another review of particle methods 2008

8/2/2019 Another Review of Particle Methods 2008

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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


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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
http://www.rsc.org/materialshttp://www.rsc.org/materials


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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.

2178 | J. Mater. Chem., 2008, 18, 21772190 This journal is The Royal Society of Chemistry 2008


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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).



12/15

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).



13/15

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).



14/15

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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2190 | J Mater Chem 2008 18 2177 2190 This journal is The Royal Society of Chemistry 2008

another review of particle methods 2008

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