Self-Assembly of TiO2 Nanofiber-Based Microcapsules by

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Self-assembly of TiO2 nanofiber-based microcapsules by spontaneously-evolved multiple emulsions Leyan Lei, Tiantian Kong, Pingan Zhu, Zhanxiao Kang, Xiaowei Tian, and Liqiu Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01472 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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Self-assembly of TiO2 nanofiber-based microcapsules by spontaneouslyevolved multiple emulsions

Leyan Lei1, Tiantian Kong2,3,*, Pingan Zhu1,3, Zhanxiao Kang1,3, Xiaowei Tian1,3, Liqiu Wang1,3,* 1

Department of Mechanical Kong SAR 999077, China.

Engineering,

University

of

Hong

Kong,

Hong

2

Guangdong Key Laboratory for Biomedical Measurements and Ultrasound Imaging, Department of Biomedical Engineering, School of Medicine, Shenzhen University, Shenzhen Guangdong 51800, China

3

HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), Hangzhou, Zhejiang 310000, China Correspondence and requests for materials should be addressed to Tiantian Kong and Liqiu Wang (Email: [email protected], [email protected])

ACS Paragon Plus Environment

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Abstract: We demonstrate hierarchical interlocked titanium dioxide (TiO2)

nest/crust-like colloidosomes composed of nanofibers using spontaneously-evolved n-

butanol/water/n-butanol (B/W/B) emulsions. We find two mechanisms to produce colloidosomes from B/W/B droplets due to mutual solubility and dewetting discrepancy. Porous TiO2 colloidal capsules with loosely intertwined nanofibers were obtained after the dewetting of nanofiber-coated B/W/B droplets, while crust-like TiO2 colloidosomes with a thin shell and large hollow interior are developed from amphiphilic polymer-stabilized B/W/B droplets. We further investigate the effect of experimental parameters, including the initial droplet size, the nanofiber concentration and the water/ butanol ratios in butanol phases, on the droplet-to-colloidosome evolution and resultant morphology of colloidosomes. Our simple and versatile approach for fabricating TiO2 colloidosomes can be extended to a range of irregular colloidal particles, and the products have great potential in performing as host systems in electrochemical catalysis, photothermal therapy or filtration materials. Keywords: spontaneous emulsion, TiO2 nanofiber, mutual diffusion, water/n-butanol.


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Introduction Microcapsules are readily found in pharmaceutical products as well as in various fields that call for the protection and encapsulation of guest materials[1], including medical imaging[2], photothermal therapy[3], agriculture[4] and cosmetics[5], [6]. In these applications, the microcapsules need to be sufficiently robust for the encapsulated materials[7], considering these microcapsules are often exposed to harsh conditions such as the acidic environment of stomach by oral administration of encapsulated antibiotics[8], [9], the constant exposure to sunlight of encapsulated pesticides[10], [11], or the elevated temperature of encapsulated therapeutics in photothermal treatments[12], [13]. Additionally, the advanced encapsulation systems allow for the incorporation of functionalities such as stimuli-triggered release[14]– [16], self-healing[17], [18], and selective permeability[19]. To ensure these attributes, the microcapsules also should provide sufficient mechanical support and act as physical barriers until the actives are delivered[1], [20]. Colloidosomes, microcapsules with shells consisted of colloidal particles, hold great potential since the colloidal particles provide means to achieve a range of functionalities[19], [21]. For instance, colloidosomes formed from gold nanoparticle show excellent surface plasmon absorption[22] and thus have been applied for surface-enhanced Raman spectroscopy[23]; colloidosomes assembled by iron oxide nanoparticles have been used to achieve

targeted delivery of drugs[24], [25].

Despite their multi-functionalities,

colloidosomes with shells of solely fused spherical nanoparticles are inherently brittle[26]. Intriguingly, the micron-sized colloidosomes fabricated from non-spherical colloidal particles have excellent mechanical properties, such as high toughness, due to the entanglement between irregular-shaped particles[27]. Thus, non-spherical inorganic colloidal particles including nanotubes and nanofibers are promising building blocks for colloidosome-based micro-encapsulation systems with tunable functionalities and mechanical stability.


