Light and Magnetic Dual-Responsive Pickering Emulsion Micro

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Light and Magnetic Dual-Responsive Pickering Emulsion Micro-Reactors Chun-Yan Xie, Shi-Xin Meng, Long-Hui Xue, Rui-Xue Bai, Xin Yang, Yaolei Wang, Zhong-Ping Qiu, Bernard P. Binks, Ting Guo, and Tao Meng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03642 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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Langmuir

Light and Magnetic Dual-Responsive Pickering Emulsion Micro-Reactors

Chun-Yan Xie,1 Shi-Xin Meng,1 Long-Hui Xue,1Rui-Xue Bai,1 Xin Yang,1 Yaolei Wang,1 Zhong-Ping Qiu,1 Bernard P. Binks,2 Ting Guo1,* and Tao Meng1,∗∗ 1

School of Life Sciences and Engineering, Southwest Jiaotong University, Chengdu, Sichuan, 610031, P.R. China

2

School of Mathematics and Physical Sciences, University of Hull, Hull HU6 7RX, UK

Submitted to: Langmuir on 16.11.17 Contains ESI

∗

Corresponding author Tel: +86-28-8760-3202 E-mail address: [email protected] (T. Guo), [email protected] (T. Meng)

ACS Paragon Plus Environment

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ABSTRACT Emulsion droplets can serve as ideal compartments for reactions. In fact, in many cases, the chemical reactions are supposed to be triggered at a desired position and time without change of the system environment. Here, we present a type of light and magnetic dual-responsive Pickering emulsion micro-reactor by co-adsorption of light-sensitive titania (TiO2) and super paramagnetic iron oxide (Fe3O4) nanoparticles at the oil-water interface of emulsion droplets. The droplets encapsulating different reactants in advance can be driven close to each other by an external magnetic field, and then chemical reaction is triggered by UV illumination due to the contact of the isolated reactants as a result of droplet coalescence. The coalescence mechanism of the Pickering emulsion micro-reactors is proposed. Insight into the simultaneous incorporation of hydrophobic TiO2 nanoparticles and hydrophilic Fe3O4 nanoparticles at the emulsion interface is also performed. Our work not only provides a novel Pickering emulsion micro-reactor platform for triggering chemical reactions in a non-intrusive and well-controlled way, but also opens a promising avenue to construct multifunctional Pickering emulsions by assembly of versatile building block nanoparticles at the interface of emulsion droplets.

Keywords: Pickering emulsion, Light and

magnetic

dual-responsive, Wettability,

Coalescence, Micro-reactors


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1. INTRODUCTION Emulsion droplets can serve as compartments for reactions.1, 2 Certain chemical reaction processes occur in emulsions, such as in pharmaceutical synthesis,3 in the preparation of functional materials,4 during food manufacturing processes5 and in cosmetics production.6 The main motivations for the use of micro-reactor systems are the gain in yield and safety and precisely controlled reaction.7 As an excellent micro-reactor, emulsions can greatly enhance reaction efficiency by virtue of their small droplet size and large reaction interfacial area.8 Meanwhile, emulsion droplets can protect reactants and products from being contaminated or released.9 Traditional emulsions are stabilized by surfactants or polymer molecules. Unfortunately, the vast majority of surfactants are organic compounds which may lead to environmental concerns and emulsion instability problems. In contrast, Pickering emulsions10 stabilized by solid particles may solve the above problems due to the advantages of high stability, easy recovery, low toxicity and low environmental pollution.11-14 However, in view of the high stability and small size of Pickering emulsion droplets, how to effectively and precisely trigger chemical reactions within emulsions remains a challenge. Responsive Pickering emulsions hold promise for addressing this challenge where the chemical reaction can be triggered due to the contact of the isolated reactants as a result of droplet coalescence. Recently, several methods have been reported to induce coalescence in Pickering emulsions using an array of external stimuli including temperature,15 shear flow,16 electric field,17, 18 microfluidic collision,19 pH-temperature,20, 21 temperature-magnetic field,22 pH-magnetic field,23 temperature-ionic strength24 and CO2/N2 and light dual stimuli.25 In most cases the chemical reactions are often required to be triggered with a non-intrusive approach


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at a certain time and location. Herein, simple and clean external stimuli (light and magnetic field) become the preferred choice. In recent years, titania (TiO2) nanoparticles have attracted much attention based on the change in their wettability using a UV-light stimulus.26-30 We also reported on the hydrophobic-hydrophilic conversion of micro-nano hierarchical titania/silica composite thin films induced by UV illumination.31 For a magnetic response, super-paramagnetic iron oxide (Fe3O4) nanoparticles have various applications in magnetic resonance imaging, targeted drug delivery, magnetic field assisted transport and separation.32-35 Emulsion droplets stabilized with iron oxide nanoparticles can also be subjected to directional movement or even coalescence under high magnetic fields.36-38 However, to the best of our knowledge, light and magnetic dual-responsive Pickering emulsions stabilized by titania and iron oxide nanoparticles for triggering chemical reaction have not been reported. Recently, it was found that different particle types will aggregate if they differ in their wettability, e.g. one is relatively hydrophilic and the other is relatively hydrophobic.39, 40 We develop a novel method to tailor the surface wettability of nanoparticles with precise control which provides a possibility to stabilize emulsions by adsorption of titania and iron oxide nanoparticles at oil-water interface. In this study, we report a facile way to construct a water-in-oil (w/o) Pickering emulsion micro-reactor system with light and magnetic dual-response by adsorption of silane-grafted titania nanoparticles and silane-grafted iron oxide nanoparticles at the oil-water interface (Figure 1). When an external magnetic field is applied, two emulsion droplets approach each other until contact (Figure 1a). Upon UV irradiation, the wettability of the titania particles changes. Subsequently, the nanoparticles detach from the droplet interfaces allowing


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coalescence and triggering chemical reaction (Figure 1b). In practical applications, raw materials including reactants, catalysts and emulsifiers are often required to feed in one batch for continuous multi-step reactions. Here, triggering the reaction at a certain time and location is necessary during industrial production. Furthermore, in the light of the non-intrusive behavior, remote control operation and isolation of harmful substances, toxic reactants can be encapsulated into two kinds of droplets, and then we can transport them to a specific location by applying the magnetic field. Afterwards the toxic and harmful reactions are triggered by UV irradiation as desired. As a result, the light and magnetic dual-responsive Pickering emulsion micro-reactors have wide applications in the chemical industry.

Figure 1. Schematic illustration of light and magnetic dual-responsive w/o Pickering emulsion stabilized by s-TiO2 (seven-carbon chain silane) and t-Fe3O4 (three-carbon chain silane) nanoparticles including (a) orientation with a magnetic field and (b) coalescence and chemical reaction following UV irradiation. The separate droplets contain reactant A or B which react to give C.


