Lipid adsorption at interfaces : a synchrotron X-ray reflectivity study

• Main Author: Sironi, Beatrice
• Published: University of Bristol 2016
• Subjects: 547
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.702268

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.