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Tuning Structural Color Changes of Porous Thermosensitive Gels through Quantitative Adjustment of the Cross-Linker in Pre-gel Solutions Yukikazu Takeoka* and Masayoshi Watanabe* Department of Chemistry and Biotechnology, Yokohama National University, 156 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Received June 26, 2003 “Smart” porous gels with different optical behaviors were synthesized by quantitative adjustment of the cross-linker in pre-gel solutions. A periodically ordered interconnecting porous structure could be created in the gels by using a closest-packing silica colloidal crystal as a template. The interconnecting porosity provides fast response to changes in temperature through the reversible swelling and shrinking of the gels, while the periodically ordered mesoscopic structure endows the porous gels with structural color, which can be tuned by simply changing the amount of the cross-linker in the pre-gel solutions.
Chemical gels, which are covalently cross-linked polymer networks swollen with solvent, freely swell and shrink in response to environmental changes.1 Much of the current research on chemical gels has focused on the use of templates or matrixes for additional functions.2 Socalled “smart” gels for molecular recognition,2a-d catalysis,2e and separation can be easily fabricated by a templating technique, because the spatial position and the threedimensional shape of the templates used can be memorized by the formation of permanent cross-links. S. Asher’s group has studied self-assembled highly charged monodisperse particles immobilized in chemical gels as a polymerized non-closest-packing colloidal crystal.3 The non-closest-packing crystal diffracts light at visible wavelengths determined by the lattice spacing, thereby exhibiting a specific color. Since this coloring is due primarily to the structures formed in the crystal, we call it “structural color”. Thus, a crystal immobilized in chemical gels can reversibly change its lattice spacing in response to environmental changes impacting the volume of the gels and will signal these changes through iridescence. Asher’s group has successfully developed novel chemical sensors for glucose, heavy metal ions, and pH. However, this type of crystal is very fragile and unsettled; its stability is affected by the addition of solute, temperature, and even the slightest applied vibration.4 Hence, * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (+81) 45-339-3956. (1) (a) Tanaka, T. Sci. Am. 1981, 244, 110-123. (b) Takeoka, Y.; Berker, N.; Du, R.; Enoki, T.; Grosberg, A.; Kardar, M.; Oya, T.; Tanaka, K.; Wang, G.; Yu, X.; Tanaka, T. Phys. Rev. Lett. 1999, 82, 4863-4865. (2) (a) Vlatakis, G.; Anderson, L. I.; Muller, R.; Mosbach, K. Nature 1993, 361, 645-647. (b) Remcho, V. T.; Tan, Z. J. Anal. Chem. 1999, 71, 248A-255A. (c) Johnson, S. A.; Ollivier, P. J.; Mallouk, T. E. Science 1999, 283, 963-965. (d) Alvarez-Lorenzo, C.; Orhan, G.; Oya, T.; Sakai, Y.; Kobayasgi, M.; Enoki, T.; Takeoka, Y.; Ishibashi, T.; Kuroda, K.; Tanaka, K.; Wang, G.; Grosberg, A. Y.; Masamune, S.; Tanaka, T. Macromolecules 2000, 33, 8693-8697. (e) Alvarez-Lorenzo, C.; Orhan, G.; Oya, T.; Sakai, Y.; Kobayasgi, M.; Enoki, T.; Takeoka, Y.; Ishibashi, T.; Kuroda, K.; Tanaka, K.; Wang, G.; Grosberg, A. Y.; Masamune, S.; Tanaka, T. J. Chem. Phys. 2001, 114 (6), 2812-2816. (f) Kumoda, M.; Takeoka, Y.; Watanabe, M. Langmuir 2003, 19, 525-528. (3) (a) Liu, L.; Asher, S. A. Nature 1998, 397, 141-144. (b) Reese, C. E.; Baltusavich, M. E.; Keim, J. P.; Asher, S. A. Anal. Chem. 2001, 73, 5038-5042. (c) Lee, K.; Asher, S. A. J. Am. Chem. Soc. 2000, 122, 9534-9537. (4) (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. (d) Woodcock, L. V. Nature 1997, 385, 141. (e) Vlasov, Y. A.; Bo, X.-Z.; Sturm, J. C.; Norris, D. J. Nature 2001, 414, 289.
