Mode Switching of a Self-Propelled Camphor Disk Sensitive to the

Article pubs.acs.org/Langmuir

Mode Switching of a Self-Propelled Camphor Disk Sensitive to the Photoisomerization of a Molecular Layer on Water Satoshi Nakata,* Tatsuya Miyaji, Yui Matsuda, Miyu Yoshii, and Manabu Abe Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan S Supporting Information *

ABSTRACT: A simple self-propelled motor on a 4-[[(dodecyloxy)benz-4-yl]azo]benzoic acid (DBA) molecular layer was investigated from the viewpoint of motor control depending on the molecular structure. The nature of the self-motion of a camphor disk on the DBA molecular layer changed depending on the surface pressure (π)−area (A) isotherm of DBA, which in turn changed by the photoisomerization between UV and green lights. The characteristic motion of the camphor disk is discussed in relation to the π−A isotherm of DBA, which changes depending on the photoisomerization as the driving force of motion.

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INTRODUCTION The creation of self-propelled motors, which can transport and manipulate matter or themselves on a small scale while sensing and adapting to the physical and chemical environments, is an important challenge in industrial and medical fields.1 All motor organs or organelles in living organisms, such as a bacterial flagellar motor,2 change the features of their motion depending on the external environment, such as in taxis. Therefore, mimicking living organisms is an important strategy for overcoming this challenge. Although several artificial autonomous motors have been studied under almost isothermal and chemical-nonequilibrium conditions as in living organisms,3−12 the characteristic features of motion depending on molecular properties have not yet been investigated. Several investigations have examined self-motion controlled by irradiation with light as a noncontact controllable system.13−17 In these systems, the driving force is the gradient of surface tension, which is induced by laser irradiation,13 photoisomerization,14−16 or a redox reaction17 under irradiation with UV or visible light. The ability to control the speed and direction of motion is an advantage of these systems. On the other hand, spontaneous changes in features of motion, such as mode switching in response to changes in the environment, are also useful, such as in taxis in bacterial motion. Thus, the demonstration of “phototactic” self-motion in an artificial system would be an important contribution to this field. © 2014 American Chemical Society

We previously investigated the mode switching of selfmotion for a camphor system18,19 that depended on the internal conditions (e.g., the scraping morphology and the chemical structure of camphor derivatives),20 external conditions (e.g., surface tension, chemical stimuli, shape of the water chamber, addition of a molecular layer, and coupling),21,22 and a chemical reaction23 as a novel self-propelled motor that is driven by a difference in surface tension. In addition, the essential features of self-motion have been qualitatively reproduced by numerical calculations.20−23 In the present study, we introduce a novel type of selfmotion that depends on the surface pressure (π)−area (A) isotherm of a 4-[[(dodecyloxy)benz-4-yl]azo]benzoic acid (DBA) monolayer which exhibits photoisomerization.24−30 When a camphor disk was floated on a molecular layer of DBA, the features of motion changed characteristically depending on the π−A isotherm. This characteristic change in self-motion and the driving force are discussed in relation to the π−A isotherm and the photoisomerization of DBA.

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EXPERIMENTAL SECTION

Camphor was purchased from Wako Chemicals (Kyoto, Japan). DBA was synthesized as reported previously.24 Water was first distilled and Received: May 1, 2014 Revised: June 4, 2014 Published: June 5, 2014 7353

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then purified with a Millipore Milli-Q filtering system (pH of the obtained water: 6.3; resistance: >20 MΩ). A camphor disk (diameter: 3 mm; thickness: 1 mm; mass: 5 mg) was prepared using a pellet die set for FTIR. The movement of the camphor disk was monitored with a digital video camera (SONY HDR-CX590, minimum time resolution: 1/30 s) and then analyzed by an image-processing system (ImageJ, National Institutes of Health). The surface pressure depending on the surface area was measured with a surface pressure meter (Kyowa Interface Science Co. Ltd., HMB, Saitama, Japan). For the water phase, 200 mL of a phosphate buffer solution (pH: 8.5; ionic strength: 0.1) was poured into a trough of the surface pressure meter (surface area: 0.005−0.021 m2; standard water level: 6 mm). To prepare a molecular layer on the aqueous phase, DBA was dissolved in chloroform, and the chloroform solution was dropped on the aqueous phase with a microsyringe (volume: several tens of microliters). The amount of DBA dropped on water was 34 × 10−9 mol. The temperature was maintained at 293 ± 1 K throughout the experiments with a circulation thermostat (Yamato Scientific Co., Ltd., CLH300, Japan). A low-pressure mercury lamp (As-one, SUV-16, 16 mW) was used as a UV light source (wavelength: 254 nm; light intensity: 202 μW mm−2). A halogen lamp (HAYASHI, LA-150TX, 180 W) passed through a green filter (LEE Filters, No. 11) was used as a green light source (wavelength: 500−560 nm). In this experiment, we selected green light as a visible light according to the literature.16 These lamps were placed 100 mm above the DBA monolayer, and UV and green lights were irradiated to the water surface, of which the area was ∼8000 mm2. The irradiation condition of UV or green light used to measure the absorbance of a DBA solution was the same as that used to measure the π−A isotherm.

