  Understanding the adsorption and physical characteristics of supported lipid
membranes is crucial for their effective use as model cell membranes. Their
morphological and thermodynamic properties at the nanoscale have traditionally
been studied on hydrophilic substrates, such as mica and silicon oxide, which
have proved to facilitate the reconstruction of biomembranes. However, in more
recent years, with the advent of the van der Waals crystals technology,
two-dimensional crystals such as graphene have been proposed as potential
substrates in biosensing devices. Membranes formed on these crystals are
expected to behave differently owing to their intrinsic hydrophobicity, however
thus far knowledge of their morphological and thermodynamic properties is
lacking. Here we present a comprehensive nanoscale analysis of the adsorption
of phosphatidylcholine lipid monolayers on two of the most commonly used van
der Waals crystals, graphite and hexagonal boron nitride. Both morphological
and thermodynamic properties of the lipid membranes were investigated using
temperature-controlled atomic force microscopy. Our experiments show that the
lipids adsorb onto the crystals, forming monolayers with their orientation
dependent upon their concentration. Furthermore, we found that the
hydrophobicity of van der Waals crystals determines a strong increase in the
transition temperature of the lipid monolayer compared to that observed on
hydrophilic substrates. These results are important for understanding the
properties of lipid membranes at solid surfaces and extending their use to
novel drug delivery and biosensing devices made of van der Waals crystals.
