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Preparations and Optical Properties of Ordered Arrays of Submicron Gel Particles: Interconnected State and Trapped State Masafumi Kumoda,† Masayoshi Watanabe,*,† and Yukikazu Takeoka*,‡ Department of Chemistry and Biotechnology, Yokohama National UniVersity, 79-5 Tokiwa-dai, Hodogaya-ku, Yokohama 240-8501, Japan, and Department of Molecular Design and Engineering, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya, 464-8603 Japan ReceiVed January 23, 2006 Two types of thermosensitive opal-structured hydrogel systems, “interconnected” and “trapped” gel particle arrays, were newly developed by extremely simple methods using silica colloidal crystal as a template. Although both systems diffract visible light following Bragg’s law combined with Snell’s law, the temperature dependences of their optical properties were quite different. The “interconnected” array exhibited a reversible change in the peak values of the reflection spectra, mainly determined by the swelling ratio of the hydrogel, as a function of the water temperature. Since the swelling ratio is dominant over the peak value, we can observe water temperature through the color of the interconnected type of gel membrane. The “trapped” array revealed a reversible change in the peak intensity of the reflection spectra with the change in temperature, whereas no change in the peak position was observed. We can interpret this phenomenon in the following ways. As the rise in temperature causes a decrease in the water content of the NIPA gel particles, the gel particles becomes stickier on the cavity wall of polystyrene PPM. This may induce a disturbance in the ordered array of the gel particles and form many layers of rough surfaces in the inverse opal structure of the PPM. This situation may lead to the stronger diffused reflection of light from the gel particles, resulting in the decrease in peak intensity at higher temperatures.
Introduction Recently, there has been an intensive effort to develop soft materials such as hydrogels having a periodic variation in refractive index to make way for new applications in optical devices.1-12 In particular, a nonclosest packing type2a and a closest packing type3,4 of monodisperse colloidal hydrogel particle array * To whom correspondence should be addressed. E-mail: ytakeoka@ apchem.nagoya-u.ac.jp. † Yokohama National University. ‡ Nagoya University. (1) (a) Satoh, N.; Tsujii, K. Langmuir 1992, 8, 581-584. (b) Yamamoto, T.; Satoh, N.; Onda T.; Tsujii, K. Langmuir 1996, 12, 3143-3150. (c) Tsujii, K.; Hayakawa, M.; Onda, T.; Tanaka, T. Macromolecules 1997, 30, 7397-7402. (2) (a) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959-960. (b) Reese, C. E.; Mikhonin, A. V.; Kamenjicki, M.; Tikhonov, A.; Asher, S. A. J. Am. Chem. Soc. 2004, 126, 1493-1496. (c) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829-832. (d) Liu, L.; Li, P.; Asher, S. A. Nature 1998, 397, 141-144. (3) (a) Debord, J. D.; Lyon, L. A. J. Phys. Chem. B 2000, 104, 6327-6331. (b) Lyon, L. A.; Debord, J. D.; Debord, S. B.; Jones, C. D.; McGrath, J. G.; Serpe, M. J. J. Phys. Chem. B 2004, 108, 19099-19108. (4) Hu, Z.; Lu, X.; Gao, J. AdV. Mater. 2001, 13, 1708-1712. (5) Lee, Y.-J.; Braun, P. V. AdV. Mater. 2003, 15, 563-566. (6) (a) Foulger, S. H.; Jiang, P.; Lattam, A. C.; Smith, D. W.; Ballato, J. Langmuir 2001, 17, 6023-6026. (b) Foulger, S. H.; Jiang, P.; Lattam, A. C.; Smith, D. W.; Ballato, J. Dausch, D. E.; Grego, S.; Stoner, B. R. AdV. Mater. 2003, 15, 685-689. (7) Tsuji, S.; Kawaguchi, H. Langmuir 2005, 21, 8439-8442. (8) (a) Fudouzi, H.; Xia, Y. AdV. Mater. 2003, 15, 892-896. (b) Fudouzi, H.; Xia, Y. Langmuir 2003, 19, 9653-9660. (9) (a) Arsenault, A. C.; Miguez, H.; Kitaev, V.; Ozin, G. A.; Manners, I. AdV. Mater. 2003, 15, 503-507. (b) Fleischhaker, F.; Arsenault, A. C.; Wang, Z.; Kitaev, V.; Peiris, F. C.; Freymann, G. v.; Manners, I.; Zentel, R.; Ozin, G. A. AdV. Mater. 2005, 17, 2455-2458. (10) Iwayama, Y.; Yamanaka, J.; Takiguchi, Y.; Takasaka, M.; Ito, K.; Shinohara, T.; Sawada, T.; Yonese, M. Langmuir 2003, 19, 977-980. (11) Yoshino, K.; Kawagishi, Y.; Ozaki, M.; Kose, A. Jpn. J. Appl. Phys. 1999, 38, 786-788. (12) (a) Takeoka, Y.; Watanabe, M. Langmuir 2002, 18, 5977-5980. (b) Takeoka, Y.; Watanabe, M. Langmuir 2003, 19, 9104-9106. (c) Takeoka, Y.; Watanabe, M. Langmuir 2003, 19, 9554-9557. (d) Takeoka, Y.; Watanabe, M.; Yoshida, R. J. Am. Chem. Soc. 2003, 125, 13320-13321. (e) Takeoka, Y.; Watanabe, M. AdV. Mater. 2003, 15, 199-201. (f) Nakayama, D.; Takeoka, Y.; Watanabe, M.; Kataoka, K. Angew. Chem. 2003, 4, 4197-4200. (g) Saito, H.; Takeoka, Y.; Watanabe, M. Chem. Commun. 2003, 2126-2127.
