One-step preparation of gold nanoparticles using 4-acylamidobenzenethiol as a reductive stabilizer

Article abstract of DOI:10.1246/cl.2010.1258

A simple one-step preparation method to prepare gold nanoparticles (AuNPs) using 4-acylamidobenzenethiol derivatives as reductive stabilizers has been developed. The stabilizers were easily prepared from 4-aminobenzenethiol and dihydroxy fatty acid. By simply mixing the stabilizer and a solution of HAuCl₄ in methanol or water, we could form AuNPs spontaneously without any additional reagents or physical treatment. The diameters of the resulting AuNPs were determined to be 25nm using TEM images.

Full text of DOI:10.1246/cl.2010.1258

doi:10.1246/cl.2010.1258
1258
Published on the web October 30, 2010
One-step Preparation of Gold Nanoparticles Using 4-Acylamidobenzenethiol
as a Reductive Stabilizer
Hiroaki Okamura,*1 Junichi Kurawaki,1 Tetsuo Iwagawa,1 Toshiyuki Hamada,1 and Masatake Kawashima2
1Department of Chemistry and Bioscience, Graduate School of Science and Engineering, Kagoshima University,
1-21-35 Korimoto, Kagoshima 890-0065
2Sanme Chemical Co., Ltd., 10-37-1-4 Hanamidai, Ranzan-machi, Hiki-gun, Saitama 355-0204
(Received September 22, 2010; CL-100815; E-mail: okam@sci.kagoshima-u.ac.jp)
A simple one-step preparation method to prepare gold
nanoparticles (AuNPs) using 4-acylamidobenzenethiol deriva-
tives as reductive stabilizers has been developed. The stabilizers
were easily prepared from 4-aminobenzenethiol and dihydroxy
fatty acid. By simply mixing the stabilizer and a solution
of HAuCl4 in methanol or water, we could form AuNPs
spontaneously without any additional reagents or physical
treatment. The diameters of the resulting AuNPs were
determined to be 2-5 nm using TEM images.
Thiol-stabilized gold nanoparticles (AuNPs) are the most
common hybrid inorganic-organic nanomaterials,1 and hence,
there is a strong demand for a practical preparation method of
highly functionalized AuNPs. A popular method is the direct
reduction of a mixture of Au3+ ions and a thiol stabilizer using
NaBH4,2 ascorbic acid,3 citric acid,3,4 or an another substance as
a reducing agent.1 However, this method yields a mixture of
AuNPs and oxidized residues of the reducing agent, which
should be removed by purification such as by centrifuging or
column chromatography.
Scheme 1.
was used for the following experiments without further
purification.
After obtaining the stabilizers, we examined the preparation
of AuNPs as shown in Table 1. A solution of the stabilizer 2
(0.20 M in methanol, 100 ¯L) was diluted with methanol
(5.0 mL), and a solution of HAuCl4¢4H2O (0.24 M in methanol,
21 ¯L) was then added at room temperature. The color of the
reaction mixture gradually darkened after 10 min and changed to
deep purple after 5 h. The vis-NIR spectrum of the resulting
colloidal suspension of AuNP showed an absorption peak at
560 nm (Entry 1, Figure 1a, solid line), and hence, compound 2
was considered to be an effective reductive stabilizer to obtain
stable AuNPs from an Au3+ solution. The profile of the vis-NIR
spectrum changed over time (Entry 2, Figure 1a, dotted line),
and no definite peak was observed after one week, although the
solution was still clear.9
A combination of a reductive thiol stabilizer and Au3+
solution would be a superior method for obtaining pure thiol-
stabilized AuNPs. Negishi and Tsukuda reported a one-pot
preparation of AuNPs using meso-2,3-disulfanylsuccinic acid as
a reductive stabilizer.5 In the course of study, they disclosed
that a highly reductive vicinal dithiol moiety was essential for
spontaneous formation of AuNPs.
Very recently, our research group has reported a simple
sonochemical synthesis of AuNPs using dendron-stabilizers
having a thiol functional group.6
In this letter, we report a novel one-step preparation of stable
AuNPs by using 4-acylamidobenzenethiol derivatives 2 and 3,
as reductive stabilizers, which spontaneously reduced Au3+ to
AuNPs at room temperature without any additional chemical or
physical treatments. The resulting AuNPs obtained by this
preparation method were dissolved in methanol or water,
depending on the molecular structure of the stabilizer, and the
diameter of the particles was 2-5 nm.
The preparation of the reductive stabilizers used in this work
is shown in Scheme 1. Stabilizer 27 was prepared by the
dicyclohexylcarbodiimide (DCC) coupling of 4-aminobenzene-
thiol and 9,10-dihydroxyoctadecanoic acid (1) that was easily
obtained from oleic acid.8 The sulfurylation of the diol moiety of
2 using chlorosulfuric acid and pyridine proceeded smoothly,
and a pyridinium salt 3 was formed. By adding water to the
reaction mixture, a yellow oil that contained compound 3 was
liberated. After drying the oil under vacuum to remove pyridine
and water, almost pure 37 was obtained as a yellow oil, which
The structure of the stabilizer bound to AuNPs was not
clarified. Since the thiol group has been oxidized into disulfide
during the course of the reaction, the following three possible
structures can be considered: a) thiolate form 4, b) disulfide form
5, and c) mixture of 4 and 5 (Scheme 1).
