Cationic nanoparticles stabilize zwitterionic liposomes better than anionic ones

Building upon the finding that zwitterionic liposomes can be stabilized against fusion up to very high surface coverage by the adsorption to submonolayer (≈25%) surface coverage of nanoparticles (Zhang, L.; Granick, S. Nano Lett. 2006, 6, 694), here we compare the efficacy of cationic and anionic nanoparticles for the stabilization of DLPC, 1,2-dilauroyl-sn-glycero-3- phosphocholine. This is rationalized by considering that because the phospholipid zwitterionic headgroup terminates with positive charge, lipids beneath an adsorbed nanoparticle bind more weakly when the nanoparticle charge is cationic. Going beyond the earlier qualitative study, here single-particle tracking using epi-fluorescence imaging is used to quantify the mobility of individual liposomes. The distribution of diffusion coefficients between different liposomes in the sample is quantified. In contrast to the colloid behavior of traditional monodisperse hard-sphere colloids, these soft, flexible, colloidalsized objects remain fluid at 50% volume fraction of liposomes.