  Encapsulation of dsDNA fragments (contour length 54 nm) by the cationic
diblock copolymer poly(butadiene-b-N-methyl 4-vinyl pyridinium) [PBd-b-P4VPQ]
has been studied with phase contrast, polarized light, and fluorescence
microscopy, as well as scanning electron microscopy. Encapsulation was achieved
with a single emulsion technique. For this purpose, an aqueous DNA solution is
emulsified in an organic solvent (toluene) and stabilized by the amphiphilic
diblock copolymer. The PBd block forms an interfacial brush, whereas the
cationic P4VPQ block complexes with DNA. A subsequent change of the quality of
the organic solvent results in a collapse of the PBd brush and the formation of
a capsule. Inside the capsules, the DNA is compacted as shown by the appearance
of birefringent textures under crossed polarizers and the increase in
fluorescence intensity of labeled DNA. The capsules can also be dispersed in
aqueous medium to form vesicles, provided they are stabilized with an osmotic
agent (polyethylene glycol) in the external phase. It is shown that the DNA is
released from the vesicles once the osmotic pressure drops below 105 N/m2 or if
the ionic strength of the supporting medium exceeds 0.1 M. The method has also
proven to be efficient to encapsulate pUC18 plasmid in sub-micron sized
vesicles and the general applicability of the method has been demonstrated by
the preparation of the charge inverse system: cationic poly(ethylene imine)
encapsulated by the anionic diblock poly(styrene-b-acrylic acid).
