Emulsion-Hydrogel Soft Motors for Water

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Self-Propelling Hydrogel/Emulsion-Hydrogel Soft Motors for Water Purification Hui Wang, Xiaoyu Gu, and Chaoyang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00197 • Publication Date (Web): 23 Mar 2016 Downloaded from http://pubs.acs.org on March 26, 2016

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ACS Applied Materials & Interfaces

Self-Propelling Hydrogel/Emulsion-Hydrogel Soft Motors for Water Purification Hui Wang, Xiaoyu Gu and Chaoyang Wang* Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, China

* Corresponding Author E-mail: [email protected]. Tel & Fax: +86-20-22236269.

ACS Paragon Plus Environment

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ABSTRACT We fabricate a kind of catalytic self-propelling hydrogel soft motors (H-motors) via a facile injection loading method with low energy consumption. The factors influencing the practicability of H-motors, including locomotive ability and reusability are investigated. The succession of rapid bubble evolution and propulsion endows the millimetre-sized columnar H-motors with length/diameter=1 a remarkable speed of 3.84 mm s-1 in 10% (w/w) hydrogen peroxide (H2O2) solution. Moreover, the H-motors maintain undiminished propulsion capability and functionality even after repeated loading for 6 times. Additionally, we also fabricate emulsion-hydrogel soft motors (E-H-motors) templated from the oil/water (O/W) emulsion for the first time, which exhibit a faster speed of 4.33 mm s-1 under the same conditions. It can be ascribed to the additional liberation of low-boiling oil phase stored in the emulsion-hydrogels caused by catalytic reaction heat, which is appropriate for larger propulsive situation. The stabilized, efficient and reusable H-motors are selected for industrial effluents purification to fit the imperious demands about the disposal of organic pollutants in water. The synergy effect between catalytic degradation and enhanced intermixing of the fluid flow around the miniaturized soft motors gives rise to an effective and exhaustive removal of organic contaminants.

KEYWORDS: hydrogels, emulsion-hydrogels, self-propelling, motors, water purification


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1. Introduction Designing new self-propelling miniaturized motors and exploring their practical applications have been regarded as an immediate area of research focus.1-4 Great efforts have been devoted to the fabrication of efficient self-motion micromotors based on different propelling mechanisms, for

instance

chemically

powered

propulsion,5-9

light-induced

motion,10-14

self-

thermophoresis,15,16 Marangoni effect,17,18 or magnetically impetus.19,20 These micromotors exhibit a prominent locomotive capability in connection to multiple potential applications such as targeted drug or protein delivery,21-25 effective biodetoxification,26 oil removal,27,28 or water purification.29-31 Despite their diversified propulsion mechanism and wide range of potential application, several challenges in self-propelling motor still must be solved prior to their practical utilization, such as the expensive and complicated fabrication process, unsatisfactory reusability and low capacity of cargo transportation. Diversiform fabrication methods of self-propelled motors have been presented, such as rolling up the catalytic thin film to form a microtube motor utilizing photolithography and nanotechnology,32,33 or plating a catalytic metal tube layer inside the tubular motor using the galvanostatic method.34 In spite of their novel fabrication, the high energy consumption and complexity of catalyst loading methods restrict the utilization of self-propelled motors in practical application. Inexpensive hydrogels have attracted much attention to be a candidate material of self-propelling soft motors to implement in some applications such as water purification, due to their inherent facile catalyst loading capacity, excellent reusability, multifunctional integration ability and admirable formability. For example, a reported millimetre-sized columnar hydrogel motor (the diameter ≈ 7 mm) was prepared by simply adding functionalized particles before gel formation, which could roll around to enhance mixing and maintain the


