pH-responsive colloidosomes and their use for controlling release
In the past eight years, several examples of colloidosome microcapsules have been developed. Such microcapsules systems are offering alternative options for the encapsulation and release of active ingredients on the basis of the size exclusion properties of their colloidal membranes. Here, we use latex particles sterically stabilised with a responsive polymer as the building blocks for the colloidosome microcapsule membranes and demonstrate that the designed microcapsules can be used as a pH-responsive system to control the release of water-soluble species on the basis of size exclusion. For their manufacture, an oil-in-water emulsion is prepared using the latex particles to stabilise the emulsion. The latex particles are reacted in situ, using an oil-soluble cross-linker, to permanently link the steric polymers on adjacent particles, thus producing a robust capsule shell. The oil core is subsequently removed from the encapsulated emulsion droplets using a solvent exchange mechanism to give water-containing microcapsules suspended in water. As the oil is removed, the microcapsules initially collapse on themselves but they can be easily re-swollen to their initial spherical shape using a combination of polymer expansion within the membrane and induced chemical potential differences between the microcapsule cores and the bulk. We show here that the responsive polymer on the surface of the constituent particles within the monolayer forming the microcapsule membrane can be induced to expand and contract through changes in the dispersion pH as a result of its weakly basic nature. Using a fluorescently-labelled dextran molecule, we demonstrate its successful uptake within the core of the microcapsules at low pH where the capsule membrane is expanded and the pore size is maximised. Altering the pH to higher values results in the contraction of the membrane and closing of the surface pores; under these conditions we show that the dextran can be successfully retained within the microcapsules. Finally, we demonstrate the pH-dependent release of the labelled sugar by readjusting the pH to acidic conditions.