Aspects of the Belousov-Zhabotinsky Reaction inside the Self

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Aspects of the Belousov-Zhabotinsky Reaction inside the Self-Oscillating Polymer Brush Tsukuru Masuda, Aya Mizutani Akimoto, Mami Furusawa, Ryota Tamate, Kenichi Nagase, Teruo Okano, and Ryo Yoshida Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03929 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on January 2, 2018

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Aspects of the Belousov-Zhabotinsky Reaction inside the Self-Oscillating Polymer Brush Tsukuru Masuda,1‡ Aya Mizutani Akimoto,1 Mami Furusawa,1 Ryota Tamate,1§ Kenichi Nagase,2§§ Teruo Okano,2 and Ryo Yoshida 1* 1) Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan *E-mail: [email protected] 2) Institute of Advanced Biomedical Engineering and Science, Tokyo Women’s Medical University (TWIns), 8-1 Kawada-cho, Shinjuku-ku, Tokyo 162-8666, Japan

ABSTRACT

We have developed a novel polymer brush surface exhibiting autonomous swellingdeswelling changes driven by the Belousov-Zhabotinsky (BZ) reaction, that is, the selfoscillating polymer brush.

In this system, the ruthenium tris(2,2’-bipyridine) [Ru(bpy)3]

catalyst-conjugated polymer chains are densely packed on the solid substrate. It is expected that the BZ reaction in the polymer brush would be influenced by the immobilization effect of the catalyst. To clarify the effect of immobilization of the catalyst on the self-oscillating polymer brush, the self-oscillating behavior of the polymer brush was investigated by comparing it with

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that of other self-oscillating polymer materials, the free polymer and the gel particle under various initial substrate concentrations. The initial substrate dependency of the oscillating period for the polymer brush was found to be different from those for the free polymer and the gel particle. Further, the oscillatory waveform was analyzed based on the Field-Körös-Noyes model. These investigations revealed that the dense immobilization of the self-oscillating polymer on the surface restricted accessibility for the Ru(bpy)3 moiety. These findings would be helpful for understanding the reaction-diffusion mechanism in the polymer brush, which is a novel reaction medium for the BZ reaction.

INTRODUCTION Belousov-Zhabotinsky (BZ) reaction is a well-known oscillating chemical reaction accompanying temporal rhythms or spatio-temporal patterns.1-5 The overall process involves the oxidization of an organic substrate by an oxidizing agent under strong acidic conditions with the aid of a metal catalyst. During the reaction, a metal catalyst, such as ferroin or ruthenium tris(2,2’-bipyridine) [Ru(bpy)3], shows periodic redox oscillation. When the size of the reaction media is sufficiently large, excited pulses of the oxidized area spontaneously propagate owing to the reaction-diffusion mechanism (“chemical wave”). These dynamic oscillatory phenomena have attracted much attention in the various research fields including nonlinear science, physical chemistry, system biology, and materials science.6-14 To date, various reaction media for the BZ reaction including polymer gels, membrane, mesoporous Vycor glass, microfluidic systems, water/oil droplet arrays, and liposomes have been investigated.15-22 For example, Toiya et al. reported that the BZ drop array separated by


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octane produced anti-phase oscillations and Turing patterns coupled with the diffusion of inhibitors in the oil phase.19 In an interesting recent progress, Ueki et al. reported hydrated protic ionic liquids (PILs) as a novel reaction medium for the BZ reaction, replacing strong acids (H2SO4 and HNO3).23 By optimizing the structure of PILs, a stable oscillatory reaction was realized in milder condition compared with the typical strong acids. These previous studies indicate that the reaction media strongly affects the diffusion of the intermediates of the BZ reaction, resulting in the tuning of rhythms or patterns. Thus, the design of the reaction media is a key point in the study of the BZ reaction. Meanwhile, our group has developed “self-oscillating” polymer gels that exhibit autonomous swelling-deswelling oscillations under constant conditions by converting the chemical energy of the BZ reaction into a mechanical motion of the gels.24,25 The self-oscillating polymer gel is a crosslinked network, composed of N-isopropylacrylamide (NIPAAm) and Ru(bpy)3. Coupled with the volume phase transition of the polymer gel, the redox changes of the Ru(bpy)3 moiety induces periodic volume oscillation of the gels. In the previous studies, it has been suggested that crosslinked networks affected the self-oscillating behavior of the gel compared with the bulk solution (i.e., a reaction solution containing a strong acid, an oxidizing agent, a reducing agent, and a non-polymerized catalyst) or uncrosslinked self-oscillating polymer chains (i.e., free polymers).26,27 This is because the diffusivity of the reaction substrates and the accessibility of the catalyst are restricted in the crosslinked network. More recently, we have designed self-oscillating polymer brushes by modifying the selfoscillating polymer into a solid substrate using surface-initiated atom transfer radical polymerization (SI-ATRP).28 In this system, the self-oscillating polymer chains are densely packed on the solid substrate, thus, densely localizing the catalyst on the surface. Owing to the