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2. EXPERIMENTAL SECTION 2.1 Materials Commercially available tetrabutyltitanate (TBT, purity > 98.5%) was purchased from Kelong Chemical Reagents Co. Ltd. (China) as hydrolysis precursor. Glacial acetic acid (purity > 97%), hydrogen peroxide (30 wt.%), hexane (purity > 97%), methylene blue and anhydrous ethanol (purity > 99.7%) were purchased from Kelong Chemical Reagents Co. Ltd. (China). Iron(II, III) oxide (Fe3O4) powder comprising of nanoparticles with a diameter of 20-30 nm and a purity of 99.5% was purchased from Ziyi Reagents Co. Ltd. (China). Polyethylene glycol diacrylate (PEGDA 4000) was purchased from Huaxia Reagents Co. Ltd. (China). 2-hydroxy-1-2-methyl-1-propanone (Irgacure 2959) was obtained from J&K Technology Co. Ltd. (China). Trichlorododecylsilane (C12H25SiCl3) was purchased from Tokyo Kasei Industry Co. Ltd. (Japan) and was used to modify the wettability of titania nanoparticles. Chlorotrimethylsilane (CTMS) from Chengdu Kelong Chemical Reagents Co. Ltd. (China) was used for the wettability modification of iron oxide particles. Deionized water was utilized throughout. 2.2. Preparation of titania nanoparticles Titania nanoparticles were prepared by a hydrothermal synthesis method as follows.41 9 mL of TBT was added to 36 mL of anhydrous ethanol with magnetic stirring as a mixed solution. 52 mL of an aqueous solution (40 mL glacial acetic acid and 12 mL water) was slowly added into this, and the solution was vigorously stirred until it became transparent at room temperature. It was then transferred to a 150 mL Teflon-lined autoclave that was sealed and heated at 150 °C for 2 h. After the autoclave was naturally cooled to room temperature,


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the as-prepared product was centrifuged at 15,000 rpm and washed three times with anhydrous ethanol. The products were taken from the bottom of the centrifuge tube and dried at 60 °C for 12 h in air. Finally, the dried products were calcined inside a Muffle furnace at 500 °C for 30 min to obtain uniform anatase nanoparticles.42 2.3 Wettability modification of titania and iron oxide particles The prepared titania particles were firstly rendered hydrophilic by adding them to 20 mL of (30 wt.%) hydrogen peroxide solution, and the solution was stirred for at least 4 h in the dark and then dried at 45 °C to obtain hydroxylated particles. For hydrophobisation, these hydroxylated particles were then added to 20 mL of 5 vol.% trichlorododecylsilane solution in hexane. The dispersion was stirred for 12 h at room temperature and centrifuged at 10,000 rpm to remove unreacted reagent and then dried at 50 °C in air. In the final step, the nanoparticles were immersed in 50 mL hexane, fully stirred and irradiated using a UV lamp (λ = 254 nm, WFH-203 three-function ultraviolet analyzer, 12 W, China) at an irradiation distance of 5 cm between light source and sample.42 In this step, the long-chain silane (trichlorododecylsilane) grafted onto the nanoparticles is degraded into a short-chain silane (seven carbon atoms, relatively hydrophobic) after UV illumination for 24 h better suited for stabilizing an emulsion (see ref. 42). In the subsequent emulsion coalescence process under UV irradiation, the carbon groups present on the surface of TiO2 nanoparticles will not be further degraded. In fact, the shorter the alkyl chain, the higher UV energy is required for its degradation and UV energy is mostly used to induce the coalescence of emulsions as a result of the wettability conversion of the nanoparticles. We believe that the short-chain silanes do not continue to degrade after a long irradiation time, verified by the X-ray photoelectron


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spectral measurements after UV irradiation (18 h and 36 h) shown in ref. 41. The purchased iron oxide nanoparticles were dispersed in CTMS solution (1 mL CTMS in 19 mL hexane) and the mixture was stirred by a mechanical agitator for 12 h at room temperature. It was then dried at 50 °C to obtain relatively hydrophilic t-Fe3O4 nanoparticles. The surface wettability of relatively hydrophobic s-TiO2 and relatively hydrophilic t-Fe3O4 are quantitatively evaluated in Section 2.5. 2.4 Preparation of light responsive and light and magnetic dual responsive Pickering emulsion micro-reactors In this experiment, the total particle concentration was kept constant at 1% w/v. The mass fraction of t-Fe3O4 nanoparticles in hexane, mp (mass Fe3O4/(mass Fe3O4 + mass TiO2) hereafter referred to as “composition” was varied from 0 (s-TiO2 only) to 1 (t-Fe3O4 only). A certain concentration of s-TiO2 and t-Fe3O4 nanoparticles (mp from 0 to 1) was first dispersed into 10 mL hexane in a glass vessel (inner volume 25 mL) using a high-intensity ultrasonic vibration processor (KQ5200DE, Kunshan Ultrasonic Instrument Co. Ltd., China) with an ultrasonic power of 200 W for 3 min. 10 mL of pure water was then added into the oil dispersions. Emulsions were obtained using a homogenizer (Scientz S10, Ningbo Xinzhi Instrument Co. Ltd., China) at 15,000 rpm for 2 min. The concentration of the emulsifiers in the total volume of oil and water (20 mL) was varied from 0.67 wt.% to 3 wt.%. A pair of water droplets of the same size was selected from a Pickering emulsion stabilized by s-TiO2 nanoparticles for the light responsive Pickering emulsion micro-reactor. Similarly, a droplet pair was selected from a Pickering emulsion stabilized by a mixture of s-TiO2 and t-Fe3O4 nanoparticles for the light and magnetic dual-responsive Pickering emulsion micro-reactor.


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2.5 Evaluation of the surface wettability of s-TiO2 and t-Fe3O4 and particle mixtures The three-phase contact angle of particles at the oil-water interface was measured by using the compressed disk method.43 Particle mixtures were prepared by blending s-TiO2 and t-Fe3O4 nanoparticles at a mass ratio of 1:1 in hexane with an ultrasonic bath. The dried nanoparticles (only s-TiO2 or only t-Fe3O4 or a mixture) were compressed by a tablet press into circular disks and then placed at the bottom of a transparent quartz vessel. Hexane was then poured into the vessel. A 3 µL water droplet was carefully placed on the disk surface. The appearance of the water droplet on the substrate was photographed when the droplet was stationary. In order to measure the UV-induced wettability conversion, the three-phase contact angle of a substrate containing only s-TiO2 particles after 8 h of UV irradiation (UV lamp, λ = 254 nm, WFH-203 three-function ultraviolet analyzer, 12 W, d = 5 cm) were recorded. All of the contact angles measured through water are the arithmetic average of at least five measurements on the same sample. 2.6 Characterization techniques The surface chemical structure of TiO2 and Fe3O4 nanoparticles before and after modification was determined by infra-red Prestige-21 Fourier transform spectroscopy (FTIR, Shimadzu, Japan). The samples were prepared by mixing the particles with KBr and pressing into a compact pellet. The morphologies of s-TiO2 particles and t-Fe3O4 nanoparticles were investigated using a field emission transmission electron microscope (FE-TEM; JEM-2100F, JEOL, Japan). All samples were prepared by dispersing in ethanol and coating on a carbon-coated copper grid. The grid was then allowed to dry before being imaged. The magnetic properties of Fe3O4 and t-Fe3O4 nanoparticles were determined using a


