Redox mediators conductic salt

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). 

Various redox compounds that fulfil catalyst characteristics have been investigated in systems with recycling of NAD by electrocatalytic methods. Quinones, formed either by oxidation of carbon surfaces [143, 145] or adsorbed to the electrode surface [146, 147], phenazines [148, 149], phenoxazine derivatives such as Meldola Blue [182], 9-naphthoyl-Nile Blue [151, 152] and l,2-benzophenoxazine-7-one [153], and also the organic conducting salt N-methyl phenazinium tetracyanoquinodimethanide (TTF TCNQ") [154, 155], ferricinium ions [156, 157] and hexacyanoferrat(IIl) ions [158, 159] can act as catalysts for the electrochemical oxidation of NADH. It is assumed that in corresponding electron-transfer reactions a charge-transfer complex between the immobilized mediator and NADH is formed. The intermediate reduced redox mediator will be reoxidized electrochemically. Most systems mentioned, however, suffer from poor electrode stabilities. 

Matsue et al. [27] were the first to explore an enzyme-based OECT biosensor. They used Diaphorase as the entrapped enzyme in a polypyrrole transducing layer for the detection of NADH via a redox mediator (the sodium salt of anthraquinone-2-sulfonic acid). The net result was the conversion of polypyrrole from its conducting state to its insulating state in the presence of NADH. The device showed a response time of 15--20 min in the presence of NADH. Later Nishizawa et al. [26] exploited the pH sensitivity of the polypyrrole film for the design and fabrication of OECT sensors for pH and for pencillin. The Penicillinase enzyme was entrapped in a membrane which was coated with a polypyrrole film, in which a decrease in pH was observed in the presence of penicillin due to the hydrolysis of penicillin by Penicillinase. 

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... 

The sharp double-barrel pipettes are then filled with electrolyte solution (typically aqueous salt solutions between 1 and 100 mM ionic strength), which can also contain other species (such as redox mediators or a nanoparticle dispersion). We have also successfully employed room temperature ionic liquids (RTILs) in SECCM. The electrolyte solution naturally forms a small liquid meniscus over the end of the pipette, connecting the two barrels. QRCEs are inserted into each barrel, and a conductance cell is formed between the QRCEs and across the liquid meniscus. Ag/ AgCl and Pd-H2 electrodes have been used as QRCEs. To help confine an aqueous meniscus, the outside walls of the pipette are often salinized using dimethyldichlorosilane [Si(CH3)2Cl2]. ... 

Mediators can be polymerized on the electrode surface prior to enzyme immobilization, co-immobilized with enzyme, or simply added to the fuel solution. Common mediators used in BFC applications include low molecular weight, polymerizable, organic dyes such as methylene green, phenazines, and azure dyes, along with other redox-active compounds such as ferrocene, ferrocene derivalives, and conductive salts [14]. These mediators are often required for nicotinamide adenine dinucleotide (NAD )- and flavin adenine dinucleotide (FAD)-dependent enzymes, such as ADH, ALDH, and GOx. MET has been achieved at both cathodic and anodic interfaces through solution-phase mediators and mediators immobilized in various ways with or near the enzymes themselves [16,17]. However, these mediated systems do have drawbacks in that the species used to assist electron transfer are often not biocompatible, have short lifetimes themselves, or cause large potential losses. Table 5.1 lists common enzyme cofactors that can mediate or undergo DET with an enzyme on the electrode. 

For electrodes based on conducting organic charge-transfer salts such as TTF + TCNQ (a complex of the radical cation of tetrathiafulvalene and the radical anion tetracyano-p-quinodimethane) or NMP +TCNQ " (N-methylphenaziniumtetracyano-p-quinodimethane), direct [155, 169] and mediated [154] electron transfer mechanisms have been described. In analogy with the theory of outer-sphere electron transfer [170], Kulys and co-workers [118,171] have developed a mathematical model which permits to evaluate the depth of the active site of some oxidoreductases from the steric requirements of inorganic redox couples (Table 14-4). 

One particular SECM experiment is the approach curve in feedback mode. In this experiment, a redox active salt, the mediator, is introduced into the electrolyte. A single potentiostat polarizes the tip to cause an electrochemical reaction however, the sample itself is not polarized. The resulting current is recorded as the tip is moved closer towards the sample. When the tip is positioned appropriately close to the sample, a local response is seen. If the specific location on the sample is conductive, the resulting nernstian response observed at the surface sample causes the current to increase when compared to the bulk current (i.e., when the tip is far from the substrate). This is called positive feedback . If the specific location on the sample is insulating, then mass transport to the electrode of the tip is hindered, and the current decreases when compared to the bulk current. This is called negative feedback . A range of intermediate types of behavior may also occur with different samples. The c uantitative analysis of such approach curves allows for a very accurate analysis of local surface kinetics to be carried out. 

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