Microfluidic Fabrication of Bubble-Propelled Micromotors for

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22761−22767

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Microfluidic Fabrication of Bubble-Propelled Micromotors for Wastewater Treatment Meng Ren, Weilin Guo,* Huaisu Guo, and Xiaohua Ren School of Water Conservancy and Environment, University of Jinan, Jinan 250022, China

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S Supporting Information *

ABSTRACT: Bubble-propelled micromotors with controllable shapes and sizes have been developed by a microfluidic method, which serves for effective wastewater treatment. Using the emulsion from microfluidics as the template, monodisperse micromotors can be fabricated in large quantities based on phase separation and UVinduced monomer polymerization. By adjusting the volume ratio of the two immiscible oils (ethoxylated trimethylolpropane triacrylate/paraffin oil) in the initial emulsion, the geometry of the resulting micromotor can be precisely controlled from nearly spherical, hemispherical to crescent-shaped. The size of the micromotor can be manipulated by varying the fluid flow parameters. In addition, by incorporating functional nanoparticles into the asymmetric structure, the micromotor can be functionalized flexibly for water remediation. In this research, Fe3O4 and MnO2 nanoparticles were successfully loaded on Janus micromotors. Fe3O4 nanoparticles can act as catalysts for pollutant degradation and also control the movement direction of micromotors. MnO2 nanoparticles on the concave of the micromotor catalyzed H2O2 to produce bubble propulsion motion in solution, which further enhanced the degradation of pollutants. Consequently, the obtained micromotor demonstrated effective degradation of methylene blue and can be easily recovered by magnets. Furthermore, this simple and flexible strategy offers a synthetic way for anisotropic Janus particles, which will broaden their potential application. KEYWORDS: bubble-propelled micromotor, microfluidic, phase separation, controllable synthesis, water treatment

1. INTRODUCTION Micromotors have drawn considerable attention in the past decade, due to their extensive practical applications in the fields of biomedical diagnostics,1−4 drug delivery,5,6 environmental monitoring, and cleaning.7−10 Self-propelled micromotors are capable of moving autonomously in solution by converting various energy into autonomous motion. In terms of the propulsion mechanism,11 the micromotors can be driven by the catalytic reaction12−18 or by external stimuli such as magnetic control,19,20 ultrasound,21 electric fields,22 and light.23−25 Among them, micromotors driven by gas bubbles can perform efficient water purification by enhancing mass transfer, which has attracted highly popular concern.26−34 To form the propulsion for bubble-propelled micromotors, the asymmetrical structure of the micromotor is crucial to generate directional bubbles.13 Mei et al. first developed catalytic tubular micromotors which propelled by unidirectional microbubbles.12,13 In general, a variety of bubble-propelled micromotors can be fabricated by template-assisted electrochemical deposition,35−37 rolling-up of nano-films,12,13,38 and selfassembly.39−42 For example, Guix et al. synthesized PEDOT/ Pt tubular micromotors by an electrodeposition technique for removing oil pollutants.43 Soler et al. utilized the rolled-up method to synthesize Fe/Pt tubular micromotors for the degradation of rhodamine 6G (R6G) via the Fenton-like reaction.44 However, these methods not only involve multiple steps but also utilize expensive equipment, and the large-scale preparation of micromotors is challenging for them. Recently, bubble-propelled micromotors with asymmetrical structures © 2019 American Chemical Society

have been achieved using the template from emulsions based on phase separation.34,45,46 However, the micromotors fabricated with the traditional mechanical stirring show polydispersity in size. Thus, it is in high demand to utilize a facile method for fabricating micromotors with multiple functions, controllable morphologies and sizes, and powerful motion. Nowadays, microfluidic techniques have been developed for the controllable fabrication of multicompartmental emulsion droplets and particles.27,47,48 By utilizing the microfluidic technique to generate controllable emulsions as templates, monodisperse microparticles with complicated compositions, diverse morphologies, and multifunctionality can be obtained, which offer candidates to fabricate the desired Janus micromotors.49 For instance, the microfluidic approach was employed to generate double emulsions as the template for nonspherical micromotors. The incorporation of functional nanoparticles endowed micromotors the ability for an efficient degradation for wastewater.47 Furthermore, on the basis of exploiting the microfluidic technology to obtain bubble-propelled micromotors, it is preferable to realize the precise control of shapes and sizes, which may extend the range of practical application. Here, we reported a flexible fabrication of the bubblepropelled micromotors with controllable sizes and shapes by combining the means of microfluidic and phase separation. A facile microfluidic device was introduced to synthesize Received: April 4, 2019 Accepted: June 5, 2019 Published: June 5, 2019 22761