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densely introduced catalysts, the BZ reaction can be stably induced inside the polymer brush. It is expected that the BZ reaction in the polymer brush would be affected by the immobilization of the catalyst to the polymer chains.

Considering the difference in the structure of the gel

(crosslinked polymer chains swollen in solvents) and the polymer brush (densely packed polymer chains immobilized to the surface, swollen in solvents), the oscillatory behaviors and the effect of immobilization of the catalyst on the polymer brush would be different from that on the gels. In this study, to clarify the effect of immobilization of the catalyst on the self-oscillating polymer brush, the dependence of the self-oscillating profiles on the initial substrate concentrations were investigated for the free polymer, the gel particle, and the polymer brush. The waveforms thus obtained were analyzed based on the Field-Körös-Noyes (FKN) model.4 Based on these investigations, the aspect of polymer brush as a novel medium for the BZ reaction was discussed.

EXPERIMENTAL Materials N-Isopropylacrylamide (NIPAAm) was kindly provided by KJ Chemicals (Tokyo, Japan), which was subsequently purified by recrystallization in toluene/hexane. N-3-(Aminopropyl) methacrylamide (NAPMAm) was purchased from Polysciences (Warrington, PA, USA) and was used as received. Tris(2-(N,N-dimethylamino) ethyl) amine (Me6TREN) was purchased from Tokyo Chemical Industries (Tokyo, Japan).

((Chloromethyl)phenylethyl) trimethoxy silane


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(abbreviated as ClMPETMS) was purchased from Gelest (Morrisville, PA, USA). Bis(2,2’bipyridine) (1-(4’-methyl-2,2’-bipyridine-4-carbonyloxy)-2,5-pyrrolidinedione) ruthenium(II) bis(hexafluorophosphate), abbreviated as Ru(bpy)3-NHS, was synthesized according to a previous report.30 Sodium bromate (NaBrO3) was purchased from Kanto Chemical (Tokyo, Japan). All other chemicals were purchased from Wako Pure Chemical Industries (Osaka, Japan). Glass coverslips (24 mm × 50 mm) were obtained from Matsunami Glass Industry (Osaka, Japan).

Preparation and characterization of poly(NIPAAm-r-NAPMAm) brush surfaces and free polymer The poly(NIPAAm-r-NAPMAm) brush and free polymer, as the precursors for the selfoscillating polymer, were prepared according to a previous study (details shown in Supporting Information).28 Briefly, as the first step, the ATRP initiator (ClMPETMS) was immobilized onto the glass surfaces by the silane coupling reaction in toluene. Subsequently, poly(NIPAAm-rNAPMAm) grafted glass substrates were prepared by SI-ATRP in DMF/water mixed solvent, using Cu/Me6TREN as the catalyst of ATRP. In this process, α-chloro-p-xylene was added to the reaction solution, thus obtaining the free poly(NIPAAm-r-NAPMAm). The amount of grafted poly(NIPAAm-r-NAPMAm) and the amount of immobilized Ru(bpy)3 were quantitatively estimated by attenuated total reflection Fourier transform infrared (ATR/FT-IR) spectroscopy (Nicolet 6700) (Thermo Fisher Scientific, Waltham, MA) and ultraviolet-visible (UV-vis) spectroscopy (UVPC-2500) (Shimadzu, Kyoto, Japan), respectively.


