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Electrochemical pH Oscillations of Ethyl Viologen/Ionic Liquid Su Ryon Shin,† Chang Kee Lee,† Sun I. Kim,† Insuk So,†,‡ Geoffrey M. Spinks,§ Gordon G. Wallace,§ and Seon Jeong Kim*,† Center for Bio-Artificial Muscle and Department of Biomedical Engineering, Hanyang UniVersity, Seoul 133-791, Korea, ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, UniVersity of Wollongong, Wollongong, New South Wales 2522, Australia, and Department of Physiology, Seoul National UniVersity, Seoul 110-744, Korea ReceiVed September 2, 2007. In Final Form: December 31, 2007 A reversible and robust electrochemical pH oscillator was achieved using an ethyl viologen/ionic liquid (IL) aqueous solution under an applied redox potential in a batch reactor, where the IL incorporated into the pH oscillator increased the stability of the pH oscillation by acting as an electron buffer solution.
Introduction Recently, several researchers have demonstrated synthetic chemical oscillators as being a method of providing a free-running chemical motor in artificial biological systems, such as DNAbased nanomachines1,2 and pH-sensitive hydrogels.3 Such pH chemical oscillators have been under investigation for some time and are inspired by spatiotemporal ordering processes in biological systems.4 The pH change is produced by a system composed of an oxidant and either one or two reductants, yielding a two- or three-component system in both quiescent and continuous-flow stirred tank reactors (CSTRs).5-7 However, while these systems are interesting as biomimetic models, their ability to function in biological environments is limited since they must be continuously supplied with toxic redox reactants, as these are irreversible reactions, and they generate accumulated waste products from the oscillating reaction.8 For application in biological systems, the challenge for synthetic pH chemical oscillators is the development of a reversible reaction that does not require a supply of fresh chemical reactants and does not create any waste product. Among the many oscillating materials that can be developed for improved synthetic pH chemical oscillators, viologen (1,1′disubstituted-4,4′-bipyridinium salts) is a good candidate due to its reversible redox behavior under a defined electrical potential.9 Viologen can be used to control the pH over a wide range in solution in a batch reactor,10 without the need for continuously supplied redox reactants and the production of waste products. To improve a viologen pH oscillatory system substantially, an * Corresponding author. Tel.: +82-2-2220-2321; fax: +82-2-2291-2320; e-mail:
[email protected]. † Hanyang University. ‡ Seoul National University. § University of Wollongong. (1) Liu, H.; Li, Y. X.; Yang, Y.; Wang, W.; Song, Y.; Liu, D. Angew. Chem., Int. Ed. 2007, 46, 2515. (2) Shu, W.; Liu, D.; Watari, M.; Riener, C. K.; Strunz, T.; Welland, M. E.; Balasubramanian, S.; Mckendry, R. A. J. Am. Chem. Soc. 2005, 127, 17054. (3) Howse, J. R.; Topham, P.; Crook, C. J.; Gleeson, A. J.; Bras, W.; Jones, R. A. L.; Ryan, A. J. Nano Lett. 2006, 6, 73. (4) Liedl, T.; Simmel, F. C. Nano Lett. 2005, 5, 1894. (5) Howse, J. R.; Topham, P.; Crook, C. J.; Gleeson, A. J.; Bras, W.; Jones, R. A. L.; Ryan, A. J. Nano Lett. 2006, 6, 73. (6) Ra´bai, G.; Orba´n, M.; Epstein, I. R. Acc. Chem. Res. 1990, 23, 258. (7) Ra´bai, G.; Hanazaki, I. Chem. Commun. (Cambridge, U.K.) 1999, 1965. (8) Kova´cs, K. M.; Ra´bai, G. Chem. Commun. (Cambridge, U.K.) 2002, 790. (9) Peon, J.; Tan, X.; Hoerner, J. D.; Xia, C.; Luk, Y. F.; Kohler, B. J. Phys. Chem. A 2001, 105, 5768. (10) Shinohara, S.; Tajima, N.; Yanagisawa, K. J. Intell. Mater. Syst. Struct. 1996, 7, 254.