6R-SH þ 2Au3þ ! 2Au0 þ 3R-S-S-R þ 6Hþ
ð1Þ
The stoichiometry of this reaction can be described as
shown in eq 1, and thus we attempted to confirm it by a
comparison of peak intensities of the vis-NIR spectra of the
AuNP solutions prepared by different molar ratios of 2/Au3+
(Table 1, Entries 1, 3, 4, and 5).9 The AuNP solutions resulting
from the reactions of 1/1, 2/1, 3/1, and 4/1 mixtures of 2/Au3+
showed peaks at around 560 nm in their spectra, but their
intensities were 2/1 > 3/1 > 4/1 > 1/1 (Figure 1b), which did
not agree with our expectation (4/1 µ 3/1 > 2/1 > 1/1). It is
speculated that the ratio of 2/Au3+ changed the structures, sizes,
and/or shapes of the resulting AuNPs that lead to different
Chem. Lett. 2010, 39, 1258-1260
© 2010 The Chemical Society of Japan
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1259
Table 1. Preparation of AuNPs using stabilizers 2 and 3
Molar ratio of
Entry
Solvent
Stabilizer
Time/h
Color of solution
-
max/nm
stabilizer/Au3+
1
2
3
4
5
6
7
8
9
10
11
12
13
14
methanol
methanol
methanol
methanol
methanol
dichloromethane
toluene
acetone
acetonitrile
water
0.10 M NaHCO3
0.10 M HCl
0.10 M NaOAc
ca. 0.10 M NH4OH
b
2
2
2
2
2
2
2
2
2
3
3
3
3
3
4/1
4/1
3/1
2/1
1/1
4/1
4/1
4/1
4/1
4/1
4/1
4/1
4/1
4/1
5
48
48
48
48
1
purple
purple
purple
blue-purple
560
559 (broad), 466 (broad)
557
572
563
®
®
®
®
460 (broad)c
465c
®
®
®
blue-purple
brown (precipitate)a
brown (precipitate)a
yellow
1
24
24
1
1
1
yellowb
reddish-brown
reddish-brown
brown (precipitate)
brown (precipitate)
brown (precipitate)
1
1
aSoluble in methanol. Small amount of precipitate was contaminated. Spectra were measured after toluene/CTAB extraction.
c
but brown precipitates were formed after 1 h, which were largely
dissolved by adding methanol to yield reddish-brown solutions.
It has been suggested that the polarity of the resulting AuNPs
was very high because the hydroxy groups of 2 concentrated on
the surfaces of the particles, and thus the particles precipitated
in these less-polar solvents and dissolved by adding polar
solvent.
Aprotic polar solvents such as acetone and acetonitrile were
less effective for this reaction (Table 1, Entries 8 and 9). The
initial pale-yellow colors of these reaction mixtures did not
change within several hours after the addition of Au3+. After
24 h, the acetone solution darkened slightly, but no peak was
observed in its vis-NIR spectrum. The acetonitrile solution
formed a very small amount of precipitate after 24 h, and the
color of the solution did not change. Consequently, the reactivity
of stabilizer 2 strongly depended on the solvent, and the protic
polar solvent was the most appropriate.
Reaction in water using polar stabilizer 3 was very
successful and yield water-soluble hydrophilic AuNPs (Table 1,
Entry 10). A solution of the stabilizer 3 (0.20 M in methanol,
100 ¯L) was diluted with water (5.0 mL), and a solution of
HAuCl4¢4H2O (0.24 M in methanol, 21 ¯L) was then added to
it at room temperature. The color of the reaction mixture
immediately changed to reddish brown. Although no definite
peak based on the surface plasmon resonance of AuNPs was
observed in the vis-NIR spectrum of the reaction mixture itself,
the toluene/cetyltrimethylammonium bromide (CTAB) extract
of the resulting AuNPs showed a broad peak around 460 nm
(Figure 1c, solid line).
Figure 1. Vis-NIR spectra of AuNP solutions. (a) Effect of
aging, solid line: stabilizer 2 and Au3+ in methanol, 5 h (Table 1,
Entry 1); dotted line: stabilizer 2 and Au3+ in methanol, 48 h
(Table 1, Entry 2). (b) Effect of molar ratio of stabilizer/Au3+
,
dotted line: 2/Au3+ = 4/1, short-dashed line: 2/Au3+ = 3/1,
solid line: 2/Au3+ = 2/1, long-dashed line: 2/Au3+ = 1/1. (c)
Effect of pH, solid line: stabilizer 3 and Au3+ in water, 1 h
(Table 1, Entry 10); dotted line: stabilizer 3 and Au3+ in 0.10 M
NaHCO3, 1 h (Table 1, Entry 11).
absorption maxima (-max) values with different intensities.
