Detection of charged macromolecules by means of fi eld-effect devices (FEDs): possibilities and limitations - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 200

4

3

Cations

n 0 0.005 M

2

0.15 M

0.5 M

1

0.5 M

0.15 M

0.005 M

Anions

0

15 20 25 30 35 40 45 50

R s , Å

FIGURE 7.7 Ratio n ds / n ss of average concentration of cations (upper part of diagram) and ani-
ons (lower part of diagram) within the intermolecular spaces before hybridization n ss and after hybridi-
zation n ds in 1:1 salt solutions with bulk ion concentration of n 0 0.005, 0.15 and 0.5 M, respectively.
R s : DNA cell radius.

of the average concentration for
cations and anions within the intermolecular spaces before hybridization

Figure 7.7 shows the calculated ratio n ds / n ss

n ss and

after hybridization is plotted as a function of the DNA

cell radius R s and for 1:1 salt solutions of different bulk-ion concentrations of n 0
0.005, 0.15 (physiological solution) and 0.5 M, respectively. The fraction of both the
ssDNA and dsDNA charge compensated by the condensed cations was taken as

n ds . The ratio n ds / n ss

θ

0.8. Two effects can be recognized from Fig. 7.7:

● the local concentration of cations and anions within the intermolecular spaces
differs strongly from that of the bulk solution (a similar effect has been experi-
mentally observed for DNA brushes in [55]), and
● the average ion concentration within the intermolecular spaces after the hybrid-
ization differs distinctly from that of before the hybridization; the effect is
dependent on the ionic strength of the bulk electrolyte solution and is stronger in
a low ionic-strength electrolyte.

In contrast to the DNA detection by its intrinsic molecular charge, even in high
ionic-strength solutions (0.5 M), where the hybridization effi ciency is high and the
hybridization event can be faster, a detectable sensor signal can be achieved. The esti-
mations performed under the assumptions presented in Fig. 7.8 predict signal values of
about 28-35 mV.
The achieved biosensor signal will be infl uenced by the following factors:

● an increase of R S implies a decreased density of immobilized ssDNA yielding a
smaller n ds / n ss ratio (more strongly in high ionic-strength solutions),