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initial sewage remediation efficiency after 9 repeated self-propelling degradation processes in water treatment.35 As a sewage processor, this millimetre-sized hydrogel motor (H-motor) possesses a unique reusability and exerts a more apparent disturbance for enhanced mixing in comparison to the present nano/micro motors ranging from 300 nm to 1 mm in length.36 Furthermore, the millimetre-sized motor has an excellent retrievability and cyclic utilization capacity compared with these nano-scale motors which are too small to be caught for recycle. The cargo load and transport ability of hydrogel-based soft motors depends on their volume, with larger volumes giving rise to higher loading capacity.37 However, the millimetre-sized motors have relatively lower speed (e.g.: 0.69  0.32 mm s−1, in 22.5% H2O2)35 in the present reports. That is probably caused by the insufficient supply of propulsive force. Therefore, great efforts ought to be devoted to developing a facile and inexpensive approach to fabricate a versatile hydrogel-based soft-motor with powerful propulsive force, excellent retrievability, favourable cyclability and multiple integrated functions for practical application. Herein, we obtain a millimetre-sized, ultrafast, and reusable soft motors through the economical and facile fabrication method based on the inexpensive polyacrylamide (PAM) hydrogels. Different from the complicated and high energy consumption fabrication process of motors reported in previous research,6,34,38 the H-motor has been prepared by simply injecting the saturated KMnO4 solution as catalyst on the tail of the truncated cylindrical hydrogel. Then the catalyst solution could be absorbed rapidly due to the intrinsic hydroscopic capacity of the hydrogel. We expect this promising H-motor prepared by this green and facile method will facilitate the industrial application of miniaturized soft motors in water purification. The KMnO4 and its product MnO2 could catalyze H2O2 to generate hydroxyl radicals which are beneficial for the effective removal of organic contaminants in water.39,40 In addition to possess a good


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catalytic degradation effect as the previous motors,9,31,41 the miniaturized H-motor also have a considerable propulsive force which could propel itself to move in a high speed to yield an excellent mixture for the acceleration of purification in the process of water treatment. The cylindrical self-propelling H-motor with the length-diameter (L/D) ratio of 1:1 (diameter: 3 mm), in 10% (w/w) H2O2 aqueous solution has a swift speed up to 3.84 ± 0.36 mm s-1. Moreover, the excellent reusability of H-motor originates in the reversible conversion of water absorption and dehydration based on the intrinsic swelling property of polymer hydrogels. Furthermore, we put forward a novel hybrid driving mechanism for the first time performed by emulsion-hydrogel motors (E-H-motors) which simultaneously utilize catalytic effect and reaction heat to release multiple bubbles for a fast propulsive speed of 4.33 ± 0.30 mm s-1. The E-H-motor is fabricated through the oil/water (O/W) emulsion template method which deposit the sprayable oil phase into the hydrogel skeleton. In this way the low-boiling oil phase can cooperate with the chemical-produced oxygen bubbles resulting in synergetic propulsion for the E-H-motor. Overall, owing to the strong propulsive force, excellent integration capability and reusability, these two kinds of hydrogel-based catalytic soft motors also display a tremendous application potential in the intelligent soft robot fields by the incorporation of corresponding functionalized particles.

2. Experimental Methods 2.1. Materials. Acrylamide (AM), the monomer to synthesize hydrogels, was recrystallized from deionized water and dried in a vacuum oven at room temperature. N,N’-Methylenebisacrylamide (MBA) serving as a chemical cross-linker of acrylamide hydrogels, was supplied from Kemiou Chemical Reagent Co. Ltd. Initiator Potassium peroxydisulfate (K2S2O8, KPS) was provided by Sinopharm Group Chemical Reagent Co. Ltd and purified by recrystallization. N,N,


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N’,N’-Tetramethylethylenediamine (TEMED) acting as an accelerator was purchased by Aladdin Industrial Corporation. Tween 80, used as an emulsifier for dichloromethane (CH2Cl2) and water, was provided from Kemiou Chemical Reagent Co. Ltd. CH2Cl2, oil phase of the emulsion serving as a kind of propellant in the E-H-motors, was purchased by Guangzhou Chemical Factory. Hydrogen peroxide (H2O2, concentration: 30% w/w) serving as chemical fuels of the miniaturized motors, was provided by Guangzhou Chemical Factory. Potassium Permanganate (KMnO4), catalyst of the driving reaction, was supplied from Guangzhou Chemical Factory. Methyl Blue (MB) used as model pollutant to measure the effect of water treatment, was provided by Tianjin Fu Chen Chemical Reagent Factory. All adopted chemicals were analytical reagents and depurated by standardized methods prior to using. Deionized water (resistivity above 18.0 MΩcm) in all experiments was purified by Millipore purification apparatus (MA, USA) utilizing deionization and filtration. 2.2. PAM Hydrogels. Typical PAM hydrogel was prepared by the steps as follows. In view of the low dosage of chemical cross-linker, the MBA solution was prepared previously and dispersed uniformly through sonication. Firstly, 1 g of AM monomer, 1 mL of MBA solution (0.25 mg/mL), 3 mg initiator of KPS, were sequentially added into 4 mL deionized water followed by ultrasonic treatment, then the mixed solution was degassed for 10 min with N 2.42 After that, 20 L of accelerator TEMED was added into the solution and shook to well-blended. Then, the solution was transferred into some glass tubes to obtain a tubulose hydrogel with 3 mm diameters. Finally, the hydrogel sealed in glass tubes would reacted at room temperature for 20 h to form a covalently cross-linked hydrogel. 2.3. Emulsion-Hydrogels.

