Triple-Responsive Pickering Emulsion Stabilized by Core Cross

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Triple-Responsive Pickering Emulsion Stabilized by Core Crosslinked Supramolecular Polymer Particles Ting Zeng,†,‡ Amin Deng,†,‡ Duanguang Yang,‡ Huaming Li,‡ Chenze Qi,† and Yong Gao*,†,‡ †

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Key Laboratory of Alternative Technologies for Fine Chemicals Process of Zhejiang Province, College of Chemistry and Chemical Engineering, Shaoxing University, Shaoxing, Zhejiang Province 312000, China ‡ College of Chemistry and Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan, Hunan Province 411105, China S Supporting Information *

ABSTRACT: It is significant to explore multiresponsive Pickering emulsions because of their flexibility in terms of demulsification in comparison with the single stimuliresponsive systems. In this study, we described a tripleresponsive oil-in-water Pickering emulsion that was stabilized by amphiphilic core cross-linked supramolecular polymer particles (CCSPs). For this purpose, β-cyclodextrin-terminated poly(N-isopropylacrylamide) (PNIPAM-β-CD) and azobenzene-capped poly(4-vinylpyridine) (P4VP-azo) were separately synthesized by reversible addition−fragmentation chain transfer polymerization. By virtue of the inclusion interaction between the β-CD host and the azobenzene guest in dimethyl sulfoxide, the amphiphilic supramolecular block copolymer, poly(4-vinylpyridine)-b-poly(N-isopropylacrylamide) (P4VP-b-PNIPAM), was formed. CCSPs were prepared through the combination of the self-assembly of P4VP-b-PNIPAM in the selective solvent, water, and the cross-linking of the P4VP core with 1,4-dibromobutane. Due to thermoresponsiveness of PNIPAM shells and the supramolecular linkages between the cross-linked hydrophobic P4VP core and hydrophilic PNIPAM shells, the as-prepared CCSPs exhibited temperature-, light-, and amantadine hydrochloride guest-triggered morphological transitions. Such triple-responsive morphological transitions gifted CCSPs stabilized oil-in-water Pickering emulsion with flexible demulsification in response to various factors, such as thermo, light, and amantadine hydrochloride or their combinations. Such triple-responsive oil-in-water Pickering emulsion also provided an ideal platform for heterogeneous reactions conducted at the oil−water interface. A large interfacial area and responsive demulsification allowed the reaction to be performed with an efficient and sustainable pattern.

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INTRODUCTION Pickering emulsion is a dispersion system stabilized by particulate emulsifiers in place of low molecular weight surfactants.1 The type of Pickering emulsion can be predicted by the three-phase contact angle of a solid particle at an oil− water interface. When the contact angle in water, θw, is smaller than 90°, an oil-in-water emulsion is favored, whereas θw > 90° favors water-in-oil emulsion.2 Under equally wetting conditions, solid particles are strongly adsorbed at the oil−water interface. ΔFads is the energy needed for the detachment of one solid particle from the oil−water interface to enter the water phase or the oil phase, which can be described as follows.3 ΔFads = πR2γo − w(1 ± cos θ )2

high stability, Pickering emulsions also exhibit a tunable droplet size and a permeable interface as well, and all of these render them promising application in wide fields where longterm stability is desired. However, in some circumstances, especially where the temporary stability is needed, such as the interfacial reaction, oil recovery, and emulsion polymerization,4−6 Pickering emulsions with on-demand demulsification or phase inversion are much preferred. Facile demulsification makes the separation of products and the recovery and reuse of emulsifiers much easier, which contributes to achieving a sustainable and efficient operation.7 Many efforts have been exerted to explore stimuli-responsive Pickering emulsion in the past decades. A broad kind of Pickering emulsions in response to external stimuli, including pH,8,9 salinity,10 temperature,11,12 N2/CO2,13 redox,14,15 light,16 etc., have been reported by many research groups. The tunable wettability of particulate emulsifiers in response to

(1)

Where γo−w is the oil−water interfacial tension. According to this equation, ΔFads of a solid particle with 10 nm is ∼1.6 × 10−17 J, which is orders of magnitude greater than the particle’s thermal energy of kT (4.1 × 10−21 J at 293 K).3 The strong adsorption of particulate emulsifiers at the oil−water interface largely prevents the coalescence of dispersed droplets, endowing Pickering emulsion with high stability. Besides the © XXXX American Chemical Society

Received: July 26, 2019 Revised: August 12, 2019

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DOI: 10.1021/acs.langmuir.9b02341 Langmuir XXXX, XXX, XXX−XXX

