Fabrication of protein microgels with spherical and urchin-like shapes within water-in-water (W/W) emulsions

Microgels are three-dimensional networks comprising crosslinked polymers. Biopolymer-based microgels, especially protein-based microgels, are finding increasing applications in various fields, for example as densely packed particles in sports beverages, as stabilizers in emulsions, and as delivery vehicles. Microgels are usually spherical and are typically produced using either solid or liquid templating methods. A popular liquid templating technique includes internal phase gelation within water-in-oil emulsions; however, it commonly requires the employment of synthetic surfactants and organic solvents. In the context of microgels, branched (urchin-like) microgels have attracted considerable attention due to their much higher surface-to-volume ratio rather than spherical microgels. Here, we report a method for fabricating urchin-like, and spherical microgels using egg yolk high-density lipoproteins (HDL) within all-aqueous (water-in-water, W/W) emulsions. HDL is compartmentalized in the droplet phase within W/W emulsions, composed of Na₂SO₄ and polyethylene glycol (PEG) solutions. HDL is crosslinked and then oxidized using gold ions, while gold ions get reduced to gold nanoparticles (AuNPs), resulting in the formation of hybrid HDL-Au microgels. The microgels with different shapes (random aggregates, urchin-like, and spherical microgels) are formed by changing HDL concentration and pH values in the emulsions. Epi-fluorescent microscopy and confocal laser scanning microscopy were used to characterize the structures of the HDL-Au microgels. The outcomes of this research may aid the development of microgels for applications in different fields, such as encapsulation, catalysis, optical switches, sensors, biomaterials for tissue engineering and in food applications.