Biomedical Engineering Reference
In-Depth Information
Fig. 6.16
Electrical signal for extended base length
probes. The common changes of the V
T
due to ambient salt and pH changes
and temperature difference in the buffer solution can be canceled out using the
differential measurement. The difference of the V
T
between the genetic FET and
the reference FET during extension reaction, V
Tdiff
, increased drastically up to
about 10 mV. This positive change in the V
Tdiff
is rightly due to negative charges
of polynucleotide extended by primer extension reaction. The V
T
of the reference
FET is considered to be mainly due to the temperature change. From Fig.
6.15
,
we could demonstrate that the primer extension event on the gate surface was
transduced directly into the electrical signal by the use of the genetic FET.
The effect of base length of the target DNA on the V
Tdiff
was investigated
(Fig.
6.16
and Table
6.1
)[
18
]. The linear relationship between the base length and
the V
Tdiff
was obtained up to 41 bases. The V
Tdiff
after extension reaction
increased to 24 mV, when target DNA with 41 bases was used. This is because
the number of charges on the gate surface increased after primer extension with
increasing template base length. However, the V
Tdiff
did not follow the linear
relationship, when the target DNA samples with 51 bases and 61 bases were used.
The reason for this nonlinearity is considered to be related to the width of the
electrical double layer at the interface between the gate insulator and an aqueous
solution. The width of the electrical double layer, the Debye length, which is
expressed in Eq.
6.1
, is about 10 nm in the diluted salt solution (approximately
1 mM) and about 1 nm in the physiological solution (approximately 100 mM).
D
©©
0
kT=2
z
2
q
2
I
1=2
•
(6.1)
