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Preparation of Multicore Colloidosomes: Nanoparticle-Assembled Capsules with Adjustable Size, Internal Structure, and Functionalities for Oil Encapsulation

Colloidosomes, also known as Pickering emulsion capsules, have attracted attention for encapsulation of hydrophilic and hydrophobic actives. However, current preparation methods are limited to single core structures and require the use of modified/engineered nanoparticles for forming the shell. Here, we report a fast, simple, and versatile method for producing multi-oil core silica colloidosomes via salt-driven assembly of purely hydrophilic commercial nanoparticles dispersed within an oil-in-water-in-oil (O/W/O) double emulsion template. The internal structure and overall diameter of the capsules can be adjusted by altering the primary and secondary emulsification conditions. With this approach, 7-35 μm diameter multicore colloidosomes containing 0.9-4.2 μm large oil cores were produced. The capsules can easily be functionalized depending on the type of nanoparticles used in the preparation process. Here, metal oxide nanoparticles, such as Fe3O4, TiO2, and ZnO, were successfully incorporated within the structure, conferring specific functional properties (i.e., magnetism and photocatalysis) to the final microcapsules. These capsules can also be ruptured by using ultrasound, enabling easy access to the internal core environments. Therefore, we believe this work offers a promising approach for producing multicore colloidosomes with adjustable structure and functionalities for the encapsulation of hydrophobic actives.

Samples reported here were prepared as follows, except when otherwise specified.
▪ Primary (O1/W) emulsion: The aqueous phase (W) was prepared by addition of silica nanoparticles (35 wt%) and Tween 80 (5 wt%) in deionized water. The internal oil phase (O1) was composed of sunflower oil. Primary emulsions were prepared by mixing the internal oil phase (2 g) with the aqueous phase (8 g) for 60 s using an Ultra Turrax mixer (T 25 digital, IKA) operating at 3,000, 5,000, 10,000 or 15,000 RPM.
▪ Secondary (O1/W/O2) emulsion: The external oil phase (O2) was composed of sunflower oil and Span 80 (1 wt%). Secondary emulsions were prepared by mixing the primary emulsion (1 g) and the external oil phase (9 g) for 60 s using a Vortex mixer (TopMix FB15024, Fischer Scientific) operating at 2,000 RPM.
▪ Multicore microcapsules: Oil core microcapsules were prepared using the O1/W/O2 double emulsions as structural templates. Capsules were obtained after dropwise addition of a calcium chloride solution (1 mL, 1 mol.L -1 ) into the O1/W/O2 emulsion, whilst maintaining 2,000 RPM stirring with a Vortex mixer (V). The capsules were recovered via centrifugation (3,000 RPM, 5 min) and cleaned twice by redispersal in deionized water and centrifugation (2,000 RPM, 5 min). Cleaned capsules were finally redispersed in deionized water.
Fe3O4 (5 wt%), TiO2 (5 wt%) or ZnO (1.5 wt%) nanoparticles were also added to the aqueous phase to create metal doped capsules. Secondary emulsification was carried out using either manual, vortex (V) or Ultra-Turrax (UT) mixing to investigate the impact of mixing conditions on the overall capsule diameter.
Capsule preparation conditions are reported as X(UT) -Y(V) with X, Y, UT, and V corresponding to primary emulsification mixing speed, secondary emulsification mixing speed, Ultra-Turrax mixer and Vortex mixer respectively.
Oil-in-Water (O1/W) emulsions were first produced to act as the initial capsule template. At low surfactant concentration (1 wt%), the oil droplet diameter was observed to be highly dependent of mixing conditions ( Figure S1). Increasing the mixing speed from 3,000 to 15,000 RPM resulted in a significant reduction in droplet diameter from 9.3 to 2.2 µm. These droplets are too large for use as an inner core template and this limits the size range of producible capsules. At high surfactant concentration (10 wt%), a limited impact of mixing conditions was observed, with the droplet diameter measured between 0.7 and 2.2 µm. The excess of surfactant molecules in the aqueous phase significantly reduces the interfacial tension at the oil/water interface, resulting in the formation of small droplets even with limited energy input. However, despite the submicron droplets, there is again minimal variability in the range of oil droplet diameters that can be attained, limiting the extent for tuneability of capsule internal structure.
Therefore, an intermediate concentration of Tween 80 (5 wt%) was utilised, which enabled production of oil droplets with diameters measured between 0.9 and 4.2 µm. Emulsions created with this composition did not exhibit any significant change in droplet diameter after 24 h The polydispersity in droplet size is directly linked to the use of a bulk emulsification approach for producing the emulsion template. This aspect could be addressed by producing emulsions one drop at a time using a microfluidic setting (i.e. emulsification in microchannels).           contamination of raw materials with additional chemicals (e.g. salt), or ageing of the solution over time. Similar agglomeration and changes of viscosity were observed in 3 different bottles and appear to increase over time, making the contamination hypothesis unlikely. All bottles used for the experiments presented in the main paper were extracted from the same batch (produced more than a year before the new bottles) and had been opened 6 to 12 months before this work was carried out. Therefore, we believe the agglomeration observed is likely to be due to a slow hydrolysis of the nanoparticle surfaces resulting in reduced surface charge.
Experiments were carried out to investigate the impact of using freshly opened Ludox bottles for producing multicore capsules. Preliminary tests, displayed in Figures S12 and S13, showed that emulsions prepared with fresh Ludox required a higher concentration of Tween 80 to offer a similar size and stability as the ones prepared with older bottles. By decreasing the viscosity of the medium, droplets are more likely to interact resulting in their coalescence. Increasing the concentration of Tween 80 to 10 wt% improved emulsion stability and hence, minimized coalescence of droplets. These emulsions were then used in the capsule preparation process.
Optical microscopy images, given in Figures S14 and S15, confirmed that at higher concentration of Tween 80, multicore capsules similar to the ones presented in the main paper could be produced. In addition, the size and core diameters of the capsules can still be controlled with new Ludox dispersions. This suggests that the age of the Ludox used in the capsule preparation process can easily be compensated for by adjusting the preparation conditions. Figure S12. Optical microscopy images of O1/W primary emulsions prepared using a fresh Ludox HS40 bottle. Scale bars are equal to 20 µm. Figure S13. Stability of O1/W emulsions prepared with a fresh Ludox HS40 bottle. Unstable mixtures are marked with an asterisk (*). Figure S14. Optical microscopy images of multicore silica capsules prepared with an aqueous phase containing 5 wt% Tween 80 and 35 wt% SiO2 nanoparticles from a fresh Ludox HS40 bottle. Scale bars are equal to 20 µm. Figure S15. Optical microscopy images of multicore silica capsules prepared with an aqueous phase containing 10 wt% Tween 80 and 35 wt% SiO2 nanoparticles from a fresh Ludox HS40 bottle. Scale bars are equal to 20 µm.