The emulsion-hydrogels were synthesized by a novel

monodispersed emulsion template.43,44 Firstly, the stabilizer Tween 80 (100 μL) was dispersed in


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deionized water (4 mL) by sonication in an ice bath for 10 min. Then 1 g of AM monomer, 1 mL of MBA solutions (0.25 mg/mL) and 3 mg of KPS were added into the dispersion. Blend them well with the assistance of ultrasonic bath. Subsequently, the O/W emulsion was obtained by adding the oil phase (5 mL, CH2Cl2) into the complex solution with high-speed shear treatment (6000 rpm, 3 min). Thereafter, catalyst TEMED was added under stirring. The polymerization was carried out at 5 oC

for 48 h. Emulsion-hydrogel samples for motor fabrication were

prepared in the glass tubes of 3.0 mm diameter  150 mm length. 2.4. Fabrication of H-Motors and E-H-Motors. To reduce the water-absorption of motors in hydrogen peroxide pool, the strategy of high-vacuum silicone surface coating was employed. The cylindrical hydrogel was cut into different length on-demand prior to smearing silicone grease on its lateral and one basal areal while keeping the other basal area uncovered for catalyst loading. The cylindrical hydrogel and emulsion-hydrogel were put on a sheet glass with the uncovered side on top respectively. Then 6 L saturated solution of KMnO4 was injected into the hydrogel after wetting the topside. The sheet glass was subsequently put in a dark and airtight environment for about 10 minutes till the solution was fully absorbed, forming the motor loaded with catalyst. The fabrication process of H-motor and E-H-motor were shown in Figure 1.


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Figure 1. The fabrication process of H-motor and E-H-motor.

2.5. Repeated Catalysis Loading of Motors.

KMnO4 solution of 6 L was injected into the

tail of H- and E-H-motors with L/D = 2:1. After complete absorption of KMnO4 solution through hydrogel’s swelling property, the motors were transferred into a specific H2O2 pool for selfpropulsion. When the motor stopped autonomously moving, it was taken out from the pool prior to erasing the H2O2 solution on its surface and then repeating the self-propulsion after processing with silicone grease and catalytic solution.

2.6. Characterizations. Tensile Test. Uniaxial stretch was performed on cylindrical hydrogel and emulsion-hydrogel (3 mm diameter and 40 mm length) with a Shimadzu AG-X plus testing system at room ambient temperature, and the crosshead speed was 100 mm min-1. The tensile stress was evaluated on the area of the original cross-section, and the tensile strain was calculated by the change in length relative to the initial length between sample and the clamps (20 mm).45 Swelling Measurement. The as-prepared hydrogels (3 mm diameter and 15 mm length) were immersed in deionized water at room temperature, and then weighed every 5 minutes for 30


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minutes in order to measure the swelling properties of the motors for a short period. The swelling ratio was calculated by the following equation: Ws  Wd (1) Wd Where Ws and Wd are the weights of the swollen hydrogels and the initial hydrogels (water Swelling ratio 

content: 80%), respectively. Optical Microscopy (OM).

Images of the CH2Cl2/water emulsions were recorded by an

Axiolab Polarizing Microscope (Carl Zeiss, Germany) equipped with a camera. Trajectory Tracking.

The videos of the motors’ trajectory were recorded with a CANON

IXUS 220 HS camera. The speed of the H-motors and E-H-motors were obtained by tracking and analyzing their motion with the NIS Elements AR 4.3 software. Scanning Electron Microscopy (SEM).

The internal morphology of the hydrogel and

emulsion-hydrogel were observed by a scanning electron microscope (SEM, Zeiss EVO 18) operated at 10 kV acceleration voltage. These as-prepared gels were quickly frozen by liquid nitrogen and then freeze-dried for 72 h to remove water thoroughly. The freeze-dried gels were sliced carefully and sputtered with gold before observation.46 The particles inside the reactive sites of motors were also examined by SEM with an X-ray energy dispersive spectrometer (EDS). Heat tests.

The reaction heat of KMnO4 and H2O2 was determined by TAM Air-8 channel

calorimeter (20 oC) with the assist of 20 mL admix ampoule. X-ray Diffraction (XRD). The catalytic sites of the motors after self-propulsion were cut off and heated to triturable state in a muffle furnace at 450 oC for 10 h. The X-ray diffraction (XRD) pattern of the calcined powders were obtained by using an X'pert PRO diffractometer (40 kV and 40 mA) equipped with a Cu Kα radiation (wavelength 0.154 nm) at room temperature.


