Inorganic Janus Particles

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Emulsion Interfacial Synthesis of Polymer/Inorganic Janus Particles Xi Chen, Jingjing Xu, Dayin Sun, Bingyin Jiang, Fuxin Liang, and Zhenzhong Yang Langmuir, Just Accepted Manuscript • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Emulsion Interfacial Synthesis of Polymer/Inorganic Janus Particles Xi Chen†,‡, Jingjing Xu†,‡, Dayin Sun⊥, Bingyin Jiang†, Fuxin Liang† and Zhenzhong Yang*,†,§ †State

Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese

Academy of Sciences, Beiing 100190, China ‡University

of Chinese Academy of Sciences, Beijing 100049, China

⊥Liaoning

Provincial Key Laboratory for Green Synthesis and Preparative Chemistry of

Advanced Materials, Liaoning University, Shenyang 110036, China §Institute

of Polymer Science and Engineering, Department of Chemical Engineering, Tsinghua

University, Beijing 100084, China

*E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: We report a method to prepare polymer/inorganic Janus particles by transferring self-assembled membrane of copolymers such as PS-b-PAA at an emulsion interface when the amine capped particles such as paramagnetic Fe3O4@SiO2 core/shell particles are preferentially adsorbed by specific interactions. While the particles are protected, the exposed side can be further modified to conjugate aldehyde capped PEO. Both connections become robust by covalent bonds. The hydrophilic PEO and hydrophobic PS chains are distinctly compartmentalized onto the opposite sides of the Fe3O4@SiO2 particles. As a magnetic responsive solid surfactant, the stabilized emulsions can be driven with a magnet for directional movement and coalescence with increasing magnetic strength. This method can be extended to other Janus particles with tunable organic materials and solid particles. KEYWORDS: emulsion interface, Janus particle, polymer/inorganic, solid surfactant


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INTRODUCTION Janus materials with two different compositions thus properties distinctly compartmentalized onto the same entity, have attracted growing academic and industrial interests. They display diversified performances and are promising in many applications.1 Many methods have been proposed to synthesize Janus materials, including site-selective modification,2 controlled surface nucleation,3 microfluidic jetting,4 space-confined assembly,5 and emulsion interfacial synthesis.6 In 1988, two-dimensional protection assisted synthesis was proposed to prepare Janus materials.7 While one side of the particles is embedded within a matrix to achieve the protection, onto the exposed side diversified materials can be grown. The method is general, and the two sides are distinct in chemistry. However, the synthesis yield is rather low. In order to increase the yield, three-dimensional surfaces including particles surface and emulsions interface are used instead of the two-dimensional protection. Many Janus materials with varied morphologies have been derived.8 Pickering emulsion interfacial synthesis has been extensively employed to prepare Janus particles with tunable compositions and microstructures. In order to restrict rotation of the particles at the Pickering emulsion interface, the internal oil phase for example in the paraffin/water emulsion system should be frozen upon formation of the Pickering emulsion.9 While one side of the particles is embedded within the inert paraffin to achieve the protection, the exposed side can be selectively modified by growing desired materials. Composition of the exposed side differs from the other side. However, the particles will aggregate at the interface when growth degree becomes higher. The Pickering emulsion synthesis was modified by selectively etching the exposed side allowing the fresh silanol group to expose.10 Recently, we have proposed synchronous ATRP grafting hydrophobic and hydrophilic polymer chains onto the corresponding sides of the anchored silica particles at a fluidic Pickering emulsion interface,


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achieving Janus particles in one step.11 Pre-modification of the particles gaining appropriate wettability is a prerequisite to form the Pickering emulsions.12 On the other hand, solid polymer spheres called Wang resins possess tunable functional groups on the surface, which are usually used as templates to synthesize Janus particles.13 Those particles with matched functional groups can be preferentially adsorbed onto the Wang resin spheres surface by specific interaction. One side of the particles is thus protected. The exposed side can be selectively modified to introduce different materials. It is required to break the specific interaction to release the Janus particles from the spheres surface. Alternatively, the emulsions stabilized with amphiphilic molecules possess functional groups at a fluidic interface, which play multiple roles in forming asymmetric materials. As a temporal Janus template, Janus inorganic shells and their derivative nanosheets were synthesized by self-organized sol-gel process of silane precursors.14 The polyacrylic acid group at one side of the interface can form a specific interaction with the functional pendant group such as amine of one silane during the sol-gel process. In the case of oil/water emulsion interface stabilized with PS-b-PAA, amine capped silica nanoparticles of ~25 nm were favorably adsorbed by hydrogen bonding, whilst PEO-b-PAA in water strongly interacted with the exposed side of the nanoparticles. A PS-silica-PEO tri-layered continuous shell forms rather than individual nanoparticles after dissolving the oil phase.15 The nanoparticles are small with the size comparable with the polymer chains, and the amine capped nanoparticle serves as a large crosslinker. Intermolecular crosslinking occurs among the monolayer at the emulsion interface. It is noticed that the monolayer is fluidic which can be transferred. It is anticipated that the self-assembly monolayer at the emulsion interface will be transferred onto the contact part of the adsorbed particles when the particles are large. A Janus particle will be achieved after releasing


