Bubble-Propelled Hierarchical Porous Micromotors from Evolved

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Materials and Interfaces

Bubble-Propelled Hierarchical Porous Micromotors from Evolved Double Emulsions Yao-Yao Su, Mao-Jie Zhang, Wei Wang, Chuan-Fu Deng, Jian Peng, Zhuang Liu, Yousef Faraj, Xiao-Jie Ju, Rui Xie, and Liang-Yin Chu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05791 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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Industrial & Engineering Chemistry Research

Bubble-Propelled Hierarchical Porous Micromotors from Evolved Double Emulsions Yao-Yao Su,† Mao-Jie Zhang,§ Wei Wang,*,†,‡ Chuan-Fu Deng,† Jian Peng,† Zhuang Liu,†,‡ Yousef Faraj,†,‡ 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 610101, P. R.

China

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT:

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Bubble-propelled micromotors with well-engineered hierarchical

porous structures are developed by a simple and flexible strategy for efficient water decontamination.

Controllably and spontaneously evolved double emulsions from

microfluidics are used as templates for continuous fabrication of the micromotors in one-step.

The micromotors possess interconnected hierarchical porous structures

with two well-aligned microscale pores incorporated in nano-porous matrix.

The

opening hole of one microscale pore at the bottom of micromotor is decorated with Fe3O4@Ag nanoparticles to decompose H2O2 for bubble-propelled motion.

Due to

their hydrophobic and magnetic hierarchical porous matrix, the micromotors can be used for capture of oil pollutants in water, and then easy recovery by using a magnetic field.

Moreover, the micromotors can be flexibly functionalized by simply

incorporating functional nanoparticles into the hierarchical porous structures for efficient water decontamination.

This is demonstrated by modifying their hierarchical

porous structures with thiol-modified SiO2 nanoparticles for efficient removal of heavy metal ions in water.

These bubble-propelled hierarchical porous micromotors show

great power as tools for efficient decontamination of polluted water.

Moreover, the

approach based on microfluidic emulsions is promising for continuous and controllable fabrication of novel micromotors with well-engineered structures and advanced functions. KEYWORDS:

porous micromotors; bubble-propelled motion; hierarchical

structures; template synthesis; water decontamination

2


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INTRODUCTION Micromotors that allow self-propelled motion by using chemical energy, are promising for myriad applications such as water remediation,1-7 drug delivery,8-15 cargo transportation,16-19 separation.26-28

detection/analysis,20-23

environmental

monitoring,24,25

and

Among them, micromotors with bubble-propelled motion for

enhanced mass transfer, and with hierarchical porous structures for improved pollutant capture are highly promising for efficient water decontamination.

With active

materials such as Ag,29,30 Mg,31,32 Zn,9,33 Pt,34,35 Pd,5 and catalase,24,36 to decompose chemical fuels into bubbles, the micromotors can achieve bubble-propelled motion to enhance convection for efficient degradation or capture of pollutants.

For example,

as compared to their counterparts without the active materials, bubble-propelled micromotors enable more efficient degradation of biological/chemical warfare agents and organic pollutants,2-3,5,29,30,37-39 and adsorption of heavy metal ions and organic pollutants.1,4,7,40-42

Meanwhile, interconnected hierarchical porous structures with

microscale and nanoscale pores can provide large functional surface area and improved mass transfer for enhanced pollutant capture.43

With the microscale pores as the easy

access to the internal of nano-porous matrix, the nanoscale pores can provide large surface area with highly accessible functional sites for capturing pollutants.

Thus,

bubble-propelled micromotors with well-engineered hierarchical porous structures are highly-potential as powerful tools for efficient decontamination of polluted water. Generally, bubble-propelled micromotors can be developed based on asymmetrically engineering their active materials and inert materials.

By shape transformation of 3



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polymersomes into stomatocytes via dialysis, supramolecular micromotors entrapped with active materials such as Pt nanoparticles and catalase in their cavity can be developed.36,44,45

However, this approach suffers from limited material choice and

relatively low efficient entrapment of active materials for micromotor construction, and remains difficult to create hierarchical porous structures into these polymersome-based micromotors.

As compared to the polymersome transformation method, more

frequently used methods for fabricating bubble-propelled micromotors are those based on electrochemical/electroless deposition techniques and film rolling-up techniques.46 By sequential deposition of different inert materials and then active material into the straight pores of sacrificed membranes,33,47,48 or onto sacrificed substrates followed with film rolling-up,46,49,50 tubular micromotors with inner layer of active material to decompose chemical fuels for bubble generation can be fabricated.

However,

engineering of hierarchical porous structures in these tubular micromotors usually remains challenging due to their thin tubular walls based on the deposition techniques. By selectively coating active materials on the top half of particulate templates, such as porous carbon microspheres,1 and porous SiO2 microspheres,29 Janus micromotors with half surface containing active materials for bubble generation can be fabricated.20,51,52 Conversely, coating of inert materials on the top half of active material particles can also produce micromotors with the inner active particle partially exposed for bubble generation.53-55

However, these approaches based on electrochemical/electroless

deposition and film rolling-up usually require troublesome multi-step processes to asymmetrically engineer the active and inert materials for micromotor construction. 4


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Although micromotors can also be simply produced by combining solvent evaporation of polymer-containing oil droplets and precipitation of their encapsulated Pt and Fe3O4 nanoparticles,56 creation of hierarchical porous structures in these micromotors still remains difficult.

