Nano-probing liquids
The atomic force microscope is currently the most valuable tool for imaging, measuring and manipulating matter. Its resolution is fraction of a nanometre and it essentially measures topography with the aid of a force probe. The probe is attached to a cantilever of micro dimensions and the radius of its curvature is basically the only factor that limits the microscope's resolution. The SPOT-NOSED project has fabricated such probes to be used for electrical measurements in liquid media. This novel type of atomic force microscope consists of a cantilever attached to the plane surface of a silicon chip with a tip, the probe, in the shape of bird's beak protruding at its end. The tip is made out of polycrystalline silicon while silicon nitride is used for the cantilever. Tip and cantilever are coated with a gold conducting layer. A metal is required to facilitate electrical measurements and specifically gold for biocompatibility reasons. Atomic force microscopes do not 'see' in the sense that they do not detect light. The cantilever when subjected to the intermolecular interaction forces in the substance that is being probed vibrates simply following Hooke's law. This deflection is the signal that is subsequently transformed into an image of the surface. Therefore spring constants characterise the cantilevers. Since biological liquid samples are targeted, project partners have fabricated rectangular and V-shaped cantilevers, with spring constants ranging from 0.01N/m to 1N/m. The experimental laboratory prototypes of atomic force microscope probes developed have been extensively and successfully tested.
