Light-Triggered Release from Pickering Emulsions Stabilized by TiO2

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Light-Triggered Release from Pickering Emulsions Stabilized by TiO2 Nanoparticles with Tailored Wettability Rui-Xue Bai, Long-Hui Xue, Rong-Kun Dou, Shi-Xin Meng, Chun-Yan Xie, Qing Zhang, Ting Guo, and Tao Meng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02329 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 17, 2016

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Figure 1. The strategy for encapsulation and release by Pickering emulsions triggered by light (b, d, f), the structural description for surface chemistry tuning of a TiO2 emulsifier (a, c, e). Figure 1 45x29mm (600 x 600 DPI)

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Figure 2. Photographs of TCA of long (a) /short (b)-chain-silane grafted TiO2 NPs via UV tailoring required. The proposed mechanism of light-induced wettability transition of TiO2 NPs resulting from degradation of grafted silane moieties (c, d). Figure 2 58x47mm (600 x 600 DPI)


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Figure 3. The XPS spectrum results of long-chain-silane-grafted (a, c) and short-chain-silane-grafted (b, d) TiO2 NPs. Figure 3 69x67mm (600 x 600 DPI)


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Figure 4. TEM image of the s-TiO2 NPs (a) and optical microscopic image (b) and CLSM microscope snapshots (c and d) of Pickering emulsion droplets stabilized by s-TiO2 NPs (1 wt % in hexane) on the green fluorescent channel (overlap of bright-field/dark-field). Figure 4 72x72mm (600 x 600 DPI)


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Figure 5. Stability evaluation of the Pickering emulsions by conductivity measurement at room temperature in dark. Insert (a) and (b), optical micrograph and interface schematic of W/O emulsions stabilized by sTiO2 NPs (1 wt % in hexane). Figure 5 56x43mm (600 x 600 DPI)


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Figure 6. Light-triggered demulsification and release of the Pickering emulsions (a1, a2), the proposed mechanism of the light-induced hydrophilic transition of s-TiO2 NPs resulting from absorbtion of hydroxyl groups (b1, b2). Photographs of WCA of s-TiO2 NPs before (c1) and after (c2) UV treatment. Figure 6 38x20mm (600 x 600 DPI)


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Figure 7. Particle concentration-dependent evolution of emulsions destabilization process. Optical microscopic images (a) and droplet coalescence kinetics plots (b) of evolution with time of the average droplet diameter of emulsions stabilized by 1, 2 and 3 wt % s-TiO2 NPs, respectively (λ = 254 nm, d = 5 cm). The cross represents phase separation occurs. Figure 7 33x15mm (600 x 600 DPI)


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Figure 8. The role of UV irradiation plays in the emulsions destabilization process. Optical microscopic images (a) and droplet coalescence kinetics plots (b) of evolution with time of the average droplet diameter of emulsions treated by UV (λ = 254 nm), visible light and dark storage (1 wt % s-TiO2 NPs, d = 5 cm). The cross represents phase separation occurs. Figure 8 33x15mm (600 x 600 DPI)


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Figure 9. Light wavelength-dependent evolution of emulsions destabilization process. Optical microscopic images (a) and droplet coalescence kinetics plots (b) of evolution with time of the average droplet diameter of emulsions under 254 nm and 365 nm UV illumination (1 wt % s-TiO2 NPs, d = 5 cm). The cross represents phase separation occurs. Figure 9 28x11mm (600 x 600 DPI)


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Figure 10. Light intensity-dependent evolution of emulsions destabilization process. Optical microscopic images (a) and droplet coalescence kinetics plots (b) of evolution with time of the average droplet diameter of emulsions under UV illumination intensity of 200 µw•cm-2 (d = 5 cm), 100 µw•cm-2 (d = 10 cm) and 50 µw•cm-2 (d = 15 cm), (1 wt % s-TiO2 NPs, λ = 254 nm). The cross represents phase separation occurs. Figure 10 33x15mm (600 x 600 DPI)


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Figure 11. Dependence of released amounts of APS on time from the Pickering emulsions: (a) stabilized by 1, 2 and 3 wt % TiO2 NPs under 254 nm UV illumination; (b) UV, visible light and dark treatment; (c) 254 and 365 nm UV illumination; (d) intensity of 200, 100 and 50 µw•cm-2 UV illumination, (d = 5cm using in a-c, 1 wt % s-TiO2 NPs using in b-d). Figure 11 62x53mm (600 x 600 DPI)


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Figure 12. (a) The XRD patterns of TiO2 and N-TiO2 NPs. (b) Dependence of released amounts of APS on time from the Pickering emulsions stabilized by s-N-TiO2 NPs under UV-Vis-Dark treatment. The inset is a comparison of the time required for complete release of APS between s-TiO2 and s-N-TiO2 NPs stabilized Pickering emulsions under UV. 34x16mm (600 x 600 DPI)


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Light-Triggered

Release

from

Pickering

Emulsions

Stabilized by TiO2 Nanoparticles with Tailored Wettability Rui-Xue Bai, Long-Hui Xue, Rong-Kun Dou, Shi-Xin Meng, Chun-Yan Xie, Qing Zhang, Ting Guo and Tao Meng∗ School of Life Sciences and Engineering, Southwest Jiaotong University, Chengdu, Sichuan, 610031, PR China

∗

Corresponding authors.

Tel: +86-28-8760-3202.