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from the interface. Herein, we propose a facile method to prepare polymer/inorganic composite Janus particles. Two different polymers are distinctly compartmentalized onto the opposite sides. As shown in Scheme 1, a PS-b-PAA Janus monolayer is achieved by self-assembling at the paraffin/water emulsion interface (a). The PAA blocks face toward the external aqueous phase, while the PS blocks face inwardly. The paraffin spheres become solid upon cooling down to room temperature. Along step (b), EDC and NHS are added to activate the carboxyl- groups of PAA. Amine group capped Fe3O4@SiO2 core/shell particles are favorably adsorbed onto the sphere surface by electrostatic interaction following by amidation between carboxyl- and amine groups. The interfacial monolayer is transferred onto the contact area of the adsorbed particles. Protection is achieved thereby to allow a selective modification of the other side. Along step (c), PEO moiety is conjugated onto the exposed side of the particles by Schiff base reaction with the aldehyde group capped poly(ethylene oxide) (PEO-CHO). The linkage becomes stable upon reduction with NaBH4. Along (d), after dissolution of the paraffin spheres, the PS-Fe3O4@SiO2-PEO Janus particle is achieved. The Fe3O4@SiO2 core/shell solid particle is chosen for magnetic manipulation of the derived Janus particles. Scheme 1. Illustrative synthesis of the PS-Fe3O4@SiO2-PEO Janus particle.

(a) PS-b-PAA copolymer Janus monolayer is formed by self-assembling at the paraffin/water interface; (b) the amine capped Fe3O4@SiO2 core/shell particles are adsorbed onto the sphere


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surface following by amidation, the PS-b-PAA Janus monolayer is transferred onto one side of the particle surface; (c) while the particles are protected, the exposed side is conjugated with PEO-CHO by Schiff base and reduction with NaBH4; (d) after dissolution of paraffin, the PS-Fe3O4@SiO2-PEO Janus particle is achieved.

EXPERIMENTAL SECTION Materials. Ferric chloride hexahydrate (FeCl3·6H2O), ethylene glycol (EG), sodium acetate (CH3COONa), sodium borohydride (NaBH4), sodium hydroxide (NaOH), hydrochloric acid (HCl), acetic acid (CH3COOH), ammonia (NH3·H2O) and the solvents of tetrahydrofuran (THF), n-decane and ethanol were purchased from Sinopharm Chemical Reagent. Tetraethoxysilane (TEOS) was purchased from Beijing Chemical Reagent. PS5.2k-b-PAA4k (PDI=1.1) and PEO5k-CHO were purchased from Polymer Source. Paraffin (Tm=50-52 °C) was purchased from Shanghai

Huayong

Paraffin

Company.

1-Ethyl-3-(3-dimethyllaminopropyl)

carbodiiehydrochlide (EDC), N-hydroxysuccinimide (NHS) and 3-aminopropyltriethoxysilane (APTES) were purchased from Aldrich. Synthesis of the Paramagnetic Fe3O4 Particle. One representative paramagnetic Fe3O4 particle was synthesized via solvothermal reaction.16 2.7 g of FeCl3·6H2O was dissolved in 100 mL of EG under ultrasonication. After 7.2 g of CH3COONa was added into the solution, the system became brown yellow. The system was transferred in a Teflon-lined stainless steel autoclave and heated to 200 °C for 12 h. After the autoclave was cooled down, the black product was collected with a magnet and washed with ethanol and water. After freeze drying, the paramagnetic Fe3O4 particle with a mean diameter of 200 nm was obtained. Synthesis of the Fe3O4@SiO2 Core/Shell Particle. After 0.1 g of Fe3O4 particle was treated


