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Feature Article

The centrosymmetric repulsive potential of volume-restricted spheres results in simple crystal lattices such as face-centered cubic (fcc), hexagonal close-packed (hcp), and body-centered cubic (bcc). The range of acces-sible crystalline packings can be extended when small and large particles are assembled into dense binary col-loidal crystals. [ 12,13 ] Non-close-packed structures are ther-modynamically not favorable for spherical particles and thus do not form spontaneously. Pathways towards more complex structures need to be elaborated to meet with the requirements of practical applications in photonics, optoelectronics, and memory storage. Using elementary units with tailored anisotropy would expand the struc-tural diversity. The reduced symmetries at the lattice points provided by anisotropic particles are thought to be essential for a large stable optical bandgap and birefrin-gence. Dense packings of asymmetric dumbbell-shaped particles are one representative of such colloidal crystals, in which shape anisotropy is responsible for a structure that is impossible for spheres. [ 14 ] The ultimate challenge is to provide the elementary subunits with geometries and valences suitable to precisely assemble them into open non-close-packed structures. [ 15 ] Experimental access to diamond-type or tetrastack structures is certainly most

DOI: 10.1002/marc.201300693

1 . Introduction

Manifold hierarchically organized structures may arise from the assembly of colloidal polymer particles (Figure 1 ). Because such supracolloidal structures [ 1 ] can be built over multiple length scales, they can be also regarded as pano-scopic materials as proposed in ref. [ 2 ]

Colloidal crystals are the most studied representa-tives of assemblies from spherical particles. They result from a long-range-ordered arrangement of particles, [ 3 ] which are usually kept together by van der Waals forces. [ 4,5 ] The organization into crystal-like structures could bring about collective material properties such as optical refraction, photonic bandgaps, and high surface-to-volume ratios. Because of the periodic replication of refractive index, density, and porosity, applications of colloidal crystals include photonic [ 4,6,7 ] and phononic materials [ 8,9 ] just as well as membranes with de ned pore sizes. [ 10,11 ]

Anisometric polymer colloids are likely to behave differently when compared with centrosym-metric particles. Their study may not only shine new light on the organization of matter; they may also serve as building units with speci c symmetries and complexity to build new mate-rials from them. Polymer colloids of well-de ned complex geometries can be obtained by packing a limited number of spherical polymer particles into clusters with de ned con g-urations. Such supracolloidal architectures can be fabricated at larger scales using narrowly dispersed emulsion droplets as templates. Assemblies built from at least two different types of particles as elementary building units open perspectives in selective targeting of colloids with speci c properties, aiming for mesoscale building blocks with tailor-made morphologies and multifunctionality. Polymer colloids with de ned geome-tries are also ideal to study shape-dependent properties such as the diffusion of complex particles.

Shape-Tailored Polymer Colloids on the Road to Become Structural Motifs for Hierarchically Organized Materials

Claudia Simone Plisch , Alexander Wittemann *

Dr. C. S. Plisch, Prof. A. WittemannColloid Chemistry , University of Konstanz, Universitaetsstrasse 10 , D-78464 , Konstanz , Germany E-mail: [email protected]

Macromol. Rapid Commun. 2013, 34, 17981814 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

MacromolecularRapid CommunicationsShape-Tailored Polymer Colloids on the Road to Become Structural Motifs . . .

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obtained. For example, a tetrahedral assembly of four spherical particles exhibits four distinct poles that if chemically modi ed could trigger the self-assembly into a colloidal crystal with diamond symmetry. [ 15 ]

In terms of their dimensions and complexity, nite par-ticle assemblies present intermediates between the single particle level and the macroscopic domain, to which col-loidal crystals can be assigned (Figure 1 ). Hence, they are interesting from a fundamental point of view because the basic principles behind their formation can provide new insights into the assembly, structure, and dynamics of hierarchically organized materials. [ 18,19 ] Particle clus-ters have received signi cant interest because they open unprecedented perspectives because of their unique rhe-ological, optical, magnetic, or electric properties, which result from the assembly of functional subunits into aggregates with tailored shapes and symmetries. [ 20,21 ] Clusters of a small number of the same or different con-stituent particles can be regarded as colloidal analogues to small molecules. Hence, expressions such as colloidal molecules or patchy particles were used to emphasize the fascinating perspectives of particle clusters. [ 2224 ] In

desirable (Figure 1 ), because they would provide 3D com-plete photonic bandgaps that enable the hierarchically organized colloids to diffract light ef ciently. [ 16 ] The prep-aration of diamond colloidal architectures was achieved by nanorobotic manipulation. [ 17 ] Particle-by-particle construction is highly time-consuming and thus limited in terms of the number of particles that can be assem-bled per hour. To tackle this subject, it was proposed to start from binary colloidal crystals, and then selectively remove one of the species. [ 7 ] Building the requisite pre-cursor structures is however challenging.

The most straightforward strategy would be breaking down the desired structure into subunits with tailored symmetries and valences that enable their directed self-assembly into the target structure. Evidently, the rst task to be tackled is the preparation of colloids with requisite shapes and symmetries. In a second step, one has to work out suitable methods to chemically modify the complex colloids at their poles in order to enable their directional assembly. The present manuscript is concerned with the rst issue, i.e., the fabrication of polymer colloids with de ned shapes and symmetries, which make them potentially interesting as non-centrosymmetric build-ings blocks for hierarchically organized materials. Joining spherical polymer particles into clusters is a promising approach towards such polymer colloids because diverse de ned geometries with outward-facing poles can be

Claudia Simone Plisch was born in 1982. She is currently a Postdoctoral Research Associate at the Chemistry Department of the University of Konstanz. In 2008, she graduated in Polymer and Colloid Chemistry from the University of Bayreuth and received her Ph.D. (2012) with a thesis on supracolloidal polymer assemblies and their physicochemical investigation from the University of Bayreuth, Germany. Her research interests focus on colloidal chemistry, materials science, and especially on assembling nanoparticles at interfaces.

Alexander Wittemann is Professor of Colloid Chemistry at the University of Konstanz. He received his Diploma degree (2000) and Ph.D. in Chemistry (2004) from the University of Karlsruhe. He then joined the Department of Physical Chemistry at the University of Bayreuth. After a postdoctoral research at McGill University, Montreal (20052006), he returned to Bayreuth where he focused his research on nanoparticle assemblies, a topic for which he received his habilitation in 2011. He was awarded from the Universittsverein Bayreuth and the Dr. Otto Rhm Gedchtnisstiftung (2008), and he received the Richard-Zsigmondy-Scholarship from the German Colloid Society (2009). Wittemanns primary research interests are in experimental colloid and polymer chemistry.

