Cationic Nanoparticles Stabilize Zwitterionic Liposomes Better than Anionic Ones
"J. Phys. Chem. C 2007, 111, 8233-8236". Yan Yu, Stephen M. Anthony, Liangfang Zhang, Sung Chul Bae, and Steve Granick.
The article describes the usage of cationic nanoparticles to stabilize zwitterionic liposomes. The zwitterionic nature of the lipid vesicles arises from the phosphorous-nitrogen dipole in the headgroup of the phophatidylcholine (PC) molecule. Anionic nanoparticles are likely to adsorb more strongly than cationic nanoparticles due to the geometry of the phosphorous-nitrogen dipole. However, the authors propose that exposing liposomes to cationic rather than anionic nanoparticles better stabilizes the vesicles and prevents aggregation. They explain that the weaker the adsorption of the nanoparticles onto the liposomes, the less likely they will tend to attract to other adjacent particles.
Positively-charged amidine and negatively-charged carboxyl modified polystyrene latex nanoparticles (20 nm in diameter) were used with PC liposomes prepared via extrusion method (200 nm diameter). Concentration of nanoparticles was chosen so that only 25% of the surface area of the vesicle was occupied by the nanoparticles, assuming 100% of the particles adsorbed. Liposomes were fluorescently tagged and diffusion of liposomes was tracked with a standard single-particle tracking algorithm. It was found that at volume fractions <math>\phi > 0.50</math>, liposomes with adsorbed anionic particles were not able to move and that liposomes with adsorbed cationic particles remained fluid.
Soft Matter Connection
Diffusion of liposomes was measured as an indicator of lipid mobility, the greater the mobility meant the vesicles remained fluid and had not associated with adjacent nanoparticles/vesicles. The stabilization of these liposomes with cationic nanoparticles is similar to the example covered in class of block copolymers attached to surfaces in order to stabilize colloidal dispersions. One disadvantage of this system mentioned on the class notes is the possible displacement by other adsorbents and the instabilities that could be caused by temperature or presence of other solvents, which are not addressed in the article.
Fig.1 A) Schematic image of PC liposomes detailing the zwitterionic headgroup and nanoparticles (yellow). B)Single liposome trajectory during 50s with each time step being 50 ms long. (<math>\phi=0.5</math>)