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superconducting quantum interference device (SQUID, MPMS XL-7, Quantum Design, USA). Magnetic hysteresis loops were recorded at 27 °C by saturating the sample in a field of 25,000.0 Oe, and then the saturation magnetization (Ms), the remnant magnetization (Mr) and the coercivity (Hc) were determined. In order to observe the droplet surface morphology of w/o Pickering emulsions stabilized by s-TiO2 and t-Fe3O4 nanoparticles, 44% w/v PEGDA 4000 as a cross-linking agent and 2% w/v Irgacure 2959 as a photoinitiator were added to the water phase before emulsification. When exposed to a 400 W high-pressure mercury lamp as a UV light source (Osram Supratec HTC 400-241) in the presence of photoinitiator, liquid PEGDA can be cross-linked into a gel. The gel microsphere samples were dried at 45 °C and coated with gold for the observation of emulsion droplet surface morphology. A JEOL-JSM 7001F scanning electron microscope (SEM) equipped with energy dispersive spectrometry (EDS) was used. The weight percentage and atomic ratios of Ti, Fe and O elements were obtained by EDS. Fluorescence images of emulsion droplets stabilized by s-TiO2 nanoparticles were obtained using a confocal laser scanning microscope (CLSM, Leica TCP SP5), exciting the green fluorescent channel at 405 nm. Emulsion droplet size distributions were examined by optical microscopy after the emulsions had been equilibrated at 25 °C for 1 h. All the droplets were viewed with an XSP-24 (Phoenix Co. Ltd, China) research microscope fitted with a Moticam 2000 camera. The images were captured using Motic Images Plus 2.0 software, then processed and analyzed by Image Pro Plus software.


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3. RESULTS AND DISCUSSION 3.1 Chemical characterization of s-TiO2 and t-Fe3O4 nanoparticles FTIR spectral analysis is performed to confirm the surface functional groups on TiO2 and Fe3O4 nanoparticle emulsifiers after preparation. The spectra of hydrophilic TiO2 and Fe3O4 particles (Figure 2a1 and b1) show the typical absorption bands of the Ti-OH group at 1650 cm-1 and the hydroxyl group at 3395 cm-1. In Figure 2a2, the appearance of the two peaks at 2921 cm-1 and 2853 cm-1 are attributed to aliphatic -CH3 and -CH2 stretching vibrations respectively, indicating the grafting of organosilane onto particle surfaces. Compared with Figure 2a1, the existence of two peaks at 1125 cm-1 and 1035 cm-1 due to Si-O-Ti stretching vibration (Figure 2a2) confirms the grafting of silane groups on the particle surfaces. In contrast to Figure 2b1, the band region 1596 and 1605 cm-1 assigned to the C-H stretching vibration of the methyl group (-CH3) (Figure 2b2) confirms the conjugation of organosilane to the Fe3O4 particle surfaces. Figure S1 shows representative TEM images of s-TiO2 and t-Fe3O4 nanoparticles obtained. A closer examination of the image shows that some of the nanoparticles form randomly shaped aggregates although the primary particle diameter ranges between 10 and 30 nm.


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Figure 2. FTIR spectra of the nanoparticles. (a1) TiO2, (a2) s-TiO2, (b1) Fe3O4, (b2) t-Fe3O4.

Three-phase contact angle measurements are useful in order to monitor the macroscopic change in the wettability of a surface composed of particles. The contact angle of a water drop in hexane on a disk composed of s-TiO2 nanoparticles is 108.4 ± 3o (Figure 3a), and thus these particles are efficient emulsifiers for w/o emulsions. In contrast, this contact angle decreases to 79.8 ± 3o after 8 h of UV irradiation (Figure 3b), such that the particles after UV irradiation cannot stabilize water-in-hexane emulsions. Figure 3 clearly demonstrates the wettability conversion of s-TiO2 nanoparticle surfaces under UV illumination, attributed to the UV-induced absorption of hydroxyl groups on particle surfaces.31 Solid particles may form an interfacial film at the oil-water interfaces within emulsions which impede coalescence when two droplets approach. The energy E required to desorb spherical particles from the interface can be estimated by equation 1: E = πR2γo/w(1±cosθ)2

(1)

where R is the particle radius, γow is the oil-water interfacial tension and θ is the three-phase contact angle. Furthermore, the solid-stabilized emulsion type is mainly dictated by the


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wettability of the particles,44 and the particle wettability has a major influence on the ability of particles to stabilize emulsions.45 Generally, spherical hydrophilic particles exhibit a contact angle less than 90o and prefer to stabilize oil-in-water (o/w) emulsions whereas hydrophobic particles for which θ is generally greater than 90o prefer to stabilize w/o emulsions. In our experiments, UV irradiation induced a change in the wettability of s-TiO2 particles where it will be seen that the emulsion droplets become unstable and ultimately coalesce.

Figure 3. Photographs of three-phase contact angle of water on s-TiO2 nanoparticle surfaces in hexane (a) before and (b) after UV irradiation.

The saturation magnetization of hydrophilic Fe3O4 and t-Fe3O4 nanoparticles is investigated by magnetic hysteresis loop analysis. In Figure 4, the field-dependent magnetization curves of the two particle types reveal negligible remnant magnetization (Mr) and coercivity (Hc), indicating super-paramagnetic behavior of the two samples. The response of the particles to a magnetic field is linear for some fields but no longer linearly proportional to the applied magnetic field at high fields because of the magnetization saturation.46 The saturation magnetization of t-Fe3O4nanoparticles is 40.3 emu g-1, which is smaller than that of unmodified nanoparticles (72.3 emu g-1). The difference between these values is attributed to the existence of grafted non-magnetic alkyl chains of chlorotrimethylsilane on the surface of


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the particles.47 After surface reaction with CTMS, t-Fe3O4 nanoparticles still display a strong magnetic response to an applied magnetic field, and are completely separated from the liquid within 6 s under the effect of a magnetic field but can be dispersed again with slight shaking (Figure 4, insert). The above experimental results confirm that the t-Fe3O4 nanoparticles exhibit super-paramagnetic characteristics at room temperature and directional movement under an external magnet.

Figure 4. Typical magnetic hysteresis loops of hydrophilic Fe3O4 and t-Fe3O4 nanoparticles at 27 oC. The insert shows the dispersion/separation behavior of t-Fe3O4 particles in ethanol under an external magnetic field.

