Mediated electron transfer overpotential

The rate of enzyme-mediator electron transfer has been described generally to vary exponentially with the electrochemical overpotential in accordance with Marcus theory such that the rate-limiting bimolecular electron transfer rate constant (A et) is given by... 

There are conflicting reports as to what precisely is the optimum overpotential. Most experimental evidence suggests a mediator potential that is between 200 mV and 400 mV beyond the redox potential of the enzyme active site will provide the minimum overpotential needed to reach the maximum enzyme-mediator electron transfer rate. Therefore, practical selection of a redox mediator to suit a given enzymatic system requires some amount of experimental optimization to find a balance between low overpotential and sufficiently high rates of enzyme-mediator electron transfer. 

When discussing the transfer of electrons from the enzyme active site to the electrode surface, thus generating catalytic current, there are two types of electron transfer mechanisms mediated electron transfer (MET) and direct electron transfer (DET) [13]. Most oxidoieductase enzymes that have been commonly used in BFC development are unable to promote the transfer of electrons themselves because of the long electron transfer distance between the enzyme active site and the electrode surface as a result, DET is slow. In such a case, a redox-active compound is incorporated to allow for MET. In this approach, a small molecule or redox-active polymer participates directly in the catalytic reaction by reacting with the enzyme or its cofactor to become oxidized or reduced and diffuses to the electrode surface, where rapid electron transfer takes place [14]. Frequently, this redox molecule is a diffusible coenzyme or cofactor for the enzyme. Characteristic requirements for mediator species include stability and selectivity of both the oxidized and reduced forms of the species. The redox chemistry for the chosen mediator is to be reversible and with minimal overpotential [15]. 

In addition, electrode reactions are frequently characterized by an irreversible, i.e., slow, electron transfer. Therefore, overpotentials have to be applied in preparative-scale electrolyses to a smaller or larger extent. This means not only a higher energy consumption but also a loss in selectivity as other functions within the molecule can already be attacked. In the case of indirect electrolyses, no overpotentials are encountered as long as reversible redox systems are used as mediators. It is very exciting that not only overpotentials can be eliminated but frequently redox catalysts can be applied with potentials which are 600 mV or in some cases even up to 1 Volt lower than the electrode potentials of the substrates. These so-called redox reactions opposite to the standard potential gradient can take place in two different ways. In the first place, a thermodynamically unfavorable electron-transfer equilibrium (Eq. (3)) may be followed by a fast and irreversible step (Eq. (4)) which will shift the electron-transfer equilibrium to the product side. In this case the reaction rate (Eq. (5)) is not only controlled by the equilibrium constant K, i.e., by the standard potential difference be-... 

For enzymatic reductions with NAD(P)H-dependent enzymes the electrochemical regeneration of NAD(P)H always has to be performed by indirect electrochemical methods. Direct electrochemical reduction, which requires high overpotentials, always leads to larger or smaller amounts of enzymatically inactive NAD dimers generated in a one-electron transfer reaction. If one-electron transfer redox catalysts are used as mediators, the same problem occurs. 

Electrocatalysis at a modified electrode is usually an electron transfer reaction, mediated by an immobilized redox couple, between the electrode and some solution substrate which proceeds at a lower overpotential than would otherwise occur at the bare electrode. This type of mediated electrocatalysis process can be represented by the scheme ... 

Since mainly the E° of the mediator dictates at what potential the heterogenous electron transfer occurs, the oxidation of NADH can now take place at a much lower potential. The different mediator structures used to produce CMEs for NADH oxidation at a decreased overpotential are summarized in Table I. As is seen in the table, not only chemically modified electrodes based on only immobilized redox mediators have been used for this purpose, but also electrodes based on the combination of redox mediators and NADH oxidizing enzymes (diaphorase and NADH dehydrogenase) as well as electrodes made of the conducting radical salts of tetrathiafulvalinium-7,7,8,8-tetracyanoquinodimethan (TTF-TCNQ) and W-methyl-phenazin-5-ium-7,7,8,8-tetracyanoquinodimethan (NMP-TCNQ). 

