  This work presents a simply methodology for coating small unilamellar
liposomes bearing different degrees of positive charge with polyelectrolyte
multilayers using the sequential Layer-by-Layer (LbL) deposition method. The
liposomes were made of mixtures of DOPC
(1,2-dioleyl-sn-glycero-3-phosphocoline) and DODAB (dimethyl dioctadecyl
ammonium bromide), and coated by alternated layers of the sodium salt of
poly(4-styrenesulfonate), PSS, and poly(allylamine), PAH, as polyanion and
polycations, respectively. The results shows that the zeta potential of the
liposomes was not very sensitive to the mole fraction of DODAB in the membrane,
XD, in the range 0.3< XD<0.8. We were able to coat the liposomes with up to
four polymer bilayers. The growth of the capsules size was followed by dynamic
light scattering, and in some cases by cryo-TEM, with good agreement between
both techniques. The thickness of the layers, measured from the hydrodynamic
radius of the coated liposome, depends on the polyelectrolyte used, so that the
PSS layers adopt a much more packaged conformation than the PAH layers. An
interesting finding is that the PSS amount needed to reach the isoelectric
point of the capsules increases linearly with the charge density of the bare
liposomes, whereas the amount of PAH does not depend on it. As expected, the
preparation of the multilayers has to be done in such a way that when the
system is close to the isoelectric point the capsules do not aggregate. For
this, we dropped the polyelectrolyte solution quickly, fast stirring and using
dilute liposome suspensions. The method is very flexible and not limited to
liposomes or polyelectrolyte multilayers, also coatings containing charged
nanoparticles can be easily made.
