Biomedical Engineering Reference
In-Depth Information
Tabl e 6. 1
Base length of probe and target DNA
Locus
Function
Sequence
factor VII
gene
.
122/
122
normal
5
0
-amino group-CGTCCTCTGAA-3
0
(11mer)
probe
5
0
-AGCTGGGGTGTTCAGAGGACG-3
0
(21mer)
5
0
-TGCAGCTCTCAGCTGGGGTGTTCAGAGGACG-3
0
(31mer)
5
0
-GGCGGCCAGGTGCAGCTCTCAGCTGGGGTGTTCAGAGG
ACG-3
0
(41mer)
5
0
-CATGGCCACTGGCGGCCAGGTGCAGCTCTCAGCTGGGG
TGTTCAGAGGACG-3
0
(51mer)
5
0
-GCAGGGGATGCATGGCCACTGGCGGCCAGGTGCAGCTC
TCAGCTGGGGTGTTCAGAGGACG-3
0
(61mer)
target
where ı is the Debye length, " is the permittivity of the electrolyte solution, "
0
is
vacuum permittivity, k is the Boltzman constant, T is the absolute temperature in
Kelvin, z is the valency of the ions in the electrolyte, q is elementary charge, and I is
the ionic strength of the electrolyte.
The charge density change induced within the Debye length can be detected with
the genetic FETs, while the charge density change induced outside the Debye length
is shielded by counter ions and cannot be detected with the genetic FETs. In the
present study, a 25 mM phosphate buffer solution was used for the V
T
measurement.
The Debye length at the gate insulator surface is therefore considered to be a few
nanometers. The length of the target DNA with 41 bases is 13.94 nm, when it is
straight. But oligonucleotide probes and the target DNA are flexible in the aqueous
solution and oligonucleotide probes are not always perpendicular to the surface
of the gate insulator. It is therefore reasonable to ascribe saturation of the linear
relationship between the V
T
shift and the base length over 51 bases to the Debye
length. Since the Debye length is dependent on the ionic strength of the aqueous
solution as shown in Eq.
6.1
, it is important to optimize the buffer concentration
used for the measurement of the V
T
shift.
6.4.1.2
Immobilization Density of Oligonucleotide Probes
The molecular recognition events such as hybridization and specific binding of DNA
binder could be directly transduced into electrical signal using the genetic FETs.
The change in the surface charge density could be detected as a shift of the V
T
of
the genetic FETs. The V
T
shift after hybridization shown in Fig.
6.14
, V
T
, can
be expressed in Eq.
6.2
,whereQ
ds
DNA
is the charge per unit area of the double-
stranded DNA after hybridization, Q
ss
DNA
is the charge per unit area of the single-
stranded oligonucleotide probes, Q
DNA
is the charge difference per unit area after
hybridization, and Ci is the gate capacitance per unit area.
V
T
D
.Q
ds
DNA
Q
ss
DNA
/=Ci
D
Q
DNA
=Ci
(6.2)
