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Published on the web February 6, 2010
Synthesis of Lipophilic Gold Nanosheets by Using Reducing Stabilizers
and Their Reversible Transformation between Golden Solid
Rempei Kuwahara, Keita Kuroiwa, and Nobuo Kimizuka*1,2
2
1,2
1
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
7
44 Moto-oka, Nishi-ku, Fukuoka 819-0395
2
JST CREST, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395
(
Received November 17, 2009; CL-091018; E-mail: n-kimi@mail.cstm.kyushu-u.ac.jp)
4
,5-Dialkylated o-phenylenediamine with oligo(ethylene
glycol) units is developed as a stabilizer with reducing property.
It reduces chloroauric acid in water, giving lipophilic gold
nanosheets whose surface is passivated by the quinoid-form
ligands generated in situ. These nanosheets show brilliant
golden luster in the solid state, whereas they are reversibly
dispersible in organic media.
Controlled synthesis of gold nanocrystals has been attract-
ing much attention because of their unique physical and
chemical properties which vary depending on their size, shape,
Figure 1. Photographs of Au nanosheets prepared by using
¢2HCl. (a) Dried nanosheets with golden luster, (b) appearance
of chloroform dispersion (reflection color), and (c) the same
sample observed in transmitted light.
1
1
and interactions. To date, synthetic methods for gold nano-
particles, nanorods, and nanosheets4 have been vigorously
explored, and several practical routes have been compiled.
However, in spite of these efforts, bridging the structural gap
between soluble nanocrystals and three-dimensionally accumu-
2
3
(
a)
(b)
220
5
lated structures has met with limited success. The two extreme
electronic structures between a single nanocrystal in solution and
bulk assemblies would be controllable if soluble nanocrystals
show reversible transformation between these architectures. In
this regard, nanosheets passivated by ultrathin molecular layers
are interesting building blocks since electronic interactions in
their accumulated brick structures are expected to make their
optical properties comparable to those of bulk gold. We describe
herein a novel one-step route to lipophilic gold nanosheets by
using 4,5-dialkylated o-phenylenediamine as a reducing stabil-
izer. These dispersible nanosheets display reversible transfor-
mation into golden solid with distinctive metallic luster.
1
/3(422)
Figure 2. (a) A transmission electron micrograph of Au
nanosheets synthesized by using reducing stabilizer 1¢2HCl
and (b) its electron diffraction pattern.
opaque orange-colored chloroform dispersion exhibited indigo-
purple color when viewed through transmitted light (Figures 1b
and 1c). The color difference observed in the reflection and
transmission light clearly indicates the presence of developed
4
,5-Dialkyloxy-o-phenylenediamine with oligo(ethylene
6
glycol) chains 1 was synthesized (Scheme 1). It was isolated
as a hydrochloride (1¢2HCl). To an aqueous solution of
chloroauric acid (TCI, 20 mM, 8 mL), an aqueous dispersion
of 1¢2HCl (80 mM, 2 mL) was quickly added at room temper-
ature under vigorous stirring. The color of the solution
immediately turned dark red-brown and black precipitates were
formed. After 5 min, the supernatant was removed by decant-
ation and the precipitate collected was washed with pure water.
After drying under reduced pressure, golden solid with brilliant
metallic luster was obtained (Figure 1a). Very interestingly, this
golden solid was easily dispersed in chloroform by ultrasonicat-
ing for a few seconds (Branson 2510J-DTH, 125 W). The
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nanocrystals in the chloroform dispersion.
The purple chloroform dispersion was placed on a carbon-
coated copper grid and dried under vacuum. In transmission
electron microscopy (TEM, JEOL JEM-2010, acceleration
voltage, 120 kV), hexagonal gold nanosheets (length of each
side: ca. 200600 nm, thickness: ca. 10 nm) are seen, together
with spherical nanoparicles (Figure 2a, Figures S2 and S3 in
Supporting Information ). All the nanosheets are oriented
parallel with respect to the TEM grid surface. The crystal
structure of gold nanosheets was further investigated by
selected-area electron diffraction (ED) measurements. The
observed ED pattern consists of hexagonal symmetry pattern
spots corresponding to the lattice spacing of 1.44 ¡, which
originates from {220} planes of the face-centered cubic (fcc)
gold single crystal (Figure 2b). The presence of spots corre-
sponding to 2.5 ¡ spacing is also noticeable, which have been
assigned to the fcc forbidden 1/3{422} reflections which occur
1
0
H N
O
O
O
O
2
O
O
O
2
HCl
H2N
O
•
1
2HCl
Scheme 1. Chemical structure of 1¢2HCl.
Chem. Lett. 2010, 39, 226227
© 2010 The Chemical Society of Japan