Molecular-level design of binary self-assembled monolayers on polycrystalline gold electrodes

Article abstract of DOI:10.1016/j.electacta.2003.12.046

In this study, we followed up, by measuring the reductive desorption patterns, the time-dependent growth of the self-assembled monolayer (SAM) of a thiol compound (typically cysteine) formed on polycrystalline Au (poly-Au) electrode. Cysteine molecules appeared to adsorb preferentially and consecutively at the Au(110), Au(100) and then at the Au(111) surface domains of the poly-Au. A 95% surface coverage of cysteine (Γ_cysteine) was attained after 5s, 120s and more than 300s for the Au(110), Au(100) and Au(111) domains, respectively, of the poly-Au electrode. The electrochemical reduction of molecular oxygen (O₂) in O₂-saturated 0.5M KOH was utilized as a probing reaction for the extent of the compactness of the SAM. A binary SAM of two thiols (cysteine and cystamine) has been successfully designed over the poly-Au electrode at which a precise positioning of cysteine was achieved, i.e., cysteine was attached to the Au(111) domains, while cystamine to the Au(100) and Au(110) domains. SEM images for the electrodeposited Ag over the cysteine sub-SAM/Au electrode gave a possible mapping of the Au(111) domains of the poly-Au electrode.

Full text of DOI:10.1016/j.electacta.2003.12.046

Electrochimica Acta 49 (2004) 2189–2194
Molecular-level design of binary self-assembled monolayers
on polycrystalline gold electrodes
Mohamed S. El-Deab1, Takeo Ohsaka∗
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering,
Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
Received 31 July 2003; received in revised form 11 December 2003; accepted 28 December 2003
Abstract
In this study, we followed up, by measuring the reductive desorption patterns, the time-dependent growth of the self-assembled monolayer
(SAM) of a thiol compound (typically cysteine) formed on polycrystalline Au (poly-Au) electrode. Cysteine molecules appeared to adsorb
preferentially and consecutively at the Au(1 1 0), Au(1 0 0) and then at the Au(1 1 1) surface domains of the poly-Au. A 95% surface coverage
of cysteine (Γ cysteine) was attained after 5 s, 120 s and more than 300 s for the Au(1 1 0), Au(1 0 0) and Au(1 1 1) domains, respectively, of the
poly-Au electrode. The electrochemical reduction of molecular oxygen (O2) in O2-saturated 0.5 M KOH was utilized as a probing reaction
for the extent of the compactness of the SAM. A binary SAM of two thiols (cysteine and cystamine) has been successfully designed over
the poly-Au electrode at which a precise positioning of cysteine was achieved, i.e., cysteine was attached to the Au(1 1 1) domains, while
cystamine to the Au(1 0 0) and Au(1 1 0) domains. SEM images for the electrodeposited Ag over the cysteine sub-SAM/Au electrode gave a
possible mapping of the Au(1 1 1) domains of the poly-Au electrode.
© 2004 Elsevier Ltd. All rights reserved.
Keywords: Binary SAMs; Partial reductive desorption; Submonolayer; Oxygen reduction reaction; Gold electrode
1. Introduction
A single-component SAM could be effectively and easily
fabricated at the gold electrode either by immersing the clean
gold electrode in a solution of the desired alkanethiol (in this
case the SAM is spontaneously formed [1,2]) or by printing
it onto the metal surface using a microstamp (microcontact
printing) [18].
The spontaneous chemisorption of alkanethiols on gold
electrodes (i.e., the formation of self-assembled mono-
layers (SAMs)) is a process of significant importance.
This phenomenon has been developed in extensive inves-
tigations aiming at the preparation, characterization, and
applications of a versatile range of SAMs of alkanethiols
or disulphides over coinage metals, particularly over gold
electrodes [1–14]. These SAMs possess highly organized
and stable structures at a molecular-level enabling a wide
applicability of these types of interfaces in several sensing
processes, e.g., as bioelectrochemical sensors at which the
biorecognition components could be immobilized at the
gold electrode through the self-assembly [15–17].
It has been reported that a multiple peak could be ob-
served during the reductive desorption of some thiols from
an Au substrate [19–28]. For instance, Porter and coworkers
[21,24] considered the appearance of a shoulder during the
reductive desorption of butanethiol from an as-evaporated
Au(1 1 1) film originated from step sites of the surface with
some Au(1 0 0) and Au(1 1 0) orientations. Furthermore,
Morin and coworkers [23] observed similar multiple re-
ductive desorption peaks during the reductive desorption of
nonanethiol self-assembled at poly-Au electrode and they
assigned the origin of the two major reductive desorption
peaks to the desorption of the thiol from the Au(1 1 1) and
Au(1 1 0) surface domains of the poly-Au electrode. On
the other hand, some authors [29–31] observed multiple
peaks during the reductive desorption of long chain alka-
nethiols self-assembled on Au(1 1 1) and they gave different
∗
Corresponding author. Tel.: +81-45-924-5404;
fax: +81-45-924-5489.
E-mail address: ohsaka@echem.titech.ac.jp (T. Ohsaka).
1
Permanent address: Department of Chemistry, Faculty of Science,
Cairo University, Cairo, Egypt.
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2003.12.046