E-mail address: [email protected] (T. Meng)


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Abstract: In this paper, a new strategy for developing light-triggered Pickering emulsions as smart soft vehicles for on-demand release is proposed. Initially, UV-induced tailored wettability allows anchoring of TiO2 nanoparticles at the interface to prepare stable water in oil emulsions. Such emulsions show the efficacy of microencapsulation and controlled release by demulsification due to the hydrophilic conversion of the TiO2 nanoparticles using a non-invasive light irradiation trigger. A molecule of interest is selected as a model cargo to quantitatively evaluate the as-prepared Pickering emulsions for their encapsulation and release behaviors. Moreover, light-responsive emulsion destabilization mechanism is studied as a function of particle concentration, light wavelength and light intensity, respectively, determined by drop diameter evolution and droplet coalescence kinetics plots. Consideration of application in life sciences, Pickering emulsions sensitive to visible light are also established based on Nitrogen-doping of TiO2 nanoparticle emulsifiers.

Keywords: Pickering emulsions, Light-responsive, Wettability, Destabilization mechanism, Release


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1. Introduction In recent years, Pickering emulsions which are emulsions stabilized by solid particles have proven to be useful alternatives of conventional emulsions in pharmaceutical[1, 2], food[3], cosmetic[4, 5] and microreactors[6] fields as vehicles for encapsulation, delivery and release. Using solid particles instead of molecular surfactants to achieve interface stabilization allows Pickering emulsions significantly differ from conventional emulsions in several key aspects. Firstly, driven by the total interfacial energy decrease, particle emulsifiers self-assemble onto the oil-water interface to form rigid layers[7-9], making the resultant Pickering droplets to be long-term stable against coalescence even under severe conditions[10, 11]. In addition, such densely-packed particle layers around the droplets could be particularly efficient as inherent barrier in storage stability[12], drug protection[2] and controlled release[4]. Moreover, by proper choice/engineer of particle emulsifiers and emulsify conditions, Pickering emulsions with desired properties such as biocompatibility and target delivery could be obtained. Despite of these attractive features, it is surprising that the application of Pickering emulsions as vehicles for controlled release has been seldom reported. So far, most presented strategies are sustained release, relying on diffusion controlled permeation from interstices between the interfacial particles[13-15]. In fact, it is difficult to release the entire payload from emulsion droplets towards an external phase only by passive diffusion. From a practical point of view, the design of emulsions with the ability of on-demand triggered release in response to external stimuli is still a challenge. Recently, temperature[16,

17]

and pH-sensitive[18]

Pickering emulsions stabilized by some “smart” particles have been proposed for stimuli-response release. However, change of system environment is inevitably in most of


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these approaches, which is unfavorable for fragile and sensitive cargoes. The efficiency of particles in stabilizing emulsions mainly depends on their wettability[19]. Generally speaking, amphipathic particles with oil-water interfacial contact angles around 90° are surface active, whereas particles either too hydrophilic or too hydrophobic are unable to stabilize emulsions[12]. TiO2 nanoparticles (NPs) are a stable, low toxicity and environmentally friendly material widely used in several applications. The wettability of TiO2 NPs can be reversibly converted by UV/dark actuation due to the adsorption and desorption of hydroxyl groups on their surfaces[20]. Recently, we have developed a novel type of light-driven switchable Pickering emulsions based on TiO2 NPs[21], and anticipate that it could be served as a kind of soft vehicle for controlled release. Here, we report the further development of Pickering emulsions as smart vehicles for encapsulation and light-triggered release as shown in Figure 1. The Astragalus polysaccharides (APS) is chosen as a model cargo encapsulated in emulsion droplets. APS with immunoregulatory, anti-aging, antiviral, antioxidant, antitumor properties has been used as a valuable medicine applied in the treatment of many diseases for human beings[22, 23]. Initially, in consideration of highly hydrophilicity of the prepared TiO2 NPs, modification with trichlorododecylsilane (Figure 1a) is carried out, which however made them too hydrophobic to be emulsifiers (Figure 1b). As we have previously shown, exposure the modified TiO2 NPs in hexane to a certain UV light results in photodegradation of long-chain-silane-grafted TiO2 (l-TiO2) NPs into short-chain-silane-grafted TiO2 (s-TiO2) NPs (Figure 1c). Such s-TiO2 NPs with tailored wettability are able to stabilize W/O emulsions for cargo encapsulation (Figure 1d). Then once triggered by light irradiation, the s-TiO2


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nanoparticle surface becomes hydrophilic due to the adsorption of hydroxyl groups (Figure 1e), leading to the coalescence and breakdown of droplets therefore burst release of the cargoes (Figure 1f). Particularly, the neat and clean light is employed to be a contactless trigger as well as the only driven energy source of emulsion breakage for controlled release. In addition, particle concentration and irradiation conditions are manipulated for the evaluation of release profile. To meet the requirements of biological applications, researches into the construction of visible-light responsive Pickering emulsions stabilized by Nitrogen-doped (N-doped) TiO2 NPs for controlled release are carried out.

Figure 1. The strategy for encapsulation and release by Pickering emulsions triggered by light irradiation (b, d, f), the structural description for surface chemistry tuning of a TiO2 emulsifier (a, c, e).


