Facile Fabrication of Bubble-Propelled Micromotors Carrying

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Materials and Interfaces

Facile Fabrication of Bubble-propelled Micromotors Carrying Nanocatalysts for Water Remediation Zhi-Lu Li, Wei Wang, Ming Li, Mao-Jie Zhang, Meng-Jiao Tang, Yao-Yao Su, Zhuang Liu, Xiao-Jie Ju, Rui Xie, and Liang-Yin Chu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04941 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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Industrial & Engineering Chemistry Research

Facile

Fabrication

of

Bubble-propelled

Micromotors Carrying Nanocatalysts for Water Remediation

Zhi-Lu Li,† Wei Wang,*,†,‡ Ming Li,† Mao-Jie Zhang,§,† Meng-Jiao Tang,† Yao-Yao Su,† Zhuang Liu,†,‡ Xiao-Jie Ju,†,‡ Rui Xie,†,‡ and Liang-Yin Chu†,‡ †

School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, P. R.

China ‡

State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu,

Sichuan 610065, P. R. China §

College of Engineering, Sichuan Normal University, Chengdu, Sichuan 610068, P. R. China

1 ACS Paragon Plus Environment

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ABSTRACT A facile and flexible approach is developed to fabricate bubble-propelled mesoporous micromotors carrying nanocatalysts for efficient water remediation.

The micromotors are

prepared by simply coating the hemispherical surface of Fe3O4-nanoparticles-containing mesoporous SiO2 microparticles with a polydopamine layer for decorating Ag nanoparticles, based on the versatile adhesion and reduction properties of polydopamine.

The Fe3O4

nanoparticles can produce OH radicals from H2O2 for pollutant degradation via Fenton reaction, while the mesoporous SiO2 matrix provides large surface area with anchored Fe3O4 nanoparticles for improved degradation performance.

Moreover, the Ag nanoparticles can

decompose H2O2 into oxygen bubbles for powering the movement of micromotors to further enhance the pollutant degradation.

The micromotors that synergistically integrate these

functions enable efficient degradation of pollutants, such as methylene blue demonstrated in this work, for water remediation.

This approach offers a simple and versatile strategy for

creating micromotors with flexible compositions and structures for applications in water remediation, drug delivery and cargo transport. KEYWORDS:

bubble-propelled micromotors; water remediation; Fenton reaction;

pollutant degradation; porous microparticles


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INTRODUCTION Micromotors that can convert chemical energy into locomotions show great potential for uses in many fields such as delivery of drugs and cargoes,1-6 detection and analysis,7-11 and water remediation.12-16

Especially, for water remediation, bubble-propelled micromotors

containing porous structures incorporated with catalysts are highly promising for efficient pollutant degradation.16-19

The catalysts in the micromotors can decompose the pollutant

molecules via chemical reactions.13,15,20

Meanwhile, the porous structures of the

micromotors can serve as absorbents for pollutant capture and provide large surface area with functional sites for pollutant degradation.21-24

Moreover, with integrated active metal

materials to decompose fuel chemicals for generating bubbles, the micromotors can achieve locomotion for enhanced mass transfer to further benefit the pollutant degradation.

Thus,

development of bubble-propelled micromotors that synergistically integrate the enhanced mass transfer, large functional surface area, and catalytic degradation property for efficient pollutant degradation are highly desired for water remediation. To achieve bubble-propelled motion for the micromotors, the basic principle for the fabrication

requires

generation.25-27

structure

asymmetry

of

micromotors for directional

bubble

Typically, such bubble-propelled micromotors can be fabricated by

selective deposition of materials on spherical or tubular templates,1,3,28-30 rolling-up of multilayered

metal

or

films,15,31,32

metal/polymer

deformation

of

assembled

polymersomes,6,33-35 and template synthesis from emulsion drops.36,37

Spherical

micromotors can be produced by selective deposition of active metals on the top surface of microspheres consisting of inert materials,23,30,28,38 or vice versa.1,3,29,39 With conical-shaped pores of membranes as templates, conical tubular micromotors can be prepared by deposition of an active metal layer on the inner surface of polymeric tubes or inert metal tubes preformed in the pores.13,32,40-43

Such tubular micromotors can also be created by rolling-up 3 ACS Paragon Plus Environment


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of multilayered metal/polymer films that are prepared by multistep deposition.15,31

However,

these methods usually require expensive instruments for precise and selective deposition. Alternatively, by osmotic folding of polymersomes into a bowl-shape to encapsulate Pt nanoparticles

in

their

bubble-generation.6,33-35

cavity,

stomatocyte

micromotors

can

be

produced

for

However, this method usually suffers from limited choice of

materials, and remains difficult to create porous structures.

