Electron transfer between ferrocene

Bioelectrochemistry and Bioenergetics 48 Ž1999. 243–247
Short communication
Electron transfer between ferrocene-modified Auroctadecanethiolrlipid
BLM electrode and redox couples in solution
Xiaoli Cui
a,1
, Dianlu Jiang
a
b
b,)
, Peng Diao a , Junxin Li b, Ruting Tong b, Xinkui Wang
a
Institute of Coal Chemistry, Chinese Academy of Science, Taiyuan 030001, China
Department of Chemistry, Hebei Teacher’s UniÕersity, Shijiazhuang 050016, China
Received 15 July 1998; revised 28 September 1998; accepted 5 October 1998
Abstract
Bilayers incorporated with ferrocene consisting of self-assembled octadecanethiol and lipid monolayer on gold substrates were
fabricated. Its electrochemical behaviors in solutions containing different redox couples were investigated by cyclic voltammetry and ac
impedance. The transmembrane electron transfer reaction across octadecanethiol self-assembled film and an adsorbed phospholipid layer
mediated by ferrocene have been observed in the solution of FeŽCN. 63yr4y. The formal potential difference between mediator in bilayer
lipid membrane ŽBLM. and redox couple in solution has a great impact on the transmembrane electron transfer behavior. The
ferrocene-modified BLM electrodes might be useful for constructing a bilayer-based electrochemical current rectifying device. q 1999
Elsevier Science S.A. All rights reserved.
Keywords: Bilayer membrane; Ferrocene; Cyclic voltammetry; Impedance spectroscopy; Transmembrane electron transfer; Current rectification
1. Introduction
Recently there has been a considerable interest in transmembrane electron transfer reaction across self-organized
structures since the understanding of membrane redox
mechanism is a central problem in bioenergetics. Lymar
and Hurst w1x, Kuhn and Hurst w2x and Hammarstrom et al.
w3,4x have studied the transmembrane electron transfer
reaction between viologen inside vesicles and reductants
outside vesicles. Researches with vesicles do not allow a
simple control of the applied potential and hence, limits
the use of well-established electrochemical techniques.
The electron transfer across biomembranes is central to
vital processes such as photosynthesis and mitochondrial
respiration, and has been the focal point of many bilayer
lipid membrane ŽBLM. studies. There have been a few
electrochemical investigations on the study of the transmembrane electron transfer. Using a planar BLM, Cheng
et al. w5x have found that the transmembrane electron
)
Corresponding author. E-mail: [email protected]
Present address: Department of Chemistry, Hebei Teacher’s University, Shijiazhuang 050016, China.
1
transfer rate was dependent on the redox potential of the
bathing solutions studied by the method of cyclic voltammetry. The determining step is the transfer of the semiubiquinone radical across the hydrophobic interior of the
membrane, instead of charge transfer reactions at the membranerelectrolyte interface. Yamada et al. w6x also used a
planar BLM incorporated with 7,7,8,8-tetracyanoquinodimethane ŽTCNQ. to study transmembrane electron
transfer by ac impedance spectroscopy. They found that
the heterogenous electron-transfer rate was determined by
three factors such as hydrophobic interaction between the
interior of the BLM and the redox species, the electrostatic
interaction between the negatively-charged surface and the
redox species in solution, the difference in formal potential
between TCNQ and the redox species in solution.
However, the fragility of conventional planar BLM
limits its applications for developing device of practical
use and validity as a media for scientists to work on. Many
attempts have been made to prepare synthetic bilayers
using a solid electrode support. For instance, Martynski
and Tien w7x and Wardak and Tien w8x reported the method
of formation of self-assembled lipid bilayers on solid
substrates Žs-BLM. which have good mechanical stability.
Unfortunately, the s-BLM prepared by the method in Refs.
0302-4598r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.
PII: S 0 3 0 2 - 4 5 9 8 Ž 9 8 . 0 0 2 0 2 - 5
244
X. Cui et al.r Bioelectrochemistry and Bioenergetics 48 (1999) 243–247
w7,8x may often have defects which make the whole process very complicated. Since the overall electron transfer
takes place not only by the mediator in the bilayer, but also
by the direct electron transfer of redox couples in solution
at the defects. In order to obtain a simple model, we
adopted the method of Plant et al. w9x, using self-assembled
layers of octadecanethiol on gold as a phospholipid support to fabricate s-BLM.
