Role of the Supporting Electrolyte Ions and Additives on the Electron Transport Properties of Electrografted Films Bearing Ferrocenyl Moieties

Abstract

The electrochemical oxidation of 6‐ferrocenylheptanoate was used to modify the surface of glassy carbon electrodes with thin films bearing ferrocene molecules. These films grow layer by layer and behave as surface redox catalyst during the electrografting. The electrografted films have a structure with pinholes or channels, whose existence was deduced from the redox behavior of the film in acetonitrile solution containing different supporting electrolytes and organic additives. The electron transport through this film requires the presence of the electrolyte anions in the film channels. However, the voltametric feature of the grafted films is sensitive to the electrolyte cation size. The supporting electrolyte anions inside the film channels stabilize the ferrocenium species during the film oxidation. However, the presence of the cations is required to keep the inner charge balance. The inclusion of both anions and cations inside the grafted film determines the surface coverage obtained from the voltammetric experiments.

Covalent Modification of Carbon Surfaces by Direct and Redox Catalysed Oxidation of Carboxylates in Acetonitrile – Concepts and Mechanisms

“…That is, the grafted film behaves as a surface redox catalyst that gradually induces a multilayer growth of the film. [50,51] Figure 6 shows the behaviour of 6-ferrocenylheptanoate as an example of the growth of a redox catalytic film, which does not passivate the electrode during the cycling protocol. The role of the number of cycles on the coverage factor, for films bearing 6-ferrocenylhexyl groups, was extended to more than 40 cycles, which yielded thin films with a thickness near 23 nm.…”

“…Contrasting with the voltammetric behaviour of other carboxylates, whose oxidation passivates the electrode during cycling procedures (Figures 1, 3–5), here the current intensity of the oxidation peak does not decrease significantly, which indicates that the grafted film is redox‐active, and allows the transmission of electrons through the grafted film. That is, the grafted film behaves as a surface redox catalyst that gradually induces a multilayer growth of the film [50,51] …”

“…Figure 6 shows the behaviour of 6‐ferrocenylheptanoate as an example of the growth of a redox catalytic film, which does not passivate the electrode during the cycling protocol. The role of the number of cycles on the coverage factor, for films bearing 6‐ferrocenylhexyl groups, was extended to more than 40 cycles, which yielded thin films with a thickness near 23 nm [51] . Another interesting aspect of the ferrocenyl‐carboxylate systems is the fact that the aliphatic chain that separates the ferrocene moiety from the carboxylate group allows the modulation of the oxidation potential of the ferrocene in the grafted film.…”

“…The role of the number of cycles on the coverage factor, for films bearing 6-ferrocenylhexyl groups, was extended to more than 40 cycles, which yielded thin films with a thickness near 23 nm. [51] Another interesting aspect of the ferrocenyl-carboxylate systems is the fact that the aliphatic chain that separates the ferrocene moiety from the carboxylate group allows the modulation of the oxidation potential of the ferrocene in the grafted film. For example, the films grafted by oxidation of 6ferroceneheptanoate show a symmetrical chemically reversible wave (0.413 V/SCE), approximately at the oxidation potential of pure ferrocene (0.392 V/SCE), whereas the films obtained by oxidation of ferroceneacetate and ferrocenecarboxylate exhibit oxidation-reduction waves at more positive potentials, 0.532 and 0.763 V/SCE respectively.…”

“…In the framework of the electrografting of carbon surfaces, examples of the use of redox catalysts are rather scarce. Until we know, there are no examples of reduction, however, such grafting possibility has been explored in the case of amines,, [45–47] alkynyl lithium compounds [48,49] and carboxylates [50,51] . The direct oxidation of these functional groups follows the potential order E amine >E alkynyl‐lithium >E carboxylate , and these potentials are generally found between 0.8 and 1.6 V/SCE.…”