Fundamentals on Bionanotechnologies - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 37

far from the conduction in crystals, which is quite well understood. The DNA is
conducting due to the interbase hybridization of z orbitals, placed perpendicular
to the planes of staked bases ( Enders et al. 2004 ). However, DNA is not a crystal;
it is not periodic. The potential barrier between two bases is very large, reaching
0.6 eV, which indicates an Anderson localization of electronic states of base pairs.
In the double-helix DNA conduction experiments, the hydrophobic bases avoid
contact with water, and the immediate environment consists of counterions formed
by positive charges, which neutralize the negatively charged backbone. These
considerations suggest that the conduction of biomolecules is strongly dependent
on environment, in contrast to many electrical experiments at room temperature on
inorganic nanodevices, where the environment plays no role. Moreover, at room
temperature, the root-mean-square vibration amplitudes of DNA bases is ten times
smaller than the distance between stacked base pairs, and one order of magnitude
greater than in any crystal, DNA being nearly in a “melted” state, essential, however,
for an easy replication or repair of its sequence. So, when dealing with biomolecules
in general, and with DNA in particular, the precautions that must be taken in
conduction measurements and the interpretation of results are far from the common
experience with usual crystals in electronic laboratories.
The studies of DNA conductance have indicated a rich variety of DNA behavior,
encompassing insulator, semiconductor, or conductor characteristics. In principle,
ssDNA molecules are insulating at room temperature, but dsDNA in the form
of short periodic structures such as poly(G)-poly(C), or bundles of -DNA, can
conduct electricity due to hybridization.
As we have mentioned, the conduction in dsDNA is due to interactions
between base pairs, which originate in the delocalized bonding and anti-
bonding orbitals separated by an energy bandgap E g of almost 4 eV, and which are
produced by the p z atomic orbitals perpendicular to the base plane. The DNA can be
doped, as can common semiconductors, the doping involving chemical oxidation or
reduction reactions. So a band engineering of DNA is possible, its transformation in
a conducting wire occurring by further functionalization with metal particles, such
as Ag.
In experiments, DNA acts as insulator, semiconductor, or conductor, depending
on the different types of electrodes used to contact DNA; the various types of
DNA strands, ssDNA or dsDNA; the length of the sample; and the specific base
sequence. Many experiments are summarized in Table 1.1 of the key reference
Enders et al. ( 2004 ). In Fig. 1.35 , we have represented the work functions of DNA
bases and metals, to get an insight into how DNA must be contacted to behave
resistively or in a Schottky-like manner. In this figure, LUMO and HOMO stand
for lowest unoccupied molecular orbital and highest occupied molecular orbital,
respectively.
The conduction of DNA could be measured due to advancements in self-
assembly and nanoscale lithographic technique, as well as in STM (scanning
tunneling microscope), which is able to measure the tunneling current with high
accuracy by approaching a sharp tip to the conductive sample (the distance
between them is 1-2 nm). The STM will be extensively described in Chap. 3 .