Polyelectrolyte multilayer capsules: nanostructure and visualisation of nanopores in the wall

Hollow polymeric capsules prepared from Layer-by-Layer coating of colloidal templates with polyelectrolyte multilayers are promising materials for drug delivery and release applications. Details of the wall structure arising from the core dissolution process are investigated by high resolution scanning electron microscopy (SEM), using both secondary (SE) and backscattered electron (BSE) imaging modes. Atomic force microscopy (AFM) of the capsules in liquid was used as an independent technique. BSE images of the coated templates may be used for a rough estimation of the wall thickness. A freeze-drying procedure allows for the first time the investigation of dried multilayer capsules with an intact shape by SEM. Details of the nanostructure of the capsule walls are obtained, the topography shows structures on the scale of several 10 nm, corresponding to single chains. These structures are confirmed by AFM in liquid. In addition, after core dissolution single holes with sizes above 10 nm can be identified in the capsule wall. These holes are the structural property controlling the permeation and release and are here visualized for the first time. The number of holes per capsule as well as their distribution and size are analysed and discussed in their relevance for release applications.