Thermally Tunable Pickering Emulsions Stabilized by Carbon-Dot

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Thermally Tunable Pickering Emulsions Stabilized by Carbon-dots Incorporated Core-shell Nanospheres with Fluorescence “On-off” Behavior Jianqiang Chen, Chenyang Zhu, Zhen Yang, Yiying Yue, Ping Wang, and Takuya Kitaoka Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03490 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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Thermally Tunable Pickering Emulsions Stabilized by Carbon-dots Incorporated Core-shell Nanospheres with Fluorescence “On-off” Behavior Jianqiang Chen,*,† Chenyang Zhu,† Zhen Yang,*,‡ Yiying Yue,† Ping Wang,† Takuya Kitaoka *,ξ † Laboratory of Advanced Environmental & Energy Materials, Department of Environment Engineering, Nanjing Forestry University, 159 Longpan Road, Nanjing 210037, P.R. China ‡ School of Chemistry and Materials Science, Jiangsu Provincial Key Laboratory of Material Cycling and Pollution Control, Nanjing Normal University, 1 Wenyuan Road, Nanjing 210023, P.R. China ξ Department of Agro-Environmental Sciences, Graduate School of Bioresource and Bioenvironmental Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan E-mail: [email protected] (Jianqiang Chen); [email protected] (Zhen Yang); [email protected] (Takuya Kitaoka)

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ABSTRACT Lack of deep understanding of nanoparticles (NPs) actions in oil/water interface set an obstacle to practical applications of Pickering emulsions. Fluorescence labels by incorporation of carbon dots (CDs) into poly(N-isopropylacrylamide) (PNIPAM) matrix can not only mark the action of PNIPAM-based NPs in the interface, but also reflect the colloidal morphologies of PNIPAM. In this work, we employed coaxial electrospraying for fabricating core-shell nanospheres of cellulose acetate encapsulated by PNIPAM and facile incorporation of CDs in PNIPAM shell was achieved simultaneously. The coaxial electrosprayed nanoparticles (CENPs) with temperature-dependent wettability can stabilize heptane and toluene in water at 25 °C, respectively, and reversible emulsion break can be triggered by temperature adjustment around LCST. Remarkably, CENPs/CDs composites exhibited a fluorescence “on-off” behavior due to the volume phase transition of PNIPAM shell. CENPs/CDs composites in Pickering emulsions clearly elucidated the motions of CENPs in response to temperature change. At temperatures below LCST, CENPs concentration played an important role in surface coverage of oil droplets. Specifically, CENPs concentration above the minimum concentration for complete emulsification of oil phase led to high surface coverage and two-domain adsorption of CENPs at the interface including primary monolayer anchoring of CENPs on droplets surrounded by interconnected CENPs networks, which contributed to the superior stability of the emulsions. Moreover, CENPs/CDs composites can be recycled with well-preserved core-shell structure and stable fluorescent properties, which offers their great potential applications in sensors and imaging. KEYWORDS: carbon dots, core-shell nanoparticles, coaxial electrospraying, Pickering emulsion, thermo-responsive, fluorescence labels 2


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INTRODUCTION In recent years, emulsions kinetically stabilized by colloidal micro-/ nanoparticles (NPs), termed Pickering emulsions, have attracted increasing attentions. The use of particles as stabilizers provides emulsions with superior stability and lower toxicity compared with conventional surfactant-stabilized emulsions. 1-3 Moreover, it provides useful templates for preparing microcapsules, 4,5 Janus microgels 6,7 and porous materials. 8,9 Meanwhile, rapid development of materials technology prompted the creation of an increasing variety of particles, such as graphene-derived particles, 10,11 biomass-based particles, 12-14 stimuli-responsive particles. 15-25 However, it remains a huge challenge to understand the mechanism of NPs actions in oil/water interface, particularly their colloidal nanostructure in response to interface-driven stability, which is regarded as one of the reasons for absence of Pickering emulsifiers in commercial formulation used for human biomedicine. 2 Poly(N-isopropylacrylamide) (PNIPAM), a classical thermo-responsive polymer, has intrinsic ability of manipulating its colloidal surface conformation in response to a small temperature variation (1~2 °C) around the low critical solution temperature (LCST). 26-29 It is well known that direct observation of PNIPAM microgels at the oil/water interface is rather difficult using conventional optical microscopy because of the very weak refractive index mismatch between the swollen microgels and the aqueous phase. 30 With recent fluorescence labeling techniques, incorporation of fluorescence labels in PNIPAM matrix could shed light on the mechanism of the particle actions in the system. Semiconductor quantum dots (QDs) with strong and tunable fluorescence have been intensively applied in bio-imaging, bio-sensing and energy harvesting. 31-33 Interestingly, incorporation of semiconductor QDs in PNIPAM nano-carriers could therefore provide new generation of fluorescence labels that can 3


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detect temperature change and make it visible by a rapid decline of fluorescent intensity. 34-39 However, due to high toxicity of semiconductor QDs, the thermo-responsive fluorescence labels were subjected to severe limitations in biomedical and environmental applications. Carbon dots (CDs) distinguish themselves from other carbon materials due to their optical properties and fluorescence emissions. 40-43

In contrast to semiconductor QDs, CDs possess the advantages of low toxicity

and excellent biocompatibility, and therefore have the potentials to replace semiconductor QDs for preparing thermo-responsive fluorescence labels. 44-46 To the best of our knowledge, thermo-responsive fluorescence labels used for Pickering emulsions have not reported yet. From the preparation methodology point of view, the incorporation of fluorescence labels in PNIPAM matrix were usually achieved by either polymerization of NIPAM monomers in the presence of fluorescence labels, 34, 36, 44, 45

or incubation reaction, 35, 38, 46 which inevitably led to complicated process, low

loading capacity of fluorescent markers, and environmental pollution. Therefore, it is of great significance to fabricate the fluorescence labels via a facile, highly efficient and less time-consuming strategy with the minimum environmental impact. Electrospraying is a simple and effective strategy for fabricating nanofibrous structures and materials with desirable engineered properties. 47An important application of electrospraying is to fabricate monodisperse spherical NPs which have been intensively investigated for biomedical applications. 48-50 More remarkably, core-shell structured NPs can be facilely fabricated by electrospraying two immiscible polymer solutions through a coaxial nozzle. 51, 52 Compared to conventional chemistry approaches, coaxial electrospraying provides not only an one-step strategy of fabricating core-shell NPs, but also well-designed core-shell nanostructures regarding shape, size and shell thickness-to-radius ratio by adjusting electrospraying conditions. 4


