Inverse Pickering Emulsions Stabilized by Cinnamate Modified

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Inverse Pickering Emulsions Stabilized by Cinnamate Modified Cellulose Nanocrystals as Templates to Prepare Silica Colloidosomes Zhen Zhang, Kam (Michael) Chiu Tam, Xiaosong Wang, and Gilles Sèbe ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04061 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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ACS Sustainable Chemistry & Engineering

Inverse Pickering Emulsions Stabilized by Cinnamate Modified Cellulose Nanocrystals as Templates to Prepare Silica Colloidosomes Zhen Zhang,†, ‡ Kam C. Tam,*,⊥ Xiaosong Wang,*,† Gilles Sèbe*,‡,§ † Department of Chemistry, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1. ‡ University of Bordeaux, LCPO, UMR 5629, F-33607 Pessac, France

⊥ Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario, Canada, N2L3G1 §CNRS, LCPO, UMR 5629, F-33607 Pessac, France

Corresponding Authors * E-mail: [email protected] * E-mail: [email protected] * E-mail: [email protected]

KEYWORDS: Cellulose nanocrystals, cinnamate, Pickering emulsions, sol-gel, colloidosomes

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ABSTRACT: A facile method to prepare colloidosomes at room temperature from inverse w/o Pickering emulsions containing silica precursors and stabilized by cinnamate modified cellulose nanocrystals (Cin-CNC) is proposed. Cin-CNC Pickering emulsifiers were prepared by acylation with an excess of cinnamoyl chloride. The Cin-CNC surface displayed partial wettability with both toluene and water, which allowed the stabilization of water-in-oil inverse Pickering emulsions. The Cin-CNC particles around the droplets were subsequently locked by crosslinking TEOS or TBOS silica precursors at the water/toluene interface, leading to an intricate network of polysiloxane within the Cin-CNC shell. In optimized conditions, the Cin-CNC/silica colloidosomes obtained displayed a robust and compact shell, which ensured a long-term encapsulation of rhodamine B or biological molecules such as fluorescent deoxyribonucleic acid (DNA).

INTRODUCTION Colloidosomes are hollow microcapsules templated from Pickering emulsions, whose shell consists of coagulated or fused colloid particles.1-4 They possess a great potential for the microencapsulation of active ingredients such as drugs, pesticides, flavors, proteins, and even deoxyribonucleic acid (DNA), enabling promising applications in the field of food, pharmacy, personal care and cosmetics.5 Colloidosomes are prepared from emulsions stabilized by colloidal particles instead of surfactants, known as Pickering emulsions.2,

4, 6-10

In these systems, the

particles display partial wettability with both the oil and the water phase and are irreversibly absorbed at the oil/water interface. Both inorganic and organic colloidal particles can be used as solid

surfactants,

including

silica,

metal

oxides,

polymers,

carbon

nanotubes

and

polysaccharides.6, 11 In colloidosomes, the Pickering emulsifiers at the interface of the emulsion droplets are locked together to form a solid shell. The properties of the colloidosomes obtained –


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i.e. the size, permeability, biocompatibility and mechanical properties – are then controlled by varying the conditions of preparation (type of colloidal particles, solvent, method used to lock the colloidal particles…).1 Many methods can be used to convert the Pickering emulsion templates into robust colloidosomes, such as thermal annealing, gel trapping, polymerization of the droplet phase, polyelectrolyte complexation and covalent cross-linking.5, 12-21 However, most of these methods are multi-steps, time-consuming, require high temperatures, or lead to shells with poor mechanical strength.5 Moreover, the colloidosomes often display insufficient encapsulation performance, due to the loose linkage between colloidal particles. The encapsulated ingredients inside the colloidosomes are generally released within one day, which is still too fast for most commercial applications.5, 12, 21 Recently, an alternative method based on the sol-gel reaction of silica precursors at room temperature has been proposed.2, 4, 10, 22 In this sol-gel method, hydrophobically modified silica nanoparticles were employed as Pickering emulsifiers, while hyperbranched polyethoxysiloxane (PEOS) was used as sol-gel precursor to lock the nanoparticles. The method resulted in outstanding encapsulation performance owing to the robust and dense silica shell obtained after the condensation of PEOS. Although the method is promising, the inorganic nature of the cross-linked shell leads to a brittle structure, and the tedious synthesis of PEOS increases the preparation steps of the colloidosomes.23 It is believed that the utilization of organic nanoparticles and simple sol-gel precursors – such as tetraethyl orthosilicate (TEOS) – could improve the process. Very recently, it has been shown that cellulose nanocrystals (CNC) – a sustainable bio-sourced material – could serve as a valuable colloidal stabilizer in Pickering emulsions.24-27 CNC are nanoparticles produced by sulfuric acid hydrolysis of cellulosic substrates such as microcrystalline cellulose, paper or pulp.28-29 The treatment provokes the hydrolysis of the


