Akihito Hashidzume

Macroscopic self-assembly through molecular recognition

Molecular recognition plays an important role in nature, with perhaps the best known example being the complementarity exhibited by pairs of nucleobases in DNA. Studies of self-assembling and self-organizing systems based on molecular recognition are often performed at the molecular level, however, and any macroscopic implications of these processes are usually far removed from the specific molecular interactions. Here, we demonstrate that well-defined molecular-recognition events can be used to direct the assembly of macroscopic objects into larger aggregated structures. Acrylamide-based gels functionalized with either host (cyclodextrin) rings or small hydrocarbon-group guest moieties were synthesized. Pieces of host and guest gels are shown to adhere to one another through the mutual molecular recognition of the cyclodextrins and hydrocarbon groups on their surfaces. By changing the size and shape of the host and guest units, different gels can be selectively assembled and sorted into distinct macroscopic structures that are on the order of millimetres to centimetres in size.

Expansion–contraction of photoresponsive artificial muscle regulated by host–guest interactions

The development of stimulus-responsive polymeric materials is of great importance, especially for the development of remotely manipulated materials not in direct contact with an actuator. Here we design a photoresponsive supramolecular actuator by integrating host–guest interactions and photoswitching ability in a hydrogel. A photoresponsive supramolecular hydrogel with α-cyclodextrin as a host molecule and an azobenzene derivative as a photoresponsive guest molecule exhibits reversible macroscopic deformations in both size and shape when irradiated by ultraviolet light at 365 nm or visible light at 430 nm. The deformation of the supramolecular hydrogel depends on the incident direction. The selectivity of the incident direction allows plate-shaped hydrogels to bend in water. Irradiating with visible light immediately restores the deformed hydrogel. A light-driven supramolecular actuator with α-cyclodextrin and azobenzene stems from the formation and dissociation of an inclusion complex by ultraviolet or visible light irradiation.

Photoswitchable gel assembly based on molecular recognition

The formation of effective and precise linkages in bottom-up or top-down processes is important for the development of self-assembled materials. Self-assembly through molecular recognition events is a powerful tool for producing functionalized materials. Photoresponsive molecular recognition systems can permit the creation of photoregulated self-assembled macroscopic objects. Here we demonstrate that macroscopic gel assembly can be highly regulated through photoisomerization of an azobenzene moiety that interacts differently with two host molecules. A photoregulated gel assembly system is developed using polyacrylamide-based hydrogels functionalized with azobenzene (guest) or cyclodextrin (host) moieties. Reversible adhesion and dissociation of the host gel from the guest gel may be controlled by photoirradiation. The differential affinities of α-cyclodextrin or β-cyclodextrin for the trans-azobenzene and cis-azobenzene are employed in the construction of a photoswitchable gel assembly system.

Redox‐Generated Mechanical Motion of a Supramolecular Polymeric Actuator Based on Host–Guest Interactions

The development of actuators based onmaterials that reversibly change their shape in response toexternalstimulishouldhelpimprovepeoplesqualityoflifeinsuch areas as medical treatment and micromachine applica-tion. Recently, stimuli-responsive materials have beenreported to create artificial muscles and actuators.

Switching of macroscopic molecular recognition selectivity using a mixed solvent system

The formation of macroscopic assemblies based on molecular recognition is promising in a wide variety of fields of materials science. Switching of the selectivity of macroscopic assemblies is of increasing importance to produce highly functional materials. Here we show macroscopic assembly based on molecular recognition using polyacrylamide gel modified with pyrenyl (Py) moiety (Py-gel) and gels possessing CD moieties (αCD-gel, βCD-gel and γCD-gel) in a mixed solvent of water and dimethyl sulfoxide. Changing the composition of the mixed solvent can switch the selectivity of Py-gel, because the fractions of the monomer and dimer of the Py moieties on the gel surface depend on the mixed solvent composition. The monomer and dimer of the Py moieties prefer βCD and γCD moieties, respectively.

