Hybrid microcapsules with tunable properties via Pickering emulsion templates for the encapsulation of bioactive volatiles

Abstract

Bioactive volatile molecules such as aldehydes, ketones, and other phytochemicals are important as flavors and fragrances in nature, as well as in industry. Because of the volatility of these molecules and their sensitivity to chemical degradation, the design of efficient delivery systems to control their stability and release has become an important research area. In this study, we fabricated organic–inorganic hybrid microcapsules with tunable properties for the encapsulation of volatile fragrance molecules by using oil-in-water emulsions stabilized with complex SiO₂ particles as templates. Two different types of SiO₂ particles, amorphous fumed SiO₂ and amino-functionalized SiO₂, were used to stabilize the oil droplets during emulsification and were immobilized in the robust shell under a subsequent interfacial reaction. These hybrid microcapsules revealed tunable droplet size and surface properties with different SiO₂ stabilizer components. The microcapsules also exhibited low permeability and excellent thermal stability for volatile molecular core liquids. Moreover, with the SiO₂ particles embedded in the shell, these hybrid microcapsules revealed different mechanical behaviors (brittleness and breakability) upon compression than in traditional polymer-based microcapsules. We believe the hybrid microcapsules presented here may be of great interest for several important applications in which the aim is to encapsulate and release bioactive volatile molecules using mechanical forces as the trigger.

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Introduction

Microcapsules composed of a polymer shell and a liquid core have attracted widespread interest for their unique properties in the protection and controlled release of encapsulated active ingredients.^1–4 Microencapsulation technology has already been used with considerable success in applications such as agriculture,⁵ pharmaceutics,⁶ home and personal care,^7,8 and construction.⁹ Bioactive volatile molecules such as aldehydes, ketones, and other phytochemicals are important as flavors and fragrances in nature, as well as in industry.^10,11 Because of the volatility of these bioactive volatiles and their sensitivity to chemical degradation, microcapsules are considered effective carriers for their sustained and controlled release in applications.^12,13

In the last decade, hybrid microcapsules with organic–inorganic composite shells have received increasing attention in both academic and industrial areas.^14–17 These hybrid microcapsules combine the desirable chemical and thermal barrier properties of conventional polymer-based microcapsules with improved mechanical and structural properties provided by the inorganic component in the capsule shell.^18,19 Pickering emulsions are surfactant-free emulsions, stabilized by solid particles via self-assembly adsorption at the liquid–liquid interface.^20,21 Owing to the high adsorption energy and permanent adsorption at the interface of these solid particles,^21–24 such emulsions have been confirmed to be ideal templates in the fabrication of organic–inorganic hybrid microcapsules via immobilization of the solid particles at the liquid–liquid interface.^25–27 Different inorganic particles have already been reported as colloidal stabilizers during emulsification to prepare oil-in-water (O/W) hybrid microcapsules. In the Armes group,²⁸ poly(ethylene imine)-modified LAPONITE® clay particles were used to stabilize a Pickering emulsion and were locked at the oil–water interface by epoxy-amine chemistry, which formed stable organic–inorganic microcapsules. These hybrid microcapsules were sufficiently robust to survive the removal of the encapsulated oil, but they showed high permeability and could therefore not provide an effective diffusion barrier for small molecules. In Rotello's group,¹⁷ amine-functionalized SiO₂ particles were applied as colloidal stabilizer during emulsification and were further immobilized at the oil–water interface via internal cross-linking. These hybrid composites showed considerable resistance to the generally interface-disrupting influence of ethanol because of their rigid capsule shell. Tong et al.²⁹ prepared organic–inorganic microcapsules by templating amorphous fumed SiO₂ nanoparticles to stabilize Pickering emulsions. Extra-low permeability of the microcapsules was achieved by introducing an additional poly (melamine formaldehyde) layer outside of the polyurea–SiO₂ hybrid wall. Preece and Zhang^30,31 reported using CaCO₃ particles as colloidal stabilizer for hybrid microcapsule preparation. Organic–inorganic hybrid microcapsules with decreased leakiness of the encapsulated oil were formed via double shell fabrication comprising ripened CaCO₃ particles as the outer shell and melamine formaldehyde resin as the inner shell.

