magnetically modulated walnut

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Biological and Environmental Phenomena at the Interface

One step fabrication of dual optically/ magnetically modulated walnut-like micromotor Dan Wang, Gang Zhao, Chunhao Chen, Hui Zhang, Ruomeng Duan, Dafeng Zhang, Mingtong Li, and Bin Dong Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02904 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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One step fabrication of dual optically/magnetically modulated walnut-like micromotor

Dan Wang,a Gang Zhao,a Chunhao Chen,a Hui Zhang,*a Ruomeng Duan,b Dafeng Zhang,c Mingtong Li,*a Bin Dong*a

aInstitute

of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-

Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P. R. China, E-mail: [email protected] bSchool

of Environment and Civil Engineering, Dongguan University of Technology, Dongguan

523808, P. R. China cSchool

of Materials Science and Engineering, Liaocheng University, Liaocheng, Shandong

252000, P. R. China

KEYWORDS: micromotor, multi-responsive, one-step fabrication, motion control, environmental remediation

ACS Paragon Plus Environment

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ABSTRACT In this paper, we report a novel multi-responsive walnut-like micromotor consisting of polycaprolactone (PCL), iron oxide nanoparticles (Fe3O4NPs) and catalase, which is constructed through a one-step electrospinning method. Based on the catalytic activity and photothermal and magnetic responsiveness originating from catalase and Fe3O4NPs, respectively, the resulting micromotor exhibits autonomous movement in the presence of hydrogen peroxide (H2O2) fuel, controlled motion velocity under light irradiation and guided movement direction upon the application of an external magnetic field. Owing to the hydrophobic nature of the PCL polymer constituent inside the micromotor, the autonomous moving micromotor can collect spilled oil inside a solution once it collides with the oil droplet. Since the micromotor could be separated out using a magnetic field, we believe the current walnut-like micromotor holds great promise in the field of environmental remediation.


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INTRODUCTION Micromotors are artificial devices that are self-propelled in a liquid environment to perform certain tasks.1 The autonomous moving micromotors have been demonstrated to have fascinating capabilities, such as increased reaction kinetics and enhanced recognition efficiency, making them potentially attractive in biomedical field,2 environmental monitoring3 or sensing applications.4 The autonomous motion of the micromotor can be realized by harnessing the energies from the chemical fuels in the solution.5 Ion gradient, concentration gradient or bubbles generated by the chemical reaction could be exploited to drive the micromotor based on different mechanisms including self-electrophoresis,6 self-diffusiophoresis,7-9 bubble propulsion,9,10 etc.11 Among others, the bubble recoil mechanism remains as an important way to propel the micromotor, which offers significant advantage as compared to the other ones, such as high locomotion velocity, long motion span, etc.12 For micromotors, in addition to the autonomous movement which is random in nature, it is also desirable to govern their motion in a precise fashion (such as controlled moving velocity, direction and trajectory, etc.). To this end, external fields are often utilized to achieve motion manipulation of a micromotor. Up till now, a number of external stimuli (such as light,13,14 magnetic,15 electricity16) have been exploited to control the micromotor’s movement behavior, which rely on the incorporated responsive materials (such as metals,17 oxides18 and semiconductors19) inside the micromotor. With the aid of these different external fields, various accurate motion behaviors and functions have been obtained. For example, when magnetic responsive materials are introduced into the micromotors, the magnetic field controlled maneuverability makes them attractive candidates for cargo capture, transportation and targeted delivery.20


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In addition, in order to obtain a micromotor, a variety of fabrication methods have been developed, such as template synthesis, photolithography, soft lithography, self-assembly method, etc. Template or lithographic methods are frequently utilized in conjunction with electrochemical,21 thermal deposition,22 roll-up technique or self-assembly method,23 etc. These combinations result in micromotors with a variety of structures (rod, tube, sphere,24,25 roll-up tube, Janus structures26) and compositions (various functional or responsive inorganics or polymers). Despite these progresses, the micromotor fabrication not only involves multiple steps but also utilizes expensive equipment. Therefore, facile and inexpensive construction technique to obtain micromotor preferably with multi-responsiveness is urgently needed. Up to now, only a few one-step fabrication strategies to obain multi-functional micromotors have been reported.2729

