Surface resistance measurements on thin gold film electrodes coated with poly( o-aminophenol) films

Abstract

The relative resistance change (Δ R/ R) of a thin gold film electrode modified by a poly( o-aminophenol) (POAP) film was monitored during the oxidation–reduction process of the polymer. These resistance changes were analysed in terms of the specularity ( r) of the metal ∣ polymer interface for the reflection of the conduction electrons from inside the gold film on its surface. When the polymer is reduced (amine sites) the metal ∣ polymer interface exhibits a higher degree of specularity than when it is oxidised (imine sites). The observed resistance changes as a function of the degree of oxidation ( θ) of the polymer allows one to differentiate between different distributions of oxidised sites at the metal ∣ polymer interface as θ increases. The distribution of oxidised sites at low degrees of oxidation ( θ<0.2) of the polymer film presents a less reflecting interface for the conduction electrons of the gold film than the corresponding distribution at higher degrees of oxidation ( θ>0.3). This was interpreted on the basis of an initial distribution of oxidised sites with neighbour distances larger than those corresponding to the distribution at higher θ values. However, for thin POAP films, the Δ R/ R change at high θ values also depends on the polymer film thickness and external electrolyte concentration. Thus, for different θ values a ratio of resistance changes (Δ R θ=0.9 /Δ R θ=0.1 ) was used to evaluate the effect of the external electrolyte on the resistometric changes due to the proper redox transformation of the polymer. The influence of the polymer film thickness and external electrolyte concentration on the specularity of the metal ∣ polymer interface was attributed to changes in the polymer morphology as the film thickness varies. Decrease in the specularity of the metal ∣ polymer interface at high θ values with decreasing polymer film thickness and increasing external electrolyte concentration was associated with open structures at low film thickness which allow incorporation of electrolyte into the polymer matrix. Also, the presence of redox active species in the electrolyte solution, such as hydroquinone–benzoquinone, which are able to reach the metal ∣ polymer interface affect the reflection of the conduction electrons on the gold film surface. Also, this last effect was more pronounced for thin polymer films.