Recently, a type of complex stabilizer which contains inorganic particles and organic surfactant/polymer was reported and used to make stable O/W emulsions.^32,33 Due to the synergistic interactions between organic portions and inorganic solids in the complex stabilizer system, such emulsions have been proven to be even more stable in comparison to emulsions stabilized by single stabilizer. While these approaches have proved promising as general concepts for delivery systems, they have not yet addressed the challenges inherent to the encapsulation of highly volatile molecular ingredients. Herein, we present a type of unique Pickering emulsions stabilized by a complex stabilizer that contained two different solid particles. Both adsorbed particles at the oil–water interface were further cross-linked and immobilized to fabricate organic–inorganic hybrid microcapsules with outstanding barrier properties for bioactive volatile payloads in the absence of additional melamine formaldehyde layers. More specifically, SiO₂ particles with different surface functionalities (amino and hydroxyl surface groups), were used to stabilize the oil droplets during emulsification; both types of particles were subsequently immobilized into the polymeric shell via interfacial reactions between isocyanates in the oil phase with the amino or hydroxyl groups on the surface of the particles. These hybrid microcapsules with a unique organic–inorganic single shell not only have low permeability and rigidity (mechanics upon compression), but also have (1) a tunable size with uniform size distribution and (2) alterable surface properties with different particle loadings in the shell of the microcapsules by adjustment of the ratio of different particle stabilizers during emulsification. These advantages of microcapsules open up opportunities to fabricate ideal delivery systems in situations where mechanical force is required to trigger the release of encapsulated volatile fragrance molecules, as occurs in many important applications, in particular in the home care or personal care fields.

Experimental

Materials

Amino-functionalized SiO₂ particles (referred to as NH₂–SiO₂) purchased from SkySpring Nanomaterials (with a primary size of 10–20 nm) and hydrophilic amorphous fumed silica HDK N20 (referred to as OH–SiO₂) purchased from Wacker Chemie (with a primary size of 5–30 nm) were used as Pickering stabilizers. When dispersed into water phase, both types of particles are significantly aggregated with an average size of several micrometers and hundreds of nanometers, respectively (Fig. S1 in ESI†). Hydrophobic aliphatic polyisocyanate (Desmodur® N 100) was supplied by Bayer and used as cross-linker in the oil phase to immobilize the particles at the oil–water interface. The encapsulated oil liquid was a mixture of an equal mass of five “model” volatile fragrance compounds:³⁴ (2z)-2-phenyl-2-hexenenitrile (Salicynile), (+-)-3-(4-isopropylphenyl)-2-methylpropanal (Cyclosal), (+-)-methyl 2,2-dimethyl-6-methylene-1-cyclohexanecarboxylate (Romascone), (+-)-2-tert-butyl-1-cyclohexyl acetate (Verdox), and cis/trans-4-tert-butyl-1-cyclohexyl acetate (Dorisyl). The pH 7 buffer solution Certipur® (di-sodium hydrogen phosphate/potassium dihydrogen phosphate), purchased from Merck Millipore, was used as dispersant.

Preparation of particle-stabilized Pickering emulsions

NH₂–SiO₂ particles or a mixture of NH₂–SiO₂ and OH–SiO₂ particles were uniformly dispersed in pH 7 buffer solution by using an ultrasonic processor (UP400S, Hielscher). For the preparation of hybrid microcapsules, the content of the particle stabilizer was fixed at 1.5 wt% in water. The oil phase was formed by dissolving Desmodur® N 100 in the model fragrance mixture, with a Desmodur® concentration of 5.0 wt% in the oil phase. The oil phase was then added to the particle suspension (with a fixed mass ratio of oil phase to water phase of 3 : 7), and immediately the Pickering emulsion was prepared by using a rotor/stator high-shear homogenizer (Ultra-Turrax, IKA T25) at a speed of 24 000 rpm for 5 min at room temperature.

Formation of hybrid microcapsules

The Pickering emulsion was transferred into a reactor after adjusting the pH value in the range of 9.5–10.0 by using a 5.0 wt% NaOH solution. The interfacial reaction was carried out at 70 °C for 3 h, and the obtained slurry was slowly cooled to room temperature.