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amorphous regions, leading to the release of nanorods bearing sulfate ester groups at their surface. They are renewable, biocompatible and display a high tensile strength and elastic modulus,29-30 which could be an asset to improve the mechanical strength of the colloidosomes shell. Moreover, they can be easily cross-linked with polysiloxane matrices via the sol-gel method, due to the abundant hydroxyl groups present at their surface.31-35 Pristine CNC have demonstrated good performance in stabilizing various oil-in-water (o/w) emulsions systems, but they generally require the utilization of salt to screen the repulsion between the negatively charged particles.24-27 Another method consists in adjusting the hydrophilic/hydrophobic balance at the CNC surface by chemical modification.36-38 In particular, remarkably stable o/w emulsions of ethyl acetate, toluene or cyclohexane were recently obtained with acetylated CNC or cinnamate-grafted CNC.37-38 Cinnamates moieties are interesting as they can undergo reversible dimerization or trans−cis isomerization upon UV irradiation.39 Inverse w/o emulsions can also be obtained if the nanocellulose surface is sufficiently hydrophobic, but studies are rare.40-42 Herein, cinnamate-grafted cellulose nanocrystals (Cin-CNC) were prepared by acylation with cinnamoyl chloride and used to prepare colloidosomes from inverse w/o Pickering emulsions containing silica precursors. The grafted nanoparticles were characterized by Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), dynamic Light Scattering (DLS) and water contact angle (WCA) measurements. The Cin-CNC nanoparticles were subsequently used as Pickering emulsifiers to stabilize inverse water-in-toluene Pickering emulsions. The impact of Cin-CNC concentration on the stability and size of the emulsion droplets was then investigated by optical microscopy. These emulsions were further cross-linked in the presence tetraethyl orthosilicate (TEOS) or tetrabutyl orthosilicate


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(TBOS) precursors to produce CNC/Silica colloidosomes. The mechanical integrity and encapsulating properties of the microcapsules obtained were particularly discussed.

EXPERIMENTAL SECTION Materials. The CNC were purchased from the University of Maine (containing 1.05% wt. % of sulfur). Dimethylaminopyridine (DMAP), triethylamine (TEA), cinnamoyl chloride, rhodamine B, tetraethyl orthosilicate (TEOS), and tetrabutyl orthosilicate (TBOS) were purchased from Sigma-Aldrich. Fluorescent DNA used here is 6-carboxyfluorescein (FAM) labeled A15 DNA (FAM-A15), which was kindly donated by Zhicheng Huang from Dr. Juewen Liu’s laboratory (sequence: FAM-AAAAAAAAAAAAAAA).43 Preparation of the Cin-CNC Pickering emulsifiers. CNC (1 g) were dispersed in DMF (80 mL) by sonication. After the addition of TEA (2.58 mL, 18.5 mmol) and DMAP (1.133 g, 9.3 mmol), the dispersion was cooled down in an ice bath. A solution of cinnamoyl chloride (3.33 g, 20 mmol) in DMF (20 mL) was then added dropwise in the mixture. The reaction was conducted at room temperature. After 24 hours, water (50 mL) was added to terminate the reaction. The resultant product was purified by Soxhlet extraction with THF for 24 hours, followed by dialysis in deionized water for 7 days. The final Cin-CNC was finally obtained after removal of the water by freeze-drying. Preparation of the inverse Pickering emulsions stabilized by Cin-CNC. A Cin-CNC stock dispersion in toluene (10 mg/mL) was prepared by dispersing 200 mg Cin-CNC in 20 ml toluene by Microson ultrasonic cell disruptor. The Cin-CNC dispersions of lower concentrations used in the paper were obtained by dilution. 7 mL of the dispersion was then mixed with 3 mL deionized water (on its own or dyed with rhodamine B) to reach a volume ratio of oil phase to water phase