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an external stimulus is the critical factor for the formation of responsive Pickering emulsion. The exploited particulate emulsifiers for responsive Pickering emulsions have been summarized in recent several reviews.17−21 In some circumstances, the fulfillment of some advanced processes relies on the combination of different stimuli factors. Meanwhile, multiresponsive systems can create synergistic effects that cannot be attainable for the single responsive systems. For instance, Liu and his co-workers22 demonstrated responsive tandem transitions between a gel-like fluid and a water-like fluid by means of the integration of two different triggers. A water-like fluid consisting of sodium benzylselanyl undecyl sulfate and N,N,N′,N′-tetramethyl-1,2-ethanediamine rapidly changed into a gel-like fluid when exposed to CO2. Upon further triggering with H2O2, the formed gel-like fluid then turned into a water-like fluid. The tandem transitions were originated from the stimuli-responsive structural changes of the molecular species presented in solution. Zhao and his co-workers23 have reported near-infrared (NIR) light-, pH-, and redox-responsive nanoparticles formed by the selfassembly of a 3-arm PEG-a-PCL-SS-P(NIPAM-co-DMA) star quaterpolymer in the selective solvent, water. The encapsulated drug in such nanoparticles could be efficiently released in response to NIR light, pH, and reduction stimuli. Furthermore, such triple-responsive nanoparticles also demonstrated synergistic anticancer efficacy in cells. The triggering of NIR light resulted in an increase of the temperature, which further enhanced photoinduced cytotoxicity and boosted pH/reduction-responsive drug release owing to an irreversible phase transition of the P(NIPAM-co-DMA) segment and caused subsequent ROS-mediated drug translocation from lysosomes to cytoplasm. All of these contributed to the remarkable enhancement in anticancer efficacy. In terms of responsive Pickering emulsion, the controllable range of multiresponsive Pickering emulsions would be largely broadened in comparison with that of single responsive Pickering emulsions.24 The demulsification of multiresponsive Pickering emulsion could be achieved by either the simultaneous triggering of several different stimuli or single optimal stimulus. A large number of Pickering emulsions having dual responsiveness, such as pH/ temperature,25−27 temperature/salinity,28 pH/glucose,29 CO2/ light,24 CO2/redox,30 pH/light,31 and light/magnetism32 have been explored up to now. Nevertheless, Pickering emulsions with two more stimuli factors are scarce. Herein, we demonstrated oil-in-water Pickering emulsion having triple stimuli-responsiveness including temperature, UV light, and the competitive guest in this study. Such a Pickering emulsion was stabilized by amphiphilic core cross-linked supramolecular polymer particles (CCSPs), which were prepared by the self-assembly of supramolecular block copolymers, poly(4-vinylpyridine)-b-poly(N-isopropylacrylamide) (P4VP-b-PNIPAM), in the selective solvent, water. P4VP-b-PNIPAM supramolecular block copolymers were synthesized via the inclusion interaction between β-cyclodextrin (β-CD)-terminated poly(N-isopropylacrylamide) hosts with azobenzene-capped poly(4-vinylpyridine) guests in a mixture of dimethyl sulfoxide (DMSO)/H2O. The prepared responsive oil-in-water Pickering emulsion was further applied as an efficient and recyclable platform for the heterogeneous reaction occurring at the oil−water interface. The structures of polymers and CCSPs were carefully characterized, and the responsive demulsification of oil-in-water Pickering emulsion was investigated in detail.