DOI: 10.1021/acsami.9b05925 ACS Appl. Mater. Interfaces 2019, 11, 22761−22767

Research Article

ACS Applied Materials & Interfaces emulsions composed of two immiscible oil phases dispersed in an aqueous continuous phase. The disperse phase was composed of photopolymerizable monomer ethoxylated trimethylolpropane triacrylate, nonpolymerizable paraffin oil (PO), and co-solvent ethyl acetate. With the volatilization of ethyl acetate, the original droplets gradually evolved into Janus structures during the phase separation process. By utilizing the Janus droplets as templates, functional nanoparticles were decorated into different regions and further fixed by UV-induced polymerization. The microparticles with various morphologies were controllably fabricated, such as near spherical, hemispherical, and crescent shapes. Moreover, the size of microparticles could be precisely controlled by manipulating the flow fluid conditions. The incorporation of functional materials into the asymmetric Janus structure offers an extensive application for micromotors. MnO2 nanoparticles depositing on the concave surface can decompose fuel H2O2 into directional bubbles to realize self-propulsion of the micromotor. Fe3O4 nanoparticles can act as the catalysts to generate hydroxyl radicals from strong oxidant H2O2 via the Fenton reaction, and control the movement direction of micromotors. The selfpropelled micromotors can be recovered by an external magnetic control. Finally, the micromotors demonstrated an efficient degradation of pollutants in aqueous solution.

Figure 1. Schematic illustration of a well-designed capillary microfluidic device for one-step generation of Janus emulsions. nonpolymerizable. Ethyl acetate (EA) was used as a co-solvent to ensure that the mixed solution was clear and transparent. Then, MnO2 nanoparticles modified with oleic acid and Fe3O4 nanoparticles were added into the mixture to form the disperse phase. An aqueous solution containing 1% (w/v) Pluronic F127 was used as the continuous phase fluid. The generated droplets were collected in the solution with composition the same as that of continuous phase fluid. After being emulsified by the water phase, the ETPTA−PO−EA homogeneous droplets can be formed continuously and steadily. The obtained emulsions can remain in contact with each other for a long time without coalescing because they were stabilized with the surfactant Pluronic F127. Following the evaporation of ethyl acetate, the disperse phase gradually formed Janus emulsions with narrow size distribution. In addition, by manipulating the volume ratio of ETPTA/PO in the oil phase or flow parameter, droplets with various shapes and sizes can be controllably fabricated. The generated Janus droplets were polymerized with ultraviolet light for 10 min. It took at least 5 h to allow the evaporation of ethyl acetate present in the droplets before exposure to UV light. The obtained microparticles were washed with ethanol and deionized water three times and then dispersed into isopropanol for extracting and removing paraffin oil. Finally, the micromotors were dried at 30 °C under vacuum conditions. 2.4. Degradation of Methylene Blue. To study the ability of the micromotors for wastewater treatment, methylene blue (MB) was used as a model contaminant. MB degradation was carried out in a cuvette (diameter 1 cm, height 4.5 cm) containing 3 mL of solution with 6.67 mg/L MB, 6.7% (w/w) H2O2, 6 mg/mL Janus micromotors, and 0.3 wt % Triton X-100. Removal of MB was measured at different remediation times using a UV−vis spectrophotometer (UV-5100B). 2.5. Characterization. Scanning electronic microscopy (SEM) images were taken through an SEM machine (FEI QUANTA FEG250, FEI Company). Energy-dispersive X-ray (EDX) (INCA Energy XMAX-50, Oxford Instruments) was characterized for element analysis on micromotors’ surfaces. Optical microscope videos and images were obtained with the optical microscope (BX53, Olympus) and analyzed by EZ-MET software. The UV−visible spectrophotometer (UV5100B, Shanghai Metash Instruments Co., Ltd.) was used to obtain the concentration of methylene blue. The wavelength for measuring is 664 nm.