Figure 1. Illustration of supracolloidal assemblies: Hierarchically organized structures with de ned shapes and speci c symme-tries can be built from spherical colloids as elementary building blocks, both variable in length scale and complexity. Further infor-mation on accessible morphologies is given in the text.


C. S. Plisch and A. WittemannMacromolecularRapid Communicationswww.mrc-journal.de

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are presented as well. Because of their de ned geometries colloidal clusters are useful model systems, which can, e.g., provide a fundamental understanding of the diffusion of complex particles. A study in this direction is presented.

2 . Shape-Tailored Polymer Colloids: State of the Art

Over the past decade promising chemical, physical, and template-assisted strategies were elaborated for the fabri-cation of polymer colloids with rationally designed shapes. The most prominent of these approaches is brie y high-lighted in the following section. More exhaustive informa-tion on this dynamically and rapidly developing research area can be found in recent review articles. [ 21,23,28 ]

2.1. Chemical Routes

Evidently, the most-straight forward approach towards shape-tailored polymer colloids would be their direct preparation in a single polymerization step. One example is the synthesis of bulb-shaped structures starting from seed polymer particles. The formation of a complex shape is a result of a controlled phase separation during seeded polymerization (Figure 2 A). This allows the synthesis of dumbbell-shaped particles, which themselves can be used as seeds for the preparation of trimer or even tetramer par-ticles. [ 29 ] Dumbbell-shaped particles obtained by such an approach have been successfully used to build dense col-loidal crystals with a partial bandgap and birefringence. [ 14 ]

A second strategy allowed the synthesis of multipod- or raspberry-like silica/polystyrene hybrids. [ 30 ] This was accomplished by the nucleation and growth of polystyrene (PS) nodules on surface-functionalized silica seed particles during emulsion polymerization of styrene (Figure 2 B). [ 31 ] Speaking in the terminology of colloidal molecules, the number of polymer nodules per silica sphere equals the number of free valences that if made reactive could act as potential binding or docking sites in organized materials.

particular, a high level of complexity can be realized when the clusters are made from constituents of different mate-rials. This offers the possibility to combine properties of two or more materials in an advantageous way at full con-trol over shape and composition. Precise control over the assembly process would allow the fabrication of a plenti-tude of compositional and surface anisotropic particles. [ 24 ] Moreover, the anisotropic nature of such heteroaggre-gates could enable directed self-assembly of the hybrids, implying perspectives as complex building blocks for the next higher level of hierarchically organized materials. [ 25 ]

Besides colloidal clusters, other supracolloidal assem-blies of nite size have been described. Compact aggre-gates made up from a large number of spherical particles are often termed as supraballs (Figure 1 ). They may bear a well-ordered layer of particles at their surface, which gives them photonic properties. [ 26 ] Hollow capsules whose walls consist of colloidal building blocks result from close packings of particles on a spherical template. These so-called colloidosomes present a further class of particle assemblies of nite size (Figure 1 ). [ 27 ]

The present article is devoted to polymer colloids with global shapes intimately connected with speci c symme-tries, which make them potentially interesting as struc-tural motifs for hierarchically organized materials. In this context, we report on nanoparticle clusters, which are obtained by joining a limited number of polymer particles with diameters in order of 100 nm into densely packed clusters with de ned con gurations. The manuscript is organized as follows: At rst, a brief survey of recent achievements and developments in the eld of polymer colloids with tailored shapes is given. In particular, selected examples of important preparative strategies that provide appropriate access to polymer colloids with de ned but complex shapes are highlighted. The main part is focused on colloidal clusters and binary heteroag-gregates with submicron-sized dimensions. We discuss their preparation by emulsion droplet evaporation nano-particle assembly and the accessible con gurations that are obtained. First approaches towards heteroaggregates

Figure 2. Synthesis of polystyrene (PS) particles with complex shapes by controlled phase separation during polymerization (A): Preparation of dumbbell-shaped particles by swelling and polymerizing spherical particles. Linear growth of a third bulb c when the crossing densities a,b of the initial blubs a and b differ. Perpendicular growth of blub c when the cross-linking densities of blubs a and b are the same. Repro-duced with permission. [ 29 ] Growth of PS nodules onto the surface of functionalized silica seeds, resulting in multipod-like structures (B). Reproduced with permission. [ 30 ]



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Pine and co-workers elaborated a directional assem-bling route, which is successfully adapting Fischers famous lock-and-key concept to the colloidal regime. [ 33 ] Site-speci c recognition and association among bowl-shaped (lock) and spherical (key) colloidal subunits can be triggered by short-range depletions attractions upon adding a non-adsorbing polymer. The entropically favored association results from minimization of the total volume around the colloidal building blocks, from which the macromolecules are excluded. In case of bowl-shaped particles, the total volume available to the macromol-ecules is at its maximum, if the cavities of the bowls are lled with spherical particles that perfectly t into the cavities. This allows the formation of complex assemblies composed of a center sphere to which a limited number of bowl-shaped colloids are bound (Figure 3 B). [ 33 ]

An alternative to geometric recognition is the direc-tional control of the depletion interactions among col-loidal building blocks of locally different surface rough-ness. [ 34 ] Association among the smooth patches is more favorable in terms of gain in free volume for the depletant (dissolved macromolecules), and thus smooth patches are preferentially attracted to each other (Figure 3 C).

The assembling strategies described in this section might not be suitable to fabricate larger scales of shape-tailored colloids eligible as structural motifs to build hierarchically organized materials from them. Nonethe-less, the principles behind their formation might be used to assemble shape-tailored colloids obtained from other methodologies into hierarchically organized structures. The spontaneous formation of supracolloidal networks of microspheres via attractive patches at their surface or the crystallization of colloidal cubes achieved by deple-tion interactions might serve as rst examples in this direction. [ 35,36 ]

2.3. Template Strategies

Alternative strategies towards particle clusters are based on the use of templates. [ 23 ] A certain number of particles is trapped in a con ned space that is given by the template. Capillary forces that occur during evaporation of the dis-persion medium trigger the formation of clusters within the con nement. The nal con guration of the assembly is kept together by van der Waals forces.

Xia and co-workers elaborated a strategy that is based on a 2D template. [ 37 ] Slow dewetting of a suspension on arrays of cylindrical holes allowed con ning a de ned number of particles in each of the holes (Figure 4 A). Limited quantities of 2D assemblies were obtained. The release of the assemblies from the substrate surface was accomplished by sonication.

