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8/2/2019 Another Review of Particle Methods 2008
1/15
www.rsc.org/materials Volume 18 | Number 19 | 21 May 2008 | Pages 216122
SSN 0959-9428
FEATURE ARTICLE
Seung-Man Yang, Gi-Ra Yiet al.
Synthesis and assembly of structuredcolloidal particles
PAPER
Eduardo Ruiz-Hitzky et al.Poly(3,4-ethylenedioxythiophene)clay nanocomposites
8/2/2019 Another Review of Particle Methods 2008
2/15
Synthesis and assembly of structured colloidal particles
Seung-Man Yang,*a Shin-Hyun Kim,a Jong-Min Lima and Gi-Ra Yi*b
Received 24th October 2007, Accepted 21st February 2008
First published as an Advance Article on the web 11th March 2008
DOI: 10.1039/b716393b
Synthesis and self-assembly of structured colloids is a nascent field. Recent advances in this area
include the development of a variety of practical routes to produce robust photonic band-gap
materials, colloidal lithography for nanopatterns, and hierarchically structured porous materials with
high surface-to-volume ratios for catalyst supports. To improve their properties, non-conventional
suprastructures have been proposed, which could be built up using binary or bimodal mixtures of
spherical particles and particles with internal or surface nanostructures. This Feature Article will
describe the state-of-the-art in colloidal particles and their assemblies. The paper consists of three main
sections categorized by the type of colloid, namely shape-anisotropic particles, chemically patterned
particles and internally structured particles. In each section, we will discuss not only synthetic routes to
uniform colloids with a range of structures, features and shapes, but also self-organization of these
colloids into macrocrystalline structures with varying nanoscopic features and functionalities. Finally,
we will outline future perspectives for these colloidal suprastructures.
I. Introduction
Following a few pioneering reports in the late 1990s that colloidal
crystals can serveas photonic bandgap structuresand as templates
for functional materials, there has been considerable interest in
colloids, and their potential use for nano- and bio-photonic
applications, from the materials chemistry community.1,2 To
date, a number of colloidal structures for photonic crystals have
been reported, including opaline face-centered cubic (fcc) assem-
blies of colloidal spheres and inverse-opal structures of various
dielectrics and metals. Recently, non-spherical and non-isotropic
sphericalparticles have been proposed as building blocksfor non-
conventional structures that may lead to better optical properties.
The formationof thesecolloidal structureshas beendemonstrated
by experimental studies that use controlled aggregation of
colloidal particles, controlled synthesis of particles, and other
physical methods. Additionally, individual non-spherical
particles can have interesting optical properties by themselves.
These properties can lead to high-efficiency diffusion or strong
scattering of light that can be useful in the development of noveloptical films for the flat panel display industry.
On the other hand, colloidal particles have been used as model
systems for the study of molecular interactions and of atomic
Seung-Man Yang
Seung-Man Yang received
a Ph.D. degree in Chemical
Engineering from Caltech in
1985. Following this, he joined
the KAIST as a Professor in
Chemical and Biomolecular
Engineering. He has served the
KAIST as a director of theComputing Center, and a
Department Chair. Currently,
Professor Yang leads the Crea-
tive Research Initiative Center
for Integrated Optofluidic
Systems. His principal contribu-
tions have been in theories and experimental methods for fabri-
cating ordered macrocrystalline structures, which can be applied
as innovative functional nanoscopic materials such as optoelectronic
devices and biosensors. He has authored over 160 peer-reviewed
papers, and a number of books and patents in related areas.
Gi-Ra Yi
Gi-Ra Yi is a senior researcher
of Korea Basic Science Institute
(KBSI) in Seoul. He received
his B.S. (1997) degree in
chemical engineering from
Yonsei University, and M.S.
(1999) and Ph.D. (2003)
degrees in chemical and bio-molecular engineering from
KAIST. After his postdoctoral
research at the University of
California, Santa Barbara, he
worked briefly for the Corporate
R&D Center of LG Chem
Research Park as a research
scientist. In 2006, Dr. Yi moved to the Nano-Bio System Research
Team of KBSI in Seoul. Currently, he is interested in self-assem-
blies of colloidal particles at micrometre or nanometre scales, as
well as multiphase microfluidics.
aNational Creative Research Initiative Center for Integrated OptofluidicSystems, Department of Chemical and Biomolecular Engineering, Korea
Advanced Institute of Science and Technology, Daejeon, 305-701, Korea.E-mail: [email protected]; Fax: +82-42-869-5962; Tel: +82-42-869-3922bKorea Basic Science Institute, Seoul, 136-713, Korea. E-mail: [email protected]
This journal is The Royal Society of Chemistry 2008 J. Mater. Chem., 2008, 18, 21772190 | 2177
FEATURE ARTICLE www.rsc.org/materials | Journal of Materials Chemistry
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3/15
crystals because their dynamic properties of phase behavior can
be easily observed with optical microscopy.35 In fact, a number
of distinct atomic crystalline phases have been discovered using
model colloidal particles, which behave like hard spheres or
interact with isotropic interparticle potentials. Recently, some
new crystalline phases were observed by the application of an
external field to colloidal crystalline phases. However, the
emergence of new model colloidal systems that have non-
isotropic directional interactions opens the possibility of
exploring molecular interactions and phase behavior in much
more complex cases.
Fig. 1 illustrates the recent revolution in colloidal particlesynthesis and how it has enabled us to control the shape of
particles, the internal structures of the systems, and the nature
of physically or chemically distinct patches on surfaces. In this
article, the key contributions to this field of research over the
past few years will be reviewed and discussed in terms of experi-
mental and theoretical progress in synthesis and assembly of the
nanostructured colloids.