With excellent flow manipulation,57-60 microfluidic techniques can

generate monodisperse phase-separated emulsions with controllable sizes, structures and compositions to well-engineer the active and inert materials in each phase for continuous template-synthesis of bubble-propelled micromotors in one step.

With

uniform precursor droplets containing magnetically-deposited Fe@Pt nanoparticles as templates, spherical chitosan micromotors with an assembled Fe@Pt-nanoparticlepatch at their bottom can be fabricated.17

Using biphasic θ-shaped droplets with each

phase containing different functional nanoparticles as templates, nonspherical polymeric micromotors coated with TiO2 and Ag nanoparticles respectively on their top and bottom surfaces can be produced.30

However, as above-mentioned, although

many micromotors have been developed by using different methods, most of them do not possess hierarchical porous structures, and suffer from troublesome multistep fabrication process.

Thus, development of a simple and efficient strategy for

continuous fabrication of bubble-propelled micromotors with controllable hierarchical porous structures is still highly desired. Here, we report on a simple and flexible strategy based on microfluidics for continuous one-step fabrication of bubble-propelled micromotors with well-engineered hierarchical porous structures for efficient water decontamination.

The controllable

double emulsions from microfluidics are spontaneously evolved via density-mismatch5



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induced droplet alignment, interfacial-energy-induced droplet dewetting, magneticguided Fe3O4@AgNPs deposition, and mass-transfer-induced nanodroplet formation and then used as templates.

The fabricated micromotors possess interconnected

hierarchical porous structures containing two well-aligned microscale pores incorporated in nano-porous matrix.

The two well-aligned and interconnected

microscale pores create a microchannel inside the micromotors, with two opening holes at their top and bottom surfaces.

The opening hole at their bottom surface is decorated

with Fe3O4@Ag nanoparticles (Fe3O4@AgNPs) to decompose H2O2 for bubblepropelled motion.

The microchannel enhances the accessibility to the internal porous

matrix, thus facilitating the efficient pollutant capture by the internal porous matrix during bubble-propelled motion.

Their hydrophobic hierarchical porous matrix

allows capture of oil pollutants in water, while the Fe3O4@AgNPs enable their easy magnetic-assisted recovery.

Moreover, the micromotors can be flexibly

functionalized by simply incorporating functional nanoparticles into their hierarchical porous structures for efficient water decontamination.

This is demonstrated by

modifying their hierarchical porous structures with thiol-modified SiO2 nanoparticles for efficient removal of toxic heavy metal ions in water.

These bubble-propelled

hierarchical porous micromotors are highly potential as powerful tools for efficient water decontamination.

Moreover, the approach based on microfluidic emulsions is

promising for facile and continuous fabrication of controllable micromotors with wellengineered porous structures and integrated functions.

6


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EXPERIMENTAL SECTION Materials.

Photo-curable ethoxylated trimethylolpropane triacrylate (ETPTA),

and surfactant Pluronic F127 were purchased from Sigma-Aldrich. hydroxy-2-methyl-1-phenyl-1-propanone

(HMPP)

and

trimethoxysilane were purchased from TCI (Shanghai). purchased from Sinopharm chemical Reagent company. purchased from Kerry Oils & Grains Co., Ltd.

Photo-initiator 2(3-mercaptopropyl)

Benzyl benzoate (BB) was Soybean oil (SO) was

Glycerin, glucose, sodium dodecyl

sulfate (SDS), hydrogen peroxide, ammonia and methanol were purchased from Chengdu Kelong Chemicals. purchased from Danisco. Guanghua Corporation.

Surfactant polyglycerol polyricinoleate (PGPR) was

Silver nitrate (AgNO3) was purchased from Guangdong Fe3O4 nanoparticles (20 nm) were purchased from Shanghai

Macklin Biochemical Co. Ltd.

SiO2 nanoparticles (30 nm) were purchased from

Aladdin Industrial Corporation.

Deionized (DI) water (18.2 MΩ, 25 °C) was prepared

from a Milli-Q water purification system (Millipore). Microfluidic Fabrication of Hierarchical Porous Micromotors.

A glass-

capillary microfluidic device was used to generate (W1+W2)/O/W1 double emulsions for template synthesis of the hierarchical porous micromotors (Figure 1).

The

microfluidic device was constructed via assembly of metal needles, cylindrical glasscapillaries, and square glass tubes on a glass plate, based on our previous work.61 Briefly, two metal needles, used as the injection tubes, were parallelly inserted into a cylindrical glass capillary, used as the transition tube.

The tapered end of the

transition tube was inserted into another cylindrical glass capillary, used as the 7



collection tube.

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The inner diameter and outer diameter of the metal tubes were 60 μm

and 240 μm, respectively.

The inner diameter and outer diameter of the cylindrical

glass-capillaries were 500 μm and 1 mm, respectively.