There is no variation in Cr(VI) photocatalytic reduction efficiency while measured over pure or platinized Ti02 (Siemon et al., 2002). If O2 competes with Cr(VI) for conduction band electrons, a faster Cr(VI) reduction should have been obtained over Pt/Ti02, as platinum reduces the overpotential for oxygen reduction. Experimental results suggest that electron transfer to metallic ion from the conduction band or from Pt is rapid and there is no oxygen mediation requirement. 

Often analytes are irreversibly (slowly) oxidized or reduced at an electrode, that is, require a substantial overpotential to be applied beyond the thermodynamic redox potential (E°) for electrolysis to occur. This problem of slow electron transfer kinetics has spawned much research in the development of electrocatalysts, which may be covalently attached to the electrode, chemisorbed, or trapped in a polymer layer. The basis of electrocatalytic CMEs is illustrated in Figure 15.5. Red is the analyte in the reduced form, which is irreversibly oxidized, and Ox is its oxidized form. The redox mediator is electrochemically reversible and is oxidized at a lower... 

For electron-transfer measurements it is very important that the formal potential of the mediator is close to that of the biomolecule in question. One can show from the Nemst equation that this formal potential for one-electron reactions should be within 118 mV of that of the biocomponent. Very often, the direct electron transfer from the biomolecule to the electrode will suffer from irreversibility and need high overpotentials. In this case, from a practical point of view, a good mediator should have a redox potential between the formal potential of the biomolecule and the observed overvoltage. Compilations of compounds that mediate biological redox systems have been published [88-90]. 

Reoxidation of the cosubstrate at an appropriate electrode surface will lead to the generation of a current that is proportional to the concentration of the substrate, hence the coenzyme can be used as a kind of mediator. The formal potential of the NADH/NAD couple is - 560 mV vs. SCE (KCl-saturated calomel electrode) at pH 7, but for the oxidation of reduced nicotinamide adenine dinucleotide (NADH) at unmodified platinum electrodes potentials >750 mV vs. SCE have to be applied [142] and on carbon electrodes potentials of 550-700 mV vs. SCE [143]. Under these conditions the oxidation proceeds via radical intermediates facilitating dimerization of the coenzyme and forming side-products. In the anodic oxidation of NADH the initial step is an irreversible heterogeneous electron transfer. The resulting cation radical NADH + looses a proton in a first-order reaction to form the neutral radical NAD, which may participate in a second electron transfer (ECE mechanism) or may react with NADH (disproportionation) to yield NAD [144]. The irreversibility of the first electron transfer seems to be the reason for the high overpotential required in comparison with the enzymatically determined oxidation potential. 

Electrocatalysis of electrode reactions at macromolecular layers involves the direct participation of the polymer material. Instead of a direct electron transfer between the Fermi level of the metallic electrode and the redox-active substance in solution (which is the classical electrocatalytic situation), the electron transfer is mediated by the surface-immobilized film. Furthermore the overpotential at which a given substrate reaction occurs at an appreciable rate can be appreciably lowered at a polymer-modified electrode compared to that obtained at an uncoated electrode. Consequently the electroactive polymer layer plays a central role. 

The electrochemical potential of a redox mediator (EJJ,) must be such that it provides a thermodynamic driving force to facilitate electron transfer with the enzyme to be used. Because of this requirement, the redox potential of the mediator determines the operational potential of the biocatalytic electrode. Thus for an enzymatic oxidation reaction, must be higher than the redox potential of the enzyme ( en) whereas the reverse is true for an enzymatic reduction reaction. The difference between and is defined as the mediator-induced overpotential (A et) and is the potential required for electron transfer to occur between the enzyme and mediator. In the context of a biosensor, a large overpotential can lead to an artificially inflated signal due the oxidation of biological interferants such as ascorbate. Additionally a large overpotential limits the open circuit potential in the context of a biofuel cell. It is therefore desirable to minimize the electrochemical overpotential however, there exists a limit to the minimum overpotential required to facilitate rapid electron exchange between the enzyme and mediator. 

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