Shape-Selective Recognition and Self-Assembly of mm-Scale Components

Molecular recognition requires the mating of two different molecular surfaces, complementary in shape and surface properties (hydrophobicity, hydrogen-bonding capability, electrical potential). Molecular recognition is ubiquitous in molecular science, with examples from protein-ligand interactions to asymmetric catalysis.1-6 This paper describes the export of ideas taken from molecular recognition to the recognition of macro-scale (∼1 mm) objects. The demonstration system examined here used “receptors” and “ligands”, fabricated as complementary shapes by joining small hexagonal plates and suspended at a perfluorodecalin/water interface. The interactions between the surfaces of these objects were controlled by manipulating the capillary forces between them. Receptors showed excellent selectivity for ligands based on complementarity of shape and juxtaposed hydrophobic surfaces. This system points to a new way of fabricating and assembling small, nonmolecular components using shapeand surface-selective recognition and self-assembly, extends a strategy demonstrated previously for self-assembly of extended arrays of objects7 to the directed assembly of different objects, and provides a macroscopic experimental model for molecular recognition that matches some aspects of the abstract lattice models used in statistical mechanical treatments of molecular phenomena.8,9 We fabricated the required shapes by gluing together small hexagonal poly(dimethylsiloxane) (PDMS) plates having dimensions f (the width of a face) ) 2.7 mm, and h (the height) ) 1 mm. In all of the experiments described here, the bottom face of the plates was hydrophobic, and the top was hydrophilic. The relative disposition of the hydrophobic sides is indicated by [x,y,...]; thus, a [1,4] hexagon is a hexagon with two opposite sides (and the bottom) hydrophobic, and all other sides (and the top) hydrophilic. The resulting objects were suspended at the interface between perfluorodecalin and water in a Petri dish, and swirled at a frequency ω ) 1.5-1.8 s-1 on an orbital shaker. Recognition and assembly of complementary shapes occurred spontaneously when the distance between them was within the distance (∼3h) required for interaction through capillarity.10,11 We have developed a number of systems in which recognition is both sizeand shape-selective. We will call the object presenting a concaVe surface a “receptor” and that presenting a conVex surface a “ligand”, by loose analogy with biological systems: for example, the receptor 1 recognized two [1,3] hexagons as ligands and formed a stable aggregate (Figure 1a). The overall process of recognition and assembly can be represented by eqs 1 and 2. A representative experiment involved