8427
2008, 112, 8427–8429 Published on Web 06/28/2008
Self-Oscillating Polymer Fueled by Organic Acid Yusuke Hara†,§ 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, and PRESTO, Japan Science and Technology Agency, Japan ReceiVed: March 7, 2008; ReVised Manuscript ReceiVed: May 23, 2008
Here we report the biomimetic polymer that causes self-oscillation driven by the addition of biorelated organic acid. We constructed the built-in system where all of the substrates of the BZ reaction other than biorelated organic substrates were incorporated into the polymer chain. The quarternary copolymer, which includes both of the pH-control and oxidant-supplying sites in the poly(N-isopropylacrylamide-co-Ru(bpy)3) chain was synthesized. By using the polymer, we first succeeded in causing the self-oscillation of the polymer only in the coexistence of organic acid. Introduction As one of characteristic behaviors in living systems, autonomous oscillation, that is, spontaneous changes with temporal periodicity (called “temporal structure”), such as heartbeat, brain waves, pulsatile secreton of hormone, cell cycle, biorhythm, and so forth, can be exemplified. From the standpoint of biomimetics, so far several functional polymer systems, for example, stimuli-responsive systems, molecular recognition systems, and so forth have been studied extensively,1 but the polymer systems undergoing self-oscillation under constant condition without any on-off switching of external stimuli are still undeveloped. If such autonomous polymer systems like a living organism acting under biological condition can be realized by using completely synthetic polymers, then unprecedented biomimetic materials will be created. Here we report the biomimetic polymer that causes self-oscillation driven by the addition of biorelated organic acid. In order to realize the autonomous polymer system by tailormade molecular design, we focused on the Belousov-Zhabotinsky (BZ) reaction, which is well-known for exhibiting temporal and spatiotemporal oscillating phenomena.2 The BZ reaction is often analogically compared with the TCA cycle (Krebs cycle), which is a key metabolic process taking place in the living body. The overall process of the BZ reaction is the oxidation of an organic substrate, such as malonic acid (MA) or citric acid, by an oxidizing agent (bromate ion) in the presence of a strong acid and a metal catalyst. In the course of the reaction, the catalyst undergoes spontaneous redox oscillation. For utilizing the chemical energy of the BZ reaction as a driving source generating mechanical oscillation of polymer chains, ruthenium tris(2,2′-bipyridine) (Ru(bpy)3) as the catalyst was covalently bonded to poly(N-isopropylacrylamide) (poly(NIPAAm)) or their cross-linked network (i.e., gel). The hydrophilicity of the polymer chain changes periodically with the redox changes of * To whom correspondence should be addressed. E-mail: ryo@ cross.t.u-tokyo.ac.jp. † The University of Tokyo. ‡ PRESTO. § Present adddress: Consolidated Research Institute for Advanced Science and Medical Care, Waseda University, Japan.
10.1021/jp802014d CCC: $40.75
Figure 1. Chemical structure of poly(NIPAAm-co-Ru(bpy)3-coAMPS-co-MAPTAC).