II. Shape-anisotropic particles
A. Ellipsoidal particles
As nature tends to enforce spherical interfaces to minimizesurface energy, it has been fairly challenging to synthesize
anisotropic particles directly. Therefore, as an alternative route
for the production of anisotropic particles with desirable proper-
ties, several clever methods have been developed. One simple
anisotropic morphology could be ellipsoids. For example,
polymeric ellipsoids were successfully prepared through the
uniform deformation of spherical latex beads in a viscous matrix
of another polymer. Ho et al. mixed polystyrene (PS) latex and
poly(vinyl acetate) (PVA) in water and prepared films by casting
them on substrates. The films were then stretched above
their glass transition temperature and rapidly quenched. The
ellipsoidal particles were separated from films by dissolving the
PVA in alcoholic solutions.6 By controlling the draw ratio and
material composition, a variety of polymeric ellipsoids could
be prepared by this technique.
Recently, Champion et al. modified the stretching method
slightly, as depicted schematically in Fig. 2a, and were able to
prepare over 20 types of distinct structures on the micrometre
scale, as shown in Fig. 2b.7 They went on to investigate the
role of these target geometries in the immune system. Adopting
the same technique, Jiang et al. obtained colloidal crystals of
ellipsoidal particles. In this case, a polymeric matrix with ellip-
soidal voids was prepared initially by stretching the polymeric
inverse opal structure. Then, inorganic particles were formed
inside the voids of the matrix, and finally removal of the polymer
by burn-out left behind well-packed ellipsoids. The SEM and
TEM images in Fig. 2c and d, respectively, show the ordered
packing of hollow titania ellipsoids.8
Another route to ellipsoidal particles is by the deformation of
inorganic particles in high-energy ion beams. As shown in Fig. 3,
silica particles were deformed along the perpendicular direction
of the ion beam. This beam consisted of Xe ions accelerated to
energies of 0.34 MeV.9 In addition, a similar approach was
used to produce crystal structures of silica oblates.10
Fig. 1 Schematic diagram of shape-anisotropic, chemically patterned,
and internally structured colloidal particles.
Fig. 2 (a) Schematic diagram of the method of film stretching for parti-
cles with various structures and (b) SEM images of the obtained particles
through scheme A and B. Scale bars are 2 mm. Reprinted with permission
from ref. 7; copyright 2007 National Academy of Sciences, USA. (c)
SEM and (d) TEM images of colloidal crystals composed of hollow
titania ellipsoids. From ref. 8. Reprinted with permission from AAAS.
Fig. 3 SEM images of deformed silica particles after Xe ion irradiation
at (a) 4 MeV and (b) 1 MeV. Reprinted with permission from ref. 9.
Copyright 2003, American Institute of Physics.
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Both deformation-based methods are limited to relatively
small quantities of particles. To produce large quantities of
anisotropic colloidal particles, direct synthetic methods have
been developed. For example, using spindle-shaped particles of
hematite (a-Fe2O3) as core materials, ellipsoidal particles of
silica are prepared by seeded solgel polymerization of tetra-
ethoxysilane (TEOS) on the surface of hematite.11 In this case,
the aspect ratio can be controlled by adjusting the thickness of
the silica coating. Furthermore, dissolving the core and perform-ing additional reactions can lead to the production of hollow
ellipsoidal and fluorescent particles.
Recently, the packing behavior of non-spherical objects have
attracted great attention in mathematics, condensed matter
physics and materials chemistry. For this, ellipsoidal particles
serve as important model systems. When the ellipsoidal objects
are packed randomly in a maximally random-jammed (MRJ)
state, the packing fraction is much higher than that of spherical
objects (0.64). An axis ratio close to 1.25 : 1 : 0.8 induces
a packing fraction of the MRJ state as high as that of the densest
packing of spheres (0.74 in the fcc lattice). Donev et al. reported
these facts based on experimental and simulation results.12 The
famous M&Ms milk chocolate candies, which have a narrowsize distribution, were used for this experiment. Several different
containers were used and the volume fraction was calculated
from their weights. In addition, Man et al. used 1.25 : 1 : 0.8 ellip-
soids prepared by stereolithography.13 Using medical magnetic
resonance imaging (MRI), the nematic order parameters were
calculated and the resulting values indicated the absence of
orientational order as MRJ states. Simulation with hard-particle
molecular dynamics algorithms showed that the packing fraction
and number of contacts per particle are functions of the aspect
ratio. The increase in packing fraction and number of contacts
originated from the additional rotational degrees of freedom
associated with ellipsoidal shape. Therefore, mass production
of ellipsoidal particles at the submicrometre scale will be essen-tial for constructing low dielectric materials with high void
fraction by inverting the structure, as well as unconventional
colloidal crystals with unique photonic band gap properties.
B. Dumbbell-like particles
For the production of polymeric dumbbell particles, the seeded
emulsion polymerization scheme was developed. The use of
swollen styrene monomers and crosslinking agents with cross-
linked seed PS latex resulted in phase separation before polymer-
ization occurred. The acceleration of phase separation during
polymerization induced the formation of dumbbell particles.14
The formation of these structures required precise control of
the experimental conditions, including the cross-linking density
of seed latex, and swelling and polymerization times. The experi-
mental results are in line with a thermodynamic model for
monomer swelling that is related to the mixing of the monomer
and polymer, the elastic energy of particles and the interfacial
tension between particles and water. Then, the Gibbs free energy
of mixing is given by:
D Gm,p RT[ln(1 vp) + vp + cmpv2p] +
RTNVm(v1/3p 1/2vp) + 2Vmg/a (1)
where R is the gas constant, T is the absolute temperature, vp is
the volume fraction of polymer in the swollen particle, cmp is the
monomerpolymer interaction parameter, N is the effective
number of chains in the network per unit volume, Vm is the
monomer molar volume, g is the interfacial tension between
the particle and the water, and a is the radius of the swollen
particle. At equilibrium, the balance between the positive contri-
bution from the elastic energy and the negative contribution
from the mixing force determines the degree of phase separation.For microspheres, the interfacial energy term is negligible
compared with the other two terms.
