Impact of surface properties on nanoparticle-induced phase change in phospholipid vesicles

Abstract

Nanoparticles (NPs) are widely used in commercial products and medical treatments for their unique properties owing to their size; however, these same properties often lead to unanticipated interactions with organisms in the environment after the products are discarded or the treatment is completed. Some NPs are known to induce a change in the lipid phase of cell membranes, which affects membrane function and may lead to further toxicity. While the phase of lipid membranes and the impact of NPs on other membrane properties are well-characterized, the impact of NPs on the lipid phase of membranes is known for only a limited number of NP and lipid systems.This work, part of a study by the NSF Center for Sustainable Nanotechnology that aims to understand the interactions between synthetic nanoparticles and model membranes or organisms at the molecular level, investigates the impact of the properties of both NPs and lipids on NP-induced membrane phase change to further this understanding. The lipid phase of model membranes was quantified by fluorescence spectroscopy of laurdan, a phase-sensitive solvatochromic fluorophore which emits at two different wavelengths for the gel and fluid lipid phases. First, laurdan fluorescence spectra from laurdan-labeled liposomes (large unilamellar vesicles or LUVs) as model cell membranes exposed to anionic polystyrene (PS(-)) NPs were deconvoluted into lognormal functions to quantify NP-induced membrane phase change more rigorously with the parameter mole fraction of gel phase (Xgel). Then, the effect of NP and lipid properties on membrane phase change was investigated by measuring the Xgel of laurdan-labeled liposomes exposed to NPs. These Xgel measurements indicate that the combination of NP hydrophobicity and anionic surface charge induces a phase change in membranes, and that properties of lipids which inhibit packing exhibit a lower amount of gelation by NPs. The measurement of Xgel via fluorescence microscopy of giant unilamellar vesicles (GUVs) was explored to move the study toward a more realistic model and to acquire per-membrane Xgel. These results increase the understanding of the mechanism of NP-induced phase change to increase control over these interactions and better inform design of benign NPs.