DISSERTATION

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Results and Discussion
One of advantages of the electrochemical impedance spectroscopy is that it enables label-free
detection of DNA hybridization, circumventing the need for dye, enzyme or redox labelling of
the target DNA 71 . However, a careful design of the electrode architecture needs to be achieved
to improve the sensitivity of the overall system. Upon DNA hybridization an increase in Rct is
generally observed. However, there are different interfacial phenomena caused by the
hybridization process that influence the access of the redox mediator to the surface.
Figure 3.9. Comparison of the Rct obtained for a MCH-modified electrode with the Rct
obtained for ssDNA/MCH-modified electrodes using different incubation times. The
MCH-modified electrode was obtained by incubation of the bare gold electrode in 10 mM
PB containing 20 mM K2SO4 and 10 mM MCH for 19 h. The ssDNA/MCH-modified
electrode was prepared by initial ssDNA immobilization followed by subsequent MCH
passivation (see Figures 3.7 and 3.8, respectively). EIS measurements were performed as
stated in Figure 3.7.
EIS measurements are performed in an intermediate ionic strength to assure that the DNA is
not fully screened by counterions and that it still manifests some effective negative charge.
Therefore, as a result of the hybridization, the amount of negative charge in front of the
electrode surface increases leading to a stronger repulsion of the redox mediator and an increase
3.2 Importance of knowing the surface 39