3.2 Pickering emulsion characterization The three-phase contact angle exhibited by particles at an oil-water interface is an important parameter governing the stabilization of a Pickering emulsion. As illustrated in


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Figure 5a, the tailored s-TiO2 nanoparticles of suitable wettability (contact angle = 110.5o) guarantee the stabilization of water-in-hexane emulsions. The t-Fe3O4 nanoparticles alone exhibiting an oil-water contact angle through water of 70.5o stabilize a hexane-in-water emulsion (Figure 5b). We find that the particle mixture at mp = 0.5 exhibits a contact angle of 109.5o (Figure 5c) close to that of s-TiO2 alone and can serve as an efficient mixed emulsifier to stabilize w/o emulsions. As shown in Figure S2, we have studied the influence of mp on the dispersion and magnetic response of w/o Pickering emulsions stabilized by mixtures of s-TiO2 and t-Fe3O4 nanoparticles. The balance of good dispersion and strong magnetic response of the emulsion is obtained at mp = 0.5. Therefore, the mixture of s-TiO2 and t-Fe3O4 nanoparticles with mp = 0.5 is chosen to prepare the dual-responsive Pickering emulsion micro-reactor sensitive to light and magnetic fields.

Figure 5. Optical microscope images of emulsions (1:1 water and hexane) prepared with (a) s-TiO2 nanoparticles (mp = 0), (b) t-Fe3O4 nanoparticles (mp = 1), (c) a mixture of s-TiO2 and t-Fe3O4 nanoparticles (mp = 0.5) all at a total particle concentration of 1% w/v. Insert: photos of water drops under hexane on a disk composed of the relevant particles.

We check the presence of s-TiO2 and t-Fe3O4 nanoparticle emulsifiers on the surface of


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w/o Pickering emulsion droplets by SEM observation. Although cryo-SEM is employed to observe the morphologies of emulsion droplets and capsules,48, 49 the sample with Fe3O4 nanoparticles is not suitable for observation by cryo-SEM. The gelling and subsequent polymerization of water droplets offers a convenient alternative.50 Hence, PEGDA 4000 and Irgacure 2959 are added to the inner water phase of the emulsion prior to emulsification as cross-linking agent and photoinitiator respectively. The water drops become microspheres due to PEGDA being cross-linked into a gel by UV illumination. We analyzed the dried spherical gel coated with nanoparticles by SEM-EDS. Figure 6a refers to an emulsion stabilized solely by s-TiO2 particles and Figure 6b to that stabilized by a mixture of s-TiO2 and t-Fe3O4 particles (mp = 0.5). Figure 6a2 and b2 clearly provide the weight percentage and atomic ratios of the elements Ti, Fe and O on microsphere surfaces. Figure 6a3 and a4 show the Ti and O elemental mapping (s-TiO2 only) whilst Figure 6b3-b5 show the Fe, Ti and O elemental mapping (s-TiO2 and t-Fe3O4 mixture). It can be seen that the arrangement of s-TiO2 nanoparticles and t-Fe3O4 nanoparticles in the area-scan matrix is reasonably uniform on the emulsion droplet surface. To further verify the effect of s-TiO2 particle concentration on the mean diameter of Pickering emulsion droplets, the emulsions are directly visualized by CLSM and optical microscopy (Figure S3). The green fluorescence from the particles (1% w/v) can be clearly observed at the interface of the emulsion droplet (Figure S3a). This provides direct evidence that the s-TiO2 particles are anchored at the oil-water interface and provide an interfacial layer responsible for stabilizing the emulsions. These emulsions are stable to coalescence for at least 3 months at room temperature. When the particle concentration is 0.67% w/v, no stable


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emulsion is formed however (Figure S3b). Increasing the particle concentration leads to the formation of stable emulsions with decreasing average drop diameter from 400 µm to 100 µm (Figure S3c-e). We can therefore obtain the desired size of Pickering emulsion micro-reactor by simply changing the particle concentration.

Figure 6. SEM images and EDS element mapping for the w/o emulsion droplet surfaces stabilized by (a) s-TiO2 nanoparticles, mp = 0 and (b) a mixture of s-TiO2 and t-Fe3O4 nanoparticles, mp = 0.5. The total particle concentration is 1% w/v.


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3.3 Light responsive Pickering emulsion micro-reactors stabilized by s-TiO2 nanoparticles In order to investigate whether the coalescence of a pair of emulsion droplets can be triggered for chemical reaction by a UV light stimulus, we arrange for two w/o Pickering emulsion droplets stabilized solely by s-TiO2 particles to be in close contact by an external force. Firstly, two kinds of w/o emulsion (A and B) with different reactants are prepared. One droplet from A and one droplet from B is removed and placed in a vessel containing hexane. Subsequently, 3 mL of hexane was slowly injected toward one of the droplets by a syringe, and the two droplets contact each other due to the relative flow of hexane. As is shown in Figure 7a, one drop is colorless containing acid (left) and the other is yellow containing base and methyl orange (right). Figure 7a2-a5 show the optical micrographs of the emulsion droplet pair after different periods of UV irradiation. After 2 hours irradiation, the photoreduction of Ti4+ to Ti3+ at definite sites on the s-TiO2 particle surfaces results in the preferential adsorption of hydroxyl groups onto corresponding vacant oxygen sites.29 This leads to an increase in the hydrophilicity of particle surfaces.51 As a result, desorption of the s-TiO2 particles from droplet interfaces occurs such that the emulsion droplets begin to coalesce (Figure 7a2). As time progresses, the emulsion droplets undergo complete coalescence (by 6 h) and the reactants from each droplet mutually diffuse for the neutralization reaction. The excess of acid causes the methyl orange to turn red. (H+ + In- ↔ HIn represents the ionization equilibrium equation of the acid-base indicator). The shape of the emulsion droplet pair is changed from that of a peanut to a sphere (Figure 7a3-a5). The morphologies of the emulsion droplet pair during the merging process can be seen in the


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Supporting Information Movie S1. In contrast, the emulsion droplet pair does not coalesce under visible light irradiation (Figure 7b1-b5). Thus, Pickering emulsions stabilized by s-TiO2 nanoparticles can act as micro-reactors which can be triggered with UV light to induce chemical reaction. There are many variables facilitating emulsion coalescence, such as temperature, pH and ionic strength.52-54 However, in this paper, UV irradiation is used as the non-intrusive trigger for coalescence and the coalescence is investigated without changing the system parameters.

Figure 7. Response of w/o Pickering emulsion stabilized by s-TiO2 nanoparticles to light stimulus. Microscopic images of water droplet pair in oil under UV irradiation (a1) – (a5) and under visible light irradiation (b1) – (b5) for different times given. The left drop contains 0.2 M acid and the right drop contains 0.1 M base and methyl orange. UV lamp: λ = 254 nm, 12 W, d = 5 cm.


