Vol.3, No.1, 39-43 (2012) Journ al of Biophysical Chemistry
Electrochemical investigation on the film of
L-cysteine self-assembled to nanoparticles
on a gold electrode
Wenting Wang1, Chunming Wang1*, Xiaoquan Lu2*
1Department of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, China;
*Corresponding Author: wangcm@lzu.edu.cn
2Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu Province, College of Chemistry & Chemical Engineer-
ing, Northwest Normal University, Lanzhou, China; *Corresponding Author: luxq@nwnu.edu.cn
Received 27 October 2011; revised 10 December 2011; accepted 21 December 2011
The film contained L-cysteine and gold nanopar-
ticles were provided by self-assembled monola-
yers (SAMs) and potentiostatic electrodeposi-
tion technology on the gold electrode. Two
methods were used to study the film: In the first,
cyclic voltammetr y (CV) was used to inspect the
functional groups of the film and the same time
hydroquinone was chosen to be a probe mole-
cule in the based solution; secondly, based on
analytical technology of scanning electrochemi-
cal microscopy (SECM), the heterogeneous rate
constant (keff) bet ween solid phase (the modified
electrode) and liquid phase (K3Fe(CN)6) was ob-
tained. As a result, the better binary catalysis of
hydroquinone was demonstrated and the het-
erogeneous rate constant (keff) is the greater at 8
h for L-cysteine self-assembled monolayers (SAMs).
Keywords: S E CM; L-Cysteine; Hydroquinone;
Self-Assembled Monolayers; Heterogeneous Rate
As well-known, L-cysteine of the favorable electro-
chemical activity is a common amino acid in the living
creature. Because of the special functional end groups,
such as -SH, -COOH and -NH2, it attracted many re-
searchers in different fields deeply. Nano-material has
been a hot topic recently. The specific surface area of the
modified electrode with nanoparticles by electrochemical
technology is larger than that of bare electrode [1]. The
new structure of L-cysteine on the Au (111) surface was
investigated by situ electrochemical scanning tunneling
microscopy [2,3]. Surface-enhanced Raman spectros-
copy (SERS) and surface-enhanced second-harmonic
generation (SESHG) were employed to investigate the
adsorption of L-cysteine on a polycrystalline silver elec-
trode [4]. What matters most was that self-assemble
monolayers (SAMs) was a simple and stable method in
the study of modified electrode, and it can obtain spon-
taneously high-order monolayer on gold via the thiol end
group [5-8]. The electrode modified directly with L-
cysteine SAMs can investigate various performances,
including the following several aspects, voltammetric
behavior of vitamin B2, selective response of dopamine in
ascorbic acid and electrochemical behavior of epineph-
rine [9-11]. Previously, the modified electrode with the
biochemical molecular L-cysteine was usually used to
catalyze the quinones in the water solution. However, the
research on the electrode modified with nanoparticles
and L-cysteine is seldom advanced. Moreover, hydro-
quinone, as electron carrier in photosynthesis and respi-
ration, plays a key role in biological energy metabolism.
The electrochemical process of quinone is usually invest-
tigated by cyclic voltammetry (CV) at glassy carbon
electrode [12,13], and electroactive quinone as terminal
group is immobilized on electrode to investigate the
well-know electron transport (ET) [14,15]. As an elec-
trochemical technology, scanning electrochemical mi-
croscopy (SECM) is a crucial tool to study electron
transfer (ET) at solid/liquid interface, liquid/liquid inter-
face, and it has several advantages over cyclic voltam-
metry (CV), such as allowing more informations to be
extracted from a single measurement [16-18].
Here, we first modified the gold electrode with the
gold nanoparticles by potentiostatic electrodeposition
and then applied self-assembly method to modify L-
cysteine on the gold nanoparticles. SECM was employed
to observe heterogeneous phase ET reaction between the
electrode and mediator. In the presence of hydroquinone
as a probe molecule, CV was used to investigate electro-
chemical response of the film, which consisted of gold
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W . T. Wang et al. / Journal of Biophysical Chemistry 3 (2012) 39-43
nanoparticles and L-cysteine on the gold electrode,
2.1. Chemicals
L-cysteine was received from the Factory of Caoyang
second middle school (Shanghai, China). Hydroquino-
neand HAuCl4·4H2O was from Fucheng Chemical Re-
gent Factory (Tianjin, China), Sinopharm Chemical Re-
gent Co., Ltd (Shanghai, China), respectively. All other
reagents were analytical-reagent grade, unless otherwise
specified. Solutions were prepared from water that had
been purified through an Ultra-pure water system Milli-
Q Plus (Millipore).
