DISSERTATION

________________________________________________________________ Experimental Work
it is important to understand the system under investigation, meaning that the electrical elements
need to have a physico-chemical meaning. Table 5.2 shows basic circuit models and their
impedance.