Article

EXPERIMENTAL SECTION

Materials. 4-Hydroxyazobenzene and 1,4-dibromobutane (DBB) were purchased from Aldrich and utilized directly without further purification; 4-vinylpyridine was bought from Aldrich, which was refluxed in the presence of calcium hydride and then distilled under reduced pressure before use. Amantadine hydrochloride (AMH) was purchased from Aldrich, which was adopted directly. Azobisisobutyronitrile (AIBN) was recrystallized twice in ethanol prior to use; Sethyl-S-(α,α-dimethyl-α-acetic acid) trithiocarbonate (EMP) and βCD-terminated poly(N-isopropylacrylamide) (PNIPAM-β-CD) were synthesized in our library according to earlier reports.29,33 All other reagents were provided by commercial suppliers and used directly. Synthesis of S-Ethyl-S′-(2-carboxy-isopropyl)trithioazobenzene (azo-EMP). azo-EMP was synthesized according to the procedures portrayed elsewhere.34 In a typical experiment, EMP (2.24 g, 0.01 mol), 4-hydroxyazobenzene (1.98 g, 0.01 mol), DCC (4.16 g, 0.02 mol), DMAP (0.245 g, 0.02 mol), and 100 mL of dry CH2Cl2 were charged into a dried round-bottom flask. After 2 h of stirring in the ice bath, the flask was transferred to room temperature. The mixture was stirred overnight under a nitrogen atmosphere. After dilution with 200 mL of CH2Cl2, the solution was extracted with a saturated NaHCO3 aqueous solution three times. The organic phase was collected and dried over magnesium sulfate. The raw product was purified by column chromatography (silica gel) using ethyl acetate/ petroleum ether (v/v 2:1) as an eluent. The product was an orange flaky crystal, which was dried in a vacuum oven overnight at room temperature. Yield: 3.13 g (74.2%). 1H NMR (400 MHz, CDCl3, δ, ppm): 1.33−1.38 (t, 3H, CH3), 1.84−1.87 (s, 6H, C(CH3)2), 3.31− 3.38 (q, 2H, CH2), 7.22−7.27 (d, 2H, azobenzene-Hα), 7.46−7.54 (m, 3H, azobenzene-Hγ), 7.87−7.96 (m, 4H, azobenzene-Hβ). 13C NMR (100 MHz, CDCl3, δ, ppm): 13.2, 25.2, 31.5, 55.9, 122, 122.8, 123.9, 129, 131, 150.4, 152.6, 153, 171.6. Synthesis of Azobenzene-Capped Poly(4-vinylpyridine) Homopolymer (P4VP-azo). P4VP-azo was synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization of 4-VP applying azo-EMP and AIBN as the chain transfer agent (CTA) and the initiator, respectively. In a typical experiment, 4-VP (630 mg, 6 mmol), azo-EMP (40.5 mg, 0.1 mmol), AIBN (1.64 mg, 0.01 mmol), and 3 mL of fresh tetrahydrofuran (THF) were charged into a Schlenk tube. After three freeze−pump−thaw cycles, the tube was sealed. After 11 h at 65 °C, the polymerization reaction was terminated by liquid N2. The reaction solution was diluted with THF and then poured into a large amount of anhydrous diethyl ether. The solid polymeric samples were first collected by filtration and then dried at room temperature in a vacuum oven overnight. Preparation of Core Cross-Linked P4VP-b-PNIPAM Supramolecular Block Copolymer Particles (CCSPs). PNIPAM-β-CD (20 mg, 4 × 10−3 mmol) and P4VP-azo (18.4 mg, 4 × 10−3 mmol) were dissolved in 5 mL of DMSO. After 1 h of stirring at room temperature, 15 mL of deionized water was pumped into the solution at a rate of 1 mL/h, followed by stirring at room temperature for another 12 h. Thereafter, DBB (17.2 mg, 8 × 10−2 mmol) was added. After 24 h of stirring at room temperature, the mixture was transferred to a bag and dialyzed against pure water to remove DMSO, affording CCSPs. The final concentration of CCSPs aqueous dispersion was 1.25 mg/mL. Responsive Morphological Transitions of CCSPs. Thermoinduced reversible morphological transitions were implemented by heating/cooling the CCSPs aqueous dispersion; UV light-triggered morphological transition was conducted by irradiating CCSPs aqueous dispersion with UV light (UV-LED light curing device, FUV-6BK) at λ = 365 nm at room temperature. The glass vial containing CCSPs aqueous dispersion was ∼5 cm away from the UV light source. The experiments were executed in the dark. The AMHtriggered morphological transition was performed by the addition of a certain amount of AMH to CCSPs aqueous dispersion. The mole ratio of β-CD to AMH was 1. Preparation of Pickering Emulsion Stabilized by CCSPs. 3 mL of CCSP aqueous dispersion and 1 mL of chloroform oil were added into a 10 mL of glass vial at room temperature. Then, Pickering B

DOI: 10.1021/acs.langmuir.9b02341 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Scheme 1. Synthesis Routes of azo-EMP and P4VP-azo

Figure 1. (A) GPC traces of P4VP40-azo (a) and P4VP110-azo (b) in DMF; (B) 1H NMR spectrum of P4VP40-azo recorded in DMSO-d6 at 20 °C. emulsion was prepared by homogenization (XHF-D high-speed disperser) at a rate of 12 000 rpm for 1 min. The glass vial was immersed in an ice-water bath during homogenization. Responsive Demulsification of Pickering Emulsion Triggered by Temperature, Light, and the Competitive Guest of AMH. Thermo-triggered demulsification of Pickering emulsion was conducted by transferring the glass vial containing Pickering emulsion from room temperature to a water bath at 38 °C. After demulsification, the glass vial was shifted out from the water bath at 38 °C. After the temperature was cooled to room temperature, Pickering emulsion was regenerated by the homogenization of the broken emulsion. UV light-triggered demulsification of Pickering emulsion was fulfilled by the exposure of Pickering emulsion to UV light (UV-LED light curing device, FUV-6BK) at λ = 365 nm at room temperature. Pickering emulsion was ∼5 cm away from the UV light source. The experiment was carried out in the dark. The competitive guest-triggered demulsification of Pickering emulsion were fulfilled by the addition of a certain amount of AMH to Pickering emulsion. The mole ratio of β-CD to AMH was 1 or 3. Interfacial Catalytic Reduction of p-Nitroanisole by NaBH4. Chloroform (2 mL) containing p-nitroanisole (1 × 10−3 mol/L) and 4 mL of Au@CCSPs aqueous dispersion (S1, Supporting Information) were charged into 10 mL of a clean glass vial at room temperature. The content of Au@CCSPs was ∼0.08 wt % versus the total weight of both oil and water for the formation of Pickering emulsion. Pickering emulsion was generated by homogenization making use of an XHF-D high-speed disperser at a stirring rate of 12 000 rpm for 1 min. Then, excess NaBH4 (10−2 mol/L) aqueous solution was added into Pickering emulsion accompanying gentle stirring. After a given time period, the glass vial was immersed into a constant water bath at 38 °C to break Pickering emulsion. The glass vial was removed and the concentration of the remaining pnitroanisole in the oil phase was analyzed with a UV−vis spectrometer. After the removal of oil, another fresh 2 mL of chloroform containing p-nitroanisole (1 × 10−3 mol/L) was introduced into the water phase. Pickering emulsion was regenerated