Using a seeded emulsion polymerization scheme, Mock et al.
reported that the surface affinity of seed latex to the monomer
affects the anisotropy of the resulting submicrometre-sized parti-
cles.15 Kegel et al. showed that the ratio of fast (initial swelling
time scale) to slow (as yet undefined) relaxation times determines
the separated volume throughout the experiment.16 It is very
important to note that the seeded emulsion technique allows
the preparation of particles that are anisotropic not only in
shape, but also in their chemical and physical properties. Using
a different monomer from that of the seed latex, PS/PMMA
and PS/PBMA dumbbell particles were synthesized.17 Througha chemical reaction on one bulb of each dumbbell, the bulbs
could be endowed with different hydrophilicities. These particles
are amphiphilic and therefore can be used for stabilizing emul-
sion drops, for example, through alignment at the interface
like surfactant molecules.
Hosein et al. showed that dumbbell-like particles can adopt
two-dimensional (2D) in-plane and out-of-plane alignments of
particles when they are confined within thin fluid layers.18 For
3D structures of dumbbell particles, Mock et al. observed
a disorderorder phase transition into a rotator phase where
centres of mass are ordered, without any increase in the orienta-
tional order of the particles, as the volume fraction of the dimer
particles increases.19 Further increases in the volume fractioninduced a body-centered tetragonal phase with orientational as
well as positional orders. This was confirmed by ultrasmall-angle
X-ray scattering and SEM in real-space, as shown in Fig. 4.
Interestingly, dumbbell particles have also been prepared by
controlled aggregations of two particles. For example, Johnson
et al. reported that silica dumbbell particles can be prepared by
Fig. 4 SEM image of colloidal crystals composed of homonuclear
dicolloids. Reprinted with permission from ref. 19. Copyright 2007
American Chemical Society.
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destabilization of a suspension followed by shell coating.20
Destabilization of a silica suspension was carried out in two
different ways: shear-induced aggregation in a high ionic
strength, and depletion-induced aggregation driven by micelles.
The aggregates were coated with silica shells using an additional
reaction. The thickness of the coated shell determined the aspect
ratio of the dumbbells. Although doublets coexist with other
aggregates, the number fraction of dumbbells at the optimum
aging time was as high as 40%. In addition, the use of a centrifu-gation process can increase the fraction. Labeling of the core and
shell with different dye molecules allows us to confirm dumbbell
formation. Ibisate et al. have shown that silica particles are
constantly colliding with neighboring particles. Furthermore,
the particles in silica suspensions of sufficiently high concentra-
tion are bound together by van der Waals attractions. Also,
they showed that the number of doublet particles was increased
with increasing aging time, and that the doublet particles could
be captured and transformed into dumbbells through subsequent
coating with silica under controlled experimental conditions.21
C. Anisotropic particles with complex morphology
The seeded emulsion polymerization scheme was developed
further, making possible more diverse particle shapes, such as
ice cream cone-like or popcorn-like particles. Recently, Kim
et al. prepared triple rod, triangle, cone and diamond particles,
in a well-controlled manner with high yield (Fig. 5) and also
showed, through simple experiments, that non-spherical parti-
cles have a higher packing density than spherical particles.22
On the other hand, Chen et al. performed molecular dynamic
simulation to study self-assembled structures of cone-shaped
particles that can be prepared by sequential use of the seeded
method.14,23 To restrict the formation of assemblies to those
desired, specific interactions between building blocks were
induced by patches with different chemical or physical properties
on each building block. The simulation showed that the confi-
guration of cone packing in small clusters is the same as that
for sphere packing in evaporation-induced self-assembly for
a wide range of cone angles (sphere packing will be discussed
below). Large clusters for some specific numbers (n) of the
constituting cones showed unique packing behavior, whereas
the packings for n 12, 32, 72, and 132 were structurally similarto those of virus configurations.
The swelling and phase separation technique can be used for
superparamagnetic coreshell particles with anisotropic shapes,
as shown in Fig. 6.24 A polymer shell is formed around an
Fe3O4 and silica core, which has acrylic functional groups on
its surface, using an emulsion polymerization scheme. By
crosslinking the polymer shell, cores can be kept at the center.
Without crosslinking, the core particles could move to an eccen-
tric position because of the interfacial tension. Additional
swelling processes produced ellipsoidal particles or asymmetric
doublets.
Particles with high aspect ratios have been synthesized by
shear-induced deformation of liquid droplets.25 When mixturesof photocurable resin and solvent were emulsified, the high shear
rates associated with stirring led to the deformation and breakup
Fig. 5 SEM images of (a) triple rod particles and (b) triangle particles
prepared by monomer swelling and phase separation during polymeriza-
tion. Schematic illustration of (c) linear growth for triple rod and (d)
perpendicular growth for triangle depending on the relative crosslinking
density of the mother particles. Copyright Wiley-VCH Verlag GmbH &
Co. KGaA. Reproduced with permission (ref. 22).
Fig. 6 (a) Schematics for various shaped core-shell particles and SEM
images of (b) eccentric, (c) concentric, (d) ellipsoidal particles and (e)
asymmetric doublets fabricated by emulsion polymerization. All scale
bars are 400 nm. Reprinted with permission from ref. 24. Copyright
2007 American Chemical Society.
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of emulsion droplets. The low interfacial tension due to good
solubility of the solvent into the continuous phase resulted in
a low capillary number and therefore deformation was not
severely limited by the minimization of interfacial energy. The
deformed shape was maintained for hours or even days owing
to the attrition of shearing that induced weak polymerization.
By controlling the shear rate, viscosity ratio, and resin concen-
tration, microrods with narrow size distributions and various
aspect ratios were prepared. These rod-like particles could be
aligned by dielectrophoretic torque from an alternating current
(AC) electric field. Additionally, assemblies of microrods at an
emulsion interface can be induced to form a hairy colloido-
some.26 This structure is a permeable capsule composed of
colloids that resembles a liposome.
