This work presents a simple methodology
for coating small unilamellar
liposomes bearing different degrees of positive charge with polyelectrolyte
multilayers using the sequential layer-by-layer deposition method.
The liposomes were made of mixtures of 1,2-dioleyl-sn-glycero-3-phosphocoline and dimethyl dioctadecyl ammonium bromide
(DODAB) and coated by alternated layers of the sodium salt of poly(4-styrenesulfonate)
(PSS) and poly(allylamine) (PAH) as polyanions and polycations, respectively.
The results show that the zeta potential of the liposomes was not
very sensitive to the mole fraction of DODAB in the membrane, X
D, in the range 0.3 ≤ X
D ≤ 0.8. We were able to coat the liposomes with
up to four polymer bilayers. The growth of the capsule size was followed
by dynamic light scattering, and in some cases, by cryo-transmission
electron microscopy, 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, stirred it fast, and used 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. Once the liposomes have
been coated, lipids can be easily eliminated, giving rise to polyelectrolyte
nanocapsules (polyelectrosomes) with potential applications as drug
delivery platforms.
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