The main objective of the present work was to study the
influence of formal potential match extent between the
mediator in membrane and the redox couple in solution on
the mechanism of transmembrane electron transfer. Because of its hydrophobicity, ferrocene was chosen as mediators for electron transfer across the membrane consisting
of self-assembled alkanethiol and lipid monolayers on gold
substrates. Cyclic voltammetry and ac impedance measurements shows that the formal potential of the redox couples
in the solution has a great impact on the electron transfer
rate. When the disparity in the formal potential of redox
couples in solution and mediator in BLM is large enough,
the current rectification behavior of voltage–current plot is
observed. The ferrocene-modified BLM electrode might be
used to construct an electrochemical current rectifier.
2. Experimental
Octadecanethiol was a generous gift from Mr. ZhongFan Liu ŽPeking University., used without further purification. Phospholipid was purchased from Sigma.
CoŽphen. 32qr3q was synthesized by the method in the
literature w10x. All other chemicals were of reagent grade
and used as received. Aqueous solutions were prepared
with deionized–distilled water.
Cyclic voltammetry and electrochemical impedance
spectroscopy were performed with an ac impedance system ŽEG & G, Princeton Applied Research, Model 388.
that includes a potentiostatrgalvanostat ŽModel 273., a
two-phase lock-in analyzer ŽModel 5208. and a 486 computer. For ac impedance measurements a 5-mV amplitude
sine wave was applied to the electrode under potentiostatic
control and the frequency range was from 0.011 Hz to 100
kHz. For cyclic voltammetry, M270 software was used.
All electrochemical experiments were carried out in a
conventional three-electrode system. The saturated calomel
electrode ŽSCE. was used as reference electrode and
large-area Pt electrode as counter electrode. All potentials
reported are referred to SCE. The electrode area was 0.05
cm2 . All experiments were carried out at room temperature
Ž20 " 28C. in solutions free from oxygen by bubbling with
nitrogen and containing equal concentration of redox
species.
Bilayer structure was prepared according to the literature w9x, using self-assembled layers of octadecanethiol on
gold as a phospholipid support. The gold substrate was
obtained by vacuum evaporation of high-purity gold
Ž99.99%. onto clean single-crystal silicon, which had been
precoated with chromium to improve adhesion. Gold substrates were treated with ‘piranha solution’ Ž3:7 vrv H 2 O 2
Ž30%.rH 2 SO4 . for 10–15 min at room temperature, rinsed
with deionized–distilled water and ethanol sequentially.
After this treatment, the gold substrates were immersed in
deposition solution of 0.1 mM octadecanethiol in ethanol
solutions for at least 24 h. After immersion, the electrode
was washed with ethanol and deionized–distilled water
and dried with a stream of nitrogen.
In our experiments, in order to avoid uncertainties in
the electrochemical responses, the octadecanethiol layer
must be structurally well-ordered and relatively defect-free.
Before the fabrication of outer layer of phospholipid,
electrochemical examination of the octadecanethiol monolayer is necessary. It should inhibit the electrochemical
reaction of dissolved ferricyanide species. Substrates, which
have a large number of pinhole defects in the monolayer,
were deemed unusable.
After the above check-up, the electrode was immersed
in deionized–distilled water for about 2 h and washed with
distilled–deionized water and dried with a stream of nitrogen. Immediately, some lipid solution saturated with ferrocene was dropped on the surface of octadecanethiol and
transferred to a bathing solution of 0.1 M KCl at once.
Because of the hydrophobicity of ferrocene, the ferrocene
moiety was located at the hydrophobic region of the
bilayer. The membrane capacitance was monitored by ac
impedance spectroscopy. After about 12 h spontaneous
thinning of the lipid layer, the bilayer was assumed as
satisfactory if its capacitance was close to 0.5 mF cmy2 , a
series of electrochemical measurements were carried out.
3. Results and discussion
Fig. 1 shows a series of cyclic voltammograms of
ferrocene-modified BLM electrode in a solution of 0.1 M
KCl. Fig. 1a was obtained at a bare gold electrode in 1
mM FeŽCN. 63yr4yq 0.1 M KCl solution. A couple of
well-defined waves of FeŽCN. 63yr4y is shown. The
voltammogram b in Fig. 1 was obtained at an unmodified
BLM electrode in a solution containing 1 mM
FeŽCN. 63yr4y. The absence of voltammetric features corresponding to FeŽCN. 64y oxidation and FeŽCN. 63y reduction
reflects the excellent barrier properties of the BLM, namely,
direct electron transfer across the membrane is impossible
because it is too remote for electroactive species to exchange electrons with the electrode. The almost same
cyclic voltammograms could be obtained in the solution of
CoŽphen. 32qr3q and Fe 2qr3q. Fig. 1c was obtained at the
same ferrocene-modified BLM electrode, but now in contact with a solution of 0.1 M KCl. Almost no oxidative
current and very small reductive current are observed in
X. Cui et al.r Bioelectrochemistry and Bioenergetics 48 (1999) 243–247
245
The ferrocene groups are buried between the SAMs and
the lipid membrane, both the oxidative current and the
reductive current are inhibited in the cyclic voltammetric
response in the KCl solution, resulting from the impermeability of the counter-ion as a consequence of the very
dense packing of the membrane and no redox species
exchanging electrons with ferrocene. With the addition of
FeŽCN. 63yr4y to the solution, a inverse sigmoidal cyclic
Fig. 1. Cyclic voltammograms of the bare Au electrode Ža. and the
modified Auroctadecanethiolrlipid BLM electrodes Žno ferrocene. Žb. in
the solution of FeŽCN . 63yr4y, the ferrocene-modified Auroctadecanethiolrlipid BLM electrode in the solution of 0.1 M KCl Žc.. The
scan rate is 0.1 Vrs.
