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Influence of Self-Assembling Redox Mediators on Charge Transfer at Hydrophobic Electrodes

Smith, Timothy J., Wang, Chenxuan, Abbott, Nicholas L.
Langmuir 2015 v.31 no.39 pp. 10638-10648
aqueous solutions, atomic force microscopy, bromides, electrodes, gold, hydrophobicity, mixing, oxidation, quartz crystal microbalance, surfactants
We report an investigation of the influence of reversible self-assembly of amphiphilic redox-mediators on interfacial charge transfer at chemically functionalized electrodes. Specifically, we employed (11-ferrocenylundecyl)-trimethylammonium bromide (FTMA) as a model self-assembling redox mediator and alkanethiol-modified gold films as hydrophobic electrodes. By performing cyclic voltammetry (CV, 10 mV/s) in aqueous solutions containing FTMA above its critical micellar concentration (CMC), we measured anodic (Iₐ) and cathodic (Ic) peak current densities of 18 ± 3 and 1.1 ± 0.1 μA/cm², respectively, revealing substantial current rectification (Iₐ/Ic= 17) at the hydrophobic electrodes. In contrast, hydroxymethyl ferrocene (a non-self-assembling redox mediator) at hydrophobic electrodes and FTMA at bare gold electrodes, yielded relatively low levels of rectification (Iₐ/Ic= 1.7 and 2.3, respectively). Scan-rate-dependent measurements revealed Iₐ of FTMA to arise largely from the diffusion of FTMA from bulk solution to the hydrophobic electrode whereas Ic was dominated by adsorbed FTMA, leading to the proposal that current rectification observed with FTMA is mediated by interfacial assemblies of reduced FTMA that block access of oxidized FTMA to the hydrophobic electrode. Support for this proposal was obtained by using atomic force microscopy and quartz crystal microbalance measurements to confirm the existence of interfacial assemblies of reduced FTMA (1.56 ± 0.2 molecules/nm²). Additional characterization of a mixed surfactant system containing FTMA and dodecyltrimethylammonium bromide (DTAB) revealed that interfacial assemblies of DTAB also block access of oxidized FTMA to hydrophobic electrodes; this system exhibited Iₐ/Ic > 80. These results and others reported in this paper suggest that current rectification occurs in this system because oxidized FTMA does not mix with interfacial assemblies of reduced FTMA or DTAB formed at hydrophobic electrodes. More broadly, these results show that self-assembling redox mediators, when combined with chemically functionalized electrodes, offer the basis of new principles for controlling charge transfer at electrode/solution interfaces.