by homogenization, which was applied for the next catalytic cycle. All of the experiments were performed at 20 °C. Characterization and Tests. NMR spectra were recorded on a 400 MHz Bruker AV-400 NMR spectrometer at room temperature. Two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY) was conducted on a Bruker AV-400 NMR spectrometer at 20 °C adopting DMSO-d6 as the solvent. The number-average molecular weight (Mn) and polydispersity index (PDI) of polymers were analyzed by gel permeation chromatography (GPC) measurements, which were performed on a Waters 1515 GPC instrument in dimethylformamide (DMF) at 80 °C with a flow rate of 1.0 mL/min, narrowly dispersed polystyrene samples were utilized as calibration standards. Dynamic light scattering (DLS) measurements were performed on a Brookhaven BI-200SM instrument equipped with a 50 mW solid-state laser operating at 532 nm at a fixed scattering angle of 90°. The number-average hydrodynamic diameter (Dh,N) of CCSPs was analyzed by a MALVERN Zetasizer Nano ZS instrument. Optical micrographs of Pickering emulsion droplets were collected with an optical microscope (Leica, DM 4500P). Transmission electron microscopy (TEM) measurements were implemented on a Tecnai G2 Spirit Bio twin (FEI, America) at an accelerating voltage of 120 kV. Samples for TEM observations were prepared by depositing a drop of aqueous suspension onto a carbon-coated copper grid. The content of Au nanoparticles in CCSPs was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). UV−vis spectroscopy was performed on a PerkinElmer Lamda-25 UV−vis spectrometer at room temperature.

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RESULTS AND DISCUSSION

Synthesis of P4VP-azo. azo-EMP was synthesized by means of the esterification reaction of EMP with 4hydroxyazobenzene. The resulting P4VP-azo was then applied as CTA for the RAFT polymerization reaction of the 4-VP monomer initiated by AIBN in THF at 65 °C. The overall synthetic route is illustrated in Scheme 1. C

DOI: 10.1021/acs.langmuir.9b02341 Langmuir XXXX, XXX, XXX−XXX

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Langmuir The structure of azo-EMP was confirmed by 1H NMR and C NMR characterization (Figures S1 and S2, Supporting Information). P4VP-azo with different molecular weights were synthesized by the adjustment of the molar ratio of 4-VP to azo-EMP. GPC traces of P4VP-azo exhibited a unimodal peak. GPC analysis indicated that Mn’s values of both the P4VP-azo samples were 6300 and 16 400 g/mol, and PDIs were 1.16 and 1.22, respectively (Figure 1A). Figure 1B displayed a typical 1H NMR spectrum of P4VP-azo, in which the respective resonances including the end groups were distinctly detected. According to the integral area ratio of the resonance signal peak at δ = 8.25 attributed to the protons of pyridine rings to that at δ = 7.51−7.53 ppm corresponding to the protons of azobenzene rings,34 the degree of polymerization (DP) of P4VP was calculated to be ∼40. Mn calculated from 1H NMR analysis was smaller than that from GPC analysis, which was the result of the different hydrodynamic volumes between polystyrene and P4VP-azo with the same molecular weight in DMF. DP of another P4VP-azo homopolymer was ∼110 according to 1H NMR analysis. Both the polymers were described as P4VP40-azo and P4VP110-azo, respectively, and the subscript represented the respective DP. Preparation of CCSPs and Their Responsive Morphological Transitions. It is well-known that there exists inclusion interaction between β-CD and azobenzene.35 P4VP-b-PNIPAM supramolecular block copolymers were synthesized via the inclusion interaction between PNIPAM26β-CD hosts and P4VP40-azo guests in DMSO. PNIPAM26-βCD was synthesized based on the procedures portrayed in our earlier study, which has been carefully characterized by 1H NMR and GPC (Figures S2 and S3, Supporting Information). In the case of the synthesis of P4VP-b-PNIPAM, the molar ratio of β-CD/azo was 1. 2D 1H NOESY was utilized to verify the inclusion interaction between PNIPAM26-β-CD and P4VP40-azo. A known small amount of PNIPAM26-β-CD and P4VP40-azo were dissolved in DMSO-d6 (β-CD/azo = 1, molar ratio). As illustrated in Figure 2, a strong NOESY cross-peak