The assembled cylindrical

glass-capillaries were inserted into a square glass tube with inner dimension of 1 mm to ensure their co-axiality. For generating (W1+W2)/O/W1 double emulsions from microfluidics for template synthesis of the hierarchical porous micromotors, typically, deionized water (50 mL) containing Pluronic F127 (0.5 g, 1% (w/v)), marked as W1 (ρW1=1.015 g mL-1), was used as outer fluid (OF), and one of the inner fluids (IF1).

Deionized water (50 mL)

containing glycerin (50 mL, 50 vol%) and Pluronic F127 (0.5 g, 0.5% (w/v)), marked as W2 (ρW2=1.128 g mL-1), was used as the other inner fluid (IF2).

Oil mixture

containing ETPTA (6 mL), BB (4 mL), PGPR (2 g, 20 % (w/v)), Fe3O4@AgNPs (0.05g, 0.5 % (w/v)) and HMPP (100 μL, 1 vol%), marked as O (ρO=1.030 g mL-1), was used as the middle fluid (MF).

To prepare the Fe3O4@AgNPs, ammonia was first

dripped into 100 mL AgNO3 solution (10 mg mL-1) under agitation.

After the solution

turned to turbid and then clear, silver-ammonia solution was obtained.

Next, 100 mL

aqueous dispersion containing Fe3O4 nanoparticles (2 mg mL-1) was added into the silver-ammonia solution.

After ultrasonic treatment for 30 min, 100 mL aqueous

solution containing glucose (20 mg mL-1) was added to the mixed solution and stirred for 1 h for synthesizing the Fe3O4@AgNPs.

Deionized water containing Pluronic

F127 (1 % (w/v)) was used as collection solution.

8


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By separately pumping the four fluids into the microfluidic device at appropriate rates, (W1+W2)/O/W1 double emulsions were generated, and then collected in a container with the collection solution for further evolution.

The typical flow rates of

IF1, IF2, MF, and OF were 2.5 mL h-1, 2.5 mL h-1, 13.2 mL h-1, and 39 mL h-1.

A

magnet was placed under the container to collect the Fe3O4@AgNPs at the bottom of the double emulsions via magnetic-guided settling.

Then, the controllably evolved

(W1+W2)/O/W1 double emulsions were converted into hierarchical porous micromotors via UV-polymerization for 5 min.

The obtained hierarchical porous

micromotors were washed with ethanol and DI water under ultrasonic treatment for further use. To fabricate the thiol-modified hierarchical porous micromotors, thiol-modified SiO2 nanoparticles (3 mg mL-1) were added into the MF containing ETPTA and BB with volume ratio of 7:3 for generating stable double emulsions for micromotor fabrication. The thiol-modified SiO2 nanoparticles were prepared by adding SiO2 nanoparticles in methanol (1 mL) containing 1 vol% (3-mercaptopropyl) trimethoxysilane.

The

mixture was kept at room temperature for 48 h under vigorous agitation.

After

washing with ethanol for three times, the thiol-modified SiO2 nanoparticles were obtained. Morphological Characterization of Emulsion Templates and Micromotors. The generation process of (W1+W2)/O/W1 double emulsions in microfluidic device was monitored by using optical microscope (IX71, Olympus) coupled with a high-speed digital camera (Phantom Miro3, Vision Research).

The morphologies of 9



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(W1+W2)/O/W1 double emulsions and hierarchical porous micromotors were respectively observed by using optical microscope (DM4000B-M, Leica) and stereoscopic microscope (SZX16, Olympus).

A self-made glass container (length: 2

cm, depth: 2 cm, width: 600 μm) was used to collect the (W1+W2)/O/W1 double emulsions without overlap for better observing their side-view morphologies.

The

morphologies of hierarchical porous micromotors, Fe3O4@AgNPs, and thiol-modified SiO2 nanoparticles were characterized by using scanning electron microscopy (JSM7500F, JEOL), with energy dispersive X-ray spectroscopy (EDX, INCA250, OXFORD) for element analysis. The size and size distribution of double emulsions and hierarchical porous micromotors were calculated based on their optical micrographs.

Coefficient of

variation (CV) for the sizes of double emulsions and micromotors was used to evaluate their size monodispersity.

Usually samples with CV < 5 % can be considered as

monodisperse samples. To evaluate the dewetting process during emulsion evolution, the interfacial tensions between W1 and O phases (γW1O), and between W2 and O phases (γW2O), were measured by interface tensiometer (DSA25, Krüss) using pendent drop method.

The tension of

the thin film (γFilm) between W1 and W2 phases was obtained from the droplet adhesion experiment based on our previous work.62 Study on the Bubble-Propelled Motion of Hierarchical Porous Micromotors. To study the bubble-propelled motion of hierarchical porous micromotors, the motion behavior of micromotors in aqueous solution containing H2O2 and SDS was recorded 10


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by using optical microscope (GE-5, Aigo).

Meanwhile, the effect of SDS

concentration (CSDS) on their bubble-propelled motion was investigated by monitoring their motion behaviors in aqueous solutions with 30 % (w/v) H2O2 and different CSDS in the range from 0.01 % (w/v) to 2 % (w/v). Hierarchical Porous Micromotors for Removal of Oil Pollutants in Water.

