Hydrophobic nanoparticles promote lamellar to inverted hexagonal transition in phospholipid mesophases

This study focuses on how the mesophase transition behaviour of the phospholipid dioleoyl phosphatidylethanolamine (DOPE) is altered by the presence of 10 nm hydrophobic and 14 nm hydrophilic silica nanoparticles (NPs) at different concentrations. The lamellar to inverted hexagonal phase transition (L_α-H_II) of phospholipids is energetically analogous to the membrane fusion process, therefore understanding the L_α-H_II transition with nanoparticulate additives is relevant to how membrane fusion may be affected by these additives, in this case the silica NPs. The overriding observation is that the H_II/L_α boundaries in the DOPE p-T phase diagram were shifted by the presence of NPs: the hydrophobic NPs enlarged the H_II phase region and thus encouraged the inverted hexagonal (H_II) phase to occur at lower temperatures, whilst hydrophilic NPs appeared to stabilise the L_α phase region. This effect was also NP-concentration dependent, with a more pronounced effect for higher concentration of the hydrophobic NPs, but the trend was less clear cut for the hydrophilic NPs. There was no evidence that the NPs were intercalated into the mesophases, and as such it was likely that they might have undergone microphase separation and resided at the mesophase domain boundaries. Whilst the loci and exact roles of the NPs invite further investigation, we tentatively discuss these results in terms of both the surface chemistry of the NPs and the effect of their curvature on the elastic bending energy considerations during the mesophase transition.