DISSERTATION

_____________________________________________________________ Results and Discussion
surface. Repetitive switching between positive and negative potentials leads to an increase in
the amount of immobilized DNA due to the created ion stirring that facilitates the approach of
DNA to the surface and the formation of the Au-S bond.
As it was shown previously, that DNA immobilization at OCP exhibits a very slow kinetics.
The proposed potential-assisted method seems to overcome diffusion limitations of the
incubation method by controlling the immobilization kinetics via migration of ions causing ion
stirring. In order to compare the developed method with the immobilization of DNA at OCP,
potential-assisted DNA immobilization was performed using the optimal pulse profile (0.5/-0.2
V with 10 ms pulse duration) while varying the total immobilization time and subsequently
passivating the surface by incubation in MCH solution for the same duration as for DNA
immobilization at OCP (19 h).
Comparing the kinetics of these two methods it is clear that the efficiency of potential-assisted
DNA immobilization is tremendously higher as compared to the incubation method (Figure
3.26). A significant increase of the Rct value is obtained after immobilization for only 15 min
as compared to the MCH-modified electrode. The DNA coverage determined for this
immobilization time using the chronocoulometric method proposed by Steel et al. 75 was
(6.85±0.47) × 10 12 DNA molecules/cm 2 , which is considered to be within an optimal DNA
coverage range for application in DNA sensors 67,75,87,88 . On the other hand, using the incubation
method only a negligible Rct increase is observed after 15 min of immobilization as compared
to the MCH-modified electrode. For 2 h of immobilization at OCP, a coverage of (4.65±0.26)
× 10 12 DNA molecules/cm 2 was determined, which is 47 % lower than the amount of
immobilized DNA achieved within 15 min by the potential-assisted method. Increasing the
incubation time results in a significant decrease in the immobilization kinetics. In order to
obtain the same Rct value that is achieved in only 15 min using the potential-assisted method,
immobilization for about 8 h is needed when the electrode is modified using the commonly
used incubation method.
Figures 3.26b and 3.27 show that using the potential-assisted DNA immobilization method
much higher Rct values, that is, higher DNA coverages, can be achieved within the investigated
immobilization duration. This may be valuable for applications such as the investigation of
DNA repair proteins 66,84 .
3.3 Importance of controlling the surface 61