X-ray Powered Micromotors - ACS Applied Materials & Interfaces

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X-ray Powered Micromotors Zhaoyi Xu, Mojun Chen, Hyeonseok Lee, Shien-Ping Feng, Jae Yeon Park, Sangsul Lee, and Ji Tae Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00174 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Materials & Interfaces

X-ray Powered Micromotors Zhaoyi Xu1, Mojun Chen1, Hyeonseok Lee1, Shien-Ping Feng1, Jae Yeon Park2, Sangsul Lee2, and Ji Tae Kim1,* 1Department

of Mechanical Engineering, The University of Hong Kong, Pokfulam road, Hong

Kong, China 2Industrial

Technology Convergence Center, Pohang Light Source, Jigokro 127, Pohang 37673,

Republic of Korea *Email: [email protected] ABSTRACT Light-powered wireless manipulation of small objects in fluids has been of interest for biomedical and environmental applications. Although many techniques employing UV-Vis-NIR light sources have been devised, new methods that hold greater penetrating power deep into medium are still in demand. Here, we develop a method to exploit X-rays to propel half-metal coated Janus microparticles in aqueous solution: The Janus particles are simultaneously propelled and visualized in real-time by using a full-field transmission X-ray microscopy. Our real-time observation discovers that the propulsive motion follows the bubble growth enhanced by water radiolysis near the particle surface under X-ray irradiation. We also show the propulsion speed is remotely controlled by varying the radiation dose. We expect this work to open opportunities to employ light-powered micro/nanomotors in opaque environments, potentially by combining with medical imaging or non-destructive testing.

KEYWORDS: micromotors; light-driven actuators; X-rays; water radiolysis; microbubbles

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INTRODUCTION Propulsive motion of small objects that outruns Brownian diffusion in fluids has long been regarded as interesting research among scientists.1-3 Furthermore, the propulsion at small scales has been employed to drive artificial micromotors4 aiming at accomplishing microscopic tasks such as drug/cell delivery,5-9 microsurgery,10,11 environmental remediation,12 nanofabrication,13,14 and so on. Propulsion mechanisms that convert various forms of energy into directed motion include catalytic self-propulsion,15-17 external stimulation by electric-, magnetic fields,18-22 light,23,24 and ultrasound,25,26 and combinations thereof.27,28

Light stimulation is wireless, reversible, and tunable, therefore emerging as a powerful method to remotely control micro/nanomotors in fluids. Light-matter interactions with high spatiotemporal resolution can trigger chemical/physical reactions that enable propulsion with high precision and individual addressability.29,30 Examples include light-driven diffusiophoresis,31 electrophoresis,32,33 thermophoresis,34-37 bubble recoil,38 interfacial tension gradient,39 and selective deformation.40,41 Most of the light-responsive micromotors have successfully employed UV-Vis-NIR light with diverse bandwidths.42-46 However, it is still a longstanding challenge to achieve the efficient transfer of light power to the micromotors in opaque environments. Conventional UV-Vis-NIR light sources hold insufficient penetrating power deep into biological medium, which is limited to a few millimeters.47 Therefore, it is necessary to develop new propulsion methods that can improve the penetration.

Since discovered by Röntgen,48 X-rays have long been regarded as the most celebrated light source for non-invasive imaging in medicine, biology, and materials science. Particularly,


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high penetrating power of X-rays enables whole-body, in vivo medical imaging and continuing scientific demands have advanced the radiation sources of X-rays – e.g. synchrotron sources – for improving brightness and coherence.49 Nevertheless, to the best of our knowledge, X-rays have never been employed as an external stimulus for propelling micromotors in fluids. We emphasize that the capability to steer and track micromotors under whole-body imaging platform50-52 may potentially accelerate their clinical use.

Here, we report on a method that exploits X-rays to generate propulsive motion of micromotors in aqueous environment. Half-copper coated silica (Cu/SiO2) Janus microparticles can be propelled under X-ray irradiation, resulting from enhanced radiolysis of water near the particle surface. To stimulate and visualize the propulsive motion at single-particle level, we use a full-field transmission X-ray microscopy (TXM)53-55 with synchrotron hard X-rays that hold excellent brightness and coherence. Our real-time microradiographic observation discovers that the radiolytic bubble growth56 propels the Janus particle and its motion coincides with the advancing of the bubble surface. We also show the propulsion speed of the Janus particle is remotely controlled by the pulse width of X-rays associated with the amount of radiation dose, resulting in on-demand manipulation.