D. Lithographically defined particles
Solution-based methods usually result in large yields, but are
restricted to simple shapes with rounded surfaces. On the other
hand, recently developed lithography-based techniques can
lead to particles with complex shapes and sharp edges. Fig. 7
shows that a pattern on a photomask has been transferred to
a photoresistant film and the subsequent removal of unexposed
regions and of the sacrificial layer from the photoresistant film
will result in the formation of various individual particles.
Badarie et al. demonstrated that cylindrical particles can be
produced with high yields (Fig. 7) and then dispersed; columnar,
parallel, orthogonal, and isotropic aggregates can be achieved by
tuning selective interactions between the particles.27 Colloidal
Fig. 7 (a) Schematics of lithographic particle fabrication and SEMimages of prepared disk particles with aspect ratios (height/diameter) of
(b) 1 and (c) 0.33. The scale bars in (b) and (c) are 1 mm. Reprinted
with permission fromref. 27. Copyright 2007American Chemical Society.
Fig. 8 (a) Schematic diagrams for first and second step assembly processes using hydrophobic interaction and van der Waals interaction, respectively
and (b) corresponding optical and SEM images showing successful binding after first- and second-step processes. Copyright Wiley-VCH Verlag GmbH
& Co. KGaA. Reproduced with permission (ref. 30).
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alphabet particles with complex geometries have been also
prepared by lithography.28 Using double exposure of different
photomasks, Janus particles can be produced from photoresis-
tant bilayer films. More recently, 2D holographic lithography
was used to produce monodisperse anisotropic particles without
the use of photomasks.29 Interference patterns of laser beams
were used to induce cylindrical particles on a substrate. Surface
chemistry could then be precisely controlled using physical graft-
ing methods and chemical reactions. The high degree of controlover the interference pattern and short exposure times enabled
the production, with high yields, of submicrometre-sized
particles.
Recently, Onoe et al. showed that trapezium- and U-shaped
microparticles prepared by lithography with controlled surface
properties can assemble into columns or microchain structures,
respectively, through a two-step assembly process.30 These
microfabricated building blocks have two different patches:
a hydrophobic self-assembled monolayer (SAM) on a gold
surface (A surface) and hydrophilic hydroxyl groups on a silica
surface (B surface). When the particles were dispersed in a weakly
acidic solvent, hydrophobic attraction between A surfaces and
electrostatic repulsion between B surfaces induced stable AAbonding. A subsequent pH change to an acidic value removed
electronic double layers from the B surface and caused BB
bonding through van der Waals interactions. This scheme and
resulting structures are shown in Fig. 8.
Recently, lithography-based particle synthesis has been
combined with microfluidic techniques, which overcomes limita-
tions associated with the batch process. The microfluidic
synthesis allows continuous high-throughput production of
specifically designed microparticles. Fig. 9a shows how the
continuous flow of a photocurable monomer through the
channel can be selectively exposed to shuttered UV light.31,32
The UV light can be screened by a photomask. As noted, the
intensity and spot size of the light are controlled by an objective
lens. Solidification was rapid, taking less than 0.1 s. Oxygen
inhibition at the PDMS wall leaves a lubricating layer and allows
continuous synthesis without problems resulting from sticking.
The resulting particles had the same shape as the photomask,
as can be seen in Fig. 9bd. Although the polymerizationoccurred in a short time (0.1 s), the boundaries of the particles
were blunt due to the flowing resin. Therefore, the resin stream
must be stopped during the exposure with UV to obtain the
particles with sharp edges. Fortunately, the pressure-driven
laminar flow of the incompressible fluid in the microfluidic
channel can be stopped by the release of pressure within a short
delay time of 0.3 s. The use of computer-controlled pressure and
exposure systems allows a sequential cycle of stopping, exposure
and flow processes to be repeated, as necessary. This method,
called stop-flow lithography (SFL), is extremely useful for the
synthesis of nanostructured microparticles owing to the high
resolution of SFL, especially when combined with the inter-
ference lithographic technique.33,34
An alternative to direct lithographic particle synthesis is the
production of particles from emulsions in microfluidic chips.
Dendukuri et al. described how photocurable resin could be
broken into droplets in a T-junction of a microfluidic chip and
then UV-curing could induce non-spherical microparticles that
were templated by the fluidic channel.35 Depending on the
relative size of the droplet in comparison with the channel size
in which UV exposure takes place, plug- or disk-type polymeric
particles could be continuously generated. Using a similar
Fig. 9 (a) Schematic illustration of flow lithography for isolated particle fabrication and SEM images of (b) triangle, (b) square and (d) triangular rod
particles. Insetsin (b)(d) represent thefeature shapes of photomaskfor the corresponding cases.Scale bars in (b)(d) are 10mm. Reprinted with permis-
sion from Macmillan Publishers Ltd: Nature Materials (ref. 31), Copyright 2006.
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concept, Xu et al. prepared particles with a variety of shapes
using flow-focusing geometry and the nature of the photo- or
thermally curable resin.36 In particular, the functionalization of
particles with dye molecules, quantum dots, magnetic nano-
particles, or liquid crystals could lead to many practical opportu-
nities in this area. The emulsion-based method for producing
biphasic droplets was used to produce non-spherical micro-
particles.37 Two inlets for the oil phase allowed the use of
a photocurable resin stream and a non-curable stream. Singleoil droplets containing the two components were generated in
a continuous water phase. The photocurable phase could be
separated from the biphasic emulsion by selective solidification.
Various non-spherical shapes were obtained by controlling the
flow ratio of the two inlet streams. In addition, particles with
optical as well as structural anisotropy were prepared using the
emulsion-based method. In particular, optically bipolar micro-
particles could be formed by UV exposure of monodisperse
emulsion droplets, which contained photopolymerizable liquid
crystals, by the breaking off of drops in co-flow systems.38 These
particles were rotated using optical trapping by circularly
polarized light. Oblate and prolate ellipsoidal particles with
high birefringence have also been prepared using a polymermatrix by the stretching and solidification method that was
described above for ellipsoidal particles.
