One-dimensional assembly of silica nanospheres mediated by block copolymer in liquid phase

Article abstract of DOI:10.1021/ja907013u

(Figure Presented) Colloidal silica spheres and their assembly processes are encountered in nature and numerous technological applications. We report here a novel and facile method to prepare highly anisotropic one-dimensional (1D) arrays of silica nanosp

Full text of DOI:10.1021/ja907013u

Published on Web 10/28/2009
One-Dimensional Assembly of Silica Nanospheres Mediated by Block
Copolymer in Liquid Phase
†
†,‡
†
§
Masashi Fukao, Ayae Sugawara, Atsushi Shimojima, Wei Fan,
§
§
,†
Manickam Adhimoolam Arunagirinathan, Michael Tsapatsis, and Tatsuya Okubo*
Department of Chemical System Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan,
Center for NanoBio Integration, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Department of
Chemical Engineering and Materials Science, UniVersity of Minnesota, Minneapolis, Minnesota 55455
Received August 19, 2009; E-mail: okubo@chemsys.t.u-tokyo.ac.jp
Colloidal silica spheres and their assembly processes are
1
encountered in nature and numerous technological applications.
The controlled organization of silica spheres into ordered arrays
2
could enable uses in optical devices, membrane and other
3
4
separations, templates for functional porous materials, and
5
functional fluids. Two- or three-dimensional colloidal silica
structures have been obtained by various techniques such as
3
,6
4
2
sedimentation, simple drying, and guided self-assembly. In
contrast, fabrication of one-dimensional (1D) arrays remains a
challenge because of the fundamental difficulty in organizing
isotropic particles into an anisotropic structure. 1D assembly of
2
,7
silica spheres was achieved in V-shaped grooves or by electro-
8
spinning only for particles over 300 nm in size, where the ‘top-
down’ approach is applicable.
Some examples of 1D assembly of metal, semiconductor, and
other nonsiliceous nanoparticles in liquid phases have been reported.
9
They are mostly based on inherent anisotropy of magnetic or
1
0
11
electric dipoles, crystal-face specific heterogeneity, and non-
1
2
uniform distribution of ligands on the surfaces. Templating
methods using linear biomacromolecules and carbon nanotubes
Figure 1. (a) Time-dependent turbidity profile of the SNSs suspension
containing F127 (pH 7.5) measured by UV-vis spectroscopy. The absor-
bance increases across the entire wavelength range of 300 to 800 nm. The
representative data at 350 nm are shown here. (b) Cryo-TEM image of the
frozen-hydrated sample at pH 7.2, after 8 days of incubation. (c,d) SEM
images of 1D chains of SNSs transferred to Si substrates. They were
obtained at pH 7.5 after (c) 1 day and (d) 7 days of incubation, respectively.
Before SEM observations, the samples on the substrates were irradiated
with UV light (172 nm) under an air pressure of 50 Pa to remove organic
13
14
are straightforward, but they usually need surface functionalization
of nanoparticles and are unsuitable for obtaining uniform, closely
arranged 1D assembly. Here, we report a novel approach to organize
silica nanospheres (SNSs) one-dimensionally in a liquid phase with
the aid of a commercially available amphiphilic block copolymer.
Uniform-sized SNSs ca. 15 nm in size are used in this study.
They have been synthesized in the emulsion system containing
20
components and were sputter-coated with Pt.
4a,b,15,16
Figure 1a shows the time-dependent turbidity profile of the
suspension after adjusting the pH at 7.5, which was taken by using
UV-visible spectroscopy at 350 nm. The turbidity gradually increases
over 2 weeks, and the suspension eventually forms a weak gel. This
indicates the slow aggregation of SNSs in a liquid phase. The cryogenic
transmission electron microscopy (Cryo-TEM) has revealed the
formation of 1D chain-like structure of SNSs in the suspension. The
typical Cryo-TEM image of the frozen-hydrated sample after 8 days
of incubation (pH 7.2) shows the presence of relatively straight 1D
chains (Figure 1b and Supporting Information, Figure S1). Each SNS
associates closely enough that no gaps are recognized between
neighboring particles. The adhesion of particles is apparently strong
enough that 1D chains can be transferred to substrates. Figure 1c and
d are SEM images of 1D chains of SNSs that were dip-coated on Si
substrates. The progress of particle connection is also evident from
these images. Two to five SNSs are connected linearly after 1 day of
incubation at 60 °C, pH 7.5 (Figure 1c), while longer chains are
observed after 7 days of incubation (Figure 1d).
tetraethoxysilane (TEOS), water, and L-lysine as a catalyst.
This method gives monodispersed, colloidally stable SNSs in
4a,b,15-17
solution.
Block copolymer Pluronic F127 is used to induce
1D assembly of SNSs in the suspension. F127 is a nonionic triblock
copolymer consisting of poly(ethylene oxide) and poly(propylene
