Nanometer Scale Surface Properties of Supported Lipid Bilayers Measured with Hydrophobic and Hydrophilic Atomic Force Microscope Probes 

Using the atomic force microscope (AFM), surface-forces measurements are made in water between chemically modified AFM probes and model membranes created by Langmuir-Blodgett (LB) deposition. A series of four different lipid monolayers with varying headgroups and monolayer properties were transferred onto monolayers on mica for analysis. Using a hydrophilic probe, the bilayers are elastically deformed at low load and punctured at a repeatable, material-dependent breakthrough force. Using hydrophobic probes, the bilayers are punctured on contact, at loads near zero, in all cases. This effect is also manifest when imaging mixed LB monolayers, which show a large topography contrast at low load when using hydrophilic tips but none when using hydrophobic tips. These results provide evidence that the forces required to deform lipid bilayers are dramatically changed in the vicinity of nonpolar surfaces, which is central to the understanding of membrane fusion processes and the function of membraneassociated proteins.