E. Colloidal clusters
Unlike anisotropic particles mentioned in the previous section,
colloidal clusters have three-dimensional (3D) complexity. These
particles can, therefore, be useful as building blocks for new
types of colloidal assemblies or as model particles for under-
standing the fundamental physics of particulate systems.
Recently, the emulsion-based colloidal assembly, which was
pioneered by Velev et al., has been further developed for
controlled synthesis of colloidal clusters using emulsion dropsas confining geometries.3942 In this method, a certain number
of particles which are bound to an emulsion interface are assem-
bled spontaneously into colloidal clusters during evaporation of
the emulsion phase.41,42 For a given number of particles, the final
configurations are all identical, as shown in Fig. 10. It is worth
noting that the configurations of clusters are similar to those
that have a minimum second moment of mass distribution for
n < 1 2 (n is the number of the constituent particles), which occurs
in many common molecules. Pure clusters of all identical
configurations from two-sphere clusters (doublets) to high order
clusters can be fractionated by density gradient centrifugation.
Lower order clusters from doublets (n 2) to octahedra (n
6) are subelements of the fcc lattice or its stacking variants.However, higher order clusters (n > 6) are not subunits of fcc
structures and form an unfamiliar set of packings. Therefore,
pure clusters of an identical structure have the potential to
form extraordinary crystalline structures through controlled
assembly.
To understand the clustering process of colloidal particles on
emulsion interfaces, Lauga et al. performed numerical calcula-
tions using Surface Evolver simulations. As the emulsion drop
shrinks, colloidal hard spheres come into contact with neigh-
boring spheres and form a critical packing structure that is
unique and forms without deformation of the emulsion interface.
With further reduction in the drop volume from the critical
packing state, the emulsion interface begins to deform and the
capillary forces pull the particles inside the emulsion drop.
Numerical simulations predict the same configurations that are
found in experimental studies, indicating that the detailed
physics of these unique configurations was correct. In addition,
simulations showed that particles with an identical contact angle
formed a unique packing structure, whereas particles with
different contact angles from each other can lead to different
configurations.
Following the developments outlined so far, the emulsion-
assisted method of colloidal self-organization has been modifiedfor use with other, more general, colloidal materials including
polymers, ceramics and metals. This has been achieved by the
inclusion of an additional reaction on the surface of the clusters
that increases the structural rigidity.43 Moreover, the micro-
fluidic technique has been applied in order to prepare mono-
disperse emulsion droplets and thereby to produce uniform
colloidal clusters.44 The uniformity of the resulting aggregations
is enhanced compared with those from polydisperse emulsion
droplets. For a higher yield, relatively large amounts of uniform
emulsions were recently prepared using a Couette cell.45 The
production rate of the resulting aggregate was high, and
enhanced uniformity was achieved.
As for molecules, colloidal clusters with directional inter-actions can form much more diverse colloidal structures that
cannot be expected for crystalline structures of isotropic spheres.
Therefore, clusters can be used as new model systems for
studying unusual optical or other physical properties if we can
create chemically or physically distinct sites in designated
positions on a colloidal particle or cluster. For example, Ngo
et al. reported that a tetrastack structure composed of tetra-
hedrons has a full photonic band gap at a refractive index
contrast as low as 2.1.46 However their simulation result has
not been confirmed experimentally yet owning to difficulty in
constructing the tetrastack structure of tetrahedrons. Zhang
Fig. 10 (a) Schematic for self-organizing particles confined in emulsion
interface and (b) experimental and Surface Evolver simulation results for
consolidated colloidal clusters for n 412. For a given n, clusters have
all identical configurations. Reprinted with permission from ref. 42.
Copyright 2004 American Physical Society.
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et al. showed by simulation that hard spheres with four attractive
patches can act as colloidal molecules and be assembled into
a diamond structure if seed crystallites are included.47 The
diamond structure is a promising candidate for a material with
a full and robust photonic band gap, and it still remains
challenging to synthesize diamond-like colloidal crystals. In
addition, the effect of the number of patches upon the phase
diagram was studied by numerical simulation for monodisperse
patchy colloidal particles and their mixtures.48 However, experi-mental studies of the assembly of patchy clusters with specific
attractions are limited due to the complex fabrication steps
required to create sticky patches.
As a first step for well-coordinated clusters, we reported that
binary colloids of different sizes in emulsion drops can lead to
coordinated patches on the outer surface of a cluster by encap-
sulating the larger particle cluster with smaller particles
partially.49,50 When the amount of small particles is not enough
for complete coverage of the entire cluster, the small particles
do not interrupt the process of assembly for the large particles.
In addition, we found that the interparticle potential between
large and small particles can play a critical role in the control
of the morphology in this process. Patchy clusters were preparedsuccessfully from the cocharged particles, while the counter-
charged particles produced different types of clusters in which
small particles covered entirely the clusters of the large particle.
Fig. 11 shows the SEM images of partially encapsulated colloidal
clusters.
Meanwhile, Yin et al. fabricated colloidal clusters using micro-
hole arrays as templates. The cluster configurations are different
from those obtained from the emulsion-based technique due to
the anisotropic geometrical confinement.51 The ratio of particle
to hole sizes and the number of stacking layers determine the
configuration of the cluster. In particular, they demonstrated
seed-induced crystallization by introducing the square tetramers
into spherical particle suspension. Although the resulting crystal
had an fcc lattice, it was meaningful in view of controlling the
crystal plane using colloidal clusters as seeds.
Realization of advanced functional structures by self-assembly
of clusters raises many challenging issues. For example, delicate
control of interactions between the patches of neighboring
particles at optimal strength will be important to avoid severeaggregation before crystallization of the desired structure and
to sustain the structure after crystallization under external
disturbances.
