A Microreactor and Imaging Platform for Studying Chemical Oscillators

Publication Date (Web): July 2, 2013 ... The coupling intensity was controlled by changing the distance between the two oscillators. ... prepared by d...
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A Microreactor and Imaging Platform for Studying Chemical Oscillators Dameng Guo, Yuefang Li, and Bo Zheng* Department of Chemistry, The Chinese University of Hong Kong, Hong Kong S Supporting Information *

ABSTRACT: We present a laser scanning confocal microscopy (LSCM) and continuous flow microreactor (CFMR)-based platform to study the Belousov− Zhabotinsky (BZ) oscillators. We demonstrated that the scanning laser light below a certain power had no detectable influence on the BZ reaction. The CFMR consisted of the poly(methyl methacrylate) (PMMA) microwell and the polydimethylsiloxane (PDMS) microchannel and maintained the oscillation with a continuous supply of the catalyst-free BZ mixture. The synchronization of the two nonidentical oscillators was studied by the platform. The coupling intensity was controlled by changing the distance between the two oscillators. Results showed that the synchronization occurred as the oscillators were closer than a critical distance. The transition from desynchronization to synchronization was observed when the distance between the oscillators was near a critical value. The results of the numerical simulation by COMSOL agreed qualitatively with the experimental observation.

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micropatterned reactor to investigate the excitation waves.11 Toiya et al. formed droplets of BZ mixture in glass capillaries to limit the characteristic time of communication between coupled oscillators below the period of the oscillation.6 Recently, Galas et al. developed a micro continuous stirred tank reactor stirred by micro pneumatic pumps and studied the chemical oscillation system at the scale of nanoliter.22 In the work presented, we used scanning light microscopy to address the issue of photosensitivity of the BZ reactions. We developed a new platform for studying coupled chemical oscillators through the combination of laser scanning confocal microscopy (LSCM) with microfluidics. We were able to finetune the coupling intensity of the oscillators in the platform, and we studied the synchronization of the oscillators.

INTRODUCTION Coupled chemical oscillators present a rich variety of dynamical behavior and are important models for naturally occurring oscillating systems, for example, cardiomyocytes1 and neural networks.2 One of the most studied chemical oscillators is based on the Belousov−Zhabotinsky (BZ) reaction,3 which proceeds in bulk solution,4 gel,5 droplets,6 resin,7 or filter paper.8−10 To study coupled chemical oscillators, it is advantageous to reduce the oscillator size to submillimeter so that the number of the oscillators and the coupling by diffusion can be enhanced.6,11 The small size of the oscillator also presents challenges including the detection method and the reactor design. Up to now, ion-selective electrode (ISE)12 and optical imaging7,13 are two common detection methods to study the BZ reaction. ISE is appropriate for studying oscillators in bulk solution. However, the behavior and the interactions of the individual micro-oscillators are difficult to be detected by ISE. In optical imaging methods, either the absorption14 or the fluorescence15 change of individual oscillators is monitored. The absorption or fluorescence is usually caused by the metal catalysts in the BZ reactions, which are also photosensitive.16 The light irradiation often interferes with the oscillation by photochemical reaction.17 To minimize the irradiation effect, one can either reduce the light intensity, which in turn reduces the optical signal, or reduce the light exposure time, for example, by using pulsed light irradiation. The design and the operation of the reactor for studying coupled micro-oscillators are in the realm of microfluidics with many microfluidic designs and techniques readily available.18−20 Microfluidic technology provides precise control of the reactor structure and the fluidic flow in the reactor,21 and the absence of turbulence in microreactors benefits the experimental design and modeling. Ginn et al. first carried out the BZ reaction in the © 2013 American Chemical Society

