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Self-Motion of a Benzoquinone Disk Coupled with a Redox Reaction Nobuhiko J. Suematsu, Yumi Miyahara, Yui Matsuda, and Satoshi Nakata* Department of Mathematical and Life Sciences, Graduate School of Science, Hiroshima UniVersity, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan ReceiVed: May 21, 2010; ReVised Manuscript ReceiVed: June 25, 2010
Self-motion is one of the most promising systems for achieving mechanical function in an inanimate system. In this Article, a change in the mode of self-motion coupled with a redox reaction is reported as a novel autonomous system. A solid disk of benzoquinone (BQ, oxidant) spontaneously moved on a solution of reductant (ascorbic acid or potassium ferrocyanide K4[Fe(CN)6]), and the mode of self-motion, that is, continuous motion f intermittent motion (repetition between rest and rapid motion) f velocity-decay mode, changed depending on the concentration of the reductant. The concentration region for the motion of K4[Fe(CN)6] was broader than that for ascorbic acid. The characteristic features of motion are discussed in relation to surface tension as the driving force and the reaction kinetics of BQ. The present BQ system can be expanded to be controlled by various external fields, such as in electrochemical, photochemical, and biochemical reaction systems. Introduction Autonomous motors may be useful for realizing complex tasks, which can be used for the transport and manipulation of matter on a small scale.1 Analogous to the strategy of biomotors,2 the conversion of chemical energy to a desired mechanical function has been demonstrated in an inanimate system. This self-motion is driven by a dipolar vortex3,4 or difference in interfacial tension working around the solid or the droplet.5-15 These systems commonly change the conditions of their surroundings, that is, surface tension or interfacial tension, through the local concentration of surfactants, and thus the mode of self-motion can be changed through coupling with a chemical reaction around a solid/liquid interface.3-5 We have previously reported self-motion systems coupled with reactions involving neutralization and complex formation, and we observed a modechange of self-motion.5,6 Nevertheless, further studies will be needed to develop a system of self-motion that can be controlled by artificially changing the external conditions. Benzoquinone (BQ) and its derivatives have been investigated in various systems, for example, BQ-hydroquinone (HQ) redox reaction, and electrochemical,16 photochemical,17 and biochemical systems.18 Such systems with BQ or its derivatives suggest that it may be possible to create a novel autonomous system that is controlled electrochemically or photochemically or is coupled with a biochemical reaction, such as a redox cycling system. In this Article, we propose the self-motion of a BQ disk coupled with a redox reaction. Experimental Methods 1,4-Benzoquinone (BQ), hydroquinone (HQ), quinhydrone, acid, potassium ferrocyanide (K4[Fe(CN)6]), and potassium ferricyanide (K3[Fe(CN)6]) were purchased from Wako Chemicals (Kyoto, Japan). Water was first filtered through active carbon and an ion-exchange resin, and then distilled. A powder of (BQ) was first ground in a mortar and then shaped
into a disk (diameter, 3 mm; thickness, 1 mm) using a pellet die set for FTIR. The disk of BQ was floated on a reductant solution at room temperature (25 ( 1 °C). Aqueous solutions of L-ascorbic acid or K4[Fe(CN)6] (concentration, 0-100 mM; volume, 20 mL) were poured into a glass Petri dish (diameter: 90 mm). The movement of the BQ disk was monitored with a digital video camera (SONY DCR-VX700, maximum frame rate: 30 fps) and then analyzed on a PC using image-analysis software “ImageJ 1.41” (National Institutes of Health, U.S.). Three or four examinations (with about 10 min for each examination) were performed for the individual experimental conditions. The surface tension on the aqueous solutions of reductant, BQ, and HQ was measured by the standard Wilhelmy method (surface tensiometer CBVP-A3, Kyowa Interface Science Co., Japan). The concentration region of reductant solutions was 0-50 mM. Aqueous solutions of BQ and HQ were prepared from 0 to 110 mM, which was nearly equal to the saturating concentration of BQ. The concentrations of ascorbic acid, BQ, and HQ were measured by UV/vis spectroscopy (UV-1650PC, Shimadzu Co., Japan). To measure the time-variation of absorbance, we prepared five samples by individual extraction from five aqueous phases where the disk moved for different elapsed times (0, 5, 10, 20, and 30 min). Infrared spectra of BQ, HQ, quinhydrone, and the black material deposited on the BQ disk in velocitydecay mode (to be hereinafter described) were recorded at room temperature on a FT-IR spectrometer (NICOLET 6700, Thermo Electron Co.). Solid samples were ground in a mortar and then mixed with KBr and pressed into a disk (10 mm in diameter). The heat of redox reaction was measured by thermography (IR FlexCam Thermoal Imager Ti45, Fluke Co., U.S.).