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. Ethoxylated trimethylolpropane triacrylate (ETPTA), polyethylene−polypropylene glycol (Pluronic F127), Triton X-100, and 2-hydroxy-2-methyl-1-phenyl-1-propanone (HMPP) were purchased from Shanghai Macklin Biochemical Co., Ltd. Paraffin oil (PO), ethyl acetate (EA), and oleic acid (OA) were purchased from Tianjin Damao Chemical Reagent Factory. Potassium permanganate (KMnO4), manganese sulfate (MnSO4), ferric chloride hexahydrate (FeCl3·6H2O), hydrogen peroxide (H2O2), sodium acetate, ethylene glycol, sodium citrate, and methylene blue (MB) were obtained from Sigma-Aldrich. The deionized water was acquired from a Milli-Q system in all experiments. Synthesis of MnO2 and Fe3O4 nanoparticles: α-MnO2 was prepared by the modified method described by Pokhrel et al.50 1.94 g of KMnO4 and 0.845 g of MnSO4 were separately dissolved in 15 mL of H2O. The solution was mixed and transported to the autoclave followed by reacting for 12 h at 160 °C. The MnO2 nanoparticles were purified by washing several times with H2O and dried at 60 °C overnight. The manganese dioxide nanoparticles were modified with oleic acid and finally dispersed in ethyl acetate to form a suspension for further use. Fe3O4 nanoparticles were synthesized by a solvothermal method.51 FeCl3·6H2O (2.6 g), sodium acetate (4.0 g), and sodium citrate (1.0 g) were dissolved in ethylene glycol (80 mL) and stirred well to form a homogeneous disperse yellow liquid, and then the prepared solution reacted in an autoclave at 200 °C for 10 h. The black solid was washed with H2O three times and finally dispersed in ethyl acetate for further use. 2.2. Microfluidic Device Fabrication and Assembly. The microfluidic device is schematically illustrated in Figure 1. To fabricate emulsions as templates, two cylindrical capillaries were aligned coaxially inside a T-junction. The inner/outer diameter of the two capillaries for injecting the disperse phase and collecting the formed emulsions were 60/230 and 250/510 μm, respectively. The joint of the capillaries, tubes, and T-junction were sealed by glues. The fluids were delivered by two microsyringe pumps (LP01-1B, Longer Precision Pump Co. Ltd.). The disperse phase and continuous phase were injected through the inlet of the capillary and T-junction, respectively. The generated emulsions were collected in the Petri-dish. 2.3. Preparation of Micromotors. To obtain the template for micromotors, two immiscible fluids were used to generate the Janus emulsions. Photocurable monomer ETPTA containing 1% (v/v) photoinitiator HMPP was polymerizable, whereas paraffin oil (PO) was

3. RESULTS AND DISCUSSION 3.1. Generation of Janus Microparticles. Uniform emulsions produced by a glass capillary microfluidic device were used as the templates for preparing multi-functional micromotors. The strategy for generating Janus emulsions was combined with solvent evaporation-induced phase separation. By subsequent UV exposure, the microparticle was fabricated with the polymerization of monomer ETPTA. The formation 22762


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ACS Applied Materials & Interfaces mechanism of Janus droplets and microparticles is shown in Figure 2.

Figure 2. Diagram describing the generation principle of Janus microparticles, involved solvent evaporation driven liquid−liquid phase separation, UV-induced polymerization, and removal of the paraffin oil. Pink and purple circles represent ETPTA and PO, respectively. The star shape represents the co-solvent, ethyl acetate. Different colored dots represent different nanoparticles. Fe3O4 is denoted by black dots, whereas MnO2 is shown as red dots. White color represents air-filled holes.

Figure 3. Schematic representation of fabricating anisotropic microparticles with Janus droplets as templates. (a−d) With decreasing ETPTA volume fraction, the shape of emulsions can be turned precisely from near-spherical “moon” to crescent shape. (e1−h1) The corresponding micromotor particles were obtained after being exposed in UV light (Volume ratios of ETPTA/PO in the disperse phase: (a) ETPTA only; (b) 2:1; (c) 1:1; (d) 1:3. (e2−h2) The optical microscope images of the corresponding micromotors. Scale bars = 200 μm. (e3−h3) SEM images of the resulting polymer microparticles. Scale bars = 200 μm.