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M.S. El-Deab, T. Ohsaka / Electrochimica Acta 49 (2004) 2189–2194
explanation that is based on the formation of a micelle of
the desorbed thiol species within the electrode double layer.
We have recently [32,33] reported that the potential-
controlled partial reductive desorption of the short alkyl
chain SAMs of thiols (like cysteine) formed over poly-
crystalline gold (poly-Au) could lead to the fabrication
of submonolayer coverage of the thiol over the poly-Au
electrode (sub-SAM/Au), at which the thiol molecules
selectively block the Au(1 0 0) and Au(1 1 0) domains
of the polycrystalline gold electrodes, while leaving the
Au(1 1 1) component bare (i.e., uncovered with the thiol).
The thus-fabricated sub-SAM poly-Au electrode was found
to behave like an Au(1 1 1) electrode, as shown by the re-
sponse towards the oxygen reduction reaction in alkaline
medium [32,33]. It has been also suggested that the uncov-
ered fraction (i.e., the Au(1 1 1)) could be further modified
by attaching another type of thiols [33], leading to the for-
mation of binary SAMs at which a precise positioning of
individual thiol species could be immobilized on specific
surface domains of the poly-Au electrode. The preparation
of phase-separated domains of mixed SAMs could lead to,
for example, the fabrication of a multianalyte sensing elec-
trode over which specific surface domains are electroactive
for certain species, while other surface domains are not.
In this paper, we report on the rate of the growth of the
SAM of cysteine on different surface domains of the poly-Au
electrodes. A binary SAM was fabricated, for the first time,
at poly-Au electrodes with a molecular-level design at which
individual thiol species were attached to specific surface do-
mains of the poly-Au electrode. A possible surface mapping
of the Au(1 1 1) domains of the poly-Au electrode is given.
lutions using Milli-Q water. Electrochemical measurements
were performed using a BAS 100 B/W electrochemical
analyzer. A conventional two-compartment, three-electrode
Pyrex glass cell was used. The current density was cal-
culated on the basis of the geometric surface area of the
ploy-Au electrodes. All the experiments were carried out
under nitrogen atmosphere. O2 gas was bubbled directly, if
necessary, into the cell for 30 min to obtain an O2-saturated
0.5 M KOH solution and during the measurements O2 gas
was flushed over the cell solution. All measurements were
performed at room temperature (25 1 ◦C). Scanning elec-
tron microscopy analysis of the electrodeposited Ag on
Au electrode was carried out using a JSM-T220 scanning
electron microscope (JEOL Optical Laboratory, Japan) at
an acceleration voltage of 15 kV and a working distance
of 4–5 mm.
3. Results and discussion
3.1. Estimation of the rate of growth of the SAM at
different surface domains of the poly-Au electrodes
It has been shown, based on the reductive desorption data,
that the poly-Au electrode surface is composed, mainly, of
2. Experimental
Poly-Au electrodes (1.6 mm in diameter, sealed in a
Teflon jacket) with an exposed geometric surface area of
2.01×10−2 cm2 were obtained from Bioanalytical Systems
Inc. (BAS) and were used as the working electrodes. A spi-
ral Pt wire and an Ag/AgCl/KCl (sat.) were the counter and
the reference electrodes, respectively. The poly-Au elec-
trodes were polished first with no. 2000 emery paper, then
with aqueous slurries of successively finer alumina powder
(down to 0.06 ␮m) and were sonicated for 10 min in Milli-Q
water. The poly-Au electrodes were then electrochemically
pretreated in 0.05 M H2SO4 solution by repeating the poten-
tial scan in the range of −0.2 to 1.5 V versus Ag/AgCl/KCl
(sat.) at 100 mV s−1 for 10 min or until the CV characteristic
for a clean Au electrode was obtained. A roughness factor
(r.f.) of 3.1 was estimated for the poly-Au electrode as cal-
culated from the charge consumed during the formation of
the surface oxide monolayer [34]. The thus-treated poly-Au
electrodes were immersed into an aqueous solution of the
desired thiol to prepare the self-assembled monolayer. Cys-
teine and cystamine were of analytical grade purity and were
used as received to prepare the aqueous thiols-containing so-
Fig. 1. CVs for the reductive desorption of SAMs of cysteine, in
N2-saturated 0.5 M KOH, formed on poly-Au electrodes immersed in
10 ␮M cysteine (aqueous solution) for (a) 5 s, (b) 10 s, (c) 20 s, (d) 30 s,
(e) 60 s, (f) 120 s, (g) 300 s, (h) 600 s and (i) 6.12 × 104 s. Potential scan
rate: 50 mV s−1
.