ionic liquid (IL) is required in a water binary system. ILs have attractive properties, such as negligible vapor pressure, low toxicity, high chemical and thermal stability, and ability to dissolve a wide range of compounds.11 Additionally, large potential windows and high conductivities, which allow studies to be undertaken without added supporting electrolytes, are additional advantages for electrochemistry.12 The IL base on N,N′-dialkylsubstituted imidazolium cations has attracted increasing attention for electrochemical applications as was previously reported.12 In this paper, we describe a reversible electrochemical pH oscillator whose rhythmic pulsations were driven by the redox reaction of ethyl viologen (1,1′-diethyl-4,4′-bipyridinium dibromide, EV) under an electrical potential in a batch reactor. Experimental Procedures The viologen-driven electrochemical oscillation was carried out in a batch reactor that was a cylindrical-shaped glass vessel containing 15 mL of solution. All solutions were carefully degassed for 30 min by nitrogen gas purge. The reaction vessel was equipped with three electrodes, and a pH microelectrode (diameter: 0.5 mm) measured the pH inside the reactor in Figure 1. Electrochemical measurements on the solutions were performed using a three-electrode electrochemical cell employing cyclic voltammetry (CV) (CH Instruments Inc., model CHI627B). The potentiostat used a low-pass RC filter at 60 Hz. The three-electrode configuration consisted of a Pt mesh (0.07 mm × 13 mm × 15 mm (100 mesh/in.)) or glassy carbon (diameter: 3 mm) as the working electrode, an Ag/AgCl reference electrode, and a Pt wire (diameter: 0.5 mm) as the counter electrode. A Teflon-covered magnetic stirrer was used to ensure uniform mixing under a nitrogen atmosphere. In our pH measurements, the most important consideration was the attainment of anaerobic conditions because reduced viologen was quickly oxidized when in contact with air. The electrolyte was placed in a vessel, and the reaction was initiated by applying a redox potential. A constant pulse potential was applied for a set time, and then the potential was turned off. The pH was measured for a period of several seconds at room temperature. The pH was repeatedly measured for various set times. The pHtime data were collected manually using a pH meter. In the oxidation cycle, the applied potential was 0.9 V, and in the reduction cycle, the potential was -0.9 V. The potential waveform was pulsed as a function of time. Two supporting electrolyte solutions were used: one was a 0.1 M EV (Sigma Chemicals) aqueous solution (deionized 18 MΩ cm water with pH 7.06 from Milli-Q reagent water system, (11) Buzzeo, M. C.; Hardacre, C.; Compton, R. G. ChemPhysChem 2006, 7, 176. (12) Schro¨der, U.; Wadhawan, J. D.; Compton, R. G.; Marken, F.; Suarez, P. A. Z.; Consorti, C. S.; Souza, R. F.; Dupont, J. New J. Chem. 2000, 24, 1009.
10.1021/la7027093 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/20/2008
pH Oscillations of Ethyl Viologen/Ionic Liquid
Figure 1. (a) Schematic drawing of the electrochemical oscillator containing a solution of EV or EV/[C4min]BF4. (b) Structure of ethyl viologen and [C4min]BF4. Millipore), and the other was a 0.1 M EV/1-butyl-3-methylimidazolium tetrafluoroborate ([C4min]BF4, Solvent Innovation) aqueous solution, which was composed of various concentrations of [C4min]BF4. The lethal dose (LD50) for [C4min]BF4 is 300-500 mg/ kg for rat oral and >2000 mg/kg for Rat dermal.13
Results and Discussion To enable a wide variation in pH values, we measured the change in pH of three types of viologen: methyl viologen (MV), EV, and benzyl viologen (BV). Among these viologens, EV was determined to be the redox reactant that exhibited the widest pH range (pH 3-11) and most stable variation in pH.10 BV exhibited a narrow pH range (pH 2-4) due to the production of a blue precipitate under a reduction potential, and MV was deemed not to be suitable because of its toxicity.10 The 0.1 M EV aqueous solution used was weakly acidic at pH 6 before the redox potential was applied because EV2+Cl2- accepted an electron pair from the water molecules. Therefore, EV2+Br2- can act as an acid interacting with water. Figure 2a, which was obtained under optimized conditions, shows that a pH of ∼5 was obtained when a 0.1 M EV solution was continuously stirred in the reaction vessel, due to the EV redox reaction occurring on the electrode’s surface. The cyclic voltammograms of EV were obtained to elucidate the mechanism of the change in pH that was due to the redox (13) Chemada Fine Chemicals Ltd., www.aprilinternet.com/cat1/items/ BMITFB.pdf
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Figure 2. (a) Observed pH-time data using an applied step potential ((0.9 V) to a 0.1 M EV solution in the batch reactor. The initial pH of the solution was 5.2 before the step potential was applied. (b) Cyclic voltammograms of a 0.1 M EV aqueous solution during potential cycling (between +1.0 and -1.6 V vs Ag/AgCl, scan rate was 25 mV/s, first scan shown).
