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