DISSERTATION

_____________________________________________________________
Results and Discussion
Taking into account that the reproducibility of the immobilization step as detected by EIS is
quite low, it is more informative to observe the ssDNA/thiol-modified surface. However, since
this response contains the contribution of both passivation and DNA immobilization, it should
be compared with the Rct obtained for an only thiol-modified electrode. Namely, Rct of the
ssDNA/thiol-modified electrode needs to be higher than the Rct of the electrode modified only
with the passivating thiol implying that a detectable amount of DNA was immobilized.
Figure 3.8. Change in the Rct value upon surface passivation with MCH. ssDNA
immobilization was performed for 15 min as stated in Figure 3.7. MCH passivation was
done by incubation for 19 h in 10 mM PB containing 20 mM K2SO4 and 10 mM MCH.
EIS measurements were performed as stated in Figure 3.7.
Therefore, after ssDNA immobilization by incubation for 15 min and subsequent passivation,
the obtained Rct is negligibly higher as compared to the Rct of the electrode modified only with
the mercaptohexanol (MCH) for a time equal to the passivation duration (Figure 3.9). This
implies that the amount of immobilized DNA is very low and not detectable by means of EIS.
On the other hand, for longer immobilization times (2 h and 8 h) a significant increase in the
Rct value is observed as compared to the thiol-modified electrode reflecting higher DNA
coverages.
3.2 Importance of knowing the surface 38