reaction (Figure 2b). The two reduction steps of EV are denoted by the CV peaks occurring at a potential of -0.65 and -0.92 V, and the two oxidation states are due to electron-transfer reactions from the Pt electrode. On applying a reduction potential, the colorless dication EV2+ solution was reduced to a blue solution of the monocation (EV•+) or a yellow solution of the neutral species (EV0) by single- or double-electron reduction, respectively, because EV2+ can accept either one or two electrons from the Pt electrode.12 The fully reduced EV0 accepts two hydrogen ions from water and two electrons from the Pt electrode or electrolyte. EV0 can produce a hydrogenated neutral form (EVH2, the yellow neutral species) and two hydroxide ions (-OH-, electrochemically inactive) in process III.14 Therefore, the pH of a 0.1 M EV solution can be increased up to 11. This increased pH solution can be reduced to the initial pH at ∼5 by applying an oxidation potential. However, the time taken to bring about this change in pH using an oxidation potential can take up to 340 s, which is a long time as compared to a reduction process, because EVH2 is insoluble in water, and this makes the reaction irreversible, and so it is difficult to return to the oxidized form.15 Moreover, EV0 is not stable in the presence of even small concentrations of oxygen. Consequently, a pH oscillator based on EV is not fully stable, and achieving a periodic pH oscillation is difficult. (14) Lieder, M.; Grzybjowski, W.; Schla¨pfer, C. W. Electroanalysis 1998, 10, 486. (15) Corbin, J. L.; Watt, G. D. Anal. Biochem. 1990, 186, 86.
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Figure 3. (a) Cyclic voltammograms of a 0.1 M EV/[C4min]BF4 solution ([C4min]BF4/H2O 4:6) with various [C4min]BF4 concentrations during potential cycling under a nitrogen atmosphere (between +1.0 and -1.2 V vs Ag/AgCl, scan rate was 25 mV/s, first scan shown). (b) Observed pH-time data during the applied step potential ((0.9 V) to a 0.1 M EV/[C4min]BF4 solution ([C4min]BF4/H2O 4:6) in the batch reactor. The initial pH was 1.5.
Thus, to construct a robust, stable, and readily reproducible redox reaction from EV in an aqueous solution, we introduced an IL, [C4min]BF4, that was water miscible. The problems of electrodeposition of EV0 and dimerization of EV•+ were removed using a [C4min]BF4-water binary system, similar to that proposed by Kim et al., who reported that one way to avoid the problems of using EV•+ was to use a H2O and DMF mixture.16 A [C4min]BF4-water binary system produces a more stable reduced EV form due to the low presence of oxygen in the electrolyte solution. Probably, [C4min]BF4 plays the role of an electron buffer in solution, while the EV redox reaction occurs because its cation consists of an imidazole group that has both a proton donor and a proton acceptor group within the same molecule. To confirm the activity as an electron buffer, we measured the CV characteristics of 0.1 M EV in a [C4min]BF4-water solution at various concentrations of [C4min]BF4, as shown in Figure 3a. In the CV curve, the reduction and oxidation peak potentials of [C4min]BF4 were observed at -0.41 and -0.3 V, respectively. That is, [C4min]BF4 was reduced before EV was and was oxidized after EV was in the EV/[C4min]BF4 aqueous solution. Therefore, these observations clearly indicate that the reduced EV form, which is known to be reactive toward dioxygen in aqueous (16) Kim, J. Y.; Lee, C.; Park, J. W. J. Electroanal. Chem. 2001, 504, 104.