In this paper, we report a multi-responsive walnut-like micromotor fabricated by one-step electrospinning method. The micromotor consists of PCL, Fe3O4NPs and catalase, which has an asymmetric structure due to the phase separation process during electrospinning. The resulting micromotor not only exhibits autonomous motion in solution containing H2O2 based on the bubble propulsion mechanism but also shows multi-responsiveness with controlled motion speed and direction. The multi-responsiveness of the current micromotor relies on the combination of the catalytic activity of the enzyme, the photo-thermal effect and magnetic property of the Fe3O4NP. And there is a synergistic interaction between the enzyme and Fe3O4NP, i.e. the photothermal effect of Fe3O4NP can boost the catalytic activity of the enzyme, thus promoting the micromotor’s motion velocity. More interestingly, thanks to the hydrophobic nature and the porous structure of the micromotors, the walnut-like micromotor can be directly utilized to collect spilled oil droplet without the introduction of multi-layered structures or additional


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surface modifications with hydrophobic materials30,31 usually required by other oil-removalmicromotors. We thus believe the one step fabrication, multi-responsiveness and oil removal capability make the current walnut-like micromotor attractive for environmental remediation.

EXPERIMENTAL SECTION Materials. Catalase, fluorescein isothiocyanate (FITC), sodium carbonate buffer, 30 nm Fe3O4NPs, mineral oil and sodium cholate were purchased from Sigma Aldrich Company. PCL (Mn = 80000), dichloromethane and dimethyl sulfoxide (DMSO) were obtained from J&K Scientific. The FITC labeled catalase was prepared according to the previous study.32 Briefly, 10 mg catalase were dissolved in 2 mL sodium carbonate buffer solution (pH = 8.5). 2 mg/mL FITC were then added to the solution and stirred for 12 h at room temperature. After 3 times centrifugation and cleaning, the obtained FITC labeled catalase solution was stored at 0 ˚C. Preparation of the walnut-like micromotor. PCL was first dissolved in dichloromethane, resulting in a solution with a concentration of 4 wt%. Catalase and Fe3O4NPs were then dispersed in the PCL’s dichloromethane solution and sonicated for 30 min. The concentrations of catalase and Fe3O4NP were 40 wt% and 0.4 wt% (unless otherwise mentioned), respectively. The polymeric solution containing catalase and Fe3O4NPs was then loaded to a syringe, which was placed on a syringe pump and pumped out at a speed of 5 mL/h. The end of the syringe was a 21 gauge needle with a blunt end, which was connected to the high voltage power source. After applying a voltage of 18 kV, the polymer solution was ejected from the syringe needle toward a grounded collector which was placed 10 cm away from the needle end.


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Characterizations. Scanning electron microscope (SEM) characterization was performed by utilizing a Carl Zeiss Supra 55 scanning electron microscope with an energy dispersive X-ray (EDX) analysis attachment. The sample for SEM observation was prepared by directly collecting the electrospun microstructure on a cover glass. A 6 nm thick gold was first sputtered onto the structure before the SEM observation. The fluorescence microscopic images of the resulting walnut-like micromotor were obtained under a Leica DM4000M fluorescence microscope with 490 nm excitation. To initiate the motion, the resulting walnut-like micromotor was placed in the aqueous solution containing 10 wt% H2O2 (unless otherwise mentioned). For the oil removal experiment, an oil dispersed solution was first prepared by dispersing mineral oil in water. 1 wt% sodium cholate was added to the solution in order to stabilize the emulsion droplet. 10 wt% H2O2 was then added to start the micromotor motion. The light source utilized to control the moving speed of the walnut-like micromotor was Model X-Cite 120Q mercury lamp (Excelitas Technologies). The autonomous and controlled movement of the walnut-like micromotor was observed and recorded by using a Nikon Eclipse 80i microscope. The motion behavior was obtained by analyzing the captured video based on the PhysVis software. UV-Vis spectra were obtained on a PerkinElmer Lambda 750 spectrophotometer. The magnetically controlled motion was achieved by placing a neodymium magnet 2 cm away from the walnut-like micromotor. The temperature around the walnut-like micromotor was measured using an I5771-01 noncontact infrared thermometer (Aladdin Company). The contact angle was measured by utilizing a Dataphysics OCA 20 contact angle system.

RESULTS AND DISCUSSION


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Figure 1. (a) Schematic of the walnut-like micromotor fabricated by electrospinning method. (b) Low magnification and (c) enlarged SEM images showing the walnut-like micromotor structure. Corresponding EDX analysis of the walnut-like micromotor shown in (c) for (d) carbon and (e) iron. (f) Optical and (g) fluorescence microscopic image of the walnut-like micromotor containing FITC labeled catalase. (h) Sizes of the walnut-like micromotor under different electrospinning voltage. Electrospinning is a method widely utilized to fabricate the polymeric microstructure based on its solutions or the blend with other functional materials, including carbon nanomaterials, enzymes, etc.33 During the fabrication process, the polymeric solutions are normally loaded to a syringe with a blunt needle, which is pumped out at a constant speed and connected to a high voltage supply. Upon the application of the high voltage, the solutions are ejected toward the grounded collector and form microstructures after the solvent evaporates. In the current study, we directly electrospin the polymer blend solution containing a biopolymer, i.e. the PCL, Fe3O4NPs and an enzyme (catalase), resulting in the formation of the microparticle illustrated in Figure 1a. The typical low magnification SEM image of the resulting microparticles is shown in


