Lateral electrical conductivity of mica-supported lipid bilayer membranes measured by scanning tunneling microscopy.

Lateral electric conductivity of mica-supported lipid monolayers and of the corresponding lipid bilayers has been studied by means of scanning tunneling microscopy (STM). The surface of freshly cleaved mica itself was found to be conductive when exposed to humid air. Lipid monolayers were transferred onto such a surface by means of the Langmuir-Blodgett technique, which makes the mica surface hydrophobic and suppresses the electric current along the surface in the experimentally accessible humidity (5-80%) and applied voltage (0-10 V) range. This is true for dipalmitoylphosphatidylethanolamine (DPPE) as well as dipalmitoylphosphatidylcholine (DPPC) monolayers. Repeated deposition of DPPC layers by means of the Langmuir-Blodgett LB technique does not lead to the formation of a stable surface-supported bilayer because of the high hydrophilicity of the phosphatidylcholine headgroups that causes DPPC/DPPC bilayers to peel off the supporting surface during the sample preparation. In contrast to this, a DPPE or a DPPC monolayer on top of a DPPE monolayer gives rise to a rather stable mica-supported bilayer that can be studied by STM. Electric currents between 10 and 100 fA, depending on the ambient humidity, flow along the DPPE bilayer surface, in the humidity range between 35 and 60%. The DPPC surface, which is more hydrophilic, is up to 100 times more conductive under comparable conditions. Anomalous high lateral conductivity thus depends on, and probably proceeds via, the surface-adsorbed water layers. The prominence of ambient humidity and surface hydrophilicity on the measured lateral currents suggests this. The combination of our STM data and previously published water adsorption isotherms as a function of the relative humidity indicate that one layer or less of adsorbed water suffices for mediating the measurable lateral currents. The fact that similar observations are also made for other hydrophilic substrates supports the conclusion that lateral conductivity via surface-adsorbed water is a rather general phenomenon.

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.

Scanning tunneling microscopy of insulators and biological specimens based on lateral conductivity of ultrathin water films.

Scanning tunneling microscopy is based on the flow of an electrical current and thus cannot be used to directly image insulating material. It has been found, however, that a very thin film of water (about one monolayer) adsorbed to a surface exhibits a surprisingly high conductivity that is sufficient to allow scanning tunneling microscope imaging at currents below 1 picoampere. Hydrophilic insulators, such as glass and mica, can thus be imaged in humid air. The same is true for biological specimens deposited on such surfaces, as demonstrated by the scanning tunneling microscope imaging of plasmid DNA on mica.

Atomic force microscopy of a hydrated bacterial surface protein.

The protein surface layer of the bacterium Deinococcus radiodurans (HPI layer) was examined with an atomic force microscope (AFM). The measurements on the air-dried, but still hydrated layer were performed in the attractive imaging mode in which the forces between tip and sample are much smaller than in AFM in the repulsive mode or in scanning tunnelling microscopy (STM). The results are compared with STM and transmission electron microscopy (TEM) data.

Scanning tunnelling microscopy of biomacromolecules.

When imaging biomacromolecules with a STM, coating of specimens with a conductive layer is a convenient preparation method which provides a good rate of success. Utilizing evaporated platinum/carbon as a coating film we have investigated two biomacromolecules of very different appearance. The first of these is the HPI-layer, a natural two-dimensional protein crystal with a period of 18 nm, which is found on the surface of the bacterium Deinococcus radiodurans. The second specimen is type IV collagen which forms long triple-helical strands approx. 1.5 nm in diameter. The resulting STM pictures compare very well with electron microscopical images.