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in 0.1 M HCl aqueous solution under ultrasonication for 2 min, the Fe3O4 particle was washed with water and collected with a magnet. The treated Fe3O4 particle was added to a mixture of 60 mL of ethanol, 15 mL of H2O and 1.2 mL of NH3·H2O under stirring for 2 h at room temperature. 0.1 g of TEOS was added to the dispersion under stirring for another 8 h. The product was washed with water and ethanol, and dried in vacuum oven at 80 °C for 12 h. The Fe3O4@SiO2 core/shell particle was obtained. Synthesis of the Amine Capped Fe3O4@SiO2 Core/Shell Particle. APTES (1%, v/v) was added to the Fe3O4@SiO2 core/shell particle dispersion in toluene at a concentration of 0.5 mg/mL containing a trace amount of CH3COOH (0.01%, v/v), and stirred at 80 °C for 12 h. The black product was washed with ethanol and collected with a magnet. After freeze drying, the amine capped Fe3O4@SiO2 core/shell particle was achieved. Paraffin/Water Emulsion by Diblock Copolymer PS-b-PAA. 0.05 g of PS-b-PAA was added into 10 mL of water, 10 μL of NaOH (2 mol/L) aqueous solution was added to improve dispersion of PS-b-PAA under ultrasonication. After the mixture was heated to 70 °C, 0.1 g of melt paraffin was added under stirring at a speed of 15000 r/min for 5 min. The paraffin/water emulsion was obtained. Upon cooling down to room temperature, the solid paraffin/water emulsion was achieved. Residual PS-b-PAA was removed by centrifugation from the emulsion. A trace of amount of aqueous HCl was added to the emulsion to adjust pH~5. Synthesis of the PS-Fe3O4@SiO2-PEO Janus Particle. 0.1 g of the paraffin spheres stabilized with PS-b-PAA were dispersed in 10 mL of water. 50 μL of the amine group capped Fe3O4@SiO2 particle aqueous dispersion (50 mg/mL) was added under stirring. Then, 10 mg of EDC and 5 mg of NHS were added to activate the carboxyl- group. After the amidation reaction, the paraffin spheres were filtered and washed with water to remove free particles, then


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redispersed in water (The free particles could be easily collected by a magnet and re-used for other paraffin emulsions). 50 μL of PEO-CHO aqueous solution (20 mg/mL) and 2 μL of acetic acid were added and stood to form the Schiff base between the aldehyde group of PEO-CHO and amine group at the exposed side of the adsorbed particles. NaBH4 aqueous solution was added to reduce the Schiff base. After dissolution of paraffin with THF and purified with toluene, the PS-Fe3O4@SiO2-PEO Janus particles were obtained. Emulsification with the PS-Fe3O4@SiO2-PEO Janus Particle. 4 mg of the Janus particle was added in a mixture of oil/water (0.4 mL/1.6 mL). The emulsification was performed under stirring at a speed of 15000 r/min for 5 min at room temperature. An oil/water (O/W) emulsion formed. In the case of paraffin, the emulsification was performed at 70 °C. The emulsion was cooled down to room temperature to solidify the internal paraffin phase. Characterization. Diluted dispersions in water were dropped onto wafer support to prepare samples for SEM observation. The samples were ambiently dried and vacuum sputtered with Pt. SEM measurement was performed with Hitachi S-4800 scanning electron microscope operated at an accelerating voltage of 15 kV. Sample morphology was characterized using a transmission electron microscope (JEOL 2100F at 200 kV) equipped with an energy-dispersive X-ray (EDS) analyzer. The samples for TEM observation were prepared by spreading very dilute dispersions onto carbon-coated copper grids. Fluorescence microscopy images were recorded using an Olympus FV1000-IX81 confocal laser scanning microscope. The crystallinity was measured by X-ray powder diffraction (XRD) using Rigaku D/max-2500. The magnetic saturation values were measured with vibrating sample magnetometer (VSM). FT-IR spectroscopy was performed with the sample/KBr pressed pellets after scanning samples for 32 times using Bruker Equinox 55 spectrometer. Zeta potential values were measured using Zeta-sizer (Nano Series, Malvern


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Instruments) at 25 °C. Thermogravimetric analysis (TGA) was performed using the PerkinElmer Pyris 1 thermogravimetric analyzer under air at a scanning rate of 10 °C/min. Type of the emulsions was determined via confocal laser scanning microscope (CLSM) FV1000 and confocal fluorescence microscope with IX-81 inverted base and PMT detector.

RESULTS AND DISCUSSION The example Fe3O4 particle was synthesized by the solvothermal reaction. The particle is relatively uniform with a diameter of 200±30 nm, and the surface is rough (Figure 1a). The Fe3O4 particle surface was coated with a thin silica layer by sol-gel process of TEOS, making the particles tolerant to environment, especially acidic media. Onto the silica layer, functional groups can be introduced by a further modification. A uniform core/shell structure is clearly distinguished with a silica layer thick ~20 nm (Figure 1b). The particle surface becomes smooth. The crystallinity of the Fe3O4 particle is less influenced with the coating of silica (Figure S1a). Both particles are paramagnetic (Figure S1b). The saturation magnetization value of the Fe3O4 particle decreases from 77.2 emu/g to 62.0 emu/g after coating the silica layer. Zeta potential becomes more negative from -4.7 to -33.4 mV at pH=7. (Figure 1c), which is consistent with the fact that the pKa (~2.6) of silica is much lower than pKa (~6.8) of Fe3O4. After a further modification with APTES to introduce amine groups onto the particle surface, zeta potential becomes positive +33.4 mV at pH=7, due to a higher pKa (~10.2) of amine group. The successful introduction of amine groups is also confirmed by FT-IR spectroscopy (Figure 1d). After coating with silica onto the Fe3O4 particle surface, the new bands around 1000-1200 cm-1 appear which are assigned to Si−O−Si vibration. After modification with APTES, the new bands at 2990 and 2810 cm-1 appear which are assigned to the −CH2− group of APTES. The


