Title:
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Lipid adsorption at interfaces : a synchrotron X-ray reflectivity study
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Lipids in aqueous environment can self-assemble to form liposomes, which adsorb readily at
the solid-aqueous interfaces. The structures formed, and thus the properties of liposomes at
interfaces, are important to many applications, ranging from the medical field to
nanotechnologies. To investigate the adsorption behaviour of liposomes, we have employed
the X-ray reflectivity (XRR) technique. Measurements were perfonned at synchrotron
radiation sources, at ESRF and Diamond Light Source, using a unique "bending mica"
method, developed here in Bristol. To analyse the collected data, a custom made program has
been coded by Dr. T. Snow, based on a previous software package from R. Thomas (Oxford).
This program took into account the effect of crystal truncation rods (CTR) due to mica's
layered structure on the overall reflectivity, and allowed the fitting of the data using tailored
models. Three main systems have been investigated: four phosphatidylcholine lipids with
different chain length (12, 14, 16, and 18 carbons per chain) in the fonn of liposomes at the
mica-water interface, pure dyoleoylphosphatidylcholine (DOPC) multilayers at the mica-air
and mica-water interface, and multilayers of mixed polyamidoamine (P AMAM) dendrimers
and Dope at the mica-air interface. All multilayers were prepared from drop casting of
liposome dispersions. Liposomes at the mica-water interface were found stable or unstable
depending on the lipid chain length, with 16 carbons being the "critical length" at which the
fonn of stable adsorbed liposomes started to be favourable over the fonnation of an adsorbed
bilayer. All multilayers showed overall an ordered lamellar structure. Pure multilayers were
mostly ordered when prepared from a homogeneous liposome dispersion, and delaminated
under water. The templating effect of a coating polymer (polyethylenimine, PEl and stearic
trimethylammonium iodide, STAI) on mica surface influenced the order of the multilayers
because of the different charge density of the substrate. Hybrid PAP AM dendrimers and
lipids multilayers revealed once again ordered structures for almost all the "nanoparticles",
but a thennal cycle disrupted in part the order. Lipid-embedded dendrimers, already reported
in literature, were also observed in some samples. Our results have provided molecular
details of these interfacial structures, which had not been investigated previously on mica
with XRR.
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