Synthesis and Directed Self-Assembly of Patterned Anisometric Polymeric Particles.
ABSTRACT A simple and versatile method for making chemically patterned anisotropic colloidal particles is proposed and demonstrated for two different types of patterning. Using a combination of thermo/mechanical stretching followed by a wet chemical treatment of a sterically stabilized latex, both patchy ellipsoidal particles with sticky interactions near the tips as well as particles with tunable fluorescent patterns could be easily produced. The potential of such model colloidal particles is demonstrated, specifically for the case of directed self-assembly.
Synthesis and Directed Self-Assembly of Patterned Anisometric Polymeric
Zhenkun Zhang,†Patrick Pfleiderer,†Andrew B. Schofield,‡Christian Clasen,†and Jan Vermant*,†
Department of Chemical Engineering, K.U. LeuVen, Belgium, and Department of Physics,
UniVersity of Edinburgh, United Kingdom
Received September 8, 2010; E-mail: firstname.lastname@example.org
Abstract: A simple and versatile method for making chemically
patterned anisotropic colloidal particles is proposed and demon-
strated for two different types of patterning. Using a combination
of thermo/mechanical stretching followed by a wet chemical
treatment of a sterically stabilized latex, both patchy ellipsoidal
particles with sticky interactions near the tips as well as particles
with tunable fluorescent patterns could be easily produced. The
potential of such model colloidal particles is demonstrated,
specifically for the case of directed self-assembly.
Controlling particle shape and imparting directionality in interac-
tions have been identified as key concepts for future progress in
colloid and nanoparticle materials design,1certainly by using
principles of directed self-assembly.2Recent advances in particle
synthesis and functionalization have proven the power of these
concepts, but the synthesis methods for suitable anisotropic particles
are normally rather cumbersome and the yields are limited.3
Compared with the relative ease of manipulating spherical particles,4
introduction of patchiness or patterns onto anisotropic particles in
a site-specific manner remains challenging and only a few examples,
typically limited to metal or semiconductor nanocrystals, have been
reported.5,6In the present work, a simple strategy for producing
anisometric colloidal particles has been developed, typically to be
used for, but not limited to, spheroids. The method yields particles
which can be chemically patterned to produce a range of materials,
going from uniform sterically stabilized hard-core ellipsoidal
particles, over fluorescently patterned stable “pom-pom” particles
to patchy ellipsoids with sticky attractive interactions near the tips
(“inverse pom-pom” particles). The method works for particle sizes
ranging from about 100 nm up to several micrometers, provides
monodisperse particles with aspect ratios ranging from 3 to 8, and
gives yields on the order of grams.
Ellipsoidal particles in the colloidal domain find numerous
applications that specifically exploit their shape, for example in
photonics, efficient stabilizing of Pickering emulsions and model
systems for understanding the shape-effect on the drug delivery, to
cite just a few examples.7Particle shape also affects fundamental
aspects of colloidal behavior such as Brownian motion, maximum
packing, crystal structures of anisotropic particles, as well as capillary
force-mediated self-assembly at a liquid-liquid interface.8-10The
ellipsoidal particles used in this work were prepared on the basis of a
principle originally developed by Keller and co-workers (Scheme 1A):
11spherical latex particles were embedded in a polymer film which
was then heated above the glass transition temperature (Tg) of the latex
and stretched to certain strain. The latex particles deformed into
ellipsoids. This method has been extended to many kind of polymeric
particles and is not limited to ellipsoids as even more complex shapes
have been obtained (e.g., worms, disks, etc.).12Here, poly(methyl
methacrylate) (PMMA) latex particles were dispersed in a poly(dim-
ethylsiloxane) (PDMS) solution in hexane, which was subsequently