With controllable emulsion

drops from microfluidics as templates,44,45 uniform microparticles with diverse compositions, structures and functions,46-48 can be fabricated as candidates for creation of micromotors.36,37 For example, chitosan hydrogel micromotors can be fabricated by magnetic-guided deposition of Fe@Pt nanoparticles at the bottom of precursor drops from microfluidics for cargo transport.36

By using Pickering emulsion drops with biphasic θ-shape as templates,

nonspherical micromotors with top and bottom surfaces respectively decorated with TiO2 and Ag nanoparticles can be fabricated for water remediation.37

However, these micromotors

show non-porous structures and provide limited amount of catalyst nanoparticles on their top surface, due to the limited surface area.

Therefore, development of a facile and versatile

strategy for fabrication of bubble-propelled micromotors with porous structures carrying nanocatalysts for efficient water remediation is still highly desired. Here we report on a facile strategy for fabricating bubble-propelled mesoporous micromotors carrying nanocatalysts for water remediation.

The proposed micromotors

contain mesoporous SiO2 structures embedded with Fe3O4 nanoparticles as nanocatalysts for water remediation, and decorated with Ag nanoparticles on one half of their surfaces as nanoengines for bubble-propelled motion.

Such a structure asymmetry is realized by

selectively coating the hemispherical surface of micromotors with a polydopamine (PDA) layer for decorating Ag nanoparticles, based on the versatile adhesion and reduction properties of PDA.49,50

The Fe3O4 nanoparticles can serve as the nanocatalysts for Fenton


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reaction, to produce OH radicals from H2O2 as strong oxidant for pollutant degradation.20 Meanwhile, the mesoporous SiO2 matrix of the micromotors provides large functional surface area for capturing pollutants, and for benefiting the interaction between incorporated Fe3O4 nanoparticles and H2O2 to produce OH radicals.

Moreover, the decorated Ag

nanoparticles can decompose H2O2 into oxygen bubbles for powering the movement of micromotors, leading to enhanced mass transfer for improved pollutant degradation.

The

micromotors that synergistically combines these functions enable efficient degradation of pollutants, such as methylene blue demonstrated in this work, for water remediation.

EXPERIMENTAL SECTION Materials.

Tetraethylorthosilicate (TEOS, 98%) and 3-aminopropyltriethoxysilane

(APTS, 98%), used for fabricating the mesoporous SiO2 microparticles, were purchased from Energy Chemicals.

Silicone oil (20 cSt) was purchased from Jinan Yingchuang Chemicals.

Dow Corning 749 (DC-749), used for stabilizing emulsions, was purchased from Dow Corning.

Dopamine (DA) hydrochloride was purchased from Sigma-Aldrich.

Ferric

chloride (FeCl3·6H2O, ≥ 99.7%) and ferrous chloride (FeCl2·4H2O, ≥ 99.7%) were purchased from Tianjin Damao Chemicals.

Tetramethylammonium hydroxide (TMA, 99%),

Tris buffer (99.5%), methylene blue (MB, ≥98.5%), sodium dodecyl sulfate (SDS) and cyclohexane (C6H12, ≥99.5%) were purchased from Chengdu Kelong Chemicals. nitrate (AgNO3) was purchased from Guangdong Guanghua Corporation.

Silver

Deionized water

from a water purification system (Elix 10, Millipore) was used throughout the experiments. Microfluidic Fabrication of Magnetic SiO2 Microparticles.

The magnetic SiO2

microparticles with mesoporous structures were fabricated by using monodisperse W/O emulsion drops from microfluidics as templates, according to our previous work.51


Briefly,


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a microfluidic device constructed by assembly of cylindrical glass-capillaries and square glass tubes on glass-plates was used for generating the W/O emulsion templates.

Deionized

water containing 5 mg mL-1 TMA-modified Fe3O4 nanoparticles and 14.3 vol.% APTS was used as the dispersed phase; while silicone oil containing 1 wt.% DC-749 and 45 wt.% TEOS was used as the continuous phase.