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Although colloidosomes from various inorganic nanoparticles have been demonstrated, the processing techniques are either time-consuming, complicated, inefficient, or delicate for operating in large scale[28]–[30]. For instance, the layer-by-layer approach is usually followed by the removal of sacrificial cores, which can involve hazardous chemicals such as hydrofluoric acid[31]. Alternatively, colloidosomes can form by gluing the colloids that assemble at liquid-liquid interfaces, including water/oil, water/water interfaces[26]. Among these soft-templated methods, the microfluidic emulsification approach provides well-defined colloidosomes with controlled shell thickness and permeability, but it is still technically challenging to produce such colloidosomes conveniently in large quantities. Herein, we present a facile method for the preparation of versatile colloidal capsules, such as colloidosomes from inorganic nanofibers and hybrid nanofiber/polymer colloidosomes in simple yet scalable approach. We generate single emulsion droplets from a miscible twophase system that consists of an aqueous colloidal suspension of titanium dioxide (TiO2) nanofibers (100 nm in diameter, 1 μm in length), and n-butanol. The sizes of resultant colloidosomes typically range from 20 μm to submillimetre. By using the partially miscible systems, the W/B emulsion droplets spontaneously evolve into B/W/B double emulsions by solvent exchange, and consequently the interfacial jamming of nanofibers converts emulsion droplets into colloidosomes. The water/n-butanol binary mixture is chosen since it has a severer micro-heterogeneity than water-ethanol and water/1-propanol mixtures that can promote phase separation [32]; TiO2 nanofibers are selected as model inorganic particles, owing to its stability, low cost and catalytic activity[33], [34], though our approach can be applied to a wide range of inorganic nanoparticles to form colloidosomes. By tuning the initial concentration of TiO2 nanofibers, the initial droplet size and using different water-containing butanol phases, we demonstrate colloidosomes with tunable sizes and nest-like morphology as a result of loosely intertwined fiber-networks. Intriguingly, by


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simply blending amphiphilic polymers in the miscible two-phase systems, we fabricate crustlike TiO2/polymer hybrid colloidosomes with a nanometer-sized shell of densely packed nanofibers. The resultant TiO2 colloidosomes can be re-suspended in water and are featured with porous network or a large hollow cavity beneficial for catalysis efficiency. Our approach can be integrated with scalable emulsification methods such as multi-nozzle electrosprays and microfluidics with paralleled channels to produce tunable colloidosomes with controlled sizes and surface morphologies in large quantities.

Experimental Materials All chemicals were used as received without further purification. TiO2 nanofibers were purchased from Nanjing XF Nano Materials Tech Co., Ltd. N-butanol (99%) was purchased from ACROS Organics; poly(ethylene glycol) (PEG, Mw~6000) and Pluronic F127 (PEOPPO-PEO block copolymer Mw~1100) were obtained from Sigma-Aldrich, Ltd.(US). Fluorescein Isothiocyanate-Dextran (FITC-dextran, Mw=500,000) was from Sigma. The deionized water was produced by a Millipore Milli-Q purification system (EMD Millipore Corporation, USA).

Methods Device Fabrication. Microfluidic coaxial flow-focusing glass capillary devices were built according to the method reported[35] (see Movie S1 in Supplementary Information). Glass capillaries (World Precision Instruments, Inc.) were tapered using a P-97 Flaming/Brown micropipette puller (Sutter Instrument). The injection capillary was dipped into a solution of 1 vol.% trimethoxy(octadecyl)silane (90%, Aldrich) and 0.5 vol.% ammonium hydroxide solution (28 vol.% NH3 in H2O, Sigma-Aldrich) in ethanol, and desiccated under 120℃ vacuum oven. The injection tube was aligned with the axes of collection capillary and the encountered cylindrical capillaries were medially placed in the square tube (Fig. 1).


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Preparation of colloidal capsules. Deionized water loaded with varied concentration of TiO2 nanofibers from 0.5 wt% to 1.5 wt% was used as dispersed phase. The continuous organic phase consists of a solution of 19.6 wt% water-containing n-butanol. Here the water/n-butanol system is chosen due to their miscibility, and the two phase have a relatively large interfacial tension around 32 mN/m. The dispersed phase flowed into injection channel and is sheared successively by the continuous phase to form single emulsion droplets. Each fluid was pumped independently at adjustable flow rates using syringe pumps connected to the capillary device via teflon microtubings. The flow rates of the dispersed phase and the continuous phase were 120 µLh-1 and 150 µLh-1, respectively. The obtained W/B emulsions were transferred to a bath of n-butanol reservoir for solidification. The microcapsules were collected and dried in the vacuum oven to remove the solvent residue. Microscopy and Scanning Electron Microscopy. The production of microcapsules was visualized and recorded by an inverted microscope (DMIL, Leica) connected with high-speed camera (Phantom V3.0, Vision Research) in bright-field mode. The size analysis of the microcapsules was carried out with Image J. The external/interior surface microstructure of capsules were imaged in a scanning electron microscope (LEO 1530, Carl-Zeiss SMT AG,; Hitachi S4800 FEG SEM) after dried in air at room temperature and kept in a vacuum oven for 3 hours at 120 °C (VT6060, Thermo Scientific). Confocal Microscopy. The release profiles from the colloidosomes were obtained by Zeiss Laser Scanning Confocal Microscope (LSM710, Oberkochen, Germany). The model of loading used in our experiments was FITC-dextran (excitation~490nm). The fluorescent imaging was performed on LSM710 with 5x objective, 488 nm Argon laser line. The fluorescent intensity density of recorded images was measured by an open software Image J.