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characteristic peak at 1573 cm-1 is assigned to amine group.

a.

c.

b.

d.

Figure 1. SEM and inset TEM images of (a) the Fe3O4 particle and (b) the Fe3O4@SiO2 core/shell particle. Zeta potential values (c) and FT-IR spectra (d) of the Fe3O4 particle (curve a), the Fe3O4@SiO2 core/shell particle (curve b), the amine capped Fe3O4@SiO2 core/shell particle (curve c).

The PS-b-PAA copolymer was used to emulsify the paraffin and water mixture forming an oil-in-water emulsion. The paraffin droplets are 5-20 μm in diameter (Figure 2a). Upon cooling down to room temperature, the melt paraffin droplets were frozen forming solid spheres. The spheres are individual without coalescence (Figure 2b). A magnified SEM image indicates that the sphere surface is smooth (Figure 2c). At the sphere surface, the PS blocks should be embedded within the paraffin while the hydrophilic PAA blocks stretching toward the external


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aqueous phase. EDC/NHS was used to activate the PAA blocks at the paraffin sphere surface. While the amine capped core/shell particles were adsorbed onto the paraffin spheres surface via electrostatic attraction, the amidation reaction occurred. The paraffin spheres become coarsening with a monolayer of the particles onto the surface (Figure 2d). Onto the exposed side of the adsorbed particles, an example hydrophilic polymer such as PEO-CHO was conjugated via Schiff base reaction. In order to strengthen the linkage, the Schiff base was reduced with NaBH4.

a.

b.

c.

d.

Figure 2. (a) Polarizing optical image, (b) SEM image of the paraffin spheres stabilized with PS-b-PAA; the paraffin sphere surface before (c) and after the amine capped particles adsorbed (d).

The saturation adsorption amount of the amine group capped Fe3O4@SiO2 particle on the paraffin sphere surface was measured to be ~25 mg/g (particle/paraffin). The time necessary to maximize the adsorption of amine group capped Fe3O4@SiO2 particles was estimated along the


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zeta potential change of the paraffin spheres in water. The zeta potential of the paraffin spheres stabilized by PS-b-PAA was negative while the zeta potential of the amine group capped Fe3O4@SiO2 particles was positive. Therefore, the zeta potential of the paraffin spheres would increase along the adsorption process until saturation. As shown in Figure S2, according to the zeta potential change with adsorption time, it is found that ~2 h was sufficient for the saturated adsorption of the particles at the paraffin spheres. Confocal laser scanning microscope (CLSM) was used to monitor the adsorption of the amine capped Fe3O4@SiO2 particles onto the paraffin sphere surface. The amine group capped particles were previously labeled with a trace amount of fluorescein isothiocyanate (FITC), which display green color in CLSM images (Figure 3a). The paraffin dyed with fluorescein Coumarin 6 shows red (Figure 3b). Upon adding the particles into the emulsion, the amine group capped particles (green) began to accumulate at the paraffin sphere surface forming a discontinuous ring (Figure 3c). Eventually, all the amine group capped particles (green) were adsorbed onto the paraffin sphere surface forming a continuous green ring (Figure 3d). No free particles were found in the continuous phase.

a.

b.


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c.

d.

Figure 3. CLSM images of (a) the amine group capped Fe3O4@SiO2 particles; (b) the paraffin spheres stabilized with PS-b-PAA; adsorption of the particles onto the paraffin sphere surface after varied time (h): (c) 0.5 and (d) 2.