Using the ETPTA−PO−EA mixture as the disperse phase, homogeneous emulsions are formed under the shear force of the continuous phase on a coaxial microfluidic device. The disperse phase was composed of ETPTA, paraffin oil, and co-solvent (ethyl acetate). Once the emulsions were generated, the volatile component, ethyl acetate, steadily spread into the air. The continuous decrease of ethyl acetate turned the emulsion from homogenous to heterogeneous. Due to the immiscibility of ETPTA and PO, a liquid−liquid phase separation is driven by evaporation occurred in the emulsion. The PO phase was separated from the mixture, and the PO-rich layer in the mother droplet was formed. The nucleation−growth−coalescence process continuously occurs in the two parts of the droplet. The droplets are evolved into two phases to form a Janus shape. As a result, Janus droplets consisting of the PO-rich phase and ETPTA-rich phase were fabricated. During the process, the MnO2 nanoparticles modified with oleic acid spontaneously migrated to the interface between the ETPTA-rich phase and the paraffin oil. To reduce the contact area between ETPTA and PO for the minimization to total interfacial energy, MnO2 nanoparticles finally bonded with the concave interface. Meanwhile, hydrophilic Fe3O4 nanoparticles tended to adsorb at the interface between the oil phase and water. With the volatilization of ethyl acetate, droplets gradually evolved into the ETPTA−PO Janus structure. When exposed to ultraviolet light, the Janus microparticle was obtained because of the solidification of the photopolymerizable monomer ETPTA and the unsolidification of PO. As shown in Figure 2, the pink region represents poly(ETPTA) and the purple color represents PO. The obtained microparticles were washed with isopropanol to remove PO and then dried at 30 °C. Finally, the air-filled holes were formed in the particle as shown with white dots in Figure 2. The initial shapes of the microparticle, which depend on the contact angle between PO and ETPTA before ultraviolet irradiation, are described in Figure 3. The contact angle of ETPTA and PO was closely related to the interfacial tensions between two oil phases and water. 1 wt % surfactant Pluronic

F127 aqueous solution was chosen in all experiments. By simply adjusting the volume ratio of the immiscible components (ETPTA/PO) in ETPTA−PO−EA ternary mixtures, various structures of Janus emulsions (Figure 3a−d) were controllably fabricated. When exposed to UV light, the corresponding microparticles can be obtained (Figure 3e1−h1). Microscopy images of the obtained micromotors with different morphologies are shown in Figure 3e2−h2. With the decreasing ETPTA volume fraction in the disperse phase, the shape of the particle was turned from nearly spherical, hemispherical to crescentshaped. 3.2. Characterization of Janus Micromotors. To investigate the impact of disperse phase composition on the morphology of Janus microparticles, a series of Janus emulsions were prepared with different volume ratios between polymerizable ETPTA and nonpolymerizable PO. The volume ratios of ETPTA and PO were manipulated at 1:0, 1:1, 2:1, and 1:3 respectively. A series of homogeneous and transparent ternary mixtures were formed by mixing. By controlling the volume ratio of the immiscible phase (ETPTA/PO) in the ternary mixtures, Janus emulsion with various shapes can be produced in one step. A variety of complex Janus morphologies can be obtained by adjusting the composition of the disperse phase. After subsequent ultraviolet polymerization and removal of paraffin oil, microparticles of poly(ETPTA) can be obtained. In the SEM images, the shapes of the resulting polymer microparticles can be accurately converted from near spherical, hemispherical to crescent-shaped with the decreasing ETPTA volume fraction, which was consistent with ETPTA proportion in the emulsions (Figure 3e3−h3). The obtained microparticles with different morphologies were chosen to be observed under the optical microscope. The size of generated microparticles was with narrow size distribution (Figure S1, Supporting Information), which clearly 22763


