Redox mediators tetrathiafulvalene

In addition to the previously described dehydrogenase-based CNT electrodes, electrochemical biosensors that employ other types of enzyme-modified CNTs have also been reported. Kowalewska and Kulesza applied CNTs with adsorbed redox mediator tetrathiafulvalene (TTF) for electrochemical detection of glucose." TTF-modified CNTs were found to facilitate electron transfer between GOx and the electrode surface for glucose detection. Jia et al. reported a similar strategy for the detection of lactate using MWCNTs modified with TTF and lactate oxidase. Since TTF does not cause skin irritation and the CNT/TTF platform also enables low-potential sensing of lactate, CNT/ TTF/lactate oxidase-based electrochemical biosensors conld be used to detect lactate in perspiration directly on human skin. This was accomplished by preparing temporary tattoos from CNT/ TTF/lactate oxidase-conductive carbon ink that was transferred onto a human subject s skin. ... 

Figure 6.8 Chemical structures of some common redox mediators (a) dimethyl ferrocene (b) tetrathiafulvalene (c) tetracyanoquinodimethane (d) Meldola Blue.

Electron mediators successfully used with oxidases include 2,6-dichlorophenolindophol, hexacyanoferrate-(III), tetrathiafulvalene, tetracyano-p-quinodimethane, various quinones and ferrocene derivatices. From Marcus theory it is evident that for long-range electron transfer the reorganization energies of the redox compound have to be low. Additionally, the redox potential of the mediator should be about 0 to 100 mV vs. standard calomel electrode (SCE) for a flavoprotein (formal potential of glucose oxidase is about -450 mV vs SCE) in order to attain rapid vectrial electron transfer from the active site of the enzyme to the oxidized form of the redox species. 

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). 

Tetrathiafulvalene (TTF), for example, is a versatile mediator, but it is not soluble in water. A simple method for its solubilization is to bind it in the CyD cavity. Hy-droxypropyl-yS-CyD (HP-j8-CyD) is even more soluble than the nonsubstituted CyD and was chosen by Schmidt et al. [45] to bind TTF with the aim of providing direct electrical communication between enzyme, GOx, and the electrode. Without it, the redox center of the enzyme is inaccessible and no redox chemistry can be observed. The cyclic voltammograms for TTF-FIP-jS-CyD in the presence of glucose, show a large anodic peak without a reduction counterpart (Fig. 16.4.4) indicating bioelec-... 

Tetrathiafulvalene (TTF) is an excellent candidate for redox-driven molecular switch because it can be easily oxidized into TTF" and TTF + ions. In 1999, Stoddart et al. reported a three-pole supramolecular switch (Figure 38), in which the mechanical action was mediated by the electrochemical adjustment of the TTF oxidation... 

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