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of 7:3. The inverse Pickering emulsion was obtained by sonicating the mixture for 30 s at the output power of 16 watts by Microson ultrasonic cell disruptor. The distribution of the diameter of the emulsion droplets was estimated from the measure of 500 emulsion droplets in the optical micrograph images. Preparation of the Cin-CNC/silica colloidosomes. TEOS or TBOS (3.5 mL) were added to toluene dispersions of Cin-CNC (3.5 mL) of different concentrations (4 mg/mL or 10 mg/mL), and the mixture was vortexed for 1 min. The resulting concentration of Cin-CNC in the oil phase was 2 mg/mL or 5 mg/mL, respectively. After the addition of water (3 mL), the inverse Pickering emulsions were obtained by sonication for 30 s at the output power of 16 W. The emulsion was kept still at room temperature for 6 or 10 days, to allow the cross-linking of the TEOS or TBOS precursors, respectively. The emulsion was then washed with ethanol three times, then dialyzed against deionized water for 5 days. The final Cin-CNC/silica colloidosomes were recovered after removal of the water by freeze-drying. The encapsulation of rhodamine B or fluorescent DNA was performed using the same procedure, with aqueous solutions of rhodamine B or fluorescent DNA (0.05 mg/mL) as the water phase. Characterization. The Fourier transform infrared spectra (FTIR) spectra were obtained with the potassium bromide technique, using a Thermo Nicolet Avatar 970 FTIR spectrometer, at a resolution of 8 cm-1 (64 scans). Transmission electron microscopy (TEM) was conducted on a Philips CM10 microscope, at an acceleration voltage of 60 keV. The TEM samples were prepared by drop coating THF dispersions of Cin-CNC onto copper grids (200 mesh coated with copper), then drying the samples overnight at ambient temperature.


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Thermogravimetric analyses (TGA) were performed on a TGA-Q50 system (TA Instruments), at a heating rate of 10 oC /min and under nitrogen atmosphere. Dynamic light scattering (DLS) and zeta-potential experiments were performed using a Malvern Instrument Zetasizer Nanoseries. The samples for water contact angle (WCA) measurements were prepared by spin coating the CNC aqueous dispersion or Cin-CNC THF dispersion on glass slides. The static water contact angle (WCA) was measured by analyzing the images of 3 water droplets using MATLAB. The optical images were captured using an inverted optical microscope (Nikon Elipse Ti-S) equipped with a CCD camera (QImaging ReTIGA 2000R). The emulsion droplets were observed using an inverted optical microscope (Carl Zeiss Axio Observer. Z1m) equipped with a CCD camera (Carl Zeiss Axio Cam 1Cm1). The minimum interfacial coverage required to stabilize our Pickering emulsions (C) was calculated using the following relation (valid in the Cin-CNC concentration range between 0.25 and 2 mg/mL): 25, 44 1/D(3,2) = [Cin-CNC]Voil/(6hρVwaterC) (1) where D(3,2) is the surface-average diameter of the emulsion droplets; [Cin-CNC] is the concentration of Cin-CNC in the oil phase; Voil is the volume of toluene (7 mL), h is the diameter of the Cin-CNC particles (8 nm) estimated from the width measured by TEM; ρ is the density of the Cin-CNC particles (1.6 g/cm3) assuming that this density is similar to that of the CNC; Vwater is the volume of the water phase (3 mL) and C is the interfacial coverage. The surface tension of the Cin-CNC toluene suspension and the interfacial tension between toluene and water were measured with a Data Physics DCAT 21 tensiometer. 45


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Scanning electron microscopy (SEM) measurements were performed on a Hitachi S4800 scanning electron microscope with an accelerating voltage of 2 kV.

