The molecular arrangement of phospholipid molecules was investigated on a hydrophilically modified gold surface within an aqueous solution by scanning tunneling microscopy. By suspending phospholipid (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, POPC) nanoparticles in the aqueous electrolyte surrounding a hydrophilically modified gold (111) substrate with 3-mercaptopropionic acid (SH-C2H4-COOH, 3-MPA), well-ordered adlattices of POPC were observed. Traces of particle fusion were visualized before formation of the adlattice. Addition of cholesterol to the suspension seems to facilitate accommodation of POPC on this surface. The observed unit cells of POPC adlattices had dimensions of 0.5 nm × 1.9-2.5 nm. By high-resolution imaging, each unit cell was discerned to be occupied by one upright POPC molecule. The POPC + cholesterol suspension also leads to formation of a flat integrated POPC layer, which may be a lipid bilayer covering the surface.
Keywords: gold single-crystal surface; high-resolution imaging; hydrophilic modification; phospholipid; scanning tunneling microscopy.