Velev and co-workers were the rst, who used emulsion droplets as 3D templates for packing particles. [ 38 ] This

2.2. Physical Strategies

Chemical routes towards shape-tailored colloids are lim-ited to a small number of systems and often demand subtle handling of experimental parameters during their syn-thesis. Joining preformed colloidal subunits into de ned aggregates offers exibility in the choice of building blocks. Such an approach is not only more fundamental from a theoretical point of view; it could provide access to an almost unlimited diversity of supracolloidal assem-blies (Figure 1 ). First preparative strategies were recently developed. The assembly into clusters can be mediated by physical interactions and/or suitable templates. [ 23 ] A successful assembling technique must set a limit for the aggregation number. Moreover, well-de ned morphologies should result from such an approach. This is challenging inasmuch colloidal particles, unlike atoms, usually do not undergo directed interactions.

One approach is based on the direct assembly of bipolar particles in solution (Figure 3 A). For this pur-pose, microspheres having oppositely charged hemi-spheres were prepared. These particles spontaneously self-assemble into clusters of de ned con gurations because of their directional electrostatic interactions. [ 32 ] However, such clusters present intermediates during the formation of larger aggregates because there is no limitation to the size of clusters formed through electro-static interactions. Moreover, the preparation of bipolar building blocks is highly demanding, which presents an obstacle for preparing complex polymer architectures at large scales.

Figure 3. A) Assembly of micron-sized particles with oppositely charged hemispheres: If the diameter of the particles exceeds the electrostatic screening length, clusters are obtained as intermedi-ates during the formation of macroscopic aggregates. [ 32 ] B) A lock and key mechanism allows colloidal particles with complemen-tary shapes to self-assemble into de ned clusters in the presence of a depletant. [ 33 ] Reproduced with permission. [ 21 ] Copyright 2011, Elsevier. C) As an alternative, site-speci c assembly can be also triggered by the surface roughness of the building units. [ 34 ]




monodisperse emulsion droplets would limit the vari-ance in number of particles per droplet. Nonetheless, this cannot result in a single set of clusters because the distri-bution of the particles on the droplets is statistical, but it would signi cantly reduce the number of resulting spe-cies and thus increase the yield of distinct clusters.

Up to now, only few studies have been devoted to the preparation of particle assemblies from monodisperse droplets. One approach is to prepare the droplets one by one using a micropipette [ 26 ] or micro uidic devices. [ 45 ] These droplets have diameters of several tens of micro-meters, and thus present suitable templates for the prep-aration of large assemblies such as supraballs described above. [ 26 ]

A rst approach towards clusters of a small number of micro-sized constituents from narrow dispersed Pick-ering emulsion droplets was presented by Bibette and co-workers. [ 46 ] Shearing a polydisperse Pickering emulsion in a Couette apparatus allowed the formation of narrow dispersed droplets bearing a small number of particles on their surface. After evaporation of the dispersed phase, large yields of single particles, particle doublets, triplets, and quadruplets were found in the suspension. [ 46 ]

2.4. Binary Assemblies and Heteroaggregates

Particles of dual complexity, i.e., compositional and/or sur-face anisotropic particles, have emerged to a frontier in materials science in recent time. [ 24 ] The best studied repre-sentatives are particles that consist of two halves with dis-similar properties. These particles are commonly referred to as Janus particles. [ 47,48 ] Only few studies have been devoted to clusters that are obtained from the combina-tion of different particles up to now. There are rst studies on homogeneous binary clusters, i.e., clusters whose con-stituents differ only in size, but also on heterogeneous binary clusters made up from dissimilar particles. [ 49,50 ] Up to now such binary heteroaggregates were prepared solely from PS, silica, titania, and gold particles. [ 50,51 ] Beyond the material itself, suitable constituents for model heteroag-gregates should be monodisperse because this facilitates the analysis of the resulting morphologies. Moreover, the wettability of the particles should be suitable for both the dispersed and the continuous phase if the clusters are tem-plated by emulsion droplets (Figure 4 B). For these reasons, one is either reliant on a limited number of simple parti-cles or one has to devote much efforts to the synthesis of suitable building blocks, with a special regard to their sur-face properties. Till now, the rst approach was followed. For this reason, the formation of colloidal heteroaggregates is still poorly explored up to now.

Aiming at innovative applications for heteroaggre-gates as colloidal molecules or patchy particles neces-sitates the use of building blocks that exhibit promising

strategy is based on the agglomeration of particles while adsorbed on emulsion droplets (Figure 4 B). So-called Pick-ering emulsions are formed by adsorption of colloidal particles at the surface of emulsion droplets. [ 39,40 ] This process is driven by a signi cant reduction of interfacial tension. The adsorption energy of the particles is gov-erned by their size, the interactions among them, and their interactions with both uid phases. [ 40 ] Subsequent evaporation of the dispersed phase of the emulsion causes capillary forces, which pack the particles together.

Pine and co-workers prepared clusters from polymer microspheres by this technique. [ 41 ] These clusters exhib-ited well-de ned con gurations, which are believed to result from an ordered arrangement of the particles at the droplet surface due to long-ranged dipoledipole repul-sion through the oil droplet and Coulomb interactions. [ 42 ] The clusters reported so far were made from a rather lim-ited number of different constituent particles, in most instances PS, poly(methyl methacrylate) or silica micro-spheres. [ 43 ] Suitable constituents of colloidal clusters do not necessarily have to be spherical. Very recently, van Blaaderen and co-workers reported on clusters built from dumbbell-shaped particles. [ 44 ]

The number of particles on a given droplet deter-mines the number of constituents of the cluster, which will result from this speci c droplet (Figure 4 B). For this reason, the distribution of the particles onto the drop-lets is crucial to the dispersity of the resulting clusters. A broad size distribution of the droplets would give rise to a broad range of different species, whereas the use of

Figure 4. Fabrication of clusters by using 2D or 3D templates. A) The constituent particles are trapped within micro-sized cylin-drical holes of a patterned substrate in a one-stage dewetting process (right). Filling the holes with microparticles results in 2D assemblies (left). Reproduced with permission. [ 37 ] B) The constit-uent particles adsorb to the oilwater interface of emulsion drop-lets acting as 3D templates due to surface tension. Subsequent evaporation of the dispersed oil phase forces the particles to pack into clusters. Reproduced with permission. [ 41 , 43 ]



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powerful ultrasonication. Several mechanisms of droplet formation by ultrasonication have been discussed. [ 55 ] Among those, acoustic cavitation is assumed to be the most important one. [ 56 ] Sonication causes cavities within the emulsion, which collapse when they gain a critical size. This causes pressure waves in the vicinity of the droplets, which rupture them into smaller ones until a steady state of droplet fusion and ssion is attained. This state is characterized by narrow-dispersed emulsion droplets. [ 57 ]

