Flexible redox polymers

Glucose Sensors. Siloxane polymers are known to be extremely flexible. This flexibility will, of course, be sensitive to the amount of side-chain substitution present along the polymer backbone. For instance, in the homopolymer used in these studies (polymer A), the presence of a ferrocenylethyl moiety bound to each silicon subunit should provide an additional degree of steric hindrance, and thus a barrier to rotation about the siloxane backbone, in comparison with the copolymers, which have ferrocene relays attached to only a fraction of the Si atoms. Because these siloxane polymers are insoluble in water, their flexibility is an important factor in their ability to facilitate electron transfer from the reduced enzyme. Relays contained within more rigid redox polymers, such as poly(vinylferrocene), cannot achieve close contact with the enzyme s redox centers and are thus less effective as electron transfer mediators (25,34). The importance of this feature can be seen quite clearly by comparing the mediating ability of the homopolymer A with that of copolymers B-D, as shown in Figures 4 and 5. 

Relevant issues still to be addressed in constructing amperometric enzyme sensors either using the electrical wiring of enzymes with redox polymers or with flexible polymeric electron mediators are sensor efficiency, accuracy, reproducibility, selectivity, insensitivity to partial pressure of oxygen, detectivity (signal-to-noise ratio) as well as sensor hfetime and biocompatibility [47]. Then we can address manufacturability and the cost of use of either in vitro or in vivo sensors. 

Ferrocene modified flexible polymeric electron transfer systems Ferrocene and its derivatives are readily available and commonly used organometalUc redox mediators, so it is quite natural that they were selected first to synthesize mediator modified polymeric electron transfer systems. Siloxane pol5uners are flexible but aqueous insoluble pol3nmers. As previously indicated, a flexible polymer backbone allows close contact between the redox center(s) of the enzyme and the mediator, and the water insoluble property of the polymer prevents not only redox polymer from leaching into bulk media but also prevents enzyme diffusion away fi-om the electrode surface by entrapping it in the polymer/carbon paste matrix. Therefore, ferrocene and... 

Initial research into the application of PMEs focused on their potential use in electrocatalysis. Much of this work centered on preformed redox polymers containing coordinated electroactive metal complexes because of the synthetic flexibility and the ability to control loading of the electrocatalytic center in the modifying layer. Electrostatic binding of electroactive ions into ionomeric polymer films is a convenient procedure for preparing electrocatalytic layers, although care must be taken to minimize leaching of the electroactive center. ... 

In some cases the polymer incorporates the mediator, producing a redox macromolecule, e.g., poly[(ferrocenyl)amidopropyl]pyrrole, ferrocene-modified poly(ethylene oxide), " and highly flexible ferrocene-modified silox-ane. These redox polymers have short chains and low molecular weights (< 20 kDa) they function as a special kind of diffusional mediators and do not alleviate the loss of a mediator to the bulk solution during amperometric detection. 

In contrast, Boguslavsky and co-workers used a flexible polymer chain to put electron relays [15]. Their polymers provide communication between redox centres in glucose oxidase (GOD) and the electrode. No mediation occurred when ferrocene was attached to a non silicone backbone. Their ferrocene-modified siloxane polymers are stable and non diffusing. Therefore, biosensors based on these redox polymers give good response and stability. 

The redox behaviors of these compounds have been characterized by the solid-state CV (Figure 13). Compound 8 showed irreversible oxidation wave at 2529 mV (vs SCE), which is ascribed to the oxidation of sulfide. On the other hand, 9 and 10 showed irreversible reduction waves ascribed to the reduction of disulfoxide at —1765 and —1760 mV (vs SCE), respectively. These results demonstrate that the coordination polymers into which chalcogen groups are incorporated are useful for the creation of flexible redox-active coordination polymers. 

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