Dynamic Colloidal Molecules Maneuvered by Light-Controlled Janus

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Dynamic Colloidal Molecules Maneuvered by Light-controlled Janus Micromotors Yirong Gao, Fangzhi Mou, Yizheng Feng, Shengping Che, Wei Li, Leilei Xu, and Jianguo Guan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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ACS Applied Materials & Interfaces

Dynamic Colloidal Molecules Maneuvered by Light-controlled Janus Micromotors Yirong Gao, Fangzhi Mou*, Yizheng Feng, Shengping Che, Wei Li, Leilei Xu and Jianguo Guan*

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, International School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China.

KEYWORDS: Colloidal Molecules, Janus Micromotors, Reversible Assembly, Light-controlled Motion, Microlens Arrays

ABSTRACT

In this work, we have proposed and demonstrated a dynamic colloidal molecule, which is capable of moving autonomously and performing swift, reversible and in-place assembly-dissociation in a high accuracy by manipulating a TiO2/Pt Janus micromotor with light irradiation. Due to the efficient motion of the TiO2/Pt Janus motor and the light-switchable electrostatic interactions between the micromotor and colloidal particles, the colloidal particles can be captured and assembled one by one on the fly, subsequently forming into swimming colloidal molecules by mimicking space-filling models of simple molecules with central atoms. The as-demonstrated

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dynamic colloidal molecules have a configuration accurately controlled and stabilized by regulating the time-depending intensity of UV light, which controls the stop-and-go motion of the colloidal molecules. The dynamic colloidal molecules are dissociated when the light irradiation is turned off due to the disappearance of light-switchable electrostatic interaction between the motor and the colloidal particles. The strategy for the assembly of dynamic colloidal molecules is applicable to various charged colloidal particles. The simulated optical properties of a dynamic colloidal molecule imply that the results here may provide a novel approach for in-place building functional microdevices, such as microlens arrays, in a swift and reversible manner.


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INTRODUCTION Colloidal molecules, an elegant term introduced by van Blaaderen in 2003,1 vividly reveal the complex materials assembled by dispersive colloidal spheres. Although the colloidal spheres are several orders of magnitude bigger than the atoms, they, to some extent, can be regarded as mesoscopic counterparts of atoms. Consequently, colloidal molecules and their assembly have provided visual models to unravel the mysteries of the atomic world, including crystal nucleation and growth and the generation of molecules. Besides, they also allow for systematic investigation of the physical properties of novel structures.2-4 To synthesize colloidal molecules, various methods have been developed, including colloidal assembly,5 controlled phase separation6-7 and controlled surface nucleation and growth.8-10 For example, taking advantages of electrostatic interaction,11-12 coalescene,13-14 and van der Waals forces,5 etc., a variety of colloidal molecules have been available by assembling diverse colloidal units under the guidance of geometric confinement.15-19 Nevertheless, the so far constructed colloidal molecules are usually based on thermal equilibrium state and cannot be dynamic.15 Micro/nanomotors could harvest various energy from environment and convert them to their own kinetic energy.20-21 They are able to effectively capture, transport and release micro/nanocargoes in liquid media based on their various interactions with the cargoes, such as electrostatic22-23 and magnetic forces,24-25 hydrophobic and hydrophilic interactions,26 hydrodynamic interactions,27 as well as strong covalent bindings.22, 28 For instance, micro/nanomotors have been recently demonstrated to capture, transport and



