Pickering Emulsions Stabilized by pH-Responsive ... - ACS Publications

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Pickering emulsions stabilized by pH-responsive microgels and their scalable transformation to robust submicrometer colloidoisomes with selective permeability Wenkai Wang, Amir H. Milani, Zhengxing Cui, Mingning Zhu, and Brian R. Saunders Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01618 • Publication Date (Web): 27 Jul 2017 Downloaded from http://pubs.acs.org on July 29, 2017

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Pickering emulsions stabilized by pH-responsive microgels and their scalable transformation to robust sub-micrometer colloidoisomes with selective permeability Wenkai Wang, Amir Milani, Zhengxing Cui, Mingning Zhu and Brian R. Saunders* Polymers and Composites Group, School of Materials, The University of Manchester, MSS Tower, Manchester, M13 9PL, U.K. Corresponding author: [email protected] ABSTRACT Colloidosomes are micrometer-sized hollow particles that have shells consisting of coagulated or fused colloid particles. Whilst many large colloidosomes with sizes well above 1.0 µm have been prepared there are fewer examples of sub-micrometer colloidosomes. Here, we establish a simple emulsion templating-based method for the preparation of robust submicrometer pH-responsive microgel colloidosomes. The colloidosomes are constructed from microgel particles based on ethyl acrylate and methacrylic acid with peripheral vinyl groups. The pH-responsive microgels acted as both a Pickering emulsion stabilizer and macrocrosslinker. The emulsion formation studies showed that the minimum droplet diameter was reached when the microgel particles were partially swollen. Microgel colloidosomes were prepared by covalently inter-linking the microgels adsorbed at the oil-water interface using thermal free-radical coupling. The colloidosomes were prepared using a standard high-shear mixer with two different rotor sizes which corresponded to high shear (HS) and very high shear (VHS) mixing conditions. The latter enabled construction of sub-micrometer pHresponsive microgel-colloidosomes at the gram-scale. The colloidosomes swelled strongly when the pH increased to above 6.0. The colloidosomes were robust and showed no evidence of colloidosome breakup at high pH. The effect of solute size on shell permeation was studied using a range of FITC-dextran polymers and size-selective permeation occurred. The average pore size of the VHS microgel-colloidosomes was estimated to be between 6.6 and 9.0 nm at pH 6.2. The microgel-colloidosomes properties suggest they have potential for future applications in cosmetics, photonics or delivery.

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INTRODUCTION Colloidosomes are microcapsules whose shells are composed of coagulated or fused colloidal particles1-6. They have often been constructed using colloidal assembly at oil/water droplet interfaces, which is a soft (emulsion) templating approach. Colloidosomes have attracted interest both for their interesting morphology and properties as well as their potential applications. The latter include drug release7, adsorbing materials8, insulation9, catalysis10, agriculture11 and energy12. Whilst there are many examples of colloidosomes with diameters much greater than 1 µm3,

13-16

, there are relatively few examples of sub-micrometer

colloidosomes17-19. However, such particles should offer improved potential for delivery and photonics applications. For delivery applications it has been reported that cellular uptake increases as the particle diameter decreases below 1 µm20. For photonics applications, as the diameter falls below 1 µm it approaches the wavelength of light which introduces opportunities for light management21. Stimulus responsive colloidosomes offer additional versatility22 when compared to non-stimulus responsive colloidosomes, and can be constructed using microgels16,

23, 24

. While colloidosomes prepared using temperature-

responsive microgels have been reported16, responsive microgels

23

there are few examples involving pH-

13, 25, 26

. The latter are crosslinked polymer particles that swell when the

pH approaches the pKa of the constituent polymer27-29. Inter-particle crosslinking within colloidosome shells is a desirable feature for increasing shell strength30 and hence robustness. For microgel-colloidosomes crosslinking between the particles has usually been achieved using two components; that is the microgel together with an added linking molecule, monomer, or polymer13, 15, 16, 25. However, such processes may complicate the construction process, require additional steps and dilute the stimulus responsive behaviour. Recently, we demonstrated that colloidosomes could be prepared using an emulsion templating method26 with vinyl-functionalized pH-responsive microgels. In that study the colloidosomes were prepared using UV-crosslinking26. However, the mechanism governing emulsion stabilization was not elucidated and the preparation method for the microgel-colloidosomes (which were much larger than 1 µm) was limited to the milligram scale. To have potential as a versatile hollow particle system pH-responsive microgel-colloidosomes are required that can be prepared in a scalable manner with sub-micrometer diameters and are able to swell at high pH without fragmentation (i.e., are robust). Their permeability should also be established. This study aimed to address these challenges and also to investigate stabilization of the emulsions by the microgels.