2.2. Preparation of Substrate and SAMs
A gold electrode (Diameter = 2 mm) was used as the
substrate for these experiments. The electrode was pol-
ished to a mirror finish using 0.05 μm alumina powder
before careful rinsing with deionized (DI) water, sonica-
tion in absolute ethanol and DI water for 10 min in turn,
and then dried with nitrogen gas. The gold electrode was
modified with gold nanoparticles by potentiostatic elec-
trodeposition at –0.2 V in 3 mM HAuCl4 solution [1].
SAMs was prepared with a solution of 1 mM L-cysteine
in phosphate-buffered solution (pH = 7).
2.3. Electrochemical Measurements
Electrochemical experiments were carried out using a
CHI 900 scanning electrochemical microscope (CH In-
struments Co. Ltd, Austin, USA) with a four-electrode
cell. The gold (or modified gold) electrode, a platinum
wire, and a KCl saturated Ag/AgCl electrode were used
as working, counter and reference electrode, respectively.
The SECM tip was a 25-m diameter Pt ultramicroelec-
trode (UME). Before each experiment, the tip was pol-
ished with 0.3-m alumina and rinsed with DI water.
3.1. Electrochemical Characterization of the
Electrochemical means of cyclic voltammetry (CV)
have been performed to characterize the SAMs. Figure
1(A) showed CVs for 1 mM K3Fe(CN)6 containing 0.1
M KCl at a) a clean gold electrode, b) a gold electrode
modified with nano gold, c) L-cysteine self-assembled to
the nanogold electrode.
For the bare Au electrode, the typical CV response
shape of ferricyanide was performed. The peak separa-
tion of 70 mV showed a reversible voltammogram for
the redox couple, indicating a diffusion-limited or elec-
trochemical quasi-reversible one electron redox process.
Figure 1. (A) Cyclic voltammograms in 1 mM
K3Fe(CN)6 at scan rate 100 mV/s. Insert: Cyclic
voltammograms in 1 M H2SO4 at scan rate 50
mV/s. (B) Impedance spectra corresponding in 1
mM K3Fe(CN)6 with 0.1 M KCl as supporting
electrolyte. Insert: The equivalent circuit model
used to obtain equations for Zre and Zim. W.E is
the working electrode, R.E is the reference elec-
trode, and C.E is the counter electrode.
When the bare gold electrode was modified with gold
nanoparticles, the peak current increased obviously in the
solution of either 1 mM K3Fe(CN)6 or 1 M H2SO4. Com-
pared with the bare and nanogold electrode, the electrode
modified with gold nanoparticles and L-cysteine showed
the better reversible CV response in K3Fe(CN) 6. This
may be the protonation of -NH2 in L-cysteine and the
electrostatic interaction with K3Fe(CN)6 [19]. Figure 1(B)
showed the electrochemical impedance spectroscopy
(EIS) of Nyquist plots. Among them, a) represented the
EIS curve of the bare gold electrode, where had a straight
line at low-frequency and a small semicircle at a high-
frequency region. The phenomenon demonstrated that
the process was essentially diffusion-controlled for the
redox couple. The electrode modified with gold nanopar-
ticles was used as the working electrode in b). Different
from the above dates, c) indicated that L-cysteine had
been self-assembled on gold electrode modified with
gold nanoparticles previously.
Copyright © 2012 SciRes. OPEN ACCESS
W . T. Wang et al. / Journal of Biophysical Chemistry 3 (2012) 39-43 41
3.2. Heterogeneous Rate Constant (keff) by
Scanning Electrochemical Microscopy
Figure 2 shows the schematic diagram of the electro-
chemical cell and the SECM tip.
The 25 m diameter Pt ultramicroelectrode (UME),
which is made by our own laboratory, were used to in-
vestigate the modified gold electrode surface and record
the feedback approach curve at the different concentra-
tion of the mediator solution (K3Fe(CN)6). In the SECM
experiments, the modified electrode was used as the sub-
strate. Because the terminal groups contain -COOH and
-NH2, we modified electrode with COOH-terminated
alkanethiol for the distinction. It was found that the feed-
back approach curve of SAM with COOH-terminated
alkanethiol was negative from 0.5 V to 0.7 V (the sub-
strate potential (Es)) vs Ag/AgCl with 1 mM 6
in 0.1 M KCl, –0.1 V (UME tip potential) vs Ag/AgCl in
Figure 2 (Inser), which was consistent with the previous
report [4]. However, the feedback curve for L-cysteine
SAMs was just positive at 0.5 V (the substrate potential
(Es)) vs Ag/AgCl with 1 mM 6 in 0.1 M KCl,
–0.1 V (UME tip potential) vs Ag/AgCl, which was at-
tribute to the terminal group: -NH2 in the L-cysteine
SAMs on the modified electrode with the nano gold. For
this reason, we conjectured that -NH2 in the L-cysteine
SAM was the key role in the ET process between the
modified electrode and the mediator (K3Fe(CN)6). Table
1 showed the heterogeneous rate constant (keff) of the
modified gold electrode with the different assembling
time by L-cysteine.
