Self-Oscillation of Polymer Chains Induced by the Belousov

A novel self-oscillating polymer was prepared by utilizing the Belousov−Zhabotinsky (BZ) reaction. In this study, a sulfonic acid group was newly in...
4 downloads 0 Views 80KB Size
Download PDF
J. Phys. Chem. B 2005, 109, 9451-9454

9451

Self-Oscillation of Polymer Chains Induced by the Belousov-Zhabotinsky Reaction under Acid-Free Conditions 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 ReceiVed: January 10, 2005; In Final Form: March 15, 2005

A novel self-oscillating polymer was prepared by utilizing the Belousov-Zhabotinsky (BZ) reaction. In this study, a sulfonic acid group was newly introduced as a pH-control site into the copolymer of N-isopropylacrylamide, and the ruthenium complex was introduced as a catalyst site. By introducing the pH-control site, we succeed in causing the soluble-insoluble self-oscillation of the polymer solution under acid-free conditions in which only two BZ substrates, malonic acid and sodium bromate, were present as added agents. The self-oscillating behavior was remarkably influenced by the temperature and polymer concentration, which reflects the intermolecular aggregative capacity of the polymer chains in the reduced state to change the lower critical solution temperature. This achievement of self-oscillation of polymer chains under acid-free conditions may lead to their practical use as novel biomimetic materials under biological conditions.

Introduction Stimuli-responsive polymers and gels, which are sensitive to external stimuli such as pH,1-3 temperature,4,5 glucose concentration,6,7 ions,8 light,9 electric fields, etc.,10,11 have attracted much attention in the field of functional polymer systems design, especially in view of their application in the biomedical and biomimetic fields as biosensors,12 drug delivery systems,13,14 actuators,15,16 etc. However, each of these conventional stimuliresponsive polymer systems (polymers and gels) provides only one unique action to stimuli. Compared with these polymer systems, many physiological systems, such as the autonomic heartbeat, brain waves, and periodic hormone secretion, show spontaneous rhythmic oscillations and respond to external stimuli under biological conditions. Namely, in the living body, the mechanisms generating autonomous and stimuli-responsive activities are launched under very mild conditions. If we can design and synthesize such intelligent polymer systems, unprecedented biomimetic materials would be realized. From this perspective, we have been trying to construct intelligent polymer systems which possess both autonomy and stimuli response under physiological conditions. We have already taken the first step in this direction, developing a novel “self-oscillating polymer system”17-22 by utilizing poly(N-isopropylacrylamide) (PNIPAAm), which is one of the most investigated thermosensitive polymers with a lower critical solution temperature (LCST) in water at 31 °C.23 Selfoscillating polymer chains and gels exhibit cyclic solubleinsoluble and swelling-deswelling changes, respectively, under the constant conditions of a closed system. These spontaneous and periodic changes are caused by the chemical energy of a nonlinear chemical reaction, the Belousov-Zhabotinsky (BZ) reaction. The BZ reaction is well-known for exhibiting temporal and spatial oscillating phenomena.24 The overall process of the BZ reaction is the oxidation of an organic substrate, such as * To whom correspondence should be addressed. Phone and fax: +813-5841-7112. E-mail: [email protected].

malonic acid and citric acid, by an oxidizing agent (bromate) in the presence of a strong acid and a metal catalyst. This process can be visualized as a cyclic reaction network of intermediates, as in the case of the TCA cycle (Krebs cycle), and it is a key metabolic process taking place in living organisms. We incorporated the ruthenium complex with tris(2,2′-bipyridine) (Ru(bpy)3), which is the catalyst for the BZ reaction, into PNIPAAm. In an aqueous solution containing the three BZ substrates (malonic acid, sodium bromate, and nitric acid) and no catalyst, spontaneous and periodic soluble-insoluble and swelling-deswelling changes of the polymer chain and gel, respectively, were observed. To cause the self-oscillation of polymer systems under physiological conditions, BZ substrates other than organic ones, such as malonic acid and citric acid, must be built into the polymer system itself. Therefore, we have attempted in this study to take the second step, namely, to design novel self-oscillating polymer chains with incorporated pH-control sites, that is, novel polymer chains that exhibit rhythmic oscillations in aqueous solutions containing only the two BZ substrates other than the metal catalyst, without using acid as an added agent. For this purpose, acrylamide-2-methylpropanesulfonic acid (AMPS) was incorporated into the poly(NIPAAm-co-Ru(bpy)3) chain as the pH-control site. Autonomous soluble-insoluble changes were observed as optical transmittance changes in the newly synthesized polymer solution under acid-free conditions. To clarify the control factors of the oscillating behavior, the influences of external temperature and polymer concentration were analyzed by comparing the changes in amplitude and oscillation period. Experimental Section Polymerization. Using NIPAAm (2.0 g), AMPS (7.0 g), Ru(bpy)3 monomer (ruthenium (4-vinyl-4′-methyl-2,2′-bipyridine)bis(2,2′-bipyridine)bis(hexafluorophosphate)) (1.0 g), and 2,2′-azobis(isobutyronitrile) (AIBN) (0.13 g) as an initiator, poly(NIPAAm-co-Ru(bpy)3-co-AMPS) (Figure 1) was synthesized by radical polymerization in a mixture of methanol

