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Controlled Multistructural Color of a Gel Membrane Yukikazu Takeoka* and Masayoshi Watanabe* Department of Chemistry and Biotechnology, Yokohama National University, 156 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Received March 11, 2003. In Final Form: June 24, 2003 A “smart” soft membrane exhibiting controlled multistructural color has been synthesized by the mechanical immobilization of a single side of a polymerized nonclosest packing colloidal crystal. A tapered gel with one fixed surface on a gel-bond film reveals monostructural color. After a partial hydrolysis of the gel to change its volume from the original size, the structural color varies depending on the thickness of the membrane. The tone can be tuned by simply changing the composition of the mixed solvent.
Photonic crystals composed of polymer gels are endowed with optical properties that can be used for sensors and displays.1,2 These photonic crystals exhibit brilliant colors, which can be changed in response to temperature,1c,2 pH,1c,d ionic species,1b,2e and glucose.1b,2c Such colors are produced primarily by the interference on the surface of the periodically ordered structures in the crystals. The ordered structures are composed of spontaneously arranged colloidal particle arrays1 or solvent-filled hole arrays,2 where the spaces of the arrays are filled with polymer gels. These arrays efficiently diffract visible light, meeting the Bragg condition; a certain monostructural color, which is determined by the spacing between diffracting planes of the crystal, can be observed from the crystals (vice versa). Thus, the lattice spacing of the crystals is synchronized with the swelling ratio of the gels. The polymerized photonic crystals demonstrate a drastic change in the structural color depending on these swelling ratios. The optical properties of the polymerized photonic crystals that exhibit monostructural color at a certain condition can be precisely tuned by adjusting the recipe for the gel synthesis.2c,d However, no report has described an attempt to control the multistructural color of a piece of polymerized photonic crystals. It is important to prepare multi-structural-colored membranes that vary their colors in response to the environmental changes to raise the utility value for applications in optical devices and displays. So far, we have prepared the gel membranes that can reveal multistructural color shown in Figure 1. We can observe the heterochromatic structural color of these gel membranes by means of prismatic radiance. However, these gel membranes do not exactly exhibit controlled colors because it is difficult to reproduce the same pattern of colors in other gel membranes. Here, we describe the details regarding the synthetic preparation of a structural-colored gel that displays controlled multicolors. In this project, we used a nonclosest packing colloidal crystal composed of 100-nm monodisperse silica particles for making a colloidal particle * Authors to whom correspondence should be addressed. Fax: (+81) 45-339-3956. E-mail:
[email protected]. (1) (a) Kamenetzky, E. A.; Mangliocco, L. G.; Panzer, H. P. Science 1994, 263, 207. (b) Liu, L.; Li, P.; Asher, S. A. Nature 1998, 397, 141. (c) Reese, C. E.; Baltusavich, M. E.; Keim, J. P.; Asher, S. A. Anal. Chem. 2001, 73, 5038. (d) Lee, K.; Asher, S. A. J. Am. Chem. Soc. 2000, 122, 9534. (e) Iwayama, Y.; Yamanaka, J.; Takiguchi, Y.; Takasaka, M.; Ito, K.; Shonohara, T.; Sawada, T.; Yonese, M. Langmuir 2003, 19, 977. (2) (a) Takeoka, Y.; Watanabe, M. Langmuir 2002, 18, 5977. (b) Takeoka, Y.; Watanabe, M. Adv. Mater. 2003, 15, 199. (c) Nakayama, D.; Takeoka, Y.; Watanabe, M.; Kataoka, K. Angew. Chem., Int. Ed., in press. (d) Takeoka, Y.; Watanabe, M. Submitted for publication. (e) Saito, H.; Takeoka, Y.; Watanabe, M. Chem. Commun., in press.