the Ru(bpy)3 moiety. As a result, autonomous and spontaneous swelling-deswelling self-oscillations for the gel or solubleinsoluble self-oscillations for the polymer chains are realized under the coexistence of the BZ substrates (MA, acid, and oxidant) other than the catalyst, respectively.3 However, in this self-oscillating polymer system, the operating conditions are limited to the nonphysiological environment where the strong acid and the oxidant coexist. For extending the application field to biomaterials, more sophisticated molecular design to cause self-oscillation under physiological condition is needed. For this purpose, we constructed the integrated polymer system where all of the BZ substrates other than biorelated organic substrate were incorporated into the polymer chain. We synthesized the quarternary copolymer, which includes both of the pH-control and oxidant-supplying sites in the poly(NIPAAm-co-Ru(bpy)3) chain at the same time (Figure 1). In the polymer, 2-acrylamido-2-methylpropanesulfonic acid (AMPS) was incorporated as a pH-control site, and methacrylamidopropyltrimethylammonium chloride (MAPTAC) with a positively charged group was incorporated as a capture site for an anionic oxidizing agent (bromate ion). By using the polymer, self-oscillation under biological conditions where only the organic acid (malonic acid) exists has been attempted. Experimental Section The poly(NIPAAm-co-Ru(bpy)3-co-AMPS-co-MAPTAC) (Figure 1) was synthesized by radical polymerization. NIPAAm 2008 American Chemical Society
8428 J. Phys. Chem. B, Vol. 112, No. 29, 2008
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(1.20 g), Ru(bpy)3 monomer (1.28 g), AMPS (13.05 g), MAPTAC (0.47 g) and 2,2′-azobis(2-methylbutyronitrile)isobutyronitrile (V-59) (0.41 g) as an initiator were dissolved in the mixture of methanol (31.80 g) and water (31.80 g), which was degassed and nitrogen-saturated under a total monomer concentration of 20 wt %. The polymerization was carried out at 80 °C for 24 h in vacuo. The resulting reaction mixture was dialyzed against methanol for 3 days and then water for 4 days. For exchanging counterions, the polymer was dissolved in NaBrO3 (1M) and NaBr (0.5M) aqueous solution and dialyzed against pure water for 30 days with repeating exchanging the water to remove excess Na+, Br-, and BrO3- ions. The counterion in the AMPS site is changed to Na+ through this counterion exchange process. Therefore, in the next step, the counterion in the AMPS site was exchanged to H+ using ion-exchange resin, and then freeze-dried. To prevent bromine formation, the polymer solution was frozen immediately after collecting it. It should be noticed that the freeze-dried polymer chain with the oxidizing agent as the counterion has explosive properties. Polymer solutions were prepared by dissolving the polymer (6.5 wt %) into an aqueous solution containing malonic acid (0.3, 0.5, and 0.7 M). The change in optical transmittance for theses polymer solutions were measured under constant temperature and stirring. Theses measurements were carried out with a spectrophotometer (Shimazu, Model UV-2500) equipped with a thermostatic controller and magnetic stirrers at 570-nm wavelength, which is the isosbestic point for the reduced and oxidized states of the Ru(bpy)3 moiety. Results and Discussion Figure 2 shows the optical transmittance change of the poly(NIPAAm-co-Ru(bpy)3-co-AMPS-co-MAPTAC) solutions under the constant temperature for several concentrations of MA as an adding agent. The soluble-insoluble self-oscillation of the polymer was observed under the coexistence of malonic acid only. This is because the polymer chain becomes the supplying source of the BZ reaction substrate; the polymer chain supplies H+, BrO3-, and Br- ions by itself as a counterion from the AMPS, the MAPTAC, and the Ru(bpy)3 sites, respectively. To cause the self-oscillation, however, enough amounts of H+, BrO3-, and Br- ions as the BZ reaction substrate were necessary. The self-oscillation did not occur when the polymer concentration was below 6.5 wt %. The soluble-insoluble self-oscillation of the polymer solution was attributed to the different solubilities between the reduced and oxidized states of the Ru(bpy)3 site.3b In the reduced state, the Ru(bpy)32+ moiety works as an hydrophobic group because the bipyridine ligands surrounding the Ru ion exert a greater influence on the solubility of the polymer chain as compared with the ionization effect of the Ru ion. On the contrary, in the oxidized state, the Ru(bpy)33+ moiety works as an hydrophilic group because the charge of the Ru increases. As shown in Figure 2, the oscillation was gradually damping with time due to aggregation among the polymer chains. In our previous investigations, the damping behaviors of the selfoscillation were observed for the poly(NIPAAm-co-Ru(bpy)3co-AMPS) solutions.4 This ternary AMPS-containing copolymer chain has the betaine type structure; that is, it has both anionic and cationic sites in a single polymer chain at the same time. This structure induces the intra- and intermolecular interaction due to the electrostatic interactions between the cationic and anionic sites in the polymer chain. On the contrary, damping behaviors were not observed for the poly(NIPAAm-co-Ru(bpy)3-
Figure 2. Oscillating profiles of the optical transmittance for the poly(NIPAAm-co-Ru(bpy)3-co-AMPS-co-MAPTAC) solutions at 12 °C for several concentrations of malonic acid. (a) [MA] ) 0.3 M, (b) [MA] ) 0.5 M, (c) [MA] ) 0.7 M.