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2. Experiments 2.1. Materials. Tetrabutyl titanate (TBOT) as a precursor of TiO2 NPs, anhydrous ethanol as reaction medium, ammonium carbonate ((NH4)2CO3) as nitrogen resource, hydrogen peroxide (30 wt %), glacial acetic acid and hexane were provided by the Chengdu Kelong Chemical Reagents Co., Ltd. (China) and used as received. Trichlorododecylsilane ((C12H25SiCl3), Tokyo KaSei Industry Co., Ltd., Japan) as a silane coupling agent was used for surface hydrophobic treatment. All reagents were analytical grade and used without further purification. Astragalus polysaccharides (APS) (Changbaishan Yipin shenrong Techan Co., Ltd.) was chosen as a model cargo. Deionized water (18.2 MΩ, Milli-Q, Millipore) was used throughout the study. 2.2. Synthesis of TiO2 and N-doped TiO2 NPs. TiO2 NPs were fabricated by the hydrothermal synthesis method. Solution A was prepared by adding TBOT (9 mL) slowly into anhydrous ethanol (36 mL) with vigorous stirring. Solution B was prepared by mixing deionized water (12 mL) and glacial acetic acid (40 mL) together with vigorous stirring. Solution B was added dropwise into solution A and the reaction mixture was stirred at room temperature until the transparent bright yellowish TiO2 sol was obtained. (For N-doped TiO2 NPs: (NH4)2CO3 was dissolved in the above transparent solution with vigorous stirring.) The TiO2 sol was transferred into a 150 mL Teflon-lined autoclave which was tightly sealed in a stainless steel tank and kept at 150 °C for 2 h to nucleate and grow TiO2 particles without any shaking or stirring during hydrothermal treatments. After the reaction was completed, the autoclave was allowed to cool to room


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temperature naturally, the precipitate were harvested by centrifugation, washed with anhydrous ethanol for three times, and dried at 60 °C for 12 h. The collected samples were calcined at 500 °C for 0.5 h in air. The final products (uniform anatase TiO2 NPs with diameters of ~ 30 nm) were obtained. 2.3. Preparation of s-TiO2 and short-chain-silane-grafted N-doped TiO2 (s-N-TiO2) NPs. Preparation procedure of s-TiO2: the synthesized TiO2 NPs were added into 20 mL of a 30 wt % hydrogen peroxide solution for 4 h with continuous stirring in the dark and then dried in air at 45 °C to gain hydroxylated TiO2 NPs. A trichlorododecylsilane solution (1 mL of trichlorododecylsilane in 20 mL of hexane) was used to treat the above hydroxylated TiO2 NPs for 12 h with continuous stirring at room temperature and then dried in air at 50 °C, then the l-TiO2 NPs were gained. Next, the l-TiO2 NPs immersed in hexane were stirred and irradiated using a UV lamp (λ = 254 nm, WFH-203 three-function ultraviolet analyzer, 12 Watt, China) with the irradiation distance (d = 5 cm) between light source and samples. After a UV illumination period of 24 h, we obtained the s-TiO2 NPs. The preparation procedure of light-tailored wettability of s-N-TiO2 NPs was the same as the s-TiO2 NPs. 2.4. Characterization techniques. X-ray photoelectron spectroscopy (XPS, XSAM800, KRATOS, UK) was used to verify the alkyl chain tailoring from l-TiO2 NPs to s-TiO2 NPs. All of the emulsion droplets were viewed using an XSP-24 (Phoenix Co., Ltd., China) research microscope equipped with a Moticam 2000 camera. The microscopic pictures were captured using Motic Images Plus 2.0 software and then processed and analyzed using Image Pro Plus software. The emulsions stability at room temperature in dark was monitored with Conductivity measurement


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(Mettler-Toledo Instruments Co., Switzerland) until 72 h. The surface morphological features of the emulsion droplets were studied with the use of an optical microscope (Olympus, BX51, Japan). Crystalline characterization of samples was identified and compared by X-ray diffraction (XRD, XL-30, Philips X’ Pert Pro, The Netherlands). The diffractometer with Cu KR radiation (λ = 1.5406A) from the copper anode source was operated at a tube current of 40 mA and a voltage of 40 kV. Data were collected over 2θ values from 10° to 60°, at a scan speed of 0.03° (2θ) per second. The morphologies of s-TiO2 NPs were investigated using a field-emission transmission electron microscope (FE-TEM; JEM-2100F, JEOL, Japan). The freshly prepared Pickering emulsions were dropped in a transparent glass container and observed by a confocal laser scanning microscope (CLSM, SP5-II, Leica). The green fluorescent channel was excited at 400 nm. 2.5. Pickering emulsions: preparation and cargo loading. Aqueous solutions (10 mL) containing 25 g·L-1 APS was as the dispersed phase, and hexane (10 mL) containing the s-TiO2 NPs (1, 2 or 3 wt %) with ultrasonic treatment to obtain a suspension with good dispersion was as the continuous phase. The hexane phase was added to the aqueous phase (1:1, v/v), followed by homogeneous emulsification at 18, 000 rpm for approximately 1 min using homogenizer (S10, NingBo Scientz Biotechnology Co., Ltd., China) to obtain W/O emulsions (Emulsion volume in total is about 12-16 mL). The emulsions sensitive to visible light were stabilized by s-N-TiO2 NPs prepared as described above. 2.6. Evaluation of TiO2 NPs surface wettability. The surface wettability characteristics of the prepared s-TiO2 NPs were investigated by