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Figure 1b. It can be seen that the microparticle has a sphere shape with rugged surface similar to a walnut (Figure 1c), which is likely formed due to the vapor-induced phase separation mechanism. During vapor-induced phase separation, the polymer solution undergoes phase separation by penetration of nonsolvent from the vapor phase. Dichloromethane is a low boiling point solvent. During the rapid solvent evaporation process, PCL surface temperature decreases significantly. When the surface cools down, moisture in the air may be condensed and grow in the form of droplets. Small imprints are thus left behind on the surface of PCL microstructures due to the difference in the evaporation rate for dichloromethane and water.34 The unique porous structure of the walnut-like micromotor is suitable for the oil removal process due to the hydrophobic nature and the rough surface morphology. The one step fabrication is in contrast to the previously developed oil-collecting micromotors, which normally require multiple step surface modification process.30,31 In addition, the one-step electrospinning method enables the fast synthesis and mass production of micromotors with unique shapes, i.e. walnut-like structure, which is also different from other reported micromotors in terms of fabrication method and morphology. In order to confirm the composition of the microparticle structure, we have performed the EDX analysis on the resulting microstructure. As shown in Figure 1d,e, the carbon and iron elements, which originate from PCL/catalase and Fe3O4NPs, respectively, distribute evenly throughout the whole structure of the microparticle. In addition, we further confirm the distribution of the catalase inside the microparticle by labeling the catalase with a fluorescence dye, i.e. FITC. As indicated in Figure 1f,g, the microparticle is transparent under the optical microscope and fluorescent under the 490 nm light irradiation. By carefully comparing Figure 1f and 1g, we find out that only part of the microstructure is fluorescent. This thus confirms the non-even distribution of the catalase inside the PCL microstructure. For the design of the


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micromotor, asymmetry is the key, which facilitates the propulsion of a micromotor. Furthermore, the size of the microstructure can be facilely tuned by changing the electrospinning parameters. For example, as shown in Figure 1h, we can alter the size of the micromotor by adjusting the electrospinning voltage. The size of the resulting walnut-like micromotor is approximately 14 µm under 18 kV. Note that other parameters are constant (4 wt% PCL solution concentration and 10 cm needle to collector distance). Furthermore, we have studied the size distribution of this micromotor. As can be seen from Figure S1 in the supporting information, this walnut-like micromotor shows low monodispersity and narrow size distribution.

Figure 2. (a) Schematic showing the autonomous movement of the walnut-like micromotor in H2O2 solution. (b-e) A series of photographs taken at 3 s intervals illustrating the moving behavior of the walnut-like micromotor in the presence of H2O2. (f) The corresponding moving trajectory. (g) The velocity distribution of the walnut-like micromotors.


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Catalase is an enzyme that is known for its high catalytic activity to degrade H2O2 into oxygen and water with high efficiency. When the walnut-like micromotor is placed in H2O2 solution, the motion behavior is observed. Note that it is motionless in the absence of H2O2. Since the PCL microstructure contains pores on its surface. H2O2 may diffuse into the microstructure and react with catalase to generate oxygen bubbles. Due to the non-even distribution of the catalase inside the walnut-like micromotor, oxygen bubbles are only generated along one side of the microstructure where catalase is located. As a result, the walnut-like micromotor is propelled toward the other end based on the bubble propulsion mechanism, as illustrated in Figure 2a and Video S1 in the supporting information. Figure 2b-e indicate a series of optical microscopic images taken from Videos S1 in the supporting information indicating the motion behavior of this walnut-like micromotor. It can be seen that the micromotor adopts a spiral moving fashion, which is likely due to the deviation of the driving force from the mass center of the micromotor, leading to the formation of the rotational force in addition to the translational one. By analyzing the motion trajectory of this walnut-like micromotor (Figure 2f), the moving velocity of this micromotor can be estimated to be approximately 20 µm/s. By studying the motion velocity of a number of micromotors (n = 50), we have obtained the average moving speed of the walnut-like micromotor, which is around 18 µm/s (Figure 2g).