Indeed, the profiles of the spectra and the colors of the resulting
Obviously, the pH of the solution was important to yield a
clear solution of AuNPs. A weak-alkaline aqueous solution,
0.10 M NaHCO3 was suitable for the reaction and yielded a
reddish-brown solution as well as the water system. Its vis-NIR
spectrum was again measured after the toluene/CTAB extrac-
tion, and a clearer and sharper peak was observed at 465 nm
(Figure 1c, dotted line).9 On the other hand, since the stabilizer 3
was hardly soluble in an acidic medium, the reaction with Au3+
in 0.10 M HCl only gave brown precipitate. Further, an ionic
species of the reaction medium was important. Reactions in
solutions were different for the different ratio of 2/Au3+
.
Consequently, a simple comparison of the peak intensities at
around 560 nm could not evaluate the stoichiometry of this
reaction.
We also examined the solvent effect on this reaction. In the
case of the reactions using less-polar solvents, dichloromethane
or toluene (Table 1, Entries 6 and 7), the colors of the solutions
changed to blue within 10 min after adding the Au3+ solution,
Chem. Lett. 2010, 39, 1258-1260
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1260
We would like to express our sincere gratitude to Professor
Junji Inanaga and Assistant Professor Hiroshi Furuno, Institute
of Materials Chemistry and Engineering, Kyushu Univerisy, for
elemental analysis of the stabilizer 2, and Professor Masaharu
Tsuji, Kyushu University, for TEM imaging. This work was
performed under the Cooperative Research Program of “Net-
work Joint Research Center for Materials and Devices (Institute
for Materials Chemistry and Engineering, Kyushu University).”
Part of this work was financially supported by KAKENHI
(Grant-in-Aid for Scientific Research) on Priority Area “Strong
Photon-Molecule Coupling Fields (No. 470)” from the Ministry
of Education, Culture, Sports, Science and Technology of Japan.
Figure 2. TEM images of AuNPs. (a) Close-up image of
stabilizer 2 and Au3+ in methanol (Table 1, Entry 2). (b) Entire
image of spherical aggregation of AuNPs. (c) Stabilizer 3 and
Au3+ in 0.10 M NaHCO3, extracted with toluene/CTAB
(Table 1, Entry 11).
References and Notes
1
a) A. C. Templeton, W. P. Wuelfing, R. W. Murray, Acc.
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0.10 M NaOAc or ca. 0.10 M NH4OH, which were both alkaline
and could dissolve stabilizer 3, yielded only a brown precipitate.
The sizes of the resulting AuNPs were established from
transmission electron microscope (TEM) images (Figure 2). The
particles obtained from the reaction listed in Table 1, Entry 2,
appeared to be spheres with diameters 2-5 nm (Figure 2a),
which formed larger spherical clusters (diameter: 20-50 nm,
Figure 2b). On the other hand, hydrophilic particles obtained
from the reaction listed in Table 1, Entry 11, were well
dispersed, and their diameters were 2-5 nm (Figure 2c).
The key factors of molecular design responsible for
obtaining an efficient reductive stabilizer are still unclear, but
it is important to note that the color of the mixture of 4-
acetamidobenzenethiol and Au3+ in methanol showed no change
even after 24 h and its vis-NIR spectrum had no absorption peak
attributable to the surface plasmon resonance of AuNPs. It can
be concluded that the chain length and/or the functionalities in
the acyl group, as well as reductivity of the thiol functional
group are the key to designing a reductive stabilizer.
2
3
M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R.
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For detailed experimental procedure and the spectra data of
the stabilizers, see Supporting Information which is available
electronically on the CSJ-Journal Web site, http://www.csj.
jp/journals/chem-lett/index.html.
M. Kawashima, M. Ninomiya, Fine Chemical, CMC
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The wavelength was shorter than that of surface plasmon
band of AuNPs (ca. 520 nm), which suggested a formation of
smaller gold clusters.5,10 Indeed, dynamic light scattering
experiments indicated that the sizes of the particles yielded
from 0.10 M NaHCO3 solution were smaller than those
obtained from methanol solution (see Supporting Informa-
tion). Detailed studies on the morphology of AuNPs are in
progress.
4
5
6
7
8
9
In conclusion, we have developed a novel preparation
method of AuNPs by mixing only a reductive stabilizer (2 or 3)
and Au3+ solution in one-step at room temperature. The
diameter of the resulting AuNPs was 2-5 nm. Further studies
on the characterization of the surface morphology of the AuNPs
obtained by this method are now in progress.
10 a) P. A. Bartlett, B. Bauer, S. J. Singer, J. Am. Chem. Soc.
1978, 100, 5085. b) G. H. Woehrle, M. G. Warner, J. E.
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Chem. Lett. 2010, 39, 1258-1260
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/