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release a single cargo or multiple cargoes, including colloidal particles,22-23 drug capsules,24 magnetic nanoplates,27 oil droplets29 etc. Inspired by this, here we propose dynamic colloidal molecules, which are defined to have an accurately controllable configuration and colloidal particle population, and to be able to transform, disassemble and even move in response to external stimuli. The as-proposed dynamic colloidal molecules are assembled out of an equilibrium state based on the electrostatic interactions between active and passive colloidal particles. They are expected to provide an intuitional and perspicuous approach to understand the assembly process of colloidal particles and even atoms. In addition, they may also offer facile and potential technologies for in-place building functional microdevices in a swift and reversible manner, including microsensors,30 microfluidic devices31 and microlens arrays.32 In analogy to the serial pick-and-place robotic assembly in macroscopic world, the assembly of dynamic colloidal molecules with controlled configurations by micromotors and cargoes is of challenge because it requires a strong motor driving force, significant motor-cargo interactions and the controllable assembly of cargoes at specific sites via the reversible motor-cargo/cargo-cargo interactions. In this work, we have demonstrated the swift, reversible and in-place assembly-dissociation and motion behaviors of dynamic colloidal molecules by maneuvering TiO2/Pt Janus micromotors with light. Due to the efficient motion of the TiO2/Pt Janus micromotor and the light-switchable electrostatic interactions between the micromotor and colloidal particles, the colloidal particles can be captured and assembled one by one on the fly, subsequently forming into swimming


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colloidal molecules with specific configurations by mimicking space-filling models of simple molecules with a central atom. The as-assembled colloidal molecules can be fixed to a desired configuration and be dissociated into individual colloidal particles by simply regulating the light intensity, which determines the stop-and-go motion of the motor or the preliminary colloidal molecules, as well as the motor-colloid/colloid-colloid interactions. Our approach potentially sheds a light on the routes to synthesize various dynamic colloidal molecules consisting of colloidal particles with a wide range of surface properties. In addition, we have also simulated the optical properties of the as-proposed dynamic colloidal molecules, which suggest that they may act as swimming microlens arrays due to the controlled and ordered configuration. RESULTS AND DISCUSSION The dynamic colloidal molecules proposed here show defined configurations and consist of a Janus micromotor as an active center and colloidal particles as building blocks. Herein, a TiO2/Pt Janus micromotor was used as an example to maneuver the assembly of the dynamic colloidal molecules as it demonstrated intriguing advantages of the efficient light-controlled propulsion and high stability.33-38 At first, we dispersed TiO2/Pt Janus micromotors with a diameter (d) of 1.2 µm and SiO2 colloidal particles (d = 2 µm) in a 0.67 wt% H2O2 solution. The TiO2/Pt Janus micromotors were fabricated by asymmetrically coating a 4-nm-thickness platinum layer on the exposed surfaces of the anatase TiO2 microspheres on a flat glass substrate via an ion sputtering process according to a previous report.35 In the H2O2 solution, they showed an efficient



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autonomous motion with a speed of 30 µm/s under the UV irradiation with a light intensity of 1 W/cm2, as shown in Figure S1. When the TiO2/Pt micromotor (M) moves to the vicinity of the SiO2 colloidal particles (X), it can serially capture and assemble SiO2 colloidal particles on its surface. When the desired number of colloidal particles are assembled on the surface of motor, the UV intensity is then tuned from 1 to 0.07 W/cm2 to quench their motions, subsequently forming into colloidal molecules with different configurations, such as heterogeneous dimer (MX), trimer (MX2), tetramer (MX3), pentamer (MX4), hexamer (MX5) and heptamer (MX6) etc., as shown in Figure 1. Evidently, for all of the observed colloidal molecules, the assembled colloidal particles are attached only at the front end (corresponding to the TiO2 end) of the motor, and contact tangentially with the motor and other already assembled particles forming into a close-packed structure. When one colloidal particle is assembled on the motor, the formed colloidal molecule MX shows a snowman-like morphology (Figure 1A). As two and three colloidal particles are attached to the front end of the motor, the assembled colloidal particles are symmetrically attached at the front apex of the motor, and the T and A-shaped trimer and tetramer MX2 and MX3 are generated (Figure 1B and C), respectively. Due to the limited surface area of the TiO2 end, the fourth, fifth and sixth colloidal particles cannot be directly assembled on the surface of the motor, but assembled at the sites of the concave interspace between the assembled colloidal particles. In this way, pentamer (MX4), hexamer (MX5) and hexagonal heptamer (MX6) are formed (Figure 1D - F). When the number of the assembled colloidal particles (n) is


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less than or equal to 4, each colloidal molecule has only one certain configuration. In contrast, isomers are observed for the colloidal molecules with n is over 4, suggesting multiple bonding sites on the primary colloidal molecules. For example, Figure S2 shows the isomers of colloidal molecule MX5 and MX6.