Characterizations

Pickering emulsions were observed with a microscope (Eclipse Ci, Nikon). The average diameters and size distributions of the SiO₂ particles and hybrid microcapsules were determined with a dynamic light scattering instrument (Zetasizer Nano ZS, Malvern) and a flow particle image analyzer (Sysmex FPIA-3000, Malvern), respectively. The zeta potential of the hybrid microcapsules was also examined by using a Zetasizer Nano ZS (Malvern). For the zeta potential measurement of the hybrid microcapsules, the capsule slurry was washed with deionized water via three centrifugation–redispersion cycles to remove the free dispersed SiO₂ particles in the water phase, and then redispersed into a 10⁻³ M KCl solution for measurement. Scanning electron microscope (SEM) micrographs of hybrid microcapsules were obtained with a JSM-6010LA SEM (Jeol). The prepared hybrid capsule samples were dried at room temperature and coated with gold plasma before observation.

The measurements of SiO₂ loading in the hybrid capsules (shell and oil core of capsules) and in the hybrid membrane (capsule shells only) were carried out on a thermogravimetric analyzer (TGA/DSC 1, Mettler-Toledo). To obtain the mass fraction of SiO₂ in the microcapsules, we washed the capsule slurry with deionized water via three centrifugation–redispersion cycles to remove the free dispersed SiO₂ particles in the water phase. The washed samples were introduced into an aluminum oxide crucible for measurement. To quantify the amount of membrane-bound SiO₂%, we broke the washed samples in a mortar and subsequently washed the ruptured capsule fragments with ethanol via six centrifugation–redispersion cycles to remove the encapsulated fragrance oil. The resulting microcapsule membranes were dried in a vacuum oven at 60 °C for 40 h. The samples were then introduced into an aluminum oxide crucible for measurement under a nitrogen atmosphere: the temperature was first increased from 25 °C to 80 °C at a rate of 10 °C min⁻¹ and subsequently held at 80 °C for 2 h, and then the temperature was further increased to 700 °C at a rate of 10 °C min⁻¹ and held at 700 °C for another 2 h. The mass fraction SiO₂% in the capsules or in the membranes was calculated as follows:

SiO₂% = W_R/W₈₀ × 100%

where W_R is the residue mass of samples at 700 °C, and W₈₀ is the constant mass at 80 °C. Assessments of the permeability of hybrid microcapsules were also performed by using TGA with an isothermal protocol in which the temperature was ramped up from 25 °C to 50 °C and then held constant at 50 °C for 250 min.

We characterized the mechanical properties of the microcapsules³⁴ by using a microforce probe, compressing the core/shell microcapsules between parallel plates while measuring the compressive force in the normal direction. A capacitive microforce sensor (FemtoTools AG) mounted on a piezo-driven three-axis translation stage was used to record force vs. distance curves. This method allowed sensitive force measurements with a resolution down to 50 nN over a wide range of large nonlinear deformations. To account for polydispersity in capsule size and shape, we measured multiple microcapsules for each sample, and for each capsule, both a compression and a retraction curve were measured at a constant absolute compression velocity of 0.5 μm s⁻¹. The force curves were analyzed with the statistical computing package R (version 3.1.2; http://www.R-project.org). Parameters analyzed were the rupture or peak force values, compressive deformation at rupture, extent of plastic deformation obtained from integration of force curves, and permanent deformation after compression.