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The incorporation of CDs in PNIPAM-coated NPs could be efficiently realized by

coaxial electrospraying. On the other hand, cellulose acetate (CA) has been widely used to produce cellulosic nanocomposites due to its biocompatibility and biodegradation. 54 Used for producing CA/PNIPAM hybrid fibers by electrospinning, CA was found to strongly interact with PNIPAM by intermolecular hydrogen-bonding which could definitely contribute to interfacial stability of PNIPAM-coated NPs when employing CA as core material. 55 In this study, with an aim of preparing fluorescent Pickering emulsifiers with optical thermo-responsiveness through a facile manner, we employed coaxial electrospraying to fabricate PNIPAM-coated NPs and the incorporation of CDs into PNIPAM matrix was achieved simultaneously. Coaxial electrosprayed nanoparticles (CENPs)/CDs composites performed as thermo-responsive stabilizers meanwhile exhibited optically thermo-responsive fluorescent property for elucidating the mechanism of Pickering emulsion. EXPERIMENTAL SECTION Materials N-isopropylacrylamide (Aladdin Co. Ltd.) was recrystallized three times in benzene/hexane mixture (the volume ratio at 1:1). PNIPAM was prepared in laboratory via free radical polymerization and then carefully fractionated by successive dissolution-precipitation cycles. The Mw of PMIPAM was 1.2×105 g/mol with Mw/Mn value was 1.63 as determined by size exclusion chromatography. Cellulose acetate of 65 wt% acetyl content was purchased from Sinopharm Chemicals Co. Ltd.. Citric acid (99.5 %; Sigma-Aldrich) and ethylenediamine (99 %; Sigma-Aldrich) were used without purification. Acetone, dimethylacetamide (DMAC), n-heptane, toluene and octanol were purchased from Nanjing Chemicals Co. Ltd. and 5


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used as received. Synthesis of CDs Carbon dots were synthesized by hydrothermal method. 43 1.0 g citric acid (5 mmol) and 335 µL ethylenediamine (5 mmol) were dissolved in 10 mL deionized water and then the solution was transferred to a polytetrafluoroethylene (Teflon)-lined autoclave (25 mL) and heated at 200 °C for 6 h. After the reaction, the reactor was cooled to room temperature naturally. The resultant brown-black and transparent solution was transferred to dialysis bag to remove the large particles. The CDs were obtained after the evaporation of water. Preparation of Core-shell NPs via Coaxial Electrospraying Core-shell NPs were one-step prepared via coaxial electrospraying and the setup is illustrated in Scheme 1. To make the inner and outer nozzles placed concentrically, we designed a coaxial needle in a bullet-shape structure (Figure S1). The outer nozzle is fixed at the center of a bullet-shape sheath where the inner nozzle was screwed up. Two stainless needles of 26G (240 µm) and 26G (450 µm) diameter were fixed concentrically to form an annular nozzle. Shell-solution was prepared by dissolving PNIPAM (0.3 wt%) and CDs (0.015 wt%) in the binary solvents of acetone and DMAC with the volume ratio of acetone to DMAC at 2:1. Core-solution was prepared by dissolving CA (0.95 wt%) in the same binary solvents as shell-solution. The shelland core-solution loaded in two plastic syringes (150 mL) were pumped by microsyringe pumps (Longer Pump, LSP01-2A) with feed rates fixed at 0.1 and 0.3 mL/h, respectively. The needle-to-collector distance was fixed at 15 cm and voltage was tuned in the range of 9.0~12.5 kV. A stainless steel sheet was placed to collect the CENPs. To prevent any possible dissolution of PNIPAM when CENPs used as emulsifiers, CENPs were exposed to UV radiation for 3 h in a SpectroLinker TM 6


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XL-1500 UV crosslinker equipped with six 15 W UV (254 nm) lamps. Preparation of Pickering emulsions CENPs aqueous suspensions were initially prepared at a series of concentrations (CCENP) i.e. 1.0~15.2 mg/mL with assistance of ultrasound. All emulsions were prepared with 5 mL CENPs aqueous suspension and 5 mL organic solvent as oil phase. The emulsification was carried out in a homogenizer (Ultraturrax T18, IKA) at 5000 rpm for 2 min. The resultant emulsion was placed into a thermostat and the stability of emulsion was determined by changes in volume fractions of water, emulsion phase and oil phase from 60 min after emulsification. Thermo-responsiveness of the emulsion was investigated by raising the temperature of thermostat from 25 to 45 °C and then cooling down to 25 °C. Recycling of CENPs was carried out by filtering the emulsions with a glass sand funnel (G6) and then flushing the residues with deionized water. The recycled CENPs were freeze-dried and weighted to calculate the recycle rate. Characterization The morphology of CENPs was investigated using a field emission-scan electron microscope (FE-SEM; JEOL, JSM-7600F). Core-shell nanostructure was observed on high-resolution transmission electron microscope (HRTEM; JEOL, JEM-2100 UHR). Besides, the core-shell structure of CENPs was optically checked on a confocal laser scanning microscope (Carl Zeiss, LSM 710 META). Fluorescent dye of coumarin-6 (green dye) and rhodamine B (red dye) were dissolved in CA and PNIPAM solution, respectively. Both fluorescent concentrations were kept at 0.01 wt%. Static water contact angle (SWCA) measurements were conducted using sessile drop method (3 µL water droplet) with a Rame-Hart-100 contact angle goniometer. A CENPs-coated film was fabricated simply by coaxial electrospraying CENPs on an aluminum foil. 7


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The film was placed on a heating panel that was precisely controlled from 25 to 45 °C and the images of the water drop on the film were recorded. According to different temperatures, size distributions of CENPs in aqueous suspension (CCENP=3.5 mg/mL) were tested on a nanoparticle characterization system (Malvern, ZEN3600) at 25 and 35 °C and on a micrometer-particle size distribution analyzer (Horiba, LA-910) at 45 °C. Emulsion drops were optically observed by CLSM and optical microscope (Olympus, BH53) equipped with fluorescent modules of UV, blue, green light excitation and a hot-stage device (Linkam, THMS 600) to precisely control the temperature of emulsion drops. The fluorescent emission spectra were recorded on a PE LS55 spectrophotometer coupled with a circulator bath to control the temperature of cuvette. CENPs in emulsion drops were observed by an environmental scanning microscope (ESEM; FEI Quanta 200). In addition, the morphology of recycled CENPs was also observed on FE-SEM and HRTEM. RESULTS AND DISCUSSION Core-shell NPs via coaxial electrospraying and incorporation of CDs into PNIPAM matrix The strategy of fabricating core-shell NPs with CDs incorporated by coaxial electrospraying was illustrated in Scheme 1. Before coaxial electrospraying, miscibility of core and shell solution was tested by slowly dripping CA solution containing green dye (coumarin-6) into transparent PNIPAM solution. CA drops hardly diffused and gradually settled down to the bottom of PINPAM solution. Unless agitated, the interface of CA drops remained for more than 5 min. The immiscibility of two solutions indicates a huge possibility of a core-shell structure forming by coaxial electrospraying.56 Under the given conditions as described in experimental section, CENPs were fabricated in stable cone-jet 8


