Patchy particle self-assembly via metal coordination

Article abstract of DOI:10.1021/ja4075979

Colloids with high-symmetry patches are functionalized with metal-coordination-based recognition units and assembled into larger chain architectures, demonstrating for the first time the use of metal coordination as a specific force in colloidal self-assembly. The cross-linked poly(styrene)-based patchy particles are fabricated by encapsulation of colloidal clusters following a two-stage swelling and polymerization methodology. The particle patches, containing carboxylic acid groups, are site-specifically functionalized either with a triblock copolymer (TBC), bearing primary alcohols, alkyl chains, and palladated pincer receptors, synthesized by ring-opening metathesis polymerization, or with a small molecule bearing a pyridine headgroup. Functionalizing with a TBC provides design flexibility for independently setting the range of the interaction and the recognition motif.

Full text of DOI:10.1021/ja4075979

Communication
pubs.acs.org/JACS
Patchy Particle Self-Assembly via Metal Coordination
Yufeng Wang,† Andrew D. Hollingsworth,‡ Si Kyung Yang,† Sonal Patel,† David J. Pine,*,‡
and Marcus Weck*,†
†Molecular Design Institute and Department of Chemistry, New York University, New York, New York 10003, United States
‡Center for Soft Matter Research, Department of Physics, New York University, New York, New York 10003, United States
S
* Supporting Information
specific DNA hybridization, and the binding directionality is
predetermined by the patch geometry.9
ABSTRACT: Colloids with high-symmetry patches are
functionalized with metal-coordination-based recognition
units and assembled into larger chain architectures,
demonstrating for the first time the use of metal
coordination as a specific force in colloidal self-assembly.
The cross-linked poly(styrene)-based patchy particles are
fabricated by encapsulation of colloidal clusters following a
two-stage swelling and polymerization methodology. The
particle patches, containing carboxylic acid groups, are site-
specifically functionalized either with a triblock copolymer
(TBC), bearing primary alcohols, alkyl chains, and
palladated pincer receptors, synthesized by ring-opening
metathesis polymerization, or with a small molecule
bearing a pyridine headgroup. Functionalizing with a
TBC provides design flexibility for independently setting
the range of the interaction and the recognition motif.
In this Communication, we describe an alternative bonding
strategy that combines synthetic polymers with metal-
coordination-based recognition units for the self-assembly of
patchy particles. Supramolecular recognition units with high
specificity and variable association constants (binding strength)
have been developed and incorporated in synthetic polymers,
yielding multifunctional polymeric structures.16−19 Functional-
izing colloids with supramolecular polymers permits the
modulation of particle interactions, which significantly expands
the tool set for anisotropic colloidal particle assembly.
We demonstrate this idea by synthesizing patchy particles
that are functionalized with triblock copolymers. The colloidal
particles have terminal carboxylic groups on the patches that
serve as functional handles for polymer attachment. The
copolymers, bearing a block of anchoring groups, a spacer
block, and a block containing metal coordination complexes,
are synthesized via ring-opening metathesis polymerization
(ROMP). The particle patches can be site-specifically function-
alized with these copolymers or pyridine containing small
molecules. Using particles with two patches, we demonstrate
that the functionalized colloids can self-assemble into larger
architectures directed by metal coordination. The assembling/
disassembling process can be triggered by the addition of small
molecules, thus providing a modular strategy for anisotropic
particle assembly and disassembly.
The use of a triblock copolymer is key to our research design
as each block can be addressed in a modular fashion (Figure
1a). The first block is designed to contain the functional groups
required to attach the block copolymer to the patchy particles.
Primary alcohol groups (−CH2OH, shown in blue, Figure 1a)
can be attached to the colloid surface by an esterification
reaction. The second block, consisting of an alkyl chain of eight
carbons on each repeat unit (−C8H17, shown in red, Figure 1a),
is designed to act as a spacer to (a) regulate the length scale of
interparticle interactions and (b) separate the recognition motif
from the patch surface. Other functionalities, such as
fluorescent tags, can also be incorporated in this block. The
final block contains the molecular recognition units as sticky
ends. We use palladated pincer metal complexes as receptors
(shown in green, Figure 1a), and, in our case, tridentate sulfur−
carbon−sulfur pincer ligands coordinated to palladium(II).
