Sustained larger-amplitude self-oscillations induced by the BZ reaction involving Fe(phen)3 catalyst
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The poly(VP-co-AA-co-CD-co-Fe(phen)3) self-oscillating polymer gels composed of 5-acrylamide-1,10-phenanthroline bis(1,10-phenanthroline) iron(II) (Fe(phen)3), vinyl-pyrrolidone (VP), beta-cyclodextrin (β-CD) and acrylic acid (AA) were synthesized by radical polymerization. The volume, color and the redox potential self-oscillations were observed synchronizing with chemical wave propagation in acid-free conditions. The amplitude of diameter oscillation was about 40 µm to approximately 10% of its initial diameter and that of the redox potential about 125 mV at the optimal conditions. Moreover, the oscillation could sustain several hours. The long-lasting oscillation gels with large amplitude under mild conditions will be useful for the development of biomimetic materials that can undergo self-oscillation without external stimuli in mild conditions.
KeywordsSelf-oscillating gels Poly(VP-co-AA-co-CD-co-Fe(phen)3) polymer BZ oscillating chemical reaction
The first self-oscillating polymeric materials based on the Belousov–Zhabtinsky (BZ) reaction was reported in 1996 . It composed of poly(N-isopropylacrylamide) (PNIPAAm) and Ru(bpy)3 which acts as the metal catalyst for the BZ reaction. The BZ reaction, a well-known oscillatory chemical reaction accompanies a rhythmical oscillation of the redox potential of a metal catalyst and spatial patterns. Up to now, it is the most extensively studied oscillating chemical reaction [2, 3]. The self-oscillation and chemical wave observed in the BZ reaction has been discovered in living systems, such as glycolytic oscillations, biorhythms, heart muscle tachycardia, self-organization of ameba cells and intracellular calcium concentrations. In addition, the BZ reaction also provided a chemical and physical model for understanding several autonomous phenomena observed in biological systems [4, 5]. The driving force for self-oscillating of the BZ gels is provided by the changing of the redox potential of metal catalyst in the BZ reaction. When the gels which copolymerized functional monomer with Ru(bpy)3 is immersed into the BZ system free of a metal catalyst, the copolymerized Ru(bpy)3 acts as the metal catalyst as in the classical BZ system and the reaction occurs inside the gels. Therefore, a periodical oscillation of the redox potential for Ru(bpy)3 moiety generates due to the change from the oxidized state to the reduced state periodically. Due to the hydrophilicity of the polymer chain increasing with Ru(bpy)3 in the oxidized state for more positive charges and decreasing in the reduced state, the change in redox states of the polymerized catalyst moiety affects the hydrophilicity of the network and induces the changes in the volume phase transition temperature of the gel as well as the swelling ratio. As a result, the gel exhibits an autonomous volume oscillation with the redox oscillation in the closed solution under constant conditions.
To date, great progress has been made in self-oscillating materials and a variety of polymeric materials have been studied for bioengineering and engineering applications [6, 7, 8]. A new kind of self-oscillating gel actuator which exhibited various behaviors of peristaltic motions without external stimuli was demonstrated. The content of each component in the polymer network were gradient distribution and which led to a successive self-walking motion like a looper in the BZ system under constant temperature . The walking velocity of the gel was approximately 170 µm/min and it could be controlled by the concentration of substrates in the BZ solution. A pendulum motion of the gel also was observed by fixing one edge . A periodic reciprocating motion was obtained when the Ru-catalyst in the gel was rapidly oxidized and slowly reduced . In addition, an object could be transported autonomously by utilizing the peristaltic motion and chemical wave propagation coupling with the diffusion of intermediates in the gels [12, 13, 14]. In fact, the self-oscillating gel has been investigated as an unprecedented biomimetic material in the past 10 years [15, 16, 17, 18]. Other works related to the oscillating chemical reactions coupling with polymers were performed. Pojman et al. demonstrated that oscillating chemical reactions might couple to polymerization processes by exploiting the oscillatory production of malony radicals in the cerium catalyzed BZ system [19, 20]. Also, theoretical and computational approaches were used to simulate the motions of the gels and provide the necessary guidelines for assembling the components into the appropriate pattern to yield the specified function [21, 22].
More recently, there were many excellent works related to self-oscillating gels providing novel concepts of biomimetic materials. For example, an autonomous functional surface composed of a self-oscillating polymer brush , an autonomous cycling artificial vesicles with synthetic block copolymers formation and fragmentation self-oscillating , a cross-linked polymersomes exhibiting self-beating motion like biological cell membranes without any external stimuli to trigger on–off switching  and an artificial cells by synthetic materials  were demonstrated. These self-oscillating polymer gels are expected to apply as novel biomimetic materials in a number of aspects including auto-mobile actuators and beating micropumps. Moreover, these achievements are also very important from the perspective of understanding the universal rhythms phenomena that sustain life and occur at all levels in physiological tissue.