extreme technical acuity is required to fabricate the non-closest-packing crystal into the appropriate polymerized colloidal crystal. A closest-packing colloidal crystal can also be a template for preparing chemical gels exhibiting a desired structural color.5 The closest-packing crystal is more stable than the non-closest-packing crystal, having an open structure between each contact particle.6 The pre-gel solution can easily be infiltrated into this space as a minivessel for making gels. After removal of the crystal component in the gels obtained, the porous gel exhibits a bright structural color under white light illumination and undergoes fast and drastic changes in color in response to a variety of environments. The structural color is caused by the Bragg diffraction of visible light from the ordered voids regarded as crystallites.6d The peak values of reflection spectra, λmax, for the porous gel are obtained by5b
λmax ) 1.633(d/m)(D/D0)(na2 - sin2 θ)1/2
(1)
where d is the diameter of a colloidal particle, m is a constant, D/D0 is the equilibrium swelling degree of the gel (D and D0 are the diameters of the gel in the equilibrium state at a certain condition and in the reference state,7 respectively), na is the refractive index of the porous gel at a certain condition, and θ is the angle measured from the normal to the plane of the gel. According to this equation, we have only to experimentally know the environmental dependence of the gel’s swelling ratio (D/D0) and the refractive index to pre-surmise the observed value of λmax of the reflection spectrum for the gel, because d and θ can be arbitrarily chosen. The swelling ratio can be estimated by monitoring the diameter of a cylindrical gel prepared in a capillary with a diameter of 100 µm using a temperature control system. The change in the refractive index of the porous gel with the varying conditions is then measured by a refract meter. Although (5) (a) Takeoka, Y.; Watanabe, M. Langmuir 2002, 18, 5977-5980. (b) Takeoka, Y.; Watanabe, M. Adv. Mater. 2003, 15, 199-201. (6) (a) Blanford, C. F.; Schroden, R. C.; Al-Daous, M.; Stein, A. Adv. Mater. 2001, 13, 26-29. (b) Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 11630-11637. (c) Xia, Y.; Gates, B.; Yin, Y.; Lu, U. Adv. Mater. 2000, 12, 693-713. (d) Park, S. H.; Xia, Y. Langmuir 1999, 15, 266-273. (7) The reference state of the gel in this paper is the diameter of a capillary to make a cylindrical gel.
10.1021/la035142w CCC: $25.00 © 2003 American Chemical Society Published on Web 10/03/2003
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Figure 1. (a) The swelling ratios of the cylindrical NIPA gels with different Cxl values are plotted as a function of water temperature. The monomer concentration ([NIPA]) of the gels upon preparation was fixed at 2 M. The concentration of crosslinker is shown in the figure (Cxl: 100 mM for [NIPA]/[BIS] ) 20, 67 mM for [NIPA]/[BIS] ) 30, 33 mM for [NIPA]/[BIS] ) 60, 20 mM for [NIPA]/[BIS] ) 100). (b) Equilibrium swelling degree of the cylindrical NIPA gels at 15.8 and 49.0 °C as a function of [NIPA]/[BIS].
the rate of change in na for the thermosensitive chemical gel composed of N-isopropylacrylamide (NIPA) is only 0.3% when the temperature is changed from 15 to 60 °C, the swelling ratio changes by about 2 times.5b Therefore, the swelling ratio is dominant over λmax of the observed reflection spectrum for the porous NIPA gel, when d and θ are known. Here, we report that the structural color from porous gels can be synthetically tuned. Specifically, we demonstrate how changing the concentration of the cross-linker (Cxl) in pre-gel solutions affects the structural color of thermosensitive porous gels. Our work aims to establish how gels of desired optical performance can be obtained. The swelling ratio of gels in swollen state can be expressed as a function of the average length of a subchain between two connected cross-links, N.8 Here, N is defined as the number of monomeric links along the subchain. The more cross-linkers there are in a gel with a certain amount of monomers, the smaller the swelling ratio is in (8) (a) Bromberg, L.; Grosberg, A. Y.; Matsuo, E. S.; Suzuki, Y.; Tanaka, T. J. Chem. Phys. 1997, 106, 2906-2910. (b) Baker, J. P.; Hong, L. H.; Blanch, H. W.; Prausnitz, J. M. Macromolecules 1994, 27, 1446-1454. (c) Grosberg, A. Y.; Khokhlov, A. R. Statistical Physics of Macromolecules; AIP Press: New York, 1994.