cast and cross-linked into a rectangular film.13The PMMA particles
are grafted with a layer of comblike stabilizers consisting of a linear
PMMA as the backbone and poly-12-hydroxystearic acid (PHSA) as
the teeth (Scheme S1, Supporting Information (SI)). These particles
have been used as model hard spheres and have played a pivotal role
in the understanding of colloidal suspensions.14The cross-linked
PDMS film loaded with PMMA particles was stretched to a given
strain and heated to a temperature above Tgof the PMMA (Scheme
1A). The particles deform into prolate ellipsoids and are recovered
from the film after solidification by cooling, using sodium methoxide
(SM) to degrade the PDMS matrix.10Stretching particles that initially
carry a fluorescently labeled stabilizer revealed ellipsoids with an
essentially constant fluorescent halo, if anything slightly denser near
the tips. This suggests the stabilizer redistributes itself during stretching
above Tg, in agreement with the earlier observations by Ottewil and
co-workers for the charge distribution of polystyrene (PS) ellipsoids
prepared from stretching PS particles.15
In the present work, by monitoring the degrading procedure, it
was observed that the originally grafted comblike stabilizers were
removed from the particle surface by sodium methoxide (SM). This
occurs because the stabilizers are linked to the surface by ester
bonds which can be attacked by SM via a transesterfication
process.16More importantly, we observed that the stabilizers near
the tips of the ellipsoids are removed faster than those in other
areas (Scheme 1B). This may be caused by the higher surface
curvature near the tips with several factors possibly contributing.
First, the brush consists of densely grafted chains which are
restricted in their motion.17Recent measurements showed that the
‡University of Edinburgh.
Scheme 1. (A) Thermo/mechanical Stretching of Polymeric
Particles in a Film. (B) Curvature-dependent Chemical Wet
Etching. (C) Tip-to-tip Assembly of Patchy Ellipsoids. (D)
Preparation of Fluorescently Patterned Particles
Published on Web 12/01/2010
10.1021/ja108099r 2011 American Chemical Society
392 9 J. AM. CHEM. SOC. 2011, 133, 392–395
maximum size of a molecule that can enter a brush for a spherical
particle is 0.65 nm.18The stretching of the particles will lead to a
decrease of the average grafting density and, enhanced by the
curvature, a locally slightly more open and more accessible brush.
Second, as recent experiments demonstrate, mass transfer depends
on the local curvature, simply because there are more molecules
per unit area available for adsorption when the curvature is higher.19
Hence, the etching of the surface-grafted groups becomes curvature
Evidence of this localized, surface-curvature-dependent etching
of the stabilizer is three-fold: Negative staining TEM reveals that,
at the initial stage of the degradation, the surface of the bare PMMA
ellipsoids is smooth and that, with increased degradation time,
defects appearing first near the tips gradually spread over the whole
surface as the protective grafted layer is removed (Figure S2 (SI)).
Second, the curvature-dependent etching produces site-specific
patchy particles: the structure of inverse pom-pom like anisotropic
polymeric particles can be inferred from their self-assembly in bulk
via their bare tips (as discussed later on). Finally, following the
procedure illustrated in Scheme 1B and D and replacing the original
stabilizer with a second fluorescent species, the localized etching
can directly be visualized and fluorescently patterned ellipsoids can
be prepared. For this purpose, the same comblike stabilizers as those
originally grafted onto the PMMA surface, but now with a
fluorescent dye (7-nitrobenzo-2-oxa-1,3-diazol (NBD)) randomly
distributed along the backbone, were used as the fluorescence
marker. This fluorescent stabilizer can physically adsorb onto the
PMMA surface and can be chemically grafted when required.20
The area of the particle from which the initial stabilizer will be
removed can be controlled by varying the concentration of the
degrading agent (SM) and, more importantly, the degradation time.