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Emulsions stabilized by microgels are a class of Pickering-Ramden emulsions and are the key precursor for microgel-colloidosomes. Microgel-stabilized emulsions (microgel-emulsions) were first reported by Ngai et al.31 and have been extensively studied32-36. An essential step for the stabilization of microgel-emulsions is the adsorption of the microgels at the oil/water interface. Microgel stabilization of emulsions benefits from the swollen nature of the particles at the oil-water interface, which depends not only on the solvent’s polarity but also on the microgel composition, crosslinking37 and morphology36. Richtering et al. showed that deformation of microgels at oil/water interfaces was important for the stabilization properties provided by microgels38. Our group proposed that interpenetration of the microgel interfaces could be used to covalently interlink the microgels in microgel-emulsions and form microgelcolloidosomes26. Here, we greatly extend our earlier work to show that sub-micrometer pHresponsive microgel-colloidosomes that are robust in terms of their pH-triggered swelling can be prepared using a new scalable method. The permeability of colloidosomes is an important aspect for colloidosomes because it offers potential for selective entrapment and/or release. For colloidosomes composed of hard particles the pore size is well known to depend on the size of the interstitial sites between the particles that comprise the shell3. Stimulus responsive microgels provide the ability to vary the pore size using environmental stimulus (such as pH or temperature). This ability is of considerable interest for a wide variety of applications including cosmetics and drug delivery22. Here, we investigate the effect of pH on shell permeability for colloidosomes constructed from pH-responsive microgels. The emulsion templating approach used to prepare our microgel colloidosomes is depicted in Scheme 1. The approach involved the use of pH-responsive poly(ethyl acrylate-comethacrylic acid-co-1,4-butanediol diacrylate)-glycidyl methacrylate (poly(EA-co-MAA-coBDDA)-GMA) microgels. We term the latter particles as MG in this work. The MGs were internally crosslinked with BDDA and were then crosslinked a second time via the peripheral GMA groups to give covalently interlinked MGs at the oil/water interface. Uniquely, the MGs had the dual roles of macro-crosslinkers and Pickering emulsion stabilizers. In a departure from our earlier approach which used UV-photocrosslinking26, we developed a scalable thermal crosslinking approach in the present study for the adsorbed MGs and decreased the colloidosome diameter to below 1.0 µm.


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O

O

HO

O

O

O

Ethylacetate

pH ~ pKa

O O

+ AIBN

OH O

O

O

MG

O

High shear (HS) Very high shear (VHS)

Poly(EA-MAA-BDDA)-GMA

Covalently interlinked

Ethyl acetate

Water

(i)

(ii)

MG-colloidosome (crumpled) MG-emulsion

(i) 50 oC + Ethylacetate (ii) pH > pKa

MG-colloidosome (swollen)

Scheme 1. Preparation of pH-responsive colloidosomes using microgels. Poly(ethylacrylateco-methacrylic acid-co-1,4-butanediol diacrylate-co-glycidyl methacrylate) (poly(EA-MAABDDA)-GMA MGs stabilized ethyl acetate-in-water emulsions containing 2,2′-azobis(2methylpropionitrile) (AIBN) and were heated to covalently interlink the MGs via free-radical coupling of surface vinyl groups. High shear (HS) or very high shear (VHS) mixing was used to control the emulsion and colloidosome diameter. The removal of ethyl acetate formed crumpled colloidsomes that subsequently swelled when the pH was increased to above the pKa. The MG-emulsion droplets and MG- colloidosomes were smallest when prepared with VHS. The study begins by investigating the MG’s ability to stabilize ethyl acetate-in-water emulsions and identifying the conditions for preparing scalable MG-colloidosomes using thermal crosslinking. Analysis of the emulsion droplet diameter variation with formulation conditions is used to provide evidence for a monolayer of MG particles at the surface of the droplets and in the shell of the MG-colloidosomes. In scaling up the preparation method the shear was increased and this together with the intrinsic properties of the MGs (which are discussed), enabled sub-micrometer microgel-colloidosomes to be prepared. DLS studies


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show the MG-colloidosomes swelled strongly and remained intact when the pH was increased. The pH-triggered permeability of the MG-colloidosomes is then studied using model solutes and the pore size and its response to pH are established. The MG-colloidosome scalability, sub-micrometer diameter, robustness to swelling, pH-responsiveness and controllable permeability raise potential for future delivery, cosmetics or photonics applications. EXPERIMENTAL Materials Ethyl acetate (99%), methacrylic acid (MAA, 99%), 1-4 butanediol diacrylate (BDDA, 90%), glycidyl methacrylate (GMA, 97%), 2,2′-azobis(2-methylpropionitrile) (AIBN, 98%) were purchased from Aldrich and used as received. Rhodamine B, Coumarin 6 and the Fluorescein isothiocyanate (FITC)-labelled dextrans were also purchased from Aldrich and used as received. The latter had average molecular weights of 4.0, 20, 40, 70 and 150 kg/mol from supplier information. All water was of ultrahigh purity de-ionised quality. Synthesis of poly(EA-MAA-BDDA)-GMA microgel The synthesis of the microgel and the vinyl-functionalization was conducted using emulsion polymerization according to a previously published method26. Briefly, a mixed solution containing EA (167 g, 1.67 mol), MAA (83 g, 0.965 mol), BDDA (2.5 g, 12.6 mmol) was prepared. A total of 31.5 g of the co-monomer solution was added to water (518 g) containing SDS (1.8 g). Subsequently, K2HPO4 (3.15 g of a 7 wt.% aqueous solution) and APS (10 g of a 2 wt.% solution) were added. The mixture was heated at 80 ˚C with mechanical stirring under nitrogen for 30 min to form the seed. The remaining monomer solution was then added at a uniform rate over 2 h. The reaction was continued for a further 2 h. After cooling, the product was purified by extensive dialysis using water. To vinyl-functionalize the microgel GMA (16.66 g) was added to the microgel dispersion (100 g, 10 wt.%) and the dispersion maintained at pH 5.2 for 8 h. Excess GMA was removed by repeated washing with chloroform. Residual chloroform was removed using rotary evaporation. All MG data presented in this work are for the vinyl-functionalised particles unless otherwise stated. Preparation of microgel-stabilized emulsions