M.S. El-Deab, T. Ohsaka / Electrochimica Acta 49 (2004) 2189–2194
2191
three low-index crystallographic orientations, i.e., Au(1 1 1),
Au(1 0 0) and Au(1 1 0) [33,35]. In this section we would like
to confine our attention to the rate of growth and the compet-
itive chemisorption of a thiol (e.g., cysteine) on the differ-
ent surface domains of the poly-Au, with the understanding
that the thiol molecules exhibit different binding strengths
towards each surface domain of the poly-Au leading to
the appearance of the multiple reductive desorption peaks.
Fig. 1 shows the cyclic voltammograms (CVs) measured in
N2-saturated 0.5 M KOH at a scan rate of 50 mV s−1, corre-
sponding to the reductive desorption patterns of the SAMs
of cysteine formed on poly-Au electrodes during different
immersion times in 10 ␮M cysteine (aqueous solution). In-
spection of Fig. 1 reveals several interesting features regard-
ing the growth of the SAM of cysteine at the Au(1 1 1),
Au(1 0 0) and Au(1 1 0) domains of the poly-Au:
1. Cysteine molecules start to quickly attach to the Au(1 1 0)
and the Au(1 0 0) surface domains within few seconds
(5 s), as can be seen from the reductive desorption peaks
observed at −1100 and −1000 mV versus Ag/AgCl,
respectively, while no peak was observed at −700 mV
[33,35].
2. The full intensity of the reductive desorption peak ap-
peared at −1100 mV (corresponding to the Au(1 1 0)
domain) was attained after 10 s (see Fig. 1, curve b),
while that observed at −1000 mV (corresponding to the
Au(1 0 0) domain) reaches its full intensity after ca. 120 s
(see Fig. 1, curve f). On the other hand, the full intensity
of the peak observed at −700 mV (corresponding to the
Au(1 1 1) domain) was attained after 600 s (see Fig. 1,
curve h). Note that the full intensity of the reductive des-
orption peak corresponds to the highest possible packing
of cysteine molecules at the specific surface domain of
the poly-Au electrode.
Fig. 2. Variation of Γ /Γ max of the different surface domains of
the poly-Au electrode with immersion time in the thiol solution
(10 ␮M cysteine), (a) Au (1 1 0) (Γmax = 2.25 × 10−10 mol cm−2),
(b) Au(1 0 0) (Γmax
=
6.29 × 10−10 mol cm−2
) and (c) Au(1 1 1)
(Γmax = 4.31 × 10−10 mol cm−2). The data were extracted from Fig. 1.
results suggest that the strength of the cysteine–Au bond
is in the order: cysteine–Au(1 1 0) > cysteine–Au(1 0 0) >
cysteine–Au(1 1 1), which is consistent with the order ob-
served for the potential needed for the reductive desorption
of cysteine from each single crystalline domain [33,35].
Fig. 3 shows the CVs for the oxygen reduction reaction in
O2-saturated 0.5 M KOH at poly-Au electrodes modified by
immersion in 10 ␮M cysteine for different times. This reac-
tion is taken as a probing criterion for the compactness of the
3. There is no significant difference in intensity between
the multiple peaks observed for the reductive desorption
patterns obtained upon the immersion of the poly-Au in
the cysteine solution for 10 min and those obtained after
17 h of immersion (compare Fig. 1, curves h and i).
Fig. 2 shows the variation of the surface coverage
of cysteine (Γ cysteine) at the individual domains of the
poly-Au with the immersion time. The surface coverage
(Γ cysteine) was normalized by dividing by the maximum
surface coverage of cysteine at each surface domain (Γ max),
Γmax(Au(1 1 1)) = 4.31 × 10−10 mol cm−2, Γmax(Au(1 0 0))
=
−10
6.29 × 10
mol cm−2 and Γmax(Au(1 1 0))
= 2.25 ×
10−10 mol cm−2. This figure shows clearly that the Au(1 1 0)
surface domain of the poly-Au electrode attains about 95%
of its full surface coverage within 5 s (curve a), while a
period of about 120 s is needed to attain the same ratio of
cysteine at the Au(1 0 0) (curve b). On the other hand, in
the case of the Au(1 1 1) some induction period (about 10 s)
elapsed without a significant adsorption of cysteine, and a
longer time is being required (>300 s) to attain a 95% of the
full packing capacity of this surface domain (curve c). These
Fig. 3. CVs obtained, in O2-saturated 0.5 M KOH, at poly-Au electrodes
immersed in 10 ␮M cysteine (aqueous solution) for (a) 0 s, (b) 10 s, (c)
20 s, (d) 30 s, (e) 60 s, (f) 120 s, (g) 300 s and (h) 600 s. Potential scan
rate: 50 mV s−1
.