Shin et al.
solution, was protected by the electron buffer properties of [C4min]BF4. This is attributed to the charged imidazolium group in [C4min]BF4, and the reduced EV form induces a facile proton exchange process. In addition, the water in the [C4min]BF4 mixture caused an increase in the voltammetric signals, which is indicative of a considerable change in the rate of diffusion.12 When comparing the data in Figure 2b with the data in Figure 3a, the oxidation reaction of EV in [C4min]BF4 occurred more rapidly than the oxidation of EV in water, which was attributed to the rapid change in the charge-transport rate due to [C4min]BF4.12 Consequently, these improved electrical properties suggest that an IL-water binary system can improve the performance of a pH oscillation due to a reversible redox reaction. The initial pH of the 0.1 M EV/[C4min]BF4 solution before the redox potential was applied was observed to be at acidic pH, which was pH 3.47, 2.8, 2.9, and 3 for 80, 60, 40, and 30% [C4min]BF4 content, respectively, in the IL-water binary system. In some cases, the IL was intentionally chosen to be less basic in water. The tetrafluoroborate anion [BF4-] is a weakly basic anion,17 indicating that under the appropriate conditions, it can be acidic. An imidazole group is a self-dissociating compound that has both a proton donor and a proton acceptor in the same molecule. This means that imidazole-based ILs not only dissociate prior to reacting with water but can donate another proton when reacted with water. Therefore, the IL-water binary system is an acidic solution, as described in a previous paper.18 As shown in Figure 3b, the pH of the 0.1 M EV/[C4min]BF4 solution increased under an oxidation potential (+0.9 V) from pH ∼2 to ∼4 and then decreased under a reduction potential (-0.9 V) to pH ∼2. However, on comparing the data in Figure 2a with the data in Figure 2b, the change in pH of a 0.1 M EV and a 0.1 M EV/[C4min]BF4 solution ([C4min]BF4/H2O 4:6) showed a reverse behavior under a redox potential of (0.9 V. In contrast to the pH behavior, the change in color of the EV/ [C4min]BF4 solution was identical to that of the EV aqueous solution under the same redox potential. When a reduction potential was applied, the EV0 species in the EV/[C4min]BF4 solution cannot accept two hydrogen ions from water. This is because [C4min]BF4 can react owing to the fact that the electron affinity of molecular [C4min]BF4 is higher than molecular water.19 The fully reduced EV may not produce the hydrogenated neutral form (i.e., the yellow product, EVH2), but instead, EV0 can form charge-transfer complexes with many electron-rich molecules as [C4min]BF4 consists of an imidazole group that has both a proton donor and a proton acceptor group within the same molecule. Generally, the cation (proton donor) and anion (proton acceptor) of the IL have an equimolar ratio and have electrostatic attraction between ions. However, the electrostatic attraction between ions is reduced by the accommodated water molecules in the water-rich region (water content >10%, mol fraction of the ionic IL (xs < 0.5)). In this state, when a reduced potential is applied (-0.9 V), the cation has accepted an electron from the Pt electrode to reduced EV, which is a very good electron-transfer mediator.19 Then, [BF4-] reaches an unconstrained state from the cation of the IL in the fully reduced EV/[C4min]BF4-water solution. The concentration of unconstrained [BF4-] gradually increased when [C4min]BF4 was reduced. [BF4-] is a typical weakly basic anion, which means that [BF4-] can be acidic. The fully reduced EV/[C4min]BF4-water solution can become more strongly acidic than the oxidized state of that solution. Therefore, (17) MacFarlane, D. R.; Forsyth, S. A. ACS Symp. Ser. 2003, 856, 264. (18) Spinks, G. M.; Lee, C. K.; Wallace, G. G.; Kim, S. I.; Kim, S. J. Langmuir 2006, 22, 9375. (19) Alvaro, M.; Aprile, C.; Atienzar, P.; Garcia, H. J. Phys. Chem. B 2005, 109, 7692.