Figure 1. (A-F) The optical microscopy images of colloidal molecules MXn (the colloid number n = 1 - 6), where the small dark spot is a TiO2/Pt micromotor (M), and the large transparent spheres are SiO2 colloidal particles (X). The white and green hemispheres represent TiO2 and Pt component, respectively. The insets focus on the definite configurations of assembled colloidal molecules with different n.



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Figure 2. (A) The motion trajectory of a TiO2/Pt micromotor under the illumination of UV light with intensity of 1 W/cm2 illustrating the step-by-step assembly process of a colloidal molecule MX6 taken from Video S1. Colored dots on the dash line indicate the place where SiO2 colloidal particles are serially captured and assembled on the TiO2/Pt Janus micromotor, forming into intermediates of the colloidal molecule MX6, and the arrow refers to the moving direction. Insets are the optical microscopic images of the various intermediates and the resultant colloidal molecules MX6 formed at different motion times. (B) The optical microscopy images illustrating that colloidal molecule MX6 assembled by a TiO2/Pt Janus micromotor at the UV intensity of 1 W/cm2 can be stabilized at an UV intensity of 0.07 W/cm2 and dissociated into dispersive particles again when the UV irradiation was turned off. (C) The motion speed of dynamic colloidal molecules MXn with different n To decipher the assembly of the colloidal molecules with different configurations, we have taken a hexagonal colloidal molecule MX6 as an example to track its formation


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process. Figure 2A and Video S1 illustrate that under the irradiation of UV light with an intensity of 1 W/cm2, the micromotor cruises in the liquid, and then serially captures and assembles passive SiO2 particles on its surface one by one, producing intermediates with increasing number of assembled passive particles. Consequently, the resultant colloidal molecule with a desired hexagonal configuration is formed when the motion time of the motor is 19 s, and it moves with a speed of 4.9 µm/s. Figure 2B and Video S2 show that once the resultant product MX6 forms (left image in Figure 2B), the configuration can remain by lowering the light intensity to 0.07 W/cm2. In this case, the motor only move at a speed of 2.1 µm/s, close to that of its Brownian motion without UV irradiation. This suggests that lowering the irradiated light intensity can quench the motor propulsion, but may still provide an electrostatic interaction force enough to stabilize the colloidal molecule. The immobile colloidal molecules do not collect additional colloidal particles and grow into large colloidal molecules. In this way, all the intermediates shown in Figure 2A can be stabilized as resultant products by reducing the light intensity to 0.07 W/cm2 once they are formed, as shown in Video S3. That is the reason why the intermediates have similar configurations as the colloidal molecules with the same n, as imaged in Figure 1. If the illumination time is prolonged with the intensity of 1 W/cm2, more complicated colloidal molecule can be obtained (Figure S3). As shown in the right image of Figure 2B and Video S2, the swimming or stabilized hexagonal colloidal molecules are instantly dissociated into separated colloidal particles when the UV is off. The above results illustrate that the assembly of colloidal particles by the TiO2/Pt



