Structural Changes to Lipid Bilayers and Their Surrounding Water upon Interaction with Functionalized Gold Nanoparticles

Structural changes of solid-supported lipid bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and of their interfacial water surrounding have been probed with vibrational sum frequency generation (SFG) upon interaction with functionalized gold NPs, carrying negative or positive surface charges. Switching the substrate/DPPC zeta potential from strongly negative to weakly negative or positive values by using SiO₂ or CaF₂ supporting surfaces has enabled elucidating interfacial charge effects, intermolecular interactions, and interaction mechanisms regulating those nano-bio interfaces. At SiO₂/DPPC surfaces (ζ = -30 mV), negative NPs have reinforced the average interfacial water alignment, while no interaction has occurred with DPPC bilayer. Oppositely, positive NPs interact with DPPC, with a probable two-step mechanism involving the formation of pores within the bilayer followed by a reorganization of a quasi-ordered DPPC bilayer around NPs. At CaF₂/DPPC surfaces (ζ = ±15 mV), both negative and positive NPs have clearly damaged the lipid bilayer structure, together with a drastic disruption of the water organization at the interface. Importantly, all major interfacial structural changes have occurred within the first 15-90 min after exposure to NPs whatever the lipid is supported on SiO₂ or CaF₂.