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Self-Oscillating Nanogel Particles Takamasa Sakai and Ryo Yoshida* Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received October 1, 2003. In Final Form: December 18, 2003 Nanogel particles composed of the cross-linked copolymer of N-isopropylacrylamide and ruthenium catalyst for the Belousov-Zhabotinsky (BZ) reaction were prepared by emulsion polymerization. In the aqueous solution containing the BZ substrates, self-oscillation of gel beads on the nanometer scale was achieved. By comparing the oscillating behaviors of the conventional BZ solution, the linear polymer solution, and the suspension of gel beads, the polymerization effect and the cross-linking effect were clarified. These self-oscillating gel beads have a potential for use in nanomachines as an oscillator.
Introduction Many kinds of stimuli-responsive gels that exhibit volume phase transition in response to temperature,1,2 pH,3,4 electric field,5,6 specific chemicals,7-9 and so forth have been extensively investigated. Their ability to swell and deswell according to external conditions makes them an interesting proposition for use in intelligent materials. In contrast to these conventional stimuli-responsive gels, we have developed a novel “self-oscillating” gel that undergoes an autonomic and periodical swelling-deswelling oscillation without external stimuli in a closed system.10-14 The mechanical oscillation is produced by the Belousov-Zhabotinsky (BZ) reaction that spontaneously generates rhythmical redox changes of the catalyst. The gels are composed of a cross-linked N-isopropylacrylamide (NIPAAm) chain to which ruthenium tris(2,2′-bipyridine) [Ru(bpy)3], as a catalyst for the BZ reaction, is covalently bonded. When the gel is immersed in the aqueous solution containing the substrate of the BZ reaction except for the catalyst, the substrates penetrate into the polymer network, and the BZ reaction occurs in the gel. Redox changes of the catalyst moiety lead to hydrophilic changes of the polymer chains, and as a result, swelling-deswelling * To whom correspondence should be addressed. E-mail:
[email protected]. Phone & fax: +81-3-5841-7112. (1) Zhang, X. Z.; Zhuo, R. X. Langmuir 2001, 17, 12-16. (2) Alvarez-Lorenzo, C.; Guney, O.; Oya, T.; Sakai, Y.; Kobayashi, M.; Enoki, T.; Takeoka, Y.; Ishibashi, T.; Kuroda, K.; Tanaka, K.; Wang, G. Q.; Grosberg, A. Y.; Masamune, S.; Tanaka, T. J. Chem. Phys. 2001, 114, 2812-2816. (3) Beebe, D. J.; Moore, J. S.; Bauer, J. M.; Yu, Q.; Liu, R. H.; Devadoss, C.; Jo, B. H. Nature (London) 2000, 404, 588-590. (4) Mistra, G. P.; Siegel, R. A. J. Pharm. Sci. 2002, 91, 2003-2015. (5) Kim, S. J.; Park, S. J.; Kim, I. Y.; Shin, M. S.; Kim, S. I. Langmuir 2002, 86, 2290-2295 (6) Kudaibergenov, S. E.; Sigitov, V. B. Langmuir 1999, 15, 42304235 (7) Kataoka, K.; Miyazaki, H.; Bunya, M.; Okano, T.; Sakurai, Y. J. Am. Chem. Soc. 1998, 120, 12694-12695. (8) Miyata, T.; Asami, N.; Uragami, T. Nature (London) 1999, 399, 766-769. (9) Oya, T.; Enoki, T.; Grosberg, A. Y.; Masamune, S.; Sakiyama, T.; Takeoka, Y.; Tanaka, K.; Wang, G.; Yilmaz, Y.; Feld, M. S.; Dasari, R.; Tanaka, T. Science 1999, 286, 1543-1545. (10) Yoshida, R.; Takahashi, T.; Yamaguchi, T.; Ichijo, H. J. Am. Chem. Soc. 1996, 118, 5134-5135. (11) Yoshida, R.; Tanaka, M.; Onodera, S.; Yamaguchi, T.; Kokufuda, E. J. Phys. Chem. A 2000, 104, 7549-7555. (12) Yoshida, R.; Takei, K.; Yamaguchi, T. Macromolecules 2003, 36, 1759-1761. (13) Sasaki, S.; Koga, S.; Yoshida, R.; Yamaguchi, T. Langmuir 2003, 19, 5595-5600. (14) Takeoka, Y.; Watanabe, M.; Yoshida, R. J. Am. Chem. Soc. 2003, 125, 13320-13321.