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The obtained polymer was analyzed by using gel permeation chromatography (GPC) (Tosoh, Tokyo, Japan) and 1H NMR (JEOL, JNM-LA400WB).

Preparation and characterization of poly(NIPAAm-r-NAPMAm) gel particle The poly(NIPAAm-r-NAPMAm) gel particle was prepared by surfactant-free precipitation polymerization, according to a previous study.30 Briefly, NIPAAm (2.67 g, 23.6 mmol), NAPMAm (0.42 g, 2.36 mmol), and N,N’-methylenebisacrylamide (0.11 g, 0.71 mmol), as a crosslinker, were dissolved in water (100 mL). The oxygen in the solution was removed by argon gas bubbling for 30 min at 70 °C. Subsequently, water (5 mL) containing potassium sulfate (0.16 g, 0.59 mmol) as an initiator was added to the solution, with the reaction being carried out for 1 h at 70 °C. After the reaction, the solution was dialyzed against water, following which, the gel particle was recovered by freeze-drying. The hydrodynamic radius (Rh) of the prepared gel particle was measured by dynamic light scattering (ALV, Germany) (Rh = 379 nm at 20 °C, in distilled water). Details are shown as Fig. S2 in Supporting Information.

Conjugation of Ru(bpy)3 to poly(NIPAAm-r-NAPMAm) brush, free polymer and gel particle Poly(NIPAAm-r-NAPMAm) grafted glass substrates were contacted with 300 µL DMSO solution, containing Ru(bpy)3-NHS (21.2 mg) and triethylamine (25 µL), for 4 h at 25 ˚C (Concentration of Ru(bpy)3-NHS under this condition: 70 mM) (Step 3 in Fig. S1). After the reaction, the substrates were washed with DMSO and water, andthen dried in a vacuum oven for 3 h at 100 °C.


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For the free polymer and the gel particle, poly(NIPAAm-r-NAPMAm) free polymer (or the gel particle: 100 mg) and Ru(bpy)3-NHS (42.4 mg) were dissolved in DMSO (1.2 mL). Triethylamine (100 µL) was added to the solution, and the conjugation reaction was carried out for 24 h at 25 °C. Subsequently, the reaction solution was dialyzed against water, and the free polymer and the gel particle were then recovered by freeze-drying.

Evaluation of the self-oscillating behaviors for the free polymer and the gel particle systems Transmittance of the free polymer solution (0.05 wt%) or the gel particle dispersion (0.05 wt%) containing the substrate of the BZ reaction (HNO3, NaBrO3, and CH2(COOH)2 (MA)) was measured by UV-vis spectroscopy at 20 °C. Soluble-insoluble oscillation for the free polymer and the flocculating-dispersing oscillation for the gel particle were detected through transmittance measurement using the wavelength of 583.5 nm, which is the isosbestic point of Ru(bpy)3.31 The total volume of the reaction solution was 2 mL, and the BZ reaction was carried out under stirring.

Observation of the BZ reaction on the self-oscillating polymer brush The BZ reaction on the self-oscillating polymer brush surface was observed using a fluorescence microscope (DFC 360FX) (Leica, Mannheim, Germany) (excitation wavelength: 425 nm, fluorescence wavelength: 585 nm). The polymer brush grafted glass substrate was immersed in a catalyst-free BZ reaction solution (2 mL) containing HNO3 NaBrO3 and MA at


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20 °C, and the BZ reaction was conducted under non-stirring condition. Spatio-temporal images were analyzed by using Image J software (National Institute of Health, Bethesda, MD, USA).

Figure 1. (a) Illustration for the self-oscillating polymer materials investigated in this study: free polymer, gel particle and polymer brush. (b) The chemical structure of for the self-oscillating polymer materials.

RESULTS AND DISCUSSIONS Characterization of the self-oscillating polymer brush, free polymer, and gel particle The self-oscillating polymer materials investigated in this study were designed as random copolymers composed of NIPAAm, NAPMAm, and Ru(bpy)3 [poly(NIPAAm-r-NAPMAm-rRu(bpy)3NAPMAm)] (Fig. 1).

