J. Fiber Sci. Technol., 76(1), 1-22 (2020) doi 10.2115/fiberst.2020-0004 ©2020 The Society of Fiber Science and Technology, Japan 【SPECIAL EDITIONS on “NANO FIBER”-Review】 Stimuli-Responsive Colloidal Assembly Consisting of Imogolite, Inorganic Nanotube Kazuhiro Shikinaka Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1, Nigatake, Miyagino-ku Sendai, Miyagi 983-8551, Japan Abstract: In this review, I describe stimuli-responsive colloidal assemblies prepared from a rigid rod-like clay mineral called imogolite, and relate them to their structural characteristics. When combined with dicarboxylic acids, imogolite formed gels that exhibited keen thixotropy, and physical anisotropy via orientation of the imogolite particles after flowing and subsequent standing. Robust hydrogels were also obtained by in-situ polymerization of vinyl monomers in imogolite aqueous dispersion. Under strain, these hydrogels showed a reversible isotropic‒anisotropic structural transition. (Received 26 September, 2019; Accepted 24 November, 2019) 1. INTRODUCTION cylindrical polymers. To create the functional colloidal assemblies in a Some living organisms consist of various biomimetic manner, I investigated a single-walled architectures of one-dimensional structures such as aluminosilicate inorganic polymer called imogolite nanofibers and nanotubes. For example, in cell (henceforth denoted as IG), which has a rigid cytoskeletons, semi-flexible rod-like proteins such as cylindrical structure and the composition (HO)3Al2O3 filamentous actin form network-like architectures via SiOH [9‒14]. With external and internal diameters of non-covalent bonding (e.g., hydrogen bonding and approximately 2 nm and 1 nm, respectively, and a electrostatic interactions) [1]. Tube-like assemblies of length ranging from several tens of nanometers to the same proteins, such as actin and microtubles, form several micrometers, IG has been noted as a perfectly bundle-like architectures by the same mechanism [1]. rigid polyelectrolyte with a high aspect ratio [15]. These architectures realize the motility and other Accordingly, it has been incorporated into inorganic‒ various functionalities of cells through their keen organic nanocomposites [16]. The outer and inner stimuli-responsive structural transitions. Similar surfaces of IG are covered with Al(OH)2 (proton- materials that respond to stimuli such as light [2] and capturing) and Si(OH) (proton-releasing) groups, temperature [3] have been designed in vitro through respectively. Thus, the charge density of IG surfaces various opportune molecular (assembling) structures, varies with the pH and ionic strength of aqueous empowering both emerging fields of scientific interest media. Consequently, the dispersibility of IG in water and unexplored applications [4]. Stimuli-responsive is highly pH-dependent; in acidic aqueous media (pH ≈ materials have also been built from rigid rod-like 4) with relatively low ionic strength, IG disperses as materials such as thermotropic/lyotropic liquid thin bundles or even as monofilaments, resulting in crystals [5‒8]. Recently, I designed novel stimuli- opaque to transparent solutions. responsive colloidal assemblies consisting of rigid In this review, I discuss some developments of cylindrical inorganic polymers that imitate the stimuli-responsive IG materials and relate them to the evolving architectures in cell cytoskeletons. This structural characteristics of IG. Section 2 describes review describes the novel functional materials the thixotropic gelation of IG nanotubes and the developed by myself and colleagues, which are based characteristics of the obtained IG gels. Section 3 on the supramolecular architectures of rigid discusses how ionic conductivity emerges in IG gel # corresponding author: Kazuhiro Shikinaka (E-mail: [email protected]) Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) 1 combined with ionic liquid. Section 4 and 5 describe well known, many kinds of inorganic and organic the structural orderings of IG gels initiated by flow- acids such as carboxylic acids [22] interact strongly shearing and chiral formation, respectively. Finally, I with the outer surface of IG nanotubes. When introduce a robust, stimuli-responsive IG gel carboxylic acids are added to IG, they establish composed of IG and organic network polymers. protonation equilibrium of the aluminol groups of the IG outer surface that form the cationic sites, creating 2. FORMATION AND CHARACTERISTICS strong hydrogen bonds or electrostatic interactions OF IG-BASED THIXOTROPIC GEL with the molar equivalent of the ‒Al(OH)2 units. When aqueous solutions of IG and maleic acid (MA, a typical 2.1 Thixotropic gelation of IGs by hydrogen bonding short-chain dicarboxylic acid) are combined and aged, I have designed stimuli-responsive IG-based opaque gels (Figure 1 a; denoted as IG‒MA gel) or materials with non-Newtonian fluid behaviors such as hard-gel particle dispersions (phase-separated liquid/ shear thinning. Shear thinning or stimuli-responsive gel mixtures) are