The TMA-modified Fe3O4 nanoparticles were prepared

using co-precipitation method according to our previous work.52 By separately pumping the two phases into the microfluidic device, monodisperse W/O emulsions were generated and then collected in a vessel containing the continuous phase for fabricating the magnetic SiO2 microparticles.

The resultant microparticles were washed with cyclohexane and then

air-dried for further fabricating the nanocatalyst-carrying micromotors. Fabrication

of

Nanocatalysts-carrying

Micromotors.

For

fabrication

of

nanocatalysts-carrying micromotors, first, one half of surface of the magnetic SiO2 microparticles were modified with PDA coating.

Briefly, dried magnetic SiO2

microparticles were dispersed on the surface of aqueous solution containing DA hydrochloride (2 mg mL-1) for 1.5 h, to coat the immersed hemispherical surface of the magnetic SiO2 microparticles with a PDA layer.

Second, after the selective PDA coating,

the microparticles were transferred into aqueous solution containing 5 mg mL-1 AgNO3 for 4 h, to decorate the PDA coating with Ag nanoparticles.

The resultant nanocatalysts-carrying

micromotors were washed by deionized water, and then dried at 30 oC under vacuum for 16 h for

further

characterization.

Moreover,

for

bubble-propelled

motion,

the

nanocatalysts-carrying micromotors were degassed in a vacuum chamber to remove the air in their mesoporous structures, enabling the micromotors to move under water. Morphological Characterization of W/O Emulsions and Microparticles.

The

morphologies of the W/O emulsion drops and magnetic SiO2 microparticles were observed by using optical microscopy (BX61, Olympus).

The morphologies of magnetic SiO2


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microparticles and nanocatalyst-carrying micromotors were characterized by scanning electron microscopy (SEM, JSM-7500F, JEOL), with energy dispersive X-ray spectroscopy (EDX, Model 550i) for element analysis on their surfaces.

The size and size distribution of

the emulsion drops and microparticles were calculated from their optical micrographs using an analytic software.

The coefficient of variation (CV) for the sizes of the emulsion drops

and microparticles was used to evaluate the size monodispersity.

Usually, samples with CV

< 5 % indicate good monodispersity. The morphologies of the TMA-modified Fe3O4 nanoparticles were observed by transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN, FEI).

The nanoparticle

sizes were measured by dynamic light scattering (DLS) (Zetasizer Nano ZS90-ZEN3690, Malvern) to analyze the size distribution, while the magnetic property was tested by vibrating sample magnetometer (VSM, LakeShore 7410) at room temperature. Study on the Bubble-propelled Motion of Nanocatalysts-carrying Micromotors.

To

study the bubble-propelled motion behavior of nanocatalysts-carrying micromotors, the effects of SDS concentration (CSDS) and H2O2 concentration (CH2O2) on their bubble-propelled motion behavior were investigated.

First, the motion behaviors of

nanocatalysts-carrying micromotors in aqueous solutions with fixed H2O2 concentration and different SDS concentrations of 0.01 %, 0.05 %, 0.1 %, 0.3 %, 0.5 %, 1 % and 2 % (w/v), were recorded by optical microscope (GE-5, Aigo).

Then, the motion behaviors of

nanocatalysts-carrying micromotors in aqueous solutions with fixed SDS concentration and different H2O2 concentrations of 5 %, 10 %, 15 %, 20 %, 25 % and 30 % (w/v), were recorded by the optical microscope. Study on the Pollutant Degradation of Nanocatalyst-carrying Micromotors.

The

ability of nanocatalysts-carrying micromotors to degrade pollutants for water remediation was studied by using methylene blue as the model pollutant.


First, 2 mL aqueous solution


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containing 20 mg L-1 MB and 25 % (w/v) H2O2, with pH = 3.4, was added in a cuvette as model wastewater.

Nanocatalysts-carrying micromotors (0.01 g) were added into the model

wastewater solution for MB degradation.

Meanwhile, model wastewater solutions with 0.01

g magnetic SiO2 microparticles and without any microparticles were used as control groups. The degradation processes of the three samples were recorded by digital camera. For quantitative characterization of the MB degradation process, typically, 10 mL model wastewater solution was added in a Petri dish.

Then, 0.05 g nanocatalyst-carrying

micromotors were added into the solution for MB degradation.