They were prepared through random copolymerization of

NIPAAm and NAPMAm, followed by post modification of Ru(bpy)3. During SI-ATRP, the


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unbound sacrificial initiator (α-chloro-p-xylene) was added to the reaction solution to obtain the free polymer from the ATRP reaction solution, while the polymer was used for the analysis. The number-averaged molecular weight (Mn) and the polydispersity index (the weight-averaged molecular weight (Mw) over Mn, i.e., Mw/Mn) determined by GPC were 2.8 × 104 and 1.4, respectively. The composition ratio of NAPMAm in the copolymer was determined to be 12 mol% by 1H NMR, which was higher than that of the feed ratio of NAPMAm (10 mol%). This may be attributed to the higher reactivity of NAPMAm.

Thus, the target copolymer,

poly(NIPAAm-r-NAPMAm) was successfully prepared by ATRP. For the characterization of the polymer brush-grafted substrate, ATR/FT-IR measurement was conducted to estimate the amount of the grafted poly(NIPAAm-r-NAPMAm). The strong absorption derived from the Si-O of glass substrate appeared at 1000 cm-1, while the absorption derived from the vibration of amide group appeared at 1650 cm-1, indicating that the target polymer was successfully modified onto the glass substrate by SI-ATRP. The amount of grafted polymer was calculated to be 7.7 ± 0.9 µg cm-2 from the intensity ratio (I1650/I1000) using the calibration curve.28

Subsequently, to estimate the amount of the conjugated Ru(bpy)3,

absorption spectrum (300 nm – 700 nm) was measured by UV-vis. The absorption peak, derived from Ru(bpy)3, appeared at 460 nm, with the amount of Ru(bpy)3 was calculated to be 3.5 ± 0.1 nmol cm-2 using the calibration curve.28 Using the composition ratio of the free polymer, the ratio of the Ru(bpy)3 in the polymer brush was estimated to be 5.7 mol%. Compared with the previous study,20 the amount of conjugated Ru(bpy)3 was expected to be sufficient to generate a stable oscillating chemical reaction. These results indicate that the target ternary copolymer, poly(NIPAAm-r-NAPMAm-r-Ru(bpy)3NAPMAm) was successfully modified onto the glass substrate by SI-ATRP and the conjugation of Ru(bpy)3.


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The free ternary self-oscillating polymer was prepared by conjugating Ru(bpy)3 to the free poly(NIPAAm-r-NAPMAm) which was obtained from the ATRP reaction solution as mentioned above. The composition of the ternary self-oscillating polymer was determined to be NIPAAm:NAPMAm:Ru(bpy)3NAPMAm = 87.8:4.7:7.5 by 1H NMR and UV-vis spectroscopy. Similarly, the gel particle, composed of the ternary copolymer, was prepared by surfactant-free precipitation polymerization of poly(NIPAAm-r-NAPMAm), followed by the conjugation of Ru(bpy)3 (the characterization of poly(NIPAAm-r-NAPMAm) gel particles is shown in the Supporting Information). The polymer brush, free polymer, and gel particle composed of ternary copolymer were used for the investigation of the self-oscillating behavior.

Effect of initial substrate concentrations on the dynamic behaviors of the self-oscillating polymer, gel particle, and polymer brush The characteristics of the BZ reaction in a supporting matrix (e.g., a polymer gel) would resemble that of the matrix-immobilized enzyme reaction. The effect of immobilization of enzymes on the kinetic behavior of the enzyme reactions can be classified as follows: (i) partitioning effects, (ii) microenvironmental effects, and (iii) diffusional or mass-transfer effects.32 These effects alter the rate of the immobilized-enzyme reaction due to differences in the concentrations of substrates, products, and other effectors. For the self-oscillating polymer brush, the catalyst for the oscillating reaction is immobilized into the polymer brush as a matrix for the reaction. Thus, a kinetic study of the BZ reaction in the bulk (the free polymer) system and the immobilized systems (the gel particle and the polymer brush), as a function of the initial substrate concentration, would be a classic and reliable approach to understanding the oscillating chemical reaction in the polymer brush.


