Photocontrol of Clustering, Retaining, and Releasing of Microbeads

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Photocontrol of Clustering, Retaining, and Releasing of Microbeads Concomitant with Phototransformation of Supramolecular Architecture of Amphiphilic Diarylethene Akira Sakaguchi, Kenji Higashiguchi, Hajime Yotsuji, and Kenji Matsuda J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b00901 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on April 4, 2017

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The Journal of Physical Chemistry

Photocontrol of Clustering, Retaining, and Releasing of Microbeads Concomitant with Phototransformation of Supramolecular Architecture of Amphiphilic Diarylethene Akira Sakaguchi,† Kenji Higashiguchi,*,†,‡ Hajime Yotsuji,† and Kenji Matsuda*,† †

Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ‡

PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan E-mail: [email protected] & [email protected]

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ACS Paragon Plus Environment


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Abstract: Photoinduced clustering of polystyrene microbeads and photocontrol of their diffusion was achieved in water with the assistance of photoinduced transformation of supramolecular architecture of amphiphilic diarylethene between sphere and fiber states. When a suspension of polystyrene beads containing the sphere state of diarylethene was UV-irradiated from beneath, clustering of the polystyrene beads by thermal convection was observed. The velocity of clustering was dependent on the amount of photogenerated nanofibers that determines the viscosity of the water. Diffusion of the clustered polymer beads was suppressed by the surrounding fibers, but was restored to regular Brownian motion upon irradiation with visible light. It was suggested that the diffusion of the microbeads was controlled by the transformation of aggregates between the more viscous fiber state and the less viscous sphere state. These results provide new insight into the photocontrol of particle motion in fluidic media.

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Introduction Energy from light is used via electronic excited state in photosynthesis,1 as well as in photocatalysis2 and solar cells.3 Energy of electromagnetic field can also be directly used for the movement of objects using optical force as in optical tweezers.4 Optical tweezers can manipulate multiple numbers of objects by increasing the number of focal points by the technique based on non-linear optics.5 The convective flow which was caused by photothermal effect can also be harnessed to move objects by light.6 Plasmon-assisted optical tweezers, which use metal nanoparticles for enhancement of an electric field, have been reported.7-10 Separately, a system was reported that uses a substrate with Au deposited on it to generate thermal convection of water, allowing the clustering of objects in the focal region based on the flow of water.11,12 Photochemical techniques may also be used to move objects in the μm-to-mm scale by illumination. Photochromic compounds13 change not only their color but also many physical properties upon photoisomerization. In the meantime, differences in surface energy around the targeted objects can induce Marangoni convection.14 Therefore, by using photochromic compound, light can be used to induce Marangoni convection; the movement are reported to be induced by photoisomerization reactions of surfactants with a photochromic core.15-17 Amphiphilic azobenzene18-24 and diarylethene25-28 show changes in their self-assembled structures in water due to the different intermolecular interactions in the two isomers. We previously synthesized various amphiphilic diarylethenes that showed photoinduced changes in their supramolecular structures.29 In particular, the amphiphilic diarylethene 1 forms a sphere state that changes to a fiber state upon irradiation with UV light (Figure 1). The resulting fiber increases the viscosity around the UV-irradiated area, which is expected to improve the efficiency of movement, because viscosity relates to the transportation of momentum in liquid flow. In this study, we employed the supramolecular architecture of diarylethene 1 to improve the photodriven movement of polystyrene (PS) beads. Photoinduced movement was observed by optical microscopy and analyzed in detail in terms of the change in mean squared displacement (MSD).30 When PS beads in the colorless spherical aggregate 1 were irradiated with UV light, the PS beads clustered in 3



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the irradiated spot. After stopping the irradiation, the diffusion was observed to be suppressed. The diffusion was restored upon irradiation with visible light due to the transformation of the supramolecular structure. A control experiment using compound 2, which has a similar molecular structure but shows no transformation of aggregates by photoirradiation, resulted in much less photoinduced movement of PS beads. open-ring isomer

closed-ring isomer

F F F F

F F F F

UV

O C9H19

O

S

S

Teg

C9H19 O

O

F F

S

S

O

vis.