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EXPERIMENTAL SECTION N-Isopropylacrylamide (NIPAm) from Kohjin was recrystallized three times from a benzene/hexane mixture before use. N,N-Methylenebis(acrylamide) (MBA) from Sigma was purified by recrystallization in methanol. Ammonium persulfate (ABS) from Beyotime, paraffin oil and tetramethylethylenediamine (TEMED) from Acros, sodium bromate (NaBrO3) from Aldrich, malonic acid (MA) from National Chemicals Import and Export, and nitric acid (63%) from BDH were used as received. Ruthenium(4-vinyl-4-methyl-2,2-bipyridine)bis(2,2bipy ridine)bis(hexafluo ropho sph at e) (Ru(vmbpy)(bpy)2(PF6)2) was provided by Prof. Guangzhao Zhang. The BZ mixture contained 84 mM malonic acid (MA), 62.5 mM NaBrO3, and 0.83 M HNO3. Received: March 27, 2013 Revised: June 30, 2013 Published: July 2, 2013 6402

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remained stable (Figure S5 of the SI). Accordingly, we set the reaction time zero as the moment when the flow rate was switched to 10 nL/min. Optimization of the Laser Power in the LSCM. Irradiation of visible light on the Ru2+/Ru3+ catalyst of the BZ reaction has been found to cause photoinhibition27 and photoinduction. 17,28 In our experiment, Ru(vmbpy)(bpy)2(PF6)2 has an absorption band around 450 nm and fluorescence emission around 610 nm.28 The 488 nm laser in the LSCM stimulated the fluorescence of Ru(vmbpy)(bpy)22+. No other chemical species in the BZ reaction were photoactive at 488 nm. The effect of the laser illumination on the chemical oscillators could be minimized through the following two ways. First, the scanning laser was used to reduce the illumination time on the oscillators. In the LSCM, one image was taken every 3 s, that is, the oscillators were illuminated by the laser once every 3 s. The scanning rate was 1.85 m/s; therefore, during the 3 s period, the illumination time of the laser beam on each spot of the oscillator was as short as 1.68 μs. Second, the power of the laser was optimized. The laser power was estimated to range from 1.2 W/cm2 to 168 W/cm2 (Figure 1

Laser scanning confocal system (Nikon C1Si) equipped with a 20 mW 488 nm laser was used to take the images of the chemical oscillators. The fluorescence intensity of the oscillators was calculated from the images by a Matlab program. The laser power was measured by a power meter (PM 100D, Thorlabs). Poly(N-isopropylacrylamide) (PNIPAm) gel particles doped with Ru(vmbpy)(bpy)2(PF6)2 were fabricated in a microfluidic chip (Figure S1 of the Supporting Information (SI)). One aqueous phase was prepared by dissolving 1 g NIPAm, 0.013 g MBA, and 0.021 g Ru(vmbpy)(bpy)2(PF6)2 in 8 mL H2O, and the other was prepared by dissolving 0.023 g APS in 2 mL H2O. The continuous phase was paraffin oil with 10% (w/w) Span 80. The droplets formed in the microfluidic chip were transferred into a polypropylene container which was filled with paraffin oil with 10% (w/w) Span 80 and 4% (v/v) TEMED. The polymerization was allowed to proceed in the container for 24 h. After rinsing by hexane with Span 80, hexane, isopropanol, methanol, and water with 0.1% (w/w) Triton 100, the particles in oil were transferred into water for storage. In the BZ mixture, the particles had a diameter of about 250 μm (Figure S2 of the SI). The gel particles used in the experiment of studying the critical distance for the synchronization were disposed after the observation at each distance. We expect all the gel particles produced in the same batch possess the same composition. The poly(methyl methacrylate) (PMMA) microwell was fabricated by hot embossing (Figure S4 of the SI). The polydimethylsiloxane (PDMS) mold with the complementary relief structure was first fabricated by soft lithography. The PDMS mold was baked at 270 °C to improve the mechanical strength and was silanized by fluorinated silane to prevent adhesion to the PMMA slab. The embossing was carried out at 180 °C for 10 min. After hot embossing, the PMMA slab with the microwell was postbaked at 65 °C for 3 h. The PDMS microchannel was fabricated by soft lithography.23 The flow in the microchannel was driven by a Harvard PHD 2000 pump.

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Figure 1. Influence of the laser power on the period of the oscillator in the CFMR.