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Fig. 2 shows the typical self-oscillating behaviors of the polymer brush and the free polymer systems. The self-oscillating behavior of the polymer brush system was evaluated by using fluorescence microscopy, because Ru(bpy)3 exhibits strong intensity only in the reduced state. For the free polymer system, transmittance oscillation was observed because solubleinsoluble oscillation occurred in synchronization with the redox changes of the Ru(bpy)3 moiety. The self-oscillating behavior of the gel particle system also exhibited transmittance oscillation attributed to the flocculation-dispersion of the gel particle, which was characterized in common with the free polymer system. Fig. 3 shows the oscillation period (Posc) as a function of the initial substrate concentrations for the gel particle and the polymer brush systems. Here, the average value of the period between the second and the sixth oscillations was regarded as the oscillation period, in accordance with a previous report on the BZ reaction in the gel particle system.27 For the free polymer, the average value of the period between the fifth and the ninth oscillations was used as the oscillation period because the transmittance oscillation fully developed after the fifth oscillation as shown in Fig. 2b. The oscillation period changed according to the initial substrate concentration, which can be expressed as the power law for these systems (Fig. S3 in Supporting Information). For the free polymer system, the obtained empirical equation was as follows: Posc, FP = 8.85[HNO3]-0.480[NaBrO3]-0.767[MA]-0.192 where [i] corresponds to the molar concentration of the substrate (i). For the gel particle system and the polymer brush systems, the empirical equations were as follows. Posc, GP = 8.17[HNO3]-0.311[NaBrO3]-0.740[MA]-0.246 Posc, PB = 8.38[HNO3]-0.220[NaBrO3]-0.388[MA]-0.579 The power indexes in the empirical equations express the degree of the substrate concentration dependence. This result suggests that the initial substrate concentration dependency of the


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polymer brush system was different from those for the free polymer and the gel particle systems. The dependency of nitric acid and sodium bromate in the polymer brush system was lower than those in the free polymer and the gel particle systems. In contrast, the dependency of malonic acid in the polymer brush system was higher than those for the free polymer and the gel particle systems. As a result, the dependency of malonic acid was found to be the highest for the selfoscillation period of the polymer brush system, which is different from the free polymer and the gel particle systems.

Figure 2. Typical waveforms for self-oscillating behaviors of (a) the polymer brush system and (b) the free polymer system. Concentrations of the substrates for the BZ reaction: [HNO3] = 0.8 M, [NaBrO3] = 150 mM, and [MA] = 100 mM. Temperature: 20 °C.


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Figure 3. Effect of the initial substrate concentrations (a: HNO3, b: NaBrO3, and c: MA) on the oscillation period for the free polymer system (black triangle), the gel particle system (blue square), and the polymer brush system (red circle).


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Effect of the initial substrate concentrations on the oscillating waveform To clarify the differences among the free polymer, the gel particle, and the polymer brush systems in detail, the self-oscillation waveforms were analyzed based on the FKN mechanism. According to the FKN mechanism, the BZ reaction can be divided into the three main processes: consumption of the inhibitor (bromide ion, Br -) (process A), autocatalytic reaction of bromous acid (HBrO2) with the oxidization of the catalyst (process B), and organic reaction with the reduction of the catalyst (process C). (A) BrO3- + 2Br- + 3H+ → 3HOBr

(B) BrO3- + HBrO2 + 2Ru(bpy)32+ + 3H+ → 2HBrO2 + 2 Ru(bpy)33+ + H2O

(C) 2 Ru(bpy)33+ + MA + BrMA → fBr- + 2 Ru(bpy)32+ + other products

Further, process A can be divided into the following processes: production of HBrO2 (A1) and degradation of HBrO2 (A2). (A1) BrO3- + Br - + 2H+ → HBrO2 + HOBr

(A2) HBrO2 + Br - + H+ → 2HOBr

During the BZ reaction, a dominant process changes in a cyclic manner, resulting in oscillations of concentrations of some intermediates. Generally, the process B occurs sharply because the process contains the autocatalytic reaction. Consequently, the sharp drop in the gray value and the sharp increase in the optical transmittance were monitored in the polymer brush system and