pH Oscillations of Ethyl Viologen/Ionic Liquid
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Figure 4. Effect of [C4min]BF4 content on the pH oscillations. The observed pH oscillations of a 0.1 M EV/[C4min]BF4 solution with various [C4min]BF4/H2O ratios: (a) 4:6, (b) 6:4, and (c) 7:3 using a cycling applied step potential of (0.9 V in the batch reactor. (d) Shift in pH range using a base 0.1 M EV/[C4min]BF4 ([C4min]BF4/H2O 6:4) solution. The initial pH of 4.17 was adjusted by adding 0.1 M NaOH. The cycling efficiency (in %) was calculated using the following equation: cycling efficiency (%) ) [∆pHshrinking/(pHmax - pHmin)] × 100, where ∆pHshrinking, pHmax, and pHmin represent the change in pH during the final shrinking step and the maximum pH and the minimum pH for the reduction and oxidation states, respectively.
the pH of a 0.1 M EV/[C4min]BF4-water solution can decrease to pH ∼1.5. However, when almost all the [BF4-] dissociated from [C4min]BF4 in the water-rich phase ([C4min]BF4/H2O 3:7), no change in pH was observed, although the pH range of the corresponding EV/[C4min]BF4 aqueous solution was much less than the EV aqueous solution. The EV/[C4min]BF4 solution showed periodic and stable pH oscillations as compared to the 0.1 M EV solution. The effect of [C4min]BF4 on the operation of the oscillator was examined under three different conditions: [C4min]BF4/ H2O 7:3, 6:4, and 4:6. In Figure 4, the data show the oscillator containing EV/[C4min]BF4 with a periodic pH value between 1 and 3 over a time period of ∼15 min for various concentrations of [C4min]BF4. This amplitude pH oscillation was 3 times higher than that of a glucose-driven chemomechanical oscillator (periodic pH value between 4.35 and 4.9).20 The period length and cycling efficiency of this oscillatory reaction was dependent on the [C4min]BF4 content. The period length decreased with increasing water content because the excess water molecules reduced the electrostatic attraction between the ions of the ionic liquid, and the viscosity of the binary system decreased markedly.21 The decrease in viscosity supported an easier electron transfer in the oscillation reaction. Each successive cycle led to a loss in cycling efficiency, with the highest cycling efficiency (58.62%) being observed in an EV/[C4min]BF4 solution where [C4min]BF4/H2O 7:3 (see the fourth cycle in Figure 4c). This result suggests that the IL medium can improve the reproducibility of the reversible redox reaction of EV in an EV/[C4min]BF4 solution, which can (20) Dhanarajan, A. P.; Misra, G. P.; Siegel, R. A. J. Phys. Chem. A 2002, 106, 8835. (21) Seddon, K. R.; Stark, A.; Torres, M. J. Pure Appl. Chem. 2000, 72, 2275.
show decomposition on repetition of the reduction step. To confirm the change in the pH region, we changed the initial pH value of the EV/[C4min]BF4 solution to pH ∼4 by titrating with 0.1 M NaOH. Figure 4d shows the shift in the range of oscillations toward an alkaline direction and the decrease in the cycling efficiency as compared to the reactor in Figure 4b operating under the same conditions.
Conclusion A reversible and robust electrochemical pH oscillator based on EV was prepared using an IL. The improved reversible reaction was due to solving the problem of electrodeposition of EV on the electrode. This indicates that [C4min]BF4 incorporated into a pH oscillator can increase the stability of the pH oscillation by acting as an electron buffer for the EV redox reactions. As a result, an EV/[C4min]BF4-based pH oscillator can be constructed to exhibit a periodic change in pH under a redox potential, which would be helpful for the cyclical control of a proton fuel DNA nanomachine or a pH-sensitive hydrogel. Furthermore, because this system does not accumulate any waste products and shows a reversible reaction in a batch reactor, our new pH oscillator may be useful as an electrolyte for bioactuators and artificial muscle systems composed of hydrogels driven by external conditions, such as changes in pH, electrical field, temperature, or solvent. Acknowledgment. This work was supported by the Creative Research Initiative Center for Bio-Artificial Muscle of the Ministry of Science and Technology and the Korea Science and Engineering Foundation in Korea. LA7027093
