Content area
Full Text
The ability to design and assemble three-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 distinct surface patches that imitate hybridized atomic orbitals, including sp, sp2, sp3, sp3d, sp3d2 and sp3d3. Functionalized with DNA with single-stranded sticky ends, patches on different particles can form highly directional bonds through programmable, specific and reversible DNA hybridization. These features allow the particles to self-assemble into 'colloidal molecules' with triangular, tetrahedral and other bonding symmetries, and should also give access to a rich variety of new microstructured colloidal materials.
The past decade has seen an explosion in the kinds of colloidal particles that can be synthesized1,2, with many new shapes, such as cubes3, clusters of spheres4-6 and dimpled particles7,8 reported. Because the self-assembly of these particles is largely controlled by their geometry, only a few relatively simple crystals have been made: face-centred and body-centred cubic crystals and variants9. Colloidal alloys increase the diversity of structures10-12, but many structures remain difficult or impossible to make. For example, the diamond lattice, predicted more than 20 years ago to have a full three-dimensional photonic bandgap13, still cannot be made by colloidal self-assembly because it requires fourfold coordination. Without directional bonds, such low-coordination states are unstable.
Unlike colloids, atoms and molecules control their assembly and packing through valence. In molecules such as methane (CH4), the valence orbitals of the carbon atom adopt sp3 hybridization and form four equivalent C-H bonds in a tetrahedral arrangement. In the colloidal domain, the kinds of structures that could be made would vastly increase if particles with controlled symmetries and highly directional interactions were available. What is needed are colloids with valence14.
One approach is to decorate the surface of colloidal particles with 'sticky patches' made of synthetic organic or biological molecules (for example) and assigned to specific locations15-19. Bonding between particles occurs through patch-patch interactions, so that in principle the location and functionality of the patches can endowparticles with bonding directionality and valence. This approach is conceptually simple, yet challenging to realize. For...