Results and discussion

Formation of hybrid microcapsules via O/W Pickering emulsion templates

A stable O/W Pickering emulsion can be obtained by using solid particles with intermediate hydrophobic/hydrophilic properties, which are estimated in terms of the contact angle. The Pickering emulsion is also influenced by the particle size/concentration, oil/water interfacial tension, and pH and salinity of the water dispersant.^35–37 SiO₂ particles are considered good stabilizer for a Pickering emulsion owing to their appropriate size and adjustable surface properties. Both amino-functionalized SiO₂ particles (NH₂–SiO₂) and amorphous fumed SiO₂ particles (OH–SiO₂) have already been reported as stabilizer to formation of stable O/W emulsions under specific conditions.^17,29 However, when a pH 7 buffer solution (as the continuous phase) and volatile fragrance oil (as entrapped oil drops) were used in our system, stable emulsions were only formed by using NH₂–SiO₂ as the stabilizer rather than the hydrophilic OH–SiO₂ under the same emulsification conditions (Fig. S2 in ESI†). It is considered that the adsorption of particles at the oil–water interface requires the partial wetting of these particles by both water and oil, which is impacted by the interfaces of solid–water, solid–oil, and oil–water, as well as the contact angle of particles.^21,37 The OH–SiO₂ is too hydrophilic to be properly wetted by the fragrance oil, so that the solid particles do not adsorb at water–fragrance oil interface. NH₂–SiO₂ with partial propylamine grafting on the surface, would meet the condition of wetting by both water and fragrance oil owing to the increased hydrophobicity. The drop size distributions and the stability of the emulsions stabilized by NH₂–SiO₂ revealed significant dependence on particle concentration as shown in Fig. 1. In the case of low amounts of solid particles (such as 0.15 wt% in the aqueous continuous phase), only a small interfacial area could be stabilized. The formed small oil droplets under high energy emulsification underwent coalescence after the shearing was stopped, and no stable emulsion was observed (Fig. 1A). With a content of 0.45 wt% NH₂–SiO₂, only limited coalescence took place during the ripening process; stable emulsions with larger oil droplets (up to 50 μm) and partial oil coalescence were observed, as shown in Fig. 1B. By further increasing the NH₂–SiO₂ content to 0.75 wt% in water, less coalescence occurred, which formed a more stable emulsion with smaller oil droplets and less dispersion of oil coalescence (Fig. 1C). A high amount of solid particles allows the formation of steric stabilized emulsion oil droplets, which are covered by cohesive particle layers. As shown in Fig. 1D and E, uniform oil droplet distributions with mean diameters of about 20 μm were obtained with a NH₂–SiO₂ content of 1.0 wt% and 1.5 wt% in the water phase. Far less coalescence occurred, and no free oil was observed, indicating that these concentrations defined the limit for sufficient particle coverage at oil–water interfaces. Further increasing the NH₂–SiO₂ content in the water phase up to 2.0 wt% reduced the oil droplet size to mean diameters of 10–15 μm (Fig. 1F). Beyond this particle content, the size of single oil droplets reached a plateau and did not lead to any further reduction upon a further increase of NH₂–SiO₂ content to 2.5 wt% and 3.0 wt% (Fig. 1G and H). However, the Pickering emulsions with higher NH₂–SiO₂ content (>2.0 wt%) showed aggregation, which might be caused by the excess particles in the continuous phase. Therefore, to match our requirements on microcapsule diameter and size distribution for practical applications, the 1.5 wt% particle content was selected to prepare hybrid microcapsules.

Fig. 1 Optical micrographs of nile-red stained O/W Pickering emulsions stabilized by NH₂–SiO₂ in pH 7 buffer with the following content: (A) 0.15 wt%, (B) 0.45 wt%, (C) 0.75 wt%, (D) 1.0 wt%, (E) 1.5 wt%, (F) 2.0 wt%, (G) 2.5 wt%, and (H) 3.0 wt%. The oil phase and water phase ratio is fixed at 3 : 7. The inserts are photographs of corresponding emulsions ripening for 3 h at room temperature after emulsification. All scale bars are 100 μm.

In the present work, OH–SiO₂ was mixed with NH₂–SiO₂, which can stabilize the fragrance oil droplets in pH 7 buffer, as complex stabilizer to stabilize the O/W emulsion. The effects of OH–SiO₂ ratio in complex stabilizer to hybrid microcapsules obtained from the emulsion templates were investigated. Stable O/W Pickering emulsions with uniform droplet size (Fig. S3 in ESI†) were obtained with fixed content of the particle stabilizer at 1.5 wt% in pH 7 buffer. The emulsion-templated microcapsules showed tunable size and surface properties by simple adjustment of the particle ratio in the NH₂–SiO₂/OH–SiO₂ mixture. As illustrated in Scheme 1, an SiO₂ suspension with uniformly dispersed bicomponent particles in pH 7 buffer was used as the continuous aqueous phase, while aliphatic polyisocyanates dissolved in a mixture of five model fragrance compounds were used as the reactive oil phase. Because of the synergistic interactions between NH₂–SiO₂ and OH–SiO₂, both particles could be adsorbed at the oil–water interface and contributed to the formation of the stable O/W emulsion during the initial emulsification process. Subsequently, the adsorbed SiO₂ particles were anchored into a robust polymer shell at the oil–water interface via an interfacial reaction between the polyisocyanates and the hydroxyl groups and/or amino groups on the surface of particles under alkaline conditions. This approach allowed us to covalently bond both types of particles into the shell, resulting in a robust capsule membrane with ultralow permeability and controllable mechanical properties.

