Mechanism of supported bilayer formation of zwitterionic lipids on SiO2 nanoparticles and structure of the stable colloids

Abstract

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 ∼100 nm silica (SiO2) nanoparticles and zwitterionic lipids in the form of ∼100 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 (SASUV) and SiO2 (SASiO2). Formation of single-SLBs is suggested to be a bi-molecular collision event, both above and below the lipid phase transition temperature (Tm), between negatively charged SiO2 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 SiO2. At low ionic strength (5 mM NaCl), colloidal metastability occurs at SASUV/SASiO2 ≥ 1/1 due to residual electrostatic repulsion, and the nanosystem exists as independent, non-interacting particles. At high ionic strength (PBS buffer), precipitation occurs at SASUV/SASiO2 = 1/1 due to charge shielding, but at SASUV/SASiO2 ≥ 2/1, excess SUVs adsorb onto the SLBs at defects sites (bare SiO2) in the first SLB, restoring undulatory/protrusion forces, and thus colloidal metastability, by portions of the SUVs not in contact with the SLBs; for SASUV/SASiO2 = 2/1 there is an approximate 1/1 pairing of SUVs and SLBs above Tm, while below Tm, aggregation of larger but meta-stable structures occurs. When the SASUV/SASiO2 = 2/1 nanosystems in PBS buffer are reheated to a temperature above Tm, separate, non-interacting particles are formed, suggesting that SUVs trapped on SiO2 defect sites are pinched off to form a SLB in the defect area, expelling a smaller SUV.