Anisotropic Self-Oscillating Reaction in Liquid Crystalline Nanosheets

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Anisotropic Self-Oscillating Reaction in Liquid Crystalline Nanosheets Hydrogels Morio Shintate, Takumi Inadomi, Shinya Yamamoto, Yusuke Kuboyama, Yutaka Ohsedo, Takashi Arimura, Tomoka Nakazumi, Yusuke Hara, and Nobuyoshi Miyamoto J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b11631 • Publication Date (Web): 17 Feb 2018 Downloaded from http://pubs.acs.org on February 17, 2018

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The Journal of Physical Chemistry

Anisotropic Self-Oscillating Reaction in Liquid Crystalline Nanosheets Hydrogels

Morio Shintate,1 Takumi Inadomi,1 Shinya Yamamoto,1 Yusuke Kuboyama,1 Yutaka Ohsedo,1 Takashi Arimura,2 Tomoka Nakazumi,2 Yusuke Hara,2* and Nobuyoshi Miyamoto1*

1

Department of Life, Environment and Materials Science, Fukuoka Institute of

Technology, 3-30-1, Wajirohigashi, Higashiku, Fukuoka 811-0295, Japan 2

National Institute of Advanced Industrial Science and Technology (AIST), AIST

Central 5–2, Tsukuba 305-8565, Japn

*

corresponding authors: [email protected], [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT.

Anisotropic

Belouzov-Zhabozinsky

chemical

(BZ)

wave

reaction

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propagation was

of

self-oscillating

demonstrated

in

the

poly(N-isopropylacrylamide gel films embedded with macroscopically aligned liquid crystalline inorganic nanosheets. While the average propagation rate of chemical wave ̅ was 3.56 mm min-1 in the gels without nanosheets, the propagation was retarded in the gels with 1 wt% of nanosheets: ∥ = 1.89 mm min-1 and

= 1.33 mm min-1

along the direction parallel and perpendicular to the nanosheet planes, respectively. Thus, the wave propagation is anisotropic with the anisotropy ratio ∥ / = 1.42 in these gels and the periodic patterns formed by the BZ-reaction were concentric ellipses, different from circles seen in isotropic gels. Furthermore, the propagation rate and degree of anisotropy were controllable by nanosheet concentration. These phenomena can be explained that the diffusion of molecules inside the gel is effectively hindered along the direction perpendicular to the nanosheet planes due to very large aspect ratio of the aligned nanosheets. The present systems will be applicable for anisotropic self-oscillating soft actuators with one-dimensional motions as well as for ideal model system of BZ reactions.


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INTRODUCTION

Soft and wet structures with anisotropy and hierarchy are regarded as key for various intriguing functions and properties of living organisms. Liquid crystal (LC) materials are often utilized to artificially fabricate such sophisticated structures like living organisms.1-2 For this purpose, lyotropic LC phases of inorganic nanosheets are emerging as a new type of LC materials due to many fascinating characteristics.3-4 Actually, inorganic nanosheet LCs have been utilized as unusual anisotropic reaction media,5-6 the materials to fabricate tough fibers 10

7-8

and inorganic-polymer composites.9

The most distinctive feature of inorganic nanosheet LCs is that they can effectively

barrier the diffusion of molecules. Since the nanosheets are two dimensional objects with huge aspect ratio (the lateral dimension of several micrometer and the thickness of ~1 nm), aligned nanosheets can effectively retard molecular diffusion compared to the materials made with conventional LC molecules.11 We recently communicated that dye ions anisotropically diffuse into the polymer gel embedded with macroscopically aligned LC nanosheets.10 Meanwhile, combining artificial soft and wet structures with particular