RESULTS AND DISCUSSION Design of the Oscillators and the CFMR. PNIPAm gel particles were used as the oscillators. The gel particles were synthesized in a microfluidic device with the advantage of uniform particle size and well-controlled chemical composition (Figure S1 of the SI). During the synthesis, Ru(vmbpy)(bpy)2(PF6)2 was doped into the gel particles as the catalyst of the BZ reaction.5,24 The diameter of the gel particles was controlled to be around 300 μm in water and smaller than the wavelength of the chemical wave of the BZ reaction.25 We constructed a continuous flow microreactor (CFMR) to maintain the oscillations.26 The CFMR contained a 2 mm × 0.5 mm × 0.5 mm PMMA microwell and a PDMS microchannel (Figure S2 of the SI). The size of the microreactor was determined according to the oscillator-to-microreactor volume ratio (Figure S3 of the SI). The PDMS microchannel with 50 μm height and 50 μm width was used for introducing the catalyst-free BZ mixture. To assemble the CFMR, PNIPAm gel particles were deposited into the PMMA microwell first, and then the PMMA microwell and the PDMS microchannel were aligned and were clamped to prevent leaking. To start the oscillation reaction, the BZ mixture was injected into the reactor at a flow rate of 0.5 μL/min at which the oscillation was suppressed. The flow rate was then switched to 10 nL/min at which the period of the oscillators inside the microwell

and SI). The period of the oscillator remained unchanged at about 93 s when the laser power was below 56 W/cm2. When the laser power was 168 W/cm2, the period of the oscillator decreased to 84 s. Previously pulsed light irradiation was utilized to study the perturbation of the light to the BZ reaction. It was found that the perturbation was dependent on the pulse energy. For example, Agladze et al. found that the pulse energy of 130 mJ would cause a phase shift of the Ru(bpy)32+ catalyzed BZ reaction in a continuous flow stirred tank reactor (CSTR) with the cell volume being 9.6 mL and the concentration of Ru(bpy)32+ being 3 mM.29 Kaminaga and Hanazaki found that the photoenergy threshold was determined by the total pulse energy instead of by the peak power.30 In their study, the pulsed energy higher than 670 mJ would excite the oscillatory state of the Ru(bpy)32+ catalyzed BZ reaction in a CSTR with the illumination area being 6 cm2 and the concentration of Ru(bpy)32+ being 1.7 mM. In the current platform, the concentration of Ru(bpy)32+ in the gel particles was about 2 mM, which was derived from the concentration of Ru(bpy)32+ in the gel precursor solution. With the laser power of 10 W/ cm2, we obtained images with fluorescence signal strong enough for studying the chemical oscillators (Figure 2 and 6403

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Figure 2. Synchronization of the two oscillators in the CFMR. (A) The two nonidentical oscillators showed the different oscillation frequencies when the oscillators were deposited in the CFMR at the distance of 550 μm initially. (B) The synchronization occurred as the oscillators contacted with each other. (C) The synchronization was lost as the oscillators were separated by 670 μm.

Figure S7 of the SI). At 10 W/cm2 laser power, the photon energy input was calculated to be about 0.4 mJ/cm2 during each 3 s of the image taking period (SI). In the next experiment, we utilized the LSCM with the laser power of 10 W/cm2 to study the synchronization of a two-oscillator system. Synchronization of the Two Oscillators. Synchronization has been studied in many theoretical and experimental works. Several systems of coupled chemical oscillators were used to study synchronizations. Kiss and co-workers developed an electrochemical system in which the coupling of the arrays of electrochemical oscillators could be controlled by the potential drop on the oscillators.31,32 Kim et al. and Bertram and Mikhailov established a system based on Pt catalyzed oxidation of CO in which the synchronizations were controlled by the supply of CO on the surface of Pt oscillators.33,34 Another typical chemical oscillating system was based on the BZ reaction. For example, Toiya et al. produced the droplets in the glass capillary, and the synchronization of the droplet oscillators relied on the diffusion of Br2 and BrO2· in the oil phase.6 Taylor and co-workers used the oscillators made of cation-exchange microbeads to study the clustering and synchronization in populations of oscillators, and the coupling between the oscillators is due to the diffusion of species in the solutions.7,13 Miyakawa et al. investigated the coupling in the descrete BZ reaction system consisting of two microbeads impregnated with the catalyst. This work delineated the dependency of the coupling state on the distance and the natural frequencies of the oscillators.35 Fukuda et al. prepared an oscillator chain also made of microbeads impregnated with catalyst to study the entrainment in the oscillators, and the result revealed that the pacemaker entrained the oscillators when the distance between the oscillators was sufficiently small.36 In these works based on the BZ reaction, the oscillators were studied in a closed milliliter-scale reactor, which was vulnerable to the undesirable convective flow. In these works, the oscillators needed hours to reach stable oscillation state. The CFMR in the current work