10.1021/jp0501704 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/14/2005

9452 J. Phys. Chem. B, Vol. 109, No. 19, 2005

Hara and Yoshida

Figure 1. Chemical structure of poly(NIPAAm-co-Ru(bpy)3-coAMPS).

(32 g) and water (8 g) under a total monomer concentration of 20 wt %. The polymerization was carried out at 60 °C for 24 h in vacuo. The resulting reaction mixture was dialyzed against water for 4 days followed by methanol for 3 days, and then freeze-dried. Measurements of LCST. The LCST of the polymer solution was measured under reduced and oxidized states, by using Ce(SO4)2 as an oxidizing agent and Ce2(SO4)3 as a reducing agent, respectively. Five polymer solutions (polymer concentrations: 0.5, 1.25, 2.0, 2.75, and 3.5 wt %) were prepared by dissolving the polymer in an aqueous solution containing 0.5 mM Ce(SO4)2 or 0.5 mM Ce2(SO4)3 per 0.5 wt % polymer concentration, respectively. LCST measurements were carried out with a spectrophotometer (Shimazu, Model UV-2500) equipped with a thermostatic controller and magnetic stirrers. To detect the optical transmittance change which is based on the soluble-insoluble change, the 570-nm wavelength was used, because it is the isosbestic point for the reduced and oxidized states of Ru(bpy)3. The transmittance (%) of the polymer solution at 570 nm was then recorded by raising the temperature at a rate of 0.5 °C/min. Measurements of Optical Oscillations. Five polymer solutions were prepared by dissolving the polymer (0.5, 1.25, 2.0, 2.75, and 3.5 wt %) into an aqueous solution containing the two BZ substrates (0.1 M malonic acid and 0.25 M sodium bromate). The optical transmittance oscillations for the polymer solutions were measured under constant temperature and stirring. The time course of transmittance at 570 nm was monitored by a spectrophotometer. Results and Discussion In the oxidized state, no LCST was observed for any of the polymer concentrations. The polymer chain in the oxidized state contained strongly hydrophilic oxidized Ru(bpy)33+ parts, in addition to the hydrophilic anionic charged AMPS component. These two strongly hydrophilic components of the polymer chain prevented the detachment of water molecules even at high temperatures. As a consequence, there were no LCSTs in the oxidized state. Generally, this phenomenon is observed in polyelectrolyte polymer chains that consist of the NIPAAm component. This tendency becomes remarkable with increasing content of the ion-charged component. The relationship between the LCST in the reduced state and the polymer concentration is summarized in Figure 2. The LCST decreased with increasing polymer concentration. These results cannot be explained by only the general mechanism of LCST, due to the detachment of the water molecules from the polymer chains near the LCST. The polymer chain containing the reduced Ru(bpy)32+ moiety becomes extremely hydrophobic, because the bipyridine ligands surrounding the Ru2+ ion exert a greater influence on the solubility of the polymer chain compared to

Figure 2. Relationship between concentration of the polymer solution and LCST for the polymer solutions in the reduced Ru(bpy)32+ state.