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recording the water contact angles (WCA) before and after UV irradiation using a contact angle instrument (DSA100, Krüss, Germany) equipped with a standard video camera. A slide glass generated s-TiO2 NPs film upon a simple dip coating in the hexane solution of the s-TiO2 NPs (3 wt %), blown with a stream of nitrogen, and then dried under atmosphere. The same procedure was repeated five times to get complete coverage of s-TiO2 NPs on slide glass. 3-µL of water droplets were employed to evaluate the WCA of the coated surface. To measure UV-induced hydrophilicity, as described above, the WCA of the s-TiO2 NPs after UV irradiation were recorded. The average value from more than five parallel measurements on different sites of the same coatings was obtained and the experiments were performed under ambient conditions. The three-phase contact angle (TCA) of NPs was measured as follows: the dried l- or s-TiO2 NPs were compressed into circular flakes with a thickness of about 2 mm and then placed at the bottom of an open, transparent quartz vessel. Subsequently, hexane was poured into the vessel and then a 3-µL deionized water droplet was dropped on the particle flake. The appearance of the water droplet on the particle flake was immediately photographed. The values of contact angle were directly measured. All the TCA data were the arithmetic average values of at least five repetitive tests on the same flake sample. 2.7. The study of light-induced cargo-release activities and droplet coalescence kinetics for the photosensitive Pickering emulsions. In order to investigate the cargo-release behaviors and droplet coalescence kinetics for photosensitive W/O Pickering emulsions stabilized by s-TiO2 NPs, the experiments were carried out as follows: three samples of the prepared W/O Pickering emulsions (encapsulated


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APS) stabilized by s-TiO2 NPs of 1 wt % were poured into quartz tubes, the samples of Pickering emulsions in the quartz tubes were irradiated using a UV lamp (λ = 254 nm, WFH-203 three-function ultraviolet analyzer, 12 Watt, China) as a UV irradiation source or a tungsten halogen lamp (QVF 133, Feilipu lamp Co., Ltd., 150 Watt, China) as a visible irradiation source with d = 5 cm, and the other samples were monitored storage in the dark respectively. To test the concentration of APS, 500 µL aqueous phases of samples were extracted from the quartz tubes and diluted to 5 mL with deionized water. Then, the concentration of APS was calculated according to a standard curve (absorbance vs. APS concentration at 194 nm). The UV-triggered cargo-release amount with time was calculated and used to evaluate the release effect according to the following equations (1):

η (% ) =

Released mass of APS into aqueous phase × 100 Total encapsulated mass of APS in emulsions

(1)

where η is the light-triggered cargo-release rate in the aqueous phase (%). To determine the influence of UV wavelength on the cargo-release property, the same measurements were then repeated using UV irradiation (λ = 365 nm). Besides UV wavelength, the d were used to investigate the influence of UV intensity on the cargo-release property (λ = 254 nm, d = 5, 10 and 15 cm, accordingly UV intensities of 200, 100 and 50 µw·cm-2). To test the impact of s-TiO2 NPs concentrations on cargo-release performance, the Pickering emulsions containing 1, 2 and 3 wt % s-TiO2 NPs were studied under UV irradiation (λ = 254 nm, d = 5 cm). Meanwhile, light-induced droplet coalescence kinetics was also performed as the described above methods. Here, in order to follow the emulsion droplet diameter evolution, microscopic pictures of droplets were captured and the change in droplet diameter with time was monitored. In this way, the evolution in time of the surface weighted average droplet


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diameter[24] defined as the equation (2):

∑N D D(t ) = ∑N D i

3 i

i

2 i

i

(2)

i

where Ni is the number of droplets with diameter Di. By measuring the scale of about 50 emulsion droplets, we evaluated the evolution of the emulsions and drew droplet coalescence kinetics plots. The uncertainty in the drop diameter is ±1µm. The test method about release behavior of Pickering emulsions stabilized by s-N-TiO2 NPs sensitive to visible light is the same as emulsions stabilized by s-TiO2 NPs. All the experiments described above were carried out at room temperature and all measurements were performed at least in duplicate.


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3. Results and discussion

Figure 2. Photographs of TCA of long (a) /short (b)-chain-silane grafted TiO2 NPs via UV tailoring required. The proposed mechanism of light-induced wettability transition of TiO2 NPs resulting from degradation of grafted silane moieties (c, d). Surface wettability of particles are thought to be crucial in stabilizing Pickering emulsions[25]. In order to make the suitable wettability of NPs to stabilize water/hexane emulsions, light tailoring chain-length is used to modify the surface wettability of TiO2 NPs (Figure 2). When the TiO2 NPs are dispersed in hexane with vigorous stirring, the long-chain silane grafted on TiO2 NPs is degraded into short-chain silanes by UV irradiation. Initially, the TCA of l-TiO2 NPs (with 12 carbons) is 140.5o (Figure 2a) because of the wettability of 12-carbon-silanes (Figure 2c), thus are unable to stabilize Pickering emulsions. In contrast, the TCA of the s-TiO2 NPs (silane-grafted with approximately 7 carbons) is 105.5o (Figure 2b) based on the wettability of 7-carbon-silanes (Figure 2d) and promises to stabilize W/O Pickering emulisons. The stability of Pickering emulsions mainly relies on the wettability of its particles. Amphipathic particles with oil-water interfacial contact angles (equal to TCA)


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around 90° are surface active, while TCA either too low or too high are unable to stabilize emulsions[12]. As is shown in Figure 2, the decrease of TCA of particles from 140.5o to 105.5o indeed shows an enhanced affinity of the tailored particles to water. Such tailored s-TiO2 NPs with suitable wettability (105.5o) guarantee stabilization of water/hexane emulsions, whereas the l-TiO2 NPs are too hydrophobic (TCA = 140.5o) to stabilize this emulsions. (a)

(b)

C1s

C1s

C-H

291

287 283 Binding Energy (eV)

C-H

279

291

(d)

(c) Si2p


Si2p Si-O

Si-O

108

Figure

3.