RESULTS AND DISCUSSION Figure 1a illustrates the experimental setup based on TXM consisting of a focusing mirrors system, a motorized sample stage, a zone plate, a phase plate, and an image detector. To propel and visualize Cu/SiO2 Janus particles, focused hard X-rays with an energy of 6.3 keV are illuminated onto a thin liquid chamber containing the particle suspension. The pulse width of X-



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rays in the range of milliseconds is controlled by a beam shutter with a constant period of 2 s. The X-ray driven propulsion mechanism is schematically illustrated in Figure 1b. When X-rays are irradiated onto a Cu/SiO2 Janus particle, a microbubble is formed on the Cu side of the particle. As the bubble grows, the particle is propelled toward the advancing of the bubble surface. We propose that the bubble growth may result from water radiolysis that is the decomposition of water molecules due to ionizing radiation. It is well known that hydrogen gas, H2(g), is one of the main radiolytic products. The chemical reactions that form H2 gas are as follows.56

― ― + eaq +2H2O →H2 +2OH ― eaq ― + H• + H2O→H2 + OH ― eaq

H• + H•→H2

(1) (2) (3)

― where eaq is the aqueous electron and H• is the hydrogen free radical. To explain the formation

mechanism of the bubble on the Cu side, it is necessary to consider the radiation dose transfer at metal/water interface.57 It has been reported that the radiation dose transfer from metal to water is greater than that from silica to water due to larger stopping power difference. Therefore, the enhanced water radiolysis at the Cu/water interface accelerates the H2 bubble formation on the Cu surface.

Sequential microradiographs in Figures 1c-f (Movie S1, Supporting Information) show the propulsive motion of a Cu/SiO2 Janus particle with 6 µm diameter (Figure S1, Supporting Information) in aqueous solution (1:1 water/glycerol with 5 M acrylamide) under X-ray irradiation. The addition of glycerol and acrylamide into water tackles two issues in this real-time observation


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study. First, the increased solution viscosity suppresses gravity-driven sedimentation of the particles. Second, acrylamide acts as a scavenger to reduce the byproducts of water radiolysis,58 therefore inhibiting undesired random generation of H2 bubbles over the solution under intense synchrotron X-rays. The width and period of the illuminated X-ray pulses are 70 ms and 2 s, respectively. Before bubble formation (Figure 1c), no propulsive motion is observed. Once a microbubble is formed on the particle, the motion of the particle drastically transforms from random motion to ballistic motion as shown in the red trajectory in Figures 1d-f. The bubble formation and subsequent growth on the particle at a single entity level are successfully visualized by phase-contrast X-ray imaging. It is not easy to visualize the microbubble surface and bubble/particle interface with a conventional optical microscope due to strong visible light scattering.59,60 We observe that the particle movement coincides with the advancing of the bubble surface, proving the propulsion mechanism. Note that such bubble-driven propulsion occurs in the presence of the Cu side. The motions of a Cu/SiO2 Janus particle and an SiO2 particle under Xrays were experimentally investigated as shown in Figure S2 and Movie S2 in Supporting information. No bubble formation on the SiO2 particle is observed (Figure S2b) while the Cu/SiO2 Janus particle is propelled by the bubble growth under the same X-ray irradiation (Figure S2a). Their representative trajectories are displayed in Figure S2c and S2d, respectively. The role of the Janus shape is to aid asymmetric bubble growth on the particle, promoting the directed motion of the particle.