III. Chemically patterned colloidal particles
In Roman mythology, Janus (the god of gates, doors, beginnings
and endings) was usually pictured with a double-faced head
looking in two opposite directions. The face originally repre-
sented the sun and the moon. Because of the two different
natures that coexisted in the head of Janus, De Gennes used
the name of this Roman god to describe a particle containing
two different chemical compositions. Fabrication methods toproduce such particles have been studied extensively over the
past few decades. The 2D-based synthesis has been one of the
most widely used methods to fabricate chemically patterned
colloidal particles. Casagrande et al. first demonstrated the
partial protection technique, as schematically illustrated in route
1 of Fig. 12a.52 In this work, cellulose film was used as a partial
protection layer for microspheres, and the unprotected regions
of particles were treated with the hydrophobic moiety, octadecyl-
trichlorosilane, in order to synthesize amphiphilic particles.
Likewise, Cui et al. used PDMS as a partial protection layer,
and the unprotected regions were modified with silver by electro-
less deposition.53 Also, Bao et al. used a photoresistant protect-
ing layer, and metals and metal oxides were deposited on theexposed parts of the silica particles using an electron-beam
evaporator.54 Finally, Paunov et al. used the gel-trapping and
replication technique for partial protection of particles.55 A
monolayer of polystyrene microspheres was formed at the inter-
face of pre-heated oil and water phases. Here, the water phase
contained a gelling agent. The contact angle of the oilwater
interface at the particle surface determined the proportion of
the polystyrene particles that was exposed. The gelling agent in
the water phase immobilized the polystyrene microspheres
when the temperature was cooled to 25 C. Subsequently, the
oil phase was replaced by PDMS elastomer, and then the particle
monolayer was transferred onto the PDMS surface from the
gelled water phase when the PDMS elastomer was peeled off.The PDMS layer acted as a partial protection layer during
gold sputtering and allowed the fabrication of Janus particles.
The Janus particles that were prepared could be detached from
the PDMS using sticky tape.
If one can manage to keep stable directional flux, the layers for
partial protection of colloids are not needed, as schematically
illustrated in route 2 of Fig. 12a. Bao et al. demonstrated that
gold-capped silica particles can be prepared through the direct
deposition of a metal layer on the non-contacting silica
monolayer followed by partial etching of the metal layer.54
Also, Hong et al. used the direct deposition of gold using
Fig. 11 SEM images of partially encapsulated clusters prepared from
emulsion drops containing bimodal silica particles. The clusters of large
silica particles have well-coordinated patches which are not covered by
small silica particles but exposed to the outside. Reprinted with permis-
sion from ref. 49. Copyright 2005 American Chemical Society.
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electron-beam deposition on the monolayer of carboxylate-
modified colloids. The gold-deposited parts were treated with
a positively charged monolayer. Therefore, the spherical
particles with opposite electric charges could be prepared and
assembled into the colloidal clusters.56 Interestingly, Cayre
et al. used microcontact printing of insoluble surfactants on
a monolayer of polystyrene microspheres to fabricate dipolar
colloidal particles without employing partial protection layers.57
First, a monolayer of monodisperse polystyrene microspheres
was prepared on a solid substrate. Then, water-insoluble surfac-
tants with opposite charges were deposited on a poly(dimethyl-
siloxane) (PDMS) elastomer. Finally, the water-insolublesurfactant film was stamped onto the polystyrene monolayer
and the fabricated dipolar colloids were redispersed in water.
Recently, colloidal masks have also been used to make exotic
Janus particles, as schematically illustrated in route 3 of Fig. 12a.
Bae et al. used the contact areas of a multilayer colloidal crystal
as a mask for octadecyltrichlorosilane treatment to prepare
chemically nanopatterned colloids.58 The hydrophilic protected
region of these colloids was used as a site for specific nucleation
and growth of titania. Similarly, Zhang et al. used a colloidal
mask to make gold-decorated particles.59 Colloidal crystals
were assembled using dip coating and upper colloidal crystal
layers etched by O2 plasma. The etched upper layers acted as
colloidal masks for gold vapor deposition to decorate micro-spheres. The final fabricated structures could be controlled by
changing the stacking structure (hcp or fcc) of the colloidal
crystal or the angle between the vapor flow and the normal to
the sample surfaces.
While the 2D-based synthesis described above allows the
preparation of well-defined Janus particles, it requires a relatively
large area and the fabrication procedures are quite complicated.
This is particularly the case if one needs to prepare larger
quantities of the samples. Most of these techniques require
monolayers of microspheres, but none of the methods that
have been reported so far were sufficiently robust and simple.
Therefore, a few groups have reported the development of an
alternative approach. Perro et al. demonstrated a solution-based
batch process capable of fabricating Janus particles in large
quantities, as shown in Fig. 12b.60 Snowman-like silicapoly-
styrene hybrid nanostructures were synthesized using emulsion
polymerization in the presence of silica colloids, the surfaces of
which had been modified by polymerizable groups. As the poly-
styrene nodules acted as partial protectors, the functional silane
coupling agents could be treated only on unprotected areas of
the silica particles. The separation of the Janus silica particles
from the polystyrene was achieved in aqueous solution by ultra-
sonication and ultracentrifugation. The silane coupling agentswith different functionality, on the other side of the Janus
particle, could then be treated. Hong et al. captured the colloids
at the liquidliquid interface between emulsified molten wax and
water at 75 C. The particles were locked by cooling to room
temperature, and further modified chemically. The emulsions
can be prepared by vigorous magnetic stirring, and consequently
the method can be applicable to large-scale fabrication of Janus
particles.61
More recently, electrohydrodynamic or microfluidic devices
have been introduced for the continuous production of Janus
particles (Fig. 12c). Roh et al. fabricated biphasic Janus particles
using electrohydrodynamic co-jetting of distinct polymer solu-
tions with small amounts of additives that had different colorsor functional groups, as can be seen in Fig. 13.62,63 Laminar
flow streams of different polymer solutions were introduced,
with side-by-side geometry, to the modified metal nozzle.