Figure 2. UV/vis spectra for a 25 μM DBA chloroform solution with a change in irradiation from (a) UV to green light and (b) green to UV light. The sample solution represented by the solid blue line in (a) was measured after UV light irradiation for 20 min, and the sample solutions represented by the green dotted and solid lines were measured after UV light irradiation and then green light irradiation for 1 and 2 min, respectively. The sample solution represented by the solid green line in (b) was measured after the green light irradiation for 20 min, and the sample solutions represented by the blue dotted and solid lines were measured after green light irradiation and then UV light irradiation for 1 and 2 min, respectively. The intensity of the green light was 700 lx.

was changed from UV to green light or vice versa, the absorbance converged to a steady value within 2 min under the present conditions; i.e., the absorbance under irradiation with green or UV light for 2 min was the same as that for 3 min or more. The time variation of UV/vis spectra for a 25 μM DBA chloroform solution under irradiation with a UV light that was switched to a weak green light are shown in Figure S2 to confirm photoisomerization based on the UV/vis spectra. Figure 3 shows the experimental results for the simultaneous measurement of the speed of a camphor disk and the surface

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RESULTS Figure 1 shows the surface pressure (π)−area (A) isotherms for DBA molecular layers under irradiation with UV and green

Figure 1. Surface pressure (π)−area (A) isotherm for a molecular layer of DBA at a compression rate of 0.09 nm2 molecule−1 min−1 under light irradiation (UV: blue dotted line; green light: green line). The pH of the water phase was buffered at 8.5.

Figure 3. Simultaneous measurement of the speed of a camphor disk and surface pressure (π) under the compression of a DBA molecular layer with a camphor disk at a rate of 0.09 nm2 molecule−1 min−1 (actual compression rate: 1460 mm2 min−1). The aqueous phase was irradiated with green light.

lights. Here, the pH of the water phase was buffered at 8.5. At A > 0.3 nm2 molecule−1, π values for UV light irradiation were higher than those for green light irradiation. In contrast, π values at A < 0.3 nm2 molecule−1 for UV light irradiation were lower than those for green light irradiation. Both the π−A isotherms for DBA molecular layers under the irradiation with UV and green lights changed depending on the pH of the water phase (see Figure S1). We selected the π−A isotherm at pH = 8.5 since the difference in π between UV and green light irradiation was the largest under the experimental conditions that we examined. Figure 2 shows the UV/vis spectra for a 25 μM DBA chloroform solution to confirm photoisomerization under irradiation with UV and green lights. When the light irradiation

pressure (π) when a molecular layer of DBA was compressed at a constant rate. Here, the speed was obtained from the translational motion. In this experiment, after the DBA molecular layer was prepared, a camphor disk was placed on the DBA molecular layer. The π−A isotherm in the presence of the camphor disk was almost the same as that in the absence of the camphor disk (data not shown). With a decrease in A, continuous motion with fluctuation was observed under the compression, and the speed of the camphor disk decreased at 7354

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Figure 4. Time variation of the speed of a camphor disk on a DBA molecular layer under irradiation with (a-1) green light, (a-2) green to UV light, (b-1) UV, and (b-2) UV to green light at A = 0.4 nm2 molecule−1. The downward-pointing arrows denote intermittent oscillatory motion.

0.35 < A < 0.45 nm2 molecule−1. No motion was observed at A < 0.35 nm2 molecule−1 at which π was larger than 15 mN m−1. Figure 4 shows the time variation of the speed of a camphor disk on a molecular layer of DBA at a constant A (= 0.4 nm2 molecule−1) (movies in the Supporting Information). In this experiment, a camphor disk was placed on the molecular layer of DBA after A reached 0.4 nm2 molecule−1 by compression. When irradiation with the green light was initiated, continuous motion was maintained (Figure 4a). The speed of continuous motion decreased with time by a change to UV light irradiation (Figure 4a-2). When irradiation with the UV light was initiated, there was no motion (Figure 4b). The absence of motion changed to intermittent oscillatory motion, i.e., repetition between rapid motion and rest, with a change to green light (Figure 4b-2). Mode switching from intermittent motion to no motion at a different surface pressure (21.9 mN m−1 under UV light, 4.3 mN m−1 under green light) is shown in Figure S3.

