DISSERTATION

_____________________________________________________________ Results and Discussion
formed DNA film is affected by the subsequent potential-pulse assisted passivation steps,
whether the passivating thiol replaces the grafted DNA from the surface, and whether the
application of potential pulses within a subsequent passivation step causes a desorption of
previously grafted DNA molecules.
Figure 3.45. Nyquist plots of ssDNA-modified electrode before and after pulsing in 10 mM
PB (450 mM K2SO4) using the 0.5/-0.2 V (vs. Ag/AgCl/3 M KCl) pulse profile with 10 ms
pulse duration. EIS measurements were performed as explained in Figure 3.7.
To assure that the passivating thiol does not remove the immobilized DNA, a special anchoring
molecule with six binding sites is used for the immobilization of ssDNA, where six Au-S bonds
are formed per individual DNA strand (Figure 3.44). Since the thiol molecules form only one
Au-S bond they cannot replace ssDNA that is much more stable 98 .
The effect of the pulsing itself on the stability of the DNA film was investigated by exposing a
ssDNA-modified electrode to potential pulsing performed in buffer solution alone (10 mM PB,
450 mM K2SO4). The same potential profile (0.5/-0.2 V, 10 ms pulse duration) that is employed
in the passivation step of short or intermediate length thiol was used. Figure 3.45 shows that
upon pulsing for 1 min the DNA layer remains unaltered, i.e., no visible desorption occurs.
After 2 min, a small decrease in Rct is observed suggesting that the DNA film starts slightly to
desorb even though it cannot be excluded that the change in Rct may originate from
3.4 Potential-assisted preparation of DNA sensors 86