When a high voltage was applied between the metal nozzle and
the collector, the polymer solutions elongated and formed
Taylor cones at the exit tip. This occurs due to the balance
between two competing forces of electric Maxwell stress and
interfacial tension. An ultrathin thread of polymer solution
was ejected from the Taylor cone, and the solvent was
evaporated during flight on the way to the collector. The final
polymeric biphasic colloids were left on the collector. The final
Fig. 12 Schematics for fabricating Janus particles. (a) 2D-based synthesis using partial protection, directional flux, and colloidal mask. (b) Solution-
based batch process. (c) Continuous synthesis using microfluidics.
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morphologies of the colloids could be controlled by varying the
properties of the polymer solution (viscosity, conductivity, and
surface tension) and the jetting conditions (applied electric field
strength, flow rate, tip-to-collector distance, etc.). Moreover,
triphasic nanocolloids could be prepared using a modified nozzle
with three inlets.64
Turning to microfluidic devices, Nisisako et al. demonstrated
synthesis of Janus particles with color and electrical anisotropy
using a microfluidic co-flow system, as shown in Fig. 14.65
Carbon black and titanium oxide were dispersed in an acrylic
monomer for the preparation of black and white pigments,
respectively. Both pigment streams were introduced into the
co-flow system with the same flow rate to generate bicolored
emulsion droplets. The fabricated bicolored pigment droplets
were polymerized at 90 C on the outside of the microfluidic
chip. Because of the difference in charge densities between
carbon black and titanium oxide, the resulting Janus particles
had electrical anisotropy. Therefore, the Janus particles were
able to be actuated or flipped by electrophoretic rotation, as
shown in Fig. 14cd. In addition, sixteen-sheath flow geometries
were also reported for mass production of Janus particles.Shepherd et al. used in situ UV curing of colloid-filled hydrogels
instead of thermal polymerization on the outside of the
microfluidic chip.66 Two different colored silica-dye coreshell
particles were prepared and dispersed in an aqueous solution
containing acrylamide, a crosslinker and a photoinitiator. The
colored aqueous colloidal dispersions were introduced with
a Y-junction, and the laminar flow of the two colored streams
was ruptured by a shear flow in order to fabricate Janus drops.
When the microfluidic channel was high enough, spherical
granules were formed. On the other hand, if the channel height
was smaller than the drop radius, discoidal granules formed.
The acrylamide in the ruptured drops was cured by UV irradia-
tion in the microfluidic channel. This photopolymerization led tothe anisotropy in shape and chemical composition by immobi-
lizing the colloids in hydrogel network. Millman et al. used a
dielectrophoretic force to entrap and transport suspended drop-
lets.67 Here, each suspended droplet acted as a microreactor for
the fabrication of, for example, eyeball particles or striped
multilayer particles.
Janus particles look very promising for use in many applica-
tions, such as for functional building blocks, emulsion stabilizers,
e-paper, bifunctional carriers for drugs, catalysis, and so on.
However, continuing research is still needed for the large-scale
production of well-defined Janus particle at low cost.68
Fig. 13 (a) Electrified co-jetting for Janus nanoparticles. Reprinted from Nature Materials with permission from Macmillan Publishers Ltd (ref. 62);
copyright 2005. (b) Schematic for bioconjugation to a single hemisphere of Janus particle and confocal laser scanning microscope images (c) before and
(d) after protein binding. Reprinted in part with permission from ref. 63. Copyright 2007 American Chemical Society.
Fig. 14 Optical microscope images of (a) the junction for Janus drop
formation and (b) the whole channel; (c) and (d) electric field induced
color switching for an e-paper application. Copyright Wiley-VCH Verlag
GmbH & Co. KGaA. Reproduced with permission (ref. 65).
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IV. Internally structured colloidal particles
The performance of microspheres and their self-assembled struc-
tures is determined by their morphology, as has been described
previously. Until this point, we have focused on the control of
the external morphology of particles. In this section, we will
discuss control of the internal structure of spherical particles.
Many research groups have demonstrated the preparation of
hollow spheres using templating methodology. As shown inFig. 15, one of the most widely used approaches has been
colloidal particle templating. Here, coreshell particles are first
prepared from colloidal particles such as polymer latex,6973
silica,7477 gold,78,79 ZnS,75 and so on. Typical template particles
are coated with various materials to prepare coreshell
composite particles by controlled surface precipitation of
inorganic materials;70,7274,78 direct surface polymerization using
functional groups on the surfaces;76,77 or layer-by-layer adsorp-
tion of a polyelectrolyte and of charged nanoparticles.69 The
core is subsequently removed by calcination at high tempera-
tures or dissolved in an appropriate solvent. Finally, hollow
spheres of various materials such as silica,6971,73,75,78 titania,72
magnetic particlesilica composites,71
gold,74
polymers,76,77,79
ZnS,75 or organicinorganic hybrids69 have been obtained.
Additionally, hollow spheres with movable gold cores have
been fabricated.80 In this case, gold nanoparticles were coated
with silica using tetraethylorthosilicate, and the goldsilica
coreshell particles were treated with silane coupling agents,
which acted as the initiators for atom transfer radical polymeri-
zation of poly(benzyl methacrylate). The silica layer between
gold core and the polymer shell was dissolved by aqueous
hydrofluoric acid (HF) solution to produce hollow polymeric
spheres with movable gold cores. However, such methods for
hollow particles using solid cores as templates are relatively
complicated in terms of the conditions required for shell coating
and for the removal the solid cores (i.e., a high temperature or
a toxic solvent) rather than those needed when liquid cores
such as emulsion droplets8184 or vesicles8588 are used. Zoldesi
et al. have fabricated highly monodisperse hollow particles
with different shapes according to the thickness of their shells,such as hollow spheres, microcapsules, and microballoons.
This is shown in Fig. 16. Monodisperse and stable oil-in-water
emulsion droplets were prepared by hydrolysis and polymeriza-
tion of dimethyldiethoxysilane with83 and without82,84 surfac-
tants. The modified Stober method was used to achieve the
encapsulation of oil droplets with a solid shell. The thicknesses
of the solid shells were controlled by changes in the time interval
between the preparation of oil droplets and the addition of
a silica precursor.
