Self-Patterned Mixed Phospholipid Monolayers for the Spatially Selective Deposition of Metals

Abstract

Metal-reactive organosulfur groups were patterned onto mica and silicon surfaces by dewetting instabilities during the Langmuir-Blodgett (LB) deposition of phase-separated mixed phospholipid monolayers. Monolayers were formed from binary mixtures of dipalmitoylphosphatidylcholine (DPPC), dilauroylphosphatidylcholine (DLPC), and their ω-methyldisulfide-dialkylphosphatidylcholine analogues, DSDPPC and DSDLPC. Patterns of highly parallel stripes of condensed DPPC or DSDPPC, protruding by 0.7-0.9 nm from a fluid matrix of DLPC or DSDLPC, were observed over areas extending at least 30 × 30 μm(2) in the LB films. The average stripe width varied from ca. 150 to 500 nm, depending on the lipid composition and deposition pressure. X-ray photoelectron spectroscopy confirmed that the phospholipid-monolayer-bound methydisulfides react with Au vapor to form a gold-thiolate species. The adsorption of thermally evaporated Au, Ag, and Cu onto DSDPPC/DLPC and DPPC/DSDLPC patterns was investigated by field emission gun scanning electron microscopy (FEGSEM) and atomic force microscopy (AFM). A change in phase contrast is observed in FEGSEM and AFM over the methyldisulfide-functionalized areas following metal deposition due to metal-thiolate bond formation. An increase in step height between the DSDPPC stripes and nonfunctionalized DLPC background following metal deposition, as well as the resistance of the metal-coated DSDPPC or DSDLPC regions to detergent extraction from the surface, attest to a selective metallization of the pattern. Our results indicate that the preferential adsorption of vapor-deposited metal onto the ω-methyldisulfide-terminated phase occurs at submonolayer coverages. The chemical reactivity exhibited by the organosulfur-modified phospholipid LB films make these templates potentially interesting for the fabrication of solid-supported patterns of metal nanostructures.