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An aqueous phase (4.0 ml) was prepared that contained buffer at specific pH values and MG dispersions with a range of particle concentrations. Ethyl acetate (1.0 ml) was added to the aqueous phase (to give an oil phase volume fraction of 20 vol.%) and the mixture was sheared at 10,500 rpm for 120 sec using a Silverson L4R mixer equipped with a 3/8” MiniMicro tubular mixing unit which had an estimated shear rate of 16,150 s-1. This system is referred to as the high shear (HS) geometry. The calculation of the shear rate is shown in the Supporting Information. To investigate the effect of shear on emulsion formation, a MGstabilized emulsion was also prepared using the very high shear geometry (discussed below). To assess emulsion type Coumarin 6 was added to ethyl acetate prior to emulsification. Colloidosome preparation using the high shear (HS) geometry These preparations were conducted using the Silverson L4R mixer with the HS geometry described above. AIBN (10 mg, 0.061 mmol) was dissolved in ethyl acetate (1.0 ml). An aqueous phase (4.0 ml) was prepared that contained buffer (pH 6.4) and MG dispersion (0.60 wt.%). The AIBN / ethyl acetate solution was added to the aqueous phase and the mixture sheared at 10,500 rpm for 120 sec. The emulsions were placed in a round bottom flask with a condenser and heated using an oil bath at 50 oC for 3 h. Ethyl acetate was subsequently removed by rotary evaporation at room temperature. The dispersion was washed with chloroform and residual chloroform was removed using rotary evaporation. The yield of dry MG-colloidosome was 20 mg. The latter particles are termed HS MG-colloidosomes. Colloidosome preparation using the very high shear (VHS) geometry Larger-scale preparations were conducted using the Silverson L4R mixer equipped with a ¾” tubular workhead and a square-hole high shear screen. This mixing assembly had an estimated shear rate of 82,000 s-1 and is referred to as the very high shear (VHS) geometry. (See Supporting Information for the shear rate calculation.) AIBN (0.60 g, 3.66 mmol) was dissolved in ethyl acetate (60.0 ml). An aqueous phase (240 ml) was prepared that contained buffer (pH 6.4) and MG dispersion (0.60 wt.%). The AIBN / ethyl acetate solution was added to the aqueous phase and sheared at 10,500 rpm for 120 s to give a MG-stabilized emulsion. The emulsions were placed in a round bottom flask which was sealed and heated using an oil bath at 50 oC for 6 h. The dispersion was washed with chloroform and residual chloroform removed using rotary evaporation. The yield of dry VHS MG-colloidosomes was 1.07 g. Physical Measurements Optical images were obtained using an Olympus BX41 microscope


