DISSERTATION

_____________________________________________________________
Results and Discussion
3.2.1 Electrochemical impedance spectroscopy. DNA assay build-up
Electrochemical impedance spectroscopy is based on applying a DC potential that is commonly
an open circuit potential superimposed with an AC potential of small amplitude. Measuring of
the resulting AC current signal allows sampling of modulus and phase of the response. The
impedance is usually presented by plotting the real and imaginary components in a Nyquist
plot. The principle of the method is explained in detail in Section 5.13.1.
In this study, a modified Randles equivalent electric circuit was used for modelling the behavior
of the electrode surface during each step of the DNA assay build-up (Figure 3.5). In the circuit,
a solution resistance (Rs) is connected in series with a constant phase element (CPE), which
represents the double layer by taking into account the roughness of a polycrystalline gold
electrode, and an impedance of a faradaic reaction (consisting of a charge transfer resistance,
Rct, and a Warburg element representing the semi-infinite linear diffusion of electroactive
species to a flat electrode, W). Since the alteration of Rct is most pronounced as compared to
other electric circuit elements, the change of Rct was followed during surface preparation.
Figure 3.5. Randles equivalent electric circuit used for fitting of Nyquist plots obtained
during the DNA assay preparation. Rs represents the solution resistance, Rct is the charge
transfer resistance, CPE is a constant phase element representing the double layer and W
is the Warburg element representing diffusion.
The interpretation of the properties of DNA-modified gold electrode surfaces by means of EIS
was based on the repulsion of a negatively charged free-diffusing redox mediator, namely
[Fe(CN)6] 3-/4- , from the DNA-modified electrode in a solution of moderate ionic strength 3 . The
extent of the DNA charge screening depends on the ionic strength of the solution above a certain
3.2 Importance of knowing the surface 35