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mode. After collected on the stainless steel sheet, CENPs were exposed by UV irradiation to make the linear PNIPAM crosslinked without additional initiators. 37, 56 Fluorescent properties of CDs would not be apparently affected due to excellent UV resistance ability of CDs. 43, 44 Crosslinking density plays an important role in the swelling properties of core-shell nanoparticles. Different crosslinking density caused by UV irradiation time led to different equilibrium swollen volume of the nanoparticles. Theoretically, the crosslinking density of CENPs can be directly reflected by the average molecular weight of PNIPAM chain between two crosslinking points ( M c ). The relation between UV irradiation time and swollen volume of CENPs by the calculation of M c are shown in Table S1 (see Supplementary Information). SEM image (Figure 1a) illustrates that all CENPs exhibited in regular spherical shape with average diameters around 100 nm. From the inset image, CENPs had very uniform and compact surface. In order to check the reproducible core-shell formation of all CENPs, we applied confocal microscope to observe the fluorescent dyes labeled CENPs. The fluorescent image obtained on confocal laser microscope (Figure 1b) shows the core particles in green fluorescence (coumarin-6) of all CENPs were confined to the shell area of red fluorescence (rhodamine B) to form obvious core-shell structures which indicated the encapsulation of CA core particles by PNIPAM shell for all CENPs observed. HRTEM image of two CENPs in comparable size (Figure 1c) illustrates a uniform core-shell structure of distinct core component tightly encapsulated by shell, which confirmed the core-shell structure reflected by the confocal microscope image. For the CENP with a prefect core-shell structure (Figure 1d), it is precise to calculate the ratio of thickness-to-radius (T/R). HRTEM images 9


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of other CENPs with T/R values were also provided (Figure S2). Note that all T/R values approximated to 0.2 which revealed the high uniformity of core-shell nanostructure obtained by coaxial electrospraying. Strong interfacial adhesions between PNIPAM and CA were reflected by the seamless morphology of core and shell components (Figure 1e). Intermolecular hydrogen-bonding between PNIPAM and CA possibly accounts for the interfacial interactions. 55 Thereby, the uniform core-shell structure of CENPs and strong interfacial adhesions between core and shell components could be the merits for the use of CENPs as emulsifiers. The morphology of CDs synthesized in this study was recorded on a HRTEM (Figure 2a). Many dark spots can be seen to uniformly disperse on a Cu grid without apparent aggregation, revealing that the CDs prepared were uniform spherical nanoparticles. DLS results (Figure S3) illustrate that CDs exhibited mono-dispersed in aqueous solution with average size of 4.7 nm similar to the CDs synthesized through the same approach. 43 After blending PNIPAM and CDs in acetone/DMAC for preparing shell solution, no phase separation occurred, which demonstrated that CDs and PNIPAM are chemically compatible in acetone/DMAC. During the coaxial electrospraying, the addition of CDs did not affect the stability of Taylor-cone. As a result, the core-shell structure of CENPs was well preserved after the incorporation of CDs into CENPs and homogeneous morphology within the shell region can be observed (Figure 2b). From the zoomed-in image (Figure 2c), it can be seen that CDs were apparently embedded in PNIPAM matrix. In addition, given that pure CDs in aqueous solution emitted blue-green light under UV light, successful incorporation of CDs into PNIPAM matrix was proved by the fluorescent CENPs/CDs composites observed on a microscope 10


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under UV light excitation (Figure 2d). CENPs/CDs composites emitting different colors were also provided (Figure S4). Thermo-responsiveness of CENPs When suspended in aqueous solution at 25 °C, colloidal CENPs were mono-dispersed with average diameter of 480 nm as shown in Figure 3. The larger size of colloidal CENPs than dry CENPs was due to swelling of crosslinked PNIPAM coating on CA below LCST in the aqueous solution. As aforementioned, the volume of swollen CENPs is connected with crosslinked density of PNIPAM shell. Such swollen morphologies of crosslinked PNIPAM will be demonstrated later by HRTEM and SEM images of recycled CENPs. As temperature rising near the LCST at 35 °C, the average diameter of CENPs declined to 310 nm indicating the crosslinked PNIPAM already transformed into a shrunk state. When temperature increased at 45 °C, the CENPs suspension became apparently cloudy indicating that aggregation of CENPs had already occurred. In this case, inhomogeneous size distribution in the inset of Figure 3 confirmed the formation of CENPs aggregates in micrometer size. The thermo-responsive volume of colloidal CENPs resembles the volume phase transition of crosslinked PNIPAM hydrogels induced by temperature rising above LCST. 34, 36, 44, 45 The wettability of CENPs at various temperatures was also studied. The profiles of a water droplet on CENPs-coated film in the range of 25~45 °C were recorded (Figure 4). With the temperature rising from 25 to 45 °C, the initially collapsed water drop on the CENPs-coated film became a complete sphere which indicated that CENPs with surface transformed from hydrophilic to highly hydrophobic wetting. Moreover, SWCA results increased from 9.33° at 25 °C to 115.35° at 45 °C, demonstrating a notable hydrophilic-to-hydrophobic transition. The 11


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thermo-responsive behaviors of CENPs verified the structural design of optically thermo-responsive labels (Scheme 2). Pickering emulsions stabilized by CENPs Using CENPs as emulsifiers, three types of emulsions with different organic solvents (i.e. n-heptane, toluene and octanol) as oil phase were prepared. Under the circumstance of the same CCENP (6.0 mg/mL), the properties of each emulsion upon various temperatures are summarized in Table 1. In the case of heptane, emulsions prepared with a series of CCENP were systematically studied. In order to form complete emulsification, a minimum CCENP is required around 3.5 mg/mL. With a higher CCENP, i.e. 6.0 mg/mL, the resultant emulsion presented remarkable stability (Figure 5), and the emulsion volume fraction remained unchanged at 25 °C for 40 days. According to the results of SWCA test, an oil-in-water emulsion is expected to form in this study which was confirmed by drop test.19 The value of 9.33° at 25 °C is different from the optimal SWCA of particles around 90 ° for the highest stabilization of Picking emulsion, based on Equation (1) for the expression of the maximum desorption energy E of particles.