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3.4 Light and magnetic dual-responsive Pickering emulsion micro-reactors stabilized by a mixture of s-TiO2 and t-Fe3O4 nanoparticles Figure 8 shows the light and magnetic dual-responsive ability of the w/o Pickering emulsion stabilized by a mixture of s-TiO2 and t-Fe3O4 nanoparticles. The prepared emulsion droplets have an average diameter of 400 µm. As shown in Figure 8a1-a4 we see that the emulsion droplets move to one side of the Petri dish when a static magnetic field is applied. Figure 8b1 and b2 show that the prepared emulsion droplets can be concentrated within 10 s by employing an external magnetic field. Once the magnetic field is removed, the emulsion droplets become dispersed in hexane again by slight shaking. From other similar experimental results, the mixed particle-stabilized Pickering emulsion has a good magnetic response performance even after long-term storage (several months). With the magnetic response, when the two droplets move in the same direction, they will be close to each other due to the uneven distribution of the magnetic force (see Movie S2). In addition, as seen from Movie S3, the two droplets also can contact closely by changing the direction of the magnetic field. The light and magnetic dual-responsive micro-reactors provide a facile and promising platform that has potential applications in a myriad of fields, such as detection analysis, micro-reactions and functional material fabrication. To show the potential, we demonstrate the drop coalescence-triggered micro-reactions for a pH indicator (Figure 7) and the synthesis of calcium carbonate (Figure 8c). In Figure 8c1, the droplet on the left contains 0.2 M calcium chloride whilst that on the right contains 0.2 M sodium carbonate. The two droplets are directed to approach each other by applying an external magnetic field (Figure 8c2) and then removing it. Upon subsequent UV irradiation causing the change in the wettability of s-TiO2


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particles, we speculate that there will be two possible scenarios: (i) One is that both t-Fe3O4 and s-TiO2 nanoparticles simultaneously desorb from the oil-water interfaces as a result of their aggregation; (ii) the other is that the Fe3O4 particles remain adsorbed but once the TiO2 particles have desorbed, the drainage of the liquid oil film between adjacent droplets results in the desorption of Fe3O4 particles leading to coalescence and chemical reaction (Figure 8c3). Subsequently the precipitation reaction forming CaCO3 can occur (Figure 8c4). The calcium carbonate particles are produced at the contact interface of the droplet pair, and then diffuse from the interface to the interior of the coalesced droplet (Figure 8c5). Figure S4 is an SEM image of the prepared CaCO3 particles. Comparing Figure 7 (mp= 0) with Figure 8 (mp= 0.5), the time for complete coalescence increases from 8 h to 16 h for the light responsive micro-reactors and light/magnetic dual-responsive micro-reactors respectively. This can be attributed to the reduced content of s-TiO2 particles at the hexane-water interface as mp increases, which require longer time in order to absorb enough hydroxyl groups via UV irradiation before particle desorption from the interface. Adsorbed particle emulsifiers form a steric barrier between adjacent droplets to impede droplet-droplet coalescence.55 The rate of coalescence is governed by the stability against drainage and rupture of the thin liquid film between droplets, and a requirement for coalescence is the detachment of these particles from the interface.56


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Figure 8. Light and magnetic dual-response of w/o Pickering emulsions stabilized by a mixture of s-TiO2 and t-Fe3O4 nanoparticles (mp = 0.5, total particle concentration = 1% w/v). (a) Snapshots of the magnetic-guided targeting behavior of two water-in-hexane droplets at 20 o

C, the droplets moved with speeds of 4 mm s-1. (b) The separation/dispersion behavior of the

emulsion droplets in excess hexane under an external magnetic field. (c) A precipitation reaction is triggered by the coalescence of the emulsion droplet pair under UV irradiation. The droplet on the left contains 0.2 M calcium chloride and that on the right contains 0.2 M sodium carbonate. UV lamp: λ = 254 nm, 12 W, d = 5 cm. A cylindrical NdFeB magnet of size 12 mm × 8 mm is placed under the Petri dish/glass vessel. The magnetic field strength is 0.4 T.


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3.5 Re-usability of particles In Figure 9, we provide experimental evidence concerning the re-usability of the particles. Figure 9a1 shows a dual-responsive Pickering emulsion stabilized by a mixture of s-TiO2 and t-Fe3O4 nanoparticles which possesses a good magnetic response performance (Figure 9a2). Upon UV irradiation, the droplets undergo complete breakage (Figure 9a3). The recycled particle emulsifiers from the first cycle can be used to stabilize Pickering emulsions, but only a few emulsion droplets are observed in Figure 9b1, which also exhibit a good magnetic response (Figure 9b2). After UV irradiation, the emulsion droplets undergo complete coalescence as seen in Figure 9b3. As expected, most of the emulsifiers can be recycled, but the recovery rate in the second cycle (R2 = 64%) is lower than in the first (R1 = 78%). As for the particle mixtures, we can recycle the iron oxide by using a magnet and the titanium dioxide can be recovered by centrifugation.

Figure 9. Light and magnetic dual-responsive Pickering emulsions stabilized by a mixture of


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s-TiO2 and t-Fe3O4 nanoparticles (a1), magnetic attraction (a2) and UV irradiation (a3), second cycle (b1-b3). Inserts are optical micrographs of initial (a4, a5) and second cycle (b4, b5).

3.6 Schematic illustration of the coalescence of light and magnetic dual-responsive Pickering emulsion micro-reactors We propose a model to explain the phenomena observed above. The partially hydrophobic s-TiO2 particles and partially hydrophilic t-Fe3O4 particles are likely to aggregate with each other at the interface due to the attractive van der Waals forces based on their nano-size.40 Figure 10a shows that two emulsion droplets can be manipulated close to each other under an external magnetic field because of the presence of paramagnetic t-Fe3O4 particles anchored at the droplet interfaces. Accordingly, the liquid oil film formed between the squeezing interfaces thins under gravity, and the hydrodynamic pressure in the liquid film continues to drain the continuous phase out of the film (Figure 10b). Once the droplet pair is irradiated by UV light, the wettability conversion of the titania particles leads to their escape from the oil-water interface due to their increased hydrophilicity. The deformation of the droplet interface brings about a non-uniform hydrodynamic pressure along the film.19 When the smallest distance between two droplet surfaces becomes less than the critical film thickness, the thin film tends to rupture and the emulsion droplet pair starts to coalesce56 (Figure 10c). As shown in Figure 10d, once the interfacial film ruptures chemical reaction occurs immediately and the products diffuse from the contact interface to the interior of the entire droplet after coalescence.


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Figure 10. Sketch of the proposed coalescence mechanism of w/o Pickering emulsion droplets stabilized by s-TiO2 and t-Fe3O4 nanoparticles. (a) Two droplets approach each other under a static magnetic field, (b) drainage of the liquid oil film occurs between adjacent droplets, (c) the liquid film becomes thinner due to the deformation of emulsion droplets, (d) nanoparticles desorb from the droplet interfaces inducing coalescence and triggering chemical reaction.