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chemical reactions that have characteristic features of biological systems is fascinating because such combinations will lead us to fabrication of artificial living organisms or molecular robots.12 Yoshida et al. reported the hydrogel of poly(N-isopropylacrylamide) (pNIPA) copolymerized with ruthenium trisbipyridine (Ru(bpy)3) units, which is a catalyst of self-oscillating Belouzov-Zhabozinsky reaction (BZ-reaction).13 Not only BZ reaction proceeded in this gel, but also the gel showed self-oscillating motion. After this pioneering work, many kinds of self-oscillating gels with improved properties such as faster and larger oscillation, were reported14-17 in view of applications for micro-robotics and micro electro mechanical systems (MEMS). The concentric circles18 or rotating spiral 19 waves that appear during BZ reaction is also interesting phenomena as a model of chemical waves in biological systems19 such as embryo, neural networks, and heart muscles. In this context, controlling and modifying the wave patterns in anisotropic, heterogeneous, or patterned media are interested. The BZ-reactions under electric field,

20

in the lamellar phase of surfactant solution

21

and in a porous glass22

were investigated to control the reaction. However, anisotropic BZ reaction has not been investigated so far.


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Herein, we demonstrate the anisotropic propagation of chemical waves of BZ reaction for the first time in anisotropic composite gel comprising aligned LC nanosheets. To realize this, we successfully synthesized very large anisotropic gel sheet in which fully aligned nanosheets are embedded.

EXPERIMENTAL

N-isopropylacrylamide (NIPA; monomer) and N,N’-methylenbisacrylamide (chemical cross-linker) were used after recrystallization from hexane/acetone mixture. The photo-initiator 2-hydroxy-2-methylpropiophenon was used without purification. The LC layered clay mineral, fluorohectorite (FHT), was supplied from Topy Industries Inc. and purified by centrifugation before use. The nanosheet/water colloid is very stable and no aggregation nor precipitation is observed for years. The average lateral size of the nanosheets is 2.2 µm as confirmed by transmission electron microscopy, while the size distributes broadly from ~100 nm to ~10 µm.23 The synthesis of the gel sheets was conducted as follows by slightly modifying the process we recently published24. First, 6.5 mmol of the monomer, 0.065 mmol of the chemical cross-linker, and 0.061 µmol of



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the photo-initiator were dissolved in 10 ml of the FHT aqueous dispersion (1 or 0.5 wt%). This solution was sealed in a 1 mm-thick cell constructed with a silicon rubber sheet, two glass plates and aluminum electrodes, followed by applying AC electric field (10 kHz and 2 V mm-1) for 1 h (Figure 1a). Thereafter, by irradiating ultraviolet light for 5 min, photo-polymerization was carried out. After polymerization, the nanosheets are immobilized in the polymer network, of which mesh size is much smaller than the lateral size of the nanosheets, so that the orientation of the nanosheets is retained even after removing the electric field. The structure of the anisotropic composite gel was characterized by observation with crossed-polarizers and the optical waveplate with retardation R of 530 nm. Small angle X - ray scattering measurements (SAXS; Rigaku NANO-PIX) were also performed. For the BZ reaction, the aqueous solution containing sodium bromate (0.24 M), sodium bromide (0.12 mM), malonic acid (0.069 mM), sulfuric acid (0.043 mM), and ferroin (0.043 mM) was prepared in a glass dish. After immersing the composite gel in the solution for 10 minutes, the gel was taken out from the dish and observed in air with a digital camera (CASIO EX-ZR 1000). The reaction was conducted in a room air-conditioned at around 23ºC to avoid the influences of


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temperature difference on the BZ reaction.15, 25-26

RESULTS AND DISCUSSION

Uniform macroscopic alignment of LC nanosheets in the pNIPA gel was assured by crossed polarizer observation and SAXS (Figure 1). In the observation of the gel with the crossed polarizers (Figure 1b), uniformly blue and yellow interference colors, which correspond to the retardation R of >530 nm and < 530 nm, are observed when the waveplate (R = 530 nm) was set with its slow axis parallel and perpendicular to the electric field direction, respectively. It is known that the total R is the sum of R for the sample and waveplate when the slow axis of the waveplate are parallel to each other, while total R is the difference between them when the axes are perpendicular to each other. Thus we assure that the nanosheets are preferentially aligned along the electric field, considering that the slow axis of the nanosheet LC domain is parallel to the nanosheet plains inside the domain.24 It is known that electric-field-induced polarization of counter cation cloud around the negatively charged nanosheet cause the nanosheet orientation.27



