Assessment of the electrical conductivity of oligonucleotides by methods of scanning probe microscopy

The physicochemical properties of DNA molecules, both double-stranded native and single-stranded synthesized (oligonucleotides), are being actively studied. Due to the invention of scanning tunneling (STM) and atomic force microscopes (AFM), it became possible to study various nanoobjects at the molecular and submolecular levels. Obviously, DNA molecules and oligonucleotides are also not an exception. Particularly interesting and significant are the studies of the physical properties of the DNA molecule, namely, electrical conductivity. Such studies have been actively carried out in the last 15 years by scientific groups working in different countries of the world. But the notable fact is that the experimental results obtained by them are very contradictory. Despite the fact that most of these scientists claim that DNA is a semiconductor or dielectric, there are results of works in which DNA molecules exhibit the properties of a conductor and a superconductor. It is possible to measure the current-voltage curves of a biomolecule by the STM. For this the molecule must be located between two electrical contacts, one of which is the conducting probe of the microscope, and the other is a fragment of the substrate surface. After passing through all the stages of the sample preparation, we proceeded to identify molecules on the surface of the gold substrate by means of AFM and STM. Then, in the mode of scanning tunneling spectroscopy, the current-voltage curves were measured at those parts of the surface where single DNA molecules were presumably located. After taking measurements at several points on the scanning area, we obtained an average current-voltage curves. According to the current-voltage curves, the electrical resistance of the molecule was determined as $R = 10^{8} \Omega$, and then the specific resistance of the DNA molecule was estimated, which turned out to be $\rho = 3.14$ $\Omega\cdot$ cm.