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Ultraviolet-Visible Absorption Spectra (UV-Vis).

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The UV-Vis spectra were measured on a

Shimadzu UV-240 spectrophotometer to record the changes in the concentration of MB during water treatment.

3. Results and Discussion 3.1. H-Motors. In order to maneuver within the various and complicated environment, this hydrogel-based soft-motors ought to have excellent performance in strength and toughness to act as mollusks, remaining pliable under external pressure.47 The stress-strain curve of PAM hydrogel was tested with different weight ratios of MBA varying from 0.025 wt% to 0.125 wt% relative to AM monomer (Figure S1). With the decrease of MBA, the ruptured stress and strain increased significantly, while the stiffness decreased appreciably. When the ratios of MBA stayed at 0.025 wt%, the ruptured stress and strain both reached an optimum value (111.1 MPa, 1380%). On account of the scarce addition of MBA, the lower crosslink density of the hydrogel network may sharply increase the swelling ratio of PAM hydrogel. Therefore, the swelling behavior of hydrogels with different contents of MBA was investigated for 30 min (Figure S2). It could be seen that the swelling ratio of hydrogel didn’t change along with the concentration of MBA in this short period of time. The swelling ratio of PAM hydrogel reached 105% after 30 min (MBA: 0.025 wt%). Therefore, it could be inferred that the motor have a good absorption of catalyst solution. However, this kind of water absorption may result in larger self-weight of hydrogel, which would influence motions. High-vacuum silicone coating is an effective measure to reduce the water absorption so that the average swelling ratio in 30 min decreased from 92% to 68% (Table S1).


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A facile and effective injection loading method with low energy consumption was applied for the fabrication of H-motors based on the inherent swelling properties of hydrogel. As shown in Figure 1, the catalytic KMnO4 solution was injected on the tail of the pre-treated hydrogel. The H-motor was obtained till the KMnO4 solution was completely absorbed into the internal network of hydrogel. The propulsion mechanism of H-motor was shown in Figure 2a. The chemical reaction between KMnO4 and H2O2 produced enough O2 bubbles to achieve a selfpropulsion. Particularly, the catalytic KMnO4 had a concentration gradient when stored inside hydrogel matrix. The concentration of KMnO4 on the surface is higher than the interior. Owing to this special catalyst storage mode, the self-propelling process of H-motor can be divided into two steps. During the inceptive self-propulsion period, the enriched catalysts (KMnO4) on the tail of H-motors reacted with H2O2 immediately, which provided a relatively large instantaneous propulsion force, resulting in a high initial velocity. However, the relative strong force could not sustain for a long time due to the rapid consumption of KMnO4 on hydrogel’s surface. The following weaker driving force is generated from the continuous reaction between H2O2 and KMnO4 which had diffused from the interior of hydrogel. The movement stopped as KMnO4 ran out. To confirm the locomotive capacity of the millimetre-sized H-motors, the self-propulsion performances of the motors are illustrated in Figure 2b-g and the corresponding video (Video S1). For example, Figure 2b-d shows the trajectory of H-motors with different length-diameter (L/D) ratios (the length increases from 3 mm, 6 mm to 9 mm to obtain L/D ratios of 1:1, 2:1, and 3:1.) in 10% (w/w) H2O2 solutions during a 30 s period. The bubble-propelled H-motors move rapidly in spiral and circular trajectories. As presented in Figure 2d, though the L/D ratio of the hydrogel increased from 1 to 3, the largest catalytic motor still has a robust driving force and sails well.


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Additionally, the generated micro-bubbles are visible in the fitted trajectories, which exhibit a high frequency of bubble generations and strong propulsion force of this millimetre-sized Hmotor. Figure 2e-g depicts the speed of H-motors with different L/D ratios in a constant concentration of the hydrogen peroxide fuel (10% w/w). It was shown that the speed fluctuated continuously within a small range in 30 seconds, which may result from the change of the sample’s movement track or the experiment environment. Furthermore, the L/D ratio has a strong influence on the velocity of the miniaturized catalytic motor. As illustrated in Figure 3, a reduction in L/D ratios of H-motors from 3:1 to 2:1 did not lead to apparent acceleration. However, when continued to decrease the L/D ratio to 1:1, the average speed of H-motors commenced to increase significantly, reaching a much higher value of 3.84 ± 0.36 mm s -1 (almost 1.3 body lengths) in 10% (w/w) H2O2 solution. The phenomenon could be explained as follows: at the beginning of self-propulsion, KMnO4 concentrated in the surface of H-motor reacted with H2O2 immediately and gave smaller motor (L/D=1) a much higher propulsion power, which was consistent with the significant improvement of light motors’ average speed. However this initial acceleration from the concentrated KMnO4 was no longer obvious with the increasing of motors’ self-weight (L/D ratio), which result in an inconspicuous speed improvement as decrease the L/D ratio from 3:1 to 2:1. Though the increase of L/D ratio would reduce the propulsive speed in certain extent, the self-propelling H-motors still have a good locomotive capability. That might be attributed to the strong bubble-based propulsion force generated from vigorous catalytic reaction between KMnO4 and H2O2 fuels (Average speed=3.36 ± 0.21 mm s-1, L/D=3).