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micromotor is a swift (tens of seconds), reversible process, which can be controlled simply by regulating the intensity and switch of the UV irradiation. In addition, the as-assembled colloidal molecules are dynamic due to the integrated behaviors of the light-controlled motion and reversible assembly-dissociation. The inert negatively charged SiO2 colloidal particles exhibit a typical Brownian motion in liquid when they do not interact with the micromotors. However, once they are attracted and captured by a micromotor, they move with the micromotor as an integral. Hence, the motion speed of the formed primary colloidal molecules or intermediates (MX1 - MXn-1) could substantially represent the assembly kinetic process of the final desired colloidal molecules (MXn), that is, the higher the motion speed of the formed primary colloidal molecules, the less the assembly time of the final desired colloidal molecules. Using Video Spot Tracker software to analyze the speed of the assembled colloidal molecules MXn with different n, we have noticed that their average velocity first increases and then decreases, finally reaches a stabled level with increasing n, as shown in Figure 2C. This can be attributed to the balance between the enhanced propulsion force and increased viscous drag force (Fd) with increasing n due to the size increment.27 In the colloidal molecules MXn, the protons may concentrate in the confined space between the motor and colloidal particles, leading to an enhanced electrical field intensity around the motor and an increased propulsion force.39 On the other hand, protons may also redistribute over negatively-charged SiO2 colloidal particles when they are captured by the Janus motor. Meanwhile, as the size of the colloidal molecule increases, Fd increases


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accordingly. Thus, when n increases from 1 to 2, the enhancement of the driving force is greater than that of Fd, but when n increases from 2 to more, the situation is reversed, resulting in the decreasing speed.

Figure 3. (A) Schematic diagram of propulsion mechanism of a catalytic TiO2/Pt Janus micromotor controlled by UV light in H2O2 solution. (B, C) Numerical simulation of the spatial distribution of (B) proton concentration and (C) electric potential () around a TiO2/Pt micromotor. The images are sectional profiles of 2D axial symmetric model simulation results. The capping Pt layer is on the left hemisphere of the TiO2 particle and labeled with a black arc. Gray curves and scattered black arrows in panel C represent equipotential line and electric field line, respectively. Color legends: concentration in mM/m3 and electric potential in mV. (D) The diagram of the electric dipole simplified from a TiO2/Pt motor under UV irradiation. The green and gray hemispheres refer to the positively charged TiO2 component and negatively charged Pt component. (E) The



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geometric diagram representing the formation of the colloidal molecule (MXn, n = 1 - 6) by minimizing Ep. The propulsion of the TiO2/Pt Janus micromotors dominantly derives from the UV light induced self-electrophoresis,34-35 as illustrated in Figure 3A. Under UV irradiation, electron-hole pairs are generated within the TiO2 part of the TiO2/Pt Janus micromotors, and then the photogenerated electrons transfer to the Pt layer.40 The holes on the TiO2 surface involve in the H2O2 oxidation, producing protons and O2, and the electrons on the Pt hemispherical surface engage in H2O2 reduction, depleting protons. Consequently, the protons highly concentrate on the TiO2 surface and thus the H+ gradient is generated across the motor, resulting in a local electric field (E). The generated E then can propel the negatively-charged TiO2/Pt micromotors moving with the TiO2 end forward. Adjusting light intensity could change the photon flux, and thus modulate the motion speed of the micromotor.35-36 Hence, when the light intensity is lowered to a certain value, such as 0.07 W/cm2, the motor propulsion is almost quenched, avoiding the continuous collection of additional particles and stabilizing the formed colloidal molecules, as shown in Figure 2B. Simultaneously, the generated E can also attract charged particles in the vicinity for the assembly of colloidal molecules. Thus, the positively charged TiO2 end can attract negatively charged SiO2 colloidal particles through electrostatic interaction between them. To further explore the formation mechanism of colloidal molecules with different


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configurations, the spatial distribution of proton concentration and electric potential () around a TiO2/Pt micromotor have been investigated by numerical simulation. Figure 3B clearly indicates that the protons are highly concentrated near the TiO2 hemisphere and have a low concentration at Pt end. This uneven distribution of protons makes the TiO2 end positively charged and the Pt end negatively charged. Figure 3C shows the simulation results of the equipotential lines and the electric field lines, suggesting that the TiO2/Pt micromotor is equivalent to an electric dipole (Figure 3D), around which the electric potential () follows equation (1).

