Gold nanoparticles prepared with a room-temperature ionic liquid-radiation irradiation method

Article abstract of DOI:10.1039/b914759d

Mass production of gold nanoparticles in room-temperature ionic liquids without any stabilizing agents has been achieved with radiation irradiation, i.e., accelerated electron beam and γ-ray.

Full text of DOI:10.1039/b914759d

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Gold nanoparticles prepared with a room-temperature ionic
liquid–radiation irradiation methodw
Tetsuya Tsuda,*ab Satoshi Seinoa and Susumu Kuwabatabc
Received (in Cambridge, UK) 22nd July 2009, Accepted 16th September 2009
First published as an Advance Article on the web 30th September 2009
DOI: 10.1039/b914759d
Mass production of gold nanoparticles in room-temperature
ionic liquids without any stabilizing agents has been achieved
with radiation irradiation, i.e., accelerated electron beam
and c-ray.
nanoparticles were observed by use of transmission electron
microscopy (TEM).
Three types of RTIL with the same anion, bis(trifluoro-
methanesulfonyl)amide (Tf2N), were used to reveal the effect
of cation structure on nanoparticle generation. Those RTILs,
which were pretreated via a standard method before use,5
were 1-butyl-3-methylimidazolium Tf2N (BuMeImTf2N),
N-methyl-N-propylpiperidinium Tf2N (MePrPipTf2N), tributyl-
methylammonium Tf2N (Bu3MeNTf2N). NaAuCl4Á2H2O was
employed as the source of Au(III). Sample vials containing
0.5 mmol LÀ1 NaAuCl4Á2H2O dissolved in an RTIL solution
(2 mL) were capped under nitrogen gas atmosphere.
The radiation sources were an accelerated electron beam
(accelerated energy: 4.8 MeV; beam current: 10 mA; irradiation
dose: 6 or 20 kGy) and g-rays (60Co (1.17 and 1.33 MeV),
irradiation dose: 6 or 20 kGy). The irradiation experiments
were carried out at an existing industrial plant for sterilizing
medical kits. The dose was less than the usual sterilization dose
for medical instruments. The UV-vis spectra of the solutions
after irradiation experiments were examined by using a
Shimadzu MultiSpec-1500 spectrometer. The morphology of
the deposited Au samples were observed with a Hitachi
H-7650 transmission electron microscope. Specimens for
TEM observation were prepared by dropping the irradiated
RTIL onto a TEM grid (+3.0 mm, copper, 400 mesh) coated
with an amorphous carbon thin layer. The composition of the
Au nanoparticles was analyzed with an EDAX Genesis XM2
energy dispersive X-ray spectroscopy system.
Metal nanoparticles, which have a large surface area, surface-
plasmon characteristics and so on, are expected to be key
materials for further development of science and technology.1
A great number of articles related to metal nanoparticle
syntheses including radiation irradiation methods have been
reported to date.2,3 Room-temperature ionic liquids (RTILs)
are known to be facile solvents for preparing functional
materials because of their anomalous physicochemical
properties, such as negligible vapor pressure and wide operating
temperature range.4,5
Within recent years, RTILs are just beginning to be applied
in the preparation of metal nanoparticles.6 Some scientists
