Non-Covalent Reconfigurable Microgel Colloidosomes with a Well-Defined Bilayer Shell

Microgels have been successfully used to stabilize emulsion droplets. Compared to conventional rigid particles, the use of microgels as stabilizers offers several distinct advantages: (1) being soft and porous, they become deformed and flattened at the oil-water interface. The deformability of the particles makes it possible to achieve larger interfacial loading, which in turn alters the interfacial tension and the rheological properties of the interface; (2) being responsive, microgels allow one to prepare emulsions that can be triggered by environmental stimuli, which is especially desirable in industrial and emerging applications.<br/><br/>Whilst soft microgels have been demonstrated as being extremely interesting stabilizers for emulsions, previous studies mainly focused on the preparation of oil-in-water (O/W) emulsions due to their intrinsic hydrophilicity and thus initially dispersed in water. Very few studies have reported on the preparation of water-in-oil (W/O) emulsions using microgels as the sole emulsifier. In addition, there have been no attempts to control over microgel-assembled structure at the interface, thus limiting our ability to exploit particle monolayers or bilayers more broadly in advanced materials applications. On the other hand, the rheological properties of colloidal bilayers from soft particles self-assembled at interfaces to resulting emulsion characteristics, have not been explored yet.<br/><br/>In this work, we show that by introducing octanol into poly(<i>N</i>-isopropylacrylamide-<i>co</i>-methacrylic acid) (PNIPAM-<i>co</i>-MAA) microgels, octanol-swollen microgels can rapidly diffuse from the initially dispersed oil phase onto the water droplet surface. This facilitates the formation of microgel-laden interfacial layers with strong elastic responses and also generates stable inverse W/O Pickering emulsions. More importantly, these emulsions can be used as templates to produce microgel colloidosomes, herein termed ‘microgelsomes’, with shells that can be fine-tuned from a particle monolayer to a well-defined bilayer consisting of self-assembled binary microgels with opposite charges via non-covalent interaction. The microgelsomes can then be used to encapsulate and/or anchor nanoparticles, proteins, vitamin C, bio-based nanocrystals or enzymes. Moreover, the programmed release of these substances can be achieved by using ethanol as a trigger to mediate the shell permeability. Thus, these reconfigurable microgelsomes with a microgel-bilayer shell can respond to external stimuli and demonstrate tailored properties, which offers novel insights into microgels and promise wider application of Pickering emulsions stabilized by soft colloids.