The


XPS

spectrum

279

96

results

108

of


96

long-chain-silane-grafted

(a,

c)

and

short-chain-silane-grafted (b, d) TiO2 NPs. XPS measurements are carried out to characterize the surface chemical composition during alkyl chain tailoring from the long-chain-silane to short-chain-silane grafted onto the TiO2 NPs. As shown in Figure 3a and 3b, the C1s peak at 284.7 eV becomes weaker, confirming the reduction of the amount carbon. Comparing Figure 3c with Figure 3d, the


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indistinctive change of the Si2p peak (102.4 eV) indicates the Si-O bonds are nearly not broken. As estimated from the XPS results, the average number of carbon atoms per silane at the surface of the degraded TiO2 NPs is nearly 7. Considering both XPS results (Figure 3 and Figure S1) and surface element contents (Table S1), we deduce that UV-induced degradation of the grafted silanes happens, leading to the long-chain-silanes with 12 carbons changing to short–chain-silanes with approximately 7 carbons. As is shown in Figure S2, we have tried a series of chloroalkylsilanes commercially available to modify TiO2 NPs with various alkyls to prepare water/hexane emulsions, such as trimethylchlorosilane (Figure S2(a) TCA = 65.5o), hexyltrichlorosilane (Figure S2(b) TCA = 138.5o) and trichlorododecylsilane (Figure S2(c) TCA = 140.5o), however all fail to stabilize. We accidentally find that, light-induced degradation of TiO2 NPs modified with 12-carbon-silanes to ~7-carbon-silanes with a moderate TCA of 105.5o (Figure S2d) can serve as efficient emulsifiers to stabilize water/hexane emulsions. All these facts suggest that the step of light tailoring chain-length is useful and necessary for silane-grafted-TiO2 NPs acting as Pickering stabilizers. Further, the alkyl-chains of the TiO2 NPs may be precisely tailored to a desired length by different degradation conditions (eg. employing certain UV intensity, wavelength, and so on) to stabilize various oil-water emulsions. In view of these characteristics, we expect the tailored wettability help to conveniently and efficiently make Pickering emulsions. Our group has been performing studies on such systems.


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Figure 4. TEM image of the s-TiO2 NPs (a) and optical microscopic image (b) and CLSM microscope snapshots (c and d) of Pickering emulsion droplets stabilized by s-TiO2 NPs (1 wt % in hexane) on the green fluorescent channel (overlap of bright-field/dark-field). The stability of Pickering emulsions is ensured by irreversible adsorption of solid particles at the oil/water interface[12]. Accordingly, Pickering emulsions can be viewed as capsules that could be used for a controlled delivery purpose[13]. The morphologies of s-TiO2 NPs and Pickering emulsions like capsule are shown in Figure 4. TEM images clearly show the shape and good dispersion state of s-TiO2 NPs with an average diameter of 30 nm (Figure 4a). After homogenization with APS-water solution and hexane (TiO2 NPs of 1 wt % in hexane) for only 1min, Pickering emulsions stabilized by the s-TiO2 NPs are obtained with a mean droplet diameter of ~300 µm (Figure 4b). As is shown in Figure 4b, the distinct dark shell enveloping their aqueous core indicates that the emulsions are covered with a dense layer of s-TiO2 NPs. These surface morphologies are also demonstrated by the CLSM microscope snapshots (Figure 4c, d). The green fluorescence from s-TiO2 NPs layers reveals


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that the emulsions droplets are thoroughly surrounded by solid s-TiO2 NPs (for details seen SI_videos data). These results imply that the s-TiO2 NPs adsorb at the oil/water interface and form densely shell-like structure to stabilize emulsions with encapsulation properties.


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Figure 5. Stability evaluation of the Pickering emulsions by conductivity measurement at room temperature in dark. Insert (a) and (b), optical micrograph and interface schematic of W/O emulsions stabilized by s-TiO2 NPs (1 wt % in hexane). As is shown in Figure 5, W/O Pickering emulsions stabilized by s-TiO2 NPs are milky-white with uniform size of ~300 µm (Insert (a)). The high emulsion stability is confirmed by the unchanged conductivity of ~400 µs·cm-1 for 72 h at room temperature in dark. An interface schematic demonstrates that the s-TiO2 NPs self-assemble at the hexane/APS-water solution interface for stabilizing W/O Pickering emulsions with hexane as oil phase and an APS-water solution as aqueous phase (Insert (b)). All these facts indicate that the s-TiO2 NPs work well as the Pickering stabilizers. As a result, the Pickering emulsion droplets stabilized by the s-TiO2 NPs are highly stable, and can robustly encapsulate active molecules.


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Figure 6. Light-triggered demulsification and release of the Pickering emulsions (a1, a2), the proposed mechanism of the light-induced hydrophilic transition of s-TiO2 NPs resulting from absorbtion of hydroxyl groups (b1, b2). Photographs of WCA of s-TiO2 NPs before (c1) and after (c2) UV treatment. Figure 6 shows the effect of the UV-induced wettability transition of the s-TiO2 NPs on the stability of Pickering emulsions. Before UV irradiation, the macroscopic view of W/O Pickering emulsions present as a homogeneous milky white solution (Figure 6a1), and they exhibit no evidence of phase separation for several weeks. The schematic is shown in Figure 6a2, the s-TiO2 NPs can stabilize W/O emulsions (disperse phase: APS-water solution, continuous phase: s-TiO2 NPs of 1 wt % in hexane), which is attributed to the suitable wettability of s-TiO2 NPs. Interestingly, after UV irradiation of 12 h, obvious stratification phenomenon is observed with the upper layer of hexane and the lower layer of APS-water solution, the s-TiO2 NPs are located at the oil-water interface and larger particles remain the bottle bottom (Figure 6b1). Consequently, the APS molecules are released from the photo-sensitive W/O emulsions to water phase (Figure 6b2) based on the change of s-TiO2 NPs’ wettability. The WCA of the s-TiO2 NPs layer before and after UV irradiation is 125.5o