An aqueous solution of an oil-in-water emulsifier was overlaid with a suspension of the building blocks of the clusters in toluene (route A in Figure 5 ). An ultra-sonic horn was dipped into the sample so that it was touching the toluene-water interphase. This in addi-tion with the geometry of the vessel used provided efficient mixing of the emulsion during sonication. Because emulsification was performed in the pres-ence of colloidal particles, narrowly dispersed Pickering emulsion droplets could be obtained by this process (Figure 6 A,B). The average droplet sizes can be tuned in the range of 360 nm to 2000 nm, depending on emul-sifier concentration, stabilizing agents, and sonication conditions. [ 54 ] Cryogenic scanning electron microscopy allowed imaging the PS particles while confined to the toluene droplets. [ 58 ] The polymer particles were signifi-cantly swollen as expressed by a larger diameter and a preferential orientation into the toluene phase. A random distribution of the PS particles at the droplet surface was found (Figure 6 C). This is corroborated by Monte Carlo (MC) computer simulations, which

properties. This could be accomplished by co-assembly of polymer particles and inorganic particles with special optical, catalytic, or superparamagnetic properties just as well as a surface-functionalization with biomolecules. [ 52 ] Then again, the higher complexity of heteroaggregates demands that their formation is fundamentally under-stood in order to predict accessible morphologies. Such a basic understanding would help to prepare hybrids of desired composition and con guration, and thus consti-tutes a basic requirement to further the perspectives of shape-tailored colloids.

3 . Approaching the Nanoscale: Templated Assembly of Polymer Nanospheres

Most of the diverse strategies towards colloidal molecules described so far are dependent on building blocks, which should not be too small. This limitation in size may result either from the preparation of well-de ned patches on col-loidal subunits or the smallest dimensions of the templates that can be produced. [ 23 ] The fabrication of colloidal mole-cules with diameters that are of the same order as the range of colloidal interactions could however be essential for their subsequent assembly into mesostructured materials. Func-tional structures in nature are often a direct result from a de ned arrangement of nanoscopic subunits, e.g., protein molecules. [ 53 ] This was a strong motivation to develop a route towards clusters consisting of a small number of con-stituents with dimensions of approximately 100 nm. For this purpose, monodisperse cross-linked PS latex particles were prepared at rst. [ 54 ] Packing of these particles into clusters should be accomplished by the agglomeration of the building blocks onto emulsion droplets while the dis-persed phase was evaporating (Figure 4 B and 5 ).

The emulsion droplet-based particle assembly was originally elaborated for micro-sized particles. [ 41 ] The macroemulsion droplets templating the assembly of the microspheres were prepared by vigorous stirring using an Ultraturrax homogenizer. We found that this procedure did not work out for polymer particles with a diameter of about 100 nm. No clusters from the nanoscale subunits were formed, most probably because of rupture of micro-sized droplets during evaporation of the dispersed phase. Hence, the diameter of the oil droplets had to be adapted to the much smaller dimensions of the particles in order to favor their assembly into clusters. Moreover, the drop-lets should exhibit a low dispersity in size to obtain a lim-ited number of different cluster species.

3.1. Emulsi cation and Droplet Stability

Control over the size and dispersity of the templating oil droplets was achieved through emulsi cation via

Figure 5. Preparation of colloidal clusters from polymer nano-particles: Ultrasonication was used to prepare narrow-sized tol-uene droplets of less than 2 m in diameter with PS particles of 154 nm diameter bound to the surface. The polymer particles can be added via the dispersed oil phase (route A) or the continuous aqueous phase (route B), which open perspectives for joining dif-ferent particles into hybrids coming from different phases. The assembly of the particles at the droplet surfaces into clusters is driven by evaporation of the dispersed toluene phase. Repro-duced with permission. [ 54 ] Copyright 2010, Springer.




indicated that the PS particles can freely diffuse at the droplet surface. [ 58 ]

For the conservation of the droplet size, it was cru-cial to suppress droplet growth mechanisms. Collision growth is prevented by the emulsi er, which stabilizes the emulsion due to the GibbsMarangoni effect. [ 59 ] Ost-wald ripening, i.e., droplet growth through diffusion of molecules of the dispersed toluene phase via the con-tinuous aqueous phase and subsequent dissolution of small droplets due to their high Laplace pressure, could be suppressed by the addition of a hydrophobe such as dodecane. [ 55 ] Because the hydrophobe is hardly soluble in water it cannot diffuse through the aqueous phase from one droplet to another. Hence, an increase in the polydispersity of the droplets would provoke a dissimilar osmotic pressure within the droplets. [ 55 ] This keeps a low dispersity of the droplets. Measurements of the turbidity of the Pickering emulsions clearly showed that adding dodecane is an ef cient way to prevent Ostwald ripening of the droplets used in this study (Figure 6 D). [ 60 ]

3.2. Cluster Formation

Gentle removal of the dispersed organic solvent by evaporation under reduced pressure forced the particles on each droplet to pack into a cluster. The drop-lets are bearing a statistically varying number of PS particles at their surfaces as indicated by MC computer simula-tions, which took a dynamical capture of the particles from the continuous phase of the emulsion into account. [ 58 ] Hence, clusters that differ in the number of con-stituents resulted. However, the use of narrow-dispersed droplets helped lim-iting the size of the largest clusters below 13 constituent spheres. All clusters had well-de ned con gurations (Figure 7 ).

Differential centrifugal sedimenta-tion (DCS) using a disk centrifuge [ 63 ] has proven to be an excellent method for analyzing the aggregation num-bers of the clusters and to quantify the yields of clusters made from the same number of constituents (Figure 8 ). The frequency distribution of cluster species was in full accord to predictions from MC simulations. [ 58 ] For this reason, DCS was used to explore various param-eters towards higher yields of clusters as compared to single particles. Raising the number of cluster building blocks, while keeping all parameters of emulsi- cation the same, increases the average

concentration of particles at the droplet surfaces. This favors the formation of larger clusters and thus helped to signi cantly increase the total yield of clusters along with the range of resulting species. [ 54 ]

The concentration of emulsi er is a second tool to fur-ther improve the total yield of clusters. Larger emulsion droplets are obtained at lower emulsi er concentrations. At a xed volume of the dispersed phase and a xed number of added particles, this reduces the number of droplets, which increases the average number of particles per droplet. A favorable combination of both parameters, i.e., particle and emulsi er concentration, allowed opti-mization of the cluster yield to a total of up to 74 wt% as compared with single particles. [ 54 ]

In a further series of experiments, the particles were ini-tially suspended in the continuous aqueous phase instead of the dispersed toluene phase (route B in Figure 5 ). As mentioned above, the resulting con gurations were the same in both cases. However, the total yield of clusters was signi cantly lower when the particles were added via

Figure 6. Templating emulsion droplets: Narrow-dispersed emulsion droplets bearing the cluster constituents on their surface were obtained through emulsi cation by ultra-sound (A: confocal micrograph, B: droplet size distribution). Colloidal particles trapped at the surface of an emulsion droplet obtained with a Cryo-FESEM image of a toluene-in-water emulsion stabilized by cross-linked PS particles (C). The average size and dispersity could be preserved over time by addition of a hydrophobe (D). In the absence of hydro-phobe (red curve), the turbidity of the emulsion increases with time because of droplet growth by Ostwald ripening. This growth process was ef ciently suppressed by adding dodecane (blue curve). A,B,D) Reproduced with permission. [ 60 ] Copyright 2008, American Chemical Society. C) Reproduced with permission. [ 58 ] Copyright 2011, AIP Publishing LLC.