Scheme 1 Schematic representation of the preparation of organic–inorganic hybrid microcapsules via a Pickering emulsion template, which is stabilized by an NH₂–SiO₂/OH–SiO₂ complex stabilizer.

Surface morphologies of hybrid microcapsules

The micrographs in Fig. 2 clearly reveal the particle adsorption on hybrid microcapsules at different OH–SiO₂ ratios in a complex particle mixture with a fixed particle stabilizer content of 1.5 wt% in the water phase. It is confirmed that the SiO₂ particles were anchored at the outer surface of the microcapsules (Fig. S4 in ESI†). Of importance, the hybrid microcapsules observed by SEM retained their spherical shape without collapsing under high vacuum and high-voltage (20 kV) electron exposure, indicating very good mechanical strength. When NH₂–SiO₂ was used as the sole stabilizer to stabilize the oil droplets, the particles did not fully cover the oil droplets and large voids appeared among the particles in the hybrid microcapsule membrane (Fig. 2A). In contrast, in the presence of OH–SiO₂, part of the OH–SiO₂ can be adsorbed and locked at the oil–water interface. As revealed in Fig. 2B, far more SiO₂ particles were embedded in the membrane when 30% OH–SiO₂ was used in the complex stabilizer. By further increasing the OH–SiO₂ ratio to 50% and 70% in the particle mixture, a more compact membrane was formed (Fig. 2C and D).

Fig. 2 SEM micrographs of hybrid microcapsules prepared with different OH–SiO₂ mass ratios in the complex stabilizer (OH–SiO₂/NH₂–SiO₂) during emulsification: (A) 0% OH–SiO₂, (B) 30% OH–SiO₂, (C) 50% OH–SiO₂, and (D) 70% OH–SiO₂. The inserts are partial amplification of corresponding samples. The scale bars are 5 μm.

Tunable properties of hybrid microcapsules

The hybrid microcapsules revealed tunable size by adjustment of the ratio of OH–SiO₂ in the complex particle mixture. As shown in Fig. 3, with a fixed stabilizer content of 1.5 wt% in the water phase, by simply adjusting the OH–SiO₂ ratio from 0 to 90%, the mean size of the hybrid microcapsules obtained was decreased from 19.6 μm to 13.4 μm. The size of the microcapsules is completely determined by the diameter of oil droplets during emulsification, since the microcapsule samples show uniform size distribution and negligible aggregations in the aqueous dispersant (as indicated by the inserts in Fig. 3). During the emulsification process with a complex stabilizer, the adsorption behavior of OH–SiO₂ particles at the oil–water interface is changed compared with that of single OH–SiO₂ stabilizers. In the presence of NH₂–SiO₂, the more hydrophilic OH–SiO₂ can also adsorb onto the oil–water interface and act as the stabilizer during emulsification, owing to the weak interactions between –NH₂ and –OH groups, as well as the changed interfacial tension of the oil droplets with NH₂–SiO₂ assembled onto the surface. OH–SiO₂ can thus also contribute to stable Pickering emulsions and enable the formation of hybrid microcapsules. This type of hydrophilic particle with a high negative surface charge will provide higher efficiency against coalescence and coagulation after being assembled on the oil–water interface. A higher OH–SiO₂ fraction in the complex stabilizer system produces smaller microcapsules.

Fig. 3 Mean diameter of hybrid microcapsules as a function of OH–SiO₂ mass ratio in the complex stabilizer during emulsification. The inserts are optical micrographs of hybrid microcapsules corresponding to OH–SiO₂ mass fractions from 10% to 90% (left to right). The scale bars are 50 μm.

Particle adsorption and embedment in the hybrid shell of microcapsules was also demonstrated in TGA measurements used to elucidate the organic vs. inorganic fragment in the shell. More compact membrane morphologies indicated higher particle loading in hybrid microcapsules, as revealed in Fig. 4A. SiO₂ particle loading (SiO₂%) revealed an increasing trend as increasing the OH–SiO₂ ratio in the complex stabilizer system. The SiO₂% on the capsules was increased from 1.1 wt% (in the absence of OH–SiO₂) to 2.0 wt% (with a fraction of 30% OH–SiO₂ in the particle mixture) and was further increased to 2.5 wt% and 2.8 wt% with OH–SiO₂ ratios of 50% and 70%, respectively. The total SiO₂ loading reached a percentage of 3.3 wt% of the whole microcapsule, to which the OH–SiO₂ contributed a relative mass ratio of up to 90% in the complex stabilizer. To eliminate the size effect of the capsules, we also used TGA to detect the SiO₂% in the hybrid membrane of the microcapsules (Fig. 4B). The increasing tendency of SiO₂% on microcapsules was confirmed, with the total SiO₂ loading in the membrane rising from ca. 38 wt% to 52 wt% as the OH–SiO₂ portion was increased from 0 to 90%.