Meanwhile, model

wastewater solution with the magnetic SiO2 microparticles were used as control group.

The

time-dependent concentration change of MB in the solution was measured by using UV-vis spectrophotometer (Shimadzu UV-1700) with the absorption at 664 nm.

200 µL of the MB

solution was taken and diluted to 3 mL for such measurements.

Aqueous solutions

containing MB with different concentrations (CMB) from 5 mg L-1 to 60 mg L-1 were used to calibrate the concentrations (Figure S1).

RESULTS AND DISCUSSION Fabrication

of

Bubble-propelled


Micromotors.

The

nanocatalysts-carrying micromotors are fabricated from magnetic mesoporous SiO2 microparticles by a two-step surface modification processes based on PDA (Figure 1). DA can self-polymerize on a wide range of materials to produce a PDA coating.

The

Then, the

PDA coating can reduce noble metal ions such as Ag+, Au3+ and Pt3+ into nanoparticles on the PDA coating for further modification.

Based on the versatile adhesion and reduction

properties of PDA, the magnetic mesoporous SiO2 microparticles can be easily decorated with Ag nanoparticles as nanoengines at low-cost for bubble-propelled motion.


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First, the magnetic SiO2 microparticles with mesoporous structures are fabricated by using monodisperse W/O emulsion drops from microfluidics as templates.

Magentic Fe3O4

nanoparticles, modified by TMA, are dispersed in the drop templates to serve as the nanocatalysts in the mesoporous SiO2 microparticles for Fenton reaction.

TEM image in

Figure 2a shows the uniform morphology of TMA-modified Fe3O4 nanoparticles in dried state, with average diameter of ~20 nm.

When dispersed in water, the average diameter

tested by DLS is ~44.64 nm (Figure 2b).

These uniform Fe3O4 nanoparticles exhibit good

superparamagnetism, with saturation magnetization of 62.74 emu g-1 (Figure 2c). Meanwhile, as compared with the one without TMA modification, the TMA-modified Fe3O4 nanoparticles can be well dispersed in water against gravity (Figure 2d,e) and external magnetic field (Figure 2f), due to the electrostatic repulsion between the TMA on the nanoparticle surface.

Such Fe3O4 nanoparticles can be well-dispersed in the precusor

solution to produce drops from microfluidics for creating the magnetic SiO2 microparticles. For microfluidic fabrication of the magnetic SiO2 microparticles, briefly, precursor solution containing APTS and TMA-modified Fe3O4 nanoparticles are emulsified into monodisperse W/O emulsion drops, by using silicone oil containing TEOS and surfactant DC-749.

The

TEOS and APTS can hydrolyze and condense at the drop interfaces to creat SiO2 matrix. With further diffusion of TEOS inward for hydrolysis, mesoporous SiO2 microparticles,51 incorporated with the Fe3O4 nanoparticles can be fabricated.

The W/O emulsion templates

and the resultant magnetic SiO2 microparticles both show uniform structures and sizes (Figure 3a, b).

Meanwhile, the magnetic SiO2 microparticles show smaller sizes as

compared with the emulsion templates, because the microparticle fabrication process based on TEOS and APTS consumes H2O in the W/O emulsion templates, leading to decreased emulsion size as well as decreased microparticle size.

The CV values of the W/O emulsion

templates and the microparticles are 1.49 % and 2.21 % respectively, indicating good size



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monodispersity (Figure 3c, d), and efficient synthesis processes. Next, the magnetic SiO2 microparticles are selectively coated with a PDA layer on their hemispherical surface, and then decorated with Ag nanoparticles for fabricating the nanocatalysts-carrying micromotors.

Typically, DA-containing aqueous solution is used to

selectively modify the dried magnetic SiO2 microparticles with PDA coating.

The dried

magnetic SiO2 microparticles can floated on the surface of DA solution due to the surface tension effect, as well as the gas remained in their mesoporous structures.

This leads to

selective modification of one half of the microparticle surface with PDA coating (Figure 4a). After that, the PDA-coated microparticles are transferred into AgNO3 solution for Ag nanoparticle decoration.

Due to the plenty of hydrophilic groups such as hydroxyl and

amine groups in PDA, for the SiO2 microparticles with air stored in the mesoporous structures, their PDA-coated is more hydrophilic than the other half.

