Fundamentals on Bionanotechnologies - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 40

Fig. 1.38 The charge
propagation in a GC periodic
structure

P S

c 2+

c 1+

G

t b +

....

b 1 b 2

t a

t b-

C

c 1- c 2-

S

P

and fits the I V curve at 18 K for t a D 0:37 eV and t b D 0:74 eV. Either the
charge propagation or the coupling backbone model described above indicates that
the bandgap is opened due to high reflections near zero energies, which correspond
to a gap in electronic states between LUMO and HOMO.
DNA is not behaving only as a semiconductor. For example, DNA behaves as
an electrical conductor with conductivity similar to that of conducting polymers,
if several DNA molecules associate in ropes with lengths of 600 nm ( Fink and
Sch onenberger 1999 ). In this case, the DNA is much longer than in the previous
example where only 30 base pairs are used. The length and shape of DNA molecules
and the number of bases dictate the conduction properties, as can be seen from the
examples collected in Di Ventra and Zwolak ( 2004 ).
Thus, if DNA is conducting, there are three possible responsible mechanisms:
thermal hopping, sequential tunneling from site to site, or coherent tunneling along
the entire length of DNA. The tunneling rate, expressed as

R Š exp. L/; (1.35)

is exponentially decreasing with the length of DNA, L.The parameter in ( 1.35 )
is 0:01 nm 1 for sequential tunneling and 0:1 nm 1 for coherent tunneling.
A step forward is to identify the conduction characteristics of the bases them-
selves. This was done by STM measurements ( Xu et al. 2007 ). The electrical signals,
and in particular the work functions of the four DNA bases, are different and, hence,
unique, as resulted from STM measurements. A simple fitting formula for the I V
signature of the four bases can be written as Dragoman and Dragoman ( 2009c )

I.V/ D V=R C aV 2 exp. b=V/;

(1.36)

where the first term in the rhs is due to a series resistance R and the last
term originates from the Fowler-Nordheim behavior, which expresses the one-
dimensional field emission from a triangular barrier of height in the presence of an
applied electric field. Thus, the DNA bases are understood as potential barriers for