Figure 2. Scheme of the modified electrode and the setup
by SECM. Insert: experimental approach curves.
Table 1. Heterogeneous rate constant (keff) of SAMs with L-
cysteine on modified electrode with gold nanoparticles.
Assembling time (h) 2 3 4 5 6 7 8
keff (10–3) cm/s 6.0 7.0 6.4 6.8 6.6 6.6 7.1
The heterogeneous rate constant (keff) was the greater
at 3 h and 8 h. During the initial stage of self-assembling,
the -SH of L-cysteine by Au-S bond attached to the gold
nanoparticles in major. The coverage of L-cysteine on
gold nanoparticles was relatively large at 3 h and Au-S
bond was the main factor. Subsequently, due to the con-
nection between -NH2 and the gold nanoparticles, the Au-
NH bond may replace the Au-S bond in a certain degree.
The film, including the gold nanoparticles and L-cysteine,
reached the new equilibrium at 8 h and performed the
greater heterogeneous rate constant (keff) once again.
3.3. Cyclic Voltammetry (CV) with
Hydroquinone as a Probe Molecule
Compared to the previous work, the research on the
self-assembled time was rarely mentioned. Here, at the
same modified condition, we paid the more attention on
the assembling time of SAMs.
Figure 3 showed the cyclic voltammograms of hy-
droquinone modified electrode at different assembling
time in acetate buffer solution (pH = 3.8). Obviously, in
the first stage (t = 2 h, 3 h, 4 h), the oxidation peak had a
negative shift and peak current increased at 3 h (Figure
3(A)). In the next stage (t = 5 h, 6 h, 7 h, 8 h), we found
that the better change at 8h and the oxidation peak had a
negative shift and peak current evidently increased (Fig-
ure 3(B)). In the further research, we have chosen the
three different assembling times, 3 h, 5 h, 8 h (Figure 3
(C)). It showed the oxidation peak position of 3h and 8h
were approximately same. However, the potential differ-
ence of 8h was the smaller than that of 3 h. Figure 4
showed the three possible SAMs and the reaction with
the hydroquinone, where L-cysteine was self-assembled
in the phosphate buffer solution (pH = 6.8).
Figure 3. Cyclic voltammograms of the modified gold elec-
rode at different assembling time.
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W . T. Wang et al. / Journal of Biophysical Chemistry 3 (2012) 39-43
Copyright © 2012 SciRes.
Figure 4. Three different possible SAM and the reaction of hydroquinone.
Because of the existence of the -COOH and -NH2, the
close to neutral solution was chosen for avoiding the
effect of pH. Moreover, we supposed that double SAMs
of L-cysteine may be formed at the appropriate pH con-
ditions, namely close to the neutral solution. As above
discussion, there were three kinds of the terminal groups
in SAMs of L-cysteine: -NH2,-COOH and -SH, and in
the assembling process, the three situations didn’t present
separately. The CVs of modified electrode in hydro-
quinone as probe molecule accorded with the heteroge-
neous rate constant (keff) between the film and the me-
diator solution. That strongly suggestted quinone could
be applied as the probe molecule, and further to learn the
film containing some special functional groups on the
sis of CVs, it was found that L-cysteine performed the
better electrochemical response at 8 h (self-assembling
time) with hydroquinone as a probe molecule in acetate
buffer solution (pH = 3.8). The heterogeneous rate con-
stant (keff) obtained from the feedback approach curves of
SECM was aslo greater at 8h. In the low pH-value solu-
tion, the functional group which reacted with hydro-
quinone was mostly -NH2, and -COOH by the free hy-
drogen ion. We confirmed that what was the optimal as-
sembling time, and which functional group mainly par-
ticipated in the reaction. Moreover, the modified elec-
trode with the film performed the binary catalysis in
acetate buffer solution of hydroquinone.
3.4. Binary Catalysis of the Modified
Electrode This work was supported by the Natural Science Foundation of
China (No. 20775060, 20875077), the Key Project of Scientific Re-
search Base of education department (08zx-07) and the Key Laboratory
of Ploymer Materials of Gansu Province.
Figure 5 showed the cyclic voltammograms of hy-
droquinone in acetate buffer solution (pH = 3.8) at the
different modification on gold electrode. It was worth to
point out that the gold electrode, which was modified
with gold nanoparticles and then self-assembled with L-
cysteine, performed the preferable capability of catalysis
for hydroquinone. From a view point of catalysis, this
catalytic reaction should be called binary catalysis, that
meant both nano gold and L-cysteine participated the
binary catalysis reaction.
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