Conductivity of DNA probed by conducting-atomic force microscopy: effects of contact electrode, DNA structure, surface interactions

TL;DR

This study investigates the electrical conductivity of DNA molecules using conducting atomic force microscopy, revealing high resistivity influenced by DNA conformation, surface interactions, and environmental conditions, with water playing a significant role.

Contribution

It provides detailed measurements of DNA conductivity under various conditions and highlights the importance of contact quality and hydration in electrical measurements.

Findings

DNA is highly resistive with conductivity around 10^-6 to 10^-5 S/cm.

Surface chemistry has little effect on DNA resistivity.

Water molecules and ions significantly affect DNA conductivity.

Abstract

We studied the electrical conductivity of DNA molecules with conducting atomic force microscopy as a function of the chemical nature of the substrate surfaces, the nature of the electrical contact, and the number of DNA molecules (from a few molecules, to ropes and large fibers containing up to ~ 106 molecules). Independent of the chemical nature of the surface (hydrophobic or hydrophilic, electrically neutral or charged), we find that DNA is highly resistive. From a large number of current-voltage curves measured at several distance along the DNA, we estimate a conductivity of about 10-6-10-5 S.cm-1 per DNA molecule. For single DNA molecules, this highly resistive behavior is correlated with its flattened conformation on the surface (reduced thickness, \~0.5-1.5 nm, compared to its nominal value, ~2.4 nm). We find that intercalating an organic semiconductor buffer film between the DNA and the metal electrode improves the reliability of the contact, while direct metal evaporation usually destroys the DNA and prevents any current measurements. After long exposure under vacuum or dry nitrogen, the conductivity strongly decreases, leading to the conclusion that water molecules and ions in the hydration shell of the DNA play a major role.