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and white transmitted light. Titration measurements were performed using a Mettler Toledo titration unit and supporting electrolyte (0.1 M NaCl). Dynamic Light Scattering (DLS) measurements were obtained using a Malvern Nano ZS instrument. All measurements were conducted using buffer solutions (0.1 M). SEM measurements were obtained using a Philips XL30 FEG SEM instrument. Dispersions of particles were deposited on SEM stubs at room temperature. The samples were coated by platinum or carbon. The number-average particle diameters were measured with Image J software (National Institutes of Health) and at least 100 particles were measured. Confocal laser scanning microscopy (CLSM) images were obtained using Broadband Confocal Leica TCS SP5 instrument. The MG-colloidomes were labelled using Rhodamine B. The latter was dissolved in water at a concentration of 2 x 10-4 wt.% MG-colloidosomes were then dispersed in the Rhodamine B solution before examination. Surface tension measurements at the air/water interface were performed using a Krüss Drop Shape Analysis (DSA100) instrument and a flow rate of 5.0 mL/min. Permeability studies CLSM was used to analyse capsule permeability of different molecular weights of FITCdextran. MG-colloidosome dispersion prepared using the VHS method (1.0 mL, 0.45 wt.%) was added to vials containing fluorescein or FITC-dextran aqueous solutions (1.0 mL) at a concentration of 0.10 wt.% in buffer and the dispersions were imaged after 24 h. RESULTS AND DISCUSSION pH-responsive microgel swelling and surface activity The microgel particles were prepared by emulsion polymerization and a minor proportion of the COOH groups subsequently functionalized with GMA via an epoxide ring-opening reaction. SEM data (Fig. 1a, inset) showed that the MG particles had a number-average diameter of 84 ± 15 nm (Fig 1a, inset). A larger image showing more particles appears in Fig. S1a. Potentiometric titration data for the MGs (Fig. S1b) showed that they had an apparent pKa of 6.2. The MGs contained 32.0 mol.% MAA and 7.0 mol.% of GMA. The MGs swelled strongly when the pH increased above 6.0 (Fig. 1a). The z-average diameter (dz) increased from ~ 82 nm at pH 5.0 to ~ 280 nm at pH 10.0. The size distributions remained monomodal as swelling occurred (Fig. S2). We used zeta potential (ζ) data (Fig. 1b) to qualitatively assess the change in the extent of charge at the periphery of the MGs. Accordingly, the MGs were negatively charged at all pH values studied. The ζ magnitude for the MGs increased


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when the pH increased and approached the pKa of 6.2 due to pH-triggered COO- formation. Simultaneously, the MGs became surface active as the pH approached the pKa (Fig. 1c) and it follows that the MGs became more amphiphilic. A study was conducted of measured surface tension variation as a function of flow rate used for droplet formation (Fig. S3). The results suggest that the surface tension data shown in Fig. 1c were obtained at sufficiently low flow rates to be considered as equilibrium values. These data are the first surface tension data reported for this class of MGs to our knowledge. The surface activity of the MGs was due to a combination of hydrophobicity from EA and the polymer backbone and polar groups from COO- groups. We propose that the increased surface active nature of the MGs at higher pH values (Fig. 1c) was due to increased segment flexibility within the swollen particles that enabled the hydrophobic portions of the MGs to more effectively adsorb at the air/water interface. The latter effect appears to have overcome any additional inter-MG electrostatic repulsion caused by deprotonation. The lowest γ value obtained (44 mN/m at pH 6.6) is similar

to

surface

tension

values

reported

by

Mourran

et

al.37

for

poly(N-

vinylcaprolactam)/poly(N-isopropylacrylamide) microgels.

Fig. 1 Effect of pH on (a) z-average diameter (dz), (b) zeta potential (ζ) and (c) the surface tension (γ). The inset shows an SEM image of the particles. The scale bar is 100 nm. The vertical line corresponds to pH 6.4, which was the pH used for MG-colloidosome construction.


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Emulsion preparation conditions, droplet diameter and interface structure implications Whilst pH-responsive Pickering emulsions can be prepared using a variety of methods which include combinations of hard nanoparticles and conventional surfactants39 reports involving pH-responsive microgels are rare. Microgels can be prepared that are well-suited to emulsion stabilization because lightly crosslinked microgels can adsorb to interfaces more rapidly than hard particles37. Whilst MG-emulsions related to those studied here were briefly investigated in the earlier study26 they were only studied at two pH values using a fixed MG concentration. Here, we studied 7 pH values and 8 MG concentrations (CMG) as well as the effect of shear. HS emulsification was used primarily because it provided large droplets that were

readily

imaged

by

optical

microscopy.

Richtering

et

al.

used

poly(N-

40-42

isopropylacrylamide)-based (poly(NIPAM)) microgels as emulsion stabilizers

. They

showed that microgel particles should carry charge to be efficient stabilizers. However, Massé et al. showed that charges did not affect the organization of poly(NIPAM)-based microgels at the oil-water interface and were not required to ensure emulsion stability34. It is important to note that the former poly(NIPAM)-based microgels are different to those studied in the present work because unlike poly(NIPAM), the EA-based microgels studied were fully collapsed solid particles when the pH decreases well below the pKa. Evidence for the latter assertion can be found by the similarity of the diameters obtained using SEM (84 ± 15 nm) and DLS at pH 5.0 (82 nm). Fig. 2a shows the MG-emulsions investigated and selected optical micrographs and tube images are shown. The complete set of optical micrographs are shown in the Supporting Information (Figures S4 and S5). Oil-in-water emulsions were present at all conditions investigated. The tendency of the MGs to form an oil-in-water emulsion was verified using the hydrophobic dye Coumarin 6 (See Fig. S6). -


