DISSERTATION

_____________________________________________________________ Results and Discussion
electrode or repelled by a negatively charged electrode is not justified for a set of parameters
usually employed for surface modification with DNA.
Figure 3.18. Nyquist plots with typical Rct values obtained for different immobilization
times where the immobilization was performed at OCP (incubation method). The MCHmodified
electrode was prepared by incubation of the bare gold electrode in a solution of
10 mM MCH with 10 mM PB and 20 mM K2SO4 for 19 h at 37 °C. ssDNA immobilization,
MCH passivation and EIS measurements were performed as stated in Figures 3.7 and 3.8.
Experiments were performed using different electrodes.
DNA immobilization performed at OCP by simple immersion of the gold electrode into a DNA
containing solution is diffusion controlled. Initial immobilization of DNA strands is relatively
fast, while with an increasing amount of immobilized ssDNA grafting of additional strands
becomes energetically unfavorable and the diffusion of new strands is hindered. Since DNA
strands can unspecifically adsorb onto the electrode, lying DNA strands sterically hinder the
approach of new strands and with this the formation of Au-S bonds. Figure 3.18 presents the
immobilization kinetics obtained by performing ssDNA immobilization at OCP followed by
EIS. The immobilization efficiency was investigated by studying the surface after the
passivation step (for reasons explained earlier, Section 3.2.1) and evaluated by comparing the
3.3 Importance of controlling the surface 52