DISSERTATION

_____________________________________________________________ Results and Discussion
of influence is attained with a higher applied potential. Conversely, anions in the same layer are
attracted towards the electrode and with this a small fraction of a DNA strand that is in
immediate proximity to the electrode is pulled towards its surface. As a result, the remaining
part of a DNA strand that was outside of the area of influence is brought closer to the electrode
surface. If the same potential is still applied the next part of the DNA strand undergoes the same
process; DNA counterions are repelled together with cations from the solution and the rest of
the DNA strand is brought closer to the surface. This process continues as long as the positive
potential pulse is applied. If the duration of a pulse is long enough, the complete DNA strand
is sequentially confined on the electrode surface. Therefore, regardless of the orientation of the
DNA strand, the anchor group is brought close enough to the electrode surface for the formation
of the Au-S bond.
Figure 3.25. Scheme presenting the zipper-like pulling of a DNA strand towards the
electrode surface during potential-assisted DNA immobilization. Figure adapted from
ref. 5 .
The positive potential pulse is followed by a potential jump to a negative potential with respect
to the pzc (DNA). If an appropriate negative potential is applied, anions in the vicinity of the
electrode are repelled from the surface together with the remaining negatively charged DNA
backbone that was left unscreened. By this, the grafted DNA strand are lifted towards the bulk
of the solution. Thus, additional space is created for new DNA strands to approach to the
3.3 Importance of controlling the surface 60