Our quantitative study in Figure 2 reveals that the propulsion dynamics of the Janus particle is governed by the radiolytic bubble growth under X-ray irradiation. Figure 2a plots the radius of the bubble 𝑅(𝑡), as a function of time 𝑡, formed on a Cu/SiO2 Janus particle (diameter: 6



µm) under pulsed X-rays with a width of 70 ms and a period of 2 s. The 𝑅 ― 𝑡 relation during the bubble growth can be approximated as61,62 𝑅(𝑡) = 𝐾𝑡𝑛

(4)

where n is the growth exponent and 𝐾 is the proportionality constant. The increase of 𝑅(𝑡) follows the power law with n = 0.39 ± 0.02, which is similar to n = 1/3, implying that volumetric bubble growth rate on the particle is constant under the given X-ray irradiation. The displacement of the Janus particle, 𝑆(𝑡), has a similar trend with the 𝑅(𝑡) as shown in Figure 2b. This result provides the opportunity to control the particle motion by varying the bubble growth. The propulsion performance can also be evaluated by the instantaneous speed of the particle. Figure 2c plots the bubble growth rate, 𝑑𝑅 𝑑𝑡 (circle), and particle propulsion speed, 𝑣(𝑡) = 𝑑𝑆 𝑑𝑡 (square), as a function of time, 𝑡. At the initial stage when the bubble starts to push the particle (𝑡 < 3 sec), the propulsion speed, 𝑣, is gradually increased and reached to the maximum value of ~ 0.85 µm/s. The trend of the initial 𝑣(𝑡) is deviated from 𝑑𝑅 𝑑𝑡 due to the drag force of the particle acting against to the bubble growth force.61 In the later stage (𝑡 > 3 sec), when the bubble is large enough, the particle speed, 𝑣, follows a similar trend to the bubble growth rate 𝑑𝑅 𝑑𝑡 and gradually decreases with time. As a result of the transition between these two regimes, the maximum propulsion speed 𝑣𝑚𝑎𝑥 at 𝑡 ~ 3 sec is observed. This transition is suppressed as the particle size decreases (as experimentally shown in Figure S3, Supporting Information). The observation time was set up to 2 mins because neighboring bubbles could bother measuring the propulsion as their sizes increased. In this experimental condition, the bubble bursting or detachment from the particle was not observed due to high viscosity in the solution although they could provide oscillatory and consistent propulsion.61 Thorough investigations to control the bubble bursting/detachment (e.g. the use of surfactant in Figure S4) are still underway.


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One central question – can X-rays tune the motion? – motivates us to investigate the dependency of the propulsion dynamics on radiation dose. To change the radiation dose, the pulse width of X-rays was controlled by a mechanical shutter. To characterize the particle motion with constant image acquisition rate, the period of X-ray pulses was fixed at 2 s. First, Figure S5 (Supporting Information) plots the required time for initial bubble formation as a function of the pulse width of X-rays at a constant period of 2 s. It is clearly shown that the bubbles are formed earlier as the pulse width increases. More importantly, the particle propulsion driven by the bubble growth is governed by the radiation dose. Figure 3a plots the propulsion speed of the Janus particle, 𝑣, as a function of time at different pulse widths of X-rays. As the pulse width increases from 50 to 110 ms, the 𝑣 ― 𝑡 profile gradually rises. As plotted in Figure 3b, the maximum propulsion speed, 𝑣𝑚𝑎𝑥 (𝑡 ~ 3 sec), increases from 0.47, 0.73, 0.92, to 1.21 µm/s as the pulse width increases from 50, 70, 90, to 110 ms. This basic investigation confirms the radiolysis-driven propulsion dynamics can be instantly tuned by varying the amount of X-ray dose.

To investigate the enhancement of radiolysis near the Cu surface of the Janus particle, we calculated the radiation dose in water/glycerol solution under hard X-ray irradiation of 6.3 keV, obtained by numerically solving the Monte Carlo simulation of electron and photon transport (described in Supporting Information).63 We used the Electron Gamma Shower, National Research Council (EGSnrc) code system.64 Figure 4 plots the profile of the energy deposited in the water/glycerol solution, i.e. radiation dose, near the Cu/SiO2 Janus particle. As the distance from the particle surface decreases, the amount of the deposited energy increases. Particularly, we point out that the average energy deposited in the water/glycerol near the Cu surface (< 1µm distance)



is 28.2 % larger than that near the SiO2 surface, which can enhance water radiolysis on the Cu side. The enhancement loses its effectiveness as the distance from the particle surface exceeds 1 µm.