Fig. 4 SiO₂ loading (SiO₂%): (A) total SiO₂ on and inside hybrid microcapsules (shell and fragrance core) and (B) membrane-bound SiO₂ content in the shell of hybrid microcapsules prepared with different OH–SiO₂ mass ratios in the complex stabilizer during emulsification. Standard deviations σ_R of the linear regression residuals for the SiO₂ loading (as indicated in Fig. S5†) are σ_R = 0.29 wt% for the SiO₂ loading on the capsules (A) and σ_R = 3.28 wt% for the SiO₂ loading in the membrane (B).

In addition to the surface morphologies, the surface charges of hybrid microcapsules were also influenced by the fraction of the OH–SiO₂ mass ratio in the particle mixture because of the differences in particle packing (different NH₂–SiO₂ and OH–SiO₂ embedment) at outer surface of the hybrid microcapsules. As demonstrated in Fig. 5, the zeta potential of hybrid microcapsules exhibited a decreasing tendency with an increasing OH–SiO₂ ratio in the particle mixture. Hybrid capsules with positive zeta potential values were obtained in the range of 60% OH–SiO₂ in the complex stabilizer. As the OH–SiO₂ ratio was further increased to 70%, the zeta potential value decreased significantly and a negative surface charge was obtained. This decreased zeta potential value of hybrid microcapsules further confirms that more OH–SiO₂ participated in the oil droplet stabilization with an increasing OH–SiO₂ mass ratio.

Fig. 5 Zeta potential of hybrid microcapsules as a function of OH–SiO₂ mass ratio in the complex stabilizer during emulsification.

Permeability detected by thermogravimetry

As microcontainers for bioactive volatile molecules, microcapsules must have a permeability that is low enough to prevent encapsulated volatiles from leaking during storage. We had already used TGA methods to study evaporation kinetics for delivery systems in our previous work,³⁸ and TGA tests with an isotherm maintained at 50 °C for 250 min were shown to be an excellent predictor of microcapsule storage stability. In general, the isotherm at 50 °C obtained for capsule suspension presents two regimes. The first regime represents weight loss dominated by the evaporation of water. The slope in the first regime depends on the rate of water evaporation, a steep slope corresponding to faster evaporation, which could be affected by the heating rate, gas flow, and surface properties of microcapsules. The second regime reflects the permeability of the microcapsule shell after it is dry and its ability to retain volatile molecules at a temperature of practical relevance in applications. As shown in Fig. 6, the hybrid microcapsules prepared with different OH–SiO₂ ratios exhibited different slopes in the first regime. Under the same heating rate and gas flow, hybrid microcapsules with more embedded OH–SiO₂ on the surface displayed lower water evaporation due to the more hydrophilic surface properties. After the water evaporated, the plateaus appeared at an average of 31–32 wt% of matter left for hybrid microcapsule samples in the second regime, corresponding to the weight percentage of the organic/inorganic hybrid membrane and the encapsulated model fragrance mixture. The plateaus in the second regime also indicated the low permeability and good storage stability of these hybrid microcapsules. In comparison, the Pickering emulsion sample gave a continued decreasing curve in the second regime, demonstrating poor storage stability.

Fig. 6 Representative isothermal thermogravimetric curves for a reference Pickering emulsion and hybrid microcapsules containing volatile fragrance oil with an isotherm at 50 °C for 250 min. The gray dotted line represents the horizontal line of 30 wt% weight loss. The dark blue dashed line shows the temperature profile.

Microcapsule mechanics

Classic core/shell microcapsules with polyurea-based polymer-only shells are typically used for the encapsulation of bioactive volatile molecules.^8,12 While they offer excellent stability against evaporation or chemical degradation, there is only limited control available over the mechanical properties of their shells via the polymerization process. In the following examples, we demonstrated how the synthesis of hybrid particle/polymer shells allows the introduction of more complex mechanical features into the shell. Specifically, we assessed how mechanical brittleness can be induced via the inclusion of SiO₂ particles in the shell.