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Figure 2. (a) The propulsion mechanism of H-motor. The trajectories of H-motors with different L/D ratios: (b) 1:1, (c) 2:1, (d) 3:1. The speed of H-motors with different L/D ratios: (e) 1:1, (f) 2:1, (g) 3:1. Time interval is 1s; concentration of H2O2 is 10% (w/w). The inset in (b-d) is the fitted trajectory obtained by NIS Elements AR 4.3 software. The inset in (e-g) is the H-motors


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with corresponding L/D ratios.

Fundamentally, the thrust of the miniaturized motors depends on the rates of catalytic reaction which results in a faster bubble generation. Therefore, the higher peroxide concentration leads to more and faster oxygen bubbles generation which is beneficial for the effective propulsion. A control experiment under lower peroxide concentration was conducted to further reveal the fuel concentration adaptability of the catalytic motors. As shown in Figure S3, the H-motor (L/D=3) achieve a speed of 2.71 ± 0.31 mm s-1 in a 5% (w/w) peroxide solution. Although the fuel concentration dropped to a half (from 10% to 5%), the average speed of miniaturized motors still maintains an acceptable declining scope, around 26% of H-motor.

Figure 3. The average speed and motion time of H-motors with different L/D ratios.

The average motion life of H-motors with different L/D ratios is illustrated in Figure 3. The moving duration of H-motors is inversely proportional to their L/D ratio, and the possible reason was: 1) at the end of complete consumption of KMnO4, the propulsion force becomes quite small


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due to the decreased generation of bubbles. 2) The bigger L/D ratio led to larger original selfweight as well as larger contact surface, which was better for water absorption and resulted in bigger increment of the sample’s weight. Hence, the thrust became insufficient for the propulsion of the motors. Although the motion life would decrease with the increase of L/D ratios, the facile catalyst supplement through repetitive injection loading is an effective way to extend the life cycle of the hydrogel-based miniaturized motors. The distinctive merit of the hydrogel-based millimetre-sized motors lies in the facile method of catalyst supplement which make the motors more readily reused. After sixth loading of catalyst, the H-motors could still propel automatically without any motion attenuation and could even reach a higher speed (average speed in 30 s: 4.8 ± 0.30 mm s-1, see Figure 4a, b and Video S2). The spontaneous propulsion rule of H-motors after catalyst loading for six times can be explained as follows: at the beginning, the enrichment of catalyst (minor KMnO4 and generated MnO2) resulting from multiple loading makes the H-motor move extremely fast. An apparent descent of moving speed is seen from the H-motor in the next 10 s. After this duration, the speed of H-motor keeps in a constant. Figure 4c reflects the micro-structure of H-motors before selfpropulsion. The excellent porosity and pore-connectivity implied a connecting and robust network structure of H-motor, which is beneficial for adsorption, transmission and releasing of some functional substance like catalyst.48 Moreover, the reversible conversion of dehydration and water absorption endows the H-motor with extraordinary reusability because of the porous polymer hydrogel network.49,50 Micro-structure of motors after propulsion was shown in Figure 4d. Compared Figure 4c with Figure 4d, we could find that the size of pores became larger due to the gas releasing during propulsion. This catching suggests that incompact structure might be beneficial for gas releasing and facilitating the propulsion which implies that the control of


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hydrogel motor could be implemented by modulating its micro-structure. (The SEM images of H-motor in smaller magnification were shown in Figure S4.)

Figure 4. The trajectory (a) and speed (b) of H-motors for the 6th cycle. The inset in (a) is the fitted trajectory obtained by NIS Elements AR 4.3 software. The inset in (b) is the H-motors after 6th cycle. SEM images of H-motors (freeze-drying for 72 h) before (c) and after (d) selfpropulsion.


