Sitting Phase Monolayers of Polymerizable Phospholipids Create Dimensional, Molecular-Scale Wetting Control for Scalable Solution-Based Patterning of Layered Materials.

The use of dimensionally ordered ligands on layered materials to direct local electronic structure and interactions with the environment promises to streamline integration into nanostructured electronic, optoelectronic, sensing, and nanofluidic interfaces. Substantial progress has been made in using ligands to control substrate electronic structure. Conversely, using the exposed face of the ligand layer to structure wetting and binding interactions, particularly with scalable solution- or spray-processed materials, remains a significant challenge. However, nature routinely utilizes wetting control at scales from nanometer to micrometer to build interfaces of striking geometric precision and functional complexity, suggesting the possibility of leveraging similar control in synthetic materials. Here, we assemble striped "sitting" phases of polymerizable phospholipids on highly oriented pyrolytic graphite, producing a surface consisting of 1 nm wide hydrophilic stripes alternating with 5 nm wide hydrophobic stripes. Protruding, strongly wetting headgroup chemistries in these monolayers enable formation of rodlike wetted patterns with widths as little as ∼6 nm and lengths up to 100 nm from high-surface-tension liquids (aqueous solutions of glycerol) commonly utilized to assess interfacial wetting properties at larger length scales. In contrast, commonly used lying-down phases of diynoic acids with in-plane headgroups do not promote droplet sticking or directional spreading. These results point to a broadly applicable strategy for achieving high-resolution solution-based patterning on layered materials, utilizing nanometer-wide patterns of protruding, charged functional groups in a noncovalent monolayer to define pattern edges.

Sitting Phases of Polymerizable Amphiphiles for Controlled Functionalization of Layered Materials.

Precisely tailoring surface chemistry of layered materials is a growing need for fields ranging from electronics to biology. For many applications, the need for noncovalently adsorbed ligands to simultaneously control interactions with a nonpolar substrate and a polar solvent is a particular challenge. However, biology routinely addresses a similar challenge in the context of the lipid bilayer. While conventional standing phases of phospholipids (such as those found in a bilayer) would not provide spatially ordered interactions with the substrate, here we demonstrate formation of a sitting phase of polymerizable phospholipids, in which the two alkyl chains extend along the surface and the two ionizable functionalities (a phosphate and an amine) sit adjacent to the substrate and project into the solvent, respectively. Interfacial ordering and polymerization are assessed by high-resolution scanning probe measurements. Water contact angle titrations demonstrate interfacial pKa shifts for the lipid phosphate but not for the amine, supporting localization of the phosphate near the nonpolar graphite surface.