DISSERTATION

_____________________________________________________________ Results and Discussion
The obtained results suggest that when the pulse duration is too short to allow for a whole DNA
strand to be pulled down via ion stirring during a single pulse (e.g. 1 ms), only a small fraction
of the DNA in the vicinity of the electrode that is already oriented with the anchor group towards
the electrode surface is grafted (Figure 3.24). On the other hand, if the pulse duration is too long
for the investigated molecule length (e.g. 100 ms), a DNA strand will be brought completely to
the surface during a fraction of a single pulse and it will remain lying on it until a negative pulse
is applied. This creates less efficient ion stirring and, in addition, reduces the number of pulse
cycles. For the investigated system a pulse time of 10 ms is apparently long enough for complete
DNA strands to be pulled down, hence allowing for the formation of the Au-S bond. Moreover,
the 10 ms pulse time is short enough to allow for a high number of potential pulse cycles.
Figure 3.24. Schematic representation of the influence of the pulse duration on the DNA
immobilization efficiency.
After presenting the effect of applied potential intensities and the duration of a single pulse on
the efficiency of potential-assisted DNA immobilization, a model that explains the processes
occurring during grafting of DNA strands supported by applied potential pulses can be proposed
(Figure 3.25). Due to the short distance to which applied potentials have an effect at a given
ionic strength during the application of a single positive potential pulse, cations that are in the
vicinity of the electrode are repelled from the electrode surface including counterions
surrounding DNA strands that are located within the layer of influence. By this, an increased
effective charge at the DNA strand is achieved. The extent of the layer of influence depends on
the intensity of applied potentials and pulse duration. If the pulse is long enough, a wider layer
3.3 Importance of controlling the surface 59