O

F F

Teg

O

1o (sphere state)

C9H19

O

Teg

F F F F

UV

O S

Teg

1c (fiber state)

F F F F

O

O

O

S

Teg

O

2o (sphere stete)

O

F F

S

S

O

vis. C9H19

O

F F

O

Teg

O

Teg

O

O

Teg

2c (sphere state) Teg=

O

O

O

Figure 1. Photoisomerization reaction of amphiphilic diarylethenes 1 and 2, and changes in the supramolecular structures.

Experimental Section A. Observation of photoinduced clustering under microscope. Optical microscopy was performed in transmission mode using an inverted microscope (Olympus, IX73) coupled to a CCD camera (Zeiss, AxioCam MRc). The objective lens was 40×/0.95 (Olympus, UPLSAPO40X2). A halogen lamp was employed for observation under transmitted path. UV irradiation (mainly 365 nm, 3.6 W/cm2) of a selected area was performed using an epifluorescence system with a 100 W mercury lamp (Olympus, ULH100HG) and filter set (interference filter BP360-370, dichroic mirror DM410, and interference filter BA420IF, OptoSigma) for the photocyclization reaction and photoinduced clustering. Decoloration by irradiation with visible light (mainly 546 nm, 6.7 W/cm2) was achieved using a filter set (interference filter BP530-550, dichroic mirror DM570, and interference filter BA575IF, OptoSigma).

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B. Preparation of sample for photoinduced movement. The mixed suspensions of spherical aggregates of 1 and PS beads (Polyscience, Polybead Microspheres, diameter 4.52  0.15 μm) was prepared as follows: a solution of amphiphilic diarylethene 1o (7.6 mg) in acetonitrile (0.97 mL) was added to pure water (7.6 mL) in one portion under vigorous stirring at room temperature. A portion of the resulting suspension (ca. 8.5 mL, 0.89 mg/mL) was diluted by an appropriate amount of water, e.g. pure water (90 μL) was added to the suspension (60 μL). PS beads (diameter 4.5 μm, 2.5 w/v% dispersed in water, 0.2 μL) was added to the diluted suspension (0.35 mg/mL, 0.15 mL), giving the mixed suspension of 1 with PS beads. The diameter of the microsphere of 1 without PS beads was determined by dynamic light scattering (DLS) to be ca. 200 nm, as shown in Figure S3a in Supporting Information (SI). A suspension of the supramolecular aggregates of 2 with PS beads was prepared in a similar manner. Concentrated suspension (0.89 mg/mL) were prepared from diarylethene 2o (4.6 mg), acetonitrile (0.59 mL), and water (4.6 mL); diluted suspension (0.35 mg/mL) was prepared from the concentrated suspension (60 μL), water (90 μL); mixed suspension 2 with PS beads was prepared from the diluted suspension (0.15 mL), PS suspension (0.2 μL). The diameter of the microsphere of 2 without PS beads was 160 nm, as shown in Figure S3c. Samples for observation under the optical microscope were prepared as follows. The mixed suspension of 1 with PS (or 2 with PS) (0.13 mL) was placed in a glass dish (Matsunami, glass-bottom dish, thickness of the bottom glass: ca. 0.17 mm, thickness of the optical path: 1 mm) and covered by thin glass. The representative method of preparation and observation for each experiment was as follows: Figure 3: The mixed suspension was placed in a glass cell and allowed to stand for 10 min to precipitate the PS beads. The images of PS bead movement were recorded automatically at 1 frame per sec (fps) until the final state. Brownian motion at the initial state was observed for 1 min. UV irradiation (wavelength λ = 365 nm, intensity I = 3.6 W/cm2, spot size ~110 μm) was carried out and clustering of the PS beads was recorded for 6 min. 5



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Figure 4: Clustering upon irradiation with UV light was carried out in the same manner. The sample was kept in the dark, and illuminated by a weak halogen lamp at 10 min intervals until 60 min for observation of suppressed diffusion. Subsequently, irradiation with visible light (λ = 546 nm, I = 6.7 W/cm2, whole field of view) was carried out for 10 sec using a mercury lamp to transform 1 to the sphere state. The diffusion of PS beads was observed at 5 sec intervals for 20 min. Figure 5: The optical-tweezers-like behavior was observed in almost the same manner as described in the method of Figure 3. The photomask was changed because the spot size of the usual optical diaphragm was too large to control the location of the beads. A square-shaped diaphragm, whose size was controlled as 25 × 25 μm, was employed. C. Tracking of particles. Tracking of PS beads under the optical microscope was carried out automatically using software. The recorded images for 1 min (61 frames, 1 fps) were picked up and recognition of the beads was achieved using the software (DIPP-MotionV, Ditect) by a method of correlation between images. Around ten beads located around the illumination spot were selected for individual tracking. The beads that did not collide with other beads and that did not enter the illumination spot were selected for tracking. The optical micrograph is shown in Figure 3b. Additional experimental details and figures on mean squared displacement (MSD) analysis, photochromism, TEM, viscosity, synthesis, and 1H and

13

C NMR spectra are described in Supporting

Information.