To

investigate the performance of hierarchical porous micromotors for removing oil pollutants in water, mixed oil containing BB and SO, with volume ratio of 1:1.2, was used as model oil pollutant.

First, 3 mL aqueous solution containing 2 % (w/v) H2O2

and 1 % (w/v) SDS was added into a cuvette.

Then, the mixed oil (20 μL), dyed with

Lumogen Red 300 (LR300), was added into aqueous solution and then shaken into small oil droplets to prepare the oil-containing solution.

Next, hierarchical porous

micromotors (0.04 g) were added into the oil-containing solution for oil removal. Meanwhile, oil-containing solution, and oil-containing solution containing hierarchical porous polyETPTA microparticles (0.04 g) without Fe3O4@AgNPs, were used as control groups.

The oil removal of the three samples were conducted in a shaking

bath, and recorded by using digital camera. Hierarchical Porous Micromotors for Removal of Heavy Metal Ions in Water. Thiol-modified hierarchical porous micromotors were used for removal of Pb2+ ions in water.

Aqueous solution containing 2 mg L-1 Pb2+ ions, 2 % (w/v) H2O2 and 1 % (w/v)

SDS was used as Pb2+-contaminated solution.

Briefly, 0.2 g thiol-modified

hierarchical porous micromotors were added into a cuvette containing the Pb2+contaminated solution (1 mL).

During the Pb2+ removal process, at a certain time, 100 11



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μL of the Pb2+-contaminated solution was taken out as sample for Pb2+ concentration measurement.

Meanwhile, 100 μL fresh Pb2+-contaminated solution was added into

the micromotor-containing Pb2+-contaminated solution for compensation.

The Pb2+

concentration in the sample was measured by using atomic absorption spectrometer (iCE 3000 SERIES, Thermo Scientific).

Thiol-modified hierarchical porous

polyETPTA microparticles without decorated Fe3O4@AgNPs, and thiol-modified SiO2 nanoparticles were used in the Pb2+-contaminated solution as control groups. Meanwhile, static hierarchical porous micromotors was also used in Pb2+-contaminated solution (without H2O2) as control group.

The weight of the thiol-modified SiO2

nanoparticles was the same as that of the thiol-modified SiO2 contained in the thiolmodified hierarchical porous micromotors.

Moreover, effect of pH on the Pb2+

removal performance was studied by using HCl for adjusting pH values in the acid pH range, since heavy metal precipitation could occur in alkaline condition.

RESULTS AND DISCUSSION Strategy for Microfluidic Fabrication of Hierarchical Porous Micromotors.

For

fabrication of the hierarchical porous micromotors, monodisperse double emulsions are generated from microfluidics as initial templates (Figure 1a).

Typically, the inner W1

and W2 phases that separately flow in the two injection tubes are sheared by the middle O phase and broken into uniform W1 and W2 droplets in the transition tube.

Then, the

middle O phase that carries droplets of W1 and W2 is emulsified into droplets under the shear of outer W1 phase in the collection tube.

This results in encapsulation of each 12


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pair of W1 and W2 droplets into each of the O droplets to generate (W1+W2)/O/W1 double emulsions (Movie S1).

The generated double emulsions are collected in a

container with collection solution for further emulsion evolution.

The double

emulsions are controllably and spontaneously evolved via density-mismatch-induced droplet alignment, interfacial-energy-induced droplet dewetting, mass-transfer-induced nanodroplet formation, and magnetic-guided Fe3O4@AgNPs deposition.

First, due to

the density mismatch among the W1, W2, and O phases (ρW1＜ρO＜ρW2), the W1 droplet floats on the top of the middle O droplet, while the W2 droplet sinks at the bottom of the middle O droplet, resulted in vertically aligned inner droplets.

Second, the middle

O phase partially dewets on the inner droplets due to the interfacial-energy-induced adhesion between the inner and outer water/oil interfaces (Figure 1b-c).62-63

Third,

the excess amount of surfactant PGPR in the middle O phase leads to formation of water nanodroplets, due to mass transfer of water molecules from W1 and W2 phases to middle O phase.43

Meanwhile, the Fe3O4@AgNPs are attracted onto the bottom of

the emulsion templates by the magnet placed under the container (Figure 1d).

After

such evolution, the double emulsions provide excellent templates for fabricating the hierarchical porous micromotors containing two microscale pores incorporated in the nano-porous matrix (Figure 1e-g).

The adherent inner droplets in double emulsions

can serve as templates for engineering the well-aligned microscale pores, with two opening holes on the top and bottom surfaces of the micromotors.

Meanwhile. the

water nanodroplets in the middle O phase can serve as templates for creating the nanoscale pores.

The Fe3O4@AgNPs, that decorated around the opening hole at the 13



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bottom of micromotors, can work as the navigator and engine for bubble-propelled motion. Bubble-Propelled Micromotors with Hierarchical Porous Structures.

First, to

demonstrate the strategy for fabricating hierarchical porous micromotors from evolved double emulsions, W1 and W2 phases, respectively labeled with rhodamine B and methylene blue, and O phase without Fe3O4@AgNPs, are used to generate double emulsions for better observation of their evolved structures.

