DISSERTATION

_____________________________________________________________ Results and Discussion
(bare) and pzc (DNA). Namely, all pulse profiles have an upper potential equal to the pzc (bare).
Furthermore, they all relate differently to the pzc (DNA). For the 300 mV pulse profile both
potentials are always positive with respect to the pzc (DNA). For the 500 mV pulse profile
pulsing occurs between positive and slightly negative potentials with respect to the pzc (DNA)
and in the 700 mV pulse profile both positive and negative potentials with similar amplitude
with respect to the pzc (DNA) are applied.
Figure 3.21. Selected potential pulse profiles with respect to the pzc (bare) and pzc (DNA):
ΔE = 300 mV (pulse profile 0.5/0.2 V), ΔE = 500 mV (pulse profile 0.5/0 V) and ΔE = 700
mV (pulse profile 0.5/-0.2 V). Figure adapted from ref. 5 .
By applying a 300 mV pulse profile a small increase in Rct as compared to the Rct of the MCHmodified
electrode is observed showing that obviously only a very small amount of DNA was
immobilized (Figure 3.22, blue curve). However, the amount of immobilized DNA still exceeds
the amount of DNA immobilized by means of the incubation method after the same time (Figure
3.22, black curve). This shows that the investigated pulse profile accelerates the immobilization
of DNA. After the initial immobilization of a minor amount of DNA, the pzc shifts from pzc
(bare) to a more negative value (pzc (DNA)). Therefore, since the pulsing occurs only between
positive potentials relative to pzc (DNA) and the potential difference is small, only a minor ion
stirring effect is achieved and upon initial DNA immobilization, the DNA strands remain lying
on the electrode surface. The electrode is physically blocked by the initially immobilized DNA
strands and the access of additional DNA molecules is impeded.
3.3 Importance of controlling the surface 56