JOURNAL OF RAMAN SPECTROSCOPY

J. Raman Spectrosc. 2007; 38: 936–942

Published online 11 April 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/jrs.1741

Self-assembled monolayers of mercaptosuccinic acid on

silver and gold surfaces designed for protein binding.
Part I: structure of the monolayer
Agata Królikowska and Jolanta Bukowska∗
Department of Chemistry, University of Warsaw, Pasteur Street 1, 02-093 Warsaw, Poland

Received 6 November 2006; Accepted 31 January 2007

Structure of the monolayers of mercaptosuccinic acid (MSA) on silver and gold electrodes, grown from
aqueous solution, has been studied using surface enhanced Raman scattering (SERS) spectroscopy. It was
found that MSA chemisorbs via both sulfur atom and one carboxylic group (more close to the sulfur atom)
dissociated on the Ag and Au at natural pH (slightly acidic). The influence of the type of the substrate,
pH of the solution and time of incubation of the thiol monolayer have been investigated. SERS results
show that acidity of one carboxylic group of MSA (more close to the sulfur) decreases on both substrates
with simultaneous increase of acidity of the second COOH group, in comparison to the solution. For silver
substrate, longer time of incubation is necessary to form a stable monolayer, whereas the structure of
monolayer on gold does not exhibit temporal evolution. Copyright  2007 John Wiley & Sons, Ltd.

KEYWORDS: SERS; self assembly; ω-terminated alkanethiols; mercaptosuccinic acid

INTRODUCTION the structure and properties of the ω-functionalized alka-

nethiols on gold and silver, e.g. cyclic voltammetry,24,25
During the last two decades, particular attention has been
scanning tunneling microscopy,25,26 X-ray photoelectron
drawn towards modifying noble metals by coating them
spectroscopy (XPS), near edge X-ray absorption fine structure
with thin, oriented molecular films.1 – 6 Among them, self-
spectroscopy (NEXAFS)27 – 29 and vibrational spectroscopy
assembled monolayers (SAMs) on silver and gold, which
techniques.29 – 33 Especially surface enhanced Raman scatter-
are formed by spontaneous adsorption of surfactant on a
ing (SERS) spectroscopy34 – 40 seems to be a sensitive surface
solid substrate, are of considerable interest due to simplicity
analytical tool for characterizing the alkanethiol monolayers
of preparation and flexibility in designing required surface
on silver and gold.
properties.6 – 11 A significant position in this class of sub-
As mentioned before, controlling the properties of
stances belongs to monolayers obtained from organosulfur
ω-functionalized alkanethiol monolayer on metal substrate
compounds. Nuzzo and Allara12 introduced this group of
can be done by a proper choice of terminal group. This
species, showing that SAMs preparation on gold can be
feature, in common with a strong sulfur–metal bond and
performed from dilute solutions of bifunctional disulfides.
alkyl chain, which mimics ion barrier in the lipid bilayers,
The great advantage of monolayers formed from sulfides,
disulfides or thiol molecules is their strong interaction with providing a biological membrane-like microenvironment,
the metal (Ag, Au and Cu) surface, resulting in the forma- predestines ω-substituted alkanethiols as linkage monolay-
tion of stable metal–sulfur chemical bonds.13 – 16 Among this ers. Particularly, carboxylic acid thiol derivative coatings are
type of coating species, the most intensively exploited are ω- very widely used to immobilize biomolecules on gold and
substituted alkanethiols HSCH2 n X, because of their ability silver substrates by covalent or noncovalent interactions to
to create an interface with a well-defined composition and eliminate metal-induced, unnecessary changes in protein or
structure and with easily controlled chemical and physical enzyme structure, including denaturation.41 – 44 The structure
properties, achieved through the selective use of functional of the alkanethiol SAMs depends on the length of the alkyl
groups.17 – 23 Many techniques can be applied to investigate chain15 and on the environment (solution pH or presence
of electrolytes).45 Mainly the long chain carboxyalkanethiols
Ł Correspondence
are believed to form highly-ordered, well organized struc-
to: Jolanta Bukowska, Department of Chemistry,
University of Warsaw, Pasteur 1, 02-093 Warsaw, Poland. tures as a result of the interchain interactions, which can
E-mail: jbukow@chem.uw.edu.pl dominate the carboxyl groups’ hydrogen bonding, leading

Copyright  2007 John Wiley & Sons, Ltd.