CONCLUSIONS In this work, we have demonstrated that X-rays can propel metal-coated Janus microparticles in aqueous environment. The real-time TXM enabled us to study X-ray driven propulsion dynamics at single-particle level. We revealed the propulsion mechanism is based on radiolysis of water: the particle motion follows the growth of H2 bubble on the particle under Xray irradiation. We showed the propulsion performance, i.e. the instantaneous speed, can be controlled by instant variation of the radiation dose. Although in this work we demonstrated only spherical particles for translational motion, the locomotion can be diversified if one utilizes 3D shaping for controlling geometry-dependent hydrodynamics and different bubble propulsion behaviors. Furthermore, the selection of diverse metals can provide the potential to control the propulsion performance. For example, as shown in Figure S6 (Supporting Information), Au-coated surface drives faster propulsion than Cu-coated surface under X-rays at 6.3 keV. We expect this study at single-particle level could build up basic knowledge enabling remote control of micro/nano-robots by whole-body computed tomography (CT). Specifically, our X-ray powered micromotors could have the potential to enhance the performance of diagnosis and radiotherapy.65 The practical materialization may necessitate thorough explorations of open questions in terms of the motor’s size/function and the radiation dose associated with risks.66


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EXPERIMENTAL METHODS Sample preparation. SiO2 beads with a diameter of 6 µm were deposited on a Si wafer by twostep spin coating (200 rpm 10 s and 8000 rpm 5 s). The Si wafer was cleaned by rinsing with acetone, isopropanol, and deionized water under sonication for 5 minutes each, and an O2 plasma process for 5 minutes. To produce Janus microparticles, 5 nm-thick Ti (as adhesion layer) and 100 nm-thick Cu were deposited on the hemispheres of the SiO2 beads by sputtering. The thickness of these deposited metals was carefully characterized by an atomic force microscope (AFM) (Bruker MultiMode 8). The wafer was sonicated for 1 minute to release the Janus microparticles into deionized water. A centrifuge process (10000 rpm and 10 minutes) was performed to obtain sufficient concentration of the Janus microparticles. The particle suspension was prepared in water/glycerol mixture (1:1 volume ratio) with 5 M acrylamide to tackle two technical challenges for stable observation: (1) High viscosity of the solution ( ~ 100 mPa·s) suppresses gravity-driven sedimentation of the particles. (2) Acrylamide acts as a scavenger to reduce the byproducts of water radiolysis,58 therefore inhibiting undesired random generation of bubbles in the solution. The prepared Janus particles were characterized using FE-SEM (LEO 1530, Zeiss) with EDX analysis.

X-ray microradiography and stimulation of micromotors. The experiment was carried out using TXM consisting of a focusing mirrors system, a motorized sample stage, a zone plate, a phase plate, a scintillator, a magnifying lens, and a CCD detector, installed at 7C X-ray Nano Imaging (XNI) beamline in Pohang Light Source (PLS-II). The beam current and storage energy of the synchrotron facility are 360 mA and 3 GeV, respectively. The X-rays were monochromated at 6.3 keV using a silicon (111) double-crystal monochromator and focused on sample using a



mirror system. The beam flux at the sample position was estimated to ~1011 photons s-1. The exposure time of X-rays was controlled by a mechanical beam shutter with a constant period of 2 s. A 100 µm-thick liquid chamber with two SiN windows (purchased from Norcada) was used to contain 0.5μL-volume particle suspension. The dimension of the SiN window was 500 μm (width) × 500 μm (height) × 100 nm (thickness). The off-axis illuminated Fresnel zone plate was used as an objective lens of X-rays with an outermost zone width of 50 nm and a diameter of 350 µm. The pinhole-type Zernike phase-contrast plate enhanced the image contrast from a 10 µm diameter hole in an aluminum film of 3.84 µm ± < 0.04 µm thick. X-ray images were detected in a Tb:LSO scintillator crystal (15 µm thickness), converted to optical images, and magnified with a 20 X objective. A CCD camera (Apogee Alta: 4096 x 4096 pixels, 9 µm pixel size, 16-bit digital resolution) was used.