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(Figure 6c1) and 86.5o (Figure 6c2), and it is attributed to wettability transition of the UV-induced absorption of hydroxyl groups on NPs surface. The change in surface wettability leads to desorption of the s-TiO2 NPs from the oil-water interface, and accordingly the droplets undergo completely breakage. The wettability of the NPs plays an important role in the type and stability of a Pickering emulsion, in general, hydrophobic NPs result in W/O emulsions, whereas the change of hydrophobicity bring out the emulsions unstability even water-oil phase separation[19]. For the photoinduced wettability transition process of silane grafted TiO2 nanoparticles, the initial and widely accepted mechanism was proposed by Wang et al. which relies on the formation of surface defects upon UV light illumination[26]. UV irradiation may create surface oxygen vacancies at bridging sites, resulting in the conversion of relevant Ti4+ sites to Ti3+ sites which are favorable for dissociative water adsorption. These defects can increase the affinity for hydroxyl groups formed by dissociation of chemisorbed water molecules and thereby forming hydrophilic domains[27]. The wettability of the NPs switched from hydrophobic to hydrophilic, resulting in Pickering emulsions are unstable and underwent complete breakage. Therefore, the light-triggered cargo-release activity of Pickering emulsions is attributed to wettability transition and the emulsions evolve slowly toward phase separation by droplet coalescence.


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Figure 7. Particle concentration-dependent evolution of emulsions destabilization process. Optical microscopic images (a) and droplet coalescence kinetics plots (b) of evolution with time of the average droplet diameter of emulsions stabilized by 1, 2 and 3 wt % s-TiO2 NPs, respectively (λ = 254 nm, d = 5 cm). The cross represents phase separation occurs. Emulsion droplet diameter evolution and drop coalescence kinetics plots are evaluated as a function of time under different particle concentrations, light wavelengths and light intensities, respectively (Figure 7-10). In brief, the light-induced coalescence behavior is considered as follows: coalescence develops slowly during the early stage (stage 1), subsequently remarkable coalescence proceeds at progressively higher rates (stage 2), and until phase separation occurs (stage 3). Figure 7 shows the effect of particle concentration on the evolution of emulsions destabilization process. As illustrated in Figure 7a, the as-prepared W/O emulsions exhibit decreasing initial diameters of ~280 µm, ~180 µm and ~130 µm, when increases the concentration of TiO2 NPs from 1 to 2 and to 3 wt %, respectively, which can be explained by the classical theory of limited coalescence[24]. Furthermore, the time of complete demulsification and phase separation prolongs from ~12 to ~24 and to ~48 h for the samples


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of 1, 2 and 3 wt %, respectively. Droplet coalescence kinetics curves according to droplet diameter evolution are illustrated in Figure 7b, and the kinetics equations (stage 1) are as follows: y = 17.39x + 280.21, k1 = 17.39 (1 wt %); y = 7.66x + 182.64, k2 = 7.66 (2 wt %) and y = 4.63x + 126.32, k3 = 4.63 (3 wt %), respectively. It is obvious that k1 ＞ k2 ＞ k3. It is well known that the particle emulsifiers adsorbed on the interface can provide a steric barrier between neighboring droplets, hindering droplet-droplet collisions, effective liquid film drainage and coalescence[28]. The rate of coalescence is governed by the stability against drainage and rupture of the thin liquid film between droplets, and necessary for coalescence is the removal these particles from interface[29]. According to classical theory of limited coalescence, as particle concentration increases, the total interfacial area increases owing to the decrease of drop diameter, which will allow more TiO2 NPs to be anchored to the interface since they are most likely irreversibly adsorbed[24,

30]

. The rate of liquid film

drainage between coalescing droplets is retarded because of the increased energy required to detach more TiO2 NPs from the interface[12]. Meanwhile, a decrease in the average droplet size inhibits coalescence due to the considerations for reduced free energy[31]. In addition, the higher scattering of the light within sample with the increase of NPs concentration also prolongs the coalescence time. Therefore, as NPs concentration progresses, k1 ＞ k2 ＞ k3 and it will take longer time to complete coalescence.


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Figure 8. The role of UV irradiation plays in the emulsions destabilization process. Optical microscopic images (a) and droplet coalescence kinetics plots (b) of evolution with time of the average droplet diameter of emulsions treated by UV (λ = 254 nm), visible light and dark storage (1 wt % s-TiO2 NPs, d = 5 cm). The cross represents phase separation occurs. In Figure 8, the role that UV irradiation plays in the emulsion destabilization process is evaluated. As is shown in Figure 8a, phase separation is induced upon UV illumination at λ = 254 nm for 12 h. However, no significant change of the average droplet diameter is observed when the emulsions are treated with visible light or dark storage instead of UV. Figure 8b shows kinetics equations (stage 1): y = 17.39x + 280.21, k1 = 17.39 under UV; y = 0.84x + 275.01, k2 = 0.84 under visible light and y = 0.65x + 272.62, k3 = 0.65 under dark storage. It is obvious that k1 ＞ k2 ≈ k3. It has been confirmed that, the hydrophilic conversion of TiO2 NPs is attributed to the adsorption of hydroxyl groups on their surfaces that is associated with the band-gap excitation of TiO2 under UV illumination. The photoinduced wettability transition of TiO2 NPs is confined to the adsorption of photon with energy being equal to or larger than its band gap (about 3.0-3.2 ev), which requires an excitation light wavelength range shorter than ca. 400


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nm (such as UV light)[20]. When light with photon energy lower than 3.0 eV applied, hydrophilic conversion not occurs even after long-term exposure. Therefore, only UV light with energy above the band gap can be used as the driven force for emulsion destabilization and coalescence via wettability transition of s-TiO2 NPs, whereas visible light and dark storage cannot.