L-ascorbic
* To whom correspondence should be addressed. Tel./fax: +81-82-4247409. E-mail:
[email protected].
Results A disk of BQ was floated on a reductant solution (ascorbic acid or K4[Fe(CN)6]). As a result, three types of self-motion, that is, continuous motion (Figure 1a), intermittent motion (repetition between rest and rapid motion, Figure 1b), and velocity-decay mode (Figure 1c), were observed on an ascorbic
10.1021/jp104666b 2010 American Chemical Society Published on Web 07/15/2010
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Figure 1. Time-series of the velocity of a BQ disk on an aqueous solution of ascorbic acid with concentrations of (a) 10 mM (continuous motion), (b) 20 mM (intermittent motion), and (c) 30 mM (velocitydecay mode). The insets show snapshots of the self-motion of BQ. Time intervals are (a,b) 0.2 s and (c) 5.0 s.
acid solution depending on the concentration. Continuous and intermittent motions were maintained for over 300 s. On the other hand, in velocity-decay mode, the disk slowed monotonically with time and stopped within 300 s after the disk was floated on the solution (Figure 1c). In contrast, only continuous and intermittent motions were observed on an aqueous solution of K4[Fe(CN)6], and the velocity-decay mode was not observed in the concentration region used in our experiments (0-100 mM). On an ascorbic acid solution, continuous and intermittent motions were observed in concentration ranges of 0-10 and 15-25 mM, respectively, and the velocity-decay mode was observed over 30 mM (Figure 2a). The concentration regions of both continuous and intermittent motions broadened on the K4[Fe(CN)6] solution (Figure 2b). The uniform velocity of the disk in continuous motion on pure water was 17.6 ( 4.1 mm s-1, and this decreased with an increase in the reductant concentration (Figure 2a,b). The maximum velocity of intermittent motion decreased with the concentration of ascorbic acid (Figure 2a), but increased with the concentration of K4[Fe(CN)6] up to 40 mM and reached a constant value over 40 mM. Moreover, while the period of intermittent motion was not very sensitive to the concentration of ascorbic acid, it increased with the concentration of K4[Fe(CN)6] (Figure 2c). The surface tension of aqueous solutions was measured by varying the concentrations to evaluate the driving force of selfmotion, as indicated in Figure 3. The surface tension decreased with BQ and HQ, did not change with ascorbic acid, and increased with K4[Fe(CN)6] and K3[Fe(CN)6] solutions. The greatest change in surface tension was observed with BQ solution. To evaluate the progress of the redox reaction, the timevariation of the UV/vis spectrum was measured. Figure 4a indicates the UV/vis spectra of the reductant solutions after floating the BQ disk for 0-30 min. Here, the absorbance of
Figure 2. Average velocities in continuous motion (b) and maximum and minimum velocities of intermittent motion (0) depending on theconcentrations of (a) ascorbic acid and (b) K4[Fe(CN)6]. The velocity-decay mode is plotted as cross-marks when the velocity is 0 mm s-1. (c) The periods of intermittent motion were plotted against the concentrations of ascorbic acid (O) and K4[Fe(CN)6] (9).
Figure 3. Surface tensions of aqueous solutions of ascorbic acid, K4[Fe(CN)6], K3[Fe(CN)6], BQ, and HQ depending on the concentrations.
ascorbic acid (or HQ) was quite larger than that of HQ (or ascorbic acid) at 270 nm (or 300 nm) (see the Supporting Information). The absorbances of BQ (50 µM) at 245 (λmax), 270, and 300 nm were 0.63, 0.01, and 0.01, respectively (see Figure S1). In contrast, the absorbances of the sample (10 min) were 0.68, 0.76, and 0.07. Therefore, the influence of BQ on the absorbance at 270 and 300 nm was significantly small, and thus the concentrations of ascorbic acid and HQ could be calculated from the absorbance. The concentration of HQ increased from zero to 50 µM, and that of ascorbic acid decreased from 70 to 40 µM (Figure 4b), and thus the redox
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Figure 4. Time-series of (a) UV/vis spectra and (b) concentration of ascorbic acid and HQ during self-motion of a BQ disk on an ascorbic acid solution.