Results and Discussion Photoclustering and suppression of diffusion of objects. The synthesis of the amphiphilic diarylethene 1 was reported previously.29 Compound 2, in which methyl group is substituted at the 4position of each thiophene ring, was synthesized in a similar manner as shown in Supporting Information. The conversion ratio from the open-ring to the closed-ring isomer (in the photostationary state, PSS, upon irradiation with 365 nm UV light) was slightly different from diarylethene 1, as described below. 6


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The preparation and observation of samples were carried out as follows. The sample cell was prepared by filling with an aqueous suspension of spherical aggregates of 1 (0.35 mg/mL) and PS beads (diameter 4.52  0.15 μm) as targets in a glass cell. Observation of the movement of PS beads was carried out using an inverted optical microscope with an objective lens (40×, NA = 0.95). The focus was set on the inner bottom of glass cell as shown in Figure 2b, where the PS beads were precipitated. A mercury lamp with a filter block was employed for UV irradiation (365 nm, 3.6 W/cm2) for photoisomerization and subsequent clustering with the epi-illuminated system. The direction of the optical path was from bottom to top, as shown in Figure 2a.

(a)

halogen lamp

(b) glass cell

UV diaphragm (365 nm) ND filter

objective 40x NA 0.95 dichroic mirror camera

1 mm

fiber state of 1 flow

sphere state of 1

PS beads

glass UV

lens

Figure 2. (a) Light source used for clustering of beads to the focused spot under an optical microscope upon irradiation with UV light, and (b) the sample cell filled with the mixed suspension of PS beads and aggregates 1. The movement of PS beads was observed before photoirradiation to investigate the initial behavior. The suspension containing spherical aggregates of 1 and PS beads was white and turbid. Optical micrographs showed larger PS beads (ca. 4.5 μm) and smaller spherical aggregates (ca. 1 μm). The mean diameter of the spherical aggregates was ca. 250 nm, by DLS measurement (Figure S3 in SI), suggesting only the maximum-sized aggregates were observed by optical microscopy. The Brownian motion of the PS beads on the glass substrate was isotropic, based on time-lapse images. The mean velocity was almost the same as that in pure water without the diarylethene aggregates. Thus, the spherical aggregates of a few hundred nanometers did not affect the Brownian motion of the PS beads. A clustering of PS beads in the region of focus was achieved upon irradiation with UV light, as shown in Figure 3. In its initial state, the suspension composed of the open-ring isomer was colorless (Figure 7



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3a). When the diameter of the spherical aggregates was large (ca. 10 μm), division of the aggregates was reported to be observed.29 In this case, morphological changes were not observed for small aggregates (ca. 250 nm) by optical microscopy due to the optical resolution; thus, homogeneous coloration to blue was observed along with a photochromic reaction from 1o to 1c. Concurrently, clustering of the PS beads took place (Figure 3b). The velocity of the PS beads was determined to be 40 μm/min when the intensity of irradiation was 3.6 W/cm2. The mechanism of movement was considered to be thermal convection and a photoinduced increase in viscosity, as described below. Finally, the large number of PS beads occupied the irradiated spot, forming a densely packed 2-D structure31 as shown in Figure 3c.

Figure 3. Photodriven clustering of PS beads using a suspension of amphiphilic diarylethene under an optical microscope upon irradiation with focused UV light. (a) At the initial state (0 min), (b) after UV irradiation for 3 min, (c) after 6 min, and (d) the expanded image of (c). The large colorless spheres are PS beads with a diameter of 4.5 μm and the smaller spheres are composed of amphiphilic diarylethene 1. The broken white circle shows the region of UV irradiation and the red dotted circles shows the resulting 2-D colloidal crystal. Green lines indicate the trajectory of selected PS beads for 1 min and the small circles in (b) show the position of the selected beads after 3 min. The average distance between sphere centers was measured as 5.2 μm (see Figure S16 in SI).