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Figure 2. (a) The reflection spectra of the porous NIPA gels, made using a closest-packing colloidal crystal as a template, in water at 15.4 °C. The spectra were measured at normal incidence. Black, [NIPA]/[BIS] ) 20; red, [NIPA]/[BIS] ) 30; green, [NIPA]/[BIS] ) 60; purple, [NIPA]/[BIS] ) 100. (b) Photograph of the porous NIPA gels with different Cxl in water at 27.6 °C. Upper left, [NIPA]/[BIS] ) 20; upper right, [NIPA]/ [BIS] ) 30; lower right, [NIPA]/[BIS] ) 60; lower left, [NIPA]/ [BIS] ) 100.
the gel’s swollen state. Bromberg et al. found that the length of subchains becomes shorter as the ratio of monomer concentration to cross-linker concentration for a pre-gel solution becomes smaller.8a Consequently, the equilibrium swelling ratio of the swollen gel can be predicted by the initial amounts of the monomer and the cross-linker in the pre-gel solution. Figure 1a shows the dependence of the swelling ratio of NIPA gels composed of 2 M NIPA and different concentrations of the cross-linker on water temperature. The swelling ratio at lower temperatures decreased with the amount of cross-linker (Figure 1b). This result agrees with the results of Bromberg et al.8a On the other hand, the size of the gel at high temperatures where the gel is collapsed is constant. This reveals that the cross-links for these gels do not prevent full condensation to a globule state when the network is collapsed. Considering these results, we proposed the hypothesis that gels exhibiting different colors can be prepared simply by changing the concentration of the cross-linker in the pre-gel solution. To verify this hypothesis, we prepared porous gels with the same recipe as the cylindrical gels shown above and by use of the same closest-packing colloidal crystal made of 210 nm silica particles as a template. Figure 2a shows the reflection spectrum of each porous gel in water at 15.4 °C. The diffracted wavelength observed
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is technically possible to prepare a gel membrane reflecting a specific color at a certain temperature by controlling the amount of the monomer and the cross-linker in the pregel solution. In summary, we have developed a family of periodically ordered porous hydrogels whose optical properties can be precisely tuned by the recipe for the gel synthesis and by thermally adjusting the size of the gel. These gels which exhibit a desired color can be tailor-made for arbitrary use. These gels have potential for use in sensors, optical devices, and displays.9 Experimental Section To prepare the porous gels, we used the closest-packing colloidal crystals composed of silica sphere particles of 210 nm diameter. The growth of the crystal has already been reported.5b The thermosensitive gels were prepared by free-radical polymerization as follows. First, certain amounts of NIPA, N,N′methylene-bis-acrylamide as a cross-linker, and benzoylperoxide, the initiator, were dissolved in degassed and nitrogen-saturated 1,4-dioxane to a final volume of 50 mL. The solutions were then infiltrated into the colloidal crystal in a Petri dish, and the polymerizations were conducted at 60 °C for 40 h. Afterward, the samples were immersed in a 5 wt % HF aqueous solution to remove the SiO2. The cylindrical gels for swelling measurement were also prepared in micropipets of 100 µm diameter. The resulting gels were washed carefully with distilled water for 1 week. Measurements. The swelling measurement was carried out by monitoring the diameter of the cylindrical gel in water. The reflection spectra of the porous membrane gels were obtained by an Ocean Optics USB2000 fiber optic spectrometer. The temperature in the measurements was controlled by using a circulating water temperature control system. The photograph of the gels was taken by a digital microscope (KEYENCE VH8000).
Figure 3. (a) Temperature dependence of the reflection spectra of the porous NIPA gel ([NIPA]/[BIS] ) 30) made using a colloidal crystal composed of silica particles of 210 nm. (b) Peak wavelength of the reflection spectra of the porous gels as a function of temperature.
from each gel shifted to a lower wavelength with the decrease in its swelling ratio. Microscopic observation shows the difference in the structural color of each gel clearly (Figure 2b). These results indeed verify our hypothesis. Furthermore, each gel exhibited changes in its structural color depending on temperature (Figure 3). Thus, it
Acknowledgment. We thank the Nippon Shokubai Company for their free gifts of silica colloidal suspension. This work was supported by the Shiseido Fund for Science and Technology and Grants-in-Aid for Scientific Research on Priority Areas (No. 413 and No. 417) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. LA035142W (9) (a) Hu, Z. B.; Lu, X.; Gao, J. Adv. Mater. 2001 13, 1708-1712. (b) Arsenault, A. C.; Mı´guez, H.; Kitaev, V.; Ozin, G. A.; Manners, I. Adv. Mater. 2003, 15, 503-507.
