Synthesis of thiol-capped gold nanoparticles with organometallic reagents as a new class of reducing agent

Article abstract of DOI:10.1246/cl.2009.562

The reaction of chloroauric(III) acid and dodecanethiol with THF solution of 2-propylmagnesium bromide furnishes thiolcapped gold nanoparticles. The obtained nanoparticles are spherical and exhibit average diameter of 5.5 ± 0.61 nm. Synthesis of the nanop

Full text of DOI:10.1246/cl.2009.562

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Chemistry Letters Vol.38, No.6 (2009)
Synthesis of Thiol-capped Gold Nanoparticles with Organometallic Reagents
as a New Class of Reducing Agent
Atsushi Sugie,1 Tomomi Hatta,1 Kiyoshi Kanie,2 Atsushi Muramatsu,2 and Atsunori Moriꢀ1
1Department of Chemical Science and Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501
2Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577
(Received February 19, 2009; CL-090176; E-mail: amori@kobe-u.ac.jp)
The reaction of chloroauric(III) acid and dodecanethiol with
THF solution of 2-propylmagnesium bromide furnishes thiol-
capped gold nanoparticles. The obtained nanoparticles are spher-
ical and exhibit average diameter of 5:5 ꢁ 0:61 nm. Synthesis of
the nanoparticles are also carried out by using other Grignard
reagents and organometallic reagents.
Gold nanoparticles (AuNPs) have attracted considerable
attention in conjunction with applications to catalysis, nonlinear
optics, and biological sensing due to characteristic magnetic,
electronic, and optical properties.1
Figure 1. TEM image of AuNP 1.
AuNPs have been generally produced by the reduction of a
Synthesis of the AuNP with several kinds of Grignard re-
agents was examined in a similar manner. The results are sum-
marized in Table 1. It was found that the formation of the AuNP
was dependent on the reaction temperature. No nanoparticle was
obtained when 2-propylmagnesium bromide was added at 0 ꢃC.
The reaction at 10 ꢃC afforded purple solution suggesting the
nanoparticle formation; however, the obtained solid was not
completely dispersible in organic solvents resulting in partial
aggregation of the nanoparticles. Though the reaction at 40 ꢃC
gave nanoparticle of a smaller particle, dispersity was found to
be inferior to that prepared at 25 ꢃC. Treatment with t-butylmag-
nesium chloride, which is a more sterically hindered reagent
than 2-propylmagnesium bromide, similarly afforded particles
of 8:1 ꢁ 0:78 nm. The reaction with sterically less hindered
Grignard reagent such as ethylmagnesium chloride also pro-
ceeded to give nanoparticles with an average diameter of
3:9 ꢁ 0:80 nm. However, no particles were obtained when phen-
ylmagnesium bromide and methylmagnesium iodide were em-
ployed. The nanoparticle size obtained was found to be similar
to the case with metal hydrides such as sodium borohydride,
superhydride, and borane–amine complex, etc.
gold salt in the presence of a capping agent which brings about
stability and functionality to the nanoparticle.2 A variety of met-
al hydrides such as sodium borohydride,3 lithium borohydride,4
superhydride (LiBHEt3),5 borane–amine complex,6 and silanes7
are used to obtain AuNPs. However, little has been studied on
the synthesis of AuNP using organometallic reagents which
bear a metal–carbon bond.8 Herein, we describe that alkyl
organometallic reagent serves as a reducing agent for the synthe-
sis of thiol-capped AuNPs.
We first examined the reaction with 2-propylmagnesium
bromide, which is a Grignard reagent widely used in organic
synthesis.9 Synthesis of AuNP was carried out as follows: A
solution of HAuCl 4H O (0.1 mmol) and 1-dodecanethiol
2
4ꢂ
(0.1 mmol) in 10 mL of THF was vigorously stirred for 3 h at
25 ꢃC under a nitrogen atmosphere. To the resulting solution,
excess THF solution (0.78 M) of 2-propylmagnesium bromide
(1.0 mmol, 1.28 mL) was added dropwise over a period of 30 s
at 25 ꢃC. The color of the solution turned yellow to dark purple
which indicates characteristic surface plasmon resonance of
AuNP.10 After stirring for 3 h at 25 ꢃC, ethanol was added to
the solution to precipitate AuNP, which was centrifuged to iso-
late the thiol-capped AuNP (1, 18.3 mg) as a dark brown powder
(eq 1). The obtained 1 was found to be dispersible in organic
solvents such as THF or CHCl3 suggesting that aggregation
did not occur during the isolation procedure.
Table 1. Synthesis of Au nanoparticles with Grignard reagentsa
RMgX
iPrMgBr
Temp/ꢃC Time/h Yield/mg Size/nm
0
10
25
40
25
25
25
25
4
10
3
6
6
6
3
3
0
—
—
b
19.0
18.3
18.2
12.6
18.1
0
MgBr
gold
5:5 ꢁ 0:61
3:0 ꢁ 0:59
8:1 ꢁ 0:78
3:9 ꢁ 0:80
—
n
ð1Þ
+
HAuCl4·4H2O
HS C12H25
nanoparticle
THF
25 °C, 3 h
tBuMgCl
CH3CH2MgBr
PhMgBr
1
TEM image of the obtained AuNP shows that nanoparticles
are spherical with an average diameter of 5:5 ꢁ 0:61 nm
CH3MgI
0
—
1
aThe reaction was carried out with HAuCl 4H O (0.1
(Figure 1). The H NMR spectrum of AuNP exhibited signals
4ꢂ
2
assigned as CH3– and –CH2– of dodecanethiol, indicating that
the 2-propyl group of the Grignard reagent was not incorporated
into the capping agent of AuNP.
mmol), 1-dodecanethiol (0.1 mmol), and THF solution of
Grignard reagent (1.0 mmol) in 10 mL of THF. bPartial
aggregation of the nanoparticle.
Copyright Ó 2009 The Chemical Society of Japan