DMSO solution. It was detected that the transparent solution gradually turned to be opaque with the addition of water, whereas no precipitations were observed during the overall process, indicating successful self-assembly of P4VP40-bPNIPAM26 supramolecular copolymers in the DMSO/water mixture. DLS was employed to monitor the size variation of P4VP40b-PNIPAM26 during the addition of water, and Dh,N values of P4VP40-b-PNIPAM26 in the DMSO/H2O mixture with various volume fractions of water (Vfw) are listed in Table S1(S4, Supporting Information). According to DLS results, Dh,N of P4VP40-b-PNIPAM26 was ∼5.70 nm in the DMSO/H2O mixture with a low Vfw, such as ∼ 9%, suggesting molecular dissolution. Dh,N exhibited an obvious increase and reached ∼14.7 nm at Vfw = ∼33%, and the size was obviously larger than the contour length of a single P4VP40-b-PNIPAM26 chain, implying that P4VP40-b-PNIPAM26 started to self-assemble and formed polymeric nanoaggregates. Dh,N of polymeric aggregates reached ∼230 nm at Vfw = 90%. The P4VP chains aggregated and formed the hydrophobic core, which was stabilized by the hydrophilic PNIPAM shells. A certain amount of DBB was added to the dispersion, and the molar ratio of DBB to the total 4-VP units was 1/2. In the DMSO/H2O mixture (v/v, 1/9), the hydrophobic P4VP core should be a swollen state. The reactive DBB would diffuse into the core and react with 4VP units, resulting in the cross-linking of the P4VP core. Only the intraparticle cross-linking reaction was certain to take place due to the shielding effect of PNIPAM shells. DLS measurement revealed that Dh,N of the polymeric aggregates decreased from ∼230 to ∼176 nm after 24 h of reaction at room temperature. The decline in Dh,N was the result of the core volume shrinkage caused by the cross-linking. After the removal of DMSO through the dialysis against pure water, Dh,N of CCSPs further decreased to ∼136 nm (Figure 3A, trace a). It was found that Dh,N of CCSPs originating from P4VP110-b-PNIPAM26 precursors with the same procedure was ∼149 nm (Figure 3A, trace b). Figure 3B,C demonstrated typical TEM images of CCSPs with different sizes. Spherical CCSPs with a core−shell structure were observed. Since the core was cross-linked by DBB, the existing bromine anion in the core led to high contrast. Based on TEM observation, the average diameters of CCSPs were ∼110 and ∼128 nm, respectively, which were almost adjacent to those from DLS measurements. CCSPs dispersed in water were extremely stable, and no obvious change was observed after 60 days of storage at room temperature (Figure 4A). Nevertheless, the transparent aqueous dispersion instantly became opaque when it was immersed in a water bath at 38 °C (Figure 4B). DLS measurement indicated that Dh,N of CCSPs identified a significant increase from ∼134 to ∼398 nm (Figure 4F). After cooling the temperature of the aqueous dispersion from 38 °C to room temperature, the aqueous dispersion recovered instantly to its original state (Figure 4C). This thermo-induced reversible morphological transition was attributed to the coilglobe transition of PNIPAM shells.36 Once the glass vial containing the aqueous dispersion of CCSPs was irradiated by UV light with λ = 365 nm for ∼40 min in the dark, macroscopic aggregates floating in water were visible to eyes, as typically indicated by arrows in Figure 4D. Upon the removal of UV light, a proportion of macroscopic aggregates could gradually disassociate and recover to the initial state of CCSPs after stirring for a long period of time at the room

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Figure 2. 2D 1H NOESY spectrum of the PNIPAM-β-CD host and the P4VP-azo guest in DMSO-d6.

between the signals at δ = 3.85 ppm belonging to the inner protons of β-CD and the signals at δ = 7.00−7.70 ppm ascribed to the protons of the azobenzene ring was observed in the spectrum of 2D 1H NOESY, revealing the formation of the P4VP40-b-PNIPAM26 supramolecular block copolymer. Thereafter, deionized water was pumped into P4VP40-b-PNIPAM26 D

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Figure 3. (A) DLS traces of CCSPs prepared with different supramolecular block copolymer precursors (a: P4VP40-b-PNIPAM26; b: P4VP110-bPNIPAM26); TEM images of CCSPs formed by P4VP40-b-PNIPAM26 (B) and P4VP110-b-PNIPAM26 supramolecular copolymers (C).