E =π rp 2γ ow（1 ± cosθ）2 where

(1)

γ ow represents the tension of the interface and rp represents the radius of a

spherical particles. However, Kaptay et al. reported that foams and emulsions stabilized by 3D network of particles can be stable at any contact angle. 57 For the emulsions stabilized by CENPs, two-domain adsorption including monolayer anchoring on oil droplets surrounded by a region of interconnected CENPs at oil/water interface were visually observed. Size distributions of heptane droplets under four CCENP, i.e. 3.5, 6.0, 10.3 and 15.2 mg/mL are shown in Figure 6. It is noted 12


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that once complete emulsification was accomplished, heptane droplets presented monodisperse at each CCENP. A general agreement has been reached that larger particle concentration led to smaller droplet size. 2, 30 Some consistent results were obtained that the average droplet size (Dmean) of 51.875 µm with CCENP at the minimal for complete emulsification (3.5 mg/mL) is apparently larger than that at a higher CCENP (6.0 and 10.3 mg/mL). For CCENP at 15.2 mg/mL, however, unexpected larger Dmean of 45.817 µm was obtained. On the other hand, under the observation of optical microscope, heptane droplets in compact distance were found to form clusters at such high particle concentration. Considering a large span of the size distribution (1.664) for CCENP at 15.2 mg/mL including small size range down to 5 µm, the distribution in large diameter range possibly reflected the droplet clusters by bridging phenomena of CENP networks. 58 Thermal responsiveness of the emulsion was investigated by optically observing the emulsion volume changes in heating and cooling process. Accordingly, the volume ratios of each phase at different temperatures were compared. Upon heating, notable changes were observed at temperatures above LCST that emulsion phase collapsed and then coalesced into the upper oil phase (Figure 5). The volume fraction of emulsion phase reduced from 60 % at 25 °C to 10 % at 38 °C. At 45 °C, the emulsion phase almost disappeared followed by the reforming of oil/water interface. In cooling process, stable emulsion can be regenerated after homogenization at 25 °C. For the case of toluene used as organic phase, complete emulsification of toluene was realized with the minimum CCENP at 5.1 mg/mL, and similar thermo-responsive property was observed (Figure S5). The fact that the emulsion could remain stable for 21 days suggested the inferior stability of toluene/water emulsion compared to heptane/water 13


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emulsion at the same CCENP. However, CENPs were not able to stabilize octanol in water at 25 °C. According to Equation (1), by using particle radius rp=3.8×10-7 m and respective θ and γow value at 25 °C in the previous report,20 calculated energy E of CENPs were 5×10-14 and 0.9× 10-15 J for heptane/water and toluene/water emulsion, respectively, which confirmed the superior stability of heptane/water emulsion stabilized by CENPs. Given that direct observation of PNIPAM particles at the oil/water interface is rather difficult using a conventional optical microscopy, we were capable of observing the motions of fluorescent CENPs in heptane/water interface by means of confocal laser scanning microscope (CLSM) using CENPs/CDs composites. Heptane/water emulsions stabilized by CENPs/CDs composites at 3.5 and 6.0 mg/mL were observed on CLSM (Figure 7 and Figure S6). In addition, the same emulsions were observed by an optical fluorescent microscope (Figure S7). By comparing oil droplets at the two CCENP, the oil droplets prepared at larger CCENP exhibited much smaller average size (Figure S7a/b) in accordance with the size distribution results. From the results obtained on CLSM, fluorescent CENPs emitting blue light were optically observed to assemble onto oil droplets (Figure 6b/f). Since CENPs presented in closed-packed organization when adsorbed on droplets, 30 individual CENPs cannot be distinguished by fluorescent labeling. However, heptane/water interface can still be clearly recognized. In the case of CCENP at 3.5 mg/mL, oil droplet adsorbed by CENPs/CDs composites reflected rather low surface coverage (Figure 6b) which can be as low as 20 % by estimating the ratio of fluorescent area to total area of the droplet in zoomed-in image (Figure 6d), which were confirmed by the results observed by optical fluorescent 14


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microscope (Figure S7e/f). Under this circumstance i.e. the minimal concentration for complete emulsification, high surface coverage of the emulsion droplets was not necessary for obtaining stable emulsions. 59 By contrast, for CCENP at 6.0 mg/mL, oil droplets of apparently increased surface coverage can be observed (Figure S7d). It is worth mentioning that under the condition of 6.0 mg/mL, some droplets were almost fully covered by particles (Figure 7f/h and Figure S7g). CLSM provided coherent evidence on the adsorption of CENPs/CDs composites onto oil droplets as shown in Figure 8. From the images with a perpendicular view to two droplets, the black dots dispersed within the circle were approved to be monolayer of CENPs adsorbed on the droplets (Figure 8 a) in combination with fluorescent image (Figure 8b/c). It is worth noting that there existed a fair amount of CENPs in the continuous phase close to heptane/water interface. From other perspective of the droplet (Figure 8d), it is likely that dense black dots were interconnected to form networks around the droplet reflected by fluorescent visualization (Figure 8e/f). Such interconnected CENPs surrounding the oil droplet were estimated to be several micrometers in thickness. Supportive information (Figure S8) obtained at high CCENP (10.3 and 15.2 mg/mL) demonstrate very consistent results. What is coincident that similar conclusions had been drawn by theoretical calculation that in addition to primary particle adsorption, other particles in clusters or aggregates protruding towards continuous phase possibly existed in Pickering emulsions. 60 The important findings can provide direct evidence for the forming of droplet clusters wrapped by the CENP networks. As a result, the formation of the interconnected CENPs networks surrounding droplets plays an important role for stabilization of the emulsion. In heating process, oil droplets were induced to break quickly at 33 °C and 15


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CENPs/CDs composites were removed from oil droplets. Only small amount of oil droplets still covered by emulsifiers can be seen (Figure S7i). Obviously, fluorescent intensity of particles in emulsion at 33 °C became apparently faded. In accordance with fluorescent spectra result of emulsion during rising of temperature (Figure 9b), such result implied the fluorescence “switch-off” state due to the volume transition of PNIPAM shell. After phase separation occurred at 38 °C, CENPs/CDs composites in the form of clusters were observed in water phase (Figure S7j) whereas very few CENPs/CDs composites appeared in oil phase. The failure of stabilizing octanol in water with CENPs can be elucidated by higher wettability of CENPs in octanol than in water. 20 That explanation was confirmed by the diffusion of fluorescent CENPs into octanol phase from initially dispersed water phase (Figure S9 and Figure S10). Fluorescence properties of CENPs/CDs composites in emulsions The tunable fluorescent emission of pure CDs was firstly measured (Figure S11). The optimal excitation appeared at 360 nm and the corresponding emission peak emerged at 441 nm. Upon heating, the fluorescent intensity of CDs solution slightly declined with the increasing temperature (Figure 9a). Compared with the pure CDs, CENPs/CDs composites had a large red shift of emission peak from 441 nm to 510 nm (Figure 9b). Such obvious red shift is possibly attributed to compact distance of adjacent CDs after incorporation into CENPs,34,35 and aggregation of CDs by crosslinking C=C bonds existed in CDs by UV irradiation.43,44 Contrary to pure CDs, CENPs/CDs composites showed an apparent fluorescence on-off behavior (Figure 9b) that reduced sharply from 450 a.u. at 25°C to 200 a.u. at 40 °C, which suggested strong quenching effect of shrunk PNIPAM network upon the emission of CDs induced by temperatures rising above LCST.48 The 16


