The electron transfer (ET) rate at the interface between two
immiscible electrolyte solutions was probed
as a function of the driving force and distance between redox centers
by scanning electrochemical microscopy. The
adsorption of phospholipids at the interface resulted in a decrease in
the rate of interfacial ET between aqueous
redox species and the oxidized form of zinc porphyrin in benzene.
The fraction of the interfacial area covered with
lipid (θ) was evaluated from the measured heterogeneous rate
constants (k
f). The dependence of θ vs
lipid
concentration in benzene fit a Langmuir isotherm. For complete
monolayers of phospholipids, k
f was a function
of
the number of methylene groups in a hydrocarbon chain. The driving
force dependencies of interfacial ET rates
(Tafel plots) were measured for several aqueous redox couples.
They were linear, with a transfer coefficient of α
≅ 0.5 when the driving force for ET (ΔG°) was not too
high, in agreement with Marcus theory, and leveled off to
the diffusion-controlled rate at larger overpotentials. For even
higher ΔG° and for the first time for
heterogeneous
ET at a polarizable interface, inverted region behavior was
observed.
The potential drop across the interface between two immiscible
electrolyte solutions (ITIES),
φ,
can be
quantitatively controlled and varied by changing the ratio of
concentrations of the potential-determining ion
in the two liquid phases. This approach was used to study the
potential dependence of the rate constant for
electron transfer (ET) at the ITIES (k
f) by
scanning electrochemical microscopy (SECM) with no
external
potential bias applied. The Tafel plot obtained for ET between
aqueous Ru(CN)6
4- and the
oxidized form of
zinc porphyrin in benzene was linear with a transfer coefficient, α
= 0.5, determined from the slope of a plot
of ln k
f vs
φ,
in agreement with conventional ET theory. The observed change in
the ET rate with the
interfacial potential drop cannot be attributed to concentration
effects and represents the potential dependence
of the apparent rate constant. This result is discussed in
relation to the interface thickness and structure.
The
SECM was also used to study solid phase formation at the interface at
high concentrations of supporting
electrolyte (tetrahexylammonium perchlorate, THAClO4) in
benzene. The precipitation of the THA+
and
Ru(CN)6
4- compound occurred
when its solubility product was exceeded. This process leads to
the formation
of a thin three-dimensional interfacial layer, which can be
unambiguously distinguished from monolayer
adsorption. The approach curve analysis yields the composition of
such a layer. Its thickness can also be
probed.
Spherical ultramicroelectrodes with diameters of 1-30 μm have been prepared by self-assembly of Au nanoparticles and 1,9-nonanedithiol molecules at the tip end of glass micropipets. The electrodes were characterized by optical and scanning electron microscopy, cyclic voltammetry in aqueous and acetonitrile solution, and scanning electrochemical microscopy approach curves. A modified theory for hemispherical electrodes was used to compute the approach curves, which agreed with the experimental results. The construction strategy represents a bottom-up approach to the fabrication of microspherical electrodes.
The rates of electron transfer (ET) between two redox species through a monolayer of saturated dipalmytoyl phosphocholine and polyconjugated 2(3-(diphenylhexatrienyl)propanoyl)-1-hexadecanoyl-sn-glycero-3phosphocholine phospholipids adsorbed at the interface of two immiscible electrolyte solutions (ITIES) were measured by scanning electrochemical microscopy. Comparison of the ET rates shows that addition of phospholipids with conjugated hydrocarbon chains increases the ET rate by at least a factor of 2 compared to films with only saturated hydrocarbon chains. This difference was sufficiently high to obtain information about distribution of lipid molecules in monolayers formed by mixing two lipids. Lateral scanning of an ultramicroelectrode tip across the lipid monolayer comprised of two different phospholipids showed that each lipid forms microsize domains. The rate of ET across a monolayer of lipid adsorbed at the ITIES was also probed as a function of temperature. A sharp decrease in ET rate with temperature suggests a phase transition of the hydrocarbon chains of the lipid molecules. The phase transition increases the ET distance between the two redox centers with a resultant decrease in ET rate.
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