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corresponding micromotor loading with nanoparticles is described in Figure 4c. Figure 4a1,b1 represent SEM images of magnified convex and concave surfaces of the hemisphericalshaped micromotor, respectively. Figure 4a2,b2 show the corresponding element analysis of convex and concave surfaces of the micromotor, which confirmed the presence of Mn, C, and Fe in the sample. EDX was used to analyze the nanoparticles loading on different surfaces of the micromotor. MnO2 nanoparticles existed on the concave surface of the hemispherical-shaped micromotor (Figure 4b1,b2). This indicated that OA-modified MnO2 nanoparticles tended to deposit on the interface between the two immiscible oil phases during the phase separation process. Meanwhile, Fe3O4 nanoparticles tended to load on the convex surface and inside ETPTA, as shown in Figure 4a1,a2, respectively. All of these characterization studies confirmed that the Janus micromotors loaded with functional nanoparticles were obtained by the process. 3.3. Study on the Self-Propelled Motion of the Micromotors. The micromotors combined with active MnO2 nanoparticles on the concave surface were used as catalysts for the decomposition of H2O2 to produce oxygen. Then, a large amount of oxygen accumulated to form bubbles, which could break away from the surface of the catalyst and enter the continuous phase of the medium. Schematic illustration of a plausible movement mechanism for the hemispherical-shaped micromotor propelled by hydrogen peroxide fuel is shown as Figure 5a. This asymmetric Janus structure of microparticles can produce asymmetric forces, which can realize the self-driven motion of micromotors in fuel solution. The motion of the bubble-propelled micromotor was determined by the shape of the micromotor, the quantity of

indicated that the droplets were monodisperse and had a narrow size distribution. Thus, precise control of dispersed phase composition allowed controllable evolution of the Janus emulsions for the synthesis of monodisperse Janus microparticles with advanced shapes. Moreover, the core surface of the microparticles could be independently modified by introducing nanoparticles into the liquids and fixed by polymerization. The results validated the successful synthesis of uniform anisotropic microparticles with different shapes using Janus droplets as templates. Additionally, the droplet size can be simply and precisely controlled by varying the flow rates of the disperse phase (Qd) and the continuous phase (Qc). The droplet size decreased with increasing the flow rate ratio (Qc/Qd), which was consistent with most emulsification processes using microfluidic devices. The size of microparticles can be simply changed by adjusting the effect of emulsification. Janus emulsions with controllable sizes were fabricated by manipulating flow fluid parameter for a certain microfluid device, aiming to obtain the corresponding microparticles. The frequency of the droplet production was controlled at Qd = 18, 28 μL/min and Qc = 100, 150, 300 μL/ min. The mean diameter of emulsion droplets decreased from 387, 313 to 280 μm with increasing the flow rate of the continuous phase (Qc) (Figure S2a−c, Supporting Information). By decreasing the flow rate of the disperse phase (Qd), the mean diameter of emulsion droplets decreased from 280 to 240 μm (Figure S2c,d, Supporting Information). Scanning electron microscopy (SEM) with EDX was performed to examine the surface and internal morphologies of the Janus micromotor. Different morphologies of the micromotors were produced after the removal of PO, from near-spherical moon, hemispherical to crescent shape, as shown in Figure 3e3−h3. In Figure 4d, the overall morphology of the micromotor (volume ratio of ETPTA and PO was 2:1) after UV exposure was hemispherical-shaped. A vivid illustration of the

Figure 5. (a) Schematic illustration of a plausible movement mechanism for the hemispherical-shaped poly(ETPTA) micromotor propelled by hydrogen peroxide fuel. The black dots and red dots represent Fe3O4 nanoparticles and MnO2 nanoparticles respectively. White color represents air-filled pores. (b) Effect of surface mass percentage of MnO2 nanoparticles on the velocity of the micromotor movement. The fuel solution is 20 wt % H2O2 and 1 wt % Triton X-100 for these circumstances. (c) Effect of fuel concentration on the velocity of the bubble-propelled micromotor. The fuel solution is H2O2 with 1 wt % Triton X-100. (d) Optical snapshots of the self-propelled micromotor in the fuel solution at time intervals of 0, 1, 2, and 3 s (Video S2). The fuel is 20 wt % H2O2 and 1 wt % Triton X-100. The white arrow direction circles show the position changes of the micromotors. Scale bars = 500 μm.

