Photochemically Controlled Supramolecular Curdlan/Single-Walled Carbon Nanotube Composite Gel: Preparation of Molecular Distaff by Cyclodextrin Modified Curdlan and Phase Transition Control

Clay Nanosheet-Based Nanocomposite Supramolecular Hydrogel Enabling Rapid, Reversible Phase Transition Only with Visible Light

High mechanical properties and rapid sol/gel phase transition are mutually exclusive in the hydrogels reported to date, most likely because the 3D crosslinked networks of mechanically robust hydrogels comprise bundled thick fibers that are not rapidly dissociable or formable. Herein, we report a visible light-responsive hydrogel that showed a rapid, reversible sol/gel phase transition despite its relatively high mechanical properties (storage modulus ~103 Pa). To construct its 3D crosslinked network, we used a design strategy analogous to that employed for our highly water-rich yet mechanically robust nanocomposite supramolecular hydrogel ("aqua material"). In this case, multiple poly(ethylene glycol) chains carrying ortho-tetramethoxyazobenzene termini (AzoPEG) were noncovalently crosslinked by clay nanosheets (CNSs) with surface-immobilized β-cyclodextrin units using their seven guanidinium ion (Gu+) pendants (GuCD) via a multivalent salt-bridge. When exposed to visible light at 625 and 450 nm, the azobenzene termini isomerized from trans-to-cis and cis-to-trans, respectively, and were detached from and attached to the surface-immobilized GuCD units. The advantage of this CNS-based nanocomposite supramolecular system is its simple 3D network structure, which forms and breaks rapidly without slow chain entangling and disentangling processes.

Development and Geometrical Considerations of Unique Conductive and Reversible Carbon-Nanotube Hydrogel without Need for Gelators

We propose a new type of CNT hydrogel that has unique conductive and reversible characteristics. We found in previous studies that CNT dispersions became gelatinous without any gelators when a specific CNT was combined with a specific dispersant. This hydrogel has conductive properties derived mainly from the CNTs it contains; and even after gelation, it can be returned to a liquid state by ultrasonic irradiation. Furthermore, the liquid is gelable again. In this study, we prepared several types of CNTs and several types of dispersants, experimentally verified the possibility of gelation by combining them, and geometrically investigated the gelation mechanism to determine how this unique hydrogel is formed. As a result, we found that the experimental results and the theory examined in this study were consistent with the combination of materials that actually become hydrogels. We expect that this study will allow us to anticipate whether or not an unknown combination of CNTs and dispersants will also become gelatinous.

Optothermally Reversible Carbon Nanotube-DNA Supramolecular Hybrid Hydrogels

Supramolecular hydrogels (SMHs) are three-dimensional constructs wherein the majority of the volume is occupied by water. Since the bonding forces between the components of SMHs are noncovalent, SMH properties are often tunable, stimuli-responsive, and reversible, which enables applications including triggered drug release, sensing, and tissue engineering. Meanwhile, single-walled carbon nanotubes (SWCNTs) possess superlative electrical and thermal conductivities, high mechanical strength, and strong optical absorption at near-infrared wavelengths that have the potential to add unique functionality to SMHs. However, SWCNT-based SMHs have thus far not realized the potential of the optical properties of SWCNTs to enable reversible response to near-infrared irradiation. Here, we present a novel SMH architecture comprised solely of DNA and SWCNTs, wherein noncovalent interactions provide structural integrity without compromising the intrinsic properties of SWCNTs. The mechanical properties of these SMHs are readily tuned by varying the relative concentrations of DNA and SWCNTs, which varies the cross-linking density as shown by molecular dynamics simulations. Moreover, the SMH gelation transition is fully reversible and can be triggered by a change in temperature or near-infrared irradiation. This work explores a new regime for SMHs with potential utility for a range of applications including sensors, actuators, responsive substrates, and 3D printing.

A high water-content and high elastic dual-responsive polyurethane hydrogel for drug delivery

Stimuli-responsive hydrogels are soft, biocompatible and smart biomaterials; however, the poor mechanical properties of the hydrogels limit their application. Herein, we prepared a reductant- and light-responsive polyurethane hydrogel which was made of polyethylene glycol, 1,6-diisocyanatohexane, azobenzene, cyclodextrin and disulfide. Attenuated Total Reflectance Infrared Spectra and ¹H NMR were used to characterize the structure of the hydrogel. The hydrogel has a high elasticity (a tensile modulus of 36.5 ± 0.5 kPa and a storage modulus of 52.9 ± 1.2 kPa) at a high water content (91.2 ± 0.4%). Swelling, mechanical and rheological properties of the hydrogel can be tuned by the content of the crosslinker, light and reductant. The hydrogel has low cytotoxicity and it can be used for drug delivery. Ultraviolet irradiation helped to load drugs and the reductant accelerated the drug release. With its high mechanical properties and light- and reductant-responsiveness, the hydrogel is hopefully to be used as a drug carrier.

Cyclodextrin-based molecular machines

This chapter overviews molecular machines based on cyclodextrins (CDs). The categories of CD-based molecular machines, external stimuli for CD-based molecular machines, and typical examples of CD-based molecular machines are briefly described.

Light‐Switchable Single‐Walled Carbon Nanotubes Based on Host–Guest Chemistry

A new type of light‐switchable “smart” single‐walled carbon nanotube (SWNTs) is developed by the reversible host–guest interaction between azobenzene‐terminal PEO (AzoPEO) and pyrene‐labeled host attached on the sidewalls of nanotubes via π–π stacking. The SWNTs hybrids not only are well dispersed in pure water, but also exhibit switchable dispersion/aggregation states upon the alternate irradiation of UV and visible light. Moreover, the SWNTs hybrids dispersion is preliminarily used as coating fluid to form transparent conductive films. The dispersant AzoPEO is removed by the contamination‐free UV treatment, decreasing the resistance of the films. This kind of light‐switchable SWNTs hybrids, possessing a ‘‘green’’ trigger and intact structure of the nanotube, may find potential applications in sensor of biomedicines, device fabrication, etc. Additionally, such a reversible host–guest interaction system may open up the possibility to control the dispersion state of SWNTs by other common polymers.

Interaction of cyclodextrins with pyrene-modified polyacrylamide in a mixed solvent of water and dimethyl sulfoxide as studied by steady-state fluorescence

The interaction of β- and γ-cyclodextrins (β-CD and γ-CD, respectively) with polyacrylamide modified with pyrenyl (Py) residues (pAAmPy) was investigated in a mixed solvent of water and dimethyl sulfoxide (DMSO) by steady-state fluorescence. In the absence of CD, the fluorescence spectra indicated that the formation of Py dimers became less favorable with increasing volume fraction of DMSO (x(DMSO)). The fluorescence spectra at varying x(DMSO) and CD concentrations indicated that β-CD and γ-CD included monomeric and dimeric Py residues, respectively. Using the fluorescence spectra, equilibrium constants of the formation of Py dimers and the complexation of β-CD and γ-CD with Py residues were roughly estimated based on simplified equilibrium schemes.

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.

Self-assembly through molecular recognition events is used in the production of functionalized materials. This study shows that macroscopic gel assembly can be regulated through photoisomerization of an azobenzene moiety that interacts differently with two host molecules.