Responsive nanostructures from aqueous assembly of rigid-flexible block molecules.

During the past decade, supramolecular nanostructures produced via self-assembly processes have received considerable attention because these structures can lead to dynamic materials. Among these diverse self-assembly systems, the aqueous assemblies that result from the sophisticated design of molecular building blocks offer many potential applications for producing biocompatible materials that can be used for tissue regeneration, drug delivery, and ion channel regulation. Along this line, researchers have synthesized self-assembling molecules based on ethylene oxide chains and peptide building blocks to exploit water-soluble supramolecular structures. Another important issue in the development of systems that self-assemble is the introduction of stimuli-responsive functions into the nanostructures. Recently, major efforts have been undertaken to develop responsive nanostructures that respond to applied stimuli and dynamically undergo defined changes, thereby producing switchable properties. As a result, this introduction of stimuli-responsive functions into aqueous self-assembly provides an attractive approach for the creation of novel nanomaterials that are capable of responding to environmental changes. This Account describes recent work in our group to develop responsive nanostructures via the self-assembly of small block molecules based on rigid-flexible building blocks in aqueous solution. Because the rigid-flexible molecules self-assemble into nanoscale aggregates through subtle anisometric interactions, the small variations in local environments trigger rapid transformation of the equilibrium features. First, we briefly describe the general self-assembly of the rod amphiphiles based on a rigid-flexible molecular architecture in aqueous solution. We then highlight the structural changes and the optical/macroscopic switching that occurs in the aqueous assemblies in response to the external signals. For example, the aqueous nanofibers formed through the self-assembly of the rod amphiphiles respond to external triggers by changing their shape into nanostructures such as hollow capsules, planar sheets, helical coils, and 3D networks. When an external trigger is applied, supramolecular rings laterally associate and merge to form 2D networks and porous capsules with gated lateral pores. We expect that the combination of self-assembly principles and responsive properties will lead to a new class of responsive nanomaterials with many applications.