全 文 ：收稿日期：2008-10-13。 收修改稿日期：2008-11-08。
国家自然科学基金资助项目(No．20874047)。
＊通讯联系人。 E-mail：jshen@njnu.edu.cn；wbzhao11@yahoo.com
第一作者：毛 春，男，36 岁，博士；研究方向：生物高分子和纳米材料。
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研究简报
红毛丹状 Au-F127纳米球的制备和表征
毛 春 赵文波＊ 肖迎红 沈 健＊
(江苏省生物功能材料重点实验室，南京师范大学化学与环境科学学院，南京 210097)
关键词： 金； 嵌段聚合物； 纳米结构； 红毛丹状
中图分类号： O614.123 文献标识码： A 文章编号： 1001-4861(2008)12-2071-04
Synthesis of Colloidal Rambutan-like Au-F127 Hybrid Nanospheres
MAO Chun ZHAO Wen-Bo＊ XIAO Ying-Hong SHEN Jian＊
(Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Environmental Science,
Nanjing Normal University, Nanjing 210097)
Abstract: The rambutan-like hybrid Au-F127 nanospheres were prepared by a self-assembly method and
characterized by transmission electron microscopy (TEM) and UV-Vis absorption spectroscopy. In addition, the
formation mechanism was discussed for the self-assembled rambutan-like Au-F127 hybrid nanospheres.
Key words: gold; triblock polymer; nanostructures; rambutan-like
It is well-known that gold is the subject of one of
the most ancient themes of investigation in science, its
renaissance now leads to an exponentially increasing
number of publications, especially in the context of
emerging nanoscience and nanotechnology with nano-
particles and self-assembled monolayers [1]. Au nano-
particles (AuNPs) have also been widely exploited for
use in catalysis, biological labeling, photonics, photo-
graphy, information storage, surface enhanced Raman
scattering and formulation of magnetic ferrofluids.
Xia et al. [2] believed that nanoparticles had
potentially useful size- and shape dependent properties.
The intrinsic properties of a metal nanoparticle are
mainly determined by its size, shape, composition,
crystallinity, and structure. In principle, one could
control any one of these parameters to fine-tune the
properties of this nanoparticle. So the synthesis of
nanoparticles with desired size/shape has, therefore,
enormous importance, especially in the compelling field
of nanotechnology[3].
Block copolymer micelles have been used as
templates to produce metal nanoparticles, permanent
nanostructures, including nanospheres, shell-cross-
linked nanospheres, star polymers, hollow nanospheres,
shaved nanospheres, nanofibers, and nanotubes [4]. The
use of block copolymer micelles is a significant
research field for the synthesis of stabilized AuNPs in
the presence or in the absence of thiol ligands[1].
Recent works of Sakai and Alexandridis [5~7] have
demonstrated the shape and size controlled synthesis of
colloidal gold in the presence of various triblock
polymers (TBP). They have shown how the structural
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modifications of TBPs influence the morphologies of
colloidal gold. Bakshi et al. [4] reported that “raspberry
type” AuNPsTBP aggregates were observed with
uniform distribution of nanoparticles about 2 ~3 nm
throughout the TBP micelle. Moreover, apart from such
aggregates, prominent ordered morphologies of nano-
particles such as rod, sphere, triangle, and hexagonal
were also observed with much larger dimensions due to
the nucleation process occurring in the mixed micelles
rather than in the micelles of single TBP components.
Here we report the synthesis of rambutan-like
hybrid Au-F127 nanospheres by a self-assembly
method and the characterization of the nanospheres by
TEM and UV-Vis techniques. This new shape of Au-
F127 hybrid nanoparticles has morphology similar to
that of Rambutan (a kind of fruit of Southeastern Asia,
bearing edible oval red fruit with soft spines). To the
best of our knowledge, this shape of AuNPs has not
been reported. The formation mechanism of the self-
assembled rambutan-like Au-F127 hybrid nanospheres
is also discussed.
1 Experimental
In a ty pical reaction, different amounts of F127
powder (MW approximately 12600, PEO(98)-PPO(67)-
PEO (98)), BASF, Germany) were added to the five
portions of freshly prepared HAuCl4 aqueous solutions
(100 cm3, 2.94×10-4 mol·L-1), respectively. All the five
mixtures were stirred for 3 hours at room temperature.
The color of all the reaction mixtures changed from co
lorless to purplish.
2 Results and discussions
Fig.1 shows the UV-Vis absorption spectra of pure
F127 aqueous solution and AuNPs in the presence of
different concentrations of TBP. A clear surface plasmon
(SP) band is observed around 535 nm confirming the
presence of AuNPs[8]. The absorption peak centered at ~
220 nm is attributed to the gold髥 chloride solutions [9].
As shown in Fig.1, with the increase in TBP
concentration, the intensity of the SP band increases,
while the intensity of the absorption peak from gold髥
chloride solution decreases. From the absorption study,
it can be concluded that gold髥 chloride reacts more
frequently with higher concentration of F127 because
the number of cavities of PEO increases with F127
concentration. As a result, the quantity of AuNPs is
larger. This result is further supported by the
comparison of the color change time in Table 1. As
shown in the table, with the increase of F127
concentration, the occurring time of the color change
keeps decreasing.