Figure 5. Schematic illustration to discuss the nature of the π−A isotherm for DBA. The surface pressure for cis-DBA (πC) is larger than that for trans-DBA (πT) at the same area, A. The surface area occupied on water for a cis-DBA molecule (SC) is larger than that for a transDBA molecule (ST) at the same surface concentration.

DISCUSSION On the basis of the present experimental results and related studies,20−23 we discuss the mechanism of self-motion of a camphor disk on a molecular layer for which the π−A isotherm changes depending on the photoisomerization. Figure 5 shows the suggested status of a DBA molecule adsorbed on water under irradiation with UV and green light. Figure 1 suggests that the surface pressure for cis-DBA (πC) is higher than that for trans-DBA (πT). The higher surface pressure of cis-DBA compared to trans-DBA is due to the increase in the molecule-occupied area on water; i.e., the cross-sectional area of a plane perpendicular to the long axis of a cis-DBA molecule (SC ∼ 0.5 nm2) adsorbed on water is larger than that of a transDBA molecule (ST ∼ 0.3 nm2). Here, SC and ST were estimated by software of modeling program (ChemBio3D, PerkinElmer). The experimental result that π for cis-DBA is lower than that for trans-DBA at A < 0.3 nm2 molecule−1 (see Figure 1 and Figure S1) may be due to instability of the cis-DBA monolayer caused by further compression in comparison with the trans-DBA monolayer.

Figure 2 suggests that the photoisomerization of DBA by irradiation with either UV or green light is completed within 2 min under the present conditions. The features of self-motion also converge within ca. 2 min, as shown in Figures 4a-2 and 4b-2. Figure 3 suggests that the threshold value of surface pressure (π) between motion and no motion is 15 mN m−1, which is similar to the difference in the surface tension around the camphor disk on pure water as the driving force.22 Figure 4 suggests that the nature and speed of self-motion are determined by the surface pressure, which depends on photoisomerization. The intermittent oscillatory motion in Figure 4b-2 can be described in terms of the following states (I−III). When the camphor disk is dropped on water with a surfactant, the camphor disk cannot move in the initial state because the surface pressure of the water phase with DBA (π) is close to that around the camphor disk (πcam) (state I). In this rest period, the accumulation of camphor molecules under the disk increases with time; i.e., πcam increases above π (state II). When πcam > π, accumulated camphor molecules can develop from the disk to the water surface since the DBA monolayer may be broken as a multilayer on water due to the shorter

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hydrophobic length. Therefore, the camphor disk can move (state III). When the camphor disk moves to another water surface, state I is reproduced, since πcam decreases to a value that is close to π. Thus, intermittent oscillatory motion occurs through the repetition of states I to III. If irradiation with UV and green light is completely reversible in the present experiment, continuous motion should converge to no motion after the change from green to UV light, and no motion should converge to continuous motion after the change from UV to green light. However, the nature of motion under UV light irradiation in Figure 4a-2 is not the same as that in Figure 4b-1, and the nature of motion under the green light irradiation in Figure 4b-2 is different from that in Figure 4a-1, even at the same area, A. We confirmed that continuous motion was observed 15 min after green light irradiation in Figure 4b-2 (data not shown). These different features of motion are due to relaxation of the photoisomerization of the DBA molecular layer with changes in the kind of irradiation.

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CONCLUSION The major finding of this study is that a camphor disk shows characteristic motion on a DBA molecular layer depending on the π−A isotherm, which is changed by photoisomerization. The mechanism of the mode switching of self-motion of a camphor disk was discussed in relation to the π−A isotherm of DBA and photoisomerization by irradiation with UV and green light. The light intensity and the perturbation become also important parameters to control the features of motion and produce synchronized motion in the future work. In addition, photomanipulation will be realized by the local irradiation around the camphor disk in the future work. Our experimental results suggest that an autonomous motor, which can change the features of its motion depending on the chemical structure of a molecular layer on water, can be controlled at a molecular level.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S3 and Movies S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +81-82-424-7409; e-mail [email protected] (S.N.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. R. Motoishi (Hiroshima University, Japan) for technical support regarding the synthesis of DBA. This work was supported in part by a Grant-in-Aid for Scientific Research (No. 25410094 to S.N. and No. 24109008 to M.A.) and the Sekisui Chemical Grant Program for Research on Manufacturing Based on Innovations Inspired by Nature to S.N. and for JSPS Fellows (No. 252039) from MEXT of Japan to Y.M.

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REFERENCES

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