(1)

Here, q is the charge value of the electric dipole, r is the radius of the TiO2/Pt micromotor, is the permittivity of the medium, is the angle formed with y-axis, and l refers to the distance to the center of TiO2/Pt micromotor (O).



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Figure 4. Schematic demonstration of the light-controlled assembly and dissociation of the colloidal molecule MXn due to the light-switchable electrostatic interactions between a TiO2/Pt micromotor (small green/white sphere) and SiO2 colloidal particles (big yellow spheres). The green lines a and the red lines b in the panels represent the bond length, and the bond angles are indexed as and . As shown in panel I of Figure 3E, a negatively charged particle near the TiO2 end of the motor would subject to an electrostatic attractive force, and thus moves along the electric field line to the area with higher until it reaches to apex of the TiO2 end where it has the highest (Figure 4I and II). In this process, the electric field does positive work (W), and the electric potential energy (Ep) decreases according to Equation 2. − −

(2)

Here, p is the charge of the colloidal particle, and represents the electric potential at ∞

infinity and is set to be zero in this model. It can be seen that the assembled colloidal molecules are in stable state only when Ep reaches to its minimum value, of which colloid should move to the position with the highest Φ. For a simple colloidal molecule assembled by one motor and one colloidal particle (panel I in Figure 3E), only when

+ " and # = 0° can the structure be stable and show the lowest electric potential

energy (%&'

()

). This corresponds to the configuration (MX) shown in

Figure 4III, consistent with that observed in Figure 1A. Because Ep is a scalar quantity,


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the total Ep of the system equals the sum of Ep of each component. In a system of colloidal molecule with a motor and two colloidal particles (panel II in Figure 3E),

* *

+

(3)

Where l1 and l2 are the distance between the motor and colloidal particle 1 and 2, respectively. # and are the angles between the y-axis and the line OA1 and OA2, respectively. The Ep of the colloidal molecule reaches the minimum value %&'

√()

(4)

(),

when l1 = l2 = R + r and #

(

#

."//01[1 − () ]. This corresponds to

the case shown in Figure 4IV, in accordance with the configuration of the colloidal molecule MX2 depicted in Figure 1B. For the colloidal molecule MX3 (panel II in Figure 3E), 56

[() )(7 ] ()8

when l1 = l2 = l3 = R + r, # 0 and :

(

."//01[1 − () ]. This requires that the three colloidal particles have a geometric arrangement on the motor according to the configuration of the colloidal molecule MX3 shown in Figure 1C and Figure 4V. The E built by an irradiated motor generally covers a range larger than the sizes of colloidal molecule MXn (n = 1, 2, 3) and a SiO2 particle (Figure 3C). It can also polarize the surface charges of the assembled SiO2 particles (Figure 4III).41 Thus, additional SiO2 particles can further be loaded on the sites between the already assembled particles to obtain more complex colloidal molecules (MXn, n ≥ 4). As for the colloidal molecule with

4

particles,

%&'

[() )(7 ]

()8

+

√ )(

√" + 2 " + √3

and >

(

."//01 [1 − () ], and thus the most stable

structure are obtained when the forth colloidal particle moves along the surface of colloidal particle 1 until it is tangent to colloidal particle 2 or 3, resulting in MX4 with a configuration shown in Figure 1D and 3VI. The fifth and sixth assembled colloidal particles follow the same principle, and the stable structures of the colloidal molecules with 5 or 6 assembled colloidal particles can accordingly be predicted, as shown in Figure 4VII - VIII. Here, to simplify the %&' calculation process, we assumed that the assembled colloidal particles on the motor did not affect the distribution of electric potential, but in real situation the proton would highly concentrate in the concave interparticle spaces (red dots in Figure 3E),29 causing high local proton concentration and high electric potential. This makes the colloidal assembly occur at more sites, producing different isomers (Figure S2) with Ep higher than the calculated %&' when n is 5 or more. In addition, the geometric restriction of the concave interparticle spaces on the attached colloidal particles may also contribute to the formation of isomers. From above analysis, we conclude that the driving force of light-controlled assembly of colloidal molecules comes from the local electric filed around the TiO2/Pt Janus micromotor and the light-switchable electrostatic interactions between the micromotor and SiO2 colloidal particles. The formation of the specific close-packed configurations of the colloidal molecules is mainly governed by the minimization of the electric potential energy. When the UV light is off, the photogenerated charges disappear in the micromotors. Thus, the local electric field vanishes. This leads to the dissociation of the