have been extensively investigated for their fundamental properties and potential applications. Although both systems can reveal iridescent color, the nonclosest type is very fragile and unsettled, as it is a liquid suspension. Since its stability can be affected by the addition of solute, by a change in temperature, and by even the slightest applied vibration, the nonclosest type is not the best system for optical devices. The closest type, i.e., the opal-type, is mechanically more stable than the nonclosest one because this type is a kind of semisolid material. The opal-type hydrogel undergoes fast and reversible color variation in response to changes in the environment under white light, owing to its periodically ordered porous structure.12a,13-15 The interconnecting porous structure provides the water paths to the gel and consequently must exhibit rapid change in volume. Such soft materials have attracted interest for use in optical sensors and displays.2c,5,8,12f,12g There are several methods for building up the opal structure from submicron hydrogel particles. One method is the fabrication of a non-crosslinked particle array consisting of thermosensitive hydrogel particles using centrifugal force.3 This array is a viscous pellet and undergoes a reversible order-disorder transition in response to temperature fluctuation. The viscous pellet displays a bright iridescence at lower temperature where the hydrogel particles are swollen and form an ordered array, while it becomes a milky-white at higher temperature. Another method utilizes surface reactive hydrogel particles to obtain a covalently bonded hydrogel particle array after the formation of an opal structure from the particles.4 The covalent linkages lead to a remarkable stability of the closepacked structure. However, it takes extensive effort to prepare these previous systems, and they have lacked adequate mechanical strength for repeated use in chemical sensors and optical devices. (13) Serizawa, T.; Wakita, K.; Kaneko, T.; Akashi, M. J. Polym. Sci. A 2002, 40, 4228-4235. (14) Zhao, B.; Moore, J. S. Langmuir 2001, 17, 4758-4763. (15) Kameyama, K.; Kishi, Y.; Yoshimura, M.; Kanzawa, N.; Sameshima, M.; Tsuchiya, T. Nature 2000, 407, 37-37.
10.1021/la060224g CCC: $33.50 © 2006 American Chemical Society Published on Web 03/25/2006
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Figure 1. Preparation of an “interconnected” gel particle array and a “trapped” gel particle array using a closest-packing silica colloidal crystal as a template.
In this paper, we present the preparation of two types of thermosensitive gel particle arrays using extremely simple templating methods, and we explain the temperature dependence of their optical properties. One was an opal-type hydrogel membrane in which submicron-sized monodisperse gel particles are regularly ordered and connected to each other; the other one was a monodisperse gel particle assembly embedded in a porous polymer membrane (PPM) having an inverse opal structure. We describe these as “interconnected” and “trapped” gel particle arrays, respectively. Both arrays diffract visible light following Bragg’s law combined with Snell’s law and exhibit structural color. The temperature dependences of their optical properties, however, were quite different: the interconnected one exhibited a reversible change in the position of the reflection spectra, while the trapped gel particle array revealed reversible change in the peak intensity of the reflection spectra, depending on the water temperature.
