Ruthenium couple, electronic mediation

A doubly metallated 15 base-pair double helix containing ruthenium and rhodium at each end of the strands [106] showed the efficiency of DNA for coupling electron donors and acceptors over a very long range, greater than 40 A. The DNA double helix was found to behave like a piece of molecular wire with fast electron-transfer rates (>1010 s l) for the photoinduced electron transfer between the metallointercalators [107-109] and semiempirical Hartree-Fock calculations of HAB for DNA mediated electron transfer [110] were described. 

So far, catalytic systems in which the mediator plays the role of both catalyst and electron carrier have been considered. Figure 4.21 shows an example where these two roles are dissociated.21 The catalyst, in the sense of a chemical catalyst, is the Co(II) porphyrin embedded in the Nafion (a trademark of Dupont) film, while the electron are shuttled by the ruthenium hexamine 3 + /2+ couple attached electrostatically to the Nafion backbone. The catalytic reaction now involves two successive steps, as expected for a chemical catalysis process (see Sections 4.2.1 and 4.3.1), calling for the definition of two characteristic currents. One has the same... 

Various fast redox couples such as ferrocene, ferro/ferri cyanide, and ruthenium hexamine have been used as mobile mediators. In order to be electron acceptors their standard potentials must be more positive than that of FADH2/FAD redox couple (E° = 0.05 V, at pH = 7). The requirement of mobility is, however, in conflict with the lifetime of the sensor. Because the mediator is of comparable size to the substrate, it cannot be confined to the electrode proper by, for example, a dialysis membrane. In fact, the only way this type of sensor can operate is in a sample containing a sufficient concentration of the mediator (Cass et al., 1984). Obviously, this requirement makes such sensors suitable only for in vitro applications. 

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