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formed colloidal molecule into individuals as the electrostatic attractive force between the micromotor and the colloidal particles no longer exists, as shown in Figure 2B and Figure 4VIII - IX. When the UV irradiation is turned on again, the TiO2/Pt Janus micromotor would move and carry out another colloidal assembly operation.

Figure 5. The optical microscopy images of the complex colloidal molecules of (A) M2X9 and (B) M3X8 combined by some simple swimming colloidal molecules under UV irradiation. For easy identification, the black dots in the microscopy images, corresponding to micromotors, are replaced by schematic diagrams of Janus microspheres and indexed as a, b and c, d, e. (C) A swimming colloidal molecule, in which positively charged amide-functionalized PS particles assembled at the rear of a TiO2/Pt Janus micromotor, demonstrating that positively charged particles can also be assembled on the micromotor to generate colloidal molecules. (D) The schematic illustration of the assembly of positively charged particles on a TiO2/Pt Janus micromotor. SiO2(-):



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Negatively

charged

SiO2

colloidal

particles.

PS(+):

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Positively

charged

amidine-functionalized PS colloidal particles.

Corresponding to the molecules with multiple central atoms, dynamic colloidal molecules can also be assembled by multiple micromotors. As illustrated in Figure S4 and Video S4, when two swimming colloidal molecules (MX3 and MX2) gradually approached to each other (0 - 3 s in Figure S4), they would form into a large and sophisticated colloidal molecule (M2X5) due to the electrostatic interaction between them derived from the generated electrical field by the motors. Once formed, the large colloidal molecule typically moved forward in a spiral way and could continuously capture and assemble SiO2 colloidal particles in its path, growing into a more complicated colloidal molecule with a configuration of M2X9, as shown in Figure 5A. Colloidal particles assembled on micromotor a also subject to an attractive force originated from micromotor b, which makes the two colloidal molecules combine together. As the resultant driving force of the two motors usually deviates from the mass center of the large colloidal molecule, the large colloidal molecule moves spirally. With the same mechanism, an even more complex colloidal molecule with more micromotors, such as M3X8 (Figure 5B), is also obtained. When positively charged PS particles (PS(+)) substitute for negatively charged SiO2 particles (SiO2(-)), the PS(+) particles are attached to the rear of the motor, which refers to the Pt end, and move forward with the micromotor, as shown in Figure 5C and Video


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S5. This could likewise be ascribed to the attractive forces between the PS(+) particles and the negatively charged Pt end of the motor (Figure 5D). Thus, swimming PS(+) colloidal molecules are generated similar to the SiO2(-) ones but at the different attached locations of the TiO2/Pt Janus micromotors under the UV irradiation. This is consistent with the electrostatic interaction mechanism, where the attraction occurs between the active micromotor and charged particles. In this way, we can employ Janus micromotors to assemble a wide range of particles with different surface properties into dynamic colloidal molecules. Recently, Palacci et al. have reported the formation of the dynamic ordered colloidal structures (“living crystals”) within a single herd of Janus micromotors.42 In comparison, this work have demonstrated the ordered assembly of active micromotors and passive colloidal particles, which shows a great potential to promote crystallization or induce phase separation of glassy passive colloidal particles with different properties according to the theoretical prediction proposed by Stenhammar et al.43 Furthermore, our strategy here may also be used to construct dynamic heterogeneous colloidal structures (molecules and crystals) with a wide range of colloidal units and sophisticated structures.