addressed these issues by providing a stable open environment in which the oscillators quickly reached stable oscillation state after the induction period. To build the two-oscillator system, two PNIPAm gel particles were deposited into the CFMR. The two gel particles contained different concentrations of Ru(vmbpy)(bpy)2(PF6)2, 1 mM and 2 mM, respectively. The two nonidentical oscillators showed different natural frequencies: ω1 = 1/52 s−1 and ω2 = 1/84 s−1 when the oscillators were initially separated in the CFMR by 550 μm (Figure 2A). Once the two oscillators were placed to contact with each other, the frequency synchronization occurred, and both frequencies became 1/72 s−1 (Figure 2B). When the two oscillators were separated again by 670 μm, the two oscillators returned to the original state with the natural frequencies (Figure 2C). By varying the separation distance between the two oscillators and by monitoring the oscillations by LSCM, we found that a critical value for the synchronization of the two oscillators in our platform was between 60 and 80 μm (Figure 3). The oscillators at the distance of 60 μm reached synchronization at 2.5 h after the flow rate was set to 10 nL/ min, which could also be viewed from the plot of the intensity of oscillator 2 versus the intensity of oscillator 1 (Figure S8 of the SI ). The relatively long desynchronization-to-synchronization transition time observed suggested that the distance was near the critical value. The transition from desynchronization to synchronization was observed only at the distance ranging from 60 to 80 μm (Figures 3 and 4A−D). When the distance was shorter than 60 μm, the oscillators synchronized immediately after the induction period. At the distances of 65 and 70 μm, the two oscillators went through a transition process to reach synchronization as highlighted in Figure 4C and D. In the transition process, oscillator 1 had a half-amplitude oscillation with a shorter period, while both oscillators self-organized through the excitable media to change their oscillation periods and reach synchronization. Figure 4E showed that a remarkable 6404

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Figure 3. The oscillations of the two oscillators at the different distances. (A) The oscillation before synchronization at the distance of 60 μm. (B) The oscillation after synchronization at the distance of 60 μm. (C, D) The oscillators did not synchronize even after 6 h at the distance of 80 μm. (E) The plot of the oscillation period of the oscillators in A and B vs time. The synchronization emerged at the point 2.5 h marked by the dashed circle. (F) The plot of the oscillation period of the oscillators in C and D vs time.

dψ = KP(ψ ) + Δ dt

increase of the standard deviation of the periods occurred at the distance between 60 and 120 μm. The result indicated that the two oscillators were in a coupled state when the distance was 120 μm or shorter. However, the coupling strength was not strong enough to cause synchronization until the distance was shorter than 80 μm. The coupling of the two oscillators can be described by the Kuramoto model.37 The coupling was achieved through the exchange of chemical species between the two oscillators. Under the current experimental condition, the coupling intensity K can be introduced into the model when the two different oscillators are coupled as38 dαp1 dt dαp2 dt

= F1(αp1 , βp1 , γp1 , ...) − K ·(αp1 − αp2)

(1)

= F2(αp2 , βp2 , γp2 , ...) − K ·(αp2 − αp1)

(2)

(3)