sensitive gel in aqueous solutions.18-20 First, 5.940 g of NIPA, 0.530 g of BIS, and 0.127 g of BPO were dissolved in 25 mL of ethanol. The solution was degassed by bubbling nitrogen gas for 30 min before the gelation to remove dissolved oxygen. The PPM was immersed into the pre-gel NIPA solution. Subsequently, polymerization was conducted by adding 0.102 g of dimethylaniline to initiate redox polymerization for 3 h at 4 °C. Consequently, the interconnected gel particle array embedded in the PPM was obtained. The freely swelling “interconnected” gel particle array membrane was prepared simply by flushing the polystyrene framework sequentially with toluene, ethanol, and distilled water. Since the regularly ordered gel particles are interconnected in the membrane, the relative position of each gel particle remained unchanged when the volume of the gel membrane was changed. The peak value of the reflection spectra, λmax, for the inverse opal-type PPM was evaluated by21
Results and Discussion
λmax ) 1.633(d/m)(na2 - sin2 θ)1/2
procedure16
Figure 1 presents a schematic of the employed to fabricate these thermosensitive gel particle arrays.17 First, we obtained a closest-packing colloidal crystal composed of monodisperse silica sphere particles 210 nm in diameter. The growth of the crystal has already been described.12e As the thickness of the crystal is approximately 1 mm, the number of layers is roughly 6000 on the assumption of fcc (111) for the crystal. A styrene monomer containing benzoylperoxide (BPO) as an initiator was infiltrated into the crystal and was polymerized at 60 °C so as to trap its crystalline structure. The composite membrane was soaked in 46% hydrofluoric acid solution for a few hours to remove the silica components completely. The resulting inverse opal-type PPM with interconnecting porous structure could then serve as a template to obtain gel particle arrays with opal-type structure. In this work, we chose N-isopropylacrylamide (NIPA) and N,N′-methylene-bis-acrylamide (BIS) as the main monomer and cross-linker, respectively, to synthesize a reversible thermo(16) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453-457. (17) Kumoda, M.; Takeoka, Y.; Watanabe, M. Langmuir 2003, 19, 525-528.
(1)
where d is the diameter of the pore that is exactly the same size of the colloidal silica particle used, m is the order of the Bragg constant, na is the average refractive index22 of the PPM, and θ is the angle measured from the normal to the plane of the PPM. Because the interconnected gel particle array changes its volume isotropically, the swelling degree of the gel has to be considered at the peak value. Thus, the peak value for the interconnected gel particle array was rewritten as follows:12b
λmax ) 1.633(d/m)(D/D0)(na2 - sin2 θ)1/2
(2)
where D/D0 is the equilibrium swelling degree of the gel. Here, (18) Hirokawa, Y.; Tanaka, T.; Matsuo, E. S. J. Chem. Phys. 1984, 81, 59775980. (19) Hirotsu, S.; Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1987, 87, 13921395. (20) Schild, H. G. Prog. Polym. Sci. 1992, 17, 6379-6380. (21) (a) Miguez, H.; Lopez, C.; Meseguer, F.; Blanco, A.; Vazquez, L.; Mayoral, R.; Ocana, M.; Fornes, V.; Mifsud, A. Appl. Phys. Lett. 1997, 71, 1148-1150. (b) Mayoral, R.; Requena, J.; Moya, J. S.; Lopez, C.; Cintas, A.; Miguez, H.; Meseguer, F.; Vazquez, L.; Holgado, M.; Blanco, A. AdV. Mater. 1997, 9, 257260.
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Figure 2. Refractive indices of bulk NIPA gel (ngel), water (nH2O), and the interconnected gel particle array in water (calculated by na2 ) Σni2Vi) are plotted as a function of temperature.
