The ability to design and assemble 3-dimensional structures from colloidal particles is limited by the absence of specific directional bonds. As a result, complex or low-coordination structures, common in atomic and molecular systems, are rare in the colloidal domain. Here we demonstrate a general method for creating the colloidal analogues of atoms with valence: colloidal particles with chemically functionalized patches that can form highly directional bonds.These "colloidal atoms" possess all the common symmetries-and some uncommon ones-characteristic of hybridized atomic orbitals, including sp, sp 2 , sp 3 , sp 3 d, sp 3 d 2 , and sp 3 d 3 . Functionalizing the patches with DNA with single-stranded sticky ends makes the interactions between patches on different particles programmable, specific, and reversible, thus facilitating the self-assembly of particles into "colloidal molecules," including "molecules" with triangular, tetrahedral, and other bonding symmetries. Because colloidal dynamics are slow, the kinetics of molecule formation can be followed directly by optical microscopy. These new colloidal atoms should enable the assembly of a rich variety of new micro-structured materials. 2 IntroductionThe past decade has seen an explosion in the kinds of colloidal particles that can be synthesized 1,2 , with many new shapes, such as cubes 3 , clusters of spheres 4-6 and dimpled particles 7,8 reported. Because the self-assembly of these particles is largely controlled by their geometry, only a few relatively simple crystals have been made: face-centered and body-centered cubic crystals and variants 9 . Colloidal alloys increase the diversity of structures [10][11][12] , but many structures remain difficult or impossible to make. For example, the diamond lattice, predicted more than 20 years ago to have a full 3-dimensional photonic band gap 13 , still cannot be made by colloidal self-assembly because it requires 4-fold coordination. Without directional bonds, such low-coordination states are unstable.
Oil-water mixtures are ubiquitous in nature and are particularly important in biology and industry. Usually additives are used to prevent the liquid droplets from coalescing. Here, we show that stabilization can also be obtained from electrostatics, because of the well known remarkable properties of water. Preferential ion uptake leads to a tunable droplet charge and surprisingly stable, additive-free, water-in-oil emulsions that can crystallize. For particle-stabilized (''Pickering'') emulsions we find that even extremely hydrophobic, nonwetting particles can be strongly bound to (like-charged) oil-water interfaces because of image charge effects. These basic insights are important for emulsion production, encapsulation, and (self-)assembly, as we demonstrate by fabricating a diversity of structures in bulk, on surfaces, and in confined geometries.colloidosomes ͉ Pickering ͉ Wigner crystal ͉ self-assembly ͉ low polar T he stabilization of emulsions and colloidal particle suspensions against aggregation and phase separation is an ancient problem. Whereas ''emulsifiers,'' for example, surfactants and small particles, are commonly used to prepare stable oil-water mixtures, solid colloids often are stabilized by a charge on their surface (1-3). Charged entities are expected in high dielectric constant ( ) liquids, such as water ( Ϸ 80), where the energetic penalty for charge separation is small. Recently, however, the focus has shifted to low dielectric constant media, in which electrostatics can also play a surprisingly dominant role (4-10), producing fascinating phenomena like the extraordinary crystal in Fig. 1A. Here, poly(methylmethacrylate) (PMMA) spheres (radius a c ϭ 1.08 m) were suspended in a density matching mixture of cyclohexyl bromide (CHB) and cis-decalin (CHBdecalin; Ϸ 5.6). They form body-centered-cubic Coulomb or ''Wigner'' crystals, with lattice constants up to 40 m (5), similar to one-component plasmas (11). Importantly, in this low-polar solvent, charge dissociation still occurs spontaneously (7), contrary to truly apolar media ( ϳ 2) that require charge stabilizing surfactants (6,8). Although electrophoresis (see Materials and Methods) shows that the particles carry a significant charge, Z Ϸ ϩ450 e (where e is the elementary charge), such extremely large lattice constants are surprising. Assuming a screened Coulomb pair potential (12) † † , the particle interaction range will depend on the ion concentration, n, in the solvent through the Debye screening length, Ϫ1 ϰ n Ϫ1/2 . Wigner crystals require an extremely low ionic strength, so that the screening length is many particle diameters and the interaction almost purely Coulombic. We estimate Ϫ1 Ϸ 1.6 m only, for CHB-decalin (see Materials and Methods), but discovered that the presence of water (even minute quantities) reproducibly induces these crystals by tremendously increasing the screening length. Apparently, the immiscible water phase acts as an ''ion sink'' for the charged species in the oily solvent. These intriguing observations led ...
The best understood crystal ordering transition is that of two-dimensional freezing, which proceeds by the rapid eradication of lattice defects as the temperature is lowered below a critical threshold. But crystals that assemble on closed surfaces are required by topology to have a minimum number of lattice defects, called disclinations, that act as conserved topological charges-consider the 12 pentagons on a football or the 12 pentamers on a viral capsid. Moreover, crystals assembled on curved surfaces can spontaneously develop additional lattice defects to alleviate the stress imposed by the curvature. It is therefore unclear how crystallization can proceed on a sphere, the simplest curved surface on which it is impossible to eliminate such defects. Here we show that freezing on the surface of a sphere proceeds by the formation of a single, encompassing crystalline 'continent', which forces defects into 12 isolated 'seas' with the same icosahedral symmetry as footballs and viruses. We use this broken symmetry-aligning the vertices of an icosahedron with the defect seas and unfolding the faces onto a plane-to construct a new order parameter that reveals the underlying long-range orientational order of the lattice. The effects of geometry on crystallization could be taken into account in the design of nanometre- and micrometre-scale structures in which mobile defects are sequestered into self-ordered arrays. Our results may also be relevant in understanding the properties and occurrence of natural icosahedral structures such as viruses.
Colloidal particles of controlled size are promising building blocks for the self-assembly of functional materials. Here, we systematically study a method to synthesize monodisperse, micrometer-sized spheres from 3-(trimethoxysilyl)propyl methacrylate (TPM) in a benchtop experiment. Their ease of preparation, smoothness, and physical properties provide distinct advantages over other widely employed materials such as silica, polystyrene, and poly(methyl methacrylate). We describe that the spontaneous emulsification of TPM droplets in water is caused by base-catalyzed hydrolysis, self-condensation, and the deprotonation of TPM. By studying the time-dependent size evolution, we find that the droplet size increases without any detectable secondary nucleation. Resulting TPM droplets are polymerized to form solid particles. The particle diameter can be controlled in the range of 0.4 to 2.8 μm by adjusting the volume fraction of added monomer and the pH of the solution. Droplets can be grown to diameters of up to 4 μm by adding TPM monomer after the initial emulsification. Additionally, we characterize various physical parameters of the TPM particles, and we describe methods to incorporate several fluorescent dyes.
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