 SAMs of mercaptosuccinic acid on silver and gold surfaces I 937

Substrate preparation
The monolayers were formed by immersion of silver
or gold substrates in 10 mM aqueous thiol solutions for
2–24 h. Before the chemical modification, the polycrystalline
silver electrodes were electrochemically roughened by three
successive positive–negative cycles in 0.1 M aqueous KCl
Figure 1. Mercaptosuccinic acid – molecular formula.
solution from
0.3 to C0.3 to
0.3 V at a sweep rate of 5 mV
s
1 in a separate cell. The cycling was finished at
0.3 V
to disorder in the monolayer.46,47 However, short chain mer- and then the silver electrode was kept for 1 min at
0.4 V.
captocarboxylic acids, like mercaptopropionic acid (MPA) For gold, 20 cycles with the potential changing from
0.5 to
can form ordered domains on gold48 and can be successfully C1.2 to
0.5 V at a sweep rate of 50 mV s
1 were applied.
used as a biocompatible coating for proteins and enzymes Platinum and Ag/AgCl/1 M KClaq electrodes were used as
immobilization.49,50 There are also reports that preparation counter and reference electrodes, respectively.
conditions have a great impact on the degree of ordering of
carboxyl terminated thiol SAMs.51 RESULTS AND DISCUSSION
In this work, we report SERS studies on the structure
of monolayers of mercaptosuccinic acid (MSA, called also Structure of MSA monolayer
thiomalic acid; for molecular formula see Fig. 1) on silver Analysis of the SERS spectra of alkanethiols around 700 cm
1
and gold surfaces. MSA is a short chain mercaptodicarboxylic can be very helpful in determination of conformation
acid often employed for preparation of monolayer protected of adsorbed molecules. In this spectral region, we can
clusters (MPCs) of gold,52 – 55 which can store charge and distinguish well-separated bands corresponding to (C–S)
are regarded as a multivalent redox species. These features stretching vibrations of different rotational isomers, gauche
directed our interest to this compound and our aim was at lower and trans at higher wavenumbers.15,32,57 – 59 In
to compare actual structure of MSA monolayer on silver Fig. 2, typical SERS spectra of MSA monolayers formed
and gold electrodes, since it has a significant impact on on roughened silver and gold in the range between
the practical applications of thiol layers.32,56 We have also 500 and 1800 cm
1 (Fig. 2(d) and (e)) are compared with
decided to apply mercaptosuccinic acid SAMs on silver and the NR spectra of solid sample (Fig. 2(a)) and aqueous
gold as linkage monolayers to control adsorption of proteins. solutions of MSA (Fig. 2(b) and (c)). SERS spectra were
Results of Raman and infrared studies of cytochrome c recorded in a natural pH of MSA solution (about 3).
immobilized on MSA SAMs are described in Part II of the Surprisingly, the spectrum on both substrates exhibited
paper. very weak enhancement of the bands in the region around
700 cm
1 , corresponding to C1 –S stretching vibrations. We
can hardly distinguish two bands around 630 and 690 cm
1
EXPERIMENTAL for MSA monolayers on silver and gold. These two bands
correspond rather to two different gauche conformations of
Chemicals the S–C1 –C2 chain than to gauche and trans conformations
All chemicals were purchased from commercial suppliers respectively.15,57 – 59 In the NR spectrum of the solid MSA
(Aldrich, POCh, and CHEMPUR) and used without fur- (Fig. 2(a)), we observe three bands in this region (at 609, 694
ther purification. Phosphate buffer solutions (typical ionic and 767 cm
1 ), which suggest that in crystalline state also
strength 50 mM) were prepared from appropriate conjugated there are more than two stretching vibrations of the C1 –S
acid–base pair of Na3 PO4 , Na2 HPO4 , NaH2 PO4 , H3 PO4 . bond: two characteristic for gauche conformer (bands below
700 cm
1 ) and one for trans conformer (above 700 cm
1 ).
Raman measurements Weak surface enhancement of the band due to C–S stretching
Raman spectra were recorded with a Jobin Yvon-Spex T64000 vibration, in comparison to the other short-chain thiols (for
Raman spectrometer equipped with a Kaiser holographic example, thioglycolic acid (TGA)59 or MPA)60 provokes the
notch filter, 1800 or 600 grooves/mm holographic grating question about the nature of bonding of MSA molecules
and a 1024 ð 256 pixels nitrogen-cooled, charge-coupled with the metal surface. This short ω-substituted alkanethiol
device detector. The Raman spectrometer was equipped contains three functional groups competing for interactions
with an Olympus BX40 microscope with a 50ð long distance with silver or gold. Therefore there is a possibility of stronger
objective. A Laser-Tech model LJ-800 mixed Ar/Kr laser interaction through carboxyl groups than through thiol
provided excitation radiation of 647.1 nm with power less moiety. However, absence of the S–H stretching band in
than 10 mW at the sample in the case of SERS spectrum and SERS spectra of MSA monolayers on Ag and Au suggests
about 100 mW at the head of the laser in the case of normal cleavage of hydrogen–sulfur bond. Potentiometric titration
Raman (NR) spectra. The integration time ranged from 30 to experiments indicating formation of 1 : 1 chelates of MSA
300 s. with silver (I),61 as well as XPS studies on silver nanoparticles

