Redox mediators tetracyanoquinodimethane

FIGURE 6-6 Chemical stmcture of some common redox mediators (a) dimethyl ferrocene (b) tetrathiafidvalene (c) tetracyanoquinodimethane (cl) Meldola Blue. 

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). 

The choline electrode usually consists of an amperometric transducer and immobilized choline oxidase. The most frequently used electrochemical transducers are hydrogen peroxide electrodes (26-28,33-36). The amperometric signal in this case is due to electrooxidation of hydrogen peroxide, which is the co-product of the enzymatic choline oxidation (equation 2). Oxygen amperometric sensors (Qark-type electrodes) have been also used as basic transducers for choline electrode construction (29, 32, 37). The signal in this case is based on the reduction of molecular oxygen which is the co-reactant in reaction (equation 2). Redox mediators hexacyanoferrate (26), ferrocene derivatives (38) and tetracyanoquinodimethane (39) have also been used in the construction of choline electrodes. 

Alternatively to carbon pastes, conducting organic salts have been used to imbed bioactive components, in particular to immobilize enzymes. The redox mediator tetrathiofulvalene acts as electron donor and forms a solid, conducting salt with the electron acceptor tetracyanoquinodimethane. This salt has a low melting point and can be mixed with proteins to give a conducting... 

Low-molecular-weight redox mediators have been used in the construction of choline electrodes for hydrogen peroxide detection. A sensor for choline and acetylcholine was described using anodic detection of hexacyanoferrate (III) in solution. Ferrocene and tetracyanoquinodimethane"" were also used in the construction of choline electrodes. The use of redox mediators facilitates the electron transfer but leads to the system complications. 

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