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SAXS measurements (Figure 1c-f) further confirm the preferential orientation of the nanosheets on macroscopic scale as well as good dispersion and well-regulated super-structure formed by the nanosheets on mesoscopic scale. The anisotropic 2D scattering pattern (Figure 1c) and the sharp peaks in the I vs χ profile (Figure 1d) indicate the strong alignment of the nanosheets. From the I vs χ profile, nematic order parameter S = 0.93 was estimated. In the I(q) vs q profile (Figure 1e), the overall profile with the power law of q-2 and q-3 are observed at χ = 0º, and 90 º, respectively. These are ascribable to the form factor of oriented nanosheets.28 In addition, the peak at q = 0.11 nm-1 (d = 2π/q = 57 nm) is observable in the I•q2 vs q plot (Figure 1f), showing the presence of a weak short-range structural order of the nanosheets. Because the structural order is low, the nanosheets are regarded as in nematic state. It is noticeable that the oriented mono-domain as large as 30 mm × 30 mm is obtained, which is suitable for the investigation of anisotropic BZ reactions. While the mono-domain of 5 mm × 30 mm was already obtained in the previous study,24 we optimized the synthetic condition to realize this larger mono-domain gel sheet.


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An important point in the present study is that the composite gel for BZ-reaction is synthesized by the very simple synthetic procedure, electric field-assisted alignment of the nanosheets, photo-polymerization, and adsorption of the catalyst by post-process, in contrast with rather complicated processes of previous BZ-gels.13 Visible absorption spectroscopy confirms that most of the ferroin cations, that are the catalyst for BZ reaction, are strongly adsorbed on the surface of the nanosheets in the gel. In the spectrum (Figure 2a) of the aqueous ferroin solution, the absorption maximum due to ferroin is observed at 510nm. In contrast, the composite gel after BZ reaction (Figure 2c) and the nanosheet colloid (1 wt%) added with ferroin (3 x 10-5 M) (Figure 2b) show the absorption maxima at the 520 nm. The 10 nm shift of the maximum is ascribed to strong adsorption of the chromophore onto nanosheet surface which cause modification of HOMO and/or LUMO level of the dye.29 Even after immersing the ferroin-adsorbed composite gel in pure water or in the acidic solution for BZ reaction, no desorption of the dye was observed; this stability allows us to use this ferroin-adsorbed gel repeatedly for BZ reactions. It is supposed that electrostatic interaction between divalent ferroin



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and negatively charged nanosheets is strong enough to keep the ferroin dications on the nanosheets although they are not fixed in the gel by covalent bond. The anisotropic propagation of BZ reaction wave in the present anisotropic gels was clearly confirmed by eye observations. Figure 3a shows the images clipped from the real-time movie (Movie S1) of the anisotropic gel in which BZ reaction is progressing. The patterns formed by the BZ-reaction are anisotropic ellipses, in contrast with the isotropic circles in the isotropic gel (Figure 3b, Movie S2). The ellipses diffuse with time, keeping their aspect ratio. We picked up one ellipse and measured the time-course of the semi-minor and semi-major axes to evaluate the propagating distance of the wave from the center (Figures 4a). With 1 wt% nanosheet (Figures 4a and a'), we find large difference in propagation rate along the two directions. From the slope, the propagation rates v// and v⊥ along the directions perpendicular and parallel to the nanosheet planes are estimated as 1.10 and 1.61 mm min-1, respectively, while anisotropy v///v⊥ = 1.46. In contrast, the wave propagation is much faster and isotropic in the gel without nanosheets (Figures 4b and b'). To confirm the validity of our experiments, we picked up 11 ellipses formed during the BZ reaction runs performed on


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two different gels with 1wt% of nanosheets synthesized in different batches (Table S1). The average propagation rates and anisotropy are evaluated as ∥ = 1.89 ± 0.77 mm -1 min-1, = 1.33 ± 0.50 mm min , and ∥ / = 1.42 ± 0.18 (the values after