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Figure 3. The influence of (a) H2O2 concentration, (b) catalase content and (c) temperature on the moving velocity of the micromotor. We have studied the influence of different factors (e.g. H2O2 concentration, catalase content and temperature) on the moving velocity of the current walnut-like micromotor. Among others, as can be seen from Figure 3a,b, the motion speed of the walnut-like micromotor increases with the increasing H2O2 or catalase concentration. This is mainly caused by the higher bubble producing rate at high H2O2 or catalase concentration, thus increasing the velocity of the micromotor. We have compared the motion velocity of the current micromotor with others.10,35-38 As summarized in Table S1 in the supporting information, the walnut-like micromotor has a moderate movement speed. However, it is possible to further increase its velocity through proper design (detailed in the description part for Table S1 in the supporting information). On the other hand, the temperature also influences the moving behavior of the walnut-like micromotor. As shown in Figure 3c, the motion speed of the walnut-like micromotor is fastest under 37 ˚C. When the temperature is lower or higher than 37 ˚C, the walnut-like micromotor shows lower speed. This is mainly because the optimum temperature for catalase with the highest catalytic activity is 37 ˚C.39 Higher or lower temperature will inhibit its catalytic efficiency, thus decreasing the motion velocity.


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Figure 4. (a) Schematic of the moving behavior of the walnut-like micromotor under light irradiation. (b) Irradiation intensity dependent moving velocity. (c) Schematic illustration and (d) the corresponding motion trajectory showing the directional movement of the walnut-like micromotor upon the application of an external magnetic field. The trajectory is obtained from Video S4 in the supporting information. Interestingly, we find out that the moving speed of the walnut-like micromotor could be manipulated by light. As can be seen from Figure 4a and Video S2 in the supporting information, when light irradiates on the autonomous walnut-like micromotor in the solution, the motion speeds up. It is possibly due to the fact that the Fe3O4NP is a material with photothermal effect,40 which raises the temperature of the micromotor, leading to acceleration of walnut-like


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micromotor. In order to further prove this mechanism, we measure the temperature around the walnut-like micromotor. As shown in Figure S2 in the supporting information, the temperature increases with the increasing light intensity. Furthermore, we have studied the influence of light intensity on the moving velocity of the walnut-like micromotor. As shown in Figure 4b, the moving speed of the walnut-like micromotor can be promoted under the appropriate light intensity due to the elevated temperature and decreased if light intensity is too high, which is similar to that shown in Figure 3c. To examine whether the speed up motion is caused by light induced H2O2 decomposition or the temperature gradient caused by the Fe3O4NP’s photothermal effect, we have performed a control experiment. We have synthesized the walnut-like microparticles (containing PCL and Fe3O4NP) without catalase (Figure S3a in the supporting information). Under light irradiation, H2O2 could decompose to produce bubbles. However, the micromotor without catalase remains motionless, as shown in Figure S3b in the supporting information. The corresponding movie is shown as Video S3 in the supporting information. This indicates that the bubbles caused by the light induced H2O2 decomposition could not propel the micromotor. Additionally, this also demonstrates that the temperature gradient caused by Fe3O4NP’s photothermal effect alone is not sufficient to drive the micromotor. Since the presence of light can make the walnut-like micromotor move faster, we have examined the lowest H2O2 concentration that could be utilized to propel the walnut-like micromotor. Under 0.7 W/cm2 light irradiation, H2O2 concentration as low as 0.1 wt% is sufficient to drive the micromotor, as indicated in Figure S4 in the supporting information. In addition, Fe3O4NP is also known as a magnetic material. As a consequence, the moving direction of the walnut-like micromotor can be remotely controlled by the application of an external magnetic field. As can be seen from Figure 4c,d and Video S4 in the supporting


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information, the autonomous moving behavior of the walnut-like micromotor in H2O2 solution is initially in a spiral fashion. When applying an external magnetic field, the walnut-like micromotor would quickly move toward the direction where the magnet is placed. Moreover, we have compared the velocities of the walnut-like micromotor before and after the application of a magnetic field, and find that there is almost no difference between them, which is about 20 µm/s. This result indicates that the magnetic field only changes the moving direction of the walnut-like micromotor, not the moving speed.

