Capsules with a core-shell structure have dimension of 10-100 nanometers. The material of the shell can be PLGA or lipid, but how do you verify its core-shell structure if the work needs to be published?
One of the best way would be stain you NPs with staining agent (osmium tertraoxide, you can find some biocompatible staining agents) and then run a TEM scan not SEM, and you will have a perfect core shell NPs.
The staining agents would induce contrast in of the either Core-shell materials