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Results and discussion Upon the generation of colloidal suspension-in-n-butanol Pickering emulsion droplets, the mutual diffusion between water and n-butanol starts, and the water/butanol system becomes the water-rich/butanol-rich two-phase system as shown in Fig. 1 a. Since the water has a larger solubility in butanol, 19.6 wt% than that of butanol in water, 6.4 wt%, the aqueous droplet loses water to the surroundings. The aqueous droplets shrink in size since generated (Fig. 1 a). When the maximum solubility of n-butanol in water ([BuOH]≈6.4wt% at 20℃) is reached, the butanol-rich phase appears as small droplets in the water-rich droplet as mutual diffusion continues (see Fig. 1 b and Movie S2 in Supplementary Information). These spontaneously- emerged tiny inner droplets tend to coalesce into a bigger one, and then dewet from the shell droplet into surroundings as shown both in Fig. 1 a-b. The cycles of droplet evolution take dozens of seconds until both the butanol-rich and water-rich phases reach equilibrium. It has been reported that the phase separation in water/alcohol systems occurs when diffusion leads to the formation of molecular clusters and fragmentation in water droplets[32], [36].


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Figure 1. a) Schematic illustration of monodisperse microcapsule fabrication using microfluidic/electrospray, followed by solvent diffusion and core dewetting; b) Optical microscope images of aqueous droplet evolution in butanol via mutual diffusion, in which n-butanol was dyed with oil red for visualization.

The TiO2 nanofibers in aqueous phase diffuse and absorb onto the water/butanol interfaces that has a tension around 32 mN/m as soon as droplets are generated in butanol environment. The packing between nanofibers become closer as the aqueous droplet shrinks. Eventually, the jamming between nanofibers leads to the formation of nest-like colloidosomes with hollow interior, which is left by the dewetted butanol-rich cores. The interior of the formed quasi-spherical TiO2 colloidosomes are porous with loosely-intertwined-nanofiber networks (see Fig. S2 in Supplementary Information), due to the continuous solvent exchange between the interior and exterior. The resultant colloidosomes with wrinkled surfaces and porous interiors have a large surface-to-volume ratio suitable for catalysis and filtration materials.


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Figure 2. The radius evolution of droplet-to-colloidosome in different surrounding butanol solution. (Insets) Scanning electron microscope images of different microcapsules for initial nanofiber concentration varied from 0.5 wt% to 1.5 wt%. NF: nanofibers.

To control the size of resultant colloidosomes, we investigated the effect of initial nanofiber concentration of the aqueous phase. We vary the initial colloidal concentration from 0.5 wt% to 1.5 wt% and observe the droplet evolution. Since the colloidosomes are formed by nanofiber jamming, the higher initial concentration of TiO2 nanofiber would lead to a shorter formation time. Indeed, the aqueous droplet with 0.5 wt% TiO2 nanofiber underwent up to 600 s until solidified while it only takes 100 s for droplets with 1.5 wt% nanofibers to become colloidal capsules, as shown in Fig. 2, in which Rx represents the varying droplet radius during its transition from droplet to colloidosome. Moreover, we found that the initial concentrations have a negligible effect on the size of resultant colloidosomes, as shown by the small disparities of ± 6% in the final sizes (Fig. 2). Note that the concentration range is selected to obtain a stable and homogeneous suspension of nanofibers. However, colloidosomes formed by nanofibers of a low concentration (0.75 wt%) have


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crumbled morphology while the surface of that formed by a high concentration (1.5 wt%) is only slightly wrinkled (see insets in Fig. 2 and Fig. S1 in Supplementary Information). The weak mechanical strength associated with low nanofiber concentration render the colloidosome easier to crumble during droplet shrinkage. Therefore, the initial concentrations of TiO2 nanofibers affect the solidification time as well as the morphology of the final capsules.