Dose calculation. We use EGSnrc code for Monte Carlo simulation. EGSnrc can simulate the transport of photons with energy lies between 1keV to several hundred GeV in any material or geometry. We use the EGSnrc C++ class library to write our application. A monoenergetic cylindrical shaped parallel beam with 10 μm radius is simulated and 1 × 109 photons’ tracks were calculated. The energy of the beam is 6.3 keV and the energy absorption cut off we set is 1 keV. The target simulated here is a water/glycerol (1:1 volume ratio, intimately mixed, density 1.14 g/cm3) mixture sphere with 500 µm radius and a 100 nm Cu coated Janus Particle with 6 µm diameter is placed in the center of the chamber.


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SUPPORTING INFORMATION Figure S1. FE-SEM and EDX images of the Cu/ SiO2 Janus particles. Figure S2. Cu/SiO2 Janus particle vs. SiO2 particle under X-rays. Figure S3. Propulsion speed of Cu/SiO2 Janus particles with different diameters. Figure S4. Bubble bursting on Cu/SiO2 Janus particle. Figure S5. Required time for bubble formation vs. pulse width of X-rays. Figure S6. Au/SiO2 Janus particle vs. Cu/SiO2 Janus particle under X-rays. Movie S1. Propulsive motion of a Cu/SiO2 Janus particle in 1:1 water/glycerol solution with 5 M acrylamide under pulsed X-rays Movie S2. Half-Cu coated SiO2 Janus particles (left) and bare SiO2 particles (right) suspended in the solution under the same X-ray irradiation.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACKNOWLEDGMENTS This work was supported by the Early Career Scheme (HKU 27207517) and General Research Fund (HKU 17208218) from the Research Grants Council of Hong Kong, the Seed Fund for Basic Research from the University of Hong Kong (201611159002), the MOTIE (Ministry of Trade, Industry & Energy) (10080526), and KSRC (Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device.



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Figure 1. X-ray powered micromotors. (a) Schematic illustration of experimental setup: A transmission X-ray microscopy is utilized to propel and visualize a Cu/SiO2 Janus particle in a liquid thin chamber. The X-ray energy is fixed at 6.3 keV and the pulse width in the range of milliseconds is controlled by a beam shutter with a constant period of 2 s. (b) The Janus particle is propelled by bubble growth from water radiolysis under X-ray irradiation. (c-f) A series of transmission X-ray microradiographs and trajectories (in red) shows propulsive motion of a Cu/SiO2 Janus particle (diameter: 6 µm, thickness of Cu: 100 nm) in 1:1 water/glycerol solution with 5 M acrylamide under pulsed X-rays (pulse width: 70 ms, period: 2 s). (c) 0 sec, (d) 2 sec; the formation of a microbubble on the particle, (e) 4 sec; translational motion driven by the bubble growth, and (f) 38 sec.


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Figure 2. Propulsion dynamics. (a) The plot of bubble radius, R(t), as a function of time, t, induced by X-ray radiolysis on a Cu/SiO2 Janus particle (the pulse width of X-rays: 70 ms, period: 2 s). The R-t relation is approximated by a power law R(t) ~ tn with n = 0.39 ± 0.02. Each plot is obtained from the individual trajectory of each bubble. (b) The plot of particle displacement, S(t), as a function of time, t. Each plot is obtained from the individual trajectory of each Janus particle. (c) The plots of the bubble growth rate (circle) and propulsion speed (square) as a function of time, t.



Figure 3. X-ray dose effect. (a) The plot of the propulsion speed of the Cu/SiO2 Janus particles driven by radiolytic bubble growth, v(t), as a function of time, t, at different pulse widths of Xrays from 50 to 110 ms. (b) The plot of the maximum speed of the Janus particles, vmax(t) as a function of pulse widths of X-rays.


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Figure 4. The energy deposited in water/glycerol mixture by hard X-rays of 6.3 keV near (square) the Cu side and (circle) the SiO2 side of the Janus particle, calculated from Monte Carlo simulation.



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X-ray Powered Micromotors - ACS Applied Materials & Interfaces

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