RESULT AND DISSCUSSION Preparation and characterization of the Cin-CNC Pickering emulsifier. The type of Pickering emulsions obtained with solid particles is determined by the particle wettability, expressed in terms of the three-phase contact angle θw that particles makes at the oil-water interface.6, 8, 40 If θw is lower than 90o, the solid particles have more affinity for the water phase (there are more hydrophilic), resulting in o/w Pickering emulsions. If θw is higher than 90o, the solid particles have more affinity for the oil phase (they are more lipophilic), leading to inverse w/o Pickering emulsions.8 CNC being hydrophilic, o/w emulsions are generally obtained with these particles,24-27 but inverse w/o emulsions can be produced if the CNC surface is chemically modified with suitable hydrophobic moieties.40-42 This was achieved in the current paper by reacting the surface hydroxyl groups with cinnamoyl chloride, an acylating agent allowing the grafting of hydrophobic cinnamate functions (Figure 1A).


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Figure 1. (A) Preparation of the Cin-CNC Pickering emulsifier by acylation of CNC with cinnamoyl chloride. (B) FT-IR absorbance spectra of the CNC and Cin-CNC materials. (C) TEM image of the Cin-CNC nanoparticles. The CNC starting material used in the experiments has been already fully characterized in a previous study of the same author.46 It consists of rod-like particles with estimated dimensions of 110.3 ± 47.7 nm in length and 4.8 ± 1.1 nm in diameter. The amount of accessible hydroxyl groups at the CNC surface was estimated to be 3.10 ± 0.11 mmol/g, according to a method based on phosphorylation coupled with

31

P NMR and FT-IR analysis.46 The CNC particles are

negatively charged (zeta-potential in water = -48.7 mV), due to the presence of sulfate ester groups at their surface (imparted by the sulphuric acid treatment). The surface modification of CNC was conducted at room temperature in DMF (for 24h), using an excess cinnamoyl chloride, (Figure 1A). The successful grafting was confirmed by the FTIR spectroscopy (Figure 1B), through the observation of the characteristic vibrations of the grafted cinnamate moieties at 1718


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cm-1 (C=O stretching), 1635 cm-1 (exocyclic C=C stretching) and 765 cm-1 (out-of-plane aromatic C-H bending).37 The TEM characterization of the Cin-CNC nanoparticles indicated that the rod-like morphology of the cellulose nanocrystals was retained after the esterification reaction (Figure 1C); the particle dimensions were estimated to be 138.9 ± 35.4 nm in length and 8.1 ± 3.2 nm in width from the TEM pictures. The impact of the treatment on the thermal stability of the CNC was investigated by TGA. The thermograms and corresponding DTG derivatives of the particles before and after esterification are presented in Figure 2.

Figure 2. TGA thermograms (A) and DTG curves (B) of the CNC and Cin-CNC particles. The thermogram of pristine CNC is consistent with the thermal behavior generally observed for CNC bearing sulfate ester groups.47-48 The main decomposition temperature range of CNC is between 250 and 320 °C, with an onset degradation temperature (Tonset) at 281 °C and a maximum degradation temperature (Tmax) at 297 °C. The 6% weight loss noted below 100 oC was associated with the evaporation of absorbed moisture. This weight loss is absent from the thermogram of Cin-CNC, indicating that the esterified nanoparticles were more hydrophobic. Their thermal stability was however decreased by the treatment (Tonset = 263 °C and Tmax = 277 °C), which could be related to the singular reactivity of the cinnamate moieties.48 Indeed,