Figure 9. Light wavelength-dependent evolution of emulsions destabilization process. Optical microscopic images (a) and droplet coalescence kinetics plots (b) of evolution with time of the average droplet diameter of emulsions under 254 nm and 365 nm UV illumination (1 wt % s-TiO2 NPs, d = 5 cm). The cross represents phase separation occurs. As seen from Figure 9, the effect of UV wavelength (λ = 254 vs. 365 nm) on the emulsions destabilization process is demonstrated. The time required for phase separation varies from ~12 h to ~36 h depending on different UV wavelength irradiation from 254 nm to 365 nm (Figure 9a). Furthermore, the kinetics equations (stage 1): y = 17.39x + 280.21, k1=17.39 (λ = 254 nm), y = 5.63x + 282.89, k2 = 5.63 (λ = 365 nm) are presented in Figure

9b. The UV energy is utilized to actuate emulsions breakage as a result of hydrophilic


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conversion of TiO2 stabilizers at interface due to the adsorption of hydroxyl groups. Obviously, UV with shorter wavelength gives higher energy, resulting in faster light-induced wettability transition of the particles[32], thus accelerates the rate of coalescence and breakage.

Figure 10. Light intensity-dependent evolution of emulsions destabilization process. Optical microscopic images (a) and droplet coalescence kinetics plots (b) of evolution with time of the average droplet diameter of emulsions under UV illumination intensity of 200 µw·cm-2 (d = 5 cm), 100 µw·cm-2 (d = 10 cm) and 50 µw·cm-2 (d = 15 cm), (1 wt % s-TiO2 NPs, λ = 254 nm). The cross represents phase separation occurs. Figure 10 shows the influence of UV intensity on the emulsions destabilization process. It is demonstrated that the complete breakage under the irradiation intensity of 200 µw·cm-2 (d = 5cm) happens in ~12 h. In contrast, longer lifetime of 32 h under 100 µw·cm-2 and 72 h under 50 µw·cm-2 are observed (Figure 10a). Accordingly, kinetics equations are as follows (Figure 10b): y = 17.39x + 280.21, k1 = 17.39 (200 µw·cm-2); y = 6.82x + 267.06, k2 = 6.82 (100 µw·cm-2) and y = 2.87x + 277.28, k3 = 2.87 (50 µw·cm-2). It is obvious that k1 ＞k2 ＞ k3 (stage 1).


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To a certain extent, the higher light intensity is applied, the more available absorbed photons, the larger quantities of photogenerated electrons and holes can be obtained[33]. Thus, increasing UV intensity by simply change the irradiation distance can remarkably accelerate the wettability conversion rate[32] and reduce coalescence time accordingly. As is shown in Figure 7-10, the surface hydrophilicity of s-TiO2 NPs elevates dramatically as light irradiation time goes up, which acts as the driving force for the destabilization of the emulsions. The coalescence behavior is the result of wettability transition of particle emulsifiers at water-oil interface[19]. In the first stage, with the NPs escaping from interface under light irradiation, droplets begin to coalesce to compensate for the incomplete coverage of NPs by the reduction of the total interfacial area, forces the liquid films between adjacent droplets to be thin and become unstable[34, 35] (stage 1). Then, the significant increase of droplet diameter is observed, indicating accelerated destabilization of the emulsions, especially, a robust removal of particles from the interface appears near the critical point of wettability (stage 2). In the last phase, the inherent instability results in the occurrence of complete phase separation (stage 3).


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Figure 11. Dependence of released amounts of APS on time from the Pickering emulsions: (a) stabilized by 1, 2 and 3 wt % TiO2 NPs under 254 nm UV illumination; (b) UV, visible light and dark treatment; (c) 254 and 365 nm UV illumination; (d) intensity of 200, 100 and 50 µw·cm-2 UV illumination, (d = 5cm using in a-c, 1 wt % s-TiO2 NPs using in b-d). To illustrate the process of light-triggered cargo release and regulating trigger time, a diagram exhibiting the release rate change with trigger time is shown in Figure 11. It is found that the APS-release is in line with a negligible release at the beginning of the time period and gives rapid and then complete release within several minutes. As is shown in Figure 11a, The rapid and complete release requires longer UV light irradiation time at 254 nm along with the concentration of the s-TiO2 NPs used as stabilizer for almost 12 h (1 wt %), 24 h (2 wt %), 48


