Self-Assembly of Lipids and Single-Walled Carbon Nanotubes: Effects of Lipid Structure and PEGylation

Self-assemblies of mixtures of a single-walled carbon nanotube (SWNT) and lipids were simulated using coarse-grained (CG) force fields previously developed and parametrized in this work. For lipids, lysophospholipids (single tail) and phospholipids (double tails) are ungrafted or grafted with poly(ethylene glycol) (PEG) at different sizes. In all simulated systems, lipids self-assemble along the SWNT, but their final conformations significantly differ. Simulations show that lysophospholipids helically wrap the SWNT as the “half-cylinder” conformation, which have been proposed by experiments but not captured by previous simulations. Phospholipids show “cylindrical micelles”, indicating the effect of the number of tails on the assembled structure. For both lysophospholipids and phospholipids, PEGylated lipids induce either “hemimicelle” or “random-adsorption” conformation, indicating dependence on the size of the lipid headgroup, because PEG chains with mushroom (solution-like) state induce the lipid-layer curvature and make lipids away from their neighbors. The number of lipids adsorbed onto SWNT increases stepwise as functions of time, implying that lipids self-assemble to their own aggregate in water modulated by the lipid headgroup and tail structure, and then the assembled lipids adsorb onto the SWNT surface. These results suggest important possible effects of PEGylation and lipid types on the self-assembled structure and mechanism, which compare favorably with experimental findings but also need to be further confirmed.