Thus, when immersed

in the AgNO3 solution, the solution prefers wetting the PDA-coated half of the microparticles for Ag nanoparticles decoration. Based on the reduction property of catechol groups on PDA,49,50 Ag+ in the solution can be reduced into Ag nanoparticles on the hemispherical PDA coating, serving as nanoengines for bubble generation.

The SEM images in Figure 4b,c and

the element analysis of Ag in Figure 4d together confirm the presence of Ag nanoparticles on the hemispherical PDA coating of the nanocatalysts-carrying micromotors.

Moreover, the

Fe element in Figure 4d is resulted from the Fe3O4 nanoparticles that distribute in the mesoporous SiO2 matrix.

These results confirm the successful fabrication of the

nanocatalysts-carrying micromotors, with mesoporous SiO2 structures containing Fe3O4 nanocatalysts, and hemispherical surfaces decorated with Ag nanoengines.


Micromotors

for

Bubble-propelled

Motion.

The

bubble-propelled motion of the nanocatalysts-carrying micromotors is achieved by using


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H2O2 as the fuel.

The Ag nanoparticles, decorated on the hemispherical surfaces of

micromotors, can decompose H2O2 via a cyclic reaction process, with no net dissolution of Ag nanoparticles;53,54 This allows directional generation of oxygen bubbles to power the movements of micromotors (Movie S1).

Since the Ag-nanoparticles-integrated PDA

coating can show good stability and reproducibility for H2O2 reduction,55 the PDA coating on the micromotors is stable enough during the bubble-propelled motion.

For the

bubble-propelled motion, the bubble generation process can be influenced by the concentrations of the fuel H2O2 and surfactant SDS in the solutions.56

Increase of H2O2

concentration can increase the bubble-generation rate under catalysis of Ag nanoparticles. Meanwhile, addition of SDS allows decrease of the interfacial free energy to benefit the generation and detachment of bubbles.56

As shown in Figure 5, with increasing SDS

concentration from 0.01 % (w/v) to 0.5 % (w/v), the bubble-generation rate of the micromotors increases, leading to a lager momentum for faster movement. When further increasing the SDS concentration higher than 0.5 % (w/v), no obvious enhancement of the movement of micromotors is observed.

The SDS-concentration-dependent velocity of the

micromotors shows that, increase of SDS concentration from 0.01 % (w/v) to 0.5 % (w/v) leads to increased velocity from 154.7 µm s-1 to 536.0 µm s-1; while further increase of SDS concentration from 0.5 % (w/v) to 2 % (w/v) leads to no obvious change in velocity.

Thus,

SDS concentration of 0.5 % (w/v) for fast bubble-propelled motion is selected for further study. The effect of H2O2 concentration on the bubble-propelled motion behaviors of nanocatalyst-carrying micromotors is investigated in solutions with fixed SDS concentration of 0.5% (w/v) and different H2O2 concentrations.

As shown in Figure 6, with increasing

H2O2 concentration from 5 % (w/v) to 30 % (w/v) to provide more fuel, the micromotors move faster with an improved momentum.

The H2O2-concentration-dependent velocity



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shows that, with increasing H2O2 concentration from 5 % (w/v) to 30 % (w/v), the velocity increases from 77.8 µm s-1 to 566.9 µm s-1 (Figure 7b).

Especially, the velocity of

micromotors shows an increase of 38.5 % with increasing H2O2 concentration from 20 % (w/v) to 25 % (w/v); while the velocity only shows an increase of 4.2 % with further increasing H2O2 concentration from 25 % (w/v) to 30 % (w/v).

Therefore, H2O2

concentration of 25 % (w/v) is selected for further studying the water-remediation performances of the micromotors.

During the bubble-propelled motion, the bubbles are

generated only from the micromotors, indicating no detached PDA coating or Ag nanoparticles from the micromotors.

Moreover, the micromotors can be repeatedly used for

bubble-propelled movement, confirming the stable PDA coating on the micromotors.

Nanocatalysts-carrying Micromotors for Water Remediation.

To demonstrate the

ability of nanocatalysts-carrying micromotors for water remediation, methylene blue (MB), an organic dye pollutant, is used as a model pollutant for degradation.