All force curves measured with the microcapsules analyzed here fell into one of three classes of force curves upon compression: (i) gradual increase with a slightly nonlinear, strain-stiffening behavior; (ii) yielding, as indicated by a force plateau; or (iii) rupture, as indicated by a sudden drop in the compressive force. All capsules were compressed far beyond the linear elastic regime, and strong irreversibility was observed via hysteresis of the compression and expansion curves.

A typical example of the mechanical characteristics of microcapsules with classic polyurea-only shells in the absence of silica particles is shown in Fig. 7A (top panel). Both the approach and retract curves are shown. Significant plasticity, but no rupture or yielding, was evident from the strong difference between the approach and retract curves, indicating permanent damage to the capsule after compression to a peak force around 1100 μN. However, these classic capsules did not exhibit any sharp drops in the force curve, as would be expected from a sudden rupture upon compression. The absence of such sudden force drops indicates that such polymer-only capsules are mechanically rather compliant and release their payload gradually, rather than in a single burst. In contrast, the particles present in the hybrid membranes provided significant brittleness, and the hybrid inorganic/organic capsules broke via a sudden rupture, as indicated by a sharp drop in the force curve (see Fig. 7A, lower panel). When we compared the averaged mechanical properties of >20 individual microcapsules synthesized with different OH–SiO₂ mass ratios in the particle mixture, it appeared that the mechanical rupture behavior can indeed be controlled by using the inorganic particle composition of the capsule shell. Fig. 7B shows that at a mass ratio of 50% of OH–SiO₂, the average peak force achieved in the compression tests was reduced, indicating that these capsules are significantly more brittle, as they tend to rupture at compressive forces around ≈650 μN. Additionally, the shape recovery behavior of individual microcapsules after mechanical testing was also found to vary strongly with the composition of the inorganic portion of the membrane, as shown in Fig. 7C. Capsules synthesized with NH₂–SiO₂ alone remained almost completely deformed after compression, and no significant elastic recovery of the membrane was observed after the compression/retraction cycle (as reflected in the high permanent deformation values of ε_perm ≈ 0.96). For all capsules synthesized with OH–SiO₂ ratios above 50%, the permanent deformation consistently remained around ε_perm ≈ 0.78–0.80 and longer, depending on the OH–SiO₂ fraction; the values were most consistent and possessed the smallest standard deviation at an OH–SiO₂ ratio around 50%. Interestingly, there appeared to be minimum permanent deformation (ε_perm ≈ 0.69) at an OH–SiO₂ ratio of 30%, meaning that these capsules elastically recover part of their original shape after a compression cycle. The mechanical properties can therefore be tailored via the OH–SiO₂ ratio in the complex particle mixture, not only with respect to the rupture force, but also with respect to the different degrees of robustness over long time scales.

Fig. 7 Mechanics of hybrid core/shell microcapsules. (A) Sample force–displacement curves for mechanically compliant but non-rupturing capsules (upper graph) and breakable capsules with burst-type release (lower graph; rupture is associated with the intermediate peak); the arrows indicate the approaching (blue) and retracting (black) curves of the compression cycle. (B) Peak force measured during compression tests for different chemically stable hybrid microcapsules with different OH–SiO₂ mass ratios in the complex stabilizer during emulsification. (C) Permanent deformation of microcapsules with different OH–SiO₂ mass ratios, obtained from single compression/retraction cycles on sets of at least 20 individual microcapsules.

Conclusions

Organic–inorganic hybrid microcapsules were prepared by immobilizing both NH₂–SiO₂ and OH–SiO₂ in the shell based on Pickering emulsion drops as the core/shell templates. The microcapsules obtained showed tunable properties by changing the OH–SiO₂ ratio in the complex stabilizer (NH₂–SiO₂/OH–SiO₂ mixture). TGA tests with an isothermal protocol demonstrated the low permeability and excellent thermal stability of these hybrid microcapsules when exposed to 50 °C for 250 minutes. Moreover, the hybrid microcapsules revealed different mechanics behaviors upon compression compared with those of polymer-based microcapsules. These hybrid microcapsules with tunable properties, low permeability, and stiff shell can significantly improve the performance of microcapsules in practical applications, in particular where volatile molecules are to be encapsulated and their release triggered by mechanical stresses.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21338c

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