After dissolution of the paraffin core with THF, the particles were collected by a magnet and redispersed in toluene. The PS-Fe3O4@SiO2-PEO Janus particles are well dispersed in toluene, while the non-modified amine group capped Fe3O4@SiO2 particles precipitate. Therefore, the PS-Fe3O4@SiO2-PEO Janus particles were easily separated and purified. The yield was calculated to be ~96%. PTA (phosphatotungstic acid) has been extensively used to selectively stain PEO without any effect on PS.17 After staining PEO at the PS-Fe3O4@SiO2-PEO Janus particle surface with PTA, one side of the particle demonstrates dark corona, implying the presence of PEO moiety (Figure 4a and S3). The elements of W and P are present at the dark corona (Figure 4b). The weight ratio of the two elements is measured 70.6:1, almost the same as theoretic one of PTA (70.8:1). The statistic occupation of the dark corona is 3/4 of the particle surface. This is because the majority of the amine capped core/shell particle surface is exposed while the minor side being protected by the paraffin sphere surface. In comparison, the PEO moiety without staining is invisible under TEM (inset Figure 4a). After another staining with RuO4, the whole surface of the particles display a dark circle (Figure 4c). This implies that the


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other 1/4 of particle surface contains PS moiety. EDS analysis indicates the presence of the element Ru at the whole dark ring (Figure 4d). In the case of the PS-Fe3O4@SiO2 Janus particle, the RuO4 stained PS moiety occupies ~1/4 of the particle surface (Figure 4e), while the other 3/4 part is bare. EDS analysis also verifies the presence of Ru at the 1/4 dark area, and no Ru is detected at the bare 3/4 part (Figure 4f). In dispersion, PEO-CHO can also be grafted onto the PS-Fe3O4@SiO2 Janus particle to achieve the PS-Fe3O4@SiO2-PEO Janus particle. The whole particle surface displays a dark ring after staining with RuO4 (Figure S4). The saturation magnetization values of the PS-Fe3O4@SiO2 and PS-Fe3O4@SiO2-PEO Janus particles are 49.7 and 40.6 emu/g, respectively (Figure S5). Accordingly, the weight ratio of grafted PS-b-PAA to the amine capped Fe3O4@SiO2 particle is calculated 0.200, while the weight ratio of PEO-CHO to the particle is 0.268. Based on the TGA results (Figure S6), the weight ratios of grafted PS-b-PAA and PEO-CHO in respect to the amine capped Fe3O4@SiO2 particle are calculated 0.208 and 0.271. Grafting degrees measured by TGA are highly consistent with the results by VSM data.

a.

b.


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c.

d.

e.

f.

Figure 4. (a) TEM image of the PS-Fe3O4@SiO2-PEO Janus particle after staining with PTA and (b) the EDS analysis; (c) TEM image of the PS-Fe3O4@SiO2-PEO Janus particle after staining with RuO4 and (d) the EDS analysis; (e) TEM image of the PS-Fe3O4@SiO2 Janus particle after staining with RuO4 and (f) the EDS analysis.

The successful amidation between PAA and the amine groups is confirmed by FT-IR spectra (Figure 5). Compared with the amine capped particles (curve a), the new peak at 1643 cm-1 is assigned to amide bond (curve b). The weak peak at 1726 cm-1 corresponds to some residual carboxyl- groups. The presence of PEO is verified by the new peaks at 1275 and 1345 cm-1 (curve c). The peak around 1675 cm-1 is assigned to the −CH=N− vibration, which disappears after reduction with NaBH4, whilst the new peak at 1290 cm-1 assigned to the −CH−N− vibration appear (curve d). The PS-Fe3O4@SiO2-PEO Janus particle is rather stable after immersion in acidic water at pH=2.0 for 6 h (curve a, Figure S7). In comparison, PEO chains escape from the


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as-prepared Janus particle without reduction with NaBH4 due to the cleavage of −CH=N− bond (curve b, Figure S7).

Figure 5. FT-IR spectra of the three representative particles: the amine capped particle (curve a); the PS-Fe3O4@SiO2 Janus particle (curve b); the PS-Fe3O4@SiO2-PEO Janus particle before (c) and after (d) reduction by NaBH4.

The success in synthesizing the Janus particles is further confirmed by the variation of zeta potential values measured in water at pH=7.0 (Figure 6). The amine capped particle displays a highly positive zeta potential (curve a). After grafting onto one side of the particle, the PS-Fe3O4@SiO2 Janus particle becomes nearly neutral (curve b). This implies that the positive charge from the amine group has been screened by the negative charge from residual PAA. When the amine capped side is conjugated with PEO, the PS-Fe3O4@SiO2-PEO Janus particle displays a negative zeta potential (curve c). The dissociated carboxyl-groups become slightly positive at pH=2.0 (Figure S8), due to its protonation. Contribution of the residual carboxyl groups to the negative zeta potential is thus verified.


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Figure 6. Zeta potential values (b) of the three representative particles: the amine capped particle (curve a); the PS-Fe3O4@SiO2 Janus particle (curve b); the PS-Fe3O4@SiO2-PEO Janus particle (curve c).