oscillation is induced. When the linear poly(NIPAAm-coRu(bpy)3) chain is dissolved into the aqueous solution of the BZ substrates, soluble-insoluble oscillations of the polymer chain occur accompanying the periodical turbidity changes of the solution.15,16 In this study, we have prepared the submicrometersized poly(NIPAAm-co-Ru(bpy)3)-gel beads and analyzed the oscillating behaviors. Self-oscillation of the swellingdeswelling for gel on the nanometer scale has been achieved for the first time. Applications to a novel chemical nano-engine or pump (nano-actuator) with a self-oscillating function may be expected. Further, to analyze the oscillating profile of the gel beads, we have compared the following three oscillating systems: (i) the conventional BZ solution using nonpolymerized catalyst, (ii) the polymer solution of polymerized catalyst by NIPAAm, and (iii) the suspension of submicrometer-sized gel beads, that is, the cross-linked polymer network of the polymerized catalyst. Through the analysis for their oscillating behaviors, the polymerization effect of catalyst and the cross-linking effect of polymer chains have been discussed. Experimental Section At first, ruthenium(4-vinyl-4′-methyl-2,2′-bipyridine)bis(2,2′bipyridine)bis(hexafluorophosphate) [Ru(bpy)3 monomer] was synthesized according to the previous work.10 For the synthesis of the linear copolymer of NIPAAm with Ru(bpy)3, purified NIPAAm (3.8 g), Ru(bpy)3 monomer (5 wt % of total monomer), and 2,2′-azobisisobutyronitrile (AIBN; 0.16 g) were dissolved in 20 mL of methanol. The solution was degassed twice by a freezethaw cycle in an ampule, which was then sealed in vacuo and immersed in a water bath at 60 °C for 24 h. The resulting mixture was purified through dialysis against methanol for 1 week then pure water for 2 days. The dialyzed solution was lyophilized for 3 days. The weight-average molecular weight of the polymer determined by static light scattering (Otsuka Electronics, DLS7000) was MW ) 141 400. The nanogel particles of NIPAAm with Ru(bpy)3 were prepared by emulsion polymerization as follows: purified NIPAAm (3.80 g), Ru(bpy)3 monomer (0.422 g), sodium dodecylbenzenesulfonate (0.700 g), N,N′-methylenebisacrylamide (0.700 g), and AIBN (0.169 g) were dissolved in 200 mL of H2O. The solution was stirred at 60 °C for 24 h under the N2-flow condition. The resulting mixture was purified through dialysis against pure water for 4 days. The dialyzed solution was lyophilized for 3 days. The poly(NIPAAm-co-Ru(bpy)3)-gel beads (0.15 g/L) were dissolved in (15) Yoshida, R.; Sakai, T.; Ito, S.; Yamaguchi, T. J. Am. Chem. Soc. 2002, 124, 8095-8098. (16) Ito, Y.; Nogawa, M.; Yoshida, R. Langmuir 2003, 19, 95779579.
10.1021/la035833s CCC: $27.50 © 2004 American Chemical Society Published on Web 01/20/2004
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Figure 1. Temperature dependence of the diameter for poly(NIPAAm-co-Ru(bpy)3)-gel beads under the different conditions of the (b) reduced Ru(II) state [in Ce(III) solution] and (O) oxidized Ru(III) state [in Ce(IV) solution]. aqueous 0.3 M HNO3 solutions containing 1 mM Ce2(SO4)3 or 1 mM Ce(SO4)2. To maintain the oxidized Ru(III) state, Ce(SO4)2 was used as an oxidizing agent. Ce2(SO4)3 was used to maintain the reduced state under the same ionic conditions. The diameters of the gel beads were measured at several temperatures under different conditions of the reduced Ru(II) and oxidized Ru(III) states by dynamic light scattering (Malvern, Zetasizer3000HSA). For the measurement of optical oscillations, Ru(bpy)3Cl2 (0.33 mM), linear poly(NIPAAm-co-Ru(bpy)3) (0.50 wt %), or the nanogel beads (0.25 wt %) were dissolved in the aqueous solution containing the reactants of the BZ reaction: malonic acid (MA), sodium bromate, and nitric acid. Under constant temperature and stirring conditions, the time course of transmittance was monitored by the spectrophotometer (Shimazu, UV-2500PC). For the polymer solution and the gel suspension, the 570-nm wavelength (isosbestic point) was used to detect the transmittance changes on the basis of conformational changes of the polymer, not on redox changes of the Ru(bpy)3 moiety. The 480-nm wavelength was used for measuring the redox change of the conventional BZ solution.
Results and Discussion Figure 1 shows the diameter changes of poly(NIPAAmco-Ru(bpy)3)-gel beads as a function of temperature under the different conditions of the reduced Ru(II) state and oxidized Ru(III) state. As a result of the characteristics of the thermosensitive NIPAAm component, the diameter decreases as the temperature increases. In the case of the bulk gel, the diameter in the oxidized state is larger than that in the reduced state all over the temperature range because the hydrophilicity of the polymer increases in the oxidized state.10 But in the case of gel beads, as they aggregate as a result of enhanced hydrophobicity, the diameter in the reduced state becomes larger than that in the oxidized state above a certain temperature (∼35 °C) as shown in Figure 1. From the deviation of the diameter between the Ru(II) and the Ru(III) states, we may expect that the gel beads undergo periodical swelling-deswelling changes on the nanometer scale when the Ru(bpy)3 moiety is oxidized and reduced periodically by the BZ reaction at constant temperature. These periodic changes of gel beads can be observed as cyclic transparent and opaque changes for the suspension accompanying color changes due to the redox oscillation of the catalyst. Figure 2 shows the oscillation profiles of transmittance for the gel-beads
Figure 2. Oscillating profiles of optical transmittance for the poly(NIPAAm-co-Ru(bpy)3)-gel-beads suspension at constant temperatures.
