Biomedical Engineering Reference
In-Depth Information
D
10 nm
1 mM
2 nm
-T-
D
3 nm
G
C
10 mM
A
T
A
T
Sugar,
phosphate
backbone
C
G
C
G
D
0.96 nm
0.1 M
A
T
A
T
D
0.3 nm
G
1 M
G
FED
FIGURE 7.5
FED with 10-bases DNA molecule oriented normal to the sensor surface and Debye length
λ
D
in the electrolyte with different ionic strengths (schematically). Increasing ionic strength of the electro-
lyte solution decreases the fraction of DNA charge that can be detected by the FED.
case, the hybridized pair (cDNA-ssDNA) produces a net-reduced or even zero charge,
although the charged target molecule has bound to the immobilized probe molecule. As
a consequence, the underlying fi eld-effect transducer is not able to deliver a measurable
sensor signal.
In order to estimate the sensor signal that is induced upon the DNA-hybridization
process, in a fi rst approximation, the hybridization of the probe molecules with their
complementary target molecules can be modeled as a transfer of a certain quantity of
charge from the test sample to the surface of the gate insulator. Since the requirement
of electroneutrality must be realized in the system, an equal quantity of the opposite
charge must either enter in the inversion layer of the FET or enter the double layer
from the solution. The estimations for typical values of double-layer and gate-insulator
capacitances (
C
dl
F cm
2
for a 10 nm thick SiO
2
layer,
respectively) imply, that only about 1.7% of the hybridization-induced charge will be
mirrored in, e.g., an FET [51]. The remaining charge will be compensated by ions in
the solution. The potential changes induced by hybridization (i.e. the expected sensor
signal) can be defi ned as [51]:
F cm
2
and
C
i
20
µ
0.35
µ
Q
CC
mN
CC
δ
(
1
θ
)
h
ϕ
(4)
i
dl
i
dl
λ
D
/
b
is the frac-
tion of DNA charge in the double layer (for simplicity, DNA molecules are considered
Here,
Q
h
is the charge change induced upon the hybridization,
m
q
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