4. CONCLUSION In conclusion, we report the development of a light and magnetic dual-responsive w/o Pickering emulsion micro-reactor. The incorporation of partially hydrophobic TiO2 and partially hydrophilic Fe3O4 nanoparticles by attractive van der Waals forces enabled their co-adsorption at droplet interfaces. Endowed with ideal magnetic responsiveness, a pair of w/o droplets driven by a magnetic field can deliver the encapsulated reactants to a given place. On condition that the two droplets approach each other closely, model reactions such an


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acid-base reaction as well as a precipitation reaction can be successfully triggered at a given time by UV illumination due to inter-droplet coalescence. Due to the UV-induced hydroxylation of the TiO2 surface, we hypothesize that this hydrophilic conversion leads to desorption of the combined nanoparticles from the oil-water interface and the liquid film drainage further accelerates this process. The proposed light and magnetic dual-responsive Pickering emulsion micro-reactor provides a smart platform for triggering chemical reactions at a desired position and time in a non-intrusive way. Future studies will probe visible light and magnetic dual-responsive Pickering emulsion micro-reactors based on the co-adsorption of nitrogen-doped TiO2 and Fe3O4 nanoparticles, which have great potential applications in the life sciences.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.****** TEM images of s-TiO2 and t-Fe3O4 nanoparticles, Effect of the mass fraction of t-Fe3O4 nanoparticles on the dispersion and magnetic response of Pickering emulsion, CLSM image and optical microscopy images of Pickering emulsions, SEM image of calcium carbonate precipitate, and original images of light responsive Pickering emulsion micro-reactors (PDF) Movie S1, the morphologies of droplet pair during the merging process; Movie S2, two droplets get close by applying an external magnetic field; and Movie S3, two


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droplets contact closely by changing the direction of magnetic field (AVI)

AUTHOR INFORMATION Corresponding Authors * (T. G.) E-mail: [email protected]. * (T. M.) E-mail: [email protected]. ORCID Bernard P. Binks: 0000-0003-3639-8041 Tao Meng: 0000-0001-5308-2861 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21406181, 21776230), the Fundamental Research Funds for the Central Universities (2682015CX050, 2682016CX101), the Science and Technology Department Foundation of Sichuan Province (2017GZ0411, 2015NZ0097) and the Sichuan Province Seedling Project (2014RZ0037). The authors gratefully acknowledge the help of Ms. Wen-Tao Wang of the Superconductivity and New Energy R&D Center at Southwest Jiaotong University for the SEM micrographs and Mr. Xiao-Tong Zheng of the Key Laboratory of Advanced Technologies of Material at Southwest Jiaotong University for TEM measurements.


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References (1) Song, H.; Chen, D. L.; Ismagilov, R. F., Reactions in Droplets in Microflulidic Channels. Angew. Chem. Int. Ed. 2006, 45, 7336-7356. (2) Dewey, D. C.; Strulson, C. A.; Cacace, D. N.; Bevilacqua, P. C.; Keating, C. D., Bioreactor Droplets from Liposome-Stabilized All-Aqueous Emulsions. Nat. Commun. 2014, 5, 4670. (3) Wang, L. J.; Li, X. F.; Zhang, G. Y.; Dong, J. F.; Eastoe, J., Oil-in-Water Nanoemulsions for Pesticide Formulations. J. Colloid Interface Sci. 2007, 314, 230-235. (4) Shchukin, D. G.; Sukhorukov, G. B., Nanoparticle Synthesis in Engineered Organic Nanoscale Reactors. Adv. Mater. 2004, 16, 671-682. (5) Rayner, M.; Marku, D.; Eriksson, M.; Sjoo, M.; Dejmek, P.; Wahlgren, M., Biomass-Based Particles for the Formulation of Pickering Type Emulsions in Food and Topical Applications. Colloids Surf. A 2014, 458, 48-62. (6) Matsuyama, K.; Mine, K.; Kubo, H.; Mae, K., Design of Micromixer for Emulsification and Application to Conventional Commercial Plant for Cosmetic. Chem. Eng. J. 2011, 167, 727-733. (7) Pohar, A.; Plazl, I., Process Intensification through Microreactor Application. Chem. Biochem. Eng. Q. 2009, 23, 537-544. (8) Wei, L. J.; Zhang, M.; Zhang, X. M.; Xin, H. C.; Yang, H. Q., Pickering Emulsion as an Efficient Platform for Enzymatic Reactions without Stirring. ACS Sustainable Chem. Eng. 2016, 4, 6838-6843. (9) Guan, X. W.; Hou, L. K.; Ren, Y. K.; Deng, X. K.; Lang, Q.; Jia, Y. K.; Hu, Q. M.; Tao, Y.;


Page 28 of 45

Page 29 of 45

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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Liu, J. W.; Jiang, H. Y., A Dual-Core Double Emulsion Platform for Osmolarity-Controlled Microreactor Triggered by Coalescence of Encapsulated Droplets. Biomicrofluidics 2016, 10, 034111-034118. (10) Pickering, S. U., Emulsions. J. Chem. Soc. Trans. 1907, 91, 2001-2021. (11) Chen, J.; Vogel, R.; Werner, S.; Heinrich, G.; Clausse, D.; Dutschk, V., Influence of the Particle Type on the Rheological Behavior of Pickering Emulsions. Colloids Surf. A 2011, 382, 238-245. (12) Tsuji, S.; Kawaguchi, H., Thermosensitive Pickering Emulsion Stabilized by Poly(N-isopropylacrylamide)-Carrying Particles. Langmuir 2008, 24, 3300-3305. (13) Yang, H. Q.; Zhou, T.; Zhang, W. J., A Strategy for Separating and Recycling Solid Catalysts Based on the pH-Triggered Pickering-Emulsion Inversion. Angew. Chem. Int. Ed. 2013, 52, 7455-7459. (14) Yu, Y. H.; Fu, L. M.; Zhang, F. W.; Zhou, T.; Yang, H. Q., Pickering-Emulsion Inversion Strategy for Separating and Recycling Nanoparticle Catalysts. ChemPhysChem 2014, 15, 841-848. (15) Sun, J.; Wang, W.; He, F.; Chen, Z. H.; Xie, R.; Ju, X. J.; Liu, Z.; Chu, L. Y., On-Chip Thermo-Triggered Coalescence of Controllable Pickering Emulsion Droplet Pairs. RSC Adv. 2016, 6, 64182-64192. (16) Whitby, C. P.; Fischer, F. E.; Fornasiero, D.; Ralston, J., Shear-Induced Coalescence of Oil-in-Water Pickering Emulsions. J. Colloid Interface Sci. 2011, 361, 170-7. (17) Chen, G.; Tan, P.; Chen, S. Y.; Huang, J. P.; Wen, W. J.; Xu, L., Coalescence of Pickering Emulsion Droplets Induced by an Electric Field. Phys. Rev. Lett. 2013, 110, 064502-1-5.


