Nano-differential scanning calorimetry (nano-DSC) is a powerful tool in the investigation of unilamellar (small unilamellar, SUVs, or large unilamellar, LUVs) vesicles, as well as lipids on supported bilayers, since it measures the main gel-to-liquid phase transition temperature (Tm), enthalpies and entropies. In order to assign these transitions in single component systems, where Tm often occurred as a doublet, nano-DSC, dynamic light scattering and cryo-transmission electron microscopy (cryo-TEM) data were compared. The two Tms were not attributable to decoupled phase transitions between the two leaflets of the bilayer, i.e. nano-DSC measurements were not able to distinguish between the outer and inner leaflets of the vesicle bilayers. Instead, the two Tms were attributed to mixtures of oligolamellar and unilamellar vesicles, as confirmed by cryo-TEM images. Tm for the oligolamellar vesicles was assigned to the peak closest to that of the parent multilamellar vesicle (MLV) peak. The other transition was higher than that of the parent MLVs for 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and increased in temperature as the vesicle size decreased, while it was lower in temperature than that of the parent MLVs for 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and decreased as the vesicle size decreased. These subtle shifts arose due to small differences in the values of ΔH and ΔS, since Tm is determined by their ratio (ΔH/ΔS). It was not possible to completely eliminate oligolamellar structures for MLVs extruded with the 200 nm pore size filter, even after 120 passes, while these structures were eliminated for MLVs extruded through the 50 nm pore size filter.

Lipid exchange/transfer has been compared for zwitterionic 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine (DMPC) small unilamellar vesicles (SUVs) and for the same lipids on silica (SiO2) nanoparticle supported lipid bilayers (NP-SLBs) as a function of ionic strength, temperature, temperature cycling, and NP size, above the main gel-to-liquid crystal phase transition temperature, Tm, using d- and h-DMPC and DPPC. Increasing ionic strength decreases the exchange kinetics for the SUVs, but more so for the NP-SLBs, due to better packing of the lipids and increased attraction between the lipid and support. When the NP-SLBs (or SUVs) are cycled above and below Tm, the exchange rate increases compared with exchange at the same temperature without cycling, for similar total times, suggesting that defects provide sites for more facile removal and thus exchange of lipids. Defects can occur: (i) at the phase boundaries between coexisting gel and fluid phases at Tm; (ii) in bare regions of exposed SiO2 that form during NP-SLB formation due to mismatched surface areas of lipid and NPs; and (iii) during cycling as the result of changes in area of the lipids at Tm and mismatched thermal expansion coefficient between the lipids and SiO2 support. Exchange rates are faster for NP-SLBs prepared with the nominal amount of lipid required to form a NP-SLB compared with NP-SLBs that have been prepared with excess lipids to minimize SiO2 patches. Nanosystems prepared with equimolar mixtures of NP-SLBs composed of d-DMPC (d(DMPC)-NP-SLB) and SUVs composed of h-DMPC (h(DMPC)-SUV) show that the calorimetric transition of the "donor" h(DMPC)-SUV decreases in intensity without an initial shift in Tm, indicating that the "acceptor" d(DMPC)-NP-SLB can accommodate more lipids, through either further fusion or insertion of lipids into the distal monolayer. Exchange for d/h(DMPC)-NP-SLB is in the order 100 nm SiO2 > 45 nm SiO2 > 5 nm SiO2.

Bare, chemically surface modified and supported lipid bilayer (SLB) nanoparticles are finding expanding uses in pharmaceutical, biomedical and materials applications, where in vivo, in vitro or in the environment they come into contact with lipids in the form of cells or cellular debris, and where clustering of the nanoparticles can affect dosages and cellular uptake. Here, the mechanism of formation, nano-structure, state of aggregation and colloidal stability of SLBs for a model system consisting of y100 nm silica (SiO 2 ) nanoparticles and zwitterionic lipids in the form of y100 nm small unilamellar vesicles (SUVs) is investigated at low and high ionic strengths, as a function of the surface area (SA) ratios of the SUVs (SA SUV ) and SiO 2 (SA SiO 2 ). Formation of single-SLBs is suggested to be a bi-molecular collision event, both above and below the lipid phase transition temperature (T m ), between negatively charged SiO 2 and neutral SUVs, with rupture of SUVs occurring at the vesicle sides where there is a small radius of curvature, followed by continued wrapping of the SiO 2 . At low ionic strength (5 mM NaCl), colloidal metastability occurs at SA SUV / SA SiO 2 ¢ 1/1 due to residual electrostatic repulsion, and the nanosystem exists as independent, noninteracting particles. At high ionic strength (PBS buffer), precipitation occurs at SA SUV /SA SiO 2 = 1/1 due to charge shielding, but at SA SUV /SA SiO 2 ¢ 2/1, excess SUVs adsorb onto the SLBs at defects sites (bare SiO 2 ) in the first SLB, restoring undulatory/protrusion forces, and thus colloidal metastability, by portions of the SUVs not in contact with the SLBs; for SA SUV /SA SiO 2 = 2/1 there is an approximate 1/1 pairing of SUVs and SLBs above T m , while below T m , aggregation of larger but meta-stable structures occurs. When the SA SUV /SA SiO 2 = 2/1 nanosystems in PBS buffer are reheated to a temperature above T m , separate, non-interacting particles are formed, suggesting that SUVs trapped on SiO 2 defect sites are pinched off to form a SLB in the defect area, expelling a smaller SUV.