These are particularly useful molecular recognition units
he self-assembly of colloidal particles into well-defined 3D
Tstructures is of great interest for potential applications in
biomaterials, catalyst supports, and photonics.1,2 Using
spherical particles with isotropic attractions or repulsions,
only a limited number of 3D structures can be obtained.3−5
Incorporating anisotropy, through either particle shape6−8 or
surface functionality,1,9 introduces bonding directionality and
can thus extend the assemblies that can be realized to motifs as
complex as those seen in atomic and molecular crystals.2,9
Numerous efforts have been devoted to the fabrication of
anisotropic particles,2 resulting in new techniques to engineer
colloidal clusters,10 Janus particles,11 patchy particles,9,12
branched particles,13 and dimpled particles.14,15 To take
advantage of particle anisotropy to create highly directional
bonds, interactions between particles should be relatively short
ranged. Several bonding schemes meet this requirement and
have been used for directional interactions in anisotropic
colloidal systems. Hydrophobic interactions induced by short
carbon chains have been utilized to drive triblock Janus particle
(spherical particles with two patches located at the opposite
poles) into a Kagome lattice.1 Dimpled particles with a
spherical cavity have been fabricated to bind by specific “lock
and key” depletion interactions.14 We have introduced a general
method to functionalize patchy particles with well-defined
symmetries with sticky-ended DNA, creating colloids with
directional interactions; the “valence” is determined by the
number of patches. Patch−patch attractions are realized by
Received: July 23, 2013
© XXXX American Chemical Society
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Figure 1. Triblock copolymer. (a) Scheme showing the synthesis of
the triblock copolymer bearing different functional groups. (b)
Palladated pincer−pyridine metal coordination and ligand exchange.
1
(c) H NMR spectrum of the triblock copolymer. Arrows indicate
characteristic signals from each monomer.
Figure 2. SEM images of (a) two- and (b) three-patch particles (insets
show the original clusters). (c) Bright-field image of a two-patch
particle in THF, showing the contrast between the patch and anti-
patch parts. (d) Confocal fluorescent images of two-patch particles.
The anti-patch parts are fluorescently labeled and shown as green.
Scale bar, 2 μm.
because they have only one open coordination site accessible
and, upon triggering, can undergo specific, fast, and quantitative
coordination with pyridine derivatives.19−21 Stronger ligands,
such as triphenylphosphine (PPh3), can replace pyridine
quantitatively (Figure 1b).17
To synthesize the triblock copolymers, we use ROMP of
functionalized norbornene monomers 1, 2, and 3 (Figure 1a).
ROMP is a living polymerization method that is fully functional
group tolerant. Norbornene-based monomers have been shown
to incorporate a wide variety of functional groups, including
molecular recognition units such as hydrogen-bonding arrays
and metal coordination complexes.17
thermal degrading of benzoyl peroxide, to yield the carboxyl-
containing patchy particles (carboxyl groups only on the
patches). We copolymerize 5% divinylbenzene (DVB) with
styrene to cross-link the shell, or “anti-patch”, producing
particles that can be dispersed in organic solvents such as
tetrahydrofuran (THF) where metal coordination can take
place (Supporting Information (SI), Figure S1). The
introduction of the cross-links, which diminishes the degree
of particle swelling and solubility, gives rise to particle
aggregation during the encapsulation step. To avoid this, we
add a non-ionic PEO-PPO-PEO surfactant (Pluronic 108) that
embeds itself in the particle matrix, leading to more stable
particles.25 The particle patches can be clearly distinguished, as
evidenced by SEM, optical microscope bright-field, and
confocal fluorescence images.
Figure 2a,b shows SEM pictures of two- and three-patch
particles purified by density gradient centrifugation. The anti-
patch part encapsulates the majority of the core clusters. The
“extremities” (i.e., vertices) of the clusters are exposed as
patches. For comparison, SEM images of over-encapsulated
clusters are shown in Figure S2, where the overall surface
appears smooth with no patches. Bright-field microscopic
images of patchy particles also display optical contrast between
the core cluster, and the anti-patch shell arises from the
difference in cross-linking density: the anti-patch (5% DVB)
appears light gray while the core cluster (3% DVB) appears as
dark dots (Figure 2c). Patchy particles of higher order can be
seen in the SI (Figure S3 and Video 1).
