DISSERTATION

_____________________________________________________________ Results and Discussion
Figure 3.27. Change of DNA coverage with the duration of potential-assisted DNA
immobilization. DNA coverage was determined as discussed in Section 5.9.3.
In order to prove that the accelerated immobilization kinetics is indeed due to a generated ion
stirring, for which the application of alternating between positive and negative potentials is
important, DNA immobilization was also investigated applying constant potentials. During the
course of the immobilization, a constant potential of 0.5 V or -0.2 V was applied and the total
immobilization time was kept constant (15 min).
As expected, applying a constant negative potential of -0.2 V (vs. Ag/AgCl/3 M KCl) during
DNA immobilization leads to a very small increase in Rct upon ssDNA immobilization (Figure
3.28, a). Under these conditions, charging of the double layer at the electrode surface occurs
relatively fast and the equilibrium of the system is reached. Therefore, no significant effect of
the applied potential is expected if the DNA is considered as a screened molecule. Similar
results are expected as in the case of immobilization at OCP. If a simple electrostatic
attraction/repulsion model is considered, no change of Rct with respect to the bare electrode
should be observed since the negatively charged electrode would repel the DNA strands and
fully prevent DNA immobilization. However, applying a constant positive potential would be
expected to make a high impact on the immobilization efficiency, since a positively charged
electrode is assumed to attract negatively charged DNA and facilitate the grafting process. This
is not observed and applying a constant potential of 0.5 V (vs. Ag/AgCl/3 M KCl) during DNA
immobilization leads to a similar result as in the case of the negatively charged electrode, where
3.3 Importance of controlling the surface 63