The top view of the

double emulsions shows uniform droplet-in-droplet structures with overlapped inner red and blue droplets (Figure S1), while their side view more clearly shows their evolved structures (Figure 2a).

In each of the double emulsions, the red W1 droplet

and blue W2 droplet vertically aligned due to density mismatch.

Moreover, partial

dewetting of middle oil phase on the two inner droplets is observed in most of the double emulsions, while the ones packing on the top, that are later collected, are still undergoing evolving (Figure 2a).

Such a dewetting process leads to adhesion between

every two of the inner W1 droplet, inner W2 droplet, and outer W1 phase, forming thin films consisting of surfactant bilayers between W1 and W1 phases and between W1 and W2 phases (Figure 2b).

Such interface adhesions are induced by adhesion energy that

associated with the thin film.62-63 For example, the adhesion between inner W2 droplet and outer W1 phase involves the adhesion between the W1/O and W2/O interfaces (Figure 2b).

The adhesion energy of such interface adhesion for forming the thin film

can be described as F=γW1O + γW2O  γFilm.

The γW1O and γW2O are respectively the

interfacial tensions between the W1 and O phases, and between the W2 and O phases, 14


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while γFilm is the tension of the thin film between W1 and W2 phases.

The values of

γW1O and γW2O are 1.32 mN m-1 and 9.55 mN m-1 respectively as measured by the interface tensiometer, while the value of γFilm is 10.58 mN m-1 as obtained from the droplet adhesion experiment.62-63

This can provide a positive F= 0.29 mN m-1 to

drive the adhesion between the W1/O and W2/O interfaces for emulsion evolution. Besides the evolution induced by density mismatch and adhesion energy, the excess amount of surfactant PGPR in the middle oil phase also leads to formation of water nanodroplets in the middle oil phase due to diffusion of water molecules from aqueous phase to the oil phase.

This diffusion process is confirmed by placing droplets, with

same composition as that of the middle oil phase, in aqueous phase dyed with rhodamine B (Figure S2). As shown in Figure S2, the fluorescent intensity

of

the

dispersed

droplet

increases

with

increasing time, due to the diffusion of water molecules from the rhodamine-B-dyed aqueous solution into the droplet to form water nanodroplets.

After the evolution, UV-polymerization of the

emulsion templates can produce polyETPTA microparticles containing hierarchical porous structures (Figure 2c), with inner W1 and W2 droplets as templates for engineering the microscale pores (Figure 2e), and water nanodroplets in the middle oil phase for engineering the nanoscale pores (Figure 2d,f).

Meanwhile, the thin films

break during UV-polymerization, which benefits the formation of two holes on the top and bottom surfaces of polyETPTA microparticles, and formation of one hole connecting the two microscale pores, to create the inner microchannel.

Moreover, the 15



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SEM images of the surface and cross-section of the microparticles confirm their interconnected porous structures through the polyETPTA matrix (Figure 2d,f). Next, for fabricating the hierarchical porous micromotors, Fe3O4@AgNPs (inset in Figure 3a) with an average diameter of ~167.1 nm are added in the middle oil phase of double emulsions to provide the navigator and engine to the hierarchical porous polyETPTA microparticles.

The SEM image of Fe3O4@AgNPs together with their

EDX analysis shown in Figure S3 confirms the presence of Ag elements in the Fe3O4@AgNPs.

The top view of the generated double emulsions containing

Fe3O4@AgNPs shows their uniform droplet-in-droplet structures (Figure 3a), with an average outer diameter of 550.8 μm, and CV value of 0.47 %, indicating high monodispersity (Figure 3b).

Meanwhile, the middle oil phase of the double emulsions

becomes non-transparent due to the addition of Fe3O4@AgNPs.

After emulsion

evolution and then magnetic-guided deposition of Fe3O4@AgNPs at the bottom of the double emulsions, hierarchical porous micromotors decorated with Fe3O4@AgNPs around the opening hole on their bottom surface can be obtained after UV-polymerizing the evolved emulsion templates.

The resultant micromotors show uniform structures

(Figure 3c), with an average size of 475.7 μm, and CV value of 0.66 % (Figure 3d). Due to the decorated Fe3O4@AgNPs around the bottom hole, the hierarchical porous micromotor shows black color at their bottom surface (inset in Figure 3c).

Moreover,

as compared with the emulsion size, the decrease in micromotor size is due to the volume contraction of the middle oil phase during UV-polymerization.

Such a

volume contraction, together with the thin film, both contribute to the formation of 16


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holes on the top and bottom surfaces of the resultant micromotors, and in between the two microscale pores.

These micromotors also exhibit interconnected hierarchical

porous structures, with nano-porous structures both on their surface and cross-section, and with two microscale pores incorporated in the nano-porous matrix (Figure 4). Moreover, the structure of the microscale pores can be further manipulated by changing the size and number of inner droplets in double emulsion templates via adjustment of flow rates and microchannel dimensions.43

Meanwhile, the nano-porous structures

can also be further engineered by changing the surfactant contents in the middle oil phase.43 Hierarchical Porous Micromotors for Bubble-Propelled Motion.