For short wet etching times, the dominant reaction is the breakage
of the siloxane bonds of the PDMS matrix, and the particles
recovered for etching times shorter than 8 h do not readsorb
significant amounts of fluorescent stabilizer (Figure 1A). For longer
wet etching times, the original stabilizers are first removed near
the tips of the ellipsoids, where the curvature is highest. When the
fluorescent stabilizer is subsequently adsorbed, the resulting particles
have two fluorescent tips and a relatively dark middle part (Figure
1B). With increased wet etching time, the area where the original
stabilizer is removed increases and progresses from the tips to the
equatorial area (Figure 1B-E), giving rise to fully fluorescently
labeled particles after etching for 48 h (Figure 1F). Figure 1G
depicts the temporal evolution of the fluorescent area, which is
representative of the area where the original stabilizer has been
etched away, as a function of etching time. The patterns at each
specific stage are reasonably uniform in size (inset of Figure 1G).
When spheres are subjected to the same procedure as the ellipsoids,
particles with homogeneous fluorescent rings are observed (Figure
S3 (SI)), confirming that the localized degradation of the stabilizer
is due to differences in curvature. The patterned ellipsoidal particles
were further characterized by confocal laser scanning microscopy
(CLSM) (Figure S4 (SI) and insets in Figure 1B-F). The variation
of the fluorescent intensity along the long axes of the ellipsoids
obtained from the CLSM images (Figure 1H) confirms the
fluorescence distribution. In conclusion, using a simple method
spatially modulated patterns are introduced onto otherwise homo-
geneous anisotropic polymeric particles at the truly colloidal length
These fluorescently patterned particles can aid the visualization
of colloidal dynamics and self-assembly processes, for example
those controlled by capillary forces occurring at a liquid-liquid
interface.2In bulk of apolar solvents such as decalin, hexane, etc.,
the produced particles interact with a well-known relatively hard
interaction from the steric repulsion,21as can be concluded from
the well-dispersed states in all pictures in Figure 1. When the
particles were dispersed at the decalin/air interface, the particles
self-assembled side-by-side into well-defined stripes images (inset
of Figure 2A), due to shape-induced capillary interactions.9Figure
2B shows that single stripes formed by particles in Figure 1C can
further self-assemble into a 2D-smectic-like structure. In this
example there is a clear benefit for in situ visualization of the details
of the local structure as fully fluorescent particles would lead to a
blurring of the image (inset of Figure 2A). The stripes can also
grow a millimeter long and evolve into morphologies of rich,
hierarchical structures (Figure 2B). When mixing fluorescently
patterned particles (Figure 1D) with fully fluorescent ones (Figure
1F), optical patterns can be introduced into the stripes (inset of
When no fluorescent stabilizer is readsorbed to the etched
ellipsoids, patchy particles with localized sticky interactions are
obtained. The patterns of chemical heterogeneity on these particles
will mimic those observed for the fluorescent particles in Figure 1,
and the etching time required for an amount of bare versus covered
areas for these “inverse pom-pom” particles can be directly inferred
from the trends of Figure 1G. For etching times shorter than 8 h,
the particles expose no bare surface yet, and the suspensions are
stable (Figure 3B). After wet etching the film and the particles for
slightly more than 8 h, the particles will have two symmetric bare
Figure 1. Fluorescently patterned anisotropic particles. (A-F) Fluorescent
microscopy images of particles recovered from varying wet etching times
and after readsorption with fluorescent stabilizers. The final concentration
the sodium methoxide is 0.04% (w/w). The particles were dispersed in
decalin. The insets are CLSM images. Under each image, the schematic
structure and the degradation time are shown (scale bar: 5 µm). (G)
Evolution of the relative fluorescent area (FA) with the etching time. (Inset)
distribution of the FA over the total area (TA) for the particles shown in
(D) based on 50 particles. (H) Fluorescence intensity profiles along the
maximum axis of the particle listed in the insets of (A-F).
J. AM. CHEM. SOC. 9 VOL. 133, NO. 3, 2011
tips and a middle part that is still sterically stabilized by the
comblike stabilizer in a good solvent for the latter, such as
decalin.21The particles will then remain sterically repelling on the
sides, while attractive interactions such as the van der Waals forces
or attractions between polar groups at the surface will dominate
the interaction near the tips. Such directionality in interactions of
anisotropic particles has so far only been reported for metallic
nanorods by taking advantage of the selectively binding of ligands
to certain crystal facets.5,6With the proposed method, the size of
the bare area at the tips can be easily controlled by varying the
degradation time as was shown by the fluorescently tagged
counterparts of the present particles in Figure 1.