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Chemistry Letters Vol.38, No.6 (2009)
563
Table 2. Synthesis of Au nanoparticles with various organometal-
lic reagentsa
This work was partially supported by a Grant-in-Aid for Sci-
entific Research for Exploratory Research, by Ministry of Edu-
cation, Culture, Sports, Science and Technology, Japan. The au-
thors thank Professor Yasukiyo Ueda for valuable suggestion on
TEM analyses.
Organometallic
reagent
Temp/ꢃC
Yield/mg
Size/nm
nBuLi
Et2Zn
iBu3Al
EtAlCl2
ꢄ78 to 25
16.6
6.8
8.3
0
2:9 ꢁ 0:40
2:4 ꢁ 0:36
4:9 ꢁ 0:67
—
25
25
0
References and Notes
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aThe reaction was carried out with HAuCl 4H O (0.1
4ꢂ
2
mmol), 1-dodecanethiol (0.1 mmol), and hexane solution of
organometallic reagent (1.0 mmol) in 10 mL of THF for 3 h.
2
It is well known that alkyl Grignard reagents release the hy-
drogen atom at the ꢀ-position to the metal, which serves as a sur-
rogate reagent of metal hydride and have been employed as a
reducing agent for carbonyl and related compounds in organic
synthesis.9 Accordingly, we consider that several Grignard re-
agents also serve as a reducing agent in the gold nanoparticle
synthesis. Indeed, the attempted nanoparticle synthesis with
MeMgX or PhMgX, which did not possess a ꢀ-hydrogen atom,
resulted in no reaction.
Encouraged by the results of Grignard reagents, we exam-
ined the synthesis of thiol-capped AuNP with other main group
alkyl organometallic reagents as shown in Table 2. The reaction
of n-butyllithium afforded nanoparticles in a reasonable yield,
and the particle size was found to be 2:9 ꢁ 0:40 nm. Treatment
of diethylzinc and triisobutylaluminum also gave AuNPs,
although removal of the metal residue by filtration was required
during the isolation. On the other hand, addition of ethylalumi-
num dichloride, which is recognized as a class of Lewis acid,
was found to be ineffective.
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In summary, we demonstrated the synthesis of thiol-capped
gold nanoparticles from HAuCl 4H O and long-chained alka-
4ꢂ
2
8
9
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nethiol with alkyl organometallic reagents. Grignard reagents
serve as a ubiquitous reagent in organic synthesis that is readily
available in laboratory scale. Treatment of organolithium, -zinc,
and -aluminum reagents were also found to be effective for the
synthesis of AuNP. These results suggest that several common
organometallic reagents are available as a facile reducing agent
for AuNP synthesis. Further studies on the nanoparticles bearing
a functional group on the capped thiol moiety will be carried out,
in which organometallic reagent would be a method of choice
concerning the tolerance of functional groups.11
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11 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
Published on the web (Advance View) May 16, 2009; doi:10.1246/cl.2009.562