Copyright  2007 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2007; 38: 936–942
DOI: 10.1002/jrs
938 A. Królikowska and J. Bukowska

Figure 2. (a) Raman spectrum of solid MSA; (b) Raman Figure 3. (a) SERS spectrum of MSA monolayers, formed on
spectrum of 0.5 M aqueous solution of MSA (pH D 2.0); roughened silver soaked for 20 h in 10 mM aqueous solution;
(c) Raman spectrum of 0.5 M solution of MSA in deuterated (b) and (c) SERS spectra of MSA monolayers on roughened
water (pH D 2.0); (d) and (e) SERS spectra of MSA monolayers, silver, formed for 2 h from 10 mM solution in deuterated water;
formed from 10 mM aqueous solution on roughened silver measured at pH D 3.0 (b) and soaked in phosphate buffer of
(d) and gold (e). Spectra were scaled and shifted for clarity of pH D 7.0 (c). Spectra were scaled and shifted for clarity of
presentation. Bands marked with asterisk are due to water presentation.
(H2 O and D2 O).

band for MSA monolayers for Ag and Au supports may

covered with MSA molecules62 and FT-IR spectra of MSA suggest direct interaction of the –COO group with the metal
modified gold nanoparticle powders,52 confirm the presence substrate. Figure 3 presents SERS spectra of MSA monolayer
of thiolate bond. Thus, weak surface enhancement of the on Ag formed from solutions in D2 O at acidic and neutral
bands corresponding to the C–S stretching vibrations may pH confronted with the one, formed from aqueous solution.
indicate almost parallel orientation of the C1 –S bond with As follows from a comparison of the respective spectra,
respect to the surface. A strong band at around 820 and the position of the band at about 820 cm
1 is insensitive
830 cm
1 for monolayers on silver and gold, respectively to both H/D exchange (compare spectra (a) and (b) in
(Fig. 2(d) and (e)), which has also been observed in the SERS Fig. 3) and pH value (compare spectra b and c in Fig. 3). It
spectra of succinic,63 tartaric and malic acids adsorbed on suggests that carboxyl groups contributing to this vibration
silver colloids,64 may originate from deformation vibration are dissociated and strongly bound to the surface. Another
of O–C–O group of atoms. This band, but at considerably strong SERS band appears slightly above 900 cm
1 at both
higher wavenumber (856 cm
1 ), is also present in the NR investigated metal surfaces (Fig. 2(d) and (e)). Its assignment
spectrum of aqueous solution of MSA in acidic pH (Fig. 2(b)) to the stretching vibration of the C2 –COOH group is based
and shifts to 835 cm
1 upon hydrogen–deuterium exchange on the analysis of the SERS spectra of MPA60 and TGA.59
(Fig. 2(c)). Significant difference in the wavenumber of this As visible in Fig. 3, this band shifts from 908 cm
1 to