suspension at a constant temperature. The oscillation of transmittance is caused by the following reasons. First, the gel particle itself becomes transparent and opaque with swelling and deswelling, respectively. Second, gel particles aggregate to increase the turbidity in the reduced state and disperse to decrease the turbidity in the oxidized state. As the temperature increases, the transmittance decreases as a whole and the period becomes short. It is a general tendency that the oscillation period of the BZ reaction decreases as the temperature increases, following the Arrhenius equation.17 The total decrease in transmittance with increasing temperature is interpreted by the aggregation of gel beads due to their temperature sensitivity. In the previous study, we investigated the oscillating profiles of redox changes when the BZ reaction occurred in the bulk gel with submillimeter size.18 The oscillation period was found to be longer in the bulk gel than in the conventional BZ system using nonpolymerized catalyst. It was suggested that several effects by gelation resulted in the elongation of the period. Here, we consider the following two effects: one is the polymerization effect of the catalyst and the other is the cross-linking effect of the polymer chains. To clarify such effects in detail, we measured the dependence of the oscillation period on the initial substrate concentration for three systems: (i) the conventional BZ solution using nonpolymerized catalyst, (ii) the solution of linear polymer, that is, polymerized catalyst by NIPAAm, and (iii) the suspension of submicrometer-sized gel beads, that is, the cross-linked polymer network of the polymerized catalyst. Figure 3 demonstrates the oscillating behavior under different concentrations of (a) MA, (b) NaBrO3, and (c) HNO3, respectively, when the other substrate concentrations are fixed. Generally, the period of the BZ reaction becomes short with the increase in the substrate concentration. The period [T(s)] can be expressed as a function of the substrate concentration, [M], in the following empirical equation with a good correlation of more than (17) Rouff, P. Physica D 1995, 84, 204-211. (18) Yoshida, R.; Onodera, S.; Yamaguchi, T.; Kokufuda, E. J. Phys. Chem. A 1999, 103, 8573-8578.
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Figure 3. Oscillating behavior under different concentration of (a) MA, (b) NaBrO3, and (c) HNO3 when the other substrate concentrations were fixed at (a) [NaBrO3] ) 84 mM, [HNO3] ) 0.3 M; (b) [MA] ) 62.5 mM, [HNO3] ) 0.3 M; and (c) [MA] ) 62.5 mM, [NaBrO3] ) 84 mM. (O) Conventional BZ solution using nonpolymerized catalyst, (4) polymer solution using polymerized catalyst, and (0) suspension of nanogel beads.
r ) 0. 99: T ) a[MA]-b[NaBrO3]-c[HNO3]-d, where a-d are constants. The concentration dependence of the period for the nonpolymerized solution was as follows:
T ) 2.97[MA]-0.414[NaBrO3]-0.794[HNO3]-0.743 (1) For the polymer solution, the equation became
T ) 2.97[MA]-0.413[NaBrO3]-0.934[HNO3]-0.567 (2) which was partly similar to eq 1. For the gel-beads suspension, the dependence was largely different from eq 1 as follows:
T ) 5.75[MA]-0.506[NaBrO3]-0.667[HNO3]-0.478 (3) Compared with the period under the same substrate concentrations, the period for each system (i-iii) increased in the following order: i < ii < iii. The difference between i and ii is attributed to the immobilization of catalysts in the polymer chains. When the catalysts are immobilized in the polymer chain, the distance between the catalysts is defined in the polymer. Then the change to the oxidized state to enhance the electrostatic repulsion is inhibited. As a result, the catalysts are likely to remain in the reduced state, and the period becomes longer. To investigate the polymer effect, we measured the oscillating profiles of the redox potential by using a Pt electrode for the conventional BZ solution and the polymer solution as shown in Figure 4. The amplitude of the polymer solution is smaller than that of the conventional BZ solution, which well supports the previous consideration. The difference between ii and iii is due to the cross-linking effect of polymer chains. By cross-linking polymer chains, polymers are forced to
Figure 4. Oscillating profiles of the redox potential for the poly(NIPAAm-co-Ru(bpy)3) solution (0.50 wt %; solid line) and the normal BZ solution using nonpolymerized catalyst, Ru(bpy)3Cl2 (0.33 mM; dotted line). A Hg2SO4 electrode utilizing a saturated K2SO4 solution was used as a reference electrode, [MA] ) 62.5 mM, [NaBrO3] ) 84 mM, [HNO3] ) 0.3 M, and temperature ) 24 °C.
oscillate cooperatively. And also, diffusion of substrates from the bulk phase to the gel phase, as well as of the products from the gel to the bulk, becomes more difficult. These effects lead to the elongation of the oscillation period. The clarified aspects of self-oscillation will serve to design nano-actuators using the nanogel particles. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research to R.Y. from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (No. 15205027). LA035833S