oxide) (PEO-b-PPO-b-PEO) with an average molecular weight of
1
2 600 and 70 wt % PEO content. F127 exhibits amphiphilicity
below the cloud point (>100 °C for 10% aqueous solution) and
forms micelles or liquid crystals depending on temperature and the
18
concentration. F127 is known to adsorb on a silica surface through
hydrogen bonding between the ether oxygens in PEO and the silanol
19
groups. For the preparation of 1D assemblies of SNSs, F127 was
added to the above suspension and dissolved with stirring at 60
°
2
C. The weight ratio of SiO to F127 was 1:1. The pH of the
suspension was lowered to 7.2-7.5 by using hydrochloric acid.
The resultant suspension was incubated at 60 °C for several days
without agitation.
To examine the effect of F127 on particle assembly, SNSs were
incubated with different amounts of F127 (SiO /F127 ) 1:x (w/w),
2
where x ) 0-2.0). In the absence of F127, the suspension remains
†
Department of Chemical System Engineering, The University of Tokyo.
‡
Center for NanoBio Integration, The University of Tokyo.
§
University of Minnesota.
1
6344 9 J. AM. CHEM. SOC. 2009, 131, 16344–16345
10.1021/ja907013u CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
transparent for more than 2 weeks at 60 °C, pH 7.2, showing good
colloidal stability of SNSs. In contrast, a small amount of F127 (x )
formation of 1D chains was induced by the transition from spherical
to wormlike micelles of the block copolymer, which also occurred
in the absence of Au nanoparticles. However, in our system, only
F127 aggregates with an average size of 30 nm can be detected by
dynamic light scattering (Figure S5). This size is comparable to
that of the typical spherical micelles of F127 reported by others.
It appears that a synergistic effect of F127 and silica nanoparticles
is contributing to the 1D assembly.
In conclusion, we have demonstrated a facile method to prepare
a 1D chain-like structure of silica nanospheres by using an
amphiphilic block copolymer. The anisotropy of chain-like assembly
can be fine-tuned using various handles like pH, ionic strength,
and temperature. This work may lead to new applications for silica
colloids and may provide useful ideas for the design of other
functional materials. Systematic investigation on the role of polymer
additives is now under progress.
0.1) strongly facilitates particle connection; precipitates consisting of
highly aggregated SNSs are generated right after the pH adjustment
to 7.2 (Figure S2a). The increase in the concentration of F127 results
in the formation of networked SNSs at x ) 0.5 (Figure S2b) and more
ordered 1D chains at x ) 0.8-2.0. Malmsten et al. reported that silica
nanoparticles of 15 nm in size aggregated in a dilute aqueous solution
2
2
19
of F127, but the formation of the anisotropic assembly has never
been reported.
The mode of particle assembly is controlled systematically by
changing the pH of the suspension. SNS colloids were mixed with
F127 (SiO /F127 ) 1:1 (w/w)) and incubated at 60 °C for 7 days
2
at various pHs ranging from 8.0 to 6.0. SNSs remain dispersed in
the suspension at pH 8.0, and only dispersed SNSs are observed
when transferred to a substrate (Figure 2a). In contrast, 1D chains
with relatively unbranched, straight structures are formed at pH
Acknowledgment. We thank Prof. Yukio Yamaguchi (The
University of Tokyo) for zeta potential and dynamic light scattering
measurements. We also gratefully acknowledge support from the
National Science Foundation (CMMI-0707610). Cryo-TEM char-
acterization was carried out at the Minnesota Characterization
Facility which receives support from the NSF through the National
Nanotechnology Infrastructure Network.
7
.5 (Figure 1d). Longer, more bent chains with branches are
observed at pH 7.0 (Figure 2b). Further lowering of pH to 6.0 gives
a more disordered chain-like assembly with increased branching
(
Figure 2c). The tendency of random assembly at lower pH can be
explained by the reduction of electrostatic repulsion between the
particles. The decrease in pH increases the number of undissociated
1
silanol groups on the silica surface and consequently reduces the
surface charge of SNSs (the zeta potentials of SNSs without F127
are -50.2, -44.2, -40.7, and -29.0 mV at pH 8.0, 7.5, 7.0, and
Supporting Information Available: Detailed experimental proce-
dures and Figures S1-S5. This material is available free of charge via
the Internet at http://pubs.acs.org.
6
.0, respectively). The undissociated silanols also facilitate the
1
9
interaction of PEO and SNSs through hydrogen bonding, which
may affect the adsorption of F127 on silica. The effect of pH on
the surface charge of F127 should be negligible since F127 is
nonionic at the present pH range.
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1
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JA907013U
J. AM. CHEM. SOC. 9 VOL. 131, NO. 45, 2009 16345