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Due to the intrinsic favorable hydroscopicity, H-motor has a superior loading capacity, which could carry some functional substances such as catalysts through the absorption of corresponding aqueous solution.51 The loading quantity relies on the volume of hydrogel-based motors. Bigger volume means more absorption. To measure the self-propelling capability of the capacious hydrogel-based motors in theory, the estimation of their propulsive force was calculated as follows. The thrust impelling the motors was balanced by the resistance on account of motion through the viscous fluid and can be predicted by using Stokes’s drag theory for a cylinder52,53 (Equation 2), which estimates an antithetic propulsive force of 0.114 N (20 oC, 101.325 kPa) for the longest hydrogel rod (9.0 mm) moving 3.36  0.21 mm s-1 in the test. Fdrag 

2 L V  2L  ln    0.72  R 

(2)

Where Fdrag is the fluid resistance, V is the speed of the motor, μ is the fluid dynamic viscosity, and L and R are the length and radius of the columned motor, respectively.34 The opposing propulsive forces of capacious miniaturized motors are sufficient for transporting more and larger cargos such as catalysts.33,54-56 This hydrogel-based motor possessing strong propulsive force and large loading capacity will be applied as self-propelled catalytic processor even utilized in toxin-neutralization in the near future.57

3.2. E-H-Motors. In this work, we also fabricated a novel faster E-H-motor based on O/W an emulsion template method to store the low-boiling oil phase (CH2Cl2) inside the hydrogel networks, thus obtaining an additional propelling bubble. The schematic of the multi-bubbles-propelled catalytic E-Hmotors was shown in Figure 5. In E-H-motors, the drastic chemical reaction between KMnO4


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and H2O2 fuel not only generated sufficient oxygen bubbles for actuation like H-motors but also produced much reaction heat which transferred the stored CH2Cl2 from liquid to gas, so as to achieve collaborative driving. Additionally, the optical microscope images of the formulated emulsion (Figure 5 a, b) exhibit an extraordinary stability of the emulsion-hydrogel based millimetre-sized motors at room temperature in spite of the low boiling point of CH2Cl2.

Figure 5. (a, b) OM photos of CH2Cl2/water emulsion at different magnification. (c) Illustration of E-H-motor’s self-propulsion mechanism. The inset in (a) is the photo of O/W emulsion. The inset in (b) is the formulated E-H-motor.


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This stabilized E-H-motor possesses remarkable self-propelling ability reflected in the moving speed and motion life. The trajectory and speed of E-H-motors with corresponding L/D ratio were tracked in the same conditions with H-motors (Figure 6 and Video S3). Compared with the H-motors, the E-H-motors obviously displayed a farther trajectory and faster speed. For example, the observed maximum speed of these motors exceeds 4.33 mm s-1 produced by E-H-motors with length-diameter ratio 1:1 as illustrated in Figure 7a. Such high speed is primarily attributed to the release of CH2Cl2 from the inner of emulsion-hydrogel, associated with the lower crosslinking density compared with the PAM H-motors. In contrast, E-H-motors also possess longer motion life (Figure 7b) due to the multi-gas eviction mechanism. Figure 7c visually shows the bubbles ejecting quantity of E-H-motors compared with H-motors. A long high-density tail of O2 and CH2Cl2 (gas) mixed micro-bubbles were generated and ejected from the catalytic sites of E-H-motors, which intuitively reflects a high frequency of bubble generation by E-H-motors. Hence, the E-H-motors which could release a certain amount of CH2Cl2 (gas) while generating identical amount of O2 as H-motor, displays a superior locomotive property in the same propulsive condition. Furthermore, the reaction heat of KMnO4 and H2O2 was measured (43.44 kJ/g KMnO4) and demonstrated that the reaction heat is enough to evict the CH2Cl2 (gas) for synergy propulsion (Figure 7d). The chemical energy provided from an E-H-motor is larger than the needed energy to change the CH2Cl2 in the reaction sites of E-H-motor from liquid into gas ( 16.68J

0.841J ). (The details of E-H-motors’ mechanical property, fuel concentration

adaptability and reusability are available in Figure S5-S8)


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Figure 6. The trajectories (a-c) and speed (d-f) of E-H-motors with different L/D ratios: (a, d) 1:1, (b, e) 2:1, (c, f) 3:1. Time interval is 1s; concentration of H2O2 is 10% (w/w). The inset in (ac) is the fitted trajectory obtained by NIS Elements AR 4.3 software.