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interactions of carbon dots with PNIPAM possibly arose from hydrogen bonding that formed between the considerable amounts of –COOH,–OH on carbon dots and acylamide moieties of PNIPAM. The fluorescence on-off behavior was more distinct in heptane/water emulsion (Figure 9c) where fluorescent intensity reduced to 30 a.u. at 45 °C, indicating that the interactions of CDs with PNIPAM were very stable when CENPs/CDs composites used as emulsifiers. Moreover, fluorescent reversibility of CENPs/CDs composites in emulsion was also tested. Fluorescent intensity of the emulsion at 22 and 52 °C changed little when the emulsion was subjected to 8 cycles of heating-cooling process (Figure 9d). The reversible fluorescence property of CENPs/CDs composites reflected stable incorporation of CDs in CENPs. Morphologies of CENPs in Pickering emulsions Environment scanning microscope (ESEM) is able to characterize water-containing samples without coating gold so that original morphology can be preserved during test.61 Heptane/water emulsion stabilized by CENPs dropped on a silicon pellet was observed by ESEM (Figure 10a~c). For the morphology of entire emulsion (Figure 10a), the dark vacant regions were heptane droplets existed before evaporation. The zoomed-in image of selected area showed that the original emulsion area was covered with colloidal NPs in loose and porous structure (Figure 10b). It is likely that these colloidal NPs interconnected by swollen PNIPAM formed networks (Figure 10c) when adsorbed onto oil droplets, which was consistent with the two domains formation of CENPs/CDs composites at the interface as shown in Figure 8. TEM images of recycled CENPs after freeze-drying illustrate well preserved core-shell structure of CENPs after emulsification (Figure 10d) and the aggregation of CENPs by interwoven shell of CENPs (Figure 10e). SEM image of recycled CENPs in the inset shows apparent swollen shells of CENPs in accordance with the results of 17


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ESEM and TEM. Furthermore, temperature has exerted great influence on the recycle rate of CENPs that up to 82.7 wt% of CENPs can be recycled when the recycling was carried out at 25 °C whereas it declined to 51.3 wt% at 45 °C. The difference in recycle rate is possibly caused by a portion of CENPs with shrunk PNIPAM shell cannot be retained by a G6 filter at 45 °C. After re-dispersion of CENPs in heptane/water mixture, recycled CENPs can still form stable emulsion which confirmed the robust and stable core-shell structure of CENPs. CONCLUSIONS In summary, we successfully fabricated fluorescent core-shell structured NPs by one-step coaxial electrospraying. A uniform core-shell structure of CA encapsulated by PNIPAM shell with CDs embedded was accordingly confirmed by SEM, TEM and confocal microscope. CENPs/CDs composites can reversibly stabilize heptane and toluene in water, respectively, indicated by their fluorescence on-off behavior. By tracing CENPs/CDs composites below LCST, as low as 20 % of surface coverage of oil droplets was observed with CCENP at 3.5 mg/mL and surface coverage increased apparently with the increase of CCENP. Almost fully covered droplets by fluorescent CENPs could be observed at CCENP of 6.0 mg/mL. Meanwhile, a mode of two-domain adsorption at the interface was clearly observed including primary monolayer anchoring of CENPs on droplets surrounded by a region of interconnected CENPs networks. At large CCENP i.e. 15.2 mg/mL, droplet clusters were observed possibly due to the bridging of CENPs networks. When temperature-induced phase separation occurred, CENPs tended to aggregate in aqueous phase. All the phenomena were demonstrated in Scheme 3. In addition, the core-shell structure and fluorescent properties of CENPs/CDs 18


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composites were very stable after the use of CENPs as emulsifiers. The strategy of coaxial electrospraying to fabricate the core-shell NPs is a facile and versatile approach that can combine multi-function into a well-designed nanostructure for extended applications from emulsifiers to bio-sensors and bio-imaging.

ASSOCIATED CONTENT Supporting Information Coaxial needle photograph, TEM images of CENPs, CDs size distribution, confocal microscope images, fluorescent images of heptane/water emulsion, photograph of toluene/water emulsion, fluorescent spectra, etc. Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author: E-mail: [email protected] (Jianqiang Chen); [email protected] (Zhen Yang); [email protected] (Takuya Kitaoka) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51603104, 51608275), Natural Science Foundation of Jiangsu Province (BK20160921), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). REFERENCES 19


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(1) Chevalier, Y.; Bolzinger, M.A. Emulsions stabilized with solid nanoparticles: Pickering emulsions. Colloids and Surfaces A: Physicochem. Eng. Aspects 2013, 439, 23-34. DOI:10.1016/j.colsurfa.2013.02.054 (2) Wu, J.; Ma, G.H. Recent studies of Pickering emulsions: particles make the difference. Small 2016, 12(34), 4633-4648. DOI:10.1002/smll.201600877 (3) Vignati, E.; Piazza, R. Pickering emulsions: interfacial tension, colloidal layer morphology, and trapped-particle motion. Langmuir 2003, 19, 6650-6656. DOI:10.1021/la034264l (4) Dinsmore, A.D.; Hsu, M.F.; Nikolaides, M.G.; Marquez, M.; Bausch, A.R.; Weitz, D.A. Colloidosomes: selectively permeable capsules composed of colloidal particles. Science 2002, 298, 1006-1009. DOI:10.1126/science.1074868 (5) Qu, Y.N.; Huang, R.L.; Qi, W.; Su, R.X.; He, Z.M. Interfacial polymerization of dopamine in a Pickering emulsion: synthesis of cross-linkable colloidosomes and enzyme immobilization at oil/water interfaces. ACS Appl. Mater. Interfaces 2015, 7, 14954-14964. DOI: 10.1021/acsami.5b03787 (6) Suzuke, D.; Tsuji, S.; Kawaguchi, H. Janus microgels prepared by surfactant-free Pickering emulsion-based modification and their self-assembly. J. Am. Chem. Soc. 2006, 129, 8088-8089. DOI:10.1021/ja072258w (7) Liu, B.; Wei, W.; Qu, X.Z.; Yang, Z.Z. Janus colloids formed by biphasic grafting at a Pickering emulsion interface. Angew. Chem. Int. Ed. 2008, 47, 3973-3975. DOI: 10.1002/anie.200705103 (8) Woodward, R.T.; Fam, D.W.H.; Anthony, D.B.; Hong, J.D.; McDonald, T.O.; Petit, C.; Shaffer, M.S.P.; Bismarck, A. Hierarchically porous carbon foams from Pickering high internal phase emulsions. Carbon 2016, 101, 253-260. DOI:10.1016/j.carbon.2016.02.002 20


