Electrochemical Techniques for the Characterization of Redox Polymers

areas in electroanalytical chemistry for the past two decades has been modification of electrode properties by the immobilization of redox-active species onto the electrode surface by their incorporation into a polymer (1-3). For example, one common application has been the immobilization of enzymes (together with mediators) for the electrochemical detection of biologically important species (e.g., glucose) (4). This article will discuss the electrochemical techniques that are used to characterize these redox polymers. It is important at this stage to differentiate between redox polymers and conducting polymers. Redox polymers contain spatially and electronically localized redox sites. These can be either covalently bound to the polymer (e.g., polyvinyl-pyridine or polymerized metal bipyridine complexes) or electrostatically bound (e.g., negatively charged metal chloride or cyanide complexes in protonated poly [vinylpyridine] or positively charged metal bipyridine complexes in Nafion). In contrast, conducting polymers (which will not be discussed in this article) contain delocalized electronic states. The current response of redox polymer films depends upon the relative sizes of the diffusion layer δ (the layer adjacent to the electrode surface where the concentrations differ from those in the bulk polymer) and the film thickness φ. There are two limiting cases to consider— semi-infinite diffusion (φ>>δ) and thin-layer behavior (φ<<δ). The current responses for semi-infinite diffusion are well-known from studies of solution redox species, although the mechanism for diffusion in polymer films is different (vide infra), and the diffusion coefficients for polymer films generally a few orders of magnitude smaller than those for species in solution. In contrast, diffusion is not important for systems exhibiting thin layer behavior, since the electroactive material immobilized on the electrode surface is electrolyzed very rapidly when the applied potential is changed. Many redox polymers exhibit behavior that is intermediate between these two limiting cases. This behavior is referred to as finite diffusion. As discussed above, the important parameters in determining the extent of diffusion effects are the thickness of the diffusion layer δ and the film thickness φ. δ depends upon the diffusion coefficient (D) and the experimental time scale te. The effects of D, δ, and φ can all be expressed using the dimensionless variable Dte/φ. If this is much larger than 1 (large D, a long experimental time scale, and/or a thin film), then thin layer behavior will be observed, whereas if it is much less than 1 (small D, a short experimental time scale, and/or a thick film), the system will exhibit semi-infinite behavior. In redox polymer films, D is related to charge transfer through the polymer; for example, the rate of Electrochemical Techniques for the Characterization of Redox Polymers