Figure 5. IR spectra of BQ, HQ, quinhydrone (QH), and the sample induced on the surface of the BQ disk.
reaction progressed on the aqueous surface. The redox reaction locally occurred around the BQ disk, and, as a result, the concentration change was very small. The time-series measurement indicated that the BQ molecules that developed on the surface were reduced by the ascorbic acid. The heat of redox reaction during self-motion was observed by thermography. No thermal change was observed around the BQ disk floating on the reductant solution (see Figure S2). Therefore, it is not necessary to consider the effect of the heat of redox reaction on the gradient of surface tension. For velocity-decay mode, the color of the bottom of the BQ disk changed from yellow to black after the disk stopped. However, we confirmed that this black product was not observed after other modes (continuous and intermittent motion). The adsorbed substance was identified using FT-IR. The IR spectrum of the black product corresponded with that of quinhydrone where the CdO band (1630 cm-1) and the C-C bands (ν19a, 1517 cm-1; ν19b, 1473 cm-1)21,22 were observed (Figure 5). Discussion The driving force of self-motion originates from the difference in surface tension around the BQ disk. Here, the anisotropic
Suematsu et al. distribution of the BQ molecular layer from the disk is caused by unidirectional motion due to the initial floating condition.19 The maximum value of the driving force can be estimated under the ideal condition that the surface concentration of BQ is 0 mM in front of the disk and is saturated at the back. In this condition, the difference in surface tension is 4 mN m-1, from which the driving force can be estimated to be 1.8 × 10-2 mN with a contact length of 4.7 mm. Because the driving force needed to achieve 20 mm s-1 (uniform velocity in continuous motion) is calculated to be 1 × 10-4 mN under a viscosity of 1 × 10-3 Pa s, the nonuniformity of surface tension can generate a driving force large enough to accelerate the BQ disk. The decrease in the driving force for the actual system is due to the isotropy of the disk, physical resistance of the disk on water, and the degree of the redox reaction, but these problems can be overcome in future studies. We propose a scenario to explain the three types of selfmotion based on the redox reaction of BQ and ascorbic acid. For continuous motion, nonreactive BQ molecules are constantly developed to the water surface due to the low concentration of ascorbic acid, and therefore the disk can move continuously (Figure 6a). For intermittent motion, most BQ molecules are reduced to HQ, which is surface-inactive, as shown in Figure 3, and the disk cannot achieve a driving force large enough to move (resting state) because of the significantly high concentration of ascorbic acid needed to reduce BQ, as shown in Figure 6b. The redox reaction and slow diffusion of the reactant molecules in the bulk induce the depletion of ascorbic acid around the disk, and the surface concentration of BQ molecules increases again. Therefore, the disk can be driven (rapid motion). After the motion of the disk, the redox reaction with ascorbic acid starts again, and the disk loses its driving force (resting state). These processes are repeated in intermittent motion, as illustrated in Figure 6b. The period of intermittent motion is almost equal to the resting time that is determined by the reaction rate and diffusion constant of reductant. Therefore, repetition of resting and rapid motion occurs with almost constant period. Next, we consider the velocity-decay mode. The viscosity of aqueous phase was almost constant independent of the concentration of reductant (data not shown). To explain the velocitydecay mode, we have to consider the formation of BQ-HQ complexes (quinhydrone).20 HQ, reacts with BQ at the surface of the disk. Because the quinhydrone is difficult to dissolve, the disk is covered with quinhydrone, and it becomes difficult to develop BQ molecules. Thus, the disk loses its driving force and stops (Figure 6c). In intermittent motion, the formation of the quinhydrone could be ignored, because the concentration of HQ is too low to form the complex. In fact, no black product of quinhydrone was observed on the surface of BQ after intermittent motion. As shown in Figure 2a,b, the chemical species of the reductant affects the mode of self-motion. Based on the mechanism of each mode, the critical concentration between continuous motion and intermittent motion, Cc-i, might be determined by the balance between the rate of development of BQ and the kinetics of the redox reaction, which depend on both the chemical species and its concentration. Thus, the difference in Cc-i between ascorbic acid and K4[Fe(CN)6] might originate from differences in the stoichiometry and reaction kinetics. In contrast to the results with ascorbic acid, the lack of velocity-decay mode in K4[Fe(CN)6] solution may be caused by the reversibility of the redox reaction of K4[Fe(CN)6]. Because the reverse reaction may inhibit an increase in the concentration of HQ, it is difficult to
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Figure 6. Schematic illustration of the mechanism of self-motion of BQ on an aqueous solution of ascorbic acid: (a) continuous motion, (b) intermittent motion, and (c) velocity-decay mode.