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the continuous water phase (Figure 8 ). [ 54 ] Given the strong af nity of the particles to the oilwater interphase and equilibration during sonication, this result seems aston-ishing inasmuch as cluster formation takes place right at the interphase. This might be related to the swelling kinetics of the cross-linked PS particles in toluene. If the particles were initially suspended in toluene, they were already swollen with toluene prior emulsi cation. How-ever, if added in water, the polymer particles could only swell at the interphase. Swelling at the interphase might be further retarded by the hydrophilic copolymer layer on the PS particles.

It turned out that the nature of the dispersed phase had a strong impact on the cluster formation as well. When going from toluene to cyclohexane, preferentially particle doublets, triplets and a small fraction of tetrahe-drons were formed. [ 54 ] This observation could be traced back to a higher number of templating droplets, resulting in smaller ratios of particles per droplet.

3.3. Cluster Morphologies

As a rst result to be highlighted, the resulting con gu-rations of the clusters did not differ regardless the phase, from which the constituents were initially added. [ 54 ] This is a clear indication that packing of the particles into clusters

takes place right at the oilwater interphase. An overview of the cluster species that differ in number of constitu-ents and con guration is given in Figure 7 . All clusters are assemblies of less than 13 spherical building blocks. The experimental cluster con gurations were reproduced in MC computer simulations, in which a clustering mecha-nism was introduced by continuous shrinking of the droplets mimicking their evaporation. [ 58 ] All clusters had compact morphologies because the packing of the particles proceeded on a spherical template. For this reason, a max-imum of two different con gurations for clusters with the same number of constituents was found (Figure 7 ).

Favorable structures for nite assemblies of spherical particles were allocated by close packings. Alternatively, theoretical studies were performed in order to predict low-energy states. These models were based on different pair potentials; the best studied being the Lennard-Jones potential [ 61 ] and the hard-sphere potential with short-range attractions. [ 62 ]

For clusters of 25 constituents, both approaches pre-dict identical structures. The predictions are in full accord with the cluster morphologies, which were found experi-mentally (Figure 7 ), expect that a small part of clusters of ve constituents had a square pyramidal shape. This con guration (eight contacts among the constituent spheres) is less favorable than the trigonal pyramidal

Figure 7. Con gurations of clusters: Joining N 4 particles on droplets led to clusters with unique con gurations, namely particle doublets, triplets, and tetrahedrons, whereas N = 5, 6, 8 particles could arrange into two different con gurations. The majority of observed packings were based on regular polyhedra. Reproduced with permission. [ 54 ] Copyright 2010, Springer.




con guration (nine contacts) that was found for the large majority of clusters.

Six particles on a droplet could assemble into clusters with either O h or C 2v symmetries. Both con gurations were found with the same frequency. The species with O h symmetry is favored by the Lennard-Jones potential as packing criteria. [ 61 ] Interestingly, the same probability for both symmetries was recently predicted for packings of hard spheres with short-range attractions. [ 62 ] Hence, the experimental results corroborate the prediction for clus-ters of hard spheres in this speci c case.

However, if going from seven to 10 constituents, the number of minimal-energy structures built from hard spheres strongly increases. [ 62 ] However, only distinct con gurations were found in the experiments. As an example, all clusters of seven constituents had a pen-tagonal dipyramidal shape, which is in full accord with the prediction for Lennard-Jones clusters. In principle, hard spheres could assemble into further structures having the same energy as a pentagonal dipyramidal

con guration. [ 62 ] Such structures are not found, most probably because of the spherical templates, which seem to make the formation of distinct particle packings more favorable than others.

To summarize this point, all experimental con gu-rations shown in Figure 7 are reproduced by a certain packing model but none of the existing models matches exactly the con gurations and their frequency for all experimentally found species. A second point to be made is that all of these con gurations are well de ned. Similar observations were made earlier for assemblies of micro-sized particles. [ 41 ] In the latter case, this was ascribed to dipoledipole repulsions, which resulted in a well-ordered arrangement of the particles already on the droplet sur-face. In the present case, the building blocks are approxi-mately 10 times smaller, which, in turn, should result in a much higher mobility of the particles at the oilwater interphase. In fact, MC simulations corroborate that the nanoparticles are randomly distributed across the droplet surfaces. [ 58 ] It is thus noteworthy that the higher dynamics of smaller building blocks does not present an obstacle for the formation of well-de ned clusters from particles with dimensions in the order of 100 nm.

3.4. Cluster Fractionation

In order to use such clusters as buildings blocks for novel materials that require particles with special symmetries or high complexity, it is crucial to separate them into frac-tions of uniform species. This was accomplished by cen-trifugation of the cluster mixture in a density gradient (Figure 9 ). The clusters were separated by their sedimen-tation rate, which depends on the number of constitu-ents. The density gradient helped eliminating streaming because throughout the fractionation the net density of the suspension, i.e., the average density of uid plus sus-pended clusters, increased continuously during sedimen-tation. [ 63 ] Well-de ned fractions of clusters were obtained during centrifugation. [ 60 ] Isolation and subsequent analysis by eld emission scanning electron microscopy (FESEM) revealed that these fractions indeed consisted of clusters made up of the same number of constituents (Figure 9 ).

Hence, the fabrication of clusters templated by emul-sion droplets, which was initially elaborated for micro-sized particles, [ 41 ] could be successfully transferred to nanoscopic building blocks. Various parameters provide control over the yields of speci c clusters. Moreover, this approach could be used to assemble different particles into hybrids even in cases were mixtures of these parti-cles would not form stable suspensions. This opens pos-sibilities for designer particles with submicron dimen-sions. Such assemblies underlie Brownian motion, which prevents sedimentation. They are attractive as com-plex building blocks for novel materials because their

Figure 8. Addition of particles either via the dispersed toluene phase (A) or via the continuous water phase (B) has a marked impact on the distribution of cluster species. Essential high yields of larger species and a higher total yield of clusters were obtained when the particles were initially suspended in the dispersed oil phase.