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Figure 7. The average speed (a) and motion time (b) of E-H-motors with different L/D ratios. The imaginary lines represent the average speed and motion time of H-motors. (c) Photo for Motors ejecting the bubbles. (d) Thermal power-time curve of reaction between KMnO4 and H2O2.

3.3. Water Treatment Performance. The advantage of E-H-motor lies in the faster propulsion speed (Fdrag= 0.129 N, V= 4.33 ± 0.30 mm s-1, L/D=1), which is appropriate for more powerful propulsive situation, while the Hmotor owning prominent repeatable loading ability exhibits better reusability and greater practical value. In view of the sustainable utilization and environmental friendliness, catalytic Hmotor was selected for water purification. The movement of H-motors during water treatment was shown in Video S4 (7.5% H2O2 w/w, L/D=1). Methyl blue (MB), an extensively used organic coloring agent appearing blue color in its oxidized state and colorless in its reduced state


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served as the model pollutant to examine the catalytic performance of H-motor in water treatment. An intense color gradient was observed in the petri dishes without (Figure 8 (1), upper panels), with 8 (Figure 8 (2), left panels) and with 16 catalytic H-motors (Figure 8 (3), right panels) during the self-propulsion degradation process. After 75 min, a complete catalyzed reduction of MB with a blue color to colorless was presented in the petri dishes with 16 catalytic H-motors. Eight self-propelling catalytic H-motors can also spontaneously remove almost model pollutant (MB) from aqueous solution which is barely colorless finally. The function of hydrogen peroxide on MB degradation exhibits certain effectiveness especially in the initial 30 min, but the catalytic efficiency of hydrogen peroxide obviously became weak, maintaining a distinct light blue till 75 min. With the participation of self-propulsion catalytic H-motors, hydrogen peroxide exhibited much enhanced degradation efficiency, which was improved by the increased number of H-motors.

Figure 8. The process of water treatment. The processes 1-3 have 0, 8, 16 H-motors respectively.


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Figure 9a-c indicates the evolution of UV-Vis spectra in the process of degradation reaction of MB corresponding to petri dishes 1-3 in Figure 8. The sharp decline in the absorption peaks of MB following the H-motors’ treatment demonstrates that the catalytic miniaturized motors could result in a significantly shorter remediation time. For example, the petri dish 2 with 8 H-motors finished degradation in 90 min, while the petri dish 3 with 16 H-motors completed the removal of MB in a shorter time (75 min). In contrast, a negligible decrement on the absorbance signal was observed in the last 90 min where individually employing hydrogen peroxide acted as the water treatment reagent, leading to a longer complete removal time of MB, up to 150 min. Catalytic rate curves of MB using H-motors as catalyst were obtained by recording the change of the absorption intensity of MB at the maximum absorption wavelength (λmax) of 604 nm58. As illustrated in Figure 9d, higher removal efficiency of MB were obtained after 30 min upon increasing the H-motors’ quantity (MB remove rate was enhanced from 72.6% to 91.7%, upon increasing the number of H-motors from 0 to 16). Overall, the motor movement and catalysis are crucial for such accelerated decontamination process. The significant superiority of catalytic H-motors in the degradation can be attributed to the enhanced mixing arising from the generation of oxygen bubbles and the flow of fluids through and around the motors.9 To further demonstrate the crucial role of mixing effect resulting from motor movements, control experiment was carried out by directly instilling the same dose of catalyst in the hydrogen peroxide pool mixed with MB, in the absence of miniaturized motors (Figure S9a, b). This experiment displays a complete removal of MB after a 135 min treatment, indicating that mixing effect acted as a crucial role on water purification. Apart from the generation of oxygen bubbles, the catalysts inside the hydrogel-based motors could simultaneously catalyse H2O2 to produce free hydroxyl radicals which served as oxidants for the


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degradation of MB. Synergy effect between mixing and catalysis of catalytic motors exerts a notable influence on the removal of MB.

Figure 9. Time-dependent UV-Vis spectra of MB reduced by H2O2 with (a) no, (b) 8, (c) 16 motors as the catalyst. (d) Catalytic rate curves of MB. Ct and C0 represent the MB concentration of at time t and the initial MB concentration, respectively. (The value of zero based on Shimadzu UV-240 spectrophotometer was used to determine the complete removal of MB.)