The

asymmetrical structure in the micromotors is engineered by only decorating Fe3O4@AgNPs around the bottom hole via the controllable emulsion evolution strategy.

The decorated Fe3O4@AgNPs allow decomposition of H2O2 into oxygen

bubbles to power the micromotor for fast bubble-propelled motion.

Figure 5 shows

the bubble-propelled motion behaviors of hierarchical porous micromotors in aqueous solution with 30 % (w/v) H2O2 and different CSDS.

The micromotors move faster with

CSDS increasing from 0.01 % (w/v) to 2 % (w/v).

As shown in Figure 6, the moving

velocity of the micromotors increases quickly upon increasing CSDS from 0.01 % (w/v) to 0.5 % (w/v), and then almost reaches a plateau upon further increasing CSDS from 0.5 % (w/v) to 2 % (w/v).

The enhancement of bubble-propelled motion is due to the

facilitation of bubble generation and detachment by the absorbed surfactants on the micromotor surface.64

With increasing CSDS, the SDS molecules with increased 17



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amount can absorb onto the micromotor surface to reduce the interfacial tension, and then facilitate the bubble generation and detachment for enhanced motion.

However,

after the micromotor surface is fully covered by the SDS molecules, further CSDS increase shows no obvious improvement on moving velocity, due to the nearly unchanged interfacial tension.64

Further improvement of the bubble-propelled motion

can be achieved by decreasing the micromotor size and increasing the content of Fe3O4@AgNPs.

For example, in aqueous solution with 30 vol% H2O2 and 0.5 % (w/v)

SDS, the micromotors with 0.5 % (w/v) Fe3O4@AgNPs can achieve a maximum velocity of 255.2 μm s-1.

During the movement, the micromotors show a reduced

velocity after ~0.5 h, and stop movement after ~ 1 h; this is mainly due to the consumption of H2O2 fuel in the solution.

Moreover, when increasing their content

of Fe3O4@AgNPs to 2 % (w/v), the micromotors can achieve a much faster bubblepropelled motion with a maximum velocity of ~1646.2 μm s-1 (Movie S2). Hierarchical Porous Micromotors for Removal of Oil Pollutants in Water. The hydrophobic hierarchical porous matrix of the micromotors can provide good affinity between the micromotors and oil for removing oil pollutants from water. H2O2 with concentration of 2 % (w/v) is used in the oil-containing solution to power the micromotors, since the peroxide levels used for environmental remediation is normally around 1 %~2 %.

As shown in Figure 7, samples A, B, C are respectively

the oil-contaminated solution (A), oil-contaminated solution with the hierarchical porous polyETPTA microparticles (B), and oil-contaminated solution with the hierarchical porous micromotors (C).

At t=0 min, the three samples with only the oil18


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containing solution show the same red color (Figure 7a).

After adding the

polyETPTA microparticles (B) and micromotors (C) for t=10 min, samples A and B remain nearly unchanged red color, but sample C becomes turbid.

This is because the

micromotors in sample C generate plenty of microbubbles and move around the solution for oil capture (Figure 7b).

After 4 h, in sample B, oil droplets are captured

by only part of the microparticles on the top of packed microparticles.

As compared

to samples A and B, although sample C still remains turbid due to the microbubbles, the red micromotors in the turbid solution with faded red color indicate their faster capture of oil droplets for oil removal (Figure 7c).

Moreover, the oil-loaded

micromotors dispersed in aqueous solution can be easily collected by using a magnet for recovery due to the decorated Fe3O4@AgNPs (Figure 7d).

After further washing

the collected micromotors with ethanol to remove their loaded oil, these micromotors can be reused for oil removal (Figure S4), showing good reusability.

These results

show the potential of the micromotors for efficient removal of oil pollutants in water. Hierarchical Porous Micromotors for Removal of Toxic Heavy Metal Ions in Water.

The hierarchical porous micromotors can be flexibly functionalized by

simply incorporating functional nanoparticles into the middle oil phase for fabricating micromotors with advanced functions for water decontamination.

To demonstrate the

use of hierarchical porous micromotors for efficient water decontamination, Pb2+ ions, a toxic heavy metal ion that can lead to immunological, cardiovascular, and neurological disorders, and particularly intellectual disabilities of children, even with trace amount, is used as the pollutant.

For removal of heavy metal ions in water, thiol19



Page 20 of 46

modified SiO2 nanoparticles are added into the middle oil phase of double emulsions for fabricating thiol-modified hierarchical porous micromotors.

The thiol groups

modified on the surface of SiO2 nanoparticles can provide strong binding affinity for capturing toxic heavy metal ions such as Pb2+, Hg2+, and Cd2+.41,65-67

As shown in the

EDX analysis of the thiol-modified SiO2 nanoparticles (Figure S5), the presence of Si, O and S elements proves the successful modification of the SiO2 nanoparticles with thiol groups.

By simply adding these thiol-modified SiO2 nanoparticles, micromotors

with their hierarchical porous matrix decorated with thiol-modified SiO2 nanoparticles can be produced.

The resultant thiol-modified micromotors exhibit interconnected

hierarchical porous structures (Figure 8a-d) as same as that of the unmodified micromotors.