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When the UV light was turned off, the clustering of the beads stopped and diffusion was suppressed due to the increase in viscosity in the irradiated region. This suppression persisted for 1 h in the dark, as shown in Figures 4a and b. Upon subsequent irradiation with visible light (mainly 546 nm, 6.7 W/cm2) for 10 sec across the whole range of field, the suspension became colorless and the diffusion rate of the beads was restored to its initial state as shown in Figure S2 in SI. Thus, the PS beads that had gathered in the irradiated region diffused after 20 min, as shown in Figures 4c and d.

(a)

(b)

in dark

UV

60 min 50 µm (c)

Vis.

in dark

10 s

20 min

(d)

Figure 4. Control of diffusivity by irradiation with visible light. After clustering upon irradiation with UV light, the PS beads were trapped (a), and the position of the PS beads hardly changed at all in the dark for 30 min (b, trapped state). Upon subsequent irradiation with visible light for 10 s (c), the PS beads showed diffusion (d, released state). The circle shows the region of UV irradiation.

The system can move microbeads similar to an optical tweezers when the location of the irradiated spot is controlled. It was expected that if the irradiated spot was moved after clustering, then the gathered particles would move into the newly irradiated area. Figure 5 shows images of this behavior using the same samples, with the same irradiation conditions except for the spot size (25 × 25 μm). First, irradiation with UV light was carried out and the PS beads clustered. The substrate was then slid to the left around 25 μm, which means that the region of focus apparently shifted to the right. The clustered PS beads did not move immediately, but re-clustered in the new region of irradiation after 6 min. This 9



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system has an advantage in the use of a low-intensity mercury lamp as a light source, compared to the reported optical tweezers with the ability to capture large numbers of particles employing spatial light modulator5 or plasmonic substrate having metal nanostructures.8

Figure 5. Behavior similar to optical tweezers. (a) Initial state: PS beads had clustered upon irradiation with focused UV light for 20 min, (b) immediately after sliding of the irradiated spot, (c) re-clustering after 1.5 min, and (d) after stepwise sliding for 52 min. The white squares, white arrows, yellow rectangles, and red circles show the irradiated area using a variable mask, sliding direction of the irradiated spot, glued beads for a mark, and the traced PS bead, respectively. The omitted images are shown in Figure S15 in SI.

The mechanism of this clustering of PS beads upon irradiation with UV light was considered to be the thermal convection due to absorption of the light by diarylethene aggregates. The point of focus was set on the inner bottom surface of the cell and the numerical aperture of the objective lens was high (NA = 0.95). Therefore, the light power density became intense at the focused point and decreased significantly along the height axis. A temperature slope was formed along the height axis, which means that thermal convection was generated. The ascending flow induced converging flow on the bottom toward the UV-irradiated region, PS beads on the glass rolled into the irradiated region. Because the

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ascending force of flow was smaller than the descending force of gravity, the PS beads sank. Ultimately, a 2-D colloidal crystal of PS beads was formed. Thermal convection was confirmed by observation from the side of a square capillary upon irradiation with UV light from the bottom, as shown in Figure S5 in SI. Small PS beads showing fluorescence were added to the sample suspension as a tracer and filled in a squared capillary which is used in place of a glass cell. The size of tracer beads was small (~1 μm), therefore the beads expected to float following the ascending flow. The rate of ascending flow was 2400 μm/min at the beginning of UV irradiation but became as slow as 300 μm/min after 5 min upon continuous irradiation with UV light. Clustering of the PS beads (4.5 μm) was also observed. The photoinduced phase transition of the supramolecular structure and the resulting movement of the PS beads may be explained as follows. Diarylethene 1 underwent a photocyclization reaction upon irradiation with UV light. The spherical aggregate, which was composed of the open-ring isomer 1o, transforms to nanofibers composed of the closed-ring isomer 1c when the conversion ratio is larger than 60% at room temperature.29 In photochemical process, the excited molecules undergo photoreaction and/or fluorescence, but the rest of the energy becomes heat due to vibrational relaxation. In this system, the increase in temperature around the irradiated spot was measured by thermal ink to be ΔT ~ 5 K. The aggregate affects not only the formation of thermal convection by absorption of UV light, but also the viscosity through the generation of fibers. The nanofibers, which had a diameter of ca. 10 nm and a length of at least 2 μm, were generated radially around the spherical aggregates. Each fiber had no dendritic structure; however, a network structure was formed because many spherical aggregates were dispersed in the suspension (Figure 6 and Figure S12 in SI). The increase in viscosity effectively transferred the momentum of the flowing water to the PS beads, therefore the rate of clustering became fast compared to less viscous media. The fibers also suppressed the diffusion of PS beads; i.e., the clustered beads were surrounded by fibers that is actually a highly viscous media.