the ionization effect of the Ru2+ ion. In our previous study,19 we observed that, as the Ru(bpy)32+ content increased, the sharpness of the change in transmittance with temperature became duller in the reduced state, due to the hydrophobic interaction between the polymer chains. Generally, hydrophobic polymer chains have a high aggregation ability originating from the hydrophobic interaction. Therefore, we consider that the concentration dependence of the LCST for AMPS-containing polymers can be attributed to the cohesion between the polymer chains in the reduced state. As the polymer concentration increases, the polymer chains can easily aggregate because the distance between the polymer chains becomes much smaller. Therefore, as shown in Figure 2, the LCST exhibited a linear dependence on the polymer concentration. When the concentration was 0.5 wt %, LCST did not exist even in the reduced state. This result demonstrates that the change in turbidity for AMPS-containing polymer chains near LCST was caused not only by a coil-to-globule transition in the polymer chain structure, but also by the cohesion between the polymer chains. Figure 3 shows the self-oscillating transmittance change for these polymer solutions at three constant temperatures (18, 21, and 24 °C). The self-oscillating soluble-insoluble changes have been attributed to the different solubilities of the reduced and oxidized states.19 Only in the 0.5 wt % polymer solution was the self-oscillating behavior not observed at any of these temperatures. As the 0.5 wt % solution did not have an LCST in the reduced state, there was no difference in solubility between the reduced and oxidized states. The waveforms of optical oscillation were remarkably influenced by the temperature and polymer concentration. In Figure 3, it was observed that the oscillations gradually damped over time. This tendency is remarkable as temperature increases. Once the polymer chain is in the globule state, the polymer chain cannot easily return to the former coil state if the aggregation or entanglement of polymer chains occurs. The damped oscillation may be due to the repeating of this irreversible aggregation or entanglement with temperature approaching the LCST. Decrease in durability of oscillation is not preferable for practical application, but the damping might be avoided by introducing hydrophilic comonomer into the polymer to suppress the hydrophobic interaction. We compared the waveforms by measuring the oscillation amplitudes (∆T%) and periods, which are summarized in Figure 4. As shown in Figure 4 (upper), only in the case of the 3.5 wt % concentration did the amplitude decrease at 24 °C. The LCST (23 °C) of the 3.5 wt % solution was lower than 24 °C. Since the polymer chains easily aggregated above the LCST, the aggregated polymer chains could not be easily elongated even

Self-Oscillating Polymer Chains

J. Phys. Chem. B, Vol. 109, No. 19, 2005 9453

Figure 4. Dependence of amplitude (upper) and period (lower) on temperature for four polymer concentrations (1.25-3.5 wt %).

Figure 3. Oscillating profiles of optical transmittance for poly(NIPAAm-co-Ru(bby)3-co-AMPS) solutions at several constant temperatures. Polymer concentrations: (a) 1.25, (b) 2.0, (c) 2.75, and (d) 3.5 wt %.

in the oxidized state, which led to a decrease in amplitude. However, as the polymer concentration decreased, this tendency became less significant. The amplitudes of the polymer solutions with lower concentrations (1.25 and 2.0 wt %) remarkably increased with increasing temperature. The LCSTs of the 1.25 and 2.0 wt % polymer solutions were 41 and 32 °C, respectively, which are higher than all the measured temperatures. There were fewer polymer chains in the aggregation state in the 1.25 and 2.0 wt % solutions than in the 3.5 wt % solution. This means that the amplitudes increased with increasing temperature, because the mobility of the polymer chains increased. These results demonstrate that the amplitude was greatly influenced by the interaction among the polymer chains, as well as by the temperature. It is known that the sulfuric acid concentration is an important parameter of the classical BZ reaction: the higher the sulfuric

acid concentration, the higher the frequency of the oscillations.25 For the catalyst concentration usually the opposite relationship holds: the higher the catalyst concentration, the lower the frequency. In the present case both the catalyst and the acid are bound to the same polymer chain; thus, their concentration cannot be varied independently. According to the experimental results, increasing the polymer concentration increases the frequency of the oscillations. This suggests that the acidity provided by the polymer is rather important. The oscillation periods were plotted as a function of temperature (Figure 4, lower). The oscillation periods exhibited a linear dependency on the temperature. It is well-known that temperature affects the periods in the BZ reaction, in accordance with the Arrhenius equation.26-28 The oscillation periods for the AMPS-containing polymer are controllable across a much wider range, in comparison with the poly(NIPAAm-coRu(bpy)3), which we studied previously.19 This is of great advantage in a variety of applications. The effect of the composition ratio on AMPS-containing polymers is under investigation. Since these polymers are sensitive to changes in external conditions, such as temperature and the concentration of BZ substrates, on-off switching of self-oscillation through these external stimuli might be possible. Smart polymer and gel systems possessing both autonomy and stimuli response may be developed. In addition, we are now attempting to introduce the oxidizing agent into the polymer as a third step, to cause self-oscillation only in the presence of bio-related organic acids. This will be presented in the next paper. If we can actuate smart polymer (gel) systems in biological environments, the practical