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h (3 wt %) respectively. These s-TiO2 NPs may lead to rigid networks at the interface which prevent coalescence behaviors[24, 30]. The different time of light-triggered release should be attributed to the increased energy which demand the longer UV light irradiation time to remove more s-TiO2 NPs from interface with the total drop interfacial area increasing. The emulsions maintain stable with negligible release during 0-24 h under the visible light/dark treatment (Figure 11b), however, once the UV irradiation time reach 12 h, the burst release takes with 4-6 min. It demonstrates that UV-light energy can be used to drive the TiO2 NPs to escape from the water-oil interfaces of emulsion, which lead to Pickering emulsions breakage and APS release. Compared with UV wavelength 254 nm (12 h) (Figure 11c), the release occurs at a longer UV light irradiation time with the wavelength 365 nm (36 h). Moreover, the intensity of UV irradiation inversely correlate with the time of light-triggered release of 12 h (200 µw·cm-2), 32 h (100 µw·cm-2), 72 h (50 µw·cm-2) respectively (Figure 11d). It can be seen from Figure 11c, d, the smaller the UV wavelength (or the higher UV energy) used the shorter the time of light-triggered release. Consequently, the time of light-triggered cargo-release are controlled by varying irradiation wavelength, irradiation intensity and NPs concentration. Light is an attractive stimulus for triggering release from emulsions owing to its high spatial resolution, which can cause release of encapsulated cargoes at a precise target moment/location[36]. Accordingly, many light-responsive emulsions release system have been designed and evaluated for such a purpose[36], however, the complexity and the use of surfactant make them difficult to replicate and scale-up for cosmetic and pharmaceutical application where surfactants often show adverse effects. Towards such domains,


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environmentally friendly Pickering emulsions have inherent advantages, the mainly stabilization mechanism for the Pickering emulsions is the existence of solid particles at its interface. In above experiments, when TiO2 NPs are exposed to UV light, the Pickering emulsions are disrupted because of the removal of the TiO2 NPs from the emulsions interface via the wettability transition of TiO2 NPs, thus the internal cargoes are completely released into the surrounding aqueous solution.


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Figure 12. (a) The XRD patterns of TiO2 and N-TiO2 NPs. (b) Dependence of released amounts of APS on time from the Pickering emulsions stabilized by s-N-TiO2 NPs under UV-Vis-Dark treatment. The inset is a comparison of the time required for complete release of APS between s-TiO2 and s-N-TiO2 NPs stabilized Pickering emulsions under UV. Figure 12a shows XRD patterns of TiO2 NPs and N-TiO2 NPs. Five high-intensity crystal peaks at 2θ = 25.3, 37.8, 48.0, and 55.1° can be perfectly indexed as (101), (004), (200), (105), and (211), respectively. All the peaks of TiO2 and N-TiO2 are attributed to the anatase TiO2 (body centered tetragonal structure, space group I41/amd)

[37]

. However, the

(101) peak of N-TiO2 samples shifts negatively (TiO2 NPs: 25.32o, N-TiO2 NPs: 25.29o). The peaks of N-TiO2 become weaker and broader revealing the incorporation of nitrogen decreases the crystallinity of TiO2. In general, N doping can efficiently yield a narrow band gap in TiO2 NPs and create oxygen vacancies, which cause the red-shift absorption threshold of TiO2 NPs and changes optical absorption to the visible light region[38, 39]. Figure 12b displays the influence of the light-triggered release time on the N doping with UV-Vis-dark condition. The light-triggered release time of Pickering emulsions (s-N-TiO2 NPs) is almost


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16 h under UV/Vis light irradiation, while these emulsions are stable with negligible release during 0-40 h in dark treatment. The inset in Figure 12b reveals that the UV-triggered behavior becomes longer for emulsions stabilized by s-N-TiO2 NPs (16 h) in contrast to those by s-TiO2 NPs (12 h). Consequently, visible light also can be used as a trigger for cargo-release of Pickering emulsions, which will benefit applications in life sciences. Taken together, by means of N-doped, the light-triggered cargo-release can adapt to UV/visible light and the different irradiation time.

4. Conclusion In summary, we design novel light-triggered Pickering emulsions for on-demand release, where the wettability transition of the TiO2 nanoparticle stabilizers via light irradiation plays a crucial role in emulsification and breakage behaviors. Initially, the wettability of TiO2 NPs can be suitably tailored by degrading the grafted alkyl chains via UV irradiation, enables them to stabilize the W/O emulsions. Once the TiO2 NPs at the water-oil interface are treated with UV, hydrophilic change due to the adsorption of hydroxyl groups on particle surface is triggered, disables their stabilizing properties: the particles are expelled out of the interface, the W/O emulsions phase separate and the encapsulated molecule is released. As is shown in the destabilization mechanism and release profiles study, either increase of intensity or reduce of wavelength of the light irradiation can efficiently accelerate the hydrophilic transition thus speed up destabilization procedure. However, as particle concentration progresses, the rate of phase separation is retarded due to the higher energy barrier which significantly inhibits drop coalescence. Furthermore, we show the possibility to trigger demulsification by visible light


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relying on N-doped of TiO2 NPs, which offers great potential in the delivery of UV-sensitive molecule of interest and in biological applications.

Acknowledgment This work is supported by the National Natural Science Foundation of China (21106115, 21406181), the Fundamental Research Funds for the Central Universities (SWJTU12CX049, 2682015CX050), 2014 Cultivation Program for the Excellent Doctoral Dissertation of Southwest Jiaotong University, 2014 Sichuan Province Seedling Project. The authors gratefully acknowledge the help of Mr. C. Xin at Chinese Academy of Science for XPS measurements.

Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at http://dx.doi.org/******.

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[12] Binks, B. P. Particles as Surfactants - Similarities and Differences. Curr. Opin. Colloid Interface Sci. 2002, 7 (1-2), 21-41.

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Light-Triggered Release from Pickering Emulsions Stabilized by TiO2

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