Monomers 1, 2, and 3 are synthesized according to the
literature.21−23 The triblock copolymer is then synthesized
using Grubbs’ first-generation ruthenium catalyst (Figure 1a).
The length of the polymer and the ratio of the different blocks
(corresponding to the numbers for m, n, and l, Figure 1a) are
1
tunable by varying the ratio of catalyst to monomers. The H
NMR spectrum of triblock copolymer 4 is shown in Figure 1c,
with the characteristic signals for all three monomers pointed
out by colored arrows, indicating successful incorporation of all
monomers. The integration of the signals from each monomer
agrees well with the feed ratio (m = 10, n = 10, l = 5).
Copolymer 4 has a molecular weight of 6.3k and a
polydispersity Mw/Mn = 1.62. Small molecule 5, a pyridine
derivative, is synthesized to act as the complementary part to
the palladated pincer complex.
We fabricate carboxyl patchy particles following the “cluster-
encapsulation” method previously reported9 with some
modifications. The carboxyl-modified poly(styrene) micro-
spheres (620 nm in diameter, 3% divinylbenzene cross-linked)
are synthesized via a surfactant-free emulsion polymerization.24
4,4′-Azobis(4-cyanopentanoic acid) is used as initiator, which
provides the carboxylic acid groups on the colloid surface. Small
clusters of the microspheres are formed using an “emulsion-
evaporation” method.10 Scanning electron microscope (SEM)
images of clusters consisting of two and three microspheres are
shown in the insets of Figure 2a,b. The clusters are then
encapsulated by first swelling them with the desired amount of
styrene followed by free radical polymerization, initiated by
To further verify that we have fabricated patchy particles, we
fluorescently label the anti-patch part (SI Scheme 1 and Figure
S4). Figure 2d shows a confocal fluorescent image of dye-
labeled two-patch particles where the anti-patch can be seen as
fluorescing green. The core clusters are invisible but depicted
by the dotted line indicating the microsphere size. No
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fluorescent signal is observed at or near the cluster particle
extremities, confirming the patchiness of the particles.
recognition units, though part of a polymer, are small enough
that they should be buried beneath the surface of the anti-patch,
except for those on the patch surfaces. In contrast, no acids
groups are available on the anti-patch surface of the particles.
This is evidenced by confocal fluorescence images of the patchy
particles (Figure 3c) where the entire cluster is visible
(functionalized), while the anti-patch (dotted line) does not
show fluorescence at all (Figure 3d and SI Figure S3c).
Using this method, two-patch particles are functionalized
with either 4, designated as pincer particles, or 5, designated as
pyridine particles. Next, we investigate the self-assembly of
these particles via palladated pincer−pyridine metal coordina-
tion. Because they are two-patch particles, we expect them to
self-assemble into chain structures (Figure 4a, middle). First,
The presence of the carboxylic acid groups on the patches
provides a convenient way to covalently attach small molecules
and polymers. We used N-(3-dimethylaminopropyl)-N′-ethyl-
carbodiimide hydrochloride (EDC) as a coupling reagent to
conjugate the carboxylic acids with primary alcohols in THF.
The functionalization of the particles is visualized by
fluorescence labeling. We synthesize a norbornene−coumarin
monomer that is doped into the second block as a fluorescent
tag during the formation of the triblock copolymer (SI Scheme
2 and Figures S5 and S6). We react the resulting triblock
copolymer with two-patch particles in the presence of EDC.
The block-copolymer-functionalized particles, after purification
by centrifugation and redispersion in THF, show strong
fluorescence (Figure 3a). We carry out a control experiment
Figure 4. Self-assembly of two-patch particles via metal coordination.
(a) Schematic representative showing the triggered assembling and
disassembling process of the two-patch particles. (b) Snapshots of
chain structures formed by the self-assembly of two-patch particles.
Scale bar, 3 μm.