Page 20 of 44

Page 21 of 44 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

Langmuir

(9) Hua, Y.; Zhang, S.M.; Chen, J.D.; Zhu, Y. Switchable release and recovery of nanoparticles via a Pickering-emulsion-templated porous carrier. J. Mater. Chem. A 2013, 1, 13970-13977. DOI:10.1039/c3ta12295f (10) Cho, H.H.; Yang, H.; Kang, D.J.; Kim, B.J. Surface engineering of graphene quantum dots and their applications as efficient surfactants. ACS Appl. Mater. Interfaces 2015, 7, 8615-8621. DOI:10.1021/acsami.5b00729 (11) Xie, P.F.; Ge, X.P.; Fang, B.; Li, Z.; Liang, Y.; Yang, C.Z. Pickering emulsion polymerization of graphene oxide-stabilized styrene. Colloid. Polym. Sci. 2013, 291, 1631-1639. DOI:10.1007/s00396-013-2897-x (12) Lam, S.; Velikov, K.P.; Velev, O.D. Pickering stabilization of foams and emulsions with particles of biological origin. Curr. Opin. Colloid In. 2014, 19, 490-500. DOI:10.1016/j.cocis.2014.07.003 (13) Gan, M.Y.; Pan, J.M.; Zhang, Y.L.; Dai, X.H.; Yin, Y.J.; Qu, Q.; Yan, Y.S. Molecularly imprinted polymers derived from lignin-based Pickering emulsions and their selectively adsorption of lambda-cyhalothrin. Chem. Eng. J. 2014, 257, 317-327. DOI:10.1016/j.cej.2014.06.110 (14) Sarkar, A.; Murray, B.; Holmes, M.; Ettelaie, R.; Abdalla, A.; Yang, X.Y. In vitro digestion of Pickering emulsions stabilized by soft whey protein microgel particles: influence of thermal treatment. Soft Matter 2016, 12, 3558-3569. DOI: 10.1039/c5sm02998h (15) Tang, J.T.; Quinlan, P.J.; Tam, K.C. Stimuli-responsive Pickering emulsions: recent advances and potential applications. Soft Matter 2015, 11, 3512-3529. DOI: 10.1039/c5sm00247h (16) 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, 21


Langmuir 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

8668-8675. DOI: 10.1021/acs.langmuir.6b01475 (17) Kim, S.H.; Kim, J.H.; Choi, H.J.; Park, J. Pickering emulsion polymerized poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/polystyrene composite particles and their electric stimuli-response. RSC Adv. 2015, 5, 72387-72393. DOI: 10.1039/c5ra10661c (18) Low, L.E.; Tey, B.T.; Ong, B.H.; Chan, E.S.; Tang, S.Y. Palm olein-in-water Pickering emulsion stabilized by Fe3O4-cellulosenanocrystal nanocomposites and their responses to pH. Carbohyd. Polym. 2017, 155, 391-399. DOI:10.1016/j.carbpol.2016.08.091 (19) Zoppe, J.O.; Venditti, R.A.; Rojas, O.J. Pickering emulsions stabilized by cellulose nanocrystals grafted with thermo-responsive polymer brushes. J. Colloid Interf. Sci. 2012, 369, 202-209. DOI:10.1016/j.jcis.2011.12.011 (20) Tsuji, S.; Kawaguchi, H. Thermosensitive Pickering emulsion stabilized by poly(N-isopropylacrylamide)-carrying particles. Langmuir 2008, 24, 3300-3305. DOI:10.1021/la701780g (21) Zhu, Y.; Fu, T.; Liu, K.H.; Lin, Q., Pei, X.M.; Jiang, J.Z.; Cui, Z.G.; Binks, B.P. Thermoresponsive Pickering emulsions stabilized by silica nanoparticles in combination with alkyl polyoxyethylene ether nonionic surfactant. Langmuir 2017, 33, 5724-5733. DOI:10.1021/acs.langmuir.7b00273 (22) Binks, B.P.; Murakami, R.; Armes, S.P.; Fujii, S. Temperature-induced inversion of nanoparticle-stabilized emulsions. Chem. Int. Ed. 2005, 44, 4795-4798. DOI:10.1002/anie.200501073

(23) Monteux, C.; Marliere, C.; Paris, P.; Pantoustier, N.; Sanson, N.; Perrin, P. Poly(N-isopropylacrylamide) microgels at the oil-water interface: interfacial

22


Page 22 of 44

Page 23 of 44 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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properties as a function of temperature, Langmuir 2010, 26, 13839-13846. DOI:10.1021/la1019982 (24) Saigal, T.; Dong, H.C.; Matyjaszewski, K. ; Tilton, R.D. Pickering emulsions stabilized by nanoparticles with thermally responsive grafted polymer brushes. Langmuir 2010, 26, 15200-15209. DOI:10.1021/la1027898. (25) Thompson, K.L.; Fielding, L.A.; Mykhaylyk, O.O.; Lane, J.A.; Derry, M.J.; Armes, S.P. Vermicious thermo-responsive Pickering emulsifiers. Chem. Sci. 2015, 6, 4207-4214. DOI:10.1039/C5SC00598A (26) Schild, H.G. Poly(N-isopropylacrylamide): experiment, theory and application. Prog. Polym. Sci. 1992, 17, 163-249. DOI:10.1016/0079-6700(92)90023-R (27) Chen, J.Q.; Xue, H.J.; Yao, Y.F.; Yang, H.; Li, A.M.; Xu, M.; Chen, Q.; Cheng, R.S. Effect of surfactant concentration on the complex structure of poly(N-isopropylacrylamide)/sodium n-dodecyl sulfate in aqueous solutions. Macromolecules 2012, 45, 5524-5229. DOI:10.1021/ma301003r (28) Chen, J.Q.; Gong, X.L.; Yang, H.; Yao, Y.F.; Xu, M.; Chen, Q.; Cheng, R.S. NMR study on the effects of sodium n-dodecyl sulfate on the coil-to-globule transition of poly(N-isopropylacrylamide) in aqueous solutions. Macromolecules 2011, 44, 6227-6231. DOI:10.1021/ma201269u (29) Chen, J.Q.; Spevacek, J.; Hanykova, L. NMR methods to study effects of additives on phase separation of thermoresponsive polymer. Macromol. Symp. 2014, 339, 24-32. DOI:10.1002/masy.201300130 (30) Destribats, M.; Lapeyre, V.; Wolfs, M.; Sellier, E.; Leal-Calderon, F.; Ravaine, V.; Schmitt, V. Soft microgels as Pickering emulsion stabilisers:

23


Langmuir 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

role of particle deformability. Soft Matter 2011, 7, 7689-7698. DOI:10.1039/c1sm05240c (31) Peng, Z.A.; Peng, X.G. Formation of high-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor J. Am. Chem. Soc. 2001, 123, 183-184. DOI:10.1021/ja003633m (32) Kamat, P.V. Quantum dot solar cells: semiconductor nanocrystals as light harvesters. J. Phys. Chem. C 2008, 112, 18737-18753. DOI:10.1021/jp806791s (33) Gao, X.H.; Cui Y.Y.; Levenson, R.M.; Chung, L.W.K.; Nie, S.M. In vivo cancer targeting and imaging with semiconductor quantum dots. Nat. Biotechnol. 2004, 22, 969-976. DOI:10.1038/nbt994 (34) Wang, Y.Q.; Zhang, Y.Y.; Zhang, F.; Li, W.Y. One-pot synthesis of thermal responsive QDs–PNIPAM hybrid fluorescent microspheres by controlling the polymerization temperature at two different polymerization stages. J. Mater. Chem. 2011, 21, 6556-6562. DOI:10.1039/c1jm10104h (35) Gong, Y.J.; Gao, M.Y.; Wang, D.Y.; Mohwald, H. Incorporating fluorescent CdTe nanocrystals into a hydrogel via hydrogen bonding: toward fluorescent microspheres with temperature-responsive properties. Chem. Mater. 2005, 17, 2648-2653. DOI:10.1021/cm047932c (36) Li, J.; Hong, X.; Liu, Y.; Li, D.; Wang, Y.W.; Li, J.H.; Bai, Y.B.; Li, T.J. Highly photoluminescent CdTe/poly(N-isopropylacrylamide) temperature-sensitive gels. Adv. Mater. 2005, 17, 163-166. DOI:10.1002/adma.200400448 (37) Janczewski, D.; Tomczak, N.; Song, J.; Long, H.; Han, M.-Y.; Vancso, G.J. Fabrication and responsive behavior of quantum dot/PNIPAM micropatterns

24


Page 24 of 44

Page 25 of 44 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

Langmuir

obtained by template copolymerization in water. J. Mater. Chem. 2011, 21, 6487-6490. DOI:10.1039/c1jm10351b (38) Salcher, A.; Nikolic, M.S.; Casado, S.; Velez, M.; Weller, H.; Juarez, B.H. CdSe/CdS nanoparticles immobilized on pNIPAm-based microspheres. J. Mater. Chem. 2010, 20, 1367-1374. DOI:10.1039/b917022g (39) GhoshMitra, S.; Diercks, D.R.; Mills, N.C.; Hynds, D.L.; Ghosh, S. Excellent biocompatibility of semiconductor quantum dots encased in multifunctional poly(N-isopropylacrylamide) nanoreservoirs and nuclear specific labeling of growing neurons. Appl. Phys. Lett. 2011, 98, 103702. DOI:10.1063/1.3562036 (40) Sun, Y.P.; Zhou, B.; Lin, Y.; Wang, W.; Fernando, K.A.S.; Pathak, P.; Meziani, M.J.; Harruff, B.A.; Wang, X.; Wang, H.F.; Luo, P.G.; Yang, H.; Kose, M.E.; Chen, B.L.; Veca, M.; Xie, S.Y. Quantum-sized carbon dots for bright and colorful photoluminescence. J. Am. Chem. Soc. 2006, 128, 7756-7757. DOI:10.1021/ja062677d (41) Lim, S.Y.; Shen, W.; Gao, Z.Q. Carbon quantum dots and their applications. Chem. Soc. Rev. 2015, 44, 362-381. DOI:10.1039/c4cs00269e (42) Song, Y.B.; Zhu, S.J.; Yang, B. Bioimaging based on fluorescent carbon dots. RSC Adv. 2014, 4, 27184-27200. DOI:10.1039/c3ra47994c (43) Zhu, S.J.; Meng, Q.N.; Wang, L.; Zhang, J.H.; Song, Y.B.; Jin, H.; Zhang, K.; Sun, H.C.; Wang, H.Y.; Yang, B. Highly photoluminescent carbon dots for multicolor patterning, sensors, and bioimaging. Angew. Chem. Int. Ed. 2013, 52, 3953-3957. DOI:10.1002/anie.201300519

25


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(44) Zhao, X.H.; Zhu, S.J.; Song, Y.B.; Zhang, J.H.; Yang, B. Thermal responsive fluorescent nanocomposites based on carbon dots. RSC Adv. 2015, 5, 15187-15193. DOI:10.1039/c4ra13417f (45) Campos, B.B.; Mutavdzic, D.; Stankovic, M.; Radotic, K.; Lazaro-Martinez, J.M. ; da Silva, J.C.G.E. ; Contreras-Caceres, R.; Pino-Gonzalez, M.S.; Rodriguez-Castellon, E.; Algarra, M. Thermo-responsive microgels based on encapsulated carbon quantum dots. New J. Chem. 2017, 41, 4835-4842. DOI: 10.1039/c6nj03893j (46) Li, D.Y.; Zhang, X.M.; Yan, Y.J.; He, X.W.; Li, W.Y.; Zhang, Y.K. Thermo-sensitive imprinted polymer embedded carbon dots using epitope approach. Biosens. Bioelectro. 2016, 79, 187-192. DOI:10.1016/j.bios.2015.12.016 (47) Zhang, M.F.; Zhao, X.N.; Zhang, G.H.; Wei, G.; Su, Z.Q. Electrospinning design of functional nanostructures for biosensor applications, J. Mater. Chem. B 2017, 5, 1699-1711. DOI: 10.1039/c6tb03121h (48) Ding, L.; Lee, T.; Wang, C.H. Fabrication of monodispersed Taxol-loaded particles using electrohydrodynamic atomization. J. Control. Release 2005, 102, 395-413. DOI:10.1016/j.jconrel.2004.10.011 (49) Bock, N.; Woodruff, M.A.; HutMacher, D.W.; Dargaville, T.R. Electrospraying, a reproducible method for production of polymeric microspheres for biomedical applications. Polymers 2011, 3, 131-149. DOI:10.3390/polym3010131 (50) Eltayeb, M.; Stride, E.; Edirishinghe, M.; Harker, A. Electrosprayed nanoparticle delivery system for controlled release. Mat. Sci. Eng. C-Mater. 2016, 66, 138-146. DOI:10.1016/j.msec.2016.04.001 26