Figure 5. (a) Schematic diagram of the oil removal process using the walnut-like micromotor. Images of the walnut-like micromotor (b) before and (c) after contact with oil obtained from the Video S5 in the supporting information. The multi-responsive walnut-like micromotor may have great potentials in a variety of applications. As a proof-of-the-concept, we have explored its potential in environmental remediation. PCL is known to be a hydrophobic material due to its hydrophobic backbone. As


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indicated in Figure S5 in the supporting information, the PCL-based micromotor deposited on the surface of the cover glass has a contact angle of approximately 109°, showing its hydrophobicity. The hydrophobic nature of this micromotor indicates that we may use it to collect the hydrophobic pollutants inside the waste water. Figure 5a and Video S5 in the supporting information show the movement of the walnut-like micromotor inside the solution containing H2O2 with dispersed mineral oil droplet. It can be seen from the captured images shown in Figure 5b-c that the walnut-like micromotor moves autonomously by itself at the beginning. As it approaches the dispersed mineral oil droplet, due to the hydrophobic nature of PCL, there is a strong hydrophobic interaction between PCL and the oil droplet. As a consequence, the oil droplet sticks to the surface of the walnut-like micromotor. As the motion of the micromotor continues, more and more droplets are collected near the micromotor. Furthermore, we have performed a control experiment by dispersing the static walnut-like micromotor in the solution containing the dispersed mineral oil droplets. As can be seen from Video S6 in the supporting information, due to the static nature in the absence of H2O2, the oil cannot be adsorbed onto the surface of the walnut-like micromotor. This demonstrates the superiority of the walnut-like micromotor when utilized in the environmental remediation applications. In addition, since the walnut-like micromotor contains Fe3O4NPs, by utilizing magnetic field, the oil-collecting micromotor inside the solution can be separated out, providing a convenient way for oil pollution removal and micromotor recycling.


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Figure 6. (a) Transmittance (at 600 nm) of the oil solution with different concentrations. (b) UVVis spectra indicating the transmittance changes of the oil solution (2 mg/mL) containing 1 wt% sodium cholate, 10 wt% H2O2 and 5 mg/mL micromotors during the removal process at different time. (c) The removal of the oil from the solution based on the walnut-like micromotor (black curve) and the stationary counterpart (red curve, in the absence of H2O2). C represents the oil concentration at different removal time, while C0 is original oil concentration. (d) The CCD images of oil solution before (left) and after (right) the oil removal by micromotors. Furthermore, we have examined the capability of the current micromotor when utilized in oil removal from the oil water mixture solution. As illustrated in Figure 6a, the oil water mixture solution containing different oil content exhibits different transparency and the transparency is inversely proportional to the oil content. The solution with 2.2 mg/mL oil is non-transparent with a transmittance (at 600 nm) of around 20 %, which increases almost linearly with the decrease of the oil content and becomes transparent with a transmittance of 84 % in the case of 0.5 mg/mL


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oil content. This transmittance changes could thus be utilized to evaluate the oil removal efficiency of the current micromotor. As shown in Figure 6b, the transmittance of 4 mL oil solution containing 2 mg/mL oil, 1 wt% sodium cholate, 10 wt% H2O2 and 5 mg/mL micromotors gradually increases during the oil removal process. Note that the micromotors are first collected at the bottom of the cuvette by applying the external magnetic field prior to each measurement. Based on Figure 6a and 6b, we can estimate the real-time oil removal efficiency, as shown in Figure 6c black curve. It can be seen clearly that the oil removed by the micromotor is more than the stationary one (Figure 6c red curve, without the addition of H2O2), demonstrating the superiority of the autonomous moving micromotor. Figure 6d shows the typical CCD images indicating the oil removal process before and after the oil removal by micromotors. These images indicate that the micromotor is effective in removing the oil droplets in the oil/water mixture solution. And meanwhile we have evaluated the re-usability of the walnut-like micromotor. This micromotor can be re-used for at least 4 times, as shown in Figure S6 in the supporting information. CONCLUSIONS In conclusion, we report a multi-responsive walnut-like micromotor consisting of PCL biopolymers, catalase enzyme and magnetic Fe3O4NPs fabricated through one step method. Interestingly, the resulting polymer-based walnut-like micromotor not only exhibits autonomous movement in the presence of H2O2 due to the catalytic activity of catalase but also is responsive to both light and magnetic field based on the photothermal effect and the magnetic property of Fe3O4NPs. This, together with the hydrophobic nature of the PCL polymer, makes the current walnut-like micromotor applicable for environmental remediation. Among others, we have explored its potential in the oil removal from the polluted water.


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ASSOCIATED CONTENT Supporting Information. Table S1, Figure S1-S6 and Video S1-S6. This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Author Contributions The 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. ACKNOWLEDGMENT This work was supported by National MCF Energy R&D Program 2018YFE0306105, the National Natural Science Foundation of China (Grant No. 21574094) and the Collaborative Innovation Center of Suzhou Nano Science & Technology. It was also supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the 111 Project, Joint International Research Laboratory of Carbon-Based Functional Materials and


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TOC

We report a multi-responsive walnut-like micromotor, which is responsive to both light and magnetic field and can be utilized for the oil removal from the polluted water.


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magnetically modulated walnut

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