Figure 3. Patchy particle functionalization. (a) Fluorescent image of
two-patch particles functionalized with triblock copolymer. Picture of
the sample with control experiment under UV lamp (inset). (b)
Fluorescent intensity measurement along the long axis of the two-
patch particles. (c) Confocal fluorescent image of functionalized two-
patch particles. (d) Schematic representation of patchy particle
functionalization and expected results under regular and confocal
microscope. Scale bars, 10 μm.
pincer and pyridine particles are combined in THF. We observe
no assembled structures (Figure 4a, left); the oval-shaped
particles are stable and well-dispersed (SI Figure S7a and Video
2). The metal coordination is triggered by adding AgBF4 to the
particle suspension (Figure 4a). Ag(I) activates the palladated
pincer complex by removing the Cl ligand from the palladium,
opening up a coordination site (Figure 1b). After the addition
of AgBF4, the particle mixture is agitated for 5 h at room
temperature. The result is shown in SI Video 3 and Figure
S7c−e. Most particles aggregate and form daisy chains
consisting of different numbers of particles within each chain.
Figure 4b shows snapshots of particle chains formed with
different lengths. The number of particles per chain varies from
2 to 12, with some branched structures being observed (see
Figure S8 for statistics). The oval-shaped particles linked
together solely in a head-to-head configuration, suggesting the
existence of the attraction interactions only between the
patches. We carry out a series of control experiments in which
single types of two-patch particles (pincer particle or pyridine
particles) are mixed with AgBF4. After agitation, the particle
suspensions stay stable and no aggregation is found (SI Figure
S7b).
where we combine the patchy particles with the triblock
copolymer but without the addition of EDC. In this case, no
fluorescence is observed. Dispersions of the fluorescently
labeled and the control samples are shown in Figure 3a inset.
Only the functionalized particles display fluorescence under a
UV lamp. Our result strongly suggests that in the presence of
EDC, primary alcohols can be attached covalently to the
particle patches through the formation of ester bonds.
Selective functionalization of the patches is essential to
endow directionality to the self-assembly of patchy particles.
We measure the fluorescence along the long axis of the elliptical
two-patch particles and plot it as a function of pixels. Figure 3b
shows that the patches are brighter than the anti-patch. The
fluorescence intensity of the anti-patch, however, is not zero,
which could be attributed to “over”-functionalization of the
cluster. In THF, poly(styrene) particles swell and become
porous. Small molecules as well as polymers might be able to
diffuse through the anti-patch and react with carboxylic acids
inside the particle matrix resulting in potential functionalization.
We do not view this “over”-functionalization as a problem, as it
will not jeopardize the directionality of the self-assembly. The
We evaluate whether the assembled architectures can be
disassembled. Triphenylphosphine (PPh3) can serve as a
competitive ligand for the palladium center and can displace
quantitatively the pyridine ligand,17 which can trigger the
disassembling of patchy particles. Indeed, upon addition of
C
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Journal of the American Chemical Society
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PPh3 to the aggregates, the chain structures fall apart
immediately (Figure 4a, right, and Figure S7f). This result
strongly suggests that the metal coordination is the driving
force for particle assembly.
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In conclusion, this report describes the first use of metal
coordination to direct the self-assembly of colloidal particles. In
particular, we describe the self-assembly of patchy particles into
larger chains. Our research strategy is based on a triblock
copolymer containing multiple functionalities to functionalize
the patches of the colloidal particles. Metal-coordination-driven
assembly of the colloidal particles is highly directional and
reversible. We view this strategy as a modular method which
enables great advantages toward colloidal particle assembly
including flexibility: polymer size (Mn), architecture, function-
ality, recognition units, and solvents can be adjusted easily to
tune particle interactions.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterization data for all new
compounds, supplementary figures, and videos. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
pine@nyu.edu
■
marcus.weck@nyu.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported partially by the MRSEC Program of
the National Science Foundation under Award No. DMR-
0820341. Additional financial support was provided by the
National Science Foundation (CHE-1213743 and DMR-
1105455) and by NASA (NNX08AK04G) to A.D.H. We
acknowledge support from the MRI program of the National
Science Foundation under Award No. DMR-0923251 for the
purchase of a Zeiss field emission SEM.
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