Fast redox couple.

Let us consider this regime for the electrochemical reaction between the oxidized (O) and reduced (R) form of a fast redox couple, when both O and R are soluble and only O is initially present in the solution. The redox couple is... 

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. 

These two concepts (control by mass transport and nernstian system) as terms are not strictly synonymous. Quasi-fast redox couples (see section 4.3.2.6) close to equilibrium conditions are nernstian, but they are not strictly controlled by mass transport. 

The concept of reversibility as defined above is sometimes used incorrectly and mixed up with the concept of the rapidity of a couple, which is defined in this section. Even if a reversible redox reaction involves a fast redox couple in many circumstances, these two notions are nonetheless different in nature. 

Finally, to complete the study, a detailed description is given to specify the interfacial conditions involved in chronoamperometry, in the case where there are unidirectional diffusion conditions with a fast redox couple (a reversible reaction) and where there are different diffusion coefficients for the eiectroactive species. 

Fast redox couple — 1 slow redox couple — [Pg.323]

The diagram below shows the changes over time in the concentration profile for an electroactive species of a fast redox couple in an experiment ... 

The concept of wired enzymes, borrowed from electrochemical biosensors [126], was recently applied to make an immunosensor for HRP [26]. Here, rabbit Ab for HRP was co-immobilized with the redox polymer [Os(bipyridyl)2poly-4-vinylpyridinejioCljCl onto a GC electrode. Upon capture at the immunosensor surface, the redox centers of the HRP molecules become electrically wired to the electrode surface via the redox polymer. Therefore, when H2O2 is added to the immunosensor following the sample incubation and wash, the HRP that is oxidized during catalysis can be electrochemically reduced (and detected) to its original form by the wired electrode. The detection of HRP in this way was shown to have the classical features of a fast redox couple strongly bound to an electrode surface. 

For a reversible (electrochemically fast) redox couple, the mid-point between the anodic and cathodic peak potentials Epa and pc is the so-called half-wave potential 1/2, which is related to the formal potential E for the Ox/Red couple by the simple expression... 

Figure 5.8 shows data for two types of -type Si photoelectrodes immersed in an electrolyte with a fast redox couple that induces a large internal potential drop in n-Si. Here the photoelectrodes are composed of a film of dense Si nanowires on top of a single-crystalline Si substrate. The photoresponses for both the composite films and just the underlying Si substrate are shown. ... 

An MET system increases the rate of electron transfer between the electrode and enzyme active site by introducing a fast redox couple capable of interacting with the electrode at a rate far higher than the enzyme itself. This redox couple is the mediator, which takes the place of one of the natural substrates of the enzyme. The choice of an appropriate mediator can result in reaction rates exceeding those of a DET system by several orders of magnitude. The cost of an MET scheme derives from increased complexity, as the mediator species must now be considered. Also, an additional overpotential is introduced the potential difference between the... 

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