Figure 3. a) The rescaled radius

Rx / Ri as a function of dimensionless time, Tx / Tf , where the

subscription x, i, f means the varying, initial and final, respectively. The purple (inverted triangles), pink (circles), green (triangles), blue (squares) and yellow (diamonds) represent initial droplets sized of 754 µm, 538 µm, 302 µm, 232 µm, 141 µm, respectively. All of the data points fall onto a linear fitting curve represented by a pink dash line; b) R / Ri is plotted with initial drop radius to show the ratio of size f

changes when droplet radius increases.

We further explore the relationship between the size of the initial aqueous droplets and that of the final colloidosomes under a constant initial concentration of TiO2 nanofibers (1 wt%). The resultant size of colloidosome is approximately 33% to 50% of that of the initial droplet as shown in Fig. 3 a-b. For instance, a droplet with 754-µm radius (purple inverted triangles) converted into a colloidosome with radius 518 µm, while a 141-µm (yellow diamonds) one ended up in with 93-µm. The initial droplet size affects the colloidosome formation time. For example, a 538 µm-radius-droplet (pink circles) takes approximately 720 s ± 0.2 s to


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complete the diffusion and solidification, whereas 302 µm- (green triangles), 232 µm- (blue squares), and 141 µm-radius droplet take 660 s ± 0.2 s, 540 s ± 0.2 s and 210 s ± 0.2 s, respectively. We rescaled the radius of droplet Rx , by the radius of initial droplet, Ri and nondimensionalize the shaping time Tx by the total duration T f . Rx / Ri is as function of Tx / Tf

.

All

data

can

be

fitted

with

the

linear

regression

equation

Rx / Ri = 0.9946 − 0.38458 * Tx / T f as illustrated in Fig. 3 a. Further analysis shows that R f / Ri value was in a negative correlation with the initial droplet radius (Fig. 2 b), in which R f is the radius of final colloidosome. The investigations on initial radius shed light upon

the size prediction, which is of great significance when producing colloidosome from size shrinking method. Since the mutual diffusion between water and butanol phase dictates the colloidosome formation, the concentration different between these two phases should affect the size or morphology of the produced microcapsules. To verify, we use butanol solutions predissolved with different weight percentage of water as surroundings. We find that the lower concentration difference between droplet and surroundings leads a longer time to form colloidosomes as shown in Fig. 4 a-c. The concentration difference shows a negligible effect on the resultant sizes, but it does affect the surface morphology of the resultant colloidosomes. As the difference in concentration decreases, intertwined networks of nanofibers that composites the shell of colloidosome become looser. Thus, the porosity of the TiO2 colloidosomes can be tuned by tuning the water/butanol ratio of the miscible systems.


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Figure 4. a) The radius evolution of droplet-microcapsule in surrounding water-butanol mixtures with different water content. b-c) Microscope and scanning electron microscope images of the corresponding microcapsules, respectively.


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Figure 5. a) A plot of the corrected total fluorescence released from colloidosomes fabricated from 0.375 wt% (red) and 0.75 wt% (blue) TiO2-nanofiber-suspension against time. b) Confocal microscope images shown the releasing process of colloidosomes fabricated from 0.375 wt% (i-iv), 0.75 wt% (v-vii) and 1.5 wt% (viii) TiO2-nanofiber-suspension droplet with the same radius of 340 ± 5 µm, respectively. Scale bar: 100 µm.

To investigate the release properties of the colloidosomes prepared from route 1, we load active ingredients by the dialysis method. We immersed the dried colloidosomes in the aqueous solution of 0.1 wt% FITC-dextran that is used as a model active ingredient. After


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incubation of 20 hours, the colloidosomes loaded with active ingredients were transferred into a micro-well plate and the redundant aqueous solution was removed by pipette. We monitored the release of FITC-dextran macromolecules from the fabricated colloidosomes under confocal microscopy (Fig. 5 a-b). The FITC-dextran macromolecules with hydrodynamic diameters around 30 nm can diffuse into the pores by the interlocked nanofiber network of the colloidosomes. Less nanofibers lead to looser network and large pores of the resultant colloidosome, thus higher loading efficiency. Indeed, as the nanofiber concentration increases from 0.375 wt%, 0.75 wt% to 1.5 wt%, the fluorescence intensity of the colloidosomes loaded with FITC-dextran shows a decrease from 8.80×106＞6.95×106＞ 1.81×106, sequentially. However, the colloidosome with loose network and high loading efficiency has an initial burst up to 34.5 % in the first 10 seconds, while the colloidosome with the intermediate loading efficiency shows a relatively steady release profile. Therefore, the loading efficiency and release profile of the formed colloidosomes can be optimized by their porosity through tuning the concentration of the precursor Pickering emulsion droplets.


