Atomic force microscope measurements and manipulation of Langmuir-Blodgett films with modified tips.

A simple method for rendering atomic force microscope tips and cantilevers hydrophilic or hydrophobic through glow discharge in an appropriate gas atmosphere is introduced. Force curves at different humidities of these modified cantilevers were taken on freshly cleaved mica (hydrophilic surface) and on a monolayer of dipalmitoylphosphatidylethanolamine transferred onto mica (hydrophobic surface) to characterize the behavior of the cantilevers on hydrophilic and hydrophobic surfaces. Furthermore, Langmuir-Blodgett bilayers, with a dipalmitoylphosphatidylethanolamine bottom layer and a dipalmitoylphosphatidylcholine top layer, were imaged in the constant force mode in a multimode atomic force microscope in air under controlled humidity conditions. The friction and elasticity signal were recorded parallel to the topography. By varying the force exerted by the tip on the sample, different layers of the Langmuir-Blodgett system could be removed or flattened. Removal exposed underlying layers that exhibited a different friction and elasticity behavior. Furthermore, force scans with tips rendered hydrophobic were taken on the different layers of the sample to characterize the hydrophilic/hydrophobic nature of the layers. Only by combining the results obtained by the different methods can the structure of the lipid layer systems be identified.