Figure 4. Scanning electron micrographs together with EDX data of the Janus micromotor. (a1, a2) The SEM image and element analysis of the magnified convex surface of the hemispherical-shaped micromotor. The scale bar is 1 μm. (b1, b2) The SEM image and element analysis of the magnified concave surface of the hemispherical-shaped micromotor. The scale bar is 4 μm. (c) Illustration of the corresponding hemispherical-shaped micromotor loading with nanoparticles. The black dots and red dots represent Fe3O4 nanoparticles and MnO2 nanoparticles, respectively. White color dots represent air-filled pores. (d) SEM image of the overall morphology of the micromotor after UV exposure. The scale bar is 100 μm. 22764


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process. Mixture solutions of hydrogen peroxide and methylene blue were prepared as the target wastewater. To quantitatively analyze the degradation process of MB, the concentration of MB in solution was measured by an ultraviolet−visible spectrophotometer. The absorption wavelength was set to 664 nm. After adding micromotors into the mixture, sufficient bubbles were continually formed upon coming into contact with H2O2 in the solution, which realized self-propulsion of the micromotor. At the same time, the Fe3O4 nanoparticles loaded on the micromotor produced hydroxyl radicals from strong oxidant H2O2 through the Fenton reaction, which was acted for the removal of MB. A digital camera was used to record the degradation process of samples containing bubble-propelled micromotors. The effectivity of the obtained micromotors was that Fe3O4 nanoparticles acted as catalysts to facilitate the degradation of H2O2, which promoted the degradation of methylene blue. MnO2 nanoparticles catalyzed H2O2 to produce bubbles (Figure 6a, cuvette C), which enhanced the mixing of the solutions, and

catalysts loaded on the concave surface, and the concentration of fuel.47 The obtained micromotors were washed with isopropanol (IPA) and ethanol to remove paraffin oil. Consequently, nanoparticles on the concave can be directly exposed to the chemical fuel such as hydrogen peroxide solution. The micromotors with different shapes were added to 20% (w/w) H2O2 solution, and then their motion was observed and recorded by the optical microscope (Olympus BX53) by taking continuous images. For the near-spherical moon-shaped micromotors (Figure S3, Supporting Information), because of the catalysis of MnO2, the oxygen bubbles were generated from H2O2, stayed on the concave surface of micromotor, and grew rapidly (Video S1). When bubbles were continuously generated from the concave surface, the reverse propulsion force prompted the movement of micromotor in solution. However, the large interfacial tension restricted the removal of bubbles from the concave surface. Consequently, the motion of micromotors was greatly limited, showing an extremely slow speed in solution. Therefore, 1 wt % surfactant Triton X-100 was added to the H2O2 solution for quick generation of smaller bubbles, to enhance the propulsion force and speed of micromotors. For a crescent micromotor, a large amount of bubbles was generated at the concave interface of ETPTA in a few seconds. Then these bubbles became large enough to prevent the contact of H2O2 and MnO2 nanoparticles. In this case, the bubbles were gathered on the large concave surface and hardly removed from the concave surface. Even with the assistance of Triton X-100, the micromotor still did not move forward in solution. Therefore, a volume ratio of ETPTA and PO in the dispersed phase of 2:1 was chosen to synthesize micromotors with a hemisphericalshape in the follow-up experiments. As shown in Figure 5b, the speed of the near-spherical shaped micromotor increased with increasing the amount of MnO2 nanoparticles. When the surface mass percentage of MnO2 nanoparticles was increased from 16 to 70%, the corresponding velocity of the micromotor increased from 75.12 to 424 μm/s. Another impacting factor on the speed of the micromotor was the concentration of the chemical fuel. These micromotors were dispersed into 5, 10, 15, and 20% (w/w) H2O2 solution, and the velocity was calculated according to the micromotor position on the moving route. The movement of the micromotor was observed by Olympus BX53, and the EZ-MET software was used for further analysis. Adequate oxygen bubbles propelling the micromotor at a certain speed confirmed that these micromotors can achieve self-driving in solution. As shown in Figure 5c, the speed of the micromotors increased with the increase in H2O2 concentration. When the concentration of hydrogen peroxide was controlled at 20%, the velocity of the micromotor was about 127 μm/s (Figure 5d and Video S2). As the micromotor was also incorporated with Fe3O4 nanoparticles, it showed the possibility of using an external magnetic field to achieve the desired direction of the motion of micromotor. The micromotor can move in the specified direction by changing according to the magnetic field control. Moreover, the recovery of micromotors with magnetic material can be easily realized. 3.4. Study on the Pollutant Degradation of the Bubble-Propelled Micromotors. The micromotor-combined Fe3O4 nanoparticles were used as the catalyst of the Fenton reaction to degrade organic contaminants. Methylene blue (MB) is a commonly used an organic dye, which appears blue in oxidized form and colorless in reduced form. Hence, methylene blue was used as a contaminant for testing the performance of micromotors in the pollutant degradation