D and D0 are diameters of the gel particle in the equilibrium state at a certain condition and in the reference state, respectively. In this work, the diameter of the gel particle in the reference state was the same as that of the silica particle used. According to this equation, we have to experimentally know the environmental dependence of D/D0 and na to pre-surmise the observed value of λmax for the interconnected gel particle array, because d and θ can be arbitrarily chosen. The swelling degree can be estimated by monitoring the diameter of a cylindrical gel prepared in a capillary with an internal diameter of 100 µm using the same recipe as that used for making the interconnected gel particle array. In this case, the reference state was the internal diameter (100 µm) of the capillary. The change in the refractive indices of the bulk NIPA gel and water at various temperatures was measured by a refractmeter with temperature control equipment. Although the rate of change in na for the interconnected gel particle array obtained by our recipe was only 6% when the temperature was changed from 15 to 45 °C (Figure 2), the swelling ratio more than doubled. In other words, the swelling ratio is dominant over the value of λmax for the interconnected gel particle array. Figure 3a shows the reflection spectra of the interconnected gel particle array in water at different temperatures. As the temperature increases, the reflected wavelength shifts toward the shorter wavelength. This shift is due to the decrease in the lattice spacing of the opal structure with the collapse of the gel. The reflected peak intensity was strengthened as the temperature increased. As the reflected peak intensity is proportional to the magnitude of an optical dielectric contrast, it must be caused by the increase in the refractive index of the gel portion and the decrease in that of water.6 The temperature dependence of the calculated values of λmax determined by the equilibrium swelling degree, using eq 2, coincided remarkably well with the experimental values (Figure 3b). This simple method of preparing the opal-type hydrogel should make possible not only the introduction of a wide variety of functional monomers but also the precise tuning of the optical properties by simple adjustments to the recipe for gel synthesis.23,24 There is, however, still room for the improvement in the mechanical strength of this system. (22) The average refractive index can be approximately calculated by na2 ) Σni2Vi where ni and Vi are the refractive indices and volume fractions of the individual components composing the crystal.
Figure 3. (a) Temperature dependence of the reflection spectra of the interconnected gel particle array in water. (b) Experimental and calculated peak wavelengths of the reflection spectra of the interconnected gel particle array in water as a function of temperature. The calculated data were determined by eq 2 using d ) 210 nm, θ ) 0, and experimentally obtained na and D/D0 values as a function of temperature.
The existence of the porous polystyrene frame provides an interesting optical property as well as robustness to the “trapped” type of a gel particle array. The “trapped” type of gel particle array was prepared as follows. The interconnected gel particle array in the PPM was immersed in 30 wt % ethanol aqueous solution at 25 °C to reach a fully collapsed state. Each gel particle underwent collapse transition in each pore by the effect of cononsolvency,25,26 thereby allowing separation of the gel particles. When mechanical stress resulting from changes in the solvent composition set in, the gel particle array was cut from the weakest place, which was the portion to which the gel particles were connected.17 Eventually, the “trapped” gel particle array in the PPM was fabricated. Each gel particle can change its volume individually in the pores. Although no change in the peak position of the reflection spectra was observed (Figure 4, panels a and b), the peak intensity decreased with elevation of temperature (23) Bromberg, L.; Grosberg, A. Y.; Matsuo, E. S.; Suzuki, Y.; Tanaka, T. J. Chem. Phys. 1997, 106, 2906-2910. (24) Baker, J. P.; Hong, L. H.; Blanch, H. W.; Prausnitz, J. M. Macromolecules 1994, 27, 1446-1454. (25) Katayama, S.; Hirokawa, Y.; Tanaka, T. Macromolecules 1984, 17, 26412643. (26) Amiya, T.; Hirokawa, Y.; Hirose, Y.; Li, Y.; Tanaka, T. J. Chem. Phys. 1987, 86, 2375-2379.
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Figure 4. (a) Temperature dependence of the reflection spectra of the trapped type of gel particle array in water. (b) The maximum peak wavelength of the reflection spectra of the trapped type in water as a function of temperature. (c) Peak intensity of the reflection spectra of the trapped type in water as a function of temperature. Inset: the swelling ratio of NIPA gel in water is plotted as a function of temperature. (d) Reversibility of the peak intensity with temperature jump. The temperature was changed between 20 and 40 °C.
(Figure 4, panels a and c). The intensity becomes constant above 35 °C, and it is much like the swelling curve of the gel (Figure 4c, inset). The observed change was also reversed with temperature reduction, as well as the temperature dependence of the swelling ratio (Figure 4d). This system is sustainable for more than hundreds of repetitive temperature changes. The peak wavelength of the reflection spectra from the trapped gel particle array corresponded with that from the PPM filled with water. No remarkable temperature-dependent change in either the peak position or the peak intensity was observed in the case of the PPM filled with water (Figure 5). Despite the decrease in the refractive index of water with rising temperature, the change has little influence on the peak position. These leads us to the interpretation that the change in the peak intensity must be associated with the swelling change of the gel particles, because the temperature dependence of the intensity shows the same tendency with the change in the size of hydrogel with the variation of temperature. If the NIPA gel particles held the center position in each pore irrespective of water temperature, a new reflection peak would appear and its peak intensity would increase as temperature increased. Such behavior has been observed previously for a nonclosest packed hydrogel particle array system by Asher’s group.2a,2b Asher’s group created a nonclosest packed NIPA gel colloidal particle array and showed its thermally controlled optical switchable behavior. At low temperature, this array diffracts light weakly, whereas it diffracts light efficiently above 32 °C. Our results, however, do not correspond to that case. We can interpret this phenomenon in the following ways.