When the paraffin spheres stabilized with PS-b-PAA were not activated by EDC/NHS, although the amine capped particles could be adsorbed onto the sphere surface via electrostatic interaction (Figure S9a), no amidation reaction occurred. No polymer chains were grafted onto the amine capped Fe3O4@SiO2 particle. The TEM image (Figure S9b), FT-IR spectrum (Figure S9c), and zeta potential (Figure S9d) are similar to the pure amine capped particle. The PS-Fe3O4@SiO2-PEO Janus particle is consisted of hydrophobic PS chains and hydrophilic PEO chains at the opposite sides. The particle can serve as a solid surfactant to stabilize emulsions. As an example, a wax-in-water emulsion was achieved at high temperature. Upon cooling, the paraffin spheres became solid. The crystalline paraffin can be clearly discerned under a polarizing optical microscope (Figure 7a). SEM image shows that the sphere surface is wrinkled corresponding to the crystalline paraffin (Figure 7b). The Janus particles form a monolayer at the sphere surface and penetrate inwardly (Figure 7c). The dispersed paraffin spheres are stable in the dispersion. The paraffin droplets can be driven with a weak


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magnet of ~0.5 T, leaving colorless transparent water phase. Upon removal of the magnet, the dispersion is recovered (Figure 7d). No coalescence among the droplets occurs. In the case of a stronger magnet such as ~0.8 T, the melt paraffin droplets become deformed and partially coalesced (Figure 7e). Eventually, all the droplets are coalesced forming the top oil continuous phase, while the Janus particles are withdrawn from the oil phase (Figure 7f). Upon cooling the top oil phase, the boundary among the spheres almost disappears. No Janus particles are found in the top oil phase.

a.

b.

c.

d.

e.

f.

Figure 7. (a) Polarizing optical image, and (b) SEM image of the paraffin spheres stabilized with


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the PS-Fe3O4@SiO2-PEO Janus particle; (c) a magnified SEM image of the paraffin sphere surface; (d) the melt paraffin/water emulsion stabilized with the PS-Fe3O4@SiO2-PEO Janus particle (left), and magnetic collection of the emulsion droplets (right); (e, f) polarizing optical images of the paraffin spheres during the de-emulsification with a stronger magnet.

By adjusting feeding amount of PEO-CHO in aqueous phase, grafting content of PEO of the PS-Fe3O4@SiO2-PEO particle can be controlled. As an example, ~0.073 weight ratio of PEO to the amine capped Fe3O4@SiO2 particle was achieved (Figure S10). Janus balance of the Janus particle is thus tunable. The emulsion (stabilized with PS-Fe3O4@SiO2-PEO) type can be controlled by the PEO grafting content. In order to observe microstructure of the emulsions under CLSM, the amine capped Fe3O4@SiO2 particle was previously labeled with a trace amount of FITC, and used to prepare PS-Fe3O4@SiO2-PEO. The PS-Fe3O4@SiO2-PEO Janus particle was also labeled with FITC, showing green color in CLSM images. The aqueous (blue) and oil (red) phases demonstrate different colors in CLSM images after selectively labeling with different fluorescein molecules. When water/n-decane weight ratio is fixed at 1.2/0.8, an oil-in-water emulsion forms at the top phase (Figure 8a) when the PEO weight ratio is 0.268 (in respect to the amine capped Fe3O4@SiO2 particle). Conversely, a water-in-oil emulsion forms at the bottom phase when the PEO weight ratio is 0.073 (Figure 8b). It is noted that the Janus particles (green) are exclusively located at the emulsion interface. In addition, control experiments were conducted by using PEO coated homogeneous amine capped Fe3O4@SiO2 particle (denoted as particle@PEO) and PS coated one (denoted as particle@PS) to stabilize emulsions. As shown in Fig S11a, an oil-in-water emulsion was achieved by using particle@PEO, and the emulsion droplets are large within 80-120 μm. In comparison, the


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emulsion droplets stabilized with the PS-Fe3O4@SiO2-PEO Janus particle are smaller within 40-60 μm (Figue 8a). In the case of particle@PS, a water-in-oil emulsion forms at the bottom phase. And the emulsion droplets are 150-200 μm in diameter (Figure S11b), much larger than the emulsion droplets stabilized with the PS-Fe3O4@SiO2-PEO Janus particle (Figure 8b). It is understood that the homogeneous particle@PEO and particle@PS demonstrate Pickering effect rather than amphiphilic performance of the Janus particle.

a.

b.

Figure 8. CLSM images of the typical emulsions stabilized with the PS-Fe3O4@SiO2-PEO Janus particles with different grafting contents of PEO: (a) n-decane-in-water emulsion when PEO/particle weight ratio is 0.268; (b) water-in-n-decane emulsion when the PEO/particle weight ratio is 0.073. The water/n-decane weight ratio is fixed at 1.2/0.8 and the Janus particle concentration is fixed at 2 mg/mL. The oil phase dyed with fluorescein Coumarin 6 shows red, the water phase dyed with fluorescein Rhodamine B shows blue, the Janus particle dyed with FITC shows green.

