  Amphiphilic phospholipids and nanoparticles functionalized with hydrophobic
capping ligands have previously been extensively investigated for their
capacity to self-assemble into Langmuir monolayers at the air/water interface.
However, understanding of composite films consisting of both nanoparticles and
phospholipids, and by extension, the complex interactions arising between
nanomaterials and biological membranes, remains limited. In this work,
dodecanethiol-capped gold nanoparticles (Au-NPs) with an average core diameter
of 6 nm were incorporated into 1,2-dipalmitoyl-sn-glycero-3-phosphocholine
(DPPC) monolayers in area ratios ranging from 0.1 to 20% area coverage at a
surface pressure of 30 mN/m. High resolution liquid surface X-ray scattering
studies revealed a phase separation of the DPPC and Au-NP components of the
composite film, as confirmed with atomic force microscopy after the film was
transferred to a substrate. At low Au-NP content, the structural organization
of the phase-separated film is best described as a DPPC film containing
isolated islands of Au-NPs. However, increasing the Au-NP content beyond 5%
area coverage transforms the structural organization of the composite film to a
long-range interconnected network of Au-NP strands surrounding small seas of
DPPC, where the density of the Au-NP network increases with increasing Au-NP
content. The observed phase separation and structural organization of the
phospholipid and nanoparticle components in these Langmuir monolayers are
useful for understanding interactions of nanoparticles with biological
membranes.
