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.

We show that coupling between a fast and a slow relaxation time causes the spontaneous formation of protrusions in colloids made of cross-linked polymers. The volume of the protrusions can be controlled by adjusting the ratio between the relaxation times. This, in principle, results in particles with levels of anisotropy that can be made "to order".Anisotropic colloidal particles may in principle form crystal structures that are completely different from their isotropic analogues. Under certain conditions, giant micelles or shell-like superstructures are expected. These particles therefore are of immense interest for fundamental study as well as for potential applications. Much progress has recently been made in methodologies to prepare anisotropic particles consisting of heterogeneous polymers. Resulting particles vary in shape from "snowman/confetti" 1 to dented 2 and egglike, 3 just to mention a few.Sheu and co-workers 4,5 made use of interpenetrating homopolymer networks of polystyrene to produce spherical crosslinked polystyrene particles with protrusions of both cross-linked and non-cross-linked polystyrene. Protrusions are formed upon swelling of cross-linked polystyrene particles with styrene, followed by heating. Heating causes the network to shrink and expel excess styrene in the form of protrusions and at the same time results in the polymerization of styrene monomers. The above method works only if the seed particles are at least several micrometers in size. Significant progress has very recently been made by modification of the surface of the seed particles, resulting in anisotropic particles with sizes on the order of 100 nm. 6 An intriguing observation 4 is the spontaneous formation of protrusions upon swelling of the particles with styrene, even without heating. This observation opens up the possibility to prepare particles with protrusions that contain material that is unstable to heating, such as fluorophores, proteins, and other biomolecules, making them in principle relevant for applications such as controlled release and artificial drug and gene delivery. Indeed, by being able to control the formation of protrusions, the relevant materials may be ejected from the polymer networks on the spot and at the right time. Here we show that the mechanism of the spontaneous formation of protrusions upon swelling is coupling between two disparate relaxation times, a fast one coupled to swelling of the network and a slow one connected to shrinking. We show that the anisotropy of the particles can be modified by systematically varying the relaxation times.Our experimental setup is as follows: 4.5 mL of an aqueous dispersion of 3% (w/w) Magsphere polystyrene particles of 1.88 µm diameter, cross linked with 5% divinylbenzene and containing 1% (w/w) SDS, was mixed with 1.1 g of styrene. The mixture was gently stirred at 20°C, and samples were taken at several time intervals and inspected under a light microscope.In Figure 1, the diameter of the primary part of the particles (i.e., not including the protrusi...

We describe a new method to covalently link a wide variety of molecules to the surface of colloidal polymer microspheres using the Cu(I)-catalyzed azide-alkyne reaction, most commonly known under the class of reactions identified by the term click chemistry. The method is generic and readily applied to a spectrum of colloidal particle systems allowing surfaces to be tailored with virtually any desired functionality. To demonstrate this method, polystyrene microspheres were functionalized with two different polyethylene oxide-based polymers, and changes in hydrodynamic radii after functionalization were measured using dynamic light scattering. Control of surface functional groups was demonstrated by fluorescently labeling the colloidal microspheres using the same Cu-catalyzed azide-alkyne cycloaddition reaction.

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