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Understanding the structure of solid supported lipid multilayers is crucial to their application as a platform for novel materials. Conventionally, they are prepared from drop casting or spin coating of lipids dissolved in organic solvents, and lipid multilayers prepared from aqueous media and their structural characterisation have not been reported previously, due to their extremely low lipid solubility (i.e.∼10(-9) M) in water. Herein, using X-ray reflectivity (XRR) facilitated by a "bending mica" method, we have studied the structural characteristics of dioleoylphosphatidylcholine (DOPC) multilayers prepared via drop casting aqueous small unilamellar and multilamellar vesicle or liposome (i.e. SUV and MLV) dispersions on different surfaces, including mica, positively charged polyethylenimine (PEI) coated mica, and stearic trimethylammonium iodide (STAI) coated mica which exposes a monolayer of hydrocarbon tails. We suggest that DOPC liposomes served both as a delivery matrix where an appreciable lipid concentration in water (∼25 mg mL(-1) or 14 mM) was feasible, and as a structural precursor where the lamellar structure was readily retained on the rupture of the vesicles at the solid surface upon solvent evaporation to facilitate rapid multilayer formation. We find that multilayers on mica from MLVs exhibited polymorphism, whereas the SUV multilayers were well ordered and showed stronger stability against water. The influence of substrate chemistry (i.e. polymer coating, charge and hydrophobicity) on the multilayer structure is discussed in terms of lipid-substrate molecular interactions determining the bilayer packing proximal to the solid-liquid interface, which then had a templating effect on the structure of the bilayers distal from the interface, resulting in the overall different multilayer structural characteristics on different substrates. Such a fundamental understanding of the correlation between the physical parameters that characterise liposomes and substrate chemistry, and the structure of lipid multilayers underpins the potential development of a simple method via an aqueous liposome dispersion route for the inclusion of hydrophilic functional additives (e.g. drugs or nanoparticles) into lipid multilayer based hybrid materials, where tailored structural characteristics are an important consideration.

BackgroundUnderstanding the structure of hybrid nanoparticle-lipid multilayers is of fundamental importance to their bioanalytical applications and nanotoxicity, where nanoparticlemembrane interactions play an important role. Poly(amidoamine) (PAMAM) dendrimers are branched polymeric nanoparticles with potential biomedical applications due to precise tunability of their physicochemical properties. Here, the effect of PAMAM dendrimers (2.9 -4.5 nm) with either a hydrophilic amine (NH 2 ) or a hydrophobic C 12 chain surface termination on the 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) multilayers has been studied for the first time.Methods DOPC multilayers were created by the liposome-rupture method via drop-casting dendrimerliposome dispersions with the dendrimers added at different concentrations and at three different stages. The multilayer structure was evaluated via the analysis of the synchrotron Xray reflectivity (XRR) curves, obtaining the bilayer d-spacing, the coherence length from the Scherrer (L s ) analysis of the Bragg peaks, and the paracrystalline disorder parameter (g). ResultsDendrimer addition led to lipid bilayer thinning and more disordered multilayer structures. Larger hydrophobic dendrimers caused greater structural disruption to the multilayers compared to the smaller dendrimers. The smallest, positively charged dendrimers at its highest concentration caused the most pronounced bilayer thinning. The dendrimer-liposome mixing method also affected the multilayer structure due to different dendrimer aggregation involved. ConclusionsThese results show the complexity of the effect of dendrimer physicochemical properties and the addition method of dendrimers on the structure of mixed dendrimer-DOPC multilayers.General significance These insights are useful for fundamental understanding of nanotoxicity and future biomedical application of nanocomposite multilayer materials in which nanoparticles are added for enhanced properties and functionality.