Δ is the natural frequency difference of the two oscillators. P(Ψ) is an odd periodic function determined from the kinetic functions in eqs 1 and 2.38 If the oscillators synchronize, the right hand of eq 3 should be 0. For nonidentical oscillators with different natural frequencies, Δ ≠ 0. According to eq 3, when the oscillators synchronize, Δ/K ∈ (0, P(Ψ)max] for a positive K. Consequently, there exists a critical value Kc which equals Δ/P(Ψ)max above which the synchronization of the oscillators emerges. The exchange of chemical species between the two oscillators is by diffusion (Figure 5); therefore, the coupling intensity K is diffusion limited, and we expect that K is a monotonic function of D/L2, where L is the distance between the two oscillators and D is the diffusion coefficient of the corresponding chemical species. Consequently, Kc is associated with a critical distance Lc if all the other experimental conditions remain the same. COMSOL Simulation. The COMSOL software was employed to simulate the experimental results. We chose the model from Gao and Försterling’s work (SI).39 Two modules were utilized to set up a model: reaction engineering and transport of diluted species. Two circles representing the two oscillators were placed in a 2.0 mm × 0.5 mm reactor (Figure S6 of the SI). The rectangle represented the reactor. Ru2+

αp, βp, and γp represent the concentration of the chemical species, such as Br−, BrO2−, and so on in the oscillators. The subscripts 1 and 2 are the oscillator index. F(αp, βp, γp, ...) is the reaction kinetic term. Because BZ reaction is a periodic reaction, according to eqs 1 and 2, the phase difference Ψ(t) = Ψ2(t) − Ψ1(t) satisfies37,38 6405

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Figure 4. The occurrence of the synchronization of the two oscillators at the different distances. (A) At the distance of 20 μm, the oscillators synchronized once the oscillations began. (B) At the distance of 45 μm, the oscillators synchronized once oscillator 2 started oscillating. (C, D) The transition from desynchronization to synchronization was observed at the distance of 65 and 70 μm, respectively. The dashed rectangle highlights the transition process. (E) The plot of standard deviation (STDEV) of the periods of the two oscillators vs the distance. All the data in E were collected from 1 to 2 h after the flow rate was set to 10 nL/min.

simulation was found only when the distance between the oscillators was near the critical value (Figure 6C). The simulation showed no transition process at the distances below 250 μm (Figure 6D). During the transition process at the distance of 265 μm, the oscillation period of oscillator 1 decreased at the eighth cycle while the period of oscillator 2 increased to reach synchronization, which qualitatively agreed with the experimental observation.

Figure 5. Scheme of the oscillators in the CFMR.

existed only inside the oscillators. The initial Ru2+ concentrations in oscillator 1 and oscillator 2 were 1 mM and 2 mM, respectively. The BZ mixture had the same composition as that in the experiments. A concentration layer was introduced to simulate the continuous supply of the catalyst-free BZ mixture. The diffusion coefficients of all the species except Br− and H+ were set as 10−9 m2/s. The diffusion coefficients of Br− and H+ were 2.08 × 10−9 m2/s and 9.31 × 10−9 m2/s, respectively.40 The simulation was carried out in time-dependent mode. A Matlab program was used to calculate the average gray scale of the oscillators to indicate the concentration of Ru2+. The simulation result showed that there existed a critical value between 265 and 275 μm for the synchronization of the two oscillators (Figure 6A−C). The difference in the critical distance between the experimental and the simulation results is likely due to the nonoptimized parameters used in the simulation. Similar to the experimental results, the transition from desynchronization to synchronization of the oscillators in the

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CONCLUSIONS A microfluidic platform equipped with the LSCM for imaging and for studying the chemical oscillating system was established. In the LSCM, as each image was taken every 3 s, the laser with the power below 56 W/cm2 was found to have no detectable effect on the oscillation period in the CFMR. The synchronization of the two nonidentical oscillators was studied by the platform. The platform provided real-time fluorescence images of the oscillators without interfering with the chemical oscillating system, which allowed us to observe the synchronization in details. The platform also allowed control of the position of the chemical oscillators for fine-tuning of the coupling intensity. The critical distance for the synchronization of the two oscillators and the coupling distance were experimentally obtained, and the transition process was 6406