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cinnamate derivatives can undergo radical trans-cis isomerization under thermal activation,39 which may promote the degradation of cellulose through free-radical mechanisms. The dispersibility of the CNC particles before and after esterification was examined in four solvents of decreasing polarity: namely, water (ε = 80; µ = 1.85 D), DMF (ε = 38; µ = 3.82 D), THF (ε = 7.5; µ = 1.63 D), and toluene (ε = 2.4; µ = 0.31 D), the polarity of each solvent being expressed by its dielectric constant ε and dipole moment µ. The quality of the dispersions in the different solvents was inferred from their visual aspect (Figure 3A) and (when possible) apparent hydrodynamic diameter (Dh) measured by DLS (Figure 3B).

Figure 3. (A) Optical images of CNC and Cin-CNC dispersions in water, DMF, THF or toluene. (B) Size distribution in intensity of the CNC and Cin-CNC particles dispersed in water, DMF or THF (0.2 mg/ml) (C) Images of the water droplets deposited on the spin-coated CNC and CinCNC films during WCA measurements.


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As expected, the dispersibility of the CNC in water was rather good: the suspension was clear and the Dh measured 2 min after sonication (90 nm) was consistent with the estimated length of the nano-rods.46 The CNC could be also dispersed in DMF (clear suspension), but a bimodal distribution of Dh was obtained in that case (Dh1 = 189 nm and Dh2 = 645), indicating that the particles were more aggregated in this aprotic solvent. On the other hand, no dispersion could be obtained in low-polar THF or toluene, even after extensive sonication. After esterification, the Cin-CNC dispersion was turbid in water (flocculation occurred within hours), while a clear and transparent suspension was obtained in DMF and THF (Figure 3A). In toluene, the dispersion was now meta-stable. In addition, the Dh increased drastically in water (two populations were identified: Dh1 = 469 nm and Dh2 = 4708), while it decreased in DMF (Dh = 93 nm), and could now be measured in THF (Dh = 189 nm) (Figure 3B). The compatibility of the cellulose nanoparticles towards DMF, THF and toluene was therefore significantly improved by the grafted cinnamate moieties, which obviously imparted hydrophobic properties to the material. The hydrophobic character of the Cin-CNC surface was further confirmed by performing water contact angle (WCA) measurements at the surface of CNC and Cin-CNC films prepared by spincoating (Figure 3C). As expected, the WCA increased significantly after the esterification treatment, rising from 14.0° before modification to 75.9° after the grafting.

Preparation and properties of the inverse Pickering emulsions stabilized by the Cin-CNC particles. The Cin-CNC nanoparticles functionalized in our experimental conditions turned out to be sufficiently hydrophobic to serve as stabilizer for inverse w/o Pickering emulsions. Waterin-toluene emulsions were indeed easily prepared by adding water to a toluene suspension of Cin-CNC and sonicating the mixture for 30 s (w/o ratio = 30/70 v/v). The emulsions were stable


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for at least 6 months. The optical micrographs of the emulsion droplets obtained with various concentrations of Cin-CNC are shown in Figure 4. The suspensions were characterized 24 hours after emulsification, to allow time for the system to stabilize through the so-called limited coalescence process.49 The w/o emulsion type was confirmed by observing what happened when a drop of the emulsion was added to a volume of pure toluene or pure water. In addition, some emulsions were prepared with water stained by rhodamine B and observed with a fluorescent microscope to visualize the water droplets (Figure 4E).

Figure 4. Optical micrographs of the inverse water-in-toluene emulsion droplets stabilized with different concentrations of Cin-CNC: A, 0.25 mg/mL; B, 0.5 mg/mL; C, 1 mg/mL; D, 2 mg/mL; E, 2 mg/mL; and F, 4 mg/mL. The water droplets in E were stained by rhodamine B and observed with a fluorescent microscope. Scale bar: 50 µm. The distribution of the number-average diameter D(1,0) of the emulsion droplets is Gaussian (Figure 5A) and the dispersity – defined as the ratio of volume-average diameter D(4,3) relative to D(1,0)50 – is quite narrow (between 1.08 and 1.14). As expected, the D(1,0) of the emulsion


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droplets decreased with increasing Cin-CNC concentration since more particles enabled the stabilization of larger w/o interfacial area (Figure 5B).

