Surface-Bound Molecular Rulers for Probing the Electrical Double Layer

Abstract

Herein, we report the first experimental investigation on the effect of varying the position of redox-active moieties, within the electrical double layer, on the apparent formal potential and on the electron transfer rate constant. This was achieved using a rigid class of molecules, norbornylogous bridges, to place redox species (ferrocene) at a fixed position above the surface of the electrode. Cyclic voltammetry and alternating current voltammetry were used to calculate the apparent formal potential and the electron transfer rate constant for the electron transfer between the ferrocene and the gold electrode. We use the effect of electric field on the apparent formal potential measurement of the surface-bound redox species to calculate the potential drop from the initiation of the electrical double layer to different distances above it. It was found that self-assembled monolayers formed from ω-hydroxyalkanethiol have a potential profile very similar to that described by classical theories for bare metal electrodes. A steep drop in potential in the Stern layer was observed followed by a smaller potential drop in the Gouy-Chapman layer. The electron transfer rate constant was found to decrease as the distance between the ferrocene moiety and the initiation of the double layer is increased. Thus, the electron transfer rate constant appears to be dependent on ion concentration.

Probing the Effect of the Solution Environment around Redox-Active Moieties Using Rigid Anthraquinone Terminated Molecular Rulers

“…By changing the length of the diluent, the redox species are essentially moved through the Stern and diffuse portions of the electrical double layer. 5 In that previous study, we showed that SAMs composed of ω-hydroxyalkanethiol has a potential profile similar to that proposed for bare unmodified metal surfaces i.e. a steep drop in potential in the Stern layer followed by a smaller potential drop in the diffuse layer or the Gouy-Chapman layer which displays an approximately exponential decay in potential out to the bulk electrolyte.…”

“…For instance, using conventional alkanethiol as the base layer of redox active monolayers, the redox species are located at the organic layer-solution interface, which is essentially within the electrical double layer. [3][4][5] In an electrochemical experiment, the redox active species will not feel the potential difference between the working electrode and the reference electrode but something in between as determined by the nature of the electrical double layer. The effect of the electrical double layer on surface-bound redox active groups was first proposed by the theory of interfacial potential distribution of Smith and White 3 and expanded by Fawcett.…”

“…3 The term that is independent of surface coverage in equation 1, is not the formal potential, as in a classical Nernst formula, but We have recently experimentally verified this theory using rigid ferrocene-terminated norbornylogous (NB) bridges that served as molecular rulers for probing the electrical double layer. 5 The importance of rigidity of the NB bridge is that the position of the redox species above the surface of the diluent, and hence its position in the electrical double layer, can be precisely controlled. By changing the length of the diluent, the redox species are essentially moved through the Stern and diffuse portions of the electrical double layer.…”

“…a steep drop in potential in the Stern layer followed by a smaller potential drop in the diffuse layer or the Gouy-Chapman layer which displays an approximately exponential decay in potential out to the bulk electrolyte. 5 The electron transfer rate constant (k et ) was found to decrease as the distance between the ferrocene moiety and the initiation of the double layer is increased. Such a 4 variation in k et is expected for redox active groups which undergoes strong ion-pairing with counterions during the redox reaction, such as ferrocene.…”

The effect of supporting electrolyte on the electron transfer at mixed self-assembled monolayers containing ferrocene moieties

“…Such Fc-terminated SAM modified electrodes show a one-electron redox reaction: Fc ⇌ Fc + + e − . So far, many reports have been published on electron-transfer kinetics [5][6][7][8], electric double-layer effects [9,10], application as an electron mediator [11] and also biosensing [12][13][14]. In these studies, however, Fc moieties were usually exposed on the surface of SAM.…”