Withal, AuNPs with different shapes were
produced via reaction with different concentrations of
F127. The TEM images for these five samples are
presented in Fig.2a ~e. As shown, the three samples
prepared at high concentrations of F127 (2% and 1%)
and the low concentration of F127 (0.25% ) contain
dispersed AuNPs with different morphologies, i.e.
sphere, triangle, and hexagonal.
However, it is very interesting to note that the new
shape, rambutan-like nanospheres are obtained in the
presence of 0.75% and 0.5% F127 (Figs.2c and 2d).
Both of the two concentrations overstep critical micelle
Concentration / % Time / min Color
2 6 Purple
1 16 Moderate purple
0.75 28 Purple and red
0.5 35 Purple and red
0.25 110 Light purple and red
Fig.1 UV-Vis spectra of pure F127 and HAuCl4+F127
at cHAuCl4=2.94×10
-4 mol·L-1
Table 1 Occurring time of the color change, and the
solutions′ final color in the presence of
difference concentrations of F127 at
cHAuCl4=2.94×10
-4 mol·L-1
Initial color of all the above solutions is colorless.
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Fig.2 (a~e) TEM images of AuNPs in the presence of different concentrations of F127: (a) 2%, (b) 1%, (c) 0.75%,
(d) 0.5% and (e) 0.25% at cHAuCl4=2.94×10
-4 mol·L-1
concentration (CMC) of F127 in water[8]. The presumed
mechanism for the formation of rambutan-like hybrid
nanospheres of Au-F127 nanoparticles is suggested as
follows:
(1) The small micelles composed of F127 are
easily formed in aqueous solution because of the
amphiphilic properties of F127 with appropriate
concentration overstep critical micelle concentration
(CMC)[10]. In this work, the PPO block can also diffuse to
the inside of the spherical F127 micelles to form
spherical micelles filled with PPO block because of the
hydrophobic nature of PPO.
(2) AuCl4- ions bind to pseudocrown ether structu-
res (cavities) that are formed from PEO coils. The
bound AuCl4- ions are reduced via the oxidation of PEO
by the metal center. This resulting Au髣 species then
migrates to other cavities where the equilibrium 3Au髣
葑 2Au + Au髥 occurs. The metal atoms coalesce to
form clusters, which then grow to yield small metal
particles[5].
(3) The new hydrophobic PPO blocks are inserted
into hydrophobic channels of AuNPs by virtue of
hydrophobic property of AuNPs as shown as Fig.3.
Furthermore, adsorption of PPO segments on AuNPs
seems to prevent particle growth from aggregation, and
results in small particle size and high colloidal stability.
As a result, Au-micelles get further growth into
organized morphologies sequentially until the raw
materials are exhausted.
There are two possible processes in the last step.
One is hydrophobic channels of AuNPs insertion by
new F127 molecule with very small AuNPs covered
onto the PEO segment (Fig.3 (1)). Another is hydroph-
obic channels of AuNPs insertion by new F127 mole-
cule without reduction of AuNPs (Fig.3(2)). This prop-
osed formation mechanism is further supported by the
formation of small rambutan-like hybrid nanospheres of
Au-F127, observed by TEM as shown in Fig.2c and d
Fig.3 Schematic representation of “Rambutan” association
between AuNPs and F127 micelle
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(indicated by white circles).
The above proposed formation mechanism
explains well the morphology of rambutan-like hybrid
nanospheres of Au-F127 obtained with intermediate
F127 concentrations. However, it could not account for
the morphology of the AuNPs observed at high F127
concentrations. For high concentrations, the formation
speed of F127 micelles and reduction speed of AuCl4- is
high, therefore, in the solution there will be numerous
amount of F127 micelles with attached small AuNPs.
These micelles will collide and interact with each other,
then AuNPs with various morphology, such as sphere,
triangle, and hexagonal, are obtained. The detailed
formation mechanism is explained by Bakshi et al. [4] At
low concentration of F127 (which is lower than CMC),
there are not enough micelles to produce rambutan-like
hybrid nanospheres of Au-F127.
A clear TEM image of a single particle is
presented in Fig.4 (a). This new shape, rambutan-like
nanospheres of polyaniline has been reported by
Wan′s group[11]. However, there has been no report on
the rambutan-like hybrid Au nanospheres. To confirm
the formation of Au nanospheres, electron diffraction
pattern for this rambutan-like nanasphere was
recorded. As shown in Fig.4(b), the diffraction pattern
indicates polycrystalline material because of the beam
focusing on a collection of individual (crystalline)
particles. The formation of Au nanoparticle is further
confirmed by high resolution TEM presented in Fig.4
(c). As shown clearly in the figure, crystal lattices of
Au are observed.
The present study clearly shows that the
rambutan-like hybrid nanospheres of Au-F127 can be
obtained from the formation of Au nanoparticles
synthesized in the presence of F127 micelles with given
concentration. Control of nanoparticle shape may
initially seem like a scientific curiosity, but its goal
goes far beyond aesthetic appeal[12].
Fig.4 (a) Representative TEM image of the rambutan-like hybrid nanospheres of Au-F127, inset: right→a photograph of rambutan;
(b) Electron diffraction patterns obtained by aligning the electron beam perpendicular to a rambutan -like hybrid
nanosphere of Au-F127; (c) TEM image of soft spines of the rambutan-like hybrid nanosphere of Au-F127
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