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Figure 6. TEM images of the diarylethene assembly for the samples of (a) the open-ring isomer of 1, (b) 1 in the photostationary state upon irradiation with 365 nm light. Black filled circles in the images were the PS beads. Inset shows the expanded images. The samples were negative-stained by sodium phosphotungstate.

Quantitative analysis of the movement of PS beads. The movement of the PS beads was analyzed quantitatively based on the mean squared displacement (MSD)30 value obtained from the trajectory, as shown in Figure 7. Before irradiation with UV light, i.e., when the PS beads and spherical aggregates of the open-ring isomer 1o were in coexistence, the PS beads showed general Brownian motion, as described in equation 1.30

MSD  4 Dt

(1)

The obtained diffusion coefficient for the PS beads on the bottom surface of the cell was D = 4.8 × 10-14 m2/s. This value is close to the value measured in pure water without spherical aggregates (D = 5.0 × 1014

) When the diffusion in bulk water was analyzed by shifting the focus from the bottom to the center of 12


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the cell, the obtained value was D = 0.98 × 10-13 m2/s, which is twice as large as the value on the bottom surface. This value agreed with the theoretical diffusion coefficient of pure water (D = kT/6ηr = 1.04 × 10-13 m2/s). Here, k, T, η, and r represent the Boltzmann coefficient, temperature, viscosity of the medium, and particle radius, respectively. These results suggest that the presence of the spherical aggregates did not affect the diffusion coefficient and the value on the bottom surface was around a half of the value in the bulk. The movement of PS beads by convection upon irradiation with UV light was observed as ballistic. MSD was increased as a quadratic function, as shown in equation 2.30 MSD   2 t 2  Kt 

(2)

where both K and α were constants. ν represents the velocity of the beads. The velocity was determined to be 40 μm/min by fitting. In some other studies using a focused laser (> 1 kW/cm2), larger values are reported.10-12 For example, in thermal convection system employing Ti:Sapphire laser (860 nm, 3 kW/cm2) and Au deposited substrate, multiple numbers of particles (ca. 10 μm) are reported to cluster with the velocity of ca. 78 μm/min.12 In our experiment, much weaker light from focused mercury lamp (3.6 W/cm2) was used. Therefore, by considering this point, our system may have even better energy efficiency. When UV irradiation was stopped, the diffusion of PS beads showed not only suppression but also subdiffusion behavior; the mean slope of MSD decreased and curved as KΔtα (α < 1). Such subdiffusion behavior,32-35 which is generally observed in heterogeneous media such as gels, liquid crystals, porous polymers, and cells, was caused by the surrounding supramolecular fibers of 1c because the closed-ring isomer did not undergo a back-reaction in the dark. Subsequent irradiation with visible light changed the fibers to spheres, and regenerated the initial state. The PS beads showed Brownian motion and the diffusion coefficient was restored to 5.1 × 10-14 m2/s. The control experiment with a different type of diarylethene suggested the effect of fibers on the movement of the PS beads. The supramolecular structure of amphiphilic diarylethene 2, which did not show a photoinduced transformation to fibers (Figures S1, S2, S3, S6, S7 and S12 in SI), was employed 13