This directional interaction forces the particles to self-assemble
into chains by connecting their tips, as shown in the in situ bright-
field microscopy image (Figure 3C). Particles with small bare area
at the tips will connect with each other with a bond angle R around
180° (inset Figure 3C, corresponding to the sticky counterparts of
the particles in Figure 1B and C). With increasing degradation time,
the bare areas at the tips increase (corresponding to the sticky
counterparts of the particles in Figure 1D and E), which makes a
wider range of bond angles possible (inset in Figure 3D) and also
allows more than two particle tips in a connection point. The
increased connectivity combined with the increase in the strength
of attraction per particle leads to the formation of an aggregated
network structure (Figure 3D). Further reduction of the area covered
by the stabilizer layer at this volume fraction results finally in a
collapse of the network and the formation of random dense
aggregates (Figure 3E). All aggregated suspensions can be brought
back to a stable state by remixing with stabilizers. These qualitative
observations demonstrate the potential applications of the inverse
pom-pom particles as a model to investigate the emerging topic of
polymerization-like self-assembly of nanocolloidal rods with di-
In summary, curvature-dependent wet chemical etching pro-
vides an easy route to produce patchy nonspherical particles,
where the size and shape of the particles and nature and extent
of the patchiness can be easily varied. In the present work,
colloidal ellipsoids were synthesized with gram-scale yield,
possessing tunable interparticle interactions ranging from stable
over patchy to fully attractive. We demonstrated this technique
by producing patchy, inverse pom-pom ellipsoids which interact
directionally and can self-assemble into chains or denser
structures in bulk, depending on the etching conditions. Similarly,
locally labeled fluorescently patterned ellipsoids are made which
offer advantages of visualization in microscopy observation.
Since curvature difference exists in any anisotropic shape, the
current method may represent a general, cost-effective and
scalable way to introduce site-directed properties onto a wide
range of model colloids, suitable as model systems for colloid
physics, for exploiting directed self-assembly, and to investigate
colloidal gels and biomimetic networks.
Acknowledgment. We gratefully thank Dr. Pavlik Lettinga,
Prof. Jan Dhont (FZJ) and Prof. Johan Hofkens (K.U. Leuven)
for their help in microscopy and discussions. This work was
supported by the EU (Nanodirect Project Grant No. CP-FP
213948, ERC Starting Grant No. 203043-NANOFIB) and the
Supporting Information Available: Detailed experimental proce-
dures, additional results from each experiment. This material is available
free of charge via the Internet at http://pubs.acs.org
Figure 2. Capillary force-mediated self-assembly of hard-core ellipsoids
at an oil/air interface. (A) Smectic-like 2D structure. (Inset) Single stripe
observed by CLSM. (B) Hierarchal structure. (Upper-right inset)
Magnified image of the side-by-side assembly of mixed fluorescently
patterned and fully fluorescent ellipsoids, scale bar: 2 µm. (Bottom-left
inset) Structure of more complicated morphology. Scale bar: 10 µm in
(A) and 5 µm in (B).
Figure 3. In situ visualization of directional self-assembly of inverse pom-
pom ellipsoids at varied degradation stages by bright-field optical micro-
scope. (A) Schematic of self-assembly of particles of varied surface
properties. (B) Stable suspension of ellipsoids fully covered with stabilizers.
(C) Linear chains formed by particles with the same 15% bare area at each
tip as in Figure 1C. (D) Aggregated network formed by particles with the
same 30% bare area as in Figure 1D. (E) Random aggregates of particles
which are fully stripped of stabilizers. Insets in (C) and (D): Typical angular
configuration of the ellipsoids in the chains. Scale bar: 10 µm.
394 J. AM. CHEM. SOC. 9 VOL. 133, NO. 3, 2011
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