array and a cross-linked polyacrylamide as a polymer network (Figure 2). The colloidal crystal is easily influenced by such small disturbances as gravitational dropping or movement.3 However, the crystalline structure can be stabilized by a polymer network.1 Consequently, the interplanar spacing of a crystal that is partially filled with the polymer network can be synchronized with the expanse of the polymer network. It is known that the polyacrylamide gel undergoes a change in volume by varying the composition of a mixedsolvent, acetone-water system.4 The polyacrylamide gel swells in water because water is a good solvent for a polyacrylamide network. If acetone, which is a poor solvent for polyacrylamide, is added to the system, the gel shrinks gradually with the increase in the amount of acetone (Figure 3a). When the concentration of acetone reaches 60%, the gel is quite contracted. Therefore, the gel membrane in which the nonclosest packing colloidal crystal is immobilized by a polyacrylamide network also represents the change in the volume in response to the solvent composition and, thus, exhibits the change in the structural color (Figure 3b,c). The peak values of reflection spectra, λmax, of the gel membrane are obtained by2
λmax ) (2d/m)(D/D0)(ng2 - sin2 θ)1/2
(1)
where d is the lattice spacing of the crystal when the gel is prepared, m is the order of diffraction, D/D0 is the equilibrium swelling ratio of the gel (D and D0 are the sizes of the gel in the equilibrium state at a certain condition and in the original state, respectively), ng is the refractive index of the polymerized nonclosest packing colloidal crystal at a certain condition, and θ is the angle measured from the normal to the plane of the gel.5 The change in ng was negligible when the composition of the mixed solvent was varied. Thus, the swelling ratio is mainly dominant over λmax of the observed reflection spectra for the gel membrane, when d and θ are known. Now, let us move on to how a structural-colored gel exhibiting controlled multicolors can be prepared. To achieve the desired result, we tried to produce a gel whose (3) (a) Ise, N. Angew. Chem., Int. Ed. Engl. 1986, 25, 323. (b) Okubo, T. Acc. Chem. Res. 1988, 21, 281. (c) Okubo, T. J. Am. Chem. Soc. 1990, 112, 5420. (c) Woodcock, L. V. Nature 1997, 385, 141. (d) Vlasov, Y. A.; Bo, X.-Z.; Sturm, J. C.; Norris, D. J. Nature 2001, 414, 289. (4) Tanaka, T. Sci. Am. 1981, 244, 110. (5) This equation can apply to the reflection spectra obtained from a gel having an ordered structure composed of a solvent-filled hole array, which is prepared by using the closest packing colloidal crystal as a template.
10.1021/la0344175 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/05/2003
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Figure 1. Photographs showing gels that display heterochromatic structural color. The gels in parts a-c are composed of a nonclosest packing colloidal crystal and a polyacrylamide network. The gels in part d are porous acrylamide-derivative hydrogels that are prepared using a closest packing colloidal crystal as a template.2 (a) Because air bubbles, foamed during gelation, disrupt a crystal structure, the polycrystalline structure is immobilized in a gel; the gel obtained reveals multicolors. (b) A cylindrical gel is removed from a micropipet. Because this gel becomes bigger than its original size, the structural color changes. (c) A thin gel membrane is folded in the manner of an accordion. Because the membrane is curved, the variation in color arises on the gel membrane. (d) A thermosensitive porous gel exhibits multicolors on the way toward another equilibrium state, after an abrupt change in the temperature.
Figure 2. Schematic view of the Bragg peak (λn) shift of the polymerized nonclosest packing colloidal crystal, depending on the degree of swelling of the gel. When the lattice spacing (dn) varies, the observed peak values of the reflection spectra, λn, of the gel membrane are also changed.
swelling ratio is heterogeneous. When a square slab gel is swollen, its size becomes bigger, whereas the square shape itself does not change: the gel swells isotropically (Figure 4a). On the other hand, let us consider a square slab gel with its upper surface free but its lower surface mechanically fixed. The gel is under a mechanical constraint. When the gel is swollen, the local degree of swelling monotonically increases from the fixed surface to the free surface (Figure 4b,c).6 Consequently, the
swelling ratio at the upper free surface is proportional to the thickness of the gel membrane. Thus, our goal will be attained if we can prepare a gel membrane whose thickness can vary while one surface is fixed. In this study, a tapered thickness with one fixed surface as the simplest model was synthesized to verify our hypothesis. Judging from the previous explanation, a gel membrane having a tapered thickness with one fixed surface can exhibit a sloping heterogeneous swelling ratio, if the size was changed from its original size. A tapered gel fixed on a gel-bond film7 obtained reveals the monostructural color in the equilibrium swelling state in water at room temperature, because D/D0 of the gel is in unity in this condition (Figure 3a). The necessary condition to demonstrate our hypothesis is that the swelling ratio of the gel membrane should be larger or smaller than that of its original size. Thus, the gel membrane was partially hydrolyzed by an alkaline buffer solution to change the size of the gel. Because the osmotic pressure in the gel is increased by hydrolysis, D/D0 of the gel in the equilibrium state becomes larger than unity in (6) Tanaka, T.; Sun, S.-T.; Hirokawa, Y.; Katayama, S.; Kucera, J.; Hirose, Y.; Amiya, T. Nature 1987, 325, 796. Tanaka, T.; Sun, S.-T.; Hirokawa, Y.; Katayama, S.; Kucera, J.; Hirose, Y.; Amiya, T. Molecular Conformation and Dynamics of Macromolecules in Condensed System 1988, 2, 203. (7) The surface of a gel-bond film is specially treated to retain polymerization-active vinyl groups.