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A special advantage for this catalytic H-motor is that the remainders inside the hydrogel motor after self-propulsion could still catalyse H2O2 to produce hydroxyl radicals which sequentially degrade the contamination in water. High magnification SEM pictures and their corresponding EDS spectra (Figure S10) of catalytic sites from H-motor (Figure 10a) and E-H-motor (Figure 10b) after self-propulsion exhibited a homogeneous and high-density dispersion of the remaining particles. To further clarify the properties of the particles inside the hydrogel, XRD experiment was carried out after heating the catalytic sites to powder in muffle furnace at 450 oC for 10 h. As depicted in Figure 10c, the diffraction peaks at 28.7°, 37.3°, 41.0°, 42.8°, 56.7°, 59.4°, and 72.3° in the treated hydrogel powders are consistent with the JCPDS(024-0735) of β-MnO2.59 Although the MnO2 formed during the chemical reaction between KMnO4 and H2O2 couldn’t eject enough oxygen bubbles to propel the motor apparently, the evicted bubbles can still produce a slight perturbance to enhance mixing, synergizing with the catalytic effect of MnO2 to accelerate purification after self-propulsion. Additionally, an interesting phenomenon was observed that the reaction product MnO2 would dissolve over time after the direct instillation of catalyst (KMnO4). However, the catalytic MnO2 inside the hydrogel motor remains intact, due to the appropriate protection of hydrogel coat (Figure S9c).


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Figure 10. SEM images of H-motors (a) and E-H-motors (b) after self-propulsion. (c) XRD pattern of H-motors after thermal disposal at 400 oC for 10 h.

In consideration of the dependency of Stokes’s drag force on the geometry of the microstructures, we designed a miniaturized hydrogel fish featuring biomimetic structures (Figure 11 and Video S5), which could synchronously achieve a surpassing locomotive capability and multiple integrated functions. Based on the superior rheological property of precursor solutions of PAM hydrogel, the fish-shaped miniaturized motor was formed by pouring forming relying on an optimized mould (Figure 11b). The molding method required little in hydrogel matrix and was more economical in comparison with the 3D-printed microfish.57 Additionally, the large dimension magnitude of self-propelled artificial fish (with the minimum


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width and thickness around 3.5 mm, length around 11 mm) could exert a more apparent mixing effect (Figure 11a) for the acceleration of water purification relative to the micro scale motors. By virtue of the large dimension and prominent repeatable loading ability, the artificial fish are facile to implement in satisfactory recycling and reusing. Furthermore, the intrinsic swelling properties of hydrogels offer convenience in fabricating self-propelled miniaturized fish for the applications ranging from toxin detection to accelerated water decontamination by simply adsorbing the functionalized substances’ solution. For example, a 3D-printed artificial microfish was applied to the toxin-neutralization by incorporating functionalized polydiacetylene nanoparticles.57 This special strategy can be easily extended to more wide applications by incorporating other functional solutions into the highly integrated motors, not confined to toxin detection or environmental conservation.

Figure 11. (a) The simulation diagram of self-propelled process presented by the miniaturized fish. (b) Photo of an artificial miniaturized fish. The inset in (b) is the mould of miniaturized fish. (c) The trajectory of miniaturized fish. Time interval is 1 s; concentration of H2O2 is 10% (w/w). The inset in (c) is the fitted trajectory obtained by NIS Elements AR 4.3 software.


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4. Conclusions In summary, a reusable H-motor has been prepared by a cost-effective, facile and low energy consumption approach which injects the saturated solution of the catalyst on the tail of the formulated tubular hydrogel. This novel hydrogel-based soft-motor possesses a pair of contradictory properties of large volume and ultrafast speed, attributed to the strong bubble-base propulsion force generated from vigorous catalytic reaction between KMnO4 and H2O2 fuels. Moreover, the ultrafast speed of this millimetre-sized soft-motor could exert an enhanced mixing effect synergizing with catalytic reaction to acquire remarkable degradation efficiency for water purification. Additionally, we also firstly report a kind of E-H-motor based on the O/W emulsion template method. The stored low-boiling oil phase (CH2Cl2) in emulsion-hydrogel would liberate under the effect of catalytic reaction heat to collaborate with generated O2 for stronger bubblebased propulsion. The E-H-motor performs better in propulsion-needed situation due to its remarkable propulsion speed, while the H-motor owning excellent repeatable loading ability is more suitable for environmental conservation and sustainable development like water treatment. Furthermore, these stabilized hydrogel-based motors can be broadened to other potential applications by a facile integration of other functionalized substance, including but not confined to water remediation.

ASSOCIATED CONTENT Supporting Information. Characterizations of H- and E-H-motors. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel & Fax: +86-20-22236269. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21274046 and 21474032) and the National Natural Basic Research Program of China (973 Program, 2012CB821500). REFERENCES (1)

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Emulsion-Hydrogel Soft Motors for Water

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