Since the rise in temperature causes a decrease in the water content of the NIPA gel particle, the gel becomes stickier on the cavity wall of polystyrene PPM.27,28 This may induce a disturbance in the ordered array of the gel particles and form many layers of rough surfaces in the inverse opal structure of the PPM (Figure 6). In other words, the hydrogel array in the PPM undergoes a reversible order-disorder transition in response to temperature variation. This situation may lead to the stronger diffused reflection of light from the gel particles, resulting in the decrease in peak intensity at higher temperatures. One of the interesting behaviors in this system is the significant change in the peak intensity at lower temperatures where the hydrogel particles cannot freely change these volumes because of the existence of the pores as hard vessels. We have to examine the exact swelling behavior of the hydrogel particles in this sort of repressed situation to investigate the optical behavior of this trapped type system. To accomplish a quantitative explanation, we have to keep in mind this contribution.
Conclusion In summary, we have demonstrated a conceptually new and exceedingly simple approach to the fabrication of two types of reversibly thermo-responsive ordered arrays composed of submicron hydrogel particles using silica colloidal crystal as a (27) Yamada, N.; Okano, T,; Sakai, H.; Karikusa, F.; Sawasaki, Y.; Sakurai, Y. Macromol. Chem., Rapid Commun. 1990, 11, 571-576. (28) Yakushiji, T.; Sakai, K.; Kikuchi, A.; Aoyagi, T.; Sakurai, Y.; Okano, T. Anal. Chem. 1999, 71, 1125-1130.
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Figure 6. Schematic aspect of hydrogel particles in pores of PPM as a function of temperature. The size of a fully collapsed hydrogel particle can be determined by the swelling ratio of NIPA gel at 60 °C. The collapsed hydrogel particles may form a rough surface and cause the diffuse reflection of light.
intensity of Bragg diffraction, depending on the swelling ratio of the gel particles. These systems may shed light on the development of optical sensors based on molecular recognizable hydrogels which reveal the change in the volume depending on the concentration of a specific molecule. Work is under way to promote such worthwhile capacities of these systems. Experimental Section
Figure 5. (a) Temperature dependence of the reflection spectra of the PPM filled with water. (b) Peak intensity of the reflection spectra of the PPM filled with water as a function of temperature.
template. Although both systems diffract visible light following Bragg’s law combined with Snell’s law, the temperature dependences of their optical properties were quite different. The “interconnected” array exhibited a reversible change in the peak values of the reflection spectra, mainly determined by the swelling ratio of the hydrogel, as a function of the water temperature. We can observe water temperature through the change in color of the interconnected type of gel membrane. On the other hand, the “trapped” array revealed a reversible change in the peak intensity of the reflection spectra with the change in temperature, whereas no change in the peak position was observed. The disturbance in the ordered array of the gel particles must cause the strong diffraction of light, resulting in the decrease in peak intensity at higher temperatures. We could fabricate two types of submicron gel particles ordered array exhibiting the change in the peak position or the peak
Materials. N-Isopropylacrylamide (NIPA) was kindly provided by Kohjin Co., Ltd. and was recrystallized using toluene-hexane mixed solvent. N-N′-Methylenebis(acrylamide)(BIS), benzoylperoxide (BPO), styrene, 46% hydrofluoric acid aqueous solution, and ethanol were purchased from Wako Pure Chemical Industries and used as received. Deionized water with a resistivity of 18.2 mΩ was used to carry out all experiments. Colloidal silica suspension was obtained from Nippon Shokubai Co., Ltd. The preparative procedure of the two types of thermosensitive opal-structured hydrogel systems was explained in the main paragraph. Measurements. The swelling measurement was carried out by monitoring the diameter of the cylindrical gel in water. The reflection spectra of the samples were obtained by a fiber optic spectrometer (Ocean Optics USB2000). The refractive index of NIPA bulk gel was determined by a digital refractometer (ATAGO RX-7000R). The temperature in the measurements was controlled by using a circulating water temperature control system.
Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research on Priority Areas (Nos. 417 and 438) to Y.T. from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. NIPA was kindly provided by KOHJIN Co., Ltd. LA060224G
