DISSERTATION

_____________________________________________________________ Results and Discussion
Figure 3.20. a) Potential-time dependence representing potential pulses during potentialassisted
immobilization and b) corresponding current-time response.
It becomes evident that in order to find an efficient pulse profile the following conditions need
to be satisfied:
- to find appropriate pulse intensities
- to find an appropriate pulse duration
Applied potential intensities need to be on the one hand within the stable potential window of
the Au-S bond and on the other hand high enough to evoke an efficient stirring to bring the
DNA towards the surface. Namely, even though according to the G-C theory the potential drop
in front of the electrode is steeper for higher applied potentials, the absolute value of the
potential Φ at a fixed distance from the electrode is higher for a higher applied potential (i.e.,
potential at the OHP, Φ0). This difference is more pronounced in close proximity to the
electrode surface. To demonstrate the influence of potential intensities in a pulse profile on the
efficiency of the immobilization process, different pulse profiles were compared (Figures 3.21
and 3.22). For the purpose of this study the total duration of the immobilization was kept
constant to 15 min and the pulse duration was kept at 10 ms. Using the same upper potential
(0.5 V vs. Ag/AgCl/3 M KCl) and varying the lower potential, three potential differences were
defined: 300 mV (0.5/0.2 V pulse profile), 500 mV (0.5/0 V pulse profile) and 700 mV (0.5/-
0.2 V pulse profile). Figure 3.21 shows how the chosen potential differences relate to both pzc
3.3 Importance of controlling the surface 55