point out that RTILs behave as an excellent medium for
formation and stabilization of metal nanoparticles; that is,
the metal nanoparticles are individually suspended in RTIL
without any additive. One of the advantages of using an RTIL
is that the resulting nanoparticles are not covered with
stabilizing agents, e.g., thiols, that often inhibit catalytic
activity. In fact, bare platinum and gold (Au) nanoparticles
prepared in RTILs show good catalytic activities for oxygen
reduction in aqueous solution.7 Several methods that employ
RTILs as a functional reaction medium have already been
shown to be an effective way of yielding stable metal
nanoparticles, but a large-scale production method for such
nanoparticles will be required in the near future from a
practical application standpoint. We are interested in using
radiation irradiation, which is already used at industrial plants
for sterilizing medical kits, in order to establish a mass
production method for nanoparticles without any stabilizing
agents. In this article, Au nanoparticles have been synthesized
in RTILs by irradiation using accelerated electron beams
and g-rays. The resulting Au nanoparticles were characterized
with UV-vis spectroscopy and energy dispersive X-ray
(EDX) spectroscopy. Morphology changes of the Au
It is well-known that radiation irradiation yields solvated
electrons and radicals in many solvents. In aqueous solution,
those are hydrated electrons, OHꢀ, Hꢀ, etc. The generated
reactive species will have a significant effect on the preparation
of metal nanoparticles in the presence of appropriate
additives.3 Most RTILs are known to have better radio-
chemical stability than aqueous solution. However it strongly
depends on the structure of the organic cation. For example,
imidazolium-based RTILs release radicals with relative
ease but non-heterocyclic ammonium-based RTILs do not.8
Therefore in this study we used three RTILs, which have
different types of cation structure.
a Frontier Research Base for Global Young Researchers, Graduate
School of Engineering, Osaka University, 2-1 Yamada-oka, Suita,
Osaka 565-0871, Japan. E-mail: ttsuda@chem.eng.osaka-u.ac.jp;
Fax: +81-6-6879-7374; Tel: +81-6-6879-7374
Fig. 1 shows UV-vis spectra of the RTILs containing
0.5 mmol LÀ1 NaAuCl4Á2H2O taken after accelerator electron
beam irradiation at 6 or 20 kGy. Irradiation with the 20 kGy
electron beam on BuMeImTf2N and MePrPipTf2N resulted in
the appearance of a broad absorption peak, which is attri-
butable to plasmon absorption of Au nanoparticles,9 whereas
irradiation to Bu3MeNTf2N caused almost no spectrum
change. In the case of electron beam irradiation at 6 kGy,
only BuMeImTf2N showed a spectral change. These results
b Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
c Japan Science and Technology Agency, CREST, Kawaguchi,
Saitama 332-0012, Japan
w Electronic supplementary information (ESI) available: EDX data
and TEM images of Au nanoparticles prepared with radiation irradiation.
See DOI: 10.1039/b914759d
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This journal is The Royal Society of Chemistry 2009
6792 | Chem. Commun., 2009, 6792–6794

