Biomedical Engineering Reference
In-Depth Information
In this situation, the spin-polarized tunneling current, which depends on the
orientation of the magnetic moment of the tip, is measured in UHV conditions.
There are also many STM image galleries, one of the richest being that provided by
the company who invented the STM:
STM is intensively used in the sequencing of DNA using tunneling currents. The
electronic properties of the four DNA bases were obtained using a UHV STM at
room temperature (
Xu et al. 2007
). As mentioned also in Chap.
1
, the experimental
STM I-V characteristics of the bases can be fitted with the expression (
Dragoman
and Dragoman 2009
)
I.V/
D
V=R
C
aV
2
exp.
b=V/;
(3.20)
in which the first term in the right-hand side is the contribution of a series
resistance R, which takes values between 500 and 1;000m for the four bases,
and the last term is a typical Fowler-Nordheim contribution. The Fowler-Nordheim
term describes field emission from a triangular barrier of height
B
in the presence
of an applied electric field. Thus, DNA bases can be regarded as potential barriers
for field-emitted electrons, their work function being determined from the relation
4L
3e
„
.2m
0
/
1=2
'
3=
B
;
b
D
(3.21)
where L is the tip-sample distance in the STM measurements and m
0
denotes the
free electron mass. The average values of the work functions for L
D
0:66 nm
obtained from data at positive and negative polarizations are
A
D
1:74 eV,
T
D
1:81 eV,
C
D
2:12eV, and
G
D
1:7 eV for adenine, thymine, cytosine, and
guanine, respectively. The electronic signatures of the DNA bases look like that of
a Zenner diode.
Starting from this unique electronic signature of the four DNA bases, obtained
with the STM, some developments were made toward STM sequencing of DNA
bases. In this respect, (
He et al. 2007
) proposed to combine the nanopore tunneling
with hydrogen-mediated molecular recognition. Each base in an ssDNA, which
translocates by chemically functionalized electrons in a nanopore integrated with
four reading heads for the identification of the four bases, is identified by a
characteristic current-distance response. The electrical reading of the base pairing
was demonstrated with a guanine functionalized STM probe by measurements of
the tunneling current decay with distance, which is able to sense and discriminate
the G-C from the G-T base pairs. Partial sequencing, i.e., the identification of the
electronic signature of the G base only, was performed for a single DNA molecule
using STM and an oblique pulse-injection method to deposit DNA on copper
(
Tanaka and Kawai 2009
). In another work, the LDOS of ssDNA molecules formed
by a poly(G)-poly(C) sequence of about 1m in length and deposited on gold was
resolved as a function of energy using the STM at cryogenic temperatures (
Shapir
et al. 2008
).
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