the cyclic voltammetric response because the counter ion
is not easy to go into or out the BLM, which not only
agree with the properties of BLM but agree with the
experimental results of the LB monolayers of 16-ferrocenylhexadecanoic acid ŽFCAC. on a self-assembled alkanethiol monolayer as well w10x. The absence of Faradaic
currents is not due to the absence of the FCAC monolayer
but rather to the blocking effect of the LB monolayer. In
our experiments, analogously, the voltammetric waves of
the ferrocene groups in hydrophobic region now become
completely inhibited because of the highly hindered counter
ion permeation into the BLM.
In order to expound the electron transfer mechanism
between ferrocene-modified Auroctadecanethiolrlipid
BLM electrode and redox species in solution, a series of
cyclic voltammetries were carried out in different redox
solution ŽFig. 2a–c..
Fig. 2. Cyclic voltammograms of the ferrocene-modified Auroctadecanethiolrlipid BLM electrode in the solution of different redox
couples Ža. in the solution of FeŽCN. 63yr4y, Žb. in the solution of
Fe 3qr2q, Žc. in the solution of CoŽphen. 32qr3q. The scan rate is 0.1 Vrs.
X. Cui et al.r Bioelectrochemistry and Bioenergetics 48 (1999) 243–247
246
voltammetric response is observed ŽFig. 2a.. This electron
transfer current observed in the solution of FeŽCN. 63yr4y
is mediated by ferrocenes. In this situation, the formal
potential for FeŽCN. 63yr4y Ž0.22 V vs. SCE. w6x are nearly
coincident with that of ferrocene Ž0.22 V vs. SCE. w11x,
both reductive and oxidative forms of the mediator in the
bilayer may exist at equilibrium according to reaction Ž1..
3y
4y
Fc Ž o . q Fe Ž CN . 6 Ž w . m Fcq Ž o . q Fe Ž CN . 6 Ž w .
Ž 1.
FeŽCN. 63yr4y
The mediated oxidation and reduction of
by FcrFcq both can occur, according to the following
mechanism depending on the applied voltage.
Anodic process:
q
y
Fc Ž o . s Fc Ž o . q e
Fcq Ž o . q Fe Ž CN .
4y
6
Ž 2.
Ž w . s Fc Ž o . q Fe Ž CN.
3y
6
Ž w.
Ž 3.
Cathodic process:
Fcq Ž o . q eys Fc Ž o .
Ž 4.
3y
4y
Fc Ž o . q Fe Ž CN . 6 Ž w . s Fcq Ž o . q Fe Ž CN . 6 Ž w .
Ž 5.
The current increases with the increasing concentration
of FeŽCN. 63yr4y shows that the whole reaction may be
determined by the reaction taken place at the interface.
When the formal potential of the mediator inside the
BLM is close to that of redox couples in solution, the
overlap of the distribution functions of redox species in
membrane and in solution is largest. The electron transfer
mediation of ferrocene is very effective in this case.
Shown in Fig. 2b is a cyclic voltammogram of the
ferrocene-modified BLM electrode in aqueous 0.1 M KCl
q Fe 2qr3q. It is different from above that only the reductive current can be observed. In this case, the formal
potential for Fe 2qr3q Ž0.77 V vs. SCE. is higher than that
of ferrocene Ž0.22 V vs. SCE.. The entire mediator in the
bilayer existed as an oxidative form, i.e., ferricenium
cation because of reaction Ž6..
Fc Ž o . q Fe 3q Ž w . ™ Fcq Ž o . q Fe 2q Ž w .
Fe 2qr3q indicates that the whole reaction may be determined by the reaction of Fc with Fe 3q.