Several works involving the volume oscillations of self-oscillating gels also were performed. However, the amplitudes aren’t satisfactory. Arimura et al.  obtained the poly(NIPAAm-co- Fe(phen)3) gel exhibiting 7% swelling/deswelling variation in the gel length. The amplitude of diameter oscillation poly(VP-co-AA-co-Fe(phen)3) gels was about 20 µm to approximately 5% of its initial diameter . In this study, we tried to construct a novel self-oscillating gel with biocompatible monomers. Therefore, vinyl-pyrrolidone (VP), beta-cyclodextrin (β-CD), acrylic acid (AA), and 5-acrylamide-1,10-phenanthroline bis(1,10-phenanthroline) iron(II) (Fe(phen)3) were used to prepare the crosslinked gel. Except for the unique biologic properties, the β-CD segments efficiently regulate the diffusion rate of the BZ substrates. And the AA segments can control the swelling process and provide H+ for the BZ reaction. The influence of the immobilized β-CD content on the period and the amplitude of the self-oscillation were investigated. The results indicated that the amplitude of both redox potential and volume oscillations were larger than the reported self-oscillating gels. Therefore, the long-lasting oscillation gels with large amplitude under mild conditions will be useful for the development of biomimetic materials.
Vinylpyrrolidone (VP) and acrylic acid (AA) were brought from Tianjin Damao Chemical Reagent Factory and used after purification. β-Cyclodextrin (β-CD) was used as received. The cross-linker N,N-methylene bis-acrylamide (MBA) and the initiator ammonium persulfate (APS) were brought from Shanghai Zhongqin Chemical Reagent Ltd. Malonic acid (MA), KBrO3, H2SO4, Ce2(SO4)3, Ce(SO4)2 and 1,10-phenanthroline were analytical reagents and used as received. 5-acrylamide-1,10-phenanthroline bis(1,10-phenanthroline) iron(II) (Fe(phen)3) was synthesized by ourselves. The distilled water was used throughout the experiments.
2.2 Preparation of poly(VP-co-AA-co-CD-co-Fe(phen)3) gel
The amounts of reactants in each prepared gel
SEM micrograph: The gels were freeze-dried before covered with a conductive coating. The SEM micrograph was obtained on a JSM-6701F scanning electron microscope(JEOL, Japan) and the surface morphology was observed at 20 kV.
Volume self-oscillation measurement: A cylindrical poly(VP-co-AA-co-CD-co-Fe(phen)3) gel was put on a slide and a drop of BZ solution (containing 0.50 mol L−1 MA and 0.15 mol L−1 KBrO3) was added to observe the volume and color self-oscillation and the chemical wave propagation. Time-lapse imaging at room temperature was recorded on a stereomicroscope (Olympus, BX53).
Potential self-oscillation measurement: The experiments were performed in a glass reactor (ca. 50 ml) coupled with an SY-601 thermostat and a Model ML-902 magnetic stirrer. The prepared gels were cut into cuboid (length: 30 mm, width: 20 mm, thickness: 2 mm) and immersed in BZ system as described above at 20 °C. A CHI-832 electrochemistry analyzer (Shanghai Chenhua Instrument Company, China) was connected directly to the reactor through a Pt-electrode (Rex, 213, China) as the working electrode and an Hg2SO4 reference electrode to record the potential change.
3 Results and discussion
3.1 SEM images analysis
3.2 Equilibrium swelling ratio
The equilibrium swelling ratio for the gels in different solutions
3.3 Self-oscillating behavior
In conclusion, a novel BZ type self-oscillating gels poly(VP-co-AA-co-CD-co-Fe(phen)3) was prepared successfully with Fe(phen)3 as the catalyst of the BZ reaction. The volume, color, and the redox potential self-oscillation were observed synchronizing with chemical wave propagation in acid-free conditions. The amplitude of diameter oscillation was about 40 µm to approximately 10% of its initial diameter and that of the redox potential about 125 mV at the optimal conditions and the oscillation could sustain several hours. Therefore, this self-oscillation gels would have wider application prospect as a novel biomimetic materials and further studies on control of oscillating behavior as well as practical applications are expected.
This work was supported in part by the Basic Project of Science and Research of Colleges and Universities of Gansu Province (5001-109), the Project for Young Teacher of Northwest Normal University (NWNU-LKQN-13-6).
Compliance with ethical standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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