Effect of Surface Ligand and Temperature on Lipid Vesicle-Gold Nanoparticle Interaction: A Spectroscopic Investigation.

We herein investigate the interactions of differently functionalized anionic and cationic gold nanoparticles (AuNPs) with zwitterionic phosphocholine (PC) as well as inverse phosphocholine (iPC) lipid bilayers via spectroscopic measures. In this study, we used PC lipids with varying phase-transition temperatures, i.e., DMPC ( Tm = 24 °C), DOPC ( Tm = -20 °C), and iPC lipid DOCP ( Tm = -20 °C) to study their interactions with AuNPs functionalized with anionic ligands citrate, 3-mercaptopropionic acid, glutathione, and cationic ligand cysteamine. We studied the interactions by steady-state and time-resolved spectroscopic studies using membrane-sensitive probes 6-propionyl-2-dimethylaminonaphthalene (PRODAN) and 8-anilino-1 naphthalenesulfonate (ANS), as well as by confocal laser scanning microscopy (CLSM) imaging and dynamic light scattering (DLS) measurements. We observe that AuNPs bring in stability to the lipid vesicle, and the extent of interaction differs with the different surface ligands on the AuNPs. We observe that AuNPs functionalized with citrate effectively increase the phase-transition temperature of the vesicles by interacting with them. Our study reveals that the extent of interaction depends on the bulkiness of the ligands attached to the AuNPs. The bulkier ligands exert less van der Waals force, resulting in a weaker interaction. Moreover, we find that the interactions are more strongly pronounced when the vesicles are near the phase-transition temperature of the lipid.  The CLSM imaging and DLS measurements demonstrate the surface modifications in the vesicles as a result of these interactions.

Spectroscopic evidence for hydration and dehydration of lipid bilayers upon interaction with metal ions: a new physical insight.

In this manuscript, we investigate the interactions of different metal ions with zwitterionic phospholipid bilayers of different chain lengths using the well-known membrane probe PRODAN and steady state and time resolved fluorescence spectroscopy. We used three zwitterionic lipids that are widely different in their phase transition temperature, namely, dipalmitoylphosphatidylcholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine (POPC) and salts of zinc (Zn), calcium (Ca) and magnesium (Mg). The steady state and time resolved studies reveal that the affinity of the metal ions follows the order Zn2+ > Ca2+ > Mg2+. The study further reveals that the lipid membrane with an unsaturated chain exhibits very small affinity towards metal ions. We find that the Zn2+ and Ca2+ metal ions induce significant gelation in the lipid bilayer possibly by dehydrating the lipid bilayer surface. The study also demonstrates that unlike Zn2+ and Ca2+, dehydration does not take place for Mg2+. The extreme hydration induced by Mg2+ is rationalized by the tight hydration of Mg2+ and very high free energy barrier of Mg2+ to bind with lipid oxygen as compared to that of water molecules.