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instead of 1. For diarylethene 2, which has methyl group at the 4-position of the thiophene rings, usual photochromic reaction was observed not only in acetonitrile solution but also in aqueous suspension, and a photoinduced phase transition from sphere to fiber was not observed. Photoinduced movement of PS beads was much less intensive in a suspension of 2 with the same number of molecules as the suspension of 1, as shown in Figure 7c. The suggested reason was that the lack of fibers caused no change in viscosity, which allowed the heated spherical aggregates to move without being affected by the surrounding environment.36 The additional reason was the low absorption cross section of the suspension of 2, which was caused by the difference in the molar absorption coefficient (ε1o,365nm = 1.03 × 103 M-1 cm-1 and ε2o,365nm = 2.67 × 102 M-1 cm-1), as shown in Figure S1 in SI. The MSD in the dark was almost the same as that of the initial state; i.e., suppression of diffusion was not observed due to the lack of fibers. Upon subsequent irradiation with visible light, no change in diffusivity occurred. As another control experiment, clustering of microbeads in bromothymol blue (BTB) ethanol solution was examined. When the viscosity of the solution was controlled by adding PEG, it was found that more viscous solution showed slower clustering (Figure S14a in SI). This phenomenon is also observed by the prolonged irradiation of UV light to diarylethene 1 as will be discussed later (Figure 9). Meanwhile, clustering velocity increased at the initial stage of photoirradiation. These results suggest that the clustering of microbeads become effective by inhomogeneous increase in viscosity around the irradiated spot due to the photogenerated fibers.

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(a)


(b) UV

Vis.

(c)

UV off

(d)

Figure 7. MSD of PS beads in a suspension of supramolecular structures of (a) 1 and (c) 2. (b) and (d) represent expansions of red-squared regions of (a) and (c), respectively. The black open squares with a broken line, black solid circles with a solid line, purple open diamonds with a solid line, and green open triangles with a solid line represent the initial conditions, during irradiation with UV light, in the dark after irradiation with UV light, and after irradiation with visible light, respectively. The lines were fitted using equations 1 for initial condition and 2 for the rest.

Optimizations of clustering ability and suppression of diffusion. The clustering ability and suppression of diffusion were optimized in terms of appropriate concentrations and irradiation times. First, the dependence of clustering velocity on the concentration of the suspension 1 was investigated, and nonlinear responsiveness was revealed under constant light intensity (3.6 W/cm2), as shown in Figure 8a. When the concentration was high (> 0.54 mg/mL) and moderate (0.35 mg/mL), the mean 15



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clustering velocities of the beads were almost the same, maybe because the photoconversion gets smaller in higher concentrated system as described below. At low concentration, clustering was scarcely observed. The threshold appeared to be around 0.27 mg/mL. The diffusion suppression is considered to be dependent on the amount of photogenerated fibers, which is generated when the content of the closed-ring isomer is high enough. At high concentration (>0.54 mg/mL), the number of photogenerated closed-ring isomers was large but the conversion ratio was small because the irradiation time, which equates to the number of photons, was not sufficient for the amount of diarylethene molecules. Fibers were obtained through phase transition when the conversion ratio was > 60% at room temperature; thus, fibers was not formed in spite of the large amount of the closed-ring isomer of 1 and diffusion in the dark was almost the same as in pure water. At moderate concentrations (0.35 and 0.27 mg/mL), the irradiation was sufficient for isomerization to the photostationary state (~75%), so that the aggregates were fully transformed to the fiber state and suppression was observed. At low concentration (0.18 mg/mL), the suspension also reached the photostationary state, but the amount of fibers was small due to the low number of molecules and the diffusion coefficient became moderate. As a result, at 0.35 mg/mL, the clustering velocity of the beads was the highest and diffusion of the clustered beads in the dark was suppressed most efficiently. In contrast, the control experiment using diarylethene 2 showed no concentration dependence on the diffusion coefficient presumably because there is no effect of the generation of fiber state (Figure 8b).36 Slight increase of clustering velocity with the increase of concentration is observed, which is attributable to the increase of absorption cross section.

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(a)


(b)

Figure 8. Concentration dependence of clustering velocity under irradiation with UV light (red filled circles) and diffusion suppression of PS beads in the dark (blue open diamonds) using (a) 1 and (b) 2. The black broken line indicates the diffusion coefficient in pure water on a glass base.

The dependence on irradiation time was investigated. Figure 9 shows the change in the clustering velocity against irradiation time at each concentration. At moderate concentration (0.35 mg/mL), the velocity increased for 5 min and then gradually decreased. At a higher concentration (0.54 mg/mL), the initial clustering velocity was greater than that of the solution with moderate concentration and the velocity did not vary during the irradiation. On the other hand, clustering of the beads barely occurred when the concentration was low (0.27 mg/mL). These results indicate that the velocity of the beads did not depend directly on the irradiation time nor the initial concentration, but on the amount of fibers, which is related to the conversion ratio around the irradiation spot as discussed above. The intensity of light was also affected linearly to the velocity, as shown in Figure S8 in SI.