Fig. 2c shows the cyclic voltammogram of the ferrocene-modified BLM electrode in aqueous 0.1 M KCl q
0.19 mM CoŽphen. 32qr3q. With the addition of
CoŽphen. 32qr3q, the whole cyclic voltammogram changes a
lot compared with the case of FeŽCN. 63yr4y and Fe 2qr3q.
In this situation, the formal potential for CoŽphen. 32qr3q
Ž0.14 V vs. SCE. w6x is lower than that of ferrocene Ž0.22
V vs. SCE., so the mediator in the bilayer existed mainly
in a reductive form, i.e., ferrocene group at equilibrium
according to reaction Ž11..
2q
3q
Fcq Ž o . q Co Ž phen . 3 Ž w . ™ Fc Ž o . q Co Ž phen . 3 Ž w .
Ž 11 .
Only the oxidative reaction is thermodynamically favorable according to reactions Ž12. – Ž13., so only oxidative
current can be observed.
Anodic process:
Fc Ž o . ™ Fcq Ž o . q ey
2q
Ž 12 .
3q
Fcq Ž o . q Co Ž phen . 3 Ž w . ™ Fc Ž o . q Co Ž phen . 3 Ž w .
Ž 13 .
Cathodic process Žno Fcq in the bilayer.:
Fcq Ž o . q ey/ Fc Ž o .
Fc Ž o . q Co Ž phen .
3q
3
Ž 14 .
Ž w . / Fcq Ž o . q Co Ž phen.
2q
3
Ž w.
Ž 15 .
Ž 6.
When the applied voltage is positive to the rest potential, the oxidative current is not observed because there is
no ferrocene in the bilayer. The mechanism can be represented with the following reactions.
Anodic process Žno Fc in the bilayer.:
Fc Ž o . / Fcq Ž o . q ey
q
Fc Ž o . q Fe
2q
Ž 7.
3q
Ž w . / Fc Ž o . q Fe Ž w .
Ž 8.
Cathodic process:
q
Fc Ž o . q ey™ Fc Ž o .
Fc Ž o . q Fe
3q
Ž 9.
q
2q
Ž w . ™ Fc Ž o . q Fe Ž w .
Ž 10 .
When the applied voltage is negative with respect to the
rest potential, the reductive current had been observed,
thereby providing the basis for a type of current rectification. The increase in current with the concentration of
Fig. 3. AC impedance spectroscopy of the ferrocene-modified Auroctadecanethiolrlipid BLM electrode in different concentrations of
FeŽCN. 63yr4y: 0.49 mM Ž(., 1.96 mM Ž Ø ., 3.84 mM Ž^., 5.66 mM
ŽI., 7.41 mM Ž'.. The frequency range was 0.011 Hz to 100 kHz. A 5
mV rms sinusoidal potential signal was applied to the electrode held at
the rest potential and five data points per decade were recorded.
X. Cui et al.r Bioelectrochemistry and Bioenergetics 48 (1999) 243–247
Because of the sufficient difference in formal potential
of redox species in membrane and in solution, the electron
transfer mediated by ferrocene between Au substrate and
solution is one direction compared with the cases of
FeŽCN. 63yr4y.
Fig. 3 shows the complex-plane impedance plot for the
ferrocene-modified BLM electrode in the solution of
FeŽCN. 63yr4y. The complex-plane impedance plot for the
BLM electrode in the solution of various concentration of
FeŽCN. 63yr4y exhibit nearly arc shapes. The impedance
decreased with the concentrations of FeŽCN. 63yr4y. In
contrast, the impedance spectroscopy at rest potential exhibit almost vertical lines and change little with the increase of concentration of Fe 2qr3q or CoŽphen. 32qr3q
reflecting that no reaction was taking place at this potential.
The above experiments have proven that the formal
potential of redox couples in the electrolyte plays an
important role in the electron transfer between ferrocenemodified Auroctadecanethiolrlipid BLM electrode and
redox species in solution. When the disparity of the formal
potential is sufficient enough, the reaction in one direction
is inhibited, providing the basis for current rectification
that might be used to construct an electrochemical electronic device.
4. Conclusion
The transmembrane electron transfer across octadecanethiol self-assembled film and an adsorbed phospholipid layer mediated by ferrocene has been observed in the
solution of FeŽCN. 63yr4y. When the disparity of the formal
potential is sufficient enough, the reaction in one direction
is inhibited. The ferrocene-modified BLM electrodes might
be useful for constructing a bilayer-based electrochemical
current rectifying device. Further studies to obtain the
kinetic parameters and to clarify the detailed electron
transfer mechanism are under way in our laboratory.
247
Acknowledgements
The authors gratefully acknowledge the support of the
Science Foundation of Hebei Province.
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