CONCLUSIONS In summary, we have proposed a facile method to synthesize magnetic responsive composite Janus particle of PS-Fe3O4@SiO2-PEO. It is key to employ an emulsion interface membrane (PS-b-PAA) to induce a preferential adsorption of target (the amine capped Fe3O4@SiO2) particles realizing the protection and transferring the interfacial membrane to the particles at the


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contact. The exposed side can be further selectively modified for example to conjugate hydrophilic PEO chains. The PS-Fe3O4@SiO2-PEO Janus particle is amphiphilic and magnetic responsive. Since the particle serves as a magnetic-responsive particle surfactant, the emulsion droplets stabilized thereby can be manipulated with a magnet. This method can be extended to other emulsion interfaces with different functional groups and solid particles. A family of Janus particles is expected with tunable compositions of the solid particle and surface.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.XXXXXXX. XRD patterns, VSM curves, FT-IR spectra, TGA traces, zeta potentials, SEM, TEM, CLSM and polarizing optical images (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Zhenzhong Yang: 0000-0002-4810-7371 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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This work is supported by National Natural Science Foundation of China (51622308, 51233007, 51703229). REFERENCES (1) (a) de Gennes, P. G. Soft Matter. Rev. Mod. Phys. 1992, 64, 645−648. (b) Walther, A.; Müller, A. H. E. Janus Particles: Synthesis, Self-Assembly, Physical Properties, and Applications. Chem. Rev. 2013, 113, 5194−5261. (c) Liang, F. X.; Zhang, C. L.; Yang, Z. Z. Rational Design and Synthesis of Janus Composites. Adv. Mater. 2014, 26, 6944−6949. (d) Liang, F. X.; Yang, Z. Z. Progress in Janus Composites toward Interfacial Engineering. Acta Polymerica Sinica 2017, 6, 883−892. (2) (a) Paunov, V. N.; Cayre, O. J. Supraparticles and “Janus” Particles Fabricated by Replication of Particle Monolayers at Liquid Surfaces Using a Gel Trapping Technique. Adv. Mater. 2004, 16, 788−791. (b) Lattuada, M.; Hatton, T. A. Preparation and Controlled Self-Assembly of Janus Magnetic Nanoparticles. J. Am. Chem. Soc. 2007, 129, 12878−12882. (3) (a) Reculusa, S.; Poncet-Legrand, C.; Perro, A.; Duguet, E.; Bourgeat-Lami, E.; Mingotaud, C.; Ravaine, S. Hybrid Dissymmetrical Colloidal Particles. Chem. Mater. 2005, 17, 3338−33344. (b) Qiang, W.; Wang, Y.; He, P.; Xu, H.; Gu, H.; Shi, D. Synthesis of Asymmetric Inorganic/Polymer Nanocomposite Particles via Localized Substrate Surface Modification and Miniemulsion Polymerization. Langmuir 2008, 24, 606−608. (4) (a) Nie, Z.; Li, W.; Seo, M.; Xu, S.; Kumacheva, E. Janus and Ternary Particles Generated by Microfluidic Synthesis: Design, Synthesis, and Self-Assembly. J. Am. Chem. Soc. 2006, 128, 9408−9412. (b) Nisisako, T.; Torii, T. Formation of Biphasic Janus Droplets in a Microfabricated Channel for the Synthesis of Shape-Controlled Polymer Microparticles. Adv. Mater. 2007, 19, 1489−1493.