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Fig. 2 TEM images of Au nanoparticles in BuMeImTf2N containing
0.5 mmol LÀ1 NaAuCl4Á2H2O after accelerator electron beam irradiation
at 6 kGy (left) and 20 kGy (right).
species. As expected from Fig. 1, in MePrPipTf2N, Au
nanoparticles were only observed at 20 kGy irradiation.
TEM observation of the Au nanoparticles revealed that the
particles have spherical structures with a mean particle size of
4.5 nm (SD: 0.8 nm).
Fig. 1 UV-vis spectra of the RTILs with 0.5 mmol LÀ1 NaAuCl4Á
2H2O after accelerator electron beam irradiation at 20 or 6 kGy.
Arrows show peaks of plasmon absorption.
g-Ray irradiation of the RTILs and NaAuCl4Á2H2O showed
very similar results to the accelerated electron beam irradiation.
Fig. 3 shows UV-vis spectra of the RTILs with 0.5 mmol LÀ1
NaAuCl4Á2H2O after g-ray irradiation experiments. The
absorbance attributed to plasmon absorption appeared in
only the BuMeImTf2N sample. This implies that only
BuMeImTf2N with Au(III) can yield Au nanoparticles. The
Au nanoparticles produced were identified with EDX (Fig. S1)
and the lattice fringe of the TEM image (Fig. S2).w The mean
particle size at 6 and 20 kGy estimated from TEM images were
2.9 nm (SD: 0.3 nm) and 10.7 nm (SD: 1.7 nm), respectively.
The g-ray irradiation method makes smaller sized Au nano-
particles with a reduced plasmon absorption in comparison to
those prepared with the accelerator electron beam irradiation.
Plausible reasons to explain these differences are the low g-ray
exposure dose rate of ca. 6 kGy hÀ1 as compared with the
accelerator electron beam irradiation, which was conducted
with ca. 10.8 Â 103 kGy hÀ1, and/or the difference in radiation
quality effect between the accelerator electron beam and
g-rays. As expected from the results of UV-vis spectroscopy,
we could not see any nanoparticles in the other RTILs
irradiated with g-rays.
It is noteworthy that Au nanoparticles prepared in this
investigation were stable without the need for a stabilizing
additive for more than three months. Several research groups
have reported a similar result that nanoparticles yielded in
RTILs show no aggregation behavior, and some stabilizing
models have been proposed.11 We are also attempting to
reveal the reason but it is not completely understood at
present. It should not be forgotten that this research was
carried out at an existing industrial plant for sterilizing
medical kits with the future mass production of nanoparticles
in RTILs in mind. The industrial plant can irradiate 200 vials
 100 mL at a time. The irradiation time of accelerated
electron beam and g-ray is 7 s and 3 h, respectively, if the
irradiation dose is 20 kGy.
indicate that Au nanoparticle generation in RTILs become
easier in the following order:
Bu3MeNTf2N o MePrPipTf2N o BuMeImTf2N
Considering the aforementioned fact that Bu3MeNTf2N is
more radiochemically-stable than BuMeImTf2N, it suggests
that radiochemically-unstable RTILs tend to generate
Au nanoparticles. Unfortunately, there is no data on the
radiochemical stability for MePrPipTf2N. However, it is likely
from a molecular structure point of view that the radio-
chemical stability of a saturated heterocyclic ammonium-
based RTIL (MePrPipTf2N) is lower than that of a saturated
non-heterocyclic ammonium-based RTIL (Bu3MeNTf2N) and
higher than that of an unsaturated heterocyclic ammonium-
based RTIL (BuMeImTf2N). It is very possible that
decomposition of the RTIL by radiation irradiation is essential
to induce the reduction of Au(III) to give Au nanoparticles.
As recognized from the absorption spectra of the produced
Au nanoparticles shown in Fig. 1, absorption peak
wavelengths are slightly altered by the RTIL used and the
irradiation dose, suggesting that there is the variation in the
nanoparticle size. Generally, plasmon absorption of Au
nanoparticles moves to higher wavelength when the nano-
particle size becomes large.10 In order to estimate the particle
size, the resulting Au nanoparticles was imaged with TEM.
Fig. 2 depicts TEM images of Au nanoparticles, which were
identified with EDX (Fig. S1),w yielded in BuMeImTf2N with
0.5 mmol LÀ1 NaAuCl4Á2H2O by accelerator electron beam
irradiation at 6 and 20 kGy. Spherical Au nanoparticles with a
mean particle size of 7.6 nm (standard deviation (SD): 1.5 nm)
were observed at 6 kGy irradiation; however, if the irradiation
dose was changed to 20 kGy, different shapes as well as
spherical crystals having a 26.4 nm mean diameter (SD: 3.7 nm)
appeared. It was the expected result because the Au nano-
particles obtained at 20 kGy showed a plasmon absorption at
higher wavelength compared to that prepared at 6 kGy. The
increase in particle size at 20 kGy would be caused by extra Au
deposition onto Au nanoparticles from unreacted Au(III)
In conclusion, mass production of Au nanoparticles can
be conducted in RTILs using accelerated electron beam and
g-ray radiation irradiation at an existing industrial plant.
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This journal is The Royal Society of Chemistry 2009
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Notes and references
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This research was supported by Core Research for
Evolution Science and Technology (CREST) from the Japan
Science and Technology Agency (JST) and Grant-in-Aid for
Scientific Research on Priority Area (452) ‘‘Science of Ionic
Liquids’’ from the Japanese Ministry of Education, Culture,
Sports, Science and Technology. The authors express their
appreciation to Mr Taro Uematsu of Osaka University for his
kind assistance.
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This journal is The Royal Society of Chemistry 2009
6794 | Chem. Commun., 2009, 6792–6794