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Figure 6. a) Schematic illustration of crust-like colloidosome fabrication formed by route 2. b-c) SEM images of TiO2 colloidosome with hollow cavities assisted by 10 wt% Pluronic F127 and 5 wt% PEG, respectively.

We hypothesize that if the spontaneously evolved B/W/B double emulsions can be stabilized with no dewetting cores, the nanofibers can continuously bind to the B/W and W/B interfaces. Consequently, as water diffuses out and these two interface gets close, nanofibers start to jam and colloidosomes are formed. As such, we can fabricate colloidosomes with thin shells and large hollow cavities by the addition of surfactant polymer in the aqueous phase. To verify, we pre-dissolve amphiphilic polymers, for instance, 10 wt% Pluronic F127 polymer in the aqueous phase, that are assumed to promote droplet stabilization by steric effects. When the initial droplet develops into B/W/B droplet, these amphiphiles combined with TiO2 nanofibers will automatically assemble at the liquid interfaces and contribute to a decrease in total free energy, thus stabilizing the spontaneous double emulsion droplets [38]. As the middle water phase diffuse out, TiO2 nanofibers and amphiphiles deposit to form the nanometer-sized shell (Fig. 6 a-c). The resultant colloidosomes show complete hollow


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structures rather than nest-like porous structure. Similarly, we use 5 wt% polyethylene glycol (PEG 2000) with nanofiber concentration of 0.15wt%, and similar crust-like structures with large hollow interior structures are obtained (Fig. 6 c). At such a low fiber concentration of 0.15 wt%, we find that the 5 wt% polymers manifest good support of the shell during the solvent exchange and droplet shrinking, demonstrating a thin yet integrated shell. The crustlike colloidosomes with large internal cavities are readily useful for encapsulating drugs, gold nanoparticles as well as dyes. The micron-sized colloidosomes by functional colloidal particles are recyclable and desirable for enhancing the efficiency of photothermal or photocatalytic reactions in medical therapy, biochemistry, and solar cell applications[34].

Conclusion We have described two facile routes for controlled fabrication of monodisperse nest/crustlike colloidosomes constructed with TiO2 nanofibers. We elaborate two different mechanisms in the mutual diffusion process, in which divergences sprang from the component in initial droplets, with or without the addition of polymer, and lead to nest/crust-like structures. We further discussed the influence of the initial droplet radius, the nanofiber concentrations and concentrations of water-containing n-butanol mixtures on the size as well as the morphology of resultant colloidosomes. These self-assembling routes provide possibilities of making colloidosomes with flexibly controlled surface morphologies and large hollow cavities. We also investigated the loading efficiency and release profile of fluorescent macromolecules of the fabricated colloidosome with different porosities. Our facile, versatile and scalable approach provides opportunities to design functional capsules in binary or ternary mixtures with various compositions and structures. The as-produced TiO2 colloidosomes hold great promise in practical applications as microcapsules for photothermal therapies, photocatalytic carriers, and energy storage.


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Supporting Information Details of the SEM images of colloidosomes, the cross-sectional morphology of sliced specimen, movie of generation of monodisperse water/n-butanol single emulsions, movie of the mutual diffusion and dewetting process of a model water/n-butanol droplet.

Acknowledgements Authors wish to thank the financial support from the Research Grants Council of Hong Kong (GRF 17211115, GRF17207914, GRF HKU717613E, GRF HKU718111E), the University of Hong Kong (URC 201511159108, 201411159074 and 201311159187), Young Scholar’s Program (NSFC 11504238) from the National Natural Science Foundation of China, and the Fundamental Research Program of Shenzhen (JCYJ20160229164007864) are gratefully acknowledged. The work is also supported in part by the Zhejiang Provincial, Hangzhou Municipal and Lin’an County Governments.


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Self-Assembly of TiO2 Nanofiber-Based Microcapsules by

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