Influence of Self-Assembling Redox Mediators on Charge
Transfer at Hydrophobic Electrodes
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Abstract
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 (<i>I</i><sub>a</sub>) and cathodic (<i>I</i><sub>c</sub>) peak
current densities of 18 ± 3 and 1.1 ± 0.1 μA/cm<sup>2</sup>, respectively, revealing substantial current rectification
(<i>I</i><sub>a</sub>/<i>I</i><sub>c</sub>= 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
(<i>I</i><sub>a</sub>/<i>I</i><sub>c</sub>= 1.7
and 2.3, respectively). Scan-rate-dependent measurements revealed <i>I</i><sub>a</sub> of FTMA to arise largely from the diffusion
of FTMA from bulk solution to the hydrophobic electrode whereas <i>I</i><sub>c</sub> 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/nm<sup>2</sup>). 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 <i>I</i><sub>a</sub>/<i>I</i><sub>c</sub> > 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