Figure 5. (A) Distribution of the number-average diameter D(1,0) of the emulsion droplets obtained at different concentrations of Cin-CNC. (B) Evolution of the number-average diameter D(1,0) as a function of Cin-CNC concentration. (C) Evolution of the reciprocal surface-average diameter 1/D(3,2) as a function of Cin-CNC concentration. (D) Evolution of the surface tension of the Cin-CNC toluene suspension (left axis) and interfacial tension between toluene and water (right axis) as a function of Cin-CNC concentration. The reciprocal surface-average diameter of the emulsion droplets 1/D(3,2) was plotted as a function of Cin-CNC concentration in Figure 5C. In agreement with the limited coalescence mechanism, 1/D(3,2) increased linearly with the particles concentration between 0.25 and 2 mg/mL (R2 = 0.996), indicating that the interfacial coverage (i.e. the packing density of the particles at the w/o interface) was constant.45 In this range of concentration, the particles content


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is too low to permit a dense coverage of the w/o interface created in the system during agitation. As a consequence, the partially unprotected droplets coalesce when the sonication is stopped, until a sufficient coverage is reached. The minimum interfacial coverage required (C) can be estimated based on a relation reported in the litterature.25, 44 After calculation (see experimental section), we found that an interfacial coverage of about 62% was sufficient to stabilize our waterin-toluene inverse emulsion system, which is in agreement with the values found in the literature for other systems stabilized by CNC Pickering surfactants.25-26, 44-45 Distinctively from the emulsions stabilized by amphiphilic surfactants, interfacial tension reduction is not the operative stabilization mechanism in Pickering emulsions.51-52 To verify this point, the surface tension of Cin-CNC toluene suspension and interfacial tension between toluene and water were measured as a function of Cin-CNC concentration (Figure 5D). The surface tension and interfacial tension did not vary with Cin-CNC concentration, confirming that the stabilization was imparted by the Cin-CNC particles adsorbed at the w/o interface.

Preparation and characterization of Cin-CNC/Silica colloidosomes. To prepare colloidosomes from our water-in-toluene emulsions, we envisaged locking the Cin-CNC particles around the droplets by cross-linking tetraethyl orthosilicate (TEOS) or tetrabutyl orthosilicate (TBOS) silica precursors at the w/o interface, following the methodology described in Figure 6A. We first investigated the impact of pH (pH 1 or 7) and Cin-CNC concentration on the droplets diameter D(1,0) (Figure 6B).


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Figure 6. (A) Schematic representation of the methodology used to prepare the Cin-CNC/Silica colloidosomes. (B) Evolution with time of the number-average diameter D(1,0) of the emulsion droplets in the Pickering systems containing TEOS or TBOS precursors (pH = 1 or 7; [Cin-CNC] = 2 or 5 mg/mL). (C-D) Evolution of the inverse water-in-toluene emulsion droplets (without silica precursors) when increasing amounts of ethanol were added to the system: 0 µL (C), 20 µL (D), 40 µL (E), and 100 µL (F) (initial volume = 200 µL; pH = 7; [Cin-CNC] = 2 mg/mL; scale bar: 100 µm). Without silica precursors ([Cin-CNC] = 2 mg/mL), the droplet diameter was stable with time (at about 9.6 µm) and did not vary with pH. When TEOS was added at pH = 7, the diameter was also stable with time, but the droplets were bigger (D(1,0) = 20.3 µm). At pH = 1 however, the droplets diameter increased significantly from 22.1 to 60.1 µm during the initial 30 min, presumably because the TEOS reacted with water at this pH. TEOS is not soluble in water, but is