Figure 4. Photographs of CCSPs aqueous dispersion at different temperatures (A: room temperature; B: 38 °C). (C) Photographs of CCSPs aqueous dispersion after cooling the temperature from 38 °C to room temperature. (D) Photographs of CCSPs aqueous dispersion after ∼40 min of UV irradiation in the dark at room temperature. (E) Photographs of CCSPs aqueous dispersion after 24 h of stirring at room temperature in the presence of AMH. (F) DLS traces of CCSPs obtained at 20 °C and 38 °C. (G) DLS traces of CCSPs. a, b, c corresponded to DLS traces of CCSPs after UV irradiation for different time periods (a: 0 min; b: 20 min; c: 40 min); d was DLS trace of CCSPs that were first subjected to ∼40 min of UV irradiation in the dark, and then 168 h of stirring under the visible light at room temperature. (H) DLS traces of CCSPs after different time periods of stirring in the presence of AMH at room temperature. The molar ratio of AMH to β-CD was 1.

chains were then grafted. With the prolonging of the time, more and more azobenzene groups embedded in the macroscopic aggregates gradually changed from cis-configuration to trans-configuration, and the grafting of more PNIPAM chains led to the disassociation of macroscopic aggregates. DLS analysis revealed that ∼80% of macroscopic aggregates could be disassociated after 7 days of stirring at room temperature under the visible light, as shown in Figure 4G (trace d). AMH-triggered disassociation of CCSPs was executed by the addition of 10 μL of AMH aqueous solution (1.95 × 10−2 mol/L) to 1.5 mL of the aqueous dispersion of CCSPs (AMH/ β-CD = 1, molar ratio). After 24 h of stirring at the room temperature, macroscopic aggregates floating in water were clearly observed, as illustrated in Figure 4E. DLS measurements revealed that Dh,N of CCSPs progressively increased with the prolonging of the time (Figure 4H), and this was

temperature. This was the outcome of the configuration change between trans-azobenzene and cis-azobenzene. Previous studies37,38 have displayed that β-CD had a high binding affinity to trans-azobenzene in aqueous solution and low, if any, binding affinity to cis-azobenzene. Upon irradiation by UV light with λ = 365 nm, the formed β-CD/azobenzene inclusions disassociated accompanying the transition of transazobenzene to cis-azobenzene. As a result, hydrophilic PNIPAM shells gradually separated themselves from the surfaces of CCSPs and entered the water phase. Without the stabilization of hydrophilic PNIPAM shells, the hydrophobic cross-linked P4VP cores aggregated and formed macroscopic precipitation in water, resulting in a significant increase in bDh,N (Figure 4G). After the removal of UV−vis light, cisazobenzene groups on the surfaces of the formed macroscopic aggregates changed first back to trans-azobenzene upon the exposure to the visible light, and the hydrophilic PNIPAM E

DOI: 10.1021/acs.langmuir.9b02341 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Scheme 2. Schematic Illustration of Thermo-, UV light-, and AMH-Triggered Morphological Transitions of CCSPs

attributed to the competitive inclusion between azobenzene and AMH with β-CD. Since the inclusion constant between βCD and azobenzene was 770 M−1, which was much smaller than that between β-CD and AMA (105 M−1).39 As a consequence, azobenzene groups entrapped in the cavities of β-CDs were gradually extruded by AMH. The detachment of hydrophilic PNIPAM shells from the surfaces of CCSPs contributed to the aggregation of hydrophobic cross-linked P4VP cores. The reason was almost the same as that caused by UV light irradiation. AMH-induced macroscopic aggregation, however, was irreversible. Triple-responsive morphological transitions of CCSPs are summarized in Scheme 2. P4VP-b-PNIPAM CCSPs Stabilized Pickering Emulsion with Triple-Responsive Demulsification. P4VP-b-PNIPAM CCSPs with responsive morphological transitions allowed them to be promising responsive Pickering emulsifiers. A series of oils including hexane, heptane, cyclohexane, toluene, xylene, styrene, benzene, and chloroform were employed for the formation of Pickering emulsion. The emulsifying outcomes suggested that stable oil-in-water Pickering emulsions were generated using styrene, benzene, and chloroform as the oil phase, respectively (Figures S3 and S5, Supporting Information). This phenomenon could be interpreted with the solubility parameter. The solubility parameter of P4VP was reported to be 21.9 MPa1/2,40 which was close to that of styrene, benzene, and chloroform. The cross-linked P4VP core, therefore, could be swollen by them, bringing about stable adsorption of CCSPs at the oil−water interface. However, oils like hexane, heptane, cyclohexane, toluene, and xylene were all poor solvents of P4VP and PNIPAM, and the cross-linked vitrified P4VP core of CCSPs could not be wetted by these oils, leading to unstable adsorption of CCSPs at the oil−water interface. For the adsorbed CCSPs at the oil−water interface, hydrophilic PNIPAM shells stretched them toward the water phase, and the hydrophobic cross-linked P4VP was located at the oil− water interface. After the solidification of the emulsion droplets, the adsorbed CCSPs on the surfaces of droplets were explicitly observed by SEM and TEM (Figures S4 and S6, Supporting Information). Figure 5A,B (insets) shows the photographs of chloroform-in-water Pickering emulsion stabilized by P4VP40-b-PNIPAM26 and P4VP110-b-PNIPAM26 CCSPs, in which the content of CCSPs was 0.08 wt % relative to the total weight of oil and water. It was observed that

