DISSERTATION

_____________________________________________________________ Results and Discussion
A similar result is observed for the pulse profile with 500 mV potential difference (Figure 3.22,
yellow curve). Although the potential difference is higher than for the 0.5/0.2 V pulse profile,
the applied negative potential is apparently not sufficiently low to induce effective enough ion
stirring and push up already immobilized DNA strands that are lying on the surface as a random
coil due to their low persistence length. Therefore, the access of new strands is hindered and a
poor immobilization yield is observed. On the other hand, further increase of the potential
difference to 700 mV (pulse profile 0.5/-0.2 V) leads to a significantly improved immobilization
yield (Figure 3.22, green curve). Evidently, this pulse profile is vigorous enough to increase the
ion flux and to induce improved ion stirring. Thus, already immobilized DNA strands can be
lifted from the electrode surface and can create space for new strands to approach the surface.
As explained earlier, even though for higher applied potentials the potential drop is steeper, at
a certain distance from the electrode, the DNA is affected by an absolute higher potential.
Figure 3.22. Comparison of different pulse profiles used for potential-assisted DNA
immobilization. MCH-modified electrode was prepared as stated in Figure 3.8. ssDNA
immobilization was performed for 15 min by incubation at OCP or using different pulse
profiles (0.5/0.2 V, 0.5/0 V and 0.5/-0.2 V) with 10 ms pulse duration. MCH passivation
and EIS measurements were performed as stated in Figures 3.7 and 3.8. Figure adapted
from ref. 5 .
3.3 Importance of controlling the surface 57