Biomedical Engineering Reference
In-Depth Information
Density of immobilized ssDNA
2 10
13
molecules/cm
2
a
s
2
R
s
~ 2.5 nm
Hybridization efficiency ~100%
Ion-concentration change
within intermolecular spaces
Factor 3-4 for cations (increased)
Factor 3-4 for anions (decreased)
Ideal Nernstian sensitivity
of FED to cations (anions)
Upon hybridization induced sensor signal
28-35 mV
FIGURE 7.8
Theoretically expected values of an FED signal due to the DNA hybridization-induced ion-
concentration redistribution within the intermolecular spaces.
●
high hybridization signals at low density of immobilized ssDNA require solu-
tions with lower ionic strength, and
●
a strong increase in
at a small separation distance (high density of
the immobilized ssDNA), on the other hand, decreases the sensor area available
for the ion interaction.
n
ds
/
n
ss
Generally, dependent on the sensor design and working conditions, the optimum
separation distance or optimum density of the immobilized ssDNA should be found in
order to achieve a maximal hybridization signal. A twice increase in the hybridization
signal can be obtained when combining a cation- and anion-sensitive FED in a differ-
ential measuring set-up.
In summary, it should be noted that in the model described above, the ssDNA is pre-
sented as a rod-like molecule oriented normal to the surface. In a more realistic picture,
the ssDNA is a fl exible coil-like molecule. If those coil-like molecules lie preferentially
fl at on the ion-sensitive gate of the FED, they can partially cover the surface active
sites for ion-binding or ion-exchange processes as well as prevent or hinder potential-
determining ions to reach the transducer surface (see Fig. 7.9a). In contrast, after the
hybridization event, a rigid rod-like dsDNA is formed. Now, the surface of the FED is
opened for ion interaction (Fig. 7.9b), resulting in an hybridization-induced alteration
of the ion sensitivity of the FED and generation of an additional sensor signal.
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