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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

(18) Hou, L.; Ren, Y.; Jia, Y.; Deng, X.; Liu, W.; Feng, X.; Jiang, H., Continuously Electrotriggered Core Coalescence of Double-Emulsion Drops for Microreactions. ACS Appl. Mater. Interfaces 2017, 9, 12282-12289. (19) Zhou, Q. Q.; Sun, Y.; Yi, S. T.; Wang, K.; Luo, G. S., Investigation of Droplet Coalescence in Nanoparticle Suspensions by a Microfluidic Collision Experiment. Soft Matter 2016, 12, 1674-1682. (20) Tang, J. T.; Lee, M. F. X.; Zhang, W.; Zhao, B. X.; Berry, R. M.; Tam, K. C., Dual Responsive Pickering Emulsion Stabilized by Poly[2-(dimethylamino)ethyl methacrylate] Grafted Cellulose Nanocrystals. Biomacromolecules 2014, 15, 3052-3060. (21) Yi, C. L.; Liu, N.; Zheng, J. C.; Jiang, J. Q.; Liu, X. Y., Dual-Responsive Poly(styrene-alt-maleic acid)-Graft-Poly(N-isopropyl acrylamide) Micelles as Switchable Emulsifiers. J. Colloid Interface Sci. 2012, 380, 90-98. (22) Chen, Y.; Bai, Y. K.; Chen, S. B.; Jup, J. P.; Li, Y. Q.; Wang, T. M.; Wang, Q. H., Stimuli-Responsive Composite Particles as Solid-Stabilizers for Effective Oil Harvesting. ACS Appl. Mater. Interfaces 2014, 6, 13334-13338. (23) Yoon, K. Y.; Li, Z. C.; Neilson, B. M.; Lee, W.; Huh, C.; Bryant, S. L.; Bielawski, C. W.; Johnston, K. P., Effect of Adsorbed Amphiphilic Copolymers on the Interfacial Activity of Superparamagnetic Nanoclusters and the Emulsification of Oil in Water. Macromolecules 2012, 45, 5157-5166. (24) Chen, Q. J.; Xu, Y. Y.; Cao, X. T.; Qin, L. J.; An, Z. S., Core Cross-Linked Star (CCS) Polymers with Temperature and Salt Dual Responsiveness: Synthesis, Formation of High Internal Phase Emulsions (HIPEs) and Triggered Demulsification. Polym. Chem. 2014, 5,


Page 30 of 45

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175-185. (25) Jiang, J. Z.; Ma, Y. X.; Cui, Z. G.; Binks, B. P., Pickering Emulsions Responsive to CO2/N2 and Light Dual Stimuli at Ambient Temperature. Langmuir 2016, 32, 8668-8675. (26) Gao, H.; Wen, D. S.; Tarakina, N. V.; Liang, J. R.; Bushby, A. J.; Sukhorukov, G. B., Bifunctional

Ultraviolet/Ultrasound

Responsive

Composite

TiO2/Polyelectrolyte

Microcapsules. Nanoscale 2016, 8, 5170-5180. (27) Nakajima, A.; Koizumi, S.; Watanabe, T.; Hashimoto, K., Photoinduced Amphiphilic Surface on Polycrystalline Anatase TiO2 Thin Films. Langmuir 2000, 16, 7048-7050. (28) Sun, R. D.; Nakajima, A.; Fujishima, A.; Watanabe, T.; Hashimoto, K., Photoinduced Surface Wettability Conversion of ZnO and TiO2 Thin Films. J. Phys. Chem. B 2001, 105, 1984-1990. (29) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T., Light-Induced Amphiphilic Surfaces. Nature 1997, 388, 431-432. (30) Chen, K. L.; Zhou, S. X., Fabrication of Ultraviolet-Responsive Microcapsules via Pickering Emulsion Polymerization using Modified Nano-Silica/Nano-titania as Pickering Agents. RSC Adv. 2015, 5, 13850-13856. (31) Li, W.; Guo, T.; Meng, T.; Huang, Y. S.; Li, X.; Yan, W. L.; Wang, S.; Li, X. R., Enhanced Reversible Wettability Conversion of Micro-Nano Hierarchical TiO2/SiO2 Composite Films under UV Irradiation. Appl. Surf. Sci. 2013, 283, 12-18. (32) Antipina, M. N.; Sukhorukov, G. B., Remote Control over Guidance and Release Properties of Composite Polyelectrolyte based Capsules. Adv. Drug Delivery Rev. 2011, 63,


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716-729. (33) Gao, G. H.; Shi, R. R.; Qin, W. Q.; Shi, Y. G.; Xu, G. F.; Qiu, G. Z.; Liu, X. H., Solvothermal Synthesis and Characterization of Size-Controlled Monodisperse Fe3O4 Nanoparticles. J. Mater. Sci. 2010, 45, 3483-3489. (34) Latham, A. H.; Williams, M. E., Controlling Transport and Chemical Functionality of Magnetic Nanoparticles. Acc. Chem. Res. 2008, 41, 411-420. (35) Sun, C.; Lee, J. S. H.; Zhang, M. Q., Magnetic Nanoparticles in MR Imaging and Drug Delivery. Adv. Drug Delivery Rev. 2008, 60, 1252-1265. (36) Lam, S.; Blanco, E.; Smoukov, S. K.; Velikov, K. P.; Velev, O. D., Magnetically Responsive Pickering Foams. J. Am. Chem. Soc. 2011, 133, 13856-13859. (37) Melle, S.; Lask, M.; Fuller, G. G., Pickering Emulsions with Controllable Stability. Langmuir 2005, 21, 2158-2162. (38) Sander, J. S.; Erb, R. M.; Denier, C.; Studart, A. R., Magnetic Transport, Mixing and Release of Cargo with Tailored Nanoliter Droplets. Adv. Mater. 2012, 24, 2582-2587. (39) Law, A. D.; Auriol, M.; Smith, D.; Horozov, T. S.; Buzza, D. M. A., Self-Assembly of Two-Dimensional Colloidal Clusters by Tuning the Hydrophobicity, Composition, and Packing Geometry. Phys. Rev. Lett. 2013, 110, 138301-138306. (40) Saha, A.; John, V. T.; Bose, A., In Situ Assembly of Hydrophilic and Hydrophobic Nanoparticles at Oil-Water Interfaces as a Versatile Strategy to Form Stable Emulsions. ACS Appl. Mater. Interfaces 2015, 7, 21010-21014. (41) Zhang, Q.; Bai, R. X.; Guo, T.; Meng, T., Switchable Pickering Emulsions Stabilized by Awakened TiO2 Nanoparticle Emulsifiers Using UV/Dark Actuation. ACS Appl. Mater.


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Interfaces 2015, 7, 18240-18246. (42) Bai, R. X.; Xue, L. H.; Dou, R. K.; Meng, S. X.; Xie, C. Y.; Zhang, Q.; Guo, T.; Meng, T., Light-Triggered Release from Pickering Emulsions Stabilized by TiO2 Nanoparticles with Tailored Wettability. Langmuir 2016, 32, 9254-9264. (43) Kostakis, T.; Ettelaie, R.; Murray, B. S., Effect of High Salt Concentrations on the Stabilization of Bubbles by Silica Particles. Langmuir 2006, 22, 1273-1280. (44) Binks, B. P.; Lumsdon, S. O., Transitional Phase Inversion of Solid-Stabilized Emulsions Using Particle Mixtures. Langmuir 2000, 16, 3748-3756. (45) Schulman, J. H.; Leja, J., Control of Contact Angles at the Oil-Water-Solid Interfaces. Emulsions Stabilized by Solid Particles (BaSO4). Trans. Faraday Soc. 1954, 50, 598-605. (46) Zhou, J.; Qiao, X. Y.; Binks, B. P.; Sun, K.; Bai, M. W.; Li, Y. L.; Liu, Y., Magnetic Pickering Emulsions Stabilized by Fe3O4 Nanoparticles. Langmuir 2011, 27, 3308-3316. (47) Yuan, Y.; Rende, D.; Altan, C. L.; Bucak, S.; Ozisik, R.; Borca-Tasciuc, D. A., Effect of Surface Modification on Magnetization of Iron Oxide Nanoparticle Colloids. Langmuir 2012, 28, 13051-13059. (48) Madivala, B.; Vandebril, S.; Fransaer, J.; Vermant, J., Exploiting Particle Shape in Solid Stabilized Emulsions. Soft Matter 2009, 5, 1717-1727. (49) Wang, Z. P.; van Oers, M. C. M.; Rutjes, F. P. J. T.; van Hest, J. C. M., Polymersome Colloidosomes for Enzyme Catalysis in a Biphasic System. Angew. Chem. Int. Ed. 2012, 51, 10746-10750. (50) Kalashnikova, I.; Bizot, H.; Cathala, B.; Capron, I., New Pickering Emulsions Stabilized by Bacterial Cellulose Nanocrystals. Langmuir 2011, 27, 7471-7479.