Microphase separation of block copolymers has been widely
used to control the internal structure of polymeric films. Okubo
et al. first used microphase separation of block copolymers to
control the internal morphologies of polymeric spheres.89 Adiblock copolymer solution was emulsified using a homogenizer
to generate oil-in-water emulsions that confine the geometry to
that of a sphere. As the volatile organic solvent evaporated,
the block copolymer self-assembled into nanostructured spheres
due to microphase separation. The morphology of the nano-
structured spheres could be controlled by changing the evapora-
tion rate of solvent (toluene), the size of confining emulsion
Fig. 15 Schematic of the method of particle templating for hollow particles and TEM images of hollow silica shells. Copyright Wiley-VCH Verlag
GmbH & Co. KGaA. Reproduced with permission (ref. 70).
Fig. 16 TEM images of: (a) and (d), hollow microspheres; (b) and (e), microcapsules; (c) and (f), microballoons prepared by emulsion droplet
templating. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission (ref. 82).
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drops and the molecular weight of the diblock copolymer, as well
as by the addition of a homopolymer. We also used an emulsion
droplet as a confining geometry for the self-assembly of block
copolymerhomopolymer blends.90 The effects of the particle
size and the content of homopolymer on the internal
morphology of the nanostructured spheres were extensively
studied by systematic investigation. Some of the representative
internal structures were reproduced in Fig. 17. Yabu et al.have demonstrated the fabrication of spherical particles with
well-developed lamellar structures using modified reprecipitation
methods.91 In conventional reprecipitation methods, small
amounts of polymer solution are dropped onto a large quantity
of poor solvent, causing the polymer particles to immediately
precipitate from the dropped polymer solution. When the
conventional reprecipitation method was applied to the fabrica-
tion of block copolymer particles, the lack of time for long-range
ordering of the block copolymer meant that copolymer particles
with well-developed lamellar phases could not be obtained.
Thus, in a modified reprecipitation method, small quantities of
poor solvent were slowly dropped into the polymer solution
in a good solvent. As the volatile good solvent graduallyevaporated, a concentration gradient was formed. In this way,
spherical particles with well-developed lamellar structure could
be produced owing to the exposure of block copolymer particles
to good solvent during reprecipitation. Nanostructured triblock
copolymers could also be produced by quenching the dilute
polymer solution.92
In a similar manner, the evaporation-induced self-assembly of
silica precursor and surfactants inside the droplets, generated
from a vibrating orifice aerosol generator, produced mono-
disperse porous silica particles of spherical shape.93 Simply
changing the orifice diameter or the concentration of the
precursor solutions allowed the diameter of the particles to be
controlled. Subsequently, inside the aerosol droplets, the surfac-tants were completely removed by annealing at high tempera-
tures, which produced internal pore structures. The porous
silica particles thus produced had uniform periodic pore struc-
tures in some regions that were aligned parallel or perpendicular
to the surface.
The preparation of nanostructured spheres has been the
subject of a great deal of attention owing to their potential
uses in a variety of applications, including controlled storage
and release of functional materials, high-performance catalysts,
sensors, and building blocks for functional self-assembled struc-
tures. Although there has been some research conducted into
their potential applications, more efforts and research are
required. This research should be aimed at enabling us to control
and optimize the morphologies that are produced, as is needed
for the development of specific applications.
V. Summary and outlook
In this Feature Article, we have discussed the general features in
colloidal particles and their assembled structures. In principle,a colloid is a functional building block in itself ranging from
several tens of nanometres to micrometre scales for 2D and
3D ordered architectures for photonic nanostructures or as
microprobes for high-throughput screening of biomolecules or
chemical substances. Depending on the external or internal
structures of the particles, they can have unique optical, mecha-
nical, or electrical properties that may be useful for the develop-
ment of novel photonic crystals, composites, plasmonic
materials, or memory devices.
For example, as shown in Fig. 18a, mixtures of binary colloids
can produce, through selective removal processes, a diamond or
pyrochlore structure of spheres.94 Although the experimental
realization of this is challenging, it is feasible since a co-crystal-lization method has already been developed95,96 and their
photonic band gaps are wide and robust in spite of a few defects
that are inevitable in self-assembling processes. Patchy particles
or colloidal clusters allow us to imagine much more diverse
structures, such as those formed by molecules. For example,
a simple cubic structure can be realized using 6-fold patchy
colloidal particles as shown in Fig. 18b, while 4-fold patchy
colloidal particles form a dodecahedral structure, as shown in
Fig. 18c.97 Extending these, we can design even more diverse
systems using non-conventional structured colloidal particles
to produce new complex structures, even though the particles
themselves would need to be more developed for such complex
colloidal suprastructures.
Fig. 17 TEM images of nanostructured microspheres for three differentratios of particle size to feature spacing of (a) 2.5, (b) 3.3 and (c) 7.0
prepared by microphase separation of a block copolymerhomopolymer
blend in an emulsion droplet. Reprinted in part with permission from
ref. 90. Copyright 2007 American Chemical Society.
Fig. 18 (a) MgCu2 structure and pyrochlore structure that could be
obtained by crystallization of binary colloidal mixture and selective
removal of large particles from MgCu2 structure. Reprinted from Nature
Materials with permission from Macmillan Publishers Ltd (ref. 94),
copyright 2007. (b) Simple cubic structure and (c) dodecahedral structure
composed of 6-fold and 4-fold patchy colloidal particles, respectively.
Reproduced by permission of the PCCPOwner Societies (ref. 97).
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Finally, spontaneous formation of well-ordered colloidal
arrays provides lithographic masks or scaffolds for creating
useful patterns. In this case, modification of the self-assembled
mask will improve the versatility of nanosphere lithography in
fabricating novel nanopatterns such as nanocups, hollow shells,
and multifaceted materials.98101
AcknowledgementsThis work was supported by a grant from the Creative Research
Initiative Program of the MOST/KOSEF for Complementary
Hybridization of Optical and Fluidic Devices for Integrated
Optofluidic Systems.
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