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in the free polymer system, respectively. The detail of the waveform analysis is shown in Fig. S4. Fig. 4 shows the waveforms for the polymer brush system obtained under different [HNO3] conditions. The durations of processes A and B decreased as [HNO3] increased for the polymer brush system (Table S1 in Supporting Information). This can be simply explained by the FKN mechanism. Since processes A and B include H+ as a reactant, an increase in [HNO3] resulted in the increase in the reaction rate of processes A and B. By comparing the polymer brush system and the free polymer system, it is suggested that the duration of process B for the polymer brush system is longer than that for the free polymer system, while the B/Posc value for the polymer brush system was larger than that for the free polymer system (Fig. 5). This is attributed to the lowered reaction rate of process B (oxidization of the catalyst) for the polymer brush system compared with the free polymer. The reason for the lowered reaction rate can be explained by the immobilization effect, i.e., the catalyst (Ru(bpy)3) is immobilized to the densely packed polymer chain, which lowers the accessibility of the reaction substrates to the catalyst. A similar result was observed for the gel particle system as shown in Fig. 5. The B/Posc value for the gel particle system was larger than that for the free polymer system, as immobilizing the catalyst to the crosslinked network resulted in lower accessibility of the reaction substrates. The increase in the B/Posc value for the polymer brush was larger than that for the gel particle, which could be attributed to the difference in the structure of the gel and polymer brush. As for the structures of these two systems, the gel particles are crosslinked polymer chains swollen in solvents, while the polymer brushes are densely packed polymer chains immobilized to the surface, swollen in solvents. The density of the polymer chains in the polymer brush is higher than that in the gel, resulting in lower accessibility for the catalyst in the polymer brush system compared with that in the gel system.


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In contrast, the durations of process A was shorter in the polymer brush system than that in the free polymer system at low [HNO3] (i.e., 0.3 M). In the polymer brush system, the catalyst is immobilized to the densely packed polymer chain as mentioned above, while the reaction medium is a non-stirring system. Thus, the autocatalytic species (HBrO2) locally generated in the polymer brush were allowed to accumulate, with the duration before the autocatalytic reaction of HBrO2 reducing. Thus, the process A of the polymer brush system became relatively shortened. According to the FKN mechanism, process A lengthened as [HNO3] decreased in the free polymer, the gel particle, and the polymer brush. Owing to the local accumulation of HBrO2, the degree of the elongation in the polymer brush was lower than that in the free polymer and the gel particle. As a result, the self-oscillating polymer brush exhibited lower dependency on [HNO3] compared with the free polymer and the gel particle.

Figure 4. Typical oscillation waveforms for the polymer brush system under different initial concentrations of HNO3.

Initial concentrations of other substrate were fixed as follows:

[NaBrO3] = 150 mM, and [MA] = 100 mM. Scale: 50 s.


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Figure 5. Typical oscillation waveforms for the free polymer system, the gel particle system, and the polymer brush system.

The concentrations of the reaction substrates were as follows:

[HNO3] = 0.3 M, [NaBrO3] = 150 mM, and [MA] = 100 mM. Scale: 50 s.

When [NaBrO3] increased, the duration of processes A and B decreased for the polymer brush system (Fig. 6 and Table S2). This dependency is also explained by the FKN mechanism, because processes A and B include BrO3- as a reactant. As mentioned above, the reaction rate of process B was affected by the immobilization of Ru(bpy)3 to the grafted polymer chain. However, the increase in [NaBrO3] resulted in the increase in the reaction rate of process B. Thus, at a higher [NaBrO3] (i.e., 400 mM), the B/Posc value became smaller, while the C/Posc value dramatically increased. For the polymer brush system, the durations of processes B and C continued to be larger at higher [NaBrO3], which can be attributed to the immobilization effect, resulting in lowered dependency on [NaBrO3].


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Figure 6. Typical oscillation waveforms for the polymer brush system under different initial concentrations of NaBrO3.

Initial concentrations of other substrate were fixed as follows:

[HNO3] = 0.8 M, and [MA] = 100 mM. Scale: 50 s.