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Figure 6. Simulation results from COMSOL. (A) The oscillators with 1 mM (oscillator 1) and 2 mM (oscillator 2) Ru2+ synchronized at the distance of 265 μm. (B) The two oscillators desynchronized when the distance was 275 μm. (C) The transition from desynchronization to synchronization of the oscillators occurred at the distance of 265 μm. (D) No transition from desynchronization to synchronization occurred at the distance of 250 μm. (4) Marek, M.; Stuchl, I. Synchronization in Two Interacting Oscillatory Systems. Biophys. Chem. 1975, 3, 241−248. (5) Yoshida, R.; Takahashi, T.; Yamaguchi, T.; Ichijo, H. SelfOscillating Gel. J. Am. Chem. Soc. 1996, 118, 5134−5135. (6) Toiya, M.; Vanag, V. K.; Epstein, I. R. Diffusively Coupled Chemical Oscillators in a Microfluidic Assembly. Angew. Chem., Int. Ed. 2008, 47, 7753−7755. (7) Taylor, A. F.; Kapetanopoulos, P.; Whitaker, B. J.; Toth, R.; Bull, L.; Tinsley, M. R. Clusters and Switchers in Globally Coupled Photochemical Oscillators. Phys. Rev. Lett. 2008, 100, 214101. (8) Steinbock, O.; Kettunen, P.; Showalter, K. Chemical Wave Logic Gates. J. Phys. Chem. 1996, 100, 18970−18975. (9) Nakata, S.; Morishima, S.; Kitahata, H. Interactive Propagation of Photosensitive Chemical Waves on Two Circular Routes. J. Phys. Chem. A 2006, 110, 3633−3637. (10) Nakata, S.; Kashima, K.; Kitahata, H.; Mori, Y. Phase Wave between Two Oscillators in the Photosensitive Belousov-Zhabotinsky Reaction Depending on the Difference in the Illumination Time. J. Phys. Chem. A 2010, 114, 9124−9129. (11) Ginn, B. T.; Steinbock, B.; Kahveci, M.; Steinbock, O. Microfluidic Systems for the Belousov−Zhabotinsky Reaction. J. Phys. Chem. A 2004, 108, 1325−1332. (12) Reddy, M. K. R.; Szlavik, Z.; Nagyungvarai, Z.; Muller, S. C. Influence of Light on the Inorganic Part of the Ruthenium-Catalyzed Belousov−Zhabotinsky Reaction. J. Phys. Chem. 1995, 99, 15081− 15085. (13) Taylor, A. F.; Tinsley, M. R.; Wang, F.; Huang, Z. Y.; Showalter, K. Dynamical Quorum Sensing and Synchronization in Large Populations of Chemical Oscillators. Science 2009, 323, 614−617. (14) Gerdts, C. J.; Sharoyan, D. E.; Ismagilov, R. F. A Synthetic Reaction Network: Chemical Amplification Using Nonequilibrium Autocatalytic Reactions Coupled in Time. J. Am. Chem. Soc. 2004, 126, 6327−6331. (15) Bolletta, F.; Balzani, V. Oscillating Chemiluminescence from the Reduction of Bromate by Malonic Acid Catalyzed by Tris(2,2′Bipyridine)Ruthenium(Ii). J. Am. Chem. Soc. 1982, 104, 4250−4251. (16) Gaspar, V.; Bazsa, G.; Beck, M. T. The Influence of Visible-Light on the Belousov-Zhabotinskii Oscillating Reactions Applying Different Catalysts. Z. Phys. Chem. (Leipzig) 1983, 264, 43−48.

observed when the distance was near the critical distance. Numerical simulation by COMSOL was qualitatively consistent with the experimental observations. We expect the platform to be useful in studying the behavior of oscillator ensembles in microreactors.

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ASSOCIATED CONTENT

S Supporting Information *

Optimization of the CFMR, calculations of the laser power and energy, the equations of the simulation by COMSOL, and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 852 39436261; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Prof. Guangzhao Zhang and Dr. Fang Zhao from University of Science and Technology of China for providing Ru(vmbpy)(bpy)2(PF6)2. We thank The Chinese University of Hong Kong (Direct Grant 2060433) and the Research Grants Council of Hong Kong (404212) for financial support.

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