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expected to hydrolyze and condense at the water/toluene interface at pH = 1, to form an intricate network of polysiloxane within the Cin-CNC shell.4, 22-23 The increase in droplets size observed can therefore be explained by the ethanol released in the course of the reaction, which is soluble in water and modify the volume of the droplet. This increase stopped after 30 min, when the cellulosic shell was sufficiently rigidified by the cross-linked silica layer. A similar behavior was observed when the Cin-CNC concentration was increased to 5 mg/mL, but the final size of the droplets was smaller in that case (24.2 µm), in consistency with the better interfacial coverage expected at this concentration. To further confirm that ethanol is responsible for the variations in droplet size, water-in-toluene emulsion droplets (without silica precursors) were observed by optical microscopy, while ethanol was progressively added in the emulsion system (Figure 6CF). As expected, an increase in droplet size was observed when more ethanol was added, in coherence with the so-called Ostwald ripening process.6, 9, 40, 53 When TEOS was replaced by TBOS at pH 7 ([Cin-CNC] = 2 mg/mL), the droplets diameter decreased to 15.3 µm (instead of 20.3 µm) (Figure 6B), supposedly because TBOS had more affinity with the Cin-CNC particles. At pH =1, the emulsion size increased only slightly during the first 30 min (from 16.0 to 17.4 µm), which could result from the limited miscibility in water of the butanol released. The locking of the Cin-CNC particles by the silica precursor was further confirmed by observing the evolution of the emulsion droplets deposited on glass slides (50 µL), in ambient environment (Figure 7). After 10 min, only the droplets stabilized with the silica precursor at pH 1 retained their spherical shape (but the aspect of the surface was crumpled), while all other emulsions quickly broke due to the evaporation of toluene.


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Figure 7. Optical micrographs of the inverse water-in-toluene emulsion droplets before (A, B, C) and after (D, E, F) evaporation of the toluene (for 6 days at room temperature): A & D, without TEOS; B & E, with TEOS at pH =7; C & F, with TEOS at pH =1 ([Cin-CNC] =2 mg/mL). Scale bar: 50 µm. Although the silica precursors were sufficiently cross-linked to prevent any further increase in droplets diameter after 30 min, the sol-gel reaction might not be completed, which would impact the mechanical strength of the colloidosomes shell.4 Indeed, the Cin-CNC/silica colloidosomes cross-linked for two days with TEOS quickly broke when ethanol was added to the suspension. In our experimental conditions, a cross-linking time of at least 6 days was actually required with TEOS, to obtained colloidosomes that retained their integrity in ethanol and supported mild sonication (Figure 8A). With TBOS on the other hand, ten days were necessary, because of the lower reactivity of this precursor (Figure 8B). The microcapsules obtained with TEOS were spherical (Figure 8C), with a shell of about 1.2 µm in thickness (estimated from the SEM images of a broken colloidosome in Figure 8F). Their FTIR spectrum (Figure 9A) displayed the characteristic vibrations of silica at 450, 800, 964 and 1006 cm-1 (Si-O vibrations),54 as expected


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after hydrolysis/condensation of TEOS. The FT-IR bands of cellulose could not be clearly distinguished, indicating that the Cin-CNC/silica colloidosomes were mostly composed of inorganic material. This was further confirmed by the TGA thermogram (Figure 9B), which displayed a weight loss of only 14.5 % after heating at 800 °C, assigned to the evaporation of adsorbed water (∼ 10 % between 25 and 120 °C) and degradation of Cin-CNC (∼ 4.5% between 150 and 500 °C).

Figure 8. (A) Optical micrograph and (C & F) SEM images of the Cin-CNC/silica colloidosomes cross-linked with TEOS for 6 days ([Cin-CNC] = 5 mg/mL; pH = 1). (B) Optical micrograph of the Cin-CNC/silica colloidosomes cross-linked with TBOS for 10 days ([CinCNC] = 5 mg/mL; pH = 1). (D) Fluorescent optical micrographs of the same colloidosomes after encapsulation of rhodamine B or (E) fluorescent DNA. The scale bars of the optical micrographs and SEM images are 50 and 5 µm respectively.