Figure 5. Optical microscopy images of emulsion droplets stabilized by 0.08 wt % of CCSPs with different sizes (A: 110 nm; B: 128 nm) at room temperature. (C) Photograph of Pickering emulsion stabilized by 0.08 wt % of CCSPs with 110 nm. (D) Thermo-triggered broken Pickering emulsion (temperature: 38 °C). (E) Photograph of the regenerated Pickering emulsion after cooling the temperature from 38 °C to room temperature. (F) Optical microscopy image of the regenerated chloroform-in-water Pickering emulsion stabilized by CCSPs that were subjected to four thermo-induced emulsification/ demulsification cycles (inset: Photograph of Pickering emulsion). The volume ratio of chloroform/water was 1/3; scale bar: 25 μm.

chloroform oil could be totally emulsified. The emulsion layer height was ∼35% relative to the total height of the water and oil phases. The mean size of emulsion droplets for both Pickering emulsions was ∼20.6 μm (Figure 5A,B). Next, the demulsification of Pickering emulsion stabilized by 0.08 wt % of P4VP40-b-PNIPAM26 CCSPs in response to thermo, light, and AMH was investigated. The thermo-induced demulsification of Pickering emulsion was conducted by the immersion of Pickering emulsion illustrated in Figure 5C into a constant water bath at 38 °C accompanying gentle shaking by hand. Pickering emulsion has been entirely demulsified within ∼20 min (Figure 5D). After the temperature was cooled to room temperature, stable chloroform-in-water Pickering emulsion could be regenerated by the homogenization of the broken emulsion, as displayed in Figure 5E. Experimental F

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Figure 6. (A) Photographs of Pickering emulsion stabilized by 0.08 wt % of CCSPs at room temperature. (B) Photograph of Pickering emulsion indicated in A after 20 min of exposure to UV light with λ = 365 nm at room temperature. (C) Photograph of Pickering emulsion displayed in A after ∼40 min of exposure to UV light with λ = 365 nm at room temperature. (D, E) Optical microscopy images of emulsion droplets illustrated in A and B. (F) Photograph of Pickering emulsion stabilized by 0.08 wt % of CCSPs at room temperature. (G) Photograph of Pickering emulsion indicated in F after 15 h of endurance in the presence of AMH (AMH/β-CD = 3:1, molar ratio). (H) Photograph of Pickering emulsion revealed in F after 24 h of the endurance in the presence of AMH (AMH/β-CD = 3:1, molar ratio). (I, J) Corresponding optical microscopy images of emulsion droplets presented in F and G; scale bar: 25 μm.

Interfacial Catalytic Reduction of p-Nitroanisole by NaBH4. Owing to its high stability, large interfacial area, and responsive demulsification, P4VP40-b-PNIPAM26 CCSP-stabilized chloroform-in-water Pickering emulsion was an ideal platform for a wealth of heterogeneous reactions occurring at the oil−water interface. Besides high reaction efficiency, the responsive demulsification would be conducive to the separation of products and the recyclability of emulsifiers. The reduction reaction of p-nitroanisole to p-anisidine catalyzed by Au nanopaticles42 was chosen as a model reaction to demonstrate these potential advantages. The catalysts of Au nanoparticles were embedded in the lightly cross-linked P4VP cores of P4VP40-b-PNIPAM26 CCSPs. The detailed procedures for the preparation of Au@CCSPs are shown in the supporting information (S1, Supporting Information). TEM observation revealed that Au@CCSP hybrids were regularly spherical structures, and Au nanoparticles embedded in P4VP cores were clearly discerned (Figure 7A,B). The presence of the Au element was also supported by the energy-dispersive X-ray (EDX) spectroscopy of Au@CCSPs (Figure 7C). The Au content in the hybrids was ∼0.21% according to ICP-AES analysis. Chloroform-in-water Pickering emulsion stabilized by ∼0.08 wt % of Au@CCSPs is presented in Figure 7D (inset), and the mean size of the emulsion droplets was ∼21 μm. Due to the adsorption of Au@CCSP hybrids, Au nanoparticle catalysts were located at the water−oil interface. Under the catalysis of Au nanoparticles, the hydrophobic p-nitroanisole dissolved in the chloroform phase was reduced by the water-soluble NaBH4, and the reaction occurred smoothly at the oil−water interface. The conversion of p-nitroanisole as a function of the reaction time is plotted in Figure 7D. The conversion of pnitroanisole reached ∼88% after 120 min of reaction based on UV spectroscopy analysis for the first reaction cycle (curve a). After thermo-triggered demulsification, the separated oil phase was carefully removed from the broken emulsion. Subsequently, the fresh chloroform containing p-nitroanisole (1 × 10−3 mol/L) was added. After homogenization, the second reaction cycle started. The conversion of p-nitroanisole against the reaction time curve is demonstrated in Figure 7D (trace of

