Molecular Recognition in the Colloidal World

Colloidal self-assembly is a bottom-up technique to fabricate functional nanomaterials, with paramount interest stemming from programmable assembly of smaller building blocks into dynamic crystalline domains and photonic materials. Multiple established colloidal platforms feature diverse shapes and bonding interactions, while achieving specific orientations along with short- and long-range order. A major impediment to their universal use as building blocks for predesigned architectures is the inability to precisely dictate and control particle functionalization and concomitant reversible self-assembly. Progress in colloidal self-assembly necessitates the development of strategies that endow bonding specificity and directionality within assemblies. Methodologies that emulate molecular and polymeric three-dimensional (3D) architectures feature elements of covalent bonding, while high-fidelity molecular recognition events have been installed to realize responsive reconfigurable assemblies. The emergence of anisotropic 'colloidal molecules', coupled with the ability to site-specifically decorate particle surfaces with supramolecular recognition motifs, has facilitated the formation of superstructures via directional interactions and shape recognition. In this Account, we describe supramolecular assembly routes to drive colloidal particles into precisely assembled architectures or crystalline lattices via directional noncovalent molecular interactions. The design principles are based upon the fabrication of colloidal particles bearing surface-exposed functional groups that can undergo programmable conjugation to install recognition motifs with high fidelity. Modular and versatile by design, our strategy allows for the introduction and integration of molecular recognition principles into the colloidal world. We define noncovalent molecular interactions as site-specific forces that are predictable (i.e., feature selective and controllable complementary bonding partners) and can engage in tunable high-fidelity interactions. Examples include metal coordination and host-guest interactions as well as hydrogen bonding and DNA hybridization. On the colloidal scale, these interactions can be used to drive the reversible formation of open structures. Key to the design is the ability to covalently conjugate supramolecular motifs onto the particle surface and/or noncovalently associate with small molecules that can mediate and direct assembly. Efforts exploiting the binding strength inherent to DNA hybridization for the preparation of reversible open-packed structures are then detailed. We describe strategies that led to the introduction of dual-responsive DNA-mediated orthogonal assembly as well as colloidal clusters that afford distinct DNA-ligated close-packed lattices. Further focus is placed on two essential and related efforts: the engineering of complex superstructures that undergo phase transitions and colloidal crystals featuring a high density of functional anchors that aid in crystallization. The design principles discussed in this Account highlight the synergy stemming from coupling well-established noncovalent interactions common on the molecular and polymeric length scales with colloidal platforms to engineer reconfigurable functional architectures by design. Directional strategies and methods such as those illustrated herein feature molecular control and dynamic assembly that afford both open-packed 1D and 2D lattices and are amenable to 3D colloidal frameworks. Multiple methods to direct colloidal assembly have been reported, yet few are capable of crystallizing 2D and 3D architectures of interest for optical data storage, electronics, and photonics. Indeed, early implications are that [supra]molecular control over colloidal assembly can fabricate rationally structured designer materials from simple fundamental building blocks.