Electrochemical response of vertically-aligned, ferrocene-functionalized mesoporous silica films: effect of the supporting electrolyte

The influence of the supporting electrolyte on the electrochemical response of ordered and oriented, ferrocene-functionalized, mesoporous silica films has been investigated. The ferrocene-functionalized films were prepared by electrochemically-assisted self-assembly, leading to azide-functionalized silica materials in a first step, which were then allowed to react with ethynylferrocene according to a Huisgen azide-alkyne cycloaddition to get the final films bearing various amounts of ferrocene groups covalently attached to the mesopore channels. In such organic-inorganic hybrid redox polymer, the charge transfer mechanism involves electron hopping between adjacent redox sites and transport of electrolyte ions through the film to maintain charge balance (i.e., ingress of anions to compensate the positive charges generated by the oxidation of ferrocene moieties). The electrochemical behavior of the ferrocene-functionalized films is thus likely to be affected by the density of redox sites in the material as well as the supporting electrolyte nature and concentration. This has been analyzed here, showing that electron transfer processes are favored in mesopore channels containing great amounts of ferrocene groups, in an electrolyte-rich medium, and for counter-anions of small size and big charge. Electron transfer kinetics are primarily governed by the density of redox sites in the material while the intensity of voltammetric signals is mainly controlled by the transport of counter-anions in the confined space of mesopore channels.