As shown in Figure

8a, three cuvettes, one with 0.01 g prepared nanocatalysts-carrying micromotors as the experimental group (sample C), and other two with 0.01 g magnetic SiO2 microparticles (sample B) and without any microparticles (sample A) as control groups are used for the demonstration.

Aqueous solution (2 mL) containing 20 mg L-1 MB and 25 (w/v) % H2O2,

with pH = 3.4, is used as the model wastewater and added into each cuvette for pollutant degradation.

Upon contacting with H2O2, the micromotors move fast in the model

wastewater due to their bubble-propelled motions.

Meanwhile, the Fe3O4 nanocatalysts in

their mesoporous structures produce OH radicals for MB degradation via Fenton reaction. Moreover, several bubbles are generated via the Fenton reaction in sample B, to lift some microparticles to the top of the solution.

However, these microparticles remain nearly

unmoved during the degradation process, showing no obvious bubble-propelled motion.


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Thus, as compared with samples A and B, the sample C containing the micromotors becomes the most transparent within the same period during MB degradation, showing the fastest degradation rate (Figure 8b-c, and Movie S2) .

The results confirm the positive effect of

bubble-propelled motion on the enhancement of MB degradation.

Moreover, a drop in the

moving speed of micromotors is observed over the operation period, because the motion of micromotors consumes H2O2 and leads to decreased H2O2 concentration as well as decreased generation frequency of bubbles for driving microparticle motion. To quantitatively evaluate the degradation rate, the time-dependent concentration changes of MB in the model wastewater with nanocatalysts-carrying micromotors and magnetic SiO2 microparticles are respectively measured by using UV-vis spectrophotometer. During the degradation process, the MB concentration in the model wastewater with nanocatalysts-carrying micromotors decreases quickly from 20.82 mg L-1 to 1.92 mg L-1 within the first 4 h (Figure 9a), showing a significant 90.79 % degradation.

By contrast, the

MB concentration in the model wastewater with magnetic SiO2 microparticles only decreases from 20.82 mg L-1 to 13.61 mg L-1 within the first 4 h (Figure 9a), showing only 34.64 % degradation.

Meanwhile, at t=8 h, the MD degraded by the nanocatalysts-carrying

micromotors show 94.49 % degradation, as compared to 59.84 % degradation by the magnetic SiO2 microparticles.

Therefore, the nanocatalysts-carrying micromotors, which

combine the enhanced mass transfer, large functional surface area, and Fenton reaction, show much better MB degradation performances than the traditional nanocatalysts-carrying microparticles for efficient water remediation.

Moreover, the microfluidic techniques,

which enable emulsion-template synthesis of microparticles from several hundreds of micrometers down to several micrometers,[25] create opportunities to fabricate much smaller micromotors for further improving the water remediation performance. With smaller sizes, the micromotors can achieve faster moving speed under relative low H2O2 concentration to



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enhance the mass transfer, and provide larger specific surface area to facilitate the access of MB molecules into the Fe3O4-nanoparticles-embedded mesoporous structures for degradation. Thus, enhanced water remediation process with shorter reaction time can be realized.

CONCLUSIONS In summary, a facile and flexible approach is developed to fabricate bubble-propelled mesoporous micromotors carrying nanocatalysts for efficient water remediation.

The

micromotors contain Fe3O4 nanoparticles incorporated in their mesoporous SiO2 structures, and Ag nanoparticles decorated on the hemispherical surfaces.

Their asymmetric structures

can be simply achieved by selective PDA coating followed with decoration of Ag nanoparticles, based on the versatile adhesion and reduction properties of PDA.

The Fe3O4

nanoparticles work as nanocatalysts for Fenton reaction to achieve the pollutant degradation.57

The mesoporous SiO2 matrix of the micromotors provides large surface area

with anchored Fe3O4 nanoparticles for improved degradation efficiency.

The Ag

nanoparticles serve as nanoengines for bubble-propelled motion to further enhance the pollutant degradation.

Thus, the micromotors enable synergistic integration of these

functions to efficiently decompose pollutants for water remediation.

Moreover, the

fabrication of the micromotors based on microfluidics can be scaled-up by using multiple microchannels for mass production of emulsion drops,58,59 e.g. >2000 t of droplets per year,59 as templates.

This allows mass production of mesoporous SiO2 microparticles, followed

with batch processes for PDA coating and Ag nanoparticles decoration to achieve scalable production of the micromotors.