In contrast to the [HNO3] dependency and [NaBrO3] dependency, the [MA] dependency for the polymer brush system was stronger than that for the free polymer system. Fig. 7 shows the waveforms for the polymer brush system obtained under different [MA]. As the [MA] increased for the polymer brush system, the duration of process C decreased according to the FKN mechanism. Further, it was found that the duration of process A also largely depended on [MA], which can be explained based on the mechanism of the BZ reaction as follows. As [MA] decreases, the reaction rate of the generation of Br- decreases, resulting in the slow degradation of HBrO2 according to the reaction (A2). Thus, the beginning of the consumption of Br-, followed by the oxidization of Ru(bpy)3,was delayed.

For the polymer brush system, the

oscillation period became dramatically longer at low [MA] than those for the free polymer and the gel particle systems, resulting in a stronger dependency for [MA]. Considering that the Ru(bpy)3 is densely introduced in the polymer brush, the reaction rate of process C would be lower than those in the free polymer and the gel particle due to the immobilization effect. As a result, both processes A and C elongate markedly in the polymer brush, since process A is


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related to process C as mentioned above. Thus, malonic acid was found to have the highest effect on the self-oscillating period for the polymer brush system among the substrates, which is different feature from other self-oscillating system.

Figure 7. Typical oscillation waveforms for the polymer brush system under different initial concentrations of MA. Initial concentrations of other substrate were fixed as follows: [HNO3] = 0.8 M and [NaBrO3] = 150 mM. Scale: 50 s.

In the investigations mentioned above, the effect of initial substrate concentrations was evaluated. However, the concentrations of HNO3 and NaBrO3 (i.e., ionic compounds) would also affect the swelling ratio of the grafted polymer chain. Thus, the concentrations of the BZ reaction substrates would affect the mechanical actuations of the grafted polymer. The attempts to clarify these effects are currently under consideration. In these ways, this investigation revealed the effect of the concentrations of reaction substrates on the self-oscillation behavior of the polymer brush, which was different from those in the free polymer and gel particle systems. These findings would be helpful for understanding the polymer brush as a novel medium for the BZ reaction and reaction-diffusion mechanism in the polymer brush surface.


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CONCLUSIONS In this study, the self-oscillating behavior of the polymer brush, which is a novel reaction medium for the BZ reaction, was investigated by comparing it with those of the free polymer and the gel particle under various initial substrate concentrations. The oscillating period changed according to the initial substrate concentration, while the dependency was expressed according to the power law. The [HNO3] dependency reduced in order, from free polymer, to the gel particle, and the polymer brush. For the [NaBrO3] dependency, the oscillating period for the polymer brush remained longer at higher [NaBrO3] compared with the free polymer and the gel particle systems. Malonic acid was found to have the strongest effect on the self-oscillating polymer brush system, because the oscillating period for the polymer brush became dramatically longer at low [MA]. From the waveform analysis based on the FKN mechanism, it is suggested that the reaction rate of the oxidization process became lower in the polymer brush system, which is the effect of immobilization of Ru(bpy)3 to the grafted polymer chain, i.e., the lower accessibility of the substrates to the catalyst. This tendency was greater for the polymer brush than that for the gel particle, which could be attributed to the difference in the structure of the gel and the polymer brush. With regard to the [MA] dependency, both processes A and C were affected, similar to the other self-oscillating systems, which could also be explained by the immobilization effect. Thus, it was revealed that the dense immobilization of the self-oscillating polymer on the surface restricted accessibility for Ru(bpy)3, resulting in a difference in the initial substrate concentration dependency. We envision that the polymer brush would play an important role in the study of oscillating chemical reaction as a novel reaction medium.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Preparation and characterization of the self-oscillating polymer brush surface, characterization of the gel particle, and the analysis of the BZ reaction. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Present Addresses (T.M.) Department of Life Science and Technology, Tokyo Institute of Technology, 4259 B-57, Nagatsuta, Yokohama 226-8501, Japan (R.T.) Department of Chemistry and Biochemistry, Yokohama National Univerity, 79-5 Tokiwaadi, Hodogaya-ku, Yokohama 240-8501, Japan (K.N.) Faculty of Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant No. 15H02198 to R.Y.), and the Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists (No. 14J09992 to T.M. and No. 14J02019 to R.T.).

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SYNOPSIS (Table of Contents).


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Aspects of the Belousov-Zhabotinsky Reaction inside the Self

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