co-MAPTAC) solution.5 Because the ternary MAPTACcontaining copolymer chain has the only cationic site, the aggregation among the polymer chains did not occur because of the electrostatic repulsive force of the MAPTAC site. Although the MAPTAC site may contribute to prevent aggregation of polymer chain as a repulsive site, in the poly(NIPAAmco-Ru(bpy)3-co-AMPS-co-MAPTAC) solution, damping was observed similar to the poly(NIPAAm-co-Ru(bpy)3-co-AMPS) solution. Further, the duration of the self-oscillation for the novel polymer solution decreased with an increase in the concentration of MA. The influence of the concentration of the BZ reaction substrates on the waveform was referable to the overall process of the BZ reaction, which can be understood by the FieldKoros-Noyes (FKN) mechanism.2c According to the mechanism, the concentration of MA significantly affects the ratio of the reduced Ru(bpy)3 moiety. It was clarified that the ratio of the reduced Ru(bpy)3 moiety in the polymer chain greatly influenced the self-oscillating behavior because the state of the Ru(bpy)3 site was a dominant factor of the solubility of the polymer chain.6 Therefore, as the concentration of MA increased, the ratio of the reduced Ru(bpy)3 site increased, and damping occurred in a short period because of decreasing solubility of the polymer chain.
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J. Phys. Chem. B, Vol. 112, No. 29, 2008 8429 applications to biomaterials are expected. We believe that the result demonstrated here will be a great breakthrough for the development of novel biomimetic materials. 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 (Grant No. 15205027). References and Notes
Figure 3. Dependence of the period on the concentration of malonic acid.
Figure 3 shows the oscillation periods plotted as a function of the concentration of MA. A good linear relationship between the period and the MA concentration was observed. This result indicates that the novel polymer has a potential as a sensor for measuring the organic acid concentration. In this study, we first succeeded in causing the self-oscillation of the polymer under the biological condition by incorporating the metal catalyst, the pH-control and the oxidant-supplying sites into the polymer at the same time. By this polymer design, selfoscillation only in the coexistence of MA becomes possible. Other than MA, citric or malic acid, which is biorelated organic acid, can be a substrate of the BZ reaction. Therefore, several
(1) Yoshida, R. Curr. Org. Chem. 2005, 9, 1617. (2) (a) Zaikin, A. N.; Zhabotinsky, A. M. Nature 1970, 225, 535. (b) Field, R. J.; Burger, M. Oscillations and TraVeling WaVes in Chemical Systems; John Wiley & Sons: New York, 1985. (c) Field, R.; Koros, E.; Noyes, R. M. J. Am. Chem. Soc. 1972, 94, 8649. (3) (a) Yoshida, R.; Takahashi, T.; Yamaguchi, T. Ichijo. J. Am. Chem. Soc. 1996, 118, 5134. (b) Yoshida, R.; Sakai, T.; Ito, S.; Yamaguchi, T. J. Am. Chem. Soc. 2002, 124, 8095. (c) Yoshida, R.; Takei, K.; Yamaguchi, T. Macromolecules 2003, 36, 1759. (d) Takeoka, Y.; Watanabe, M.; Yoshida, R. J. Am. Chem. Soc. 2003, 125, 13320. (e) Sakai, T.; Yoshida, R. Langmuir 2004, 20, 1036. (f) Maeda, S.; Hara, Y.; Sakai, T.; Yoshida, R.; Hashimoto, S. AdV. Mater. 2007, 19, 3480. (g) Suzuki, D.; Sakai, T.; Yoshida, R. Angew. Chem., Int. Ed. 2008, 47, 917. (4) (a) Hara, Y.; Yoshida, R. J. Phys. Chem. B 2005, 109, 9451. (b) Hara, Y.; Yoshida, R. Langmuir 2005, 21, 9773. (5) Hara, Y.; Sakai, T.; Maeda, S.; Hashimoto, S.; Yoshida, R. J. Phys. Chem. B 2005, 109, 23316. (6) Yoshida, R.; Onodera, S.; Yamaguchi, T.; Kokufuda, E. J. Phys. Chem. A 1999, 103, 8573.
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