Release Behavior of Thin-Walled Microcapsules Composed of Polyelectrolyte Multilayers

The release properties of the fluorescent probe pyrene, PYR, encapsulated in polyelectrolyte multilayer capsules were examined by fluorescence spectroscopy. PYR was encapsulated by the layer-by-layer deposition of oppositely charged polyelectrolytes onto PYR microcrystal templates, and its release was accomplished by exposing the polyelectrolyte-coated microcrystals to ethanol solutions. Solubilization of the hydrophobic probe by ethanol resulted in its expulsion from the core through the permeable polyelectrolyte multilayer shells. The PYR release rate, quantified by following the monomer fluorescence of the solubilized PYR as a function of time, decreased with increasing number of polyelectrolyte layers deposited onto the microcrystal cores. In addition, two amphiphiles (sodium dodecyl sulfate and dipalmitoyl- dl -α-phosphatidylcholine) used to disperse the microcrystals in aqueous solution prior to coating with polyelectrolytes significantly influenced the PYR release rate. The maximum release time, defined as the time required to achieve saturation release, could be varied by a factor of 2−5 depending on the nature of the first adsorbed layer and the number of polyelectrolyte layers composing the shell wall. The influence of flow (i.e., centrifugation vs magnetic stirring) as well as solvent on the PYR release rate is also discussed.