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M.S. El-Deab, T. Ohsaka / Electrochimica Acta 49 (2004) 2189–2194
formed SAM during different immersion times. This figure
shows that at the bare poly-Au electrode (curve a) the oxy-
gen reduction reaction proceeds irreversibly as previously
observed [32,33], while this irreversible reaction is gradually
retarded by the increase of the time of the pre-immersion
into the cysteine solution and a quasi-reversible behavior is
obtained at poly-Au electrode with a pre-immersion time
of 60 s (Fig. 3, curve e). The oxygen reduction reaction at
this electrode resembles that observed at poly-Au electrodes
at which a submonolayer coverage of cysteine is fabricated
via the potential-controlled partial reductive desorption (i.e.,
by desorbing the cysteine molecules from the Au(1 1 1) do-
mains of the poly-Au electrode and keeping the Au(1 1 0)
and Au(1 0 0) domains covered) [32,33]. Consequently the
time-controlled immersion of the poly-Au electrode in the
thiol solution and/or the potential-controlled partial reduc-
tive desorption of the compact SAM can be utilized to con-
trol the surface design of the poly-Au electrode at which
a precise positioning of thiol molecules can be tailored to
certain surface domains. Fig. 3 shows also that the oxy-
gen reduction reaction is greatly retarded at poly-Au elec-
trodes pre-immersed in the cysteine solution for 10 min,
that is, this immersion time is quite sufficient to the forma-
tion of a compact monolayer of cysteine over the poly-Au
electrode.
mains become bare while the other surface domains of the
poly-Au are blocked. This electrode (with the sub-SAM)
is transferred into the Ag electrodeposition solution (0.1 M
H2SO4 + 1 mM Ag2SO4) and the potential was swept from
+350 to −290 mV versus Pt plate [38,39]. Fig. 4 shows
a comparative SEM images for the electrodeposited Ag
over the bare poly-Au (image b), SAM/Au (image c) and
sub-SAM/Au (image d) electrodes. Image a represents the
SEM of a bare poly-Au. Inspection of this figure reflects
two significant points:
(i) The Ag deposition is greatly retarded in the case of
the SAM/Au electrode, compared to that over the bare
poly-Au (compare the white spots in images b and c).
(ii) For the Ag electrodeposited over the sub-SAM/Au elec-
trode (imaged), we observed smaller particles in the
nanometer scale (the white spots of ca. 10 nm in size).
These Ag particles are believed to be electrodeposited
at the pin-holes of the SAM of cysteine (which are
purposely generated at the Au(1 1 1) domains of the
poly-Au). Consequently, we can safely argue that the
distribution of the electrodeposited Ag reflects that of
the bare fraction of the poly-Au surface (which repre-
sents the Au(1 1 1) domains).
3.3. Fabrication of the binary SAMs at the poly-Au
electrodes
3.2. Surface mapping of the poly-Au electrodes
The estimation of the distribution of the different single
crystalline surface domains that constitute the structure of
the poly-Au electrode has not been explored yet, to the best
of our knowledge. This estimation cannot be done by the
scanning tunneling microscopy (STM) since the poly-Au
surface is very rough (r.f. = 3.1) as it contains a huge
number of kinks and steps, while this application (i.e., the
STM) is limited only to homogeneously smooth surfaces like
single crystalline electrodes. In this section we introduce a
simple method to estimate the distribution of the Au(1 1 1)
domains of the poly-Au electrode.
The oxygen reduction reaction as well as some other elec-
trochemical reactions are known to be greatly inhibited at
a compact SAM/Au electrode [32,36,37], since the thiol
layer acts as a barrier for the charge transfer across the elec-
trode/electrolyte interface. Consequently if a poly-Au elec-
trode with some pin-holes within its SAM is positioned in
an electroplating bath of Ag, the Ag will be deposited onto
the bare fraction of the poly-Au surface within the SAM.
We utilized the same concept with the exception that the
pin-holes are generated within the SAM by the selective
partial reductive desorption of cysteine molecules from the
Au(1 1 1) domains of the poly-Au electrode by applying
three successive CV cycles in the potential region from −300
to −900 mV versus Ag/AgCl/KCl(sat) (cf. Fig. 5, curve c)
in N2-saturated 0.5 M KOH. This procedure was found to
result in the poly-Au electrode with submonolayer cover-
age of cysteine (sub-SAM/Au), at which the Au(1 1 1) do-
The molecular-level control of the structure of the SAMs
formed on the Au electrodes is a promising criterion that
could be utilized to fabricate multi-functional Au elec-
trodes, for example, to analyze several bioactive species.
In this section, we prepared the poly-Au electrode with
molecular-level design of a binary SAM, at which the first
thiol molecules (i.e., cysteine) are being attached to the
Au(1 1 1) surface domains of the poly-Au electrode, while
the second thiol species (i.e., cystamine) are being attached
to the Au(1 0 0) and Au(1 1 0) surface domains of the same
poly-Au electrode. Fig. 5 shows the strategy employed to
fabricate such a binary SAM/Au electrode. Curves a and
b of this figure show the CVs for the reductive desorption
of cysteine SAM/Au and cystamine SAM/Au electrodes,
respectively, in N2-saturated 0.5 M KOH solution at a scan
rate of 50 mV s−1. The single-component SAMs of cys-
teine and cystamine were fabricated by immersing the clean
poly-Au electrodes in 1 mM aqueous solution of the de-
sired thiol for 20 min. Curve c represents the CV performed
to selectively desorb the cystamine molecules from the
Au(1 1 1) domains of the poly-Au electrode by sweeping
the potential between −300 and −900 mV for three cycles
at a scan rate of 50 mV s−1, that is, to prepare a cystamine
sub-SAM/Au electrode. Curve d shows the reductive des-
orption pattern of the thus-prepared cystamine sub-SAM/Au
electrode, indicating that the cystamine molecules attaching
to Au(1 1 0) and Au(1 0 0) domains are not affected by the
selective desorption shown in curve c, on the time scale of