Page 26 of 44

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Langmuir

(51) Jing, Y.J.; Zhu, Y.H.; Yang, X.L.; Shen, J.H.; Li, C.Z. Ultrasound-triggered smart drug release from multifunctional core−shell capsules one-step fabricated by coaxial electrospray method. Langmuir 2011, 27, 1175-1180. DOI:10.1021/la1042734 (52) Cui, L.; Liu, Z.; Yu, D.; Zhang, S.; Bligh, S.; Zhao, N. Electrosprayed core-shell nanoparticles of PVP and shellac for furnishing biphasic controlled release of ferulic acid. Colloid Polym. Sci. 2014, 292, 2089-2096. DOI:10.1007/s00396-014-3226-8 (53) Gao, Y.; Zhao, D.; Chang, M.W.; Ahmad, Z.; Li, J.S. Optimising the shell thickness-to-radius ratio for the fabrication of oil-encapsulated polymeric microspheres. Chem. Eng. J. 2016, 284, 963-971. DOI:10.1016/j.cej.2015.09.054 (54) Sridhar, R.; Lakshminarayanan, R.; Madhaiyan, K.; Barathi, V.A.; Lim, K.H.C.; Ramakrishna, S. Electrosprayed nanoparticles and electrospun nanofibers based on natural materials: applications in tissue regeneration, drug delivery and pharmaceuticals. Chem. Soc. Rev. 2015, 44, 790-814. DOI:10.1039/c4cs00226a (55) Ganesh, V.; Ranganath, A.; Sridhar, R.; Raut, H.; Jayaraman, S.; Sahay, R.; Ramakrishna, S.; Baji, A. Cellulose acetate–poly(N-isopropylacrylamide)-based functional surfaces with temperature-triggered switchable wettability. Macromol. Rapid Commun. 2015, 36, 1368-1373. DOI:10.1002/marc.201500037 (56) Hwang, Y.; Jeong, U.; Cho, E. Production of uniform-sized polymer core-shell microcapsules by coaxial electrospraying. Langmuir 2008, 24, 2446-2451. DOI:10.1021/la703546f 27


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(57) Kaptay, G. On the equation of the maximum capillary pressure induced by solid particles to stabilize emulsions and foams and on the emulsion stability diagrams. Colloids and Surfaces A: Physicochem. Eng. Aspects 2006, 282–283, 387-401. DOI:10.1016/j.colsurfa.2005.12.021 (58) Kalashnikova, I.; Bizot, H.; Bertoncini, P.; Cathala, B.; Capron, I. Cellulosic nanorods of various aspect ratios for oil in water Pickering emulsions. Soft Matter 2013, 9, 952-959. DOI:10.1039/c2sm26472b (59) Midmore, B.R. Preparation of a novel silica-stabilized oil/water emulsion. Colloids and Surfaces A: Physicochem. Eng. Aspects 1998, 132, 257-265. DOI:10.1016/S0927-7757(97)00094-0 (60) Arditty, S.; Whitby, C.P., Binks, B.P.; Schmitt, V.; Leal-Calderon, F. Some general features of limited coalescence in solid-stabilized emulsions. Eur. Phys. J. E 2003, 11, 273-281. DOI:10.1140/epje/i2003-10018-6 (61) Danilatos, G.D. Introduction to the ESEM Instrument. Microsc. Res. Techniq.1993, 25, 354-361. DOI:10.1002/jemt.1070250503

Figure captions Scheme 1. The strategy of fabricating core-shell nanospheres with CDs embedded in the shell matrix via coaxial electrospraying. Scheme 2. Schematic description of structural design of thermo-responsive labels. Scheme 3. Mechanisms of particle actions in oil/water emulsions stabilized by CENPs. Figure 1. (a) FE-SEM image of CENPs and zoomed-in morphology in the inset; (b) fluorescent image of CENPs recorded on a confocal laser microscope; (c) HRTEM image of two CENPs; (d) a CENP with shell thickness of 41.6 nm; (e) the core-shell interface of a CENP. 28


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Figure 2. (a) HRTEM images of pure CDs; (b) TEM image of a CENP shell region; (c) CDs embedded in PNIPAM matrix within the dotted rectangle in (b); (d) fluorescence microscope image of CENPs/CDs composites, the inset shows pure CDs in aqueous solution under visible (left) and UV light (right). Figure 3. Size distributions of CENPs dispersed in water at various temperatures. Figure 4. Schematic illustration of wettability of CENPs: the profile of a water drop on CENPs-coated film at various temperatures with corresponding static water contact angle result. Figure5. Heptane and water emulsion stabilized by CENPs (CCENP=6.0 mg/mL)(left); the profile of the emulsion in heating process (right). Figure 6. Size distributions of heptane droplets at 25 °C under various CCENP. Figure 7. Confocal laser scanning microscope images of heptane droplets stabilized by CENPs/CDs composites (CCENP=3.5 mg/mL) (a~d) and (CCENP= 6.0 mg/mL) (e~h) at 25 °C. Figure 8. Confocal laser scanning microscope images of heptane/water interface (CCENP= 6.0 mg/mL) from different perspectives. Figure 9. Fluorescent spectra of pure CDs (a) and CENPs/CDs composites (b) in aqueous solution at various temperatures, (c) heptane/water emulsion stabilized by CENPs/CDs composites; (d) fluorescent intensity of CENPs/CDs composites subjected to 8 heating-cooling cycles. Figure 10. (a) ESEM images of heptane/water emulsion stabilized by CENPs dropped on a silicon pellet, and colloidal CENPs in scale bar of 10 µm (b) and 2 µm (c); TEM images of recycled CENPs from heptane/water emulsion in scale bar of 1 µm (d) and 200 nm (e), the inset shows SEM image of recycled CENPs.

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Table 1. The properties of emulsions stabilized by CENPs Oil

Type

heptane

O/W

toluene

O/W

Temp.

Volume ratio a

25 °C 32 °C 35 °C 38 °C 45 °C 25 °C 32 °C 35 °C 38 °C 45 °C

0:6:4 1:5:4 2:4:4 4:1:5 5:0:5 1:5:4 1:5:4 4:1:5 5:0:5 5:0:5

Droplets size 10~50 µm 10~50 µm 5~20 µm 5~20 µm -10~100 µm 5~20 µm 5~20 µm --N. A.

octanol c a Volume ratio of oil/emulsion/water. b Duration of volume ratio remained unchanged at 25°C. c CENPs cannot stabilize octanol in water.

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CENPs concentration

Stability b

6.0 mg/mL

40 days

6.0 mg/mL

21 days

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Scheme 1.

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Scheme 2.

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Scheme 3. 33


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Figure 1.

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Langmuir

Figure 2.

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Figure 3.

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Langmuir

Figure 4.

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Figure 5.

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Langmuir

a

b

c

d

Figure 6. 39


Langmuir 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

Figure 7.

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Langmuir

Figure 8.

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Figure 9.

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Langmuir

Figure 10.

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Table of Contents: CENPs/CDs composites exhibit fluorescence “on-off” behavior for Pickering emulsion.

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Thermally Tunable Pickering Emulsions Stabilized by Carbon-Dot

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