Meanwhile, as shown in Figure 8e and 8f, the well-distribution of Si

and S elements in the cross-section of the micromotor confirms the good decoration of thiol-modified SiO2 nanoparticles in their hierarchical porous matrix.

Moreover, the

presence of Si, S, Fe and Ag elements around the bottom hole proves the decoration of thiol-modified SiO2 nanoparticles and Fe3O4@AgNPs on the bottom surface of the micromotors (Figure S6).

All the results show the successful modification of the

micromotors with the functional SiO2 nanoparticles from their outside to inside. Moreover, the facile and flexible surface modification of SiO2 nanoparticles also offers possibilities to endow the micromotors with diverse functionalities to fulfill on-demand tasks.68 For removal of Pb2+ from water, 0.2 g thiol-modified hierarchical porous micromotors are added into the Pb2+-contaminated solution (2 mg L-1).

The thiol20


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modified micromotors can provide large functional surface area with highly accessible thiol groups for bonding Pb2+, and bubble-propelled motion for improving mass transfer to achieve efficient water decontamination.

As shown in the time-dependent changes

of Pb2+ removal efficiency in Figure 9, for the thiol-modified hierarchical porous polyEPTPA microparticles, only 59.5 % of the Pb2+ ions in the solution are captured at t =180 min.

Meanwhile, at t =180 min, 93.9 % of the Pb2+ ions are captured by the

thiol-modified hierarchical porous micromotors, due to their bubble-propelled motion for enhanced mass transfer (Figure 9).

Meanwhile, the static micromotors show a

similar removal efficiency as that of the polyEPTPA microparticles due to their similar hierarchical porous structures, while the thiol-modified SiO2 nanoparticles show the lowest removal efficiency due to their close packing at the bottom of the cuvette.

The

results show the power of hierarchical porous micromotors for efficient decontamination of Pb2+ polluted water. Changes of the pH value of Pb2+-contaminated solution and the micromotor number both influence the Pb2+ removal performance.

As shown in Figure 10a, when

decreasing the pH value from pH=6.7 (Figure 9) to lower values of pH=5 and pH=3, the Pb2+ removal efficiency at t =180 min decreases from 93.9 % to 80.8 % and 72.7 % respectively, due to the protonation of more thiol groups in lower pH values. Moreover, when increasing the micromotor number from ~1800 to ~4800, the removal efficiency of Pb2+ ions at 180 min increases from 74.8 % to 97.9 % (Figure 10b), showing enhanced Pb2+ removal performance.

21



Page 22 of 46

Upon bubble-propelled motion, the micromotors enhance the convection in the solution to benefit the contact between the modified thiol groups and Pb2+. Meanwhile, the inner microchannel created by the two microscale pores serves as an easy access to the inner surface of the porous matrix.

This microchannel allows flow

of Pb2+-contaminated solution through the micromotor body upon the bubble-propelled motion.

Thus, the micromotors can provide both the outer and inner surfaces for the

Pb2+ ions to enter their porous matrix for fast capture.

Both the bubble-propelled

motion and the hierarchical porous structures can lead to efficient Pb2+ capture.

CONCLUSIONS In summary, a simple and flexible strategy based on microfluidics are developed for continuous fabrication of bubble-propelled micromotors with well-engineered hierarchical porous structures in one-step for efficient water decontamination.

Double

emulsions with controllably evolved morphologies are generated from microfluidics for template synthesis of the micromotors.

The fabricated micromotors containing

well-designed hierarchical porous structures decorated with Fe3O4@AgNPs, can decompose H2O2 into oxygen bubbles for bubble-propelled motion.

The micromotors

with hydrophobic and magnetic hierarchical porous matrix allow efficient removal of oil pollutants in water, and easy magnetic-assisted recovery.

Meanwhile, the

micromotors can be flexibly functionalized by simply incorporating functional nanoparticles into their hierarchical porous structures.

As demonstrated here, by

simply modifying their hierarchical porous structures with thiol-modified SiO2 22


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nanoparticles, the micromotors enable efficient removal of toxic heavy metal ions in water.

Based on the flexibility for surface modification of SiO2 nanoparticles,68 the

micromotors integrated with SiO2 nanoparticles can be further endowed with diverse functionalities for many uses in environmental and biomedical fields.42

Control of the

structure of hierarchical porous micromotors, such as size of the micromotors and their pores, can be achieved by manipulating the structure and composition of emulsion templates for optimizing their motion behavior for enhanced performances.

Since the

micromotor size is mainly determined by the size of their emulsion templates, the micromotor size can be manipulated in the range from several tens to hundreds of micrometers based on double emulsions from microfluidics.58,69

Moreover, based on

this fabrication strategy, other materilas such as poly(methyl methacrylate),43 can also be used for engineering different hierarchical porous micromotors.

Besides, by

parallelly multiplying the microchannels, microfluidic techniques enable mass production of emulsion droplets,70,71 for example, at a rate of >1 trillion droplets per hour;71 this provides opportunities for mass production of the micromotors. Therefore, these bubble-propelled hierarchical porous micromotors show great power as tools for efficient water decontamination.

Moreover, the approach based on

microfluidic emulsion-template synthesis is promising for continuous and controllable fabrication of novel micromotors with well-engineered structures and integrated functions.

