Molecular-resolution images of Langmuir-Blodgett films and DNA by atomic force microscopy

An atomic force microscope (AFM) operating with the tip in contact with the surface has given molecular-resolution images of monomolecular Langmuir-Blodgett (LB) films consisting of the lipids DMPE (DL-{alpha}-dimyristoylphosphatidylethanolamine), DMPG (L-{alpha}-dimyristoylphosphatidylglycerol), DODAB (dioctadecyldimethylammonium bromide), or 1:1 DODAB/DPPG (L-{alpha}-dipalmitoylphosphatidylglycerol). All the LB films were submerged in a buffer solution during imaging. Precoated mica served as a substrate for the LB films, which were transferred onto it by vertical dipping. The molecular arrangement of the lipids in the LB films agrees with the known 3D crystal data to within the accuracy of our measurements, about 20%, and shows a clear difference from the substrate mica, whose hexagonal lattice could be seen by increasing the force applied by the AFM if the bottom monomolecular film was not covalently interlinked. The AFM also gave images of single-stranded DNA bound to LB films, thus illustrating the potential of LB films as substrates for binding and imaging macromolecules.