Copyright  2007 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2007; 38: 936–942
DOI: 10.1002/jrs
 SAMs of mercaptosuccinic acid on silver and gold surfaces I 939

880 cm
1 upon H/D exchange. A strong SERS band present

at around 1570 cm
1 can be assigned to the antisymmetric
stretching vibration of the COO
groups.65 Appearance of
the 1570 cm
1 band in the SERS spectrum of MSA may be
surprising since, as mentioned before, we simultaneously
observe the band at around 900 cm
1 , assigned to the
C2 –COOH vibrations of protonated carboxylic groups
(Fig. 2(d) and (e)). Therefore, we may conclude that in an
adsorbed MSA molecule, one carboxylic group remains
undissociated, while the other one, which is more close
to the sulfur atom, is bound to the surface in dissociated
form, since interaction with the metal may facilitate the loss
of proton. At the high-wavenumber side of the 1570 cm
1
band in the SERS spectra in Fig. 2(d) and (e), we can also
distinguish a weakly enhanced band (at about 1650 cm
1 ),
which can be attributed to the CO streching vibrations of
the hydrogen bonded COOH groups. This wavenumber is
similar to that observed in the NR spectrum of solid MSA
at 1647 cm
1 (Fig. 2(a)), where we expect a dimeric structure
of the thiol.52 In the normal Raman spectrum of acidic MSA
solution in H2 O (Fig. 2(b)), this band is shifted to 1728 cm
1
and corresponds to the C O groups hydrogen bonded with
water molecules. This spectrum also exhibits the 1650 cm
1
component most probably ascribed to deformation vibration
of water molecules. As follows from the SERS spectra on
silver and gold (Fig. 2(d) and (e)), recorded at natural,
acidic pH, the structure of MSA monolayers does not
differ significantly on these two substrates. The band
positions are however slightly different, reflecting difference
in interaction with the metal support. Moreover, the relative
intensity of the bands ascribed to the COO
groups (at Figure 4. SERS spectra of MSA monolayer on gold soaked in
820–830 cm
1 and at about 1570 cm
1 ) with respect to the various phosphate buffer solutions: (a) pH D 1.2, (b) pH D 3.2,
bands corresponding to the C2 –COOH vibration (slightly (c) pH D 5.2, (d) pH D 6.2, (e) pH D 9.2, (f) pH D 10.6 and
above 900 cm
1 ) depends on the type of the substrate. These (g) pH D 12.3. To prevent desorption of MSA, measurements
spectral features suggest different orientation of the MSA were carried out in solutions containing thiol at a concentration
alkyl chain for the studied metal surfaces. As reported by of 1 mM. Spectra were scaled and shifted for clarity of
Ulmann9 for unsubstituted alkanethiols, it may arise from presentation.
the interplay of several factors such as different substrate-
adsorbate interactions, intermolecular interactions between
alkyl chains of adsorbed molecules and intramolecular MSA monolayers on gold, the band at 945 cm
1 appears
interactions, resulting in different surface coverage for silver at pH around 6.0 (Fig. 4(d)), but the presence of the low-
and gold substrate. The higher relative intensity of the band wavenumber component at 917 cm
1 points out that there is
due to C2 –COOH stretching to the ones related to the still a portion of protonated carboxyl groups. The situation
carboxylate groups vibrations on Ag support is consistent changes in more alkaline pH, e.g. at pH of 9.2 (Fig. 4(e)),
with less tilted carbon backbone observed for alkanethiols where only vibrations characteristic for COO
group are
adsorbed on Ag, in comparison to the Au substrate.15 observed. These spectral changes may be attributed to
The noticeable changes in the SERS spectra of MSA dissociation of the second carboxylic group of MSA molecule.
monolayers are observed under variation of pH value Alkalization of the solution leads also to disappearance of
(Figs 4 and 5). The most striking changes induced by the band near 1505 cm
1 , which is accompanied by the
alkalization of the solution are obviously connected with changes in the 1383–1395 cm
1 region. A band closer to
the vibrations involving carboxylic groups. First, we observe 1385 cm
1 , visible as a single band at acidic pH (up to 5.2,
a progressive substitution of the band at around 910 cm
1 , as can be seen from Fig. 4(c)) has been also detected in SERS
assigned to C2 –COOH with the band about 940 cm
1 , spectrum of TGA monolayer on gold59 and was ascribed
which we attribute to the C2 –COO
, upon increasing to the vibration (of unknown assignment) of undissociated
pH of the solutions in which monolayers were soaked. For carboxylic groups. At pH around 6.2 and higher, the second