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Figure 9. FT-IR spectrum (A) and TGA thermogram (B) of the freeze-dried Cin-CNC/silica colloidosomes cross-linked with TEOS for 6 days ([Cin-CNC] = 5 mg/mL; pH = 1).

The encapsulating properties of the Cin-CNC/silica colloidosomes were investigated by preparing samples with water solutions of rhodamine B or fluorescent DNA (FAM-A15). The colloidosomes obtained were observed by fluorescent optical microscopy, which confirmed the encapsulation of the molecules (Figures 8D & 8E). To probe the barrier properties of the capsules, the colloidosomes containing rhodamine B or fluorescent DNA were dispersed in water. No fluorescence was detected in the external medium after one month, indicating that the structure of the Cin-CNC/Silica shell was sufficiently compact to ensure the long-term retention of rhodamine B, or biological molecules such as fluorescent DNA. A timely release of the incorporated compounds can then be later envisaged, by breaking the thin shell under mechanical forces.2 It might be also possible to control the release of the encapsulated molecules by varying the Cin-CNC or TEOS concentrations during the preparation of the colloidosomes. The variation of these parameters should indeed lead to Cin-CNC/Silica shells of different porosity and


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permeability, which should impact the releasing kinetics of the encapsulated compounds. The Cin-CNC/silica hollow microspheres could alternatively find application as reinforcing agents,5455

flame retardants,55 radiation shielding agents,56 particles for hydrogen gas storage,57 etc.58-59

CONCLUSION We have presented a novel method to prepare colloidosomes from inverse w/o Pickering emulsions containing silica precursors and stabilized by chemically modified cellulose nanocrystals (CNC). The grafting of cinnamate moieties at the surface of the CNCs allowed producing particles of increased hydrophobicity and improved compatibility with low-polar solvents, which could be later used to stabilize water-in-toluene inverse Pickering emulsions. The size of the water emulsion droplets was found to decrease with increasing Cin-CNC concentration, in agreement with the limited coalescence process. A minimum interfacial coverage of about 62% was sufficient to stabilize our water-in-toluene emulsion system. To obtain colloidosomes from these dispersions, the Cin-CNC particles around the droplets were locked by cross-linking TEOS or TBOS silica precursors at the water/toluene interface, leading to the formation of an intricate network of polysiloxane within the Cin-CNC shell. In optimized conditions, the Cin-CNC/silica colloidosomes obtained displayed a robust and compact shell, which allowed the long-term encapsulation of molecules such as rhodamine B or fluorescent DNA. The encapsulating properties of the Cin-CNC/silica colloidosomes have been therefore demonstrated in our study, but the releasing properties of these capsules require now to be explored. Further breakthrough could be also made in areas where these hollow microspheres could find original applications (reinforcing agents, flame retardants, radiation shielding agents, hydrogen gas storage, etc). This will be the object of our future investigations.


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ACKNOWLEDGMENT The authors are thankful to the University of Waterloo for providing financial support to this project. The authors are also grateful to Zhicheng Huang from Pr Juewen Liu's lab (department of Chemistry, University of Waterloo) who supplied the DNA. The authors also want to thank Jian Zhi from Pr Pu Chen‘s lab (Department of Chemical Engineer, University of Waterloo) for his help with FTIR analysis.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] * E-mail: [email protected] * E-mail: [email protected] Notes The authors declare no competing financial interest.


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For Table of Contents Use Only

Inverse Pickering emulsion

Cinnamate-g- CNC + Toluene/water + TEOS

Cross-linking of TEOS

Encapsulation of active molecules

Colloidosome

DNA

Sustainable encapsulating colloidosomes were prepared by cross-linking inverse w/o Pickering emulsions containing silica precursors and stabilized by cellulose nanoparticles.


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Inverse Pickering Emulsions Stabilized by Cinnamate Modified

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