ASSOCIATED CONTENT 23



Page 24 of 46

Supporting Information Top-view optical micrograph of the evolved double emulsions, formation process of water nanodroplets in oil droplets, SEM image and EDX analysis of Fe3O4@AgNPs, thiol-modified SiO2 nanoparticles, and the bottom hole of thiol-modified hierarchical porous micromotors, and movies showing the generation process of double emulsions and the bubble-propelled motion of micromotors, are included as Supporting Information.

This material is available free

of charge via the Internet at http://pubs.acs.org.

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

24


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The authors gratefully acknowledge support from the National Natural Science Foundation of China (91434202), the State Key Laboratory of Polymer Materials Engineering (sklpme2014-1-01), and Scientific Research Fund of Sichuan Provincial Education Department (18ZB0495).

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(70) Romanowsky, M. B.; Abate, A. R.; Rotem, A.; Holtze, C.; Weitz, D. A., High Throughput Production of Single Core Double Emulsions in a Parallelized Microfluidic Device. Lab Chip 2012, 12, 802-807. (71) Yadavali, S.; Jeong, H. H.; Lee, D.; Issadore, D., Silicon and Glass Very Large Scale Microfluidic Droplet Integration for Terascale Generation of Polymer Microparticles. Nat Commun 2018, 9, 1222.

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Figures

Figure 1. Schematic illustration of fabrication of hierarchical porous micromotors from controllably evolved double emulsions.

(a) Microfluidic generation of monodisperse

(W1+W2)/O/W1 double emulsions as initial templates.

(b-g) Fabrication strategy

based on controllably emulsion evolution (b-c), magnetic-guided Fe3O4@AgNPs deposition (d), and UV-initiated polymerization (e) to produce micromotors with hierarchical porous structures (f,g).

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Figure 2. Morphological characterization of hierarchical porous polyETPTA microparticles.

(a) Optical micrographs showing the side view of controllably

evolved (W1+W2)/O/W1 double emulsions.

(b) Schematic illustration showing a

double emulsion droplet with interfacial-energy-induced adhesion among inner W1 and W2 droplets and outer W1 phase.

(c,d) SEM images of hierarchical porous

polyETPTA microparticles (c) with porous structures on their surfaces (d).

(e,f) SEM

images of a ruptured hierarchical porous polyETPTA microparticle (e) with porous structures on its cross-section (f).

Scale bars are 200 μm in (a-c) and (e), and 2 μm in

the rest.

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Figure 3. Fabrication of monodisperse hierarchical porous micromotors.

Page 38 of 46

(a,b)

Optical micrograph (a) and size distribution (b) of controllably evolved (W1+W2)/O/W1 double emulsions containing Fe3O4@AgNPs, and SEM image of Fe3O4@AgNPs (inset in (a)). (c,d) SEM image (c), optical micrograph (inset in (c)) and size distribution (d) of hierarchical porous micromotors.

Scale bars are 300 μm in (a,c), and 1 μm in inset

of (a).

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Figure 4. Morphological characterization of hierarchical porous micromotors.

(a-d)

SEM images of a hierarchical porous micromotor before (a) and after (c) rupturing, with porous structures on its surface (b) and cross-section (d).

Scale bars are 100 μm

in (a,c), and 1 μm in (b,d).

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Figure 5. Optical snapshots of the bubble-propelled motion of hierarchical porous micromotors in aqueous solutions containing 30 % (w/v) H2O2 and different CSDS values.

Scale bar is 500 μm.

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Figure 6. Effect of CSDS on the moving velocity of hierarchical porous micromotors in aqueous solutions containing 30 % (w/v) H2O2.

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Figure 7. Hierarchical porous micromotors for oil removal in water.

(a-c) Optical

images showing the oil-containing solution (A), oil-containing solution with hierarchical porous polyETPTA microparticles (B), and hierarchical porous micromotors (C), at 0 min (a), 10 min (b) and 4 h (c).

(d) Easy recovery of oil-

absorbed hierarchical porous micromotors by using a magnet.

Scale bar is 1 cm.

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Figure 8. Morphological characterization of thiol-modified hierarchical porous micromotors.

(a,b) SEM images of thiol-modified hierarchical porous micromotors

(a) with porous structures on their surface (b).

(c,d) SEM images of a ruptured thiol-

modified hierarchical porous micromotor (c) with porous structures on its crosssection (d).

(e,f) EDX analysis showing the distribution of Si (e) and S (f) elements

on the cross-sectional part of the micromotor shown in (d).

Scale bars are 100 μm

in (a,c) and 2 μm in the rest.

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Figure 9. Time-dependent changes of Pb2+ removal efficiency of the hierarchical porous micromotors.

Static hierarchical porous micromotors (without H2O2), and

hierarchical porous polyETPTA microparticles and thiol-modified

SiO2

nanoparticles (both with H2O2), are used as control groups.

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Figure 10. Effect of pH (a) and micromotor number (b) on the Pb2+ removal efficiency of hierarchical porous micromotors.

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Graphic for TOC

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Bubble-Propelled Hierarchical Porous Micromotors from Evolved

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