Copyright  2007 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2007; 38: 936–942
DOI: 10.1002/jrs
940 A. Królikowska and J. Bukowska

vibration of the COO
groups from around 1575–1580 cm
1

to around 1590 cm
1 (Figs 4 and 5).
The values of pKa for MSA in solution determined from
potentiometric titration experiments amount to pKa1 D 3.30
and pKa2 D 4.94 and pKa3 D 10.64, where pKa1 and pKa2 refer
to carboxylic acid groups dissociation and pKa3 refers to the
thiol group dissociation, respectively.65 In the presence of
metals like silver, copper, and gold, the SH group loses a
proton easily even in strongly acidic pH, forming stable
metal mercaptans.15,16 In the case of COOH groups we
assume a different change in the pKa value after adsorption,
dependant on the position of the carboxylic group in MSA
molecule. As follows from the SERS spectra on both silver
and gold surface, which exhibit the band around 1575 cm
1 ,
assigned to the antisymmetric stretching vibration of the
COO
group, a portion of carboxylic groups of MSA is
dissociated even in acidic solutions (pH 1.0–2.0; Figs 4(a) and
5(a) as example). It is obvious from geometric reasons that
attachment of MSA molecule to the surface through the sulfur
atom diminishes the distance between adjacent carboxylic
groups and the surface, thus facilitating their dissociation.
Similar increase of the strength of the mercaptocarboxylic
acid after chemisorption on a metal surface compared to
the solution was found also for 3,30 -thiodipropionic acid
(TDPA) in our previous studies.66 The lowering of the pKa1
value for adsorbed MSA molecules can be explained by
the interaction of the carboxylic groups with the silver
or gold substrate, which probably possesses a number of
metal cations, which are able to coordinate with COO

groups, thus facilitating deprotonation of COOH groups
Figure 5. SERS spectra of MSA monolayer on silver soaked in and/or by electric field in the electrochemical double layer.
various phosphate buffer solutions: (a) pH D 1.2, (b) pH D 3.2, As deduced from the SERS spectra in Figs 4 and 5, the
(c) pH D 5.2, (d) pH D 7.2, (e) pH D 9.2. To prevent desorption second carboxylic group, which stays further away from
of MSA, measurements were carried out in solutions the metal substrate, is mostly dissociated on gold at pH
containing thiol at a concentration of 1 mM. Spectra were higher than 6 (the band at 945 cm
1 is considerably stronger
scaled and shifted for clarity of presentation. than that at 917 cm
1 ) and totally dissociated at pH higher
than 5 on silver (only the band at 942 cm
1 is observed).
Since in the case of Au support at pH D 5.2, only the band
component becomes visible near 1395 cm
1 , which is due corresponding to COOH groups (at 915 cm
1 ) is visible in
to symmetric stretching vibration of COO
groups. The the SERS spectrum, the acid strength of the MSA seems to
appearance of the second nonprotonated carboxylic group be slightly reduced upon binding to this metal. A decrease
in the MSA molecule may be followed by a variation in of the strength after adsorption was also observed for TGA
the thiol geometry at the metal surface. With increasing molecules, which additionally was dependent on the type
pH of the solution, we observe the high-wavenumber shift of the substrate and was significantly more pronounced
(about 20 cm
1 and 15 cm
1 for gold and silver in Figs 4 for gold than for silver.59 The hindered loss of proton for
and 5, respectively) of the band corresponding to the C–S chemisorbed thiol can be explained in terms of hydrogen
stretching vibration. This shift may appear as a result of bonds formed between neighboring carboxylic groups of
the pH induced conformational changes in adsorbed MSA MSA molecules, which additionally stabilize the protonated
molecules. It can be explained in terms of change of torsion form at the surface. The weak band around 1665 cm
1 , visible
angle of the thiol molecule at the surface upon loss of at acidic pH for monolayers on silver (Fig. 5(a)), can be
the proton, which provokes stronger repulsion between because of the stretching vibration of the carbonyl group
the same-sign-charged carboxylate groups. Increasing the involved in hydrogen bonding, thus providing additional
number of dissociated carboxylic groups with concomitant support for that hypothesis. The changes in the electric
rearrangement of the thiol monolayer also results in a shift of field caused by already dissociated carboxylic groups, which
the wavenumber characteristic for antisymmetric stretching make dissociation of the next portion of neutral groups more