formed, depending on the mixing liquid-to-solid phase transitions (also known as ratio. When the mixing ratio is imbalanced (> 2:1 or < thixotropy) are often found in muscle and protoplasm 1:4), the resulting mixtures undergo an apparent [17], and are also important in many industrial phase separation, yielding hard-gel particles in processes (e.g., paints and ceramic sols) [18]. Shear aqueous medium. thinning is thought to emerge from assemblies of At an approximately 1:1 molar ratio of ‒Al(OH)2 colloidal particles, generally called hydroclusters and MA (not the molar quantity of a single ‒COOH [19,20]. In our study [21], IGs purified by appropriate group), the resulting sol‒state mixture gradually procedures were sonicated in pure water to obtain turns into an opaque gel with a thixotropic nature. slightly opaque solutions of nanotubes at a This transition occurs after approximately 1 h at 25 ̊C concentration of 6.4 wt% (i.e. 0.16 mol/L of aluminol (Figure 1 a). In the rotating rheometric test, the aged groups). The average length of the nanotubes was IG‒MA gel (sol) transitioned to the sol (gel) state 68.5 nm. These aqueous solutions were used as within 6 s of agitation (rest), accompanied by perfect starting materials throughout the experiments. As is recovery of its elastic modulus [22]. A 1:1 combination Fig. 1 (a) Preparative procedure of IG‒MA gel and photographs of the sample tube inversion test for solid and liquid states. [‒Al(OH)2 of IG] = [MA] = 0.08 mol/L. (b) Cryo HAADF‒STEM images of quick-frozen IG‒MA gel. [‒Al(OH)2 of IG] = [DA] = 0.08 mol/L. (c) HAADF‒STEM image and EDS elemental mapping of a dried mixture of IG, bis‒2-carboxyethyl germanium(IV)] sesquioxide, andMA. The Ge atoms (i.e., carboxylic acids) localized on the IG nanotubes clearly indicate an interaction between the dicarboxylic acids and the IG nanotubes. Reproduced with permission from [69]. Copyright 2016, Nature Publishing Group 2 Journal of Fiber Science and Technology (JFST), Vol.76, No. 1 (2020) of IG and dicarboxylic acids with 4‒6 main-chain carbons forms similar thixotropic gels, but requires a longer aging period to obtain the gel state. When IG is combined with oxalic or malonic acid at a molar ratio of 1:1, the mixture instantaneously forms turbid hard- gel particles dispersed in the aqueous solution. Thus, the gelation speed of the mixtures and their stimuli- responsiveness sensitively depends on the number of main-chain carbons, configurations, and steric hindrance of the dicarboxylic acids. 2.2 Gelation/structural transition process of IG thixotropic gel Here, I relate the microscopic structural changes to the thixotropic properties of the well-aged 1:1 IG‒ MA gel (sol). To observe the microscopic structures of Fig. 2 Scattering curves obtained by time-lapse the wet-state IG‒MA gels (including the depth profile), SAXS measurements of a well-aged IG‒MA I directly observed the wet-state gel. The IG portions mixture ([‒Al(OH)2 of IG] = [MA] = 0.08 mol/L) during solid - to - liquid transition. These of the gel comprise the Al and Si atoms, which measurements were performed under 0.1 s X- provided sufficient Z contrast (defined as the contrast ray irradiation at 0.1-s intervals. Reproduced in proportion to atom number Z) for their observation. with permission from [69]. Copyright 2016, The gels were observed by high-angle annular dark- Nature Publishing Group field scanning transmission electron microscopy equipped with a cryo-sample transfer system (cryo high contrast and resolution. Periodic rapid HAADF‒STEM), which obtained the Z contrast measurements were obtained at 0.2-s interval by a images of the silicon, aluminum, and oxygen atoms. tandem vertical undulator synchrotron radiation Figure 1 b shows cryo HAADF‒STEM images of a apparatus with a high photon flux and an X-ray torn sample of a quick-frozen IG‒MA gel, presenting counting two-dimensional pixel detector. Figure 2 the bulk and whole structures. The IG‒MA gel illustrates the scattering profiles (i.e., the SAXS consisted of interconnected spongy frameworks. scattering curves of scattering intensity I(q) versus A contrast-intensified cryo HAADF‒STEM the scattering vector q) of a well-aged IG‒MA gel image (Figure 1 b) partially verifies the individual IG recorded at 0.2-s intervals during the cycles between nanotubes in the frameworks of the IG‒MA gel. As the rest period (2.0 s) and the following vortex-mixer shown in the highly magnified STEM image (Figure agitation period (2.0 s). Here, a rod-like substance can 1b), the IG‒MA gel locally appeared as crossed be inferred from the steep slope of the I(q) ~ q－E nanotubes. Furthermore, as shown in the HAADF‒ relation [23,24]. As the region q = 0.08‒0.3 nm－1 STEM images of a dried mixture of germanium corresponds to the real-space size of IG nanotubes bonded dicarboxylic acids and IG (Figure 1 c), the (average length 68.5 nm; average external diameter ~ dicarboxylic acids attached to the whole IG surface in 2.0nm),E=1and2inthisregionindicate that the IG the mixture.