± denote the standard deviation), ensuring the validity of our data. The propagation rate and the anisotropy both strongly depended on the nanosheet concentration as shown in Figure 5. In the gel with 1 wt% of nanosheets, the wave propagation rate is 47% and 73% retarded along the direction parallel and perpendicular to the nanosheet planes compared to the gel without nanosheets (̅ = 356 mm min-1). The gel with 0.5 wt% nanosheets showed the intermediate values. It is also ensured that the averaged anisotropy ∥ / increases with increasing nanosheet concentration. Anisotropic propagation of the chemical wave is explained by the difference in diffusion rate of the molecules in the gel along the two directions,10 caused by the barrier effect of the 2D nanosheets with huge aspect ratio (> 2 x 103). The formation of rotating spiral patterns is also a noticeable feature in the present anisotropic gels. The spirals rotated clockwise or anti-clockwise at many points (Figure 3a), which were not found in the isotropic gel (Figure 3b). It has been reported that



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rotating spirals appears in the BZ reactions in inhomogeneous media.19 Although the present composite gels have uniform monodomain on macroscopic scale, it is considered that the inhomogeneous structures on mesoscopic and nanoscopic scales causes the spirals.

CONCLUSION

Unusual anisotropic Belouzov-Zhabozinsky (BZ) reactions was demonstrated in the anisotropic mono-domain gel films of nanosheet LC/pNIPA composites, in which molecular diffusion is effectively retarded along the direction perpendicular to the nanosheets planes. Due to the easy preparation and controllable anisotropy, the present system will be an ideal model system for fundamental studies of self-oscillating BZ-reactions that are related to curious pattern formations in natures such as embryo and neural networks. The present system is also applicable to develop self-oscillating soft actuators with uniaxial motion.

Acknowledgement


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This research was supported by: Research Center for Materials and Energy Devices of Fukuoka Institute of Technology (FIT-ME) (Strategic Research Foundation Grant-Aided Project for Private University from MEXT; #S1511036L); KAKENHI (#24104005, #15K05657, and #17H03209); Canon Foundation; Network Joint Research Center for Materials and Devices (#201507 and #20166009); and Electronics Research Laboratory of Fukuoka Institute of Technology.

Supporting Information Available: Movies S1 and S2 of the anisotropic and isotropic gel films in which BZ reaction is running. Table S1 that show the propagation rate and anisotropy of each experimental runs. This material is available free of charge via the Internet at http://pubs.acs.org.



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TABLE OF CONTENTS IMAGE



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Figure 1. Synthesis and characterization of the nanosheet LC/pNIPA composite gel with

anisotropy: (a) the schematic representation of the cell setup for the synthesis, (b) the

photograph of the gel observed with crossed polarizers and a wave plate, and (c)-(f) small angle

X-ray scattering results. The (d) I vs χ, (e) I vs q and (f) I•q2 vs q profiles were obtained from

the (c) 2D scattering pattern.


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Figure 2. Visible spectra of (a) aqueous ferroin solution (12 mM), (b) the flocculates obtained

by mixing nanosheet colloid (1 wt%, 10mL) and ferroin solution (0.03 M, 10 µL), and (c) the

nanosheet LC/pNIPA composite gel adsorbed with ferroin.



Page 22 of 24

Figure 3. Progression of BZ reaction in the anisotropic composite gel sheet with 1 and (b) 0.5

wt% of the nanosheets and (c) chemically crosslinked isotropic gel sheet. The double arrow

shows the direction of aligned nanosheet plane.


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Figure 4. The time-dependence of the distance that the chemical wave progressed along the

direction (a) parallel (//) and (a') perpendicular (⊥) to the nanosheet planes in the anisotropic

gel with 1.0 wt% of nanosheets. The propagation along two orthogonal directions in the

isotropic gel without nanosheets is also shown as (b) and (b'). The solid and dashed lines were

obtained by linear fitting to evaluate the propagation rates from the slope.



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Figure 5. The nanosheet concentration dependence of the average propagation rates along the

direction parallel ( ∥ ) and perpendicular ( ) to the nanosheet planes and the anisotropy

( ∥ / ).


Anisotropic Self-Oscillating Reaction in Liquid Crystalline Nanosheets

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