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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
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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
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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.
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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
Nanocatalysts-carrying
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.
Nanocatalysts-carrying
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: wangwei512@scu.edu.cn. 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).
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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.
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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.
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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.
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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.
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