Polymers, osmium

A compound which is a good choice for an artificial electron relay is one which can reach the reduced FADH2 active site, undergo fast electron transfer, and then transport the electrons to the electrodes as rapidly as possible. Electron-transport rate studies have been done for an enzyme electrode for glucose (G) using interdigitated array electrodes (41). The following mechanism for redox reactions in osmium polymer—GOD biosensor films has... 

Figure 2.27 Double catalytic cycle that gives rise to the current in the presence of p-D-glucose. COx(FAD) oxidizes the enzyme substrate p-D-glucose to yield the reduced form of the enzyme GOx(FADH2) that is then reoxidized by the osmium polymer mediator PAH-Os(ll I). This process yields PAH-Os(ll). Electrons should diffuse to the electrode surface by electron...

The analogous osmium polymers have also been studied in great detail. The synthetic procedures required for these metallopolymers are the same as those described above for ruthenium however, the reaction times are longer. The similarity between the analogous mononuclear and polymeric species is further illustrated by the fact that the corresponding osmium polymers have considerably lower redox potentials and are also photostable, as expected on the basis of the behavior observed for osmium polypyridyl complexes. 

Tables 5.4 and 5.5 provides the homogeneous charge transport diffusion coefficients, Dct, for osmium polymers with different loadings in HCIO4 and H2SO4. Further information about the nature of these processes can be obtained by determining the thermodynamic parameters. These parameters are also summarized...

Given the formal potential of the osmium polymers (about 250 mV (vs. SCE)) [94], electrodes modified with these polymers can be use to mediate the reduction of Fe(m) as follows ... 

First of all, we must realize that the most important property of the electroactive polymer, as far as mediation is concerned, is its redox potential. To mediate a reduction of a solution species, the redox potential of the electroactive layer must be less positive than that of the analyte for the mediated oxidation process, the reverse is the case. This means that the osmium polymers under consideration here which have a redox potential of about 250 mV, are thermodynamically able to mediate the reduction of Fe(III) to FeCII), but not the reverse process (see Fig. 8.24), since the formal potential of the Fe(III/II) couple is 450 mV. The difference in the two redox potentials can be considered the driving force for the mediating process. On the basis of these considerations, it is clear that the mediated reduction of Fe(III) [as in Eq. (38)] is irreversible. 

The redox polymer has three functions, it communicates with the active site in the enzyme, transfers the electrons to the electrode surface and it forms a matrix in which the enzyme is immobilized [3], This is the reasons which makes the osmium polymer such a successful mediator. A very sensitive and fast biosensor is achieved with the redox polymer bound enzyme. There are no membrane passages that will delay the reaction and the mediator is immobilized in the same matrix and will not diffuse away. An amperometric glucose sensor with the mediator electrostatically bound to a PVP polymer has been studied in a previous paper [4], where the mediator loss seemed to be a problem. The hydrophilicity of the osmium redox polymer also contributes to the rapid response because of the fast transport of water soluble substrates and products. 

A specific example worth exploring is the EC reaction. Such an example of this process is the modification of a glassy carbon electrode with an Osmium polymer, [Os(bpy)2(PVP)ioCl]Cl and Nafion prepared by drop-coating, producing a doublelayer membrane modified electrode [9]. In this case the modified electrode was explored towards the sensing of the neurotransmitter epinephrine (EP) [9]. Figure 2.19 shows the voltammetric response of the electroactive polymer (curves A and B in Fig. 2.19) where upon contact with epinephrine (curve C in Fig. 2.19) a reduction in the back peak coupled with an increase in the forward wave is evident. The process can be described as ... 

Antiochia R, Gorton L. Development of a carbon nanotube paste electrode osmium polymer-mediated biosensor for determination of glucose in alcoholic beverages. Biosens Bioelectron 2007 22 2611-2617. 

Scodeller P, Carballo R, Szamocki R, Levin L, Forchiassin F, Calvo EJ. I.ayer-by-layer self-assembled osmium polymer-mediated laccase oxygen cathodes for biofuel cells the role of hydrogen peroxide. J Am Chem Soc 132 11132-11140. 

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