Copyright  2007 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2007; 38: 936–942
DOI: 10.1002/jrs
 SAMs of mercaptosuccinic acid on silver and gold surfaces I 941

difficult, may be another factor responsible for the surface the other group remaining in undissociated state, forming
pKa increase. hydrogen bonds with neighbouring molecules. The last
It is worth stressing that at around neutral pH, a number conclusion is supported by appearance of the band around
of MSA carboxylic groups are deprotonated (Fig. 5(d)), 1680 cm
1 , which can be attributed to the C O stretching
which make them very suitable for electrostatic attraction vibration of the hydrogen bonded COOH groups. This
of biomolecules possessing positively charged domains like band significantly broadens and shifts to lower wavenumber
cytochrome c, which is biologically active at this pH value.67 values when the monolayer is grown for 24 h. This effect may
be connected with the increasing hydrogen bonds strength
Kinetics of monolayer formation due to increasing coverage of the surface by MSA molecules.
The kinetics of adsorption of thiol is very important The most significant changes in the SERS spectrum, occurring
from the point of view of influence of structural features with increasing time of incubation, are high-wavenumber
of the monolayer on the attachment of biomolecules. shifts of two bands. The band at 1576 cm
1 moves gradually
Figure 6 presents typical temporal evolution of the SERS to 1594 cm
1 within the first 3 h with simultaneous intensity
spectrum during self-assembling of the monolayer from increase and at the same time the band around 820 cm
1
10 mM aqueous solution of MSA on the silver surface. As shifts to about 830 cm
1 . These spectral changes can be a
can be seen from Fig. 6(a), formation of the MSA monolayer consequence of the increasing number of deprotonated MSA
is a very fast process and the first step is adsorption in a form carboxylic groups at the metal substrate, connected with
with one dissociated carboxylic group in close proximity some changes in the orientation of the thiol molecule at the
to the metal surface, which facilitates its deprotonation and surface, as a result of restructurization of the Ag surface. After
24 h of incubation the SERS spectrum reminds that, observed
at initial stage of monolayer incubation (compare Fig. 6(f)
and (a)). From this experiment, we can deduce that during
the first hours of adsorption we observe restructurization of
the MSA monolayer on Ag. We can expect that the more
stable MSA monolayers on Ag substrate are obtained on
longer times of incubation. Monolayers on gold revealed
stable structure within the whole period of the incubation
time (data not shown). These observations suggest that for
the purpose of cytochrome immobilization on MSA coated
silver, SAMs formed over at least a few hours should be
applied to avoid structural instability of the monolayer.

CONCLUSIONS
MSA monolayers on silver and gold exhibit similar structure.
Chemisorption of MSA from aqueous solutions takes place
in natural, acidic pH with carboxylic group in proximity
to the sulfur atom dissociated, as a result of interaction
with the metal surface, and the other group protonated
and stabilized by hydrogen bonding. For both substrates,
we observe similar lowering of acidity compared to the
solution for one of the carboxylic groups and increase for the
second one. More stable SAMs are formed on gold surface;
monolayers on silver require longer times of incubation to
obtain stable structure. Fast kinetics of adsorption, relative
time stability of the monolayers and presence of functional
carboxylic groups, which enables simple electrostatic or
covalent immobilization of biomolecule, make MSA an
attractive partner for cytochrome c attachment.

Acknowledgements
Figure 6. Evolution of SERS spectrum during formation of This work was financially supported by KBN grant 1 T09A 031 30.
MSA monolayer on Ag from 10 mM MSA aqueous solution.
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DOI: 10.1002/jrs
942 A. Królikowska and J. Bukowska

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Copyright  2007 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2007; 38: 936–942
DOI: 10.1002/jrs