22

Page 23 of 26 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

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(5) (a) Deng, R. H.; Liang, F. X.; Zhou, P.; Zhang, C. L. Qu, Z. X.; Yang, Z. Z. Janus Nanodisc of Diblock Copolymers. Adv. Mater. 2014, 26, 4469−4472. (b) Deng, R. H.; Zhu, J. T.; Li, B. H.; Liang, F. X.; Yang, Z. Z. Janus Nanoparticles of Block Copolymers by Emulsion Solvent Evaporation Induced Assembly. Macromolecules 2016, 49, 1362−1368. (6) (a) Boker, A.; He, J.; Emrick, T.; Russel, T. P. Self-Assembly of NanoParticles at Interfaces. Soft Matter 2007, 3, 1231−1248. (b) Wang, Y. H.; Zhang, C. L.; Qu, Z. X.; Li, J. L.; Yang, Z. Z. Emulsion Interfacial Synthesis of Asymmetric Janus Particles. Macromolecules 2011, 44, 3787−3794. (7) Casagrande, C.; Veyssié, M. C. R. Equilibrium of a Janus Bead at a Water Oil Interface. Acad. Sci. (Paris) 1988, 2, 1423−1424. (8) (a) Pickering, S. U. Pickering: Emulsions. J. Chem. Soc. 1907, 91, 2001−2021. (b) Aveyard, R.; Binks, B. P.; Clint, J. H. Emulsions Stabilised Solely by Colloidal Particles. Adv. Colloid Interface Sci. 2003, 100-102, 503−546. (c) Yang, T. Y.; Wei, L. J.; Jing, L. Y.; Liang, J. F.; Monteiro, M. J.; Wang, Y.; Gu, S.; Zhao D.Y.; Yang, H. Q.; Liu, J. Dumbbell-Shaped Bi-component Mesoporous Janus Solid Nanoparticles for Biphasic Interface Catalysis. Angew. Chem. Int. Ed. 2017, 56, 8459−8583. (d) Zhao, T. C.; Zhu, X. H.; Hung, C. T.; Wang, P. Y.; Elzatahry, A.; Li, X. M.; Zhao, D. Y. Spatial Isolation of Carbon and Silica in a Single Janus Mesoporous Nanoparticle with Tunable Amphiphilicity. J. Am. Chem. Soc. 2018, 140, 10009−10015. (9) (a) Hong, L.; Jiang, S.; Granick, S. Simple Method to Produce Janus Colloidal Particles in Large Quantity. Langmuir 2006, 22, 9495−9499. (b) Jiang, S.; Granick, S. Controlling the Geometry (Janus Balance) of Amphiphilic Colloidal Particles. Langmuir 2008, 24, 2438−2445.


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Page 24 of 26

(10) Liu, B.; Zhang, C. L.; Liu, J. G.; Qu, X. Z.; Yang, Z. Z. Janus Non-Spherical Colloids by Asymmetric Wet-Etching. Chem. Commun. 2009, 45, 3871−3873. (11) Liu, B.; Wei, W.; Qu, X. Z.; Yang, Z. Z. Janus Colloids Formed by Biphasic Grafting at a Pickering Emulsion Interface. Angew. Chem., Int. Ed. 2008, 120, 4037−4039. (12) Berger, S.; Synytska, S.; Ionov, L.; Eichhorn, K.; Stamm, M. Stimuli-Responsive Bicomponent Polymer Janus Particles by “Grafting from”/“Grafting to” Approaches. Macromolecules 2008, 41, 9669−9676. (13) (a) Worden, J. G.; Shaffer, A. W.; Huo, Q. Controlled Functionalization of Gold Nanoparticles through a Solid Phase Synthesis Approach. Chem. Commun. 2004, 518−519. (b) Worden, J. G.; Shaffer, A. W.; Huo, Q. Monofunctional Group-Modified Gold Nanoparticles from Solid Phase Synthesis Approach: Solid Support and Experimental Condition Effect. Chem. Commun. 2004, 16, 3746−3755. (14) Liang, F. X.; Shen, K.; Qu, X. Z.; Zhang, C. L.; Wang, Q.; Li, J. L.; Yang, Z. Z. Inorganic Janus Nanosheets. Angew. Chem., Int. Ed. 2011, 50, 2379−2382. (15) Wang, Q. G.; Liang, F. X.; Wang, Q.; Qu, X. Z.; Yang, Z. Z. Responsive Composite Janus Cage. Chinese J. Polym. Sci. 2015, 33, 1462−1468. (16) Deng, H.; Li, X.; Peng, Q.; Wang, X.; Chen, J.; Li, Y. Monodisperse Magnetic Single-Crystal Ferrite Microspheres. Angew. Chem. Int. Ed. 2005, 117, 2842−2845. (17) (a) Lei, L. C.;

Gohy, J. F.; Willet, N.;

Zhang, J. X.; Varshney, S.; Jérôme, R.

Dependence of the Structure of Core–Shell–Corona Micelles on the Composition of Water/Toluene Mixtures. Polymer, 2006, 47, 2723–2727. (b) Zhou, P.; Liang, F. X.; Liu, Y. J.; Deng, R. H.; Yang, Z. Z. Janus Colloidal Copolymers. Sci. China Mater. 2015, 58, 961−968. (c) Jia, F.; Liang, F. X.; Yang, Z. Z. Janus Mesoporous Nanodisc from Gelable Triblock Copolymer.


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ACS Macro Lett. 2016, 5, 1344−1347.


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For Table of Contents Use Only:

Title: Emulsion Interfacial Synthesis of Polymer/Inorganic Janus Particles Authors: Xi Chen, Jingjing Xu, Bingyin Jiang, Fuxin Liang, Zhenzhong Yang*


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Inorganic Janus Particles

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