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(51) Zhang, X. T.; Jin, M.; Liu, Z. Y.; Nishimoto, S.; Saito, H.; Murakami, T.; Fujishima, A., Preparation and Photocatalytic Wettability Conversion of TiO2-Based Superhydrophobic Surfaces. Langmuir 2006, 22, 9477-9479. (52) Binks, B. P.; Murakami, R.; Armes, S. P.; Fujii, S., Temperature-Induced Inversion of Nanoparticle-Stabilized Emulsions. Angew. Chem. Int. Ed. 2005, 44, 4795-4798. (53) Bizmark, N.; Ioannidis, M. A., Effects of Ionic Strength on the Colloidal Stability and Interfacial Assembly of Hydrophobic Ethyl Cellulose Nanoparticles. Langmuir 2015, 31, 9282-9289. (54) Liu, K. H.; Jiang, J. Z.; Cui, Z. G.; Binks, B. P., pH-Responsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with a Conventional Zwitterionic Surfactant. Langmuir 2017, 33, 2296-2305. (55) Tambe, D. E.; Sharma, M. M., Factors Controlling the Stability of Colloid-Stabilized Emulsions .1. An Experimental Investigation. J. Colloid Interface Sci. 1993, 157, 244-253. (56) Stancik, E. J.; Kouhkan, M.; Fuller, G. G., Coalescence of Particle-Laden Fluid Interfaces. Langmuir 2004, 20, 90-94. (57) Yeo, L. Y.; Matar, O. K.; de Ortiz, E. S. P.; Hewitt, G. E., Film Drainage between Two Surfactant-Coated Drops Colliding at Constant Approach Velocity. J. Colloid Interface Sci. 2003, 257, 93-107.


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Figure 1. Schematic illustration of light and magnetic dual-responsive w/o Pickering emulsion stabilized by sTiO2 (seven-carbon chain silane) and t-Fe3O4 (three-carbon chain silane) nanoparticles including (a) orientation with a magnetic field and (b) coalescence and chemical reaction following UV irradiation. The separate droplets contain reactant A or B which react to give C. 34x16mm (600 x 600 DPI)


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Figure 2. FTIR spectra of the nanoparticles. (a1) TiO2, (a2) s-TiO2, (b1) Fe3O4, (b2) t-Fe3O4. 38x20mm (600 x 600 DPI)


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Figure 3. Photographs of three-phase contact angle of water on s-TiO2 nanoparticle surfaces in hexane (a) before and (b) after UV irradiation. 18x4mm (600 x 600 DPI)


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Figure 4. Typical magnetic hysteresis loops of hydrophilic Fe3O4 and t-Fe3O4 nanoparticles at 27 oC. The insert shows the dispersion/separation behavior of t-Fe3O4 particles in ethanol under an external magnetic field. 63x56mm (600 x 600 DPI)


Langmuir

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Figure 5. Optical microscope images of emulsions (1:1 water and hexane) prepared with (a) s-TiO2 nanoparticles (mp = 0), (b) t-Fe3O4 nanoparticles (mp = 1), (c) a mixture of s-TiO2 and t-Fe3O4 nanoparticles (mp = 0.5) all at a total particle concentration of 1% w/v. Insert: photos of water drops under hexane on a disk composed of the relevant particles. 20x5mm (600 x 600 DPI)


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Langmuir

Figure 6. SEM images and EDS element mapping for the w/o emulsion droplet surfaces stabilized by (a) sTiO2 nanoparticles, mp = 0 and (b) a mixture of s-TiO2 and t-Fe3O4 nanoparticles, mp = 0.5. The total particle concentration is 1% w/v. 98x132mm (600 x 600 DPI)


Langmuir

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Figure 7. Response of w/o Pickering emulsion stabilized by s-TiO2 nanoparticles to light stimulus. Microscopic images of water droplet pair in oil under UV irradiation (a1) – (a5) and under visible light irradiation (b1) – (b5) for different times given. The left drop contains 0.2 M acid and the right drop contains 0.1 M base and methyl orange. UV lamp: λ = 254 nm, 12 W, d = 5 cm. 32x14mm (600 x 600 DPI)


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Langmuir

Figure 8. Light and magnetic dual-response of w/o Pickering emulsions stabilized by a mixture of s-TiO2 and t-Fe3O4 nanoparticles (mp = 0.5, total particle concentration = 1% w/v). (a) Snapshots of the magneticguided targeting behavior of two water-in-hexane droplets at 20 oC, the droplets moved with speeds of 4 mm s-1. (b) The separation/dispersion behavior of the emulsion droplets in excess hexane under an external magnetic field. (c) A precipitation reaction is triggered by the coalescence of the emulsion droplet pair under UV irradiation. The droplet on the left contains 0.2 M calcium chloride and that on the right contains 0.2 M sodium carbonate. UV lamp: λ = 254 nm, 12 W, d = 5 cm. A cylindrical NdFeB magnet of size 12 mm × 8 mm is placed under the Petri dish/glass vessel. The magnetic field strength is 0.4 T. 46x30mm (600 x 600 DPI)


Langmuir

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Figure 9. Light and magnetic dual-responsive Pickering emulsions stabilized by a mixture of s-TiO2 and tFe3O4 nanoparticles (a1), magnetic attraction (a2) and UV irradiation (a3), second cycle (b1-b3). Inserts are optical micrographs of initial (a4, a5) and second cycle (b4, b5). 53x39mm (600 x 600 DPI)


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Langmuir

Figure 10. Sketch of the proposed coalescence mechanism of w/o Pickering emulsion droplets stabilized by s-TiO2 and t-Fe3O4 nanoparticles. (a) Two droplets approach each other under a static magnetic field, (b) drainage of the liquid oil film occurs between adjacent droplets, (c) the liquid film becomes thinner due to the deformation of emulsion droplets, (d) nanoparticles desorb from the droplet interfaces inducing coalescence and triggering chemical reaction. 54x41mm (600 x 600 DPI)


Light and Magnetic Dual-Responsive Pickering Emulsion Micro

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