Polymer-modified electrodes transport properties

Perhaps the original hope for these polymers was that they would act simultaneously as immobilisation matrix and mediator, facilitating electron transfer between the enzyme and electrode and eliminating the need for either O2 or an additional redox mediator. This did not appear to be the case for polypyrrole, and in fact while a copolymer of pyrrole and a ferrocene modified pyrrole did achieve the mediation (43), the response suggested that far from enhancing the charge transport, the polypyrrole acted as an inert diffusion barrier. Since these early reports, other mediator doped polypyrroles have been reported (44t45) and curiosity about the actual role of polypyrrole or any other electrochemically deposited polymer, has lead to many studies more concerned with the kinetics of the enzyme linked reactions and the film transport properties, than with the achievement of a real biosensor. 

The examples described in this chapter clearly show the potential of modified electrodes based on redox-active osmium-containing polymers. The redox potentials of these materials can be manipulated by varying the nature of the polymer-bound redox couple, which allows us to tailor polymers to particular application, especially in the sensors area. Furthermore changes can also be made in the polymer composition both with respect to polymer loading and the nature of the polymer backbone. This will allow control of such parameters as substrate diffiision and charge transport through the layer. This flexibility allows the systematic investigation of electrochemical properties of electrodes modified with such materials. 

Amphophilic pyrroles modified with biotin have b n used to attach a number of biotinylated oxidases and flavins to electrode surfeces by means of an avidin/biotin interaction. This approach not only avoids use of large amounts of enzyme but also r ults in a polymer with excellent swelling properties and thus improved mass transport in the polymer film 51). 

CHARGE TRANSPORT PROPERTIES OF ELECTRODES MODIFIED WITH OSMIUM CONTAINING POLYMER FILMS,... 

Charge Transport Properties of Electrodes Modified with Osmium Containing Polymer Films R.J. Forster and J.G. Vos... 

A powerful and elegant way to overcome the limitations encountered in redox polymers makes use of ion-exchange polymers (5). This class of modified electrode can be constructed using any polymer with ion-exchange properties by exchanging the native counter-ion for an appropriately charged electroactive ion. This ion will serve as the electron transfer site within and on the surface of the film. The result is a layer with the ability to transport electrons as illustrated in Figure 8.17B. 

Modeling an electrochemical interface by the equivalent circuit (EC) representation approach has been exceptionally popular in studies of electrodes modified with polymer membranes, although an analytical approach based on transport equations derived from irreversible thermodynamics was also attempted [6,7]. ECs are typically composed of numerous ideal electrical components, which attempt to represent the redox electrochemistry of the polymer itself, its highly developed morphology, the interpenetration of the electrolyte solution and the polymer matrix, and the extended electrochemical double layer established between the solution and the polymer with variable localized properties (degree of oxidation, porosity, conductivity, etc.). 

A thin layer deposited between the electrode and the charge transport material can be used to modify the injection process. Some of these arc (relatively poor) conductors and should be viewed as electrode materials in their own right, for example the polymers polyaniline (PAni) [81-83] and polyethylenedioxythiophene (PEDT or PEDOT) [83, 841 heavily doped with anions to be intrinsically conducting. They have work functions of approximately 5.0 cV [75] and therefore are used as anode materials, typically on top of 1TO, which is present to provide lateral conductivity. Thin layers of transition metal oxide on ITO have also been shown [74J to have better injection properties than ITO itself. Again these materials (oxides of ruthenium, molybdenum or vanadium) have high work functions, but because of their low conductivity cannot be used alone as the electrode. 

Figure 3.9 illustrates the electrochemical and mass transport events that can occur at an electrode modified with a interfacial supramolecular assembly [9]. For monolayers in contact with a supporting electrolyte, the principal process is heterogeneous electron transfer across the electrode/monolayer interface. However, as discussed later in Chapter 5, thin films of polymers [10] represent an important class of interfacial supramolecular assembly (ISA) in which the properties of the redox center are affected by the physico-chemical properties of the polymer backbone. To address the properties of these thin films, mass transfer and reaction kinetics have to be considered. In this section, the properties of an ideally responding ISA are considered. 

Further studies have also appeared in the mass transfer properties of the modifying layers. The mass transfer behavior of the polymer [Os(bipy)2(PVP)ioCl]Cl in para-toluene sulphonic acid has been studied in de-tail. In this study the effect of the non-rigidity of the layer on the mass transport data is discussed in some detail. A detailed study of the pH dependence of the mass transfer of a glassy carbon electrode modified with the [Ru(bipy)2(PVP)io(H20)] is reported. This study shows that the EQCM can be used very effectively for the study of redox reactions which show complicated (in this case pH dependent) features. (See also Section 8.2). 

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