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dimensions are of the same range as colloidal interactions (e.g., van der Waals, electrostatic, steric interactions). [ 64,65 ]

4 . Colloidal Heteroaggregates

4.1. Polymer Clusters with Inorganic Nanoparticles at their Surface

A straightforward route to such designer colloids is the direct adsorption of oppositely charged nanoparticles onto pre-formed assemblies of polymer particles. For this

purpose, clusters from amine-modi ed cross-linked PS particles were prepared by emulsion droplet-assisted nano-particle assembly along the lines given in the previous sec-tion. [ 66 ] The positively charged clusters were gently added to suspensions of narrowly dispersed negatively charged silica, gold, or maghemite nanoparticles, resulting in the formation of hybrids (Figure 10 ). These heteroaggregates exhibit compositional and interfacial anisotropy just as well as two speci c radii of curvature. Hence, they can be considered as nanoparticle assemblies of dual complexity.

Binary mixtures of oppositely charged colloids were exhaustively studied during the last decades. [ 67,68 ] In most cases, phase separation into a concentrated phase consisting of fractal aggregates and its equilibrium solu-tion was observed. [ 67 ] A fundamental understanding of the adsorption process as shown in Figure 10 was thus crucial to get stable suspensions of binary colloids from oppositely charged particles. Interestingly, stable sus-pensions of composite particles were obtained at low and high doses of added nanoparticles, whereas imme-diate occulation was observed in between. [ 66 ] Electro-phoretic measurements showed that occulation occurs near the isoelectric point of the composites, i.e., if the number of positive charges of the polymer particles is exactly balanced by the number of negative charges of the adsorbed nanoparticles. [ 66 ] If only few silica nano-particles were bound, the composites were stabilized by the excess of positive charges onto the polymer clusters, whereas high loading with nanoparticles resulted in het-eroaggregates with a negative net charge. In both cases, long-term stable suspensions of heteroaggregates were obtained. Almost all nanoparticles were bound onto the polymer clusters until maximum adsorption was reached. This strong af nity was corroborated by calori-metric measurements, which showed that the adsorption is exothermic. [ 66 ]

To demonstrate the versatility of the route as outlined in Figure 10 , the uptake of three different types of inor-ganic nanoparticles that differ in their chemical nature, size, and surface charge was probed. [ 66 ] Figure 11 indi-

cates that in all cases the nanoparti-cles are quite evenly dispersed over the surface of the PS clusters. Evidently, the arrangement of the nanoparticles is governed by electrostatic repulsions. However, it has to be kept in mind that electrostatic repulsions among nano-particles are less strong than in the bulk when the particles are located within the electric double layer of larger colloids. [ 67 ]

Moreover, rst experiments pointing towards potential applications were performed. In recent years, it has been

Figure 10. Preparation of hybrids with dual-surface roughness through the deposition of oppositely charged inorganic nanoparticles (NPs) onto the surface of pre-formed clus-ters of PS particles.

Figure 9. Fractionation by cluster size was accomplished by centrifugation of the cluster suspension in a density gradient (photograph). Isolation of the fractions resulted in suspensions containing clusters of the same number of constituents (FESEM images: 1) single particles; 2) doublets; 3) triplets; 4) tetrahedrons; 5) triangular dipyramids; 6) octahedrons; 7) clusters consisting of seven or more building blocks). Scale bars are 200 nm. Repro-duced with permission. [ 60 ] Copyright 2008, American Chemical Society.




a challenge to keep gold nanoparticles physically sepa-rated in surface-enhanced Raman scattering. [ 69,70 ] The spatially separated arrangement of gold nanoparticles onto the polymer clusters (Figure 11 ) should allow for col-lective plasmon modes, implying perspectives for sensing applications. [ 71,72 ]

Deposition of superparamagnetic nanoparticles offers perspectives for complex ferro uids. [ 73 ] The superpara-magnetic entities on the surface of the complex colloids allows for ef cient separation in external magnetic elds. Moreover, such tailor-made heteroaggregates open new avenues as complex building units for complex materials. [ 74 ]

Hence, this study contributes to earlier promising studies in this direction that were based on micro-sized particles. [ 74 ] Control over shape, composition, and sur-face roughness can be managed by the size of the con-stituents and their aggregation numbers offering new avenues for various rationally designed functional col-loids, which are of interest for sensing, photonic, or mag-netic devices.

4.2. Towards Nanoparticle Capsules via Polymer/Inorganic Hybrids

The previous section reported on the physisorption of inorganic nanoparticles onto shape-tailored polymer col-loids. The physisorption of small particles on a surface can be treated as a random sequential adsorption, if one de nes an effective size of the highly charged nanoparti-cles, which is larger than their true diameter to account for electrostatic repulsions. [ 75,76 ] While physisorption

results in spatially separated nanoparticles, we will now present a strategy that yields clusters of polymer particles encased in a densely packed monolayer of nanoparticles in a one-step process. This approach is also interesting from a fundamental point of view, because it extends emulsion droplet-assisted nanoparticle assembly to the co-assembly of oppositely charged particles. In doing so, the particles of opposite charge were kept in different phases, i.e., the dispersed and the continuous phase of the emulsion, to ensure that they are brought together at the oilwater interface. Hence, this procedure avoids the for-mation of fractal aggregates, which are usually obtained in mixtures of oppositely charged colloids in the dilute regime. [ 77 ]

Figure 12 outlines the fabrication of the binary hetero-aggregates. An aqueous suspension of negatively charged silica nanoparticles and positively charged cross-linked PS particles in toluene was emulsi ed using a high-shear device yielding a narrowly dispersed oil-in-water emul-sion. [ 78 ] The polymer particles are bound to the droplet surfaces because of the Pickering effect. Since the inor-ganic nanoparticles are initially dispersed in the con-tinuous phase, they can either adsorb onto the polymer colloids, obviously to the sides exposed to the continuous phase, or adhere to the droplets. Because the dispersed toluene presents a good solvent for PS, the polymer par-ticles are signi cantly swollen during the physisorp-tion of the nanoparticles. Hence, evaporation of the dispersed phase induces both shrinkage of the polymer particles and their assembly into clusters. The progres-sive shrinkage of their polymer support brings the nanoparticles together. MC computer simulations showed

Figure 11. Assembly of narrow-dispersed polymer and inorganic nanoparticles (NPs) gives access to mesostructured hybrids with full control over shape, composition, and surface roughness. Small functional entities such as gold or maghemite NPs on the surface of polymer col-loids with speci c symmetries open new possibilities for sensing applications or as complex ferro uids. Reproduced with permission. [ 66 ] Copyright 2011, Elsevier.



