacterial cell is reduced to Pu(IV) by the bacterially mediated electron transfer process during the accumulation of Pu(VI) in the mixture.

Mediated Enzyme Electrodes for Biological Fuel Cell and Biosensor Applications

Integration of enzymes, redox mediators and high surface area materials to provide bioelectrochemical platforms capable of efficiently transferring electrons between chemicals and electrodes is a dynamic and rapidly expanding research area. These bioelectrochemical reactions are enabled by designing efficient mediated electron transfer processes between selected enzymes and electrode materials. Targeted synthesis of a library of osmium based redox mediator complexes possessing a range of redox potentials is presented whereby these mediators are designed to possess the dual function of mediation and covalent immobilization. In addition to designing inbuilt immobilization strategies for mediators, anchoring a support to surfaces can improve the stability of an enzyme/mediator layer compared to physisorbed films. An overview of some of the strategies we have adopted towards the fabrication of these anchored redox active biocatalytic films on supported pre-functionalized electrodes is presented.

Solution phase electron transfer versus bridge mediated electron transfer across carboxylic acid terminated thiols

Self-assembled monolayers (SAMs) of thiols with carboxylic acid terminal groups were formed on gold substrates. The electron transfer characteristics of redox species on the above SAM-modified electrodes were studied in acid and neutral media with the help of voltammetry under two different conditions: (1) solution phase electron transfer and (2) bridge mediated electron transfer. Two redox systems, viz., [Fe(CN)6]4-/3− and Ru[(NH3)6]2+/3+ were chosen for the solution phase study. Investigations of bridge mediated electron transfer were carried out by functionalising the SAM with redox moieties and then studying their redox behaviour. For this study, ferrocene carboxylic acid and 1,4-diamino anthraquinone were used and they were linked to carboxylic acid terminated thiols by covalent linkage. The voltammetric results with mercaptoundecanoic acid SAM demonstrate the difference in behaviour between solution phase and bridge mediated electron transfer processes.

Shuttle electron transfer in tethered mediator biosensor

Theoretical model of mediated electron transfer processes in redox enzyme biosensor is presented. We have analyzed the charge transfer from substrate to enzyme-modified electrodes via a small mediator molecule anchored on the electrode surface by long flexible polymer. In the system considered, a mediator communicates with redox enzyme center and “shuttles” electrons to the electrode. We have shown that in this case, the output current depends on tether length and that the smaller tethers enhance the electron transfer rate.

Gold electrode modification with a monolayer of 1,4,5,8-naphthalenetetrone as a four-electron transfer mediator: electrochemical reduction of dioxygen

The electrochemical properties of an unusual diquinone containing four carbonyl groups in the molecule (a carboxylic derivatized 1,4,5,8-naphthalenetetrone) were studied in aqueous solution and in monolayer form on a gold electrode surface. Monolayer covalent immobilization of the diquinone was achieved owing to the carbodiimide coupling of its carboxylic group with amino groups of cystamine chemisorbed previously onto the gold electrode surface. Cyclic voltammetry, constant potential electrolysis and rotating ring-disk electrode measurements were used to reveal the four-electron transfer process for dioxygen reduction mediated by the monolayer of immobilized diquinone. A very small amount of hydrogen peroxide was produced and a very high current efficiency, ca. 95%, for water formation was evaluated. These four-electron transfer properties are observed for quinonoid compounds for the first time. However, the diquinone redox potential is too negative (−0.48 V vs. SCE, pH 7.0) to provide efficient electrocatalysis for the four-electron reduction of dioxygen and the diquinone mediated electron transfer process could be observed only on the electrode surface blocked for the normal non-mediated dioxygen reduction. New compounds with a similar structure but with much more positive redox potentials are suggested as a new class of catalyst for the four-electron reduction of dioxygen.

Preparation, characterisation and electrochemistry of electrodes coated with poly (4-vinylpyridine) bound ruthenium complexes

Strong catalytic effects are observed for the reduction of Ce(IV) and the oxidation of Fe(II) when electrodes are coated with [Ru(bipy)2Cl poly(4-vinylpyridine)]Cl or [Ru(bipy)2pyridine poly(4-vinylpyridine)]Cl2. The interfacial reaction can be described as a mediated electron-transfer process. The attached poly(4-vinylpyridine) ruthenium complexes act as the intermediate medium, and the heterogeneous rate constant is connected to the second-order reaction between the attached polymer and the redox couple in solution. Depending on the magnitude of the rate constant of this second-order reaction, catalytic or inhibition effects are observed.

Preparation, characterisation and electrochemistry of electrodes coated with poly (4-vinylpyridine) bound ruthenium complexes

Strong catalytic effects are observed for the reduction of Ce(IV) and the oxidation of Fe(II) when electrodes are coated with [Ru(bipy) 2Cl poly(4-vinylpyridine)]Cl or [Ru(bipy) 2pyridine poly(4-vinylpyridine)]Cl 2. The interfacial reaction can be described as a mediated electron-transfer process. The attached poly(4-vinylpyridine) ruthenium complexes act as the intermediate medium, and the heterogeneous rate constant is connected to the second-order reaction between the attached polymer and the redox couple in solution. Depending on the magnitude of the rate constant of this second-order reaction, catalytic or inhibition effects are observed.