Figure 6. Degradation of MB under different conditions (V = 3 mL; [MB]0 = 6.67 mg/L; [H2O2]0 = 6.7% (w/w); [micromotors] = 6 mg/ mL; [Triton X-100] = 0.3 wt %; T = 25 °C). Optical images of the degradation processes of MB over time. (a) 0 h, (b) 4.5 h, (c) collection of the micromotors by magnetic attraction.

further accelerated the degradation process of MB. With the addition of bubble-propelled micromotors, the MB concentration in the model wastewater decreased significantly during the first 4.5 h, showing 93.8% degradation, as the purple line shown in Figure 6. H2O2 was catalyzed to continuously generate oxygen, which pushed the micromotor to move around, thereby facilitating the spread of methylene blue. Finally, the color of the model wastewater became almost colorless (Figure 6b, cuvette C). At the same time, the model wastewater containing microparticles without any catalyst exhibited almost no change, shown as the blue line. Microparticles containing Fe3O4 nanoparticles showed only 47.41% degradation of MB, within the first 4.5 h (Figure 6). This result confirmed the promotion of MB degradation with self-propelled micromotors. In addition, micromotors loaded with Fe3O4 nanoparticles can be recovered using magnets (Figure 6c). All in all, the bubble-propelled micromotors played an active role in the degradation process. 22765



4. CONCLUSIONS In this paper, a universal microfluidic method was presented to fabricate bubble-propelled micromotors using Janus droplets as templates, which exhibited efficient water remediation. Microfluidics, evaporation-induced liquid−liquid phase separation, and subsequent UV-induced polymerization, were combined to obtain the desired micromotors. By flexibly controlling the fabrication conditions, Janus micromotors loaded with MnO2 and Fe3O4 nanoparticles simultaneously on their different regions were prepared. By simply adjusting the volume ratio of two immiscible components (ETPTA/paraffin oil) in ternary mixtures, the micromotors with various morphologies can be achieved. The shapes of micromotors can be precisely controlled from near-spherical moon, hemispherical to crescent shape with decreasing the ETPTA volume fraction. The size can also be precisely controlled by manipulating the flow rate ratio (Qc/Qd) for a certain microfluidic device. The movement of the micromotor can be influenced by the micromotors’ shape, the quantity of catalysts loaded on the concave surface, and the concentration of fuel. The bubble-propelled micromotor with multifunctionality was applied for wastewater remediation and collected by a magnetic field after the degradation process. These micromotors as new devices exhibited a considerable opportunity in the future treatment of industrial wastewater, in view of the easy removal from the system, reusability, and the potential in heavy elements recovery. In addition, this strategy offered a novel pathway to synthesize anisotropic materials with controllable shapes and sizes. The current droplet microfluidic technology combining with evaporation-induced phase separation showed an appealing prospect for fabricating large-scale monodisperse particles. These particles with advanced shapes and functions will be allowed for practical application in wastewater treatment and other fields.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the National Natural Science Foundation, China (Grant Nos 51578264, 41877132).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05925. Microscopy images of micromotors with different morphologies; optical microscopy images of particles with different sizes and the optical microscopy images of the self-propelled micromotor (PDF) Bubble propulsion of Janus micromotor in the fuel (AVI) Optical snapshots of the self-propelled micromotor in the fuel solution (AVI)

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86 531 8276 9233. ORCID

Weilin Guo: 0000-0002-5684-8954 Author Contributions

This manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 22766


Research Article

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