Moreover, since the microfluidic techniques allow

fabrication of controllable microspheres and microcapsules with flexible structures and functions,46,48,60 diverse micromotors can be simply created by using the proposed strategy


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based on the PDA chemistry.

Therefore, the proposed strategy provides a facile and flexible

strategy to fabricate bubble-propelled micromotors for myriad applications.

AUTHOR INFORMATION Corresponding Author * 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.

ACKNOWLEDGMENTS The authors gratefully acknowledge support from the National Natural Science Foundation of China (91434202, 81621062), the Program for Changjiang Scholars and Innovative Research Team in University (IRT15R48), and State Key Laboratory of Polymer Materials Engineering (sklpme2014-1-01).

Supporting Information A Figure and Movies as described in the text.

This material is available free of charge via

the Internet at http://pubs.acs.org.



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Figures

Figure 1. Schematic illustration of the fabrication process for bubble-propelled nanocatalysts-carrying micromotors.

(a,b) Fe3O4-nanoparticles-containing mesoporous

SiO2 microparticles (a) floated on DA solution for selective hemispherical coating (b). Decorating of Ag nanoparticles on the PDA coating via Ag+ reduction.

(c) (d,e)

Nanocatalysts-carrying micromotors containing Fe3O4 nanoparticles in the mesoporous SiO2 structures (d) and Ag nanoparticles decorated on the PDA coating (d,e).



Figure 2. Morphological characterization and magnetic property of TMA-modified Fe3O4 nanoparticles.

(a-c) TEM image (a), size distribution (b), and magnetization hysteresis loop

(c) of TMA-modified Fe3O4 nanoparticles.

(d-f) Optical images showing aqueous

dispersion of Fe3O4 nanoparticles with (A) and without (B) TMA modification before (d) and after (e) being placed for 1 h, and attracted by a magnet (f).


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Figure 3. Fabrication of magnetic SiO2 microparticles from W/O emulsions.

Optical

micrographs (a,b) and size distributions (c,d) of W/O emulsion drops (a,c) and resultant magnetic SiO2 microparticles (b,d).

Scale bars are 200 µm.



Figure 4. Morphology and element analysis of PDA-coated magnetic SiO2 microparticles before and after Ag+ reduction.

(a) SEM images of the microparticles before Ag+ reduction.

(b,c) SEM images of the microparticles after Ag+ reduction (b), with magnified surface (c) for element analysis (d).

Scale bars are 50 µm in (a,b) and 3 µm in (c).


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Figure 5. Effect of SDS concentration on the bubble-propelled motion behaviors of nanocatalysts-carrying micromotors.

(a-d) Optical snapshots showing the bubble-propelled

motion behaviors of the micromotors in 25 % (w/v) H2O2 solutions with SDS concentrations of 5 % (a), 1 % (b), 0.5 % (c) and 0.01 % (w/v) (d). location changes of the micromotors.

The colorful circles indicate the

Scale bar is 500 µm.



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Figure 6. Effect of H2O2 concentration on the bubble-propelled motion behaviors of nanocatalysts-carrying micromotors.

(a-d) Optical snapshots showing the bubble-propelled

motion behaviors of the micromotors in aqueous solutions with 0.5 % (w/v) SDS and different H2O2 concentrations of 30 % (a), 25 % (b), 15 % (c) and 5 % (w/v) (d) H2O2. colorful circles indicate the location changes of the micromotors.


Scale bar is 500 µm.

The

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Figure 7. SDS-concentration-dependent (a) and H2O2-concentration-dependent (b) velocity of the nanocatalysts-carrying micromotors.



Figure 8. Nanocatalysts-carrying micromotors for water remediation.

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(a-f) Optical images

showing the degradation processes of methylene blue in aqueous solutions. H2O2 is added in samples A, B, and C.

25 % (w/v)

Fe3O4-nanoparticles-containing mesoporous SiO2

microparticles are added in sample B, and nanocatalysts-carrying micromotors are added in sample C.

Scale bar is 1 cm.


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Figure 9. Time-dependent changes of methylene blue concentration (a) and the degradation ratio (b) in aqueous solutions dispersed with nanocatalysts-carrying micromotors and Fe3O4-nanoparticles-containing mesoporous SiO2 microparticles.



Graphic for TOC


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Facile Fabrication of Bubble-Propelled Micromotors Carrying

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