9454 J. Phys. Chem. B, Vol. 109, No. 19, 2005 scope of these systems as biomachines could be greatly expanded; we could create, for instance, autonomous ciliary motion actuators29 for the transport of cells or other bioactive substances, self-oscillators for nanomachines, self-beating micropumps mimicking heart muscle, etc. Conclusions Under acid-free conditions and in the presence of only two BZ substrates (malonic acid and sodium bromate), we succeeded in causing the soluble-insoluble self-oscillation of a polymer solution. The effect of the polymer concentration and temperature on the self-oscillating behavior was investigated. By introducing a pH-control site into the polymer, it was not necessary to add a strong acid to the surrounding solution. Thus, the first step in using self-oscillating polymers and gels under biological conditions as novel smart materials has been made. 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). References and Notes (1) Tanaka, T.; Fillmore, D.; Sun, S. T.; Nishio, I.; Swislow, G.; Shah, A. Phys. ReV. Lett. 1980, 45, 1636. (2) Siegal, R. A.; Firestone, B. A. Macromolecules 1988, 21, 3254. (3) Beebe, D. J.; Moore, J. S.; Bauer, J. M.; Yu, Q.; Liu, R. H.; Devadoss, C.; Jo, B. H. Nature 2000, 404, 588. (4) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (5) Hu, Z.; Zhang, X.; Li, Y. Science 1995, 218, 525. (6) Kataoka, K.; Miyazaki, H.; Bunya, H.; Okano, T.; Sakurai, Y. J. Am. Chem. Soc. 1998, 120, 12694.

Hara and Yoshida (7) Baldi, A.; Gu, Y.; Loftness, P. E.; Siegel, R. A.; Ziaie, B. J. Microelectromech. Syst. 2003, 12, 613. (8) Armentrout, R. S.; McCormick, C. L. Macromolecules 2000, 33, 419. (9) Suzuki, A.; Tanaka, T. Nature 1990, 346, 345. (10) Tanaka, T.; Nishio, I.; Sun, S. T.; Ueno-nishio, S. Science 1982, 218, 467. (11) Osada, Y.; Okuzaki. H.; Hori, H. Nature 1992, 355, 242. (12) Miyata, T.; Asami, N.; Uragami, T. Nature 1999, 399, 766. (13) Kwon, I. C.; Bae, Y. H.; Kim, S. W. Nature 1991, 354, 291. (14) Kataoka, K.; Harada, A.; Nagasaki, Y. AdV. Drug DeliVery ReV. 2001, 47, 113. (15) Kwon, I. C.; Bae, Y. H.; Kim, S. W. Nature 1991, 354, 291. (16) Kakugo, A.; Shigimoto, S.; Gong, J. P.; Osada, Y. AdV. Mater. 2002, 14, 1124. (17) Yoshida, R.; Takahashi, T.; Yamaguchi, T.; Ichijo, J. Am. Chem. Soc. 1996, 118, 5134. (18) Yoshida, R.; Tanaka, M.; Onodera, S.; Yamaguchi, T.; Kokufuda, E. J. Phys. Chem. A 2000, 104, 7549. (19) Yoshida, R.; Sakai, T.; Ito, S.; Yamaguchi, T. J. Am. Chem. Soc. 2002, 124, 8095. (20) Ito, Y.; Nogawa, M.; Yoshida, R. Langmuir 2003, 19, 9577. (21) Yoshida, R.; Takei, K.; Yamaguchi, T. Macromolecules 2003, 36, 1759. (22) Sakai, T.; Yoshida, R. Langmuir 2004, 20, 1036. (23) Heskins, M.; Guillet, J. E. J. Macromol. Sci. Chem. 1968, A2 (8), 1441. (24) Oscillations and TraVeling WaVes in Chemical Systems; Field, R. J., Burger, M., Eds.; John Wiley & Sons: New York, 1985. (25) Smoes, M.-L. J. Chem. Phys. 1979, 71, 4669. (26) Ruoff, P. Physica D 1995, 84, 204. (27) Yoshikawa, K. Bull. Chem. Soc. Jpn. 1982, 55, 2042. (28) Yoshida, R.; Otoshi, G.; Yamaguchi, T.; Kokufuta, E. J. Phys. Chem. A 2001, 105, 3667. (29) Tabata, O.; Hirasawa, H.; Aoki, S.; Yoshida, R.; Kokufuta, E. Sens. Actuators, A 2002, 95, 234.