results implied that the emulsifying performances of CCSPs almost remained constant after multiple emulsification/ demulsification cycles. Figure 5F revealed that the regenerated chloroform-in-water Pickering emulsion that was stabilized by CCSPs underwent four thermo-induced emulsification/demulsification cycles. The regenerated Pickering emulsion was almost the same in both the height of the emulsion layer and the average size of the emulsion droplet as the original one indicated in Figure 5A. This thermo-induced reversible emulsification/demulsification cycle was the result of the reversible coil-globule transition of PNIPAM chains. The collapse of PIPAM chains at the oil−water interface led to the rapid decline of the surface coverage,41 promoting the coalescence of the dispersed oil droplets. UV-induced demulsification was implemented by the exposure of Pickering emulsion to UV light with λ = 365 nm. It was found that the emulsion layer progressively disappeared with the prolonging of the irradiation time. Approximately 80% of the emulsion layer of the original one (Figure 6A) has been broken within ∼20 min, as shown in Figure 6B. The average size of the droplets of the remaining emulsion increased from ∼20 to ∼24 μm (Figure 6D,E). The total disappearance of the emulsion layer was attained after ∼40 min (Figure 6C). The AMH-triggered demulsification could be achieved by the addition of a known amount of AMH solution to Pickering emulsion. The results revealed that the time for the demulsification of Pickering emulsion was related to AMH/β-CD (molar ratio). The demulsification was fully fulfilled within 48 h when the molar ratio of AMH/β-CD was 1. When the molar ratio of AMH/β-CD was improved to 3, most of the original emulsion broke within 15 h (Figure 6F,G). The average size of droplets of the remaining emulsion increased from ∼20 μm (Figure 6I) to ∼34 μm (Figure 6J). The complete demulsification could be ended within 24 h (Figure 6H). Both of the demulsification were derived from the UV light− and AMH−triggered detachment of PNIPAM shells of CCSPs from the cross-linked P4VP core. The unbalanced hydrophilicity−hydrophobicity made cross-liked P4VP cores desorbed from the oil−water interface, causing the dispersed droplets to coalesce rapidly. G

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good platform for the heterogeneously catalytic reduction reaction of p-nitroanisole by NaBH4. The reaction was conducted with an efficient and sustainable pattern.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b02341.

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Figure 7. (A, B) Typical TEM images of Au@CCSPs hybrids. (C) Energy-dispersive X-ray (EDX) spectroscopy of Au@CCSP hybrids. (D) Conversion of p-nitroanisole against the reaction time curve (a: the first cycle; b: the second cycle; c: the third cycle); Inset of D: optical microscopy image of emulsion droplets and the photograph of chloroform-in-water Pickering emulsion stabilized by 0.08 wt % of Au@CCSPs hybrids; scale bar: 25 μm.

NMR characterization of azo-EMP; 1H NMR and GPC characterization of PNIPAM26-β-CD; 2D 1H NOESY proof for the formation of supramolecular P4VP-bPNIPAM block copolymer; Styrene-in-water and benzene-in-water Pickering emulsions stabilized by CCSPs; TEM proof for the adsorption of CCSPs at the oil−water interface and preparation of Au@CCSPs (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Huaming Li: 0000-0003-4575-9945 Yong Gao: 0000-0001-7962-7196 Notes

The authors declare no competing financial interest.

b), ∼80% of conversion was attained after 3 h. The similar procedures were adopted for the third catalytic cycle. The conversion of p-nitroanisole was found to be less than 60% after 4 h of reaction, as indicated in Figure 7D (trace of c). All the same, the reaction efficiency at the oil−water emulsion interface was far higher than that at the plane oil−water interface. Control trial showed that the conversion of pnitroanisole was not more than 10% after 5 h when the same reaction was conducted at the plane chloroform−water interface (data not shown). Neglecting the experimental error, the decrease in conversion should be mainly caused by the leakage of Au nanoparticles during the reaction cycle. AESICP revealed that the content of Au nanoparticles decreased from ∼0.21% to ∼0.16% after the first reaction cycle, which further declined to ∼0.13% after the second reaction cycle.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (21574112 and 21971220) and the Project of Xiangtan University (2017XZZ36).

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REFERENCES

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CONCLUSIONS By virtue of host−guest inclusion interaction, amphiphilic supramolecular diblock copolymers, P4VP-b-PNIPAM, were successfully synthesized. CCSPs were prepared through the combination of self-assembly of P4VP-b-PNIPAM in solution and the chemical cross-linking of the P4VP core with DBB. CCSPs exhibited thermo-, light-, and competitive guestresponsive morphological transitions. Employing CCSPs as particulate emulsifiers, stable oil-in-water Pickering emulsions could be generated at a low content of particulate emulsifiers, in which oils that could swell the cross-linked P4VP core, such as chloroform, St, and benzene were suitable for the disperse phase. The responsive morphological transitions endowed the generated chloroform-in-water Pickering emulsion with thermo-, light-, and AMH-responsive demulsification. By means of the complexation interaction between 4VP and AuCl4−, Au nanoparticles were embedded in the lightly crosslinked P4VP cores of CCSPs. The Au@CCSP hybrids stabilized chloroform-in-water Pickering emulsion provided a H

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