M.S. El-Deab, T. Ohsaka / Electrochimica Acta 49 (2004) 2189–2194
2193
Fig. 4. SEM micrographs of (a) bare poly-Au and Ag particles electrodeposited onto (b) bare poly-Au, (c) cysteine SAM/Au and (d) cysteine sub-SAM/Au
electrodes. The electrodeposition of Ag was performed in a 0.1 M H2SO4 + 1 mM Ag2SO4 solution and a single potential scan from +350 to −290 mV
versus Pt plate was used.
the experiments (compare the intensity of the peaks around
−1050 and −1250 mV in curves b and d). The cystamine
sub-SAM/Au electrode was then introduced into the cys-
teine solution to attach the cysteine molecules to the bare
Au(1 1 1) domains of the cystamine sub-SAM/Au electrode.
The resulting poly-Au electrode has a definite composition
of cysteine and cystamine, i.e., a binary SAM/Au electrode,
at which the cysteine molecules are attached to the Au(1 1 1)
domains while the cystamine molecules are attached to the
Au(1 1 0) and Au(1 0 0) domains of the poly-Au (see curve
e). To the best of our knowledge, this is the first report con-
cerning a preparation of a binary SAM-modified poly-Au
electrode with a precise positioning and a known surface
coverage of different thiols.

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A binary SAM was fabricated, for the first time, at
poly-Au electrodes with a molecular-level design at which
individual thiol species were attached to specific surface
domains of the poly-Au electrode. The rate of the forma-
tion of saturated SAM at the different surface domains of
the poly-Au electrode is demonstrated for the first time.
A 95% surface coverage of cysteine was attained within
5, 120 s and more than 300 s for the Au(1 1 0), Au(1 0 0)
and Au(1 1 1) surface domains of the poly-Au electrode,
respectively. The oxygen reduction reaction was taken as a
probing reaction to monitor the compactness of the SAMs
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The present work was financially supported by Grant-in
Aids for Scientific Research on Priority Areas (No. 417),
Scientific Research (Nos. 12875164 and 14050038) and
“Scientific Research (A)” (No. 10305064) from the Min-
istry of Education, Culture, Sports, Science and Technol-
ogy, Japan. M.S. El-Deab thanks the Japan Society for the
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