produce quinhydrone. In fact, quinhydrone was not observed on the surface of the disk after the examinations. The positive correlation between the period of intermittent motion and the concentration of K4[Fe(CN)6] shown in Figure 2c can be explained on the basis of the mechanism described above. As shown in Figure 1b, the period is determined mainly by the resting time because rapid motion is a short-term phenomenon, and the duration of the resting state depends on the amount of reductant molecules around the disk, which have to be consumed for the transition to rapid motion (Figure 6b). Therefore, the period increases with the concentration of K4[Fe(CN)6]. With the addition of ascorbic acid, it is difficult to clarify the relationship between the period and the concentration because the concentration range of intermittent motion is too narrow (Figure 2c). The sizes of BQ disk and container are found to influence the mode of self-motion, although these parameters are kept constant in this Article. If the BQ disk was large, the velocitydecay mode might be observed at the lower concentration, because the rapid development rate of BQ induced an amount of HQ. This size effect is also one topic of future plan as is an electro- or photochemical control of the mode of self-motion. Conclusion We have proposed a novel self-motion system coupled with a redox reaction. The self-motion of a BQ disk depended on the species of the reductant and its concentration in an aqueous phase. Continuous motion and intermittent motion can be explained by a mechanism analogous to a previous system.5 In addition, velocity-decay mode was observed at a high concentration of ascorbic acid, which reacted with BQ irreversibly. In this novel mode, poorly-soluble quinhydrone was deposited on the BQ disk and prevented self-motion, and as a result the velocity of self-motion decreased with time. This report is only the first step in describing the self-motion of BQ and only introduces the mode-change of self-motion of a BQ disk. Because BQ is related to electrochemical, photochemical, and biochemical reactions, our system may be able to evolve into a novel self-motion system that is sensitive to the environmental conditions. Acknowledgment. We thank Prof. T. Amemiya (Yokohama National University, Japan) for his kind comments on the
properties of BQ, and Prof. Y. Katsumoto (Hiroshima University) for his technical support of the measurement of FT-IR. This work was supported in part by Grants-in-Aid for Scientific Research (no. 20550124) and by the Meiji University Global COE Program “Formation and Development of Mathematical Sciences Based on Modeling and Analysis”. Supporting Information Available: UV/vis spectra of ascorbic acid, BQ, and HQ solutions, and images observed by thermography. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Paxton, W. F.; Sundararajan, S.; Mallouk, T. E.; Sen, A. Angew. Chem., Int. Ed. 2006, 45, 5420–5429. (2) Schmidt, J. J.; Montemagno, C. D. Annu. ReV. Mater. Res. 2004, 34, 315–337. (3) Kitahata, H.; Aihara, R.; Magome, N.; Yoshikawa, K. J. Chem. Phys. 2002, 116, 5666–5672. (4) Hanczyc, M. M.; Toyota, T.; Ikegami, T.; Packard, N.; Sugawara, T. J. Am. Chem. Soc. 2007, 129, 9386–9391. (5) Nakata, S.; Arima, Y. Colloids Surf., A 2008, 324, 222–227. (6) Nagayama, M.; Yadome, M.; Murakami, M.; Kato, N.; Kirisaka, J.; Nakata, S. Phys. Chem. Chem. Phys. 2009, 11, 1085–1090. (7) Bush, J. W. M.; Hu, D. L. Annu. ReV. Fluid Mech. 2006, 38, 339– 369. (8) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Science 2000, 288, 1624– 1541. (9) Que´re´, D.; Ajdari, A. Nat. Mater. 2006, 5, 429–430. (10) Sumino, Y.; Magome, N.; Hamada, T.; Yoshikawa, K. Phys. ReV. Lett. 2005, 94, 068301. (11) Nagai, K.; Sumino, Y.; Kitahata, H.; Yoshikawa, K. Phys. ReV. E 2005, 71, 065301(R). (12) Schulz, O.; Markus, M. J. Phys. Chem. B 2007, 111, 8175–8178. (13) Soh, S.; Bishop, J. M. K.; Grzybowski, B. A. J. Phys. Chem. B 2008, 112, 10848–10853. (14) Bassik, N.; Abebe, B. T.; Gracial, D. H. Langmuir 2008, 24, 12158– 12163. (15) Okawa, D.; Pastine, S. J.; Zettl, A.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2009, 131, 5396–5398. (16) Rafiee, M.; Nematollahi, D. Electroanalysis 2007, 19, 1382–1386. (17) Go¨rner, H. J. Phys. Chem. A 2003, 107, 11587–11595. (18) Uchiyama, S.; Hasebe, Y.; Shimizu, H.; Ishihara, H. Anal. Chim. Acta 1993, 276, 341–345. (19) Nagayama, M.; Nakata, S.; Doi, Y.; Hayashima, Y. Physica D 2004, 194, 151–165. (20) Li, N.; Wang, J. J. Phys. Chem. A 2008, 112, 6281–6284. (21) Kanesaka, I.; Nagami, H.; Kobayashi, K.; Ohno, K. Bull. Chem. Soc. Jpn. 2006, 79, 406–412. (22) Kubinyi, M.; Keresztury, G. Spectrochim. Acta 1989, 45A, 421–429.
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