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that given an appropriate number of nanoparticles per polymer particle, a dense random monolayer of nanopar-ticles is formed tightly encasing an inner cluster of the polymer spheres (Figure 13 B). [ 78 ] Size and charge selec-tivity during the co-assembly at the droplet interface resulted in heteroaggregates, whose global shapes are precisely determined by the number of polymer spheres forming the inner core (Figure 13 A). For this reason, the con gurations of the polymer colloids are identical to those of aggregates formed in the absence of the oppo-sitely charged nanoparticles.

Having access to such coreshell supracolloids raises the question if they may serve as precursors for nano-porous capsules after removal of the inner polymer core. For this purpose, the heteroaggregates were heated at 500 C until the polymer core under-went thermal decomposition. Elemen-tary analysis showed that the polymer was completely removed during this thermal treatment. [ 78 ]

The complex shape brought by the polymer colloids resulted in com-partmentalized nanoporous capsules (Figure 13 C,D). Remarkably, all nano-particle capsules were intact after the decomposition of the core despite they consist of single layer of nanoparticles kept together by nanoscopic contact areas. It has to be noted that the silica

nanoparticles did not bake together during the thermal treatment, because the temperature was kept below the sintering temperature of silica (>600 C). [ 79 ] This was cor-roborated by BET analysis of the free surface area, which did not change. MC computer simulations revealed that the stability of the nanoparticle capsules can be explained by interplay of short-range (van der Waals) attractions and long-range (electrostatic) repulsions. [ 78,80 ] Hence, the nanoparticles are trapped in local energy minima. For this reason, the capsules could neither burst nor could single constituents leave the monolayer shell.

This section has shown that a facile template-based assembling strategy can open new perspectives for the prep-aration of heteroaggregates with both tailored composition and morpholo-gies. Moreover, such heteroaggregates open avenues as precursors for colloidal capsules that exhibit a high density of nanopores and complex shapes. The nanoparticle capsules might serve as model systems for biological cells or drug delivery vehicles. [ 81 ]

5 . Diffusion of Clusters from Polymer Nanospheres

The diffusion of particles that exhibits complex shapes is fundamental to the understanding of many practical problems such as biodistribution, sedi-mentation, coagulation, oatation, and rheology. [ 82 ] While the diffusion of

Figure 12. Illustration of the fabrication of heteroaggregates, which exhibit coreshell morphologies (A B) and serve as precursors for the production of nanoparticle capsules with tailored shapes (B C): Positively charged PS particles and negatively charged silica nanoparticles (NPs) were assembled at the oilwater interface of emulsion droplets by successive evaporation of the dispersed oil phase (B). The supracolloidal assemblies had well-de ned coreshell morphologies, with clusters of the polymer colloids as the core and a dense monolayer of inorganic nanoparticles as the shell. Subsequent thermal decomposition of the polymer resulted in nanoporous capsules (C), whose global shapes are determined by the number of polymer particles that were forming the core. Repro-duced with permission. [ 78 ] Copyright 2011, The Royal Society of Chemistry.

Figure 13. Synopsis of experimental coreshell heteroaggregates (A) and nanoparticle capsules (C) together with their theoretical counterparts (B & D); The variable para-meter is the number of polymer particles ( N ) forming the core of the heteroaggregates and thus determining the global shapes of the supracolloids. Reproduced with permis-sion. [ 78 ] Copyright 2011, The Royal Society of Chemistry.




colloidal particles with simple shapes such as spheres, [ 83,84 ] ellipsoids, [ 85,86 ] rods, [ 87,88 ] and platelets [ 89 ] was extensively studied by experiment, [ 90 ] simulation, [ 91 ] and theory [ 92 ] over the past decades, studies on the diffusion of complex colloids are still rather scarce. [ 93 ] Well-de ned colloids with complex but known shapes could serve as model systems for studying diffusion of objects with arbitrary shapes. For this reason, Granick and co-workers developed the idea to use planar clusters of micron-sized spherical particles. [ 94 ] Particle clusters are ideal candidates because they exhibit well-de ned geometries. [ 41 ] If made from microspheres, the 2D diffusion of the assemblies could be monitored by video microscopy. [ 94 ] This technique is however limited to micron-sized objects. Hence, gravitational forces are in u-encing the motion of microparticles. Moreover, the diffu-sion characteristics might differ from the bulk behavior because the microscope slide can bring strong wall effects on the diffusion. [ 95 ]

If the dimensions of the clusters could be kept in the submicron regime, Brownian motion would prevail over gravitational forces. [ 96 ] Particle assemblies exhibiting such dimensions could be studied by scattering tech-niques. Today polarized dynamic light scattering (DLS) is a routine technique to measure translational diffusion coef cients of submicron-sized particles in solution. In a polarized DLS experiment, vertically polarized light is shined on the suspension of particles. [ 97 ] The scattered light is dominated by vertically polarized light as well, but anisotropic polarizability of the particles can arouse different orientations of the incident and the scattered electric elds. This results in an additional horizontally polarized contribution. The intensity of the latter contri-bution scales with the mean-squared optical anisotropy of the particles. [ 87 ] Usually, it is rather weak, but it can be selectively measured in a depolarized dynamic light scattering (DDLS) experiment through a horizontally ori-ented polarizer, such as a Glan-Thompson prism. Pecora and co-workers could demonstrate that both transla-tional and rotational diffusion coef cients can be derived from the contribution of the horizontally polarized scat-tered light. [ 87 ] The studies described above showed that monodisperse polymer latex particles with dimensions of approximately 100 nm can be assembled into stable clus-ters of well-de ned con gurations. In the following, the diffusion of uniform clusters of two, three, and four con-stituents was studied because these assemblies showed only one speci c con guration (Figure 7 ). A combina-tion of DLS and DDLS allowed probing both translational and rotational diffusion of the clusters. The polarized intensity autocorrelation functions as obtained by DLS were governed by a single exponential decay, whereas the depolarized autocorrelation functions measured by DDLS were the sum of two discrete exponential decays. [ 98 ] Hence, two discrete modes associated with the motion of

the clusters were observed in the DDLS experiment. The slow mode was a direct result of the limited extinction ratio of the horizontally oriented polarizing unit. Hence, a portion of vertically polarized light could pass the detector in addition to horizontally polarized light. For this reason, the relaxation rates of the slow mode were identical to the relaxation rates from DLS, which were related to the translational diffusion of the clusters. [ 98 ] Hence, the slow mode of DDLS gave an additional access to the translational diffusion coef cient of the clusters. The second mode, i.e., the fast mode, constituted the true depolarized signal, which arouse from the horizon-tally polarized component of the scattered light. Because its relaxation rate is associated to the translational and the rotational diffusion coef cients, both characteristics could be determined from the fast mode.