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Letters
Figure 3. (a) Equilibrium swelling degree (D/D0) of disk gels and (b) wavelength of the peak of the reflection spectra from the gels as a function of the temperature. (c) The photograph shows the appearance of the disk gels in aqueous acetone solutions. Figure 5. Change in the structural color of the tapered gel with one surface fixed on a gel-bond film observed at the same portion of the gel in different solvents. (a) The tapered gel that is partially hydrolyzed by CAPS buffer exhibits structural multicolor that varies gradually with the change in its thickness in an aqueous 33% acetone solution. (b) The tapered gel turns the structural multicolors to the shorter wavelength region because the swelling ratio of the gel in a 35% aqueous acetone solution becomes smaller than that in an aqueous 33% acetone solution.
Figure 4. Effect of unilateral fixing on the swelling behavior of a polymer gel.
water. After the hydrolysis of the gel for 2 days, the structural color observed at the upper free surface shifts to the near-IR region. Therefore, the appearance of the
membrane is opaque and colorless. However, the gel immersed in an aqueous 33% acetone solution exhibits multicolors (Figure 5a). Because the swelling ratio of the slab gel having a tapered thickness with one fixed surface gradually changes, the structural color becomes that of a rainbow. The color tone varies to the lower wavelength with the increase in the acetone composition of the mixed solvent (Figure 5b). The structural color of the gel shifts to the visible region as a result of a contraction of the gel in an aqueous acetone solution. Thus, the gel that shows controlled multistructural colors depending on the degree of swelling change can be obtained, if the gel in which the film thickness beforehand differs is prepared. This project constitutes the first step in the construction of a gel membrane that displays controlled multicolors. It is technically possible to inscribe characters and pictures with multicolors on the gel membrane. Work is underway to promote such worthwhile capacities of the gels.8 Experimental Section Synthesis of Structural-Colored Gels. To prepare nonclosest packing colloidal crystals immobilized in a gel, an aqueous 15.5 wt % solution containing silica spheres having diameters
Letters of 100 nm (Nippon Shokubai Co. Ltd.) was used. This solution was desalinated by shaking with ion-exchange resin (Bio-Rad mixed bed, AG501-X8) to obtain the crystals. All the gels were prepared by free-radical polymerization as follows. First, acrylamide (1.5 g), N,N′-methylene-bis-acrylamide (0.04 g) as a crosslinker, and diethoxyacetophenone (0.025 g), the light-sensitive initiator, were dissolved in the degassed and nitrogen-saturated colloidal suspension to a final volume of 30 mL. The solution was then stored in a refrigerator for 3 days. Thus, we obtained a pre-gel solution that exhibited structural color. The photopolymerization was conducted at 25 °C for 3 h with UV irradiation by an ultrahigh-pressure mercury lamp. The slab gels for swelling measurement were prepared in a quartz-glass cell framed by two flat quartz glasses and a Teflon spacer with a thickness of 1 mm. A slab gel with a thickness tapered from 0 to 1 mm and its one surface mechanically fixed to a gel-bond film (Bio-Rad) was prepared in a glass cell, as is shown in Figure 6. The resulting gels were washed carefully with distilled water for 1 week. Hydrolysis of the tapered gel was carried out so that the swelling ratio of the gel membranes increased. It is well-known that hydrolysis can be achieved by immersing polyacrylamide gel membranes for a period of hours in basic solutions. However, the silica component is also hydrolyzed by a basic solution, one with a pH of more than 11.9 In this study, we employed 3-(cyclohexylamino)-1-propansulfonsyre (CAPS) buffer (pKa ) 10.5) to prevent the hydrolysis of the silica component. Measurements. The swelling measurement was carried out by monitoring the diameter of the disk gel that was cut from the slab-gel membrane. The temperature was controlled by using a circulating water temperature control system. The reflection (8) To synthesize a tapered gel composed of a nonclosest packing colloidal crystal is easier than to make a porous gel with a tapered thickness obtained by using a closest packing colloidal crystal. Therefore, we used the nonclosest packing colloidal crystal in this research. We are preparing the porous gel exhibiting a sloping swelling ratio. (9) Technical report from Nippon Shokubai Co. Ltd.
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Figure 6. Assembly of a tapered glass cell to create a slab gel having a thickness tapered from 0 to 1 mm with one surface mechanically fixed to a gel-bond film. The structural color was observed from the free surface of the tapered gel. spectra were obtained by means of an Ocean Optics USB2000 fiber-optic spectrometer. Photographs were taken by a digital microscope (KEYENCE VH-8000).
Acknowledgment. We wish to thank the Nippon Shokubai Co. Ltd. for their gift of the silica colloidal suspension. This work was supported by the Shiseido Fund for Science and Technology and a Grant-in-Aid for Scientific Research on Priority Areas of “Dynamic Control of Strongly Correlated Softmaterials” (No. 413) to Y.T. from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. LA0344175
