The electrical conductivity of biomaterials on a molecular scale is of fundamental interest in the life sciences. We perform first principles electronic structure calculations, which clearly indicate that $\lambda$-DNA chains should present large resistance values. We also present two direct procedures to measure electrical currents through DNA molecules adsorbed on mica. The lower limit for the resistivity is ${10}^6\Omega ̇\mathrm{cm}$, in agreement with our calculations. We also show that low energy electron bombardment induces a rapid contamination and dramatically affects the measured conductivity, thus providing an explanation to recent reports of high DNA conductivity.

The electrostatic interaction between a model probe and a sample in a scanning probe microscope is analyzed. A simple model for a real experimental setup is proposed and solved by means of an appropriate approximation. In addition, a quantitative definition for resolution is presented. We find that generally the total force between tip and sample is dominated by contributions which are not confined to a nanometer-sized region under the tip apex. From our analysis we conclude that such a confinement is only obtained either with specially designed probes or by using the force gradient as signal source. We show that reliable experimental data acquired by local Kelvin probe microscopy can only be obtained if these considerations are taken into account. Finally, we propose an experimental setup which optimizes resolution and gives the correct local surface potential in the case of Kelvin probe microscopy.

A fundamental requirement for a molecule to be considered a molecular wire (MW) is the ability to transport electrical charge with a reasonably low resistance. We have carried out two experiments that measure first, the charge transfer from an electrode to the molecule, and second, the dielectric response of the MW. The latter experiment requires no contacts to either end of the molecule. From our experiments we conclude that adsorbed individual DNA molecules have a resistivity similar to mica, glass, and silicon oxide substrates. Therefore adsorbed DNA is not a conductor, and it should not be considered as a viable candidate for MW applications. Parallel studies on other nanowires, including single-walled carbon nanotubes, showed conductivity as expected.