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Figure 6. Numerical simulation results illustrating the light convergence effect and the formation of patterned spots by an assembled hexagonal colloidal molecule (A) when light propagates along Z direction: (B) The ray trajectories, (C, D) the time-averaged energy intensity of the incident light in a slice of the X-Z plane along line a-a in panel A with different incident angle of (C) 0o and (D) 32o. The radius of motor and SiO2 colloidal particles are 0.6 µm and 1 µm, respectively. The wavelength and power of UV light are 365 nm and 1 W, respectively. In panel B, the colors of the rays represent their optical path length.

As dynamical colloidal molecules possess a particular refractive index, spherical morphology and motion behaviors, Figure 6 demonstrates the numerical simulation of light propagation in a hexagonal colloidal molecule. As shown in the numerical simulation obtained by Ray Optics Module of COMSOL in Figure 6B, UV light with a


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wavelength of 365 nm show a convergence phenomenon when it propagates through the assembled colloidal molecule, suggesting the potential application of the colloidal molecules as dynamic microlens arrays. To further probe how the incident light is manipulated by the colloidal molecule, we also have simulated the distribution of the time-averaged energy intensity of the incident light in a slice of the X-Z plane along line a-a, as shown in Figure 6C and D. It can be seen that the light intensity reaches a much higher value directly beneath the colloidal lens, referring focusing spot of the light. The UV light cannot penetrate the motor because the light intensity exponentially attenuates with increasing the propagation distance in TiO2 due to the strong light absorption.38 Changing the incident angle, pronounced changes in the direction of light propagation are observed, as shown in Figure 6C and D. The above results indicate that the intensity of the incident light can be manipulated into controlled patterns corresponding to the configuration of the colloidal molecules. Hence, combining the capabilities of light manipulation and movement, the swimming colloidal molecules developed in this work may act as swimming microlens arrays for the mask-free lithography.44 Although the micro/nano-electromechanical systems provide many effective methods and techniques to fabricate small devices, such as microsensors, mirror arrays, microfluidic devices and microlens arrays, through a top-down approach, they still have limitations including high cost, complex fabrication steps and inflexibility etc.45-46 In contrast, due to the capability of collecting and assembling different functional colloidal



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microparticles, the TiO2/Pt Janus micromotor may play as a novel platform to fabricate dynamic microdevices through a bottom-up process to break these limitations. CONCLUSION In summary, we have demonstrated the swift, reversible and in-place assembly and dissociation of dynamic colloidal molecules by light-controlled TiO2/Pt Janus micromotors in a high accuracy. The UV-induced charge separation in TiO2/Pt Janus micromotors and the charge polarization in colloidal particles make primary contributions to the assembly of the colloidal molecules. The as-assembled dynamic colloidal molecules possess configurations finely controlled and stabilized by regulating the intensity of UV light, which governs the stop-and-go motion of the colloidal molecules and the number of the assembled colloidal particles. The dissociation of the colloidal molecules can be easily executed by turning off the light irradiation because of the light-switchable electrostatic interaction between the motor and colloidal particles. Due to the inherent optical properties of colloidal microparticles, the assembled dynamic colloidal molecules may act as swimming microlen arrays. The results demonstrated in this work not only shed a light on the routes to synthesize various dynamic colloidal molecules consisting of colloidal particles with a wide range of chemical properties, but also may provide a novel approach for in-place building functional microdevices in a swift and reversible manner, such as microlens arrays. Experimental section Preparation of TiO2 microspheres