It turned out that the clusters undergo independent rotational and translational diffusion because the relaxa-tion rates of the fast mode, which depend on both char-acteristics followed linear relationships if plotted versus the square of the absolute value of the scattering vector q . [ 98 ] This decoupling of rotational from translational dif-fusion indicates that the orientation of the cluster is inde-pendent of its position, which, in turn, is evoked by the small dimensions of the clusters. This makes such clusters ideal model systems for the diffusion of complex parti-cles because data analysis can be done based on linear relationships.

Interestingly, even the spherical building blocks of the clusters showed a nite depolarized signal. [ 98 ] This is remarkable inasmuch an ideal sphere is optically iso-tropic and should thus not give a depolarized contribu-tion. The weak depolarized signal might arise from slight anisotropies in either composition or shape. Fortunately, it gave access to the rotational diffusion coef cients of the building blocks of the clusters, which, in turn, enabled a direct comparison to the rotational characteristics of the clusters (Figure 14 ).

The particle doublets showed the highest depolarized contribution because of their high shape anisotropy. [ 98 ] The shape anisotropy of particle triplets was already less pronounced but still suf cient to result in a strong depo-larized signal. A tetrahedral packing is the one character-ized by the lowest shape anisotropy for assemblies of four spheres. For tetrahedral clusters, the depolarized signal was less strong than for the particle doublets and triplets but signi cantly stronger than for the single particles.

Evaluation of the DLS data showed that the transla-tional diffusion of the clusters is widely determined by their mean radius. Hence, the decrease of the transla-tional diffusion coef cient with the number of constitu-ents follows widely the decreasing reciprocal value of the mean radius. However, the observation that shape plays a secondary role on the translational motion does not hold



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for the rotational diffusion. As a clear evidence of this, the values for the rotational diffusion coef cient of particle doublets and triplets were almost the same (Figure 14 ).

To gain more insight into the experimental results, modern hydrodynamic models for objects having com-plex shapes were probed. Comprehensive overviews of model building and hydrodynamic calculation are given in refs. [ 99101 ] These models are based on beads as sources of friction. The beads are treated as point-like with fric-tion acting at their centers, whereas in reality friction takes place at the bead surface. [ 101 ] Hence, a model in which each constituent sphere is considered as an indi-vidual bead is not appropriate without adequate correc-tion factors. This problem can be neatly circumvented by using the so-called shell model, in which the surface

of complex particles is subdivided into a large number of tiny non-overlapping frictional elements. [ 101 ] Replacing the cluster surface through a shell of numerous friction elements should give the correct diffusion coef cients; provided that the diffusion matrix can be calculated numerically for a large number of frictional elements. Indeed, an excellent agreement between experimental data and theoretical prediction was found (Figure 14 ). This accordance of theory and experiment was achieved by assumption of stick boundary conditions, whereas treatments based on slip boundary conditions do not agree with the experimental data. [ 98 ] The lower orienta-tion times of the clusters as compared to small molecules make stick boundary conditions work much better than slip boundary conditions. [ 102 ]

The shell model was also successfully used to con rm the nature of rotation that was monitored by DDLS. As mentioned above, the spherical building blocks of the clusters and particle tetrahedrons showed only moderate shape anisotropies. Hence, the rotation of these two spe-cies could not be assigned to a speci c axis. However, particle doublets and triplets show strong shape anisotro-pies. In these cases, both diffusion coef cients related to the rotation around the main symmetry axis and the axis perpendicular to it were calculated and compared to the experimental result. DDLS monitors the rotation around the axis perpendicular to the main symmetry axis in both cases (Figure 14 ). The scattering experiment is insensitive to the rotation of the clusters around their main sym-metry axis because of the low shape anisotropies of their constituent spheres. This also explains why the rotational diffusion coef cients of doublets and triplets are similar (Figure 14 ). The volume that is displaced by a rotation around the axis perpendicular to the main symmetry axis is the same for an ideal particle doublet and triplet.

6 . Conclusions and Perspectives

Nanoparticle assembly into nite size supracolloids with tailored shapes and composition is fuelled by the aspira-tion to fabricate designer colloids, which do not only com-bine the properties of their constituents in a single entity but also provide new collective properties resulting from their closely packed building units. Shape control is a direct result, which might rst come to our minds. Both de ned symmetries resulting in non-centrosymmetric interaction potentials and coding of polymer colloids by site-speci c incorporation of functional constituents would allow for directional interactions that could open unprecedented possibilities in building novel hierarchically organized materials and mesostructures. An ideal strategy towards such designer colloids should be facile, scalable, applicable to a large variety of building blocks, allow the co-assembly

Figure 14. Synopsis of experimental and theoretical results on cluster diffusion: Below the clusters which are oriented with their main body parallel to the plane of the gure, the transla-tional ( D T ) and rotational ( D R ) diffusion coef cient as obtained by DLS and DDLS are given (left column). The right column gathers theoretical predications for the translational diffusion coef -cient D T and the rotational diffusion coef cients perpendicular and parallel to the main symmetry axis (DR, DR ) using the shell model, which allowed to take the exact shapes of the clusters into account. Reproduced with permission. [ 98 ] Copyright 2009, American Chemical Society.




of different building blocks, and after all deliver various con gurations. Recently, many ingenious strategies were elaborated in this direction, which are different in terms of versatility, scalability, and accessible morphologies. Most of these approaches are limited by the smallest size of parti-cles that can be used as building blocks. A scalable strategy towards supracolloidal assemblies is the aggregation of particles as elementary building units assisted by geomet-rical con nement to emulsion droplets. This technique ini-tially elaborated for the assembly of micro-sized particles can be successfully adapted to fabricate submicron-sized clusters through the agglomeration of polymer particles of dimensions in the order of 100 nm. The size distribution of the templating oil droplets was controlled by ultrasonica-tion. The particles adhere to the surface of the oil droplets and pack together in a well-de ned way during evapora-tion of the dispersed oil phase. By systematic optimization of particle and emulsi er concentrations and selection of suitable emulsi cation conditions, high yields of clusters with a complex, yet well de ned form and colloidal dimen-sions (


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a plenitude of hierarchically organized (multi)functional materials.

Acknowledgements : Financial support from the Deutsche Forschungsgemeinschaft (DFG) within SFB 840/A3 and the Fonds der Chemischen Industrie (FCI) is gratefully acknowledged.

Received: September 6, 2013; Revised: October 9, 2013; Published online: November 8, 2013; DOI: 10.1002/marc.201300693

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Keywords: colloidal clusters ; depolarized dynamic light scattering ; nanoparticle assemblies ; polymer particles ; supracolloids

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