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Figure 9. Dependence of the clustering velocity on UV irradiation time. Blue triangles, black circles, and red squares indicate high (0.54 mg/mL), moderate (0.35 mg/mL), and low (0.27 mg/mL) concentration, respectively.

Clustering behavior with photomasks. The use of certain types of photomask employed in the irradiation process indicated that the PS beads were clustered not into the irradiated region but into the fluidic center. The photomasks had four holes at the corners of a square, in which diameter of hole was fixed as 50 μm and the distance between the holes was 25 to 100 μm. When the distance was narrow (25 μm), PS beads clustered in the center of the square, as shown in Figure 10a. When a medium distance was used (50 μm), PS beads clustered between the irradiated spots and the center of square in linear fashion (Figure 10b). When the distance was long (100 μm), the PS beads were attracted in the irradiated spots, but more populated near the center of the square (Figure 10c). The results indicated that the PS beads clustered not to the center of the irradiated spot but to the balanced place of water flow. When the distance between the spots was narrow, the water flux from the inside of the square to the irradiated spots was small. Therefore, the PS beads were clustered only by the influx of water from the outside of the square. In contrast, when the distance was wide, a flux from the center of the square to the spots was able to form. Therefore, the PS beads were clustered inside the spots.

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(a)

(b)

(c)

50 µm

Figure 10. Optical image of clustering of PS beads using photomasks illuminating the four corners of square. The diameter of each spot was 50 μm and the distance between the edges, which was shown by white arrows, was (a) 25, (b) 50, and (c) 100 μm, respectively. Red circles show the regions where PS bead clustered.

Conclusions We achieved the clustering of objects in a focused region, in a manner similar to optical tweezers, by irradiation with UV light using a relatively weak light source with the assistance of supramolecular fibers of an amphiphilic diarylethene. When a suspension of PS beads containing the sphere state was irradiated from below with UV light, an ascending flow was generated by the photothermal effect of the absorption of UV light. The transformation from sphere to nanofiber state increased the viscosity of the water. Diffusion of the clustered PS beads was suppressed by the surrounding fibers, but was restored upon irradiation with visible light due to a reverse transformation from the fiber to the less viscous sphere state. These phenomena were not observed when different type of amphiphilic diarylethene was used, which did not show a phase transition upon irradiation with UV and visible light. This system is expected to be applicable as an optical actuator for manipulating multiple numbers of objects.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ***/***. Additional experimental details and figures of MSD, photochromism, TEM, viscosity, 19



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synthesis, 1H and 13C NMR spectra, and movies. Movie S1. Photodriven clustering of PS beads using a suspension of amphiphilic diarylethene under an optical microscope upon irradiation with focused UV light (see Figure 3). The movie has a 440 × 330 μm field of view for 367 s at a frame rate of 0.03 s/frame by 33-fold speed of reproduction. Movie S2. Behavior similar to optical tweezers. (see Figure 5). The movie has a 440 × 330 μm field of view for 52 min at a frame rate of 0.03 s/frame by 780-fold speed of reproduction. Movie S3. Side view of the ascending flow in a squared capillary with fluorescent tracers. (see Figure S5 in SI). The movie has a 700 × 560 μm field of view for 10 s at a frame rate of 0.03 s/frame by real speed.

Author Information Corresponding Authors Kenji Matsuda, *E-mail: [email protected]. Phone:+81-75-383-2738. Kenji Higashiguchi, *E-mail: [email protected]. Phone:+81-75-383-2745. Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by PRESTO, JST; and a Grant-in-Aid for Scientific Research on Innovative Areas “Photosynergetics” (JSPS KAKENHI Grant Number JP26107008), from MEXT, Japan. This work was also supported by a Grant-in-Aid for Scientific Research (C) (JSPS KAKENHI Grant Number JP26410090) from JSPS, Japan. This work was performed under the Research Program of "Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials" in "Network Joint Research Center for Materials and Devices" (Number 20162025).

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Photocontrol of Clustering, Retaining, and Releasing of Microbeads

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