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Monodisperse TiO2 microspheres with a size of 1.2 um were prepared according to the previous report.47 Briefly, 100 µl NaCl solution with concentration of 0.1 M was added into 25 ml ethanol, followed by adding 0.425 ml tetrabutyl titanate (TBT), the solution was stirred for 18 minutes and then sedimentated for 24 h. The white precipitation was collected by centrifuged and washed with water for three times. The product was dried under 60 °C for 24 h and then calcined at 500 °C for 2 h. Then 0.01g anatase TiO2 microspheres dispersed into 4 mL 0.129 mM chloroplatinic acid hexahydrate aqueous solution. The dispersion aqueous liquid was illuminated under xenon lamp for 20 min with stirring simultaneously. The resulting Pt-TiO2 products, which refer to TiO2 microspheres loaded with platinum nanoparticles, were collected by centrifugation, washed with deionized water several times. Preparation of TiO2/Pt microspheres A 50 µL Pt-TiO2 ethanol suspension was dropped onto a modified glass slide (25*25 mm), which was previously cleaned with piranha solution, washed with deionized water for several times, and dried at ambient temperature. The anatase Pt-TiO2 microspheres were partially covered with a platinum layer (about 4 nm in thickness) by ion sputtering for 40 s under a pressure of 0.6 Pa (Model E-1020 Hitachi ion sputter). Finally, the TiO2/Pt Janus microspheres were obtained by sonicating the glass slide in deionized water for about 10 s. Recording microscopy videos and analysis



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100 µL aqueous suspension containing 50 µL 2 wt% hydrogen peroxide aqueous solution, and 50 µL aqueous suspension of SiO2 microspheres with the size of 2 µm were dropped on a glass slide, followed by some TiO2/Pt Janus microspheres were added into it, in which the hydrogen peroxide concentration of the system was 0.67 wt%. A UV light source with a wavelength of 365 nm was placed above the surface of suspension, and the UV intensity on the surface of the suspension was 1 W/cm2. The motions of the TiO2/Pt Janus microspheres and the assembly process of SiO2 microspheres under the continuous UV irradiation were observed and recorded at room temperature through an optical microscope (Leica DMI 3000 M). All videos were analyzed by Video Spot Tracker V08.01 software and edited by Corel Video studio software. Numerical simulation All numerical simulations were conducted by using the commercial software COMSOL MultiphysicsTM. We used 2D simulation to solve the proton concentration distribution and electric potential around a TiO2/Pt Janus micromotors. The Electrostatics Module and Transport of Diluted Species Module were the physics used in building the whole model. In our 2D simulation, the radium of micromotor was 1.2 µm and the motor was placed in a square of 100 µm which was filled with H2O2. The fluxes on TiO2 and Pt ends were set to be 7×10-6 mol/(m2·s) and -7×10-6 mol/(m2·s), in which the plus and minus represented the output and input. Zeta potential (ζ) of the TiO2/Pt micromotors was set to be -8 mV. For numerical simulation of the light convergence effect when it propagated through the assembled colloidal molecule, the Ray Optics Module was used to compute the ray


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trajectories. A wave with a wavelength of 365 nm was executed as a source wave in the simulation. For the numerical simulation of different incident angle when it propagated through the assembled colloidal molecule, we used the Electromagnetic Wave Module and Perfect matched layer with the thickness of 1 µm to simulate the distribution of the time-averaged energy intensity of the incident light in X-Z cross-section plane. ASSOCIATED CONTENT Supporting Information

The following files are available free of charge via the Internet at http://pubs.acs.org.

Supporting videos description and supporting figures (PDF)

Supporting videos VideoS1 to Video S5 (AVI)

AUTHOR INFORMATION

Corresponding Author

E-mail: [email protected], [email protected]

Notes

The authors declare no competing financial interests.

ACKNOWLEDGEMENTS



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This work was supported by the National Natural Science Foundation of China (21474078, 51303144 and 51521001), the Top Talents Lead Cultivation Project and Natural Science Foundation of Hubei Province (2015CFA003), the Yellow Crane talents plan of Wuhan municipal government and the Fundamental Research Funds for the Central Universities (WUT: 2016III009 and 2015III060). We also acknowledge Prof. Wei Wang and Mr. Chao Zhou from Harbin Institute of Technology (Shenzhen), Mr. Chang Long and Mr. Dawei Hu for their valuable discussion about the manuscript.

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