(569f) Temperature and Electrolyte Dependence of Liposome Aggregation Caused by Nanoparticle Binding

The binding of hydrophilic nanoparticles to biomembranes is important for designing drug delivery systems as well as studying nanoparticle toxicity. Lipid bilayer vesicles or liposomes provide a simplified model to understand the fundamental mechanisms governing this binding. In this study, the effect of temperature and electrolytes on the binding of anionic superparamagnetic iron oxide nanoparticles to zwitterionic dipalmitoylphosphatidylcholine (DPPC) and partially anionic DPPC/dipalmitoylphosphatidylglycerol (DPPC/DPPG at molar ratio 3:1) was investigated. Experiments were conducted using nanoparticles with an average hydrodynamic diameter of 16 nm, which is below the cutoff (~20 nm) for supported lipid bilayer formation. Dynamic light scattering (DLS) results showed nanoparticle-induced liposome-nanoparticle aggregates at 25 ^oC in deionized water that could be de-aggregated by inducing a gel to fluid phase transition (25 ^oC to 45 ^oC) or by adding salt (NaCl or phosphate buffered saline, PBS). Cryogenic transmission electron microscopy (cryo-TEM) imaging further showed that the nanoparticles remained bound to the liposomes in the gel and fluid phases, which suggests that bilayer undulation forces reduced aggregation. Also revealed by cryo-TEM was nanoparticle-induced liposome fusion in the gel phase that was significantly reduced in the fluid phase. The binding mechanism at different electrolyte concentrations was determined by isothermal titration calorimetry (ITC) and revealed a shift from entropic to enthalpic binding with increasing salt concentration. This work shows that nanoparticles can bind to net-neutral and like-charged liposomes, and that the effects of binding on liposome aggregation and fusion can be modulated with temperature or salt.