Sensing of Biomolecules - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 77

30% of the average current for a homogenous sequence. So, single-base resolution
is not reached, and various methods are studied to obtain this resolution. The
temperature, the pH, the introduction of a molecule in the pore, and the use of
a special pore with a molecular affinity that confers it the property of interacting
distinctively with each of the bases are ways to alleviate the lack of sensitivity of
the blockade ionic current, such that the sensitivity of this DNA sequence method
can increase up to 95%.
The transverse electronic current method is used to improve the sensitivity of
DNA sequence detection by adding two dc biased electrodes on the two sides of
the nanopore and by monitoring the transverse current, as shown in Fig. 2.21 .The
parallel electric field, E jj , pulls the DNA through the nanopore, while the transverse
field, E ? , is applied for DNA sequencing purpose, since each base has a very
distinct current signature, as displayed in Table 2.1 ( Zwolak and Di Ventra 2005 ).
From Table 2.1 , it can be observed that, when nucleotides translocate through the
nanopore, the transverse current provides a direct and distinct electronic signature
of DNA bases. The current for different nucleotides varies with almost three orders
of magnitude between T and A, while between G and C, the difference is of a
factor of 2. The origin of the distinct currents associated to different DNA bases
is the Fermi level position with respect to HOMO and LUMO, and the DOS
values ( Zwolak and Di Ventra 2005 ). The bases, more precisely their corresponding
molecular orbitals, have different spatial extensions. The largest base is adenine,
which is easily detected by this method. Considering that the transverse bias is
nearly zero, the tunneling current through the nanopore at a single energy level E N

DNA

E ┴

E ║

μ A

-

+

Fig. 2.21 The DNA sequencing using transverse current method

Tabl e 2. 1 DNA sequencing
based on base tunneling
current signature

Base Ratio I base / I adenine

A 1

G 20

C 40

T 660