Switching between Supramolecular Dimer and Nonthreaded Supramolecular Self-Assembly of Stilbene Amide-α-Cyclodextrin by Photoirradiation

3-trans-Stilbene amide-alpha-cyclodextrin (3-trans-Sti-alpha-CD) formed a double-threaded dimer in aqueous solutions. In contrast, the photoisomerization of the stilbene moiety in 3-Sti-alpha-CD from trans to cis leads to the structural changes from the double-threaded dimer to nonthreaded supramolecular assemblies in aqueous solutions. The structures of these supramolecular complexes have been found to be controllable by photoirradiation.

A metal–ion-responsive adhesive material via switching of molecular recognition properties

Common adhesives stick to a wide range of materials immediately after they are applied to the surfaces. To prevent indiscriminate sticking, smart adhesive materials that adhere to a specific target surface only under particular conditions are desired. Here we report a polymer hydrogel modified with both β-cyclodextrin (βCD) and 2,2′-bipyridyl (bpy) moieties (βCD–bpy gel) as a functional adhesive material responding to metal ions as chemical stimuli. The adhesive property of βCD–bpy gel based on interfacial molecular recognition is expressed by complexation of metal ions to bpy that controlled dissociation of supramolecular cross-linking of βCD–bpy. Moreover, adhesion of βCD–bpy gel exhibits selectivity on the kinds of metal ions, depending on the efficiency of metal–bpy complexes in cross-linking. Transduction of two independent chemical signals (metal ions and host–guest interactions) is achieved in this adhesion system, which leads to the development of highly orthogonal macroscopic joining of multiple objects.

pH-Responsive Self-Assembly by Molecular Recognition on a Macroscopic Scale

Macroscopic pH-responsive self-assembly is successfully constructed by polyacrylamide(pAAm)-based gels carrying dansyl (Dns) and β-cyclodextrin (βCD) residues, which are represented as Dns-gel and βCD-gel, respectively. Dns-gel and βCD-gel assemble together at pH ≥ 4.0, but disassemble at pH ≤ 3.0. The adhesion strengths for pairs of Dns-gel/βCD-gel increase with increasing pH. The fluorescence study on the model system of pAAm modified with 1 mol% Dns moieties (pAAm/Dns) reveals that Dns residues are protonated at a lower pH, which results in the reduction in binding constant (K) for Dns residues and βCD.

Reversible self-assembly of gels through metal-ligand interactions

Metal-ligand interactions with various proteins form in vivo metal assemblies. In recent years, metallosupramolecular approaches have been utilized to forge an assortment of fascinating two- and three-dimensional nano-architectures, and macroscopic materials, such as metal-ligand coordination polymeric materials, have promise in artificial systems. However to the best of our knowledge, the self-assembly of macroscopic materials through metal-ligand interactions has yet to be reported. Herein we demonstrate a gel assembly formed via metal-ligand interactions using polyacrylamide modified with Fe-porphyrin and L-histidine moieties. The stress values for the assembly increase as the concentration of Fe-porphyrin or L-histidine in the gels increases. Moreover, agitation of Fe-porphyrin gel, Zn-porphyrin gel, and L-histidine gel in an 80 mM Tris-acetate buffer (pH 9.0) results in selective adhesion of the Fe-porphyrin gel to the L-histidine gel based on the affinities of Fe-porphyrin and Zn-porphyrin with L-histidine.

Recognition of polymer side chains by cyclodextrins

This review article describes the interaction of cyclodextrins (CDs) with polymer side chains as model systems for biological molecular recognition, focusing on the steric effect of the polymer chains, the effect of the conformation of the polymer main chain or competition with association of polymer side chains, and the effect of multi-site interaction. Some typical examples of stimuli-responsive systems, nanoparticles for drug delivery systems and macroscopic self-assembly based on the interaction of CDs with polymer side chains are also reviewed briefly.