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Fig. 2 (a) MG-emulsions investigated for emulsion stability and droplet diameter. Optical micrographs and tube images are shown in for selected ethyl acetate-in-water emulsions (oil phase volume fraction = 0.20 and tube diameters = 23 mm). The optical microscopy images and tube photographs were obtained ~ 30 min and 1 h after emulsification, respectively. The scale bars are 5 µm. (b) Effect of emulsion pH on the number-average emulsion droplet diameter (Demul). A data point was also obtained using very high shear (VHS). CMG = 0.60%. (c) Effect of MG concentration on Demul (pH = 6.4). Fig. 2b shows the variation of the number-average droplet diameter with pH for HS and VHS MG-stabilized emulsions (Demul) prepared using CMG = 0.6%. The Demul value decreased from ~ 6.9 µm at pH 6.0 to ~ 3.3 µm at pH 6.4 for the HS MG-emulsions and did not change


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significantly at higher pH values. Stable emulsions could not be prepared at pH values lower than 6.0 due to the lack of charge on the MGs as judged by the zeta potential data (Fig. 1b) and their poor surfactant-like properties as judged by the surface tension data (Fig. 1c). The ability of the MGs to stabilize the ethyl acetate-in-water emulsions benefits from their amphiphilic character produced by –COO- formation. We selected pH 6.4 for the pH to prepare MG-emulsions (and MG-colloidosomes) in this study. This pH corresponded to the minimum surface tension (Fig. 1c). For microgel-emulsions the interfacial structure determines emulsions stability38. The DLS (Fig. 1a) and zeta potential (Fig. 1b) data show that pH 6.4 corresponded to MG particles that were partially swollen with intermediate charge. We propose that these conditions resulted in inter-MG overlap for the MGs adsorbed to the droplet surfaces. The dependence of Demul on CMG was also investigated at pH 6.4. Whilst the data in Fig. 2c show considerable scatter it appears as though Demul decreased with increasing MG concentration and that a constant value was achieved when the MG concentration reached 0.36 wt.% It is likely that at higher CMG values there were too many MGs to cover the oil/water interface and the excess MGs remained in the dispersed phase. This conjecture raises the interesting question of the likely arrangement of the MGs at the ethyl acetate/water interface. From Fig. 2b a constant minimum diameter was first achieved at pH 6.4 and CMG = 0.60%. If the reasonable assumption is made that all of the MG particles were irreversibly adsorbed at the oil/water interface a nominal fractional coverage (θnom) can be calculated. For this calculation we used a geometric approach and the simplifying assumptions that the diameter of the MG particles adsorbed to the O/W interface was the same as that measured by DLS at pH 6.4 and that the MGs were present as spheres at the surface. It is understood that (a) the MG particles probably deformed at the oil/water interface, (b) part of the MG particles would have penetrated the oil phase37, 38 and (c) the Demul value used in the following calculation had significant error. Equation (1) enabled θnom to be estimated for a fixed CMG. The derivation is given in the Supporting Information. మ ଵ ௗಾಸ ௗಾಸಶ ଵିథ ஼ಾಸ ൰ ቀ థ ೚ ቁ ቀ ଵ଴଴ ቁ య ௗಾಸ(೎) ೚

ߠ௡௢௠ = ସ ൬

(1)


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The parameters dMG and dMG(c) are the diameters of the adsorbed MGs at pH 6.4 and that in the collapsed state, respectively. For these values we used the dz values for the MGs at pH 6.4 (178 nm) and in the collapsed state (82 nm at pH 5.4), respectively. The values used for the parameters dMGE (MG-emulsion droplet diameter), φo (oil-phase volume fraction) and CMG were 3.4 µm (from Fig. 2b), 0.20 and 0.60%, respectively. The θnom value calculated was 1.2. This value is comparable to unity and supports the assumption that a monolayer of MG particles were adsorbed to the droplets at pH 6.4 (CMG = 0.60%). There may have been residual (non-adsorbed) MGs present for this HS MG-emulsion. The analysis give above assumed that in the collapsed state (pH 5.4) the MG particles did not contain water. The MG particles contained ~ 67 mol.% of EA and 32 mol.% of MAA, with the remainder being BDDA. PEA is hydrophobic and poly(MAA) is also hydrophobic at low pH43. Furthermore, the dz value at pH 5.4 agreed well with the average diameter obtained using SEM (discussed above). Hence, the particles can be assumed to have not contained significant water at pH 5.4. A VHS emulsion was also prepared using pH 6.4 and CMG = 0.60%. The droplets were much smaller (see Fig. S4(i)) with an average Demul value of 1.7 µm (Fig. 2b), which was ~ half the value obtained for the HS MG-stabilized emulsion (above). The value for θnom calculated using equation (1) was 0.58 and is about half the value calculated for the HS MG-stabilized emulsion. This value is also comparable to unity. It is proposed that there was a monolayer of MG particles at the surface of the VHS MG-colloidosomes. The effect of pH (Fig. S8) and CMG (Fig. S9) on the stability of the MG-emulsions to creaming and phase separation was examined visually after 5 min, 1 h and 24 h. In all cases the droplets had creamed after 24 h. Importantly, there was no evidence of phase separation, which suggests that the MGs remained strongly adsorbed to the oil/water interface. The aqueous phases (bottom of tubes) for the emulsions prepared using relatively high CMG values (1.08 and 1.32%) remained turbid after 24 h (Fig. S9), which is attributed to residual MG particles. The VHS MG-stabilized emulsion also showed evidence of creaming (Fig. S8); however, there was greater turbidity in the aqueous phase after 24 h which indicates that creaming was, in general, slower than for the HS MG-stabilized emulsion. This difference in creaming is due to the smaller Demul value for the VHS MG-emulsion (Fig. 2b).


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Colloidosome preparation and properties The formation of covalently-interlinked MGs at the MG-emulsion surface likely requires significant overlap of the MG peripheries. Robust MG-colloidosome formation success was judged by the response to pH-triggered swelling. If the MG-colloidosomes swelled without fragmentation then MG-colloidosome formation was considered successful. It was found that MG-colloidosomes could not be successfully prepared at pH values below the pKa, presumably due to a lack of MG overlap. Surprisingly, the use of pH values greater than 0.5 pH units from the pKa also did not provide robust MG-colloidosomes. The latter observation was presumably the result of increased inter-MG electrostatic repulsion, increased MG hydrophilicity (due to a higher proportion of COO- groups) and poor overlap of interfacial MGs. However, a pH value of 6.4 gave a reliable MG-colloidosomes synthesis. This pH corresponded to intermediate values for particle swelling (Fig. 1a) and the zeta potential magnitude (Fig. 1b). We propose that MG peripheral overlap occurred at the oil/water interface when a balance was reached between particle swelling (which favoured overlap), electrostatic repulsion between neighbouring MGs which opposed overlap and particle amphiphilicity which drove the MGs to adsorb to the interface. MG-colloidosome diameter for the two colloidosome types (VHS and HS) was first investigated using optical microscopy (Fig. 3a and b). The number-average diameters measured for the VHS and HS MG-colloidsomes measured using optical microscopy (Dcoll) were 0.90 µm and 1.9 µm, respectively. The latter value is approximately half the diameter of the pre-cursor MG-stabilised emulsion droplets (Demul = 3.4 µm, Fig. 2b). Similarly, the Dcoll value for the VHS MG is approximately half the diameter for the precursor VHS MGstabilized emulsion (Demul = 1.7 µm). We attribute this substantial diameter decrease to a combination of ethyl acetate removal and inter-MG shell crosslinking which resulted in crumpling of the shells. Furthermore, the Dcoll value decreased by a factor of ~ 2 using the VHS method compared to the HS method, which is due to greater shear being used during VHS emulsification. Furthermore, the VHS MG-colloidosomes were successfully prepared at the gram scale with a dry mass yield of 1.07 g. In contrast the yield of dry MGcolloidosomes prepared using the HS method was only 20 mg.


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Fig. 3. Optical micrographs (a and b), CLSM images (c and d) and SEM images (e and f) for MG-colloidosomes prepared using the VHS (left column) and HS (right column) methods. The scale bars for the main images and insets for (a) to (d) represent 10 and 5 µm, respectively. The pH was 6.4. For (e) and (f) The scale bars for the left and right hand panels are 500 nm and 100 nm, respectively. The arrow in (f) identifies MG particles in the colloidosome shell.


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The morphologies of the VHS and HS MG-colloidosomes were also investigated using confocal laser scanning microscopy CLSM (Fig. 3c and d) at pH 6.4. In each case hollow particles can be seen which confirms that MG-colloidosomes were successfully prepared. SEM images of MG-colloidosomes obtained by drying pH 6.4 dispersions were obtained and representative high magnification images are shown in Fig. 3e and f. (Lower magnification images showing more particles are shown in Fig. S10.). The HS MG-colloidosomes had a crumpled and wrinkled appearance (Fig. 3f). In contrast SEM images for the MGcolloidosomes prepared using the VHS method (Figures 3e, 4b and c) showed that the surfaces of these MG-colloidosomes had less surface wrinkling. We propose that the latter effect was due to their smaller diameter with smaller colloidosomes having less space for wrinkling. The pH-triggered swelling of the VHS and HS MG-colloidosome dispersions at pH 6.4, 8.0 and 10.0 was measured using DLS and the data are compared in Fig. 4a. Data for the MG particles are also shown for comparison. Because the MG-colloidosomes had polydisperse size distributions we used the peak diameters (dp) in the discussion below. The dp value for the VHS MG-colloidosomes at pH 6.4 was 825 nm. This value can be compared to the dp value for the parent MG particles (Fig. 4a) at the same pH of 190 nm. It follows that the VHS MG-colloidosome diameter was equivalent to only ~ 4.5 MG particle diameters at the preparation pH. The data show a clear increase in the diameter as the pH increased to 1480 nm at pH 8.0 and this value was unchanged at pH 10.0. It is noted that there was no evidence of residual MGs at any of the pH values studied. This result indicates that as far as could be detected by our DLS measurements there were not significant excess MGs and also indicates that fragmentation did not occur. Our VHS method appears to have produced robust, submicrometer MG-colloidosomes that were pH-responsive.


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Fig. 4. (a) DLS data for VHS and HS MG-colloidosomes as well as the parent MGs measured at pH 6.4, 8.0 and 10.0. SEM images are shown for the VHS MG-colloidosomes deposited from dispersions with pH values of 6.4 (b and c) and pH 10 (d and e). Scale bars: (b) and (d) are 2 µm; (c) and (e) are 500 nm. The arrow in (e) highlights a MG particle in the colloidosome shell. The DLS data for the HS MG-colloidosomes (Fig. 4a) shows a strong peak at 1490 nm at pH 6.4 which increased to 4,800 nm at pH 8.0 and 5,560 nm at pH 10.0. Relatively small maxima were evident at dp values of 220 nm at pH 8 and 342 nm at pH 10. These maxima were not apparent at pH 6.4 and are ascribed to fragments. To probe the significance of pHtriggered HS MG-colloidosome fragmentation optical micrographs were obtained for the HS MG-colloidosomes at pH 6.4 and 10.0 (Fig. S11). The images confirm pH-triggered swelling for the HS MG-colloidosomes and demonstrate that the majority of the colloidosomes did not fragment at pH 10.0. We suggest that the synthesis for the HS MG-colloidosomes provided reasonably robust particles. Interestingly, deposition of the VHS MG-colloidosomes at pH 10 (from the fully swollen state) showed doughnut-like morphology when examined using SEM (Fig. 4d and e). The expanded shells appear to have collapsed during sample drying for SEM. Fig. S12 shows a lower magnification image containing more VHS MG-colloidosomes and it is can be seen that the shells remained intact after appearing to flatten at the centre of the particles. From


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Fig. 4d, the shell folded thickness was about 400 - 600 nm. The latter range is about twice the swollen MG diameter at pH 10 measured from DLS of 270 nm (Fig. 1b). Closer scrutiny of the shell walls (Fig. 4e) shows that MG particles were present (see arrow). We therefore propose the MG-colloidosomes had the morphology depicted in Scheme 1, with shells of mostly monolayer MG particles. The remarkably small diameter of the as-prepared VHS MG-colloidosomes warrants further comment. The SEM data for the VHS MG-colloidosomes deposited at pH 6.4 (Fig. 4b and c) gave an average collapsed number-average diameter of 0.67 ± 0.17 µm. Furthermore, the dp (0.825 µm) and Dcoll (0.90 µm) values discussed above were also below 1.0 µm at pH 6.4 (as prepared). Fig. 5 provides a survey of reported diameters for microgel-colloidosomes. Many of the colloidosomes surveyed were polydisperse as a consequence of emulsion-based preparation methods and the data shown in Fig. 5 are our best estimates of the average diameters. The VHS MG-colloidosomes have the smallest diameter of any microgel-based colloidosomes reported to our knowledge. We propose that the remarkably small diameter achieved for the VHS MG-colloidosomes was aided by the strong surface activity and small diameter of the MGs, inter-MG linking and the shear used.

Shah

100 Diameter / µm

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

Morse

Bon Berger

10

Gong

Lawrence Argawal

1

0.1 2000

Gong

Wang VHS

2005

2010

2015

2020

Year

Fig. 5. Survey of microgel-colloidosome diameters14-16, 24-26, 44-46 including the VHS MGcolloidosomes from this study. The diameters have been taken from the text or images reported. pH-responsive MG-colloidosome permeability The permeability of the VHS MG-colloidosomes was probed using CLSM after mixing the colloidosomes with FITC-Dextran of different molecular weights using pH 5.5, 6.2 and 7.4


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(Fig. 6). The images shown in Fig. 6 reveal dramatically different behaviours. At pH 5.5 and 6.2 the MG-colloidosomes were relatively small and appeared bright green for dextran molecular weights less than or equal to 40 kg/mol-1 (at pH 5.5) or 20 kg/mol-1 (at pH 6.2). The MG particles were negatively charged at pH 5.5 and 6.2 (Fig. 1b). Whilst it is not entirely certain why hollow cores could not be seen, it appears that the cores were relatively small compared to the colloidosome diameter at pH 5.5 and pH 6.2. We take the green colloidosomes to indicate adsorption and / or shell permeability. Fluorescein has a pKa of 6.3547 and consequently charge repulsion between the dye and the negatively charged colloidsomes would have been relatively low at pH 5.5 and pH 6.2. Because these pH values are below the MG pKa hydrogen bonding between the fluorescein group and MG–COOH groups likely occurred. For F-Dex70k and F-Dex150k (and F-Dex40k at pH 6.2) the MGcolloidosomes appeared black, which is because the solutes were excluded. Therefore, at pH 6.2 the F-Dex 20k and F-Dex40 species correspond to a permeability / impermeability transition. F-Dex20k and F-Dex40k have hydrodynamic diameters of 6.6 and 9.0 nm, respectively48. Therefore, we estimate the pore size for the VHS MG-colloidosomes at pH 6.2 to be between 6.6 and 9.0 nm. There are indications from the pH 5.5 data that the pore size may have increased. However, more study on that aspect is required in future work.


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Fig. 6. CLSM images of VHS MG-colloidosomes mixed with FITC-Dextran of various molecular weights (shown in the codes at the top as kg/mol) for 24 h. The pH values are shown on the left-hand side. The scale bars are 5 µm. When the pH was increased to 7.4. The images (lower row of Fig. 6) show that all of the dextrans studied were excluded from the interior of the MG-colloidosomes. Consequently, the pores appear to have fully closed. Examination of the images shows that the MGcolloidosomes had strongly swollen considerably at this pH. It is therefore proposed that in the fully swollen state the MG particles had fully inter-meshed preventing solute penetration. On the other hand one might expect the pores to expand as the pH increased due to greater electrostatic repulsion between neighbouring MGs. However, if this were the case the permeability to the F-Dex solutes should have increased, which was not observed. We speculate that MG swelling within the shell was frustrated to some extent by inter-MG crosslinking and this forced the pores to contract as the pH increased. Mechanism for sub-micrometer MG-colloidosome formation and selective permeability Based on the results above we can propose the following mechanism for formation of the sub-micrometer VHS MG-colloidosomes. The MGs became strongly surface active at pH 6.4 and adsorbed strongly to the oil-water interface in a partially swollen and charged state. Their peripheries inter-penetrated bringing the vinyl groups from neighboring MGs into close


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proximity. Thermolysis of the initiator resulted in crosslinking of these vinyl groups and formation of an interlinked MG shell. However, the shell only formed if the MGs were sufficiently swollen to inter-penetrate. If the pH was too high then the MGs fully swelled and inter-MG electrostatic repulsion was too strong for inter-penetration The fully swollen particles would have been more hydrophilic and more able to desorb from the droplets. Hence, a pH close to the pKa was required for covalent interlinking. The inter-MG crosslinking combined with the removal of the volatile droplet template resulted in crumpling of the MG-colloidosomes (Scheme 1). The combined effect of small and surface-active MGs and VHS conditions together will crumpling resulted in the collapsed VHS MGcolloidosomes having sub-micrometer diameters. The overlap between the interfacial MGs was not perfect as indicated by the permeability of the shell at pH 6.2. The as-made MGcolloidosomes subsequently swelled when the pH was increased beyond the pKa and this process also resulted in pH-triggered closing of the pores.


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CONCLUSIONS Here, we have investigated MG stabilization of ethyl acetate-in-water emulsions and demonstrated that sub-micrometer pH-responsive MG-colloidosomes can be prepared at the gram scale using a new scalable method. The optimum conditions for formation of the MGstabilised emulsions corresponded to the point where the MGs first reached their minimum surface tension and were able to inter-penetrate at the interface. An optimum balance between pH-triggered swelling, inter-penetration of the polymer chains and electrostatic repulsion occurred at pH 6.4. The diameter of the VHS MG-colloidosomes is the smallest reported for microgel-colloidosomes. The small diameter was due to the small MG diameter, high surface activity, inter-MG crosslinking at the droplet interface, solvent removal as well as the use of very high shear. Nanogels with similar compositions to the MGs used in this study have recently been reported49 which raises the possibility of preparing even smaller nanogelcolloidosomes in the future. The shell of the MG-colloidosomes was porous and the extent of the porosity was pH dependent. At pH 6.2 permeation experiments indicated a pore diameter of between 6.6 to 9.0 nm. The MGs contained abundant COOH groups which raises the potential for functionalization in the future which should increase their versatility. The family of MGs used to prepare the MG-colloidosomes here have already been shown to be biocompatible in the macroscopic hydrogel form50 and hence there is potential for delivery applications of the present MG-colloidosomes in future. Furthermore, the ability to prepare these sub-micrometer MG-colloidosomes at gram scale demonstrates scalability which should facilitate use by other researchers and also potential applications for cosmetics and photonics. ASSOCIATED CONTENT Supporting Information description Shear rate calculations and values, SEM and potentiometric titration data for the MG; DLS diameter distributions vs. pH; surface tension data measured as a function of droplet formation speed; optical micrographs of MG-emulsions with varying pH and MG concentrations; optical micrographs of ethyl acetate (Courmarin 6)–in-water emulsion; method for calculating nominal fractional coverage of oil droplets by microgel particles; images for emulsions as a function of pH, microgel concentration and time; SEM images for VHS and HS-MG colloidosomes using pH 6.4; optical micrographs for HS MG-


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colloidosomes at pH 6.4 and 10.0; SEM image for VHS MG-colloidosome deposited at pH 10.0. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGEMENTS BRS gratefully acknowledges a 5 year EPSRC Established Career Fellowship (M002020/1). REFERENCES 1.

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Pickering Emulsions Stabilized by pH-Responsive ... - ACS Publications

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