Directional self-assembly of a colloidal metal-organic framework.

Materials science is at an early stage of learning to employ anisotropic interaction in self-assembly. One important approach towards this goal involves the study of nm-sized particles, but we note two limitations: first that this is a rather special case as the range of inter-particle interaction in suspensions is typically likewise nm in size, and secondly that the diffraction limit precludes visualizing adjacent nm-sized particles optically at the single-particle level. Among the various nm-sized particles that are known, polyhedra are particularly interesting. That new structures might result from the self-assembly of polyhedra was suggested by Monte Carlo simulation predictions of various non-trivial ordered structures or complex mesophase structures. Here, we report an experimental approach to extend the self-assembly of polyhedra to larger, colloidal scales. Metal-based particles have already been designed to this end; here we consider a different class of materials, which may offer a greater variety of colloidal building blocks and attainable applications. Metal–organic frameworks (MOFs) or porous coordination polymers (PCPs) are recently developed materials prepared by co-crystallization of metal ions and bridging organic ligands; they offer a wide range of potential applications, among them gas storage, separation, catalysis, and drug delivery. These might be augmented if combined with aspects of particle self-assembly, which presents several potential advantages. First, the extension of MOF features to mm-size scales may allow interesting interactions with light, enabling photonic and sensing applications. Second, the extension to the volume included in mm-sized particles may allow interesting gas and other adsorption/ release features that because the positions of the particles can be controlled, goes beyond present-day applications in which MOF elements are fixed in space. Third, the nontrivial, nonspherical shapes of mm-sized MOFs may be useful for surface patterning and surface templating. While it is true that these applications remain to be realized, they provide motivation for studies of this kind. The robustness and high porosity of MOFs are suited for colloidal self-assembly, in which materials should have chemical/physical stability in liquid dispersion; moreover, sedimentation should be a minor issue as the density of MOFs is close to that of most solvents. Here we show the first example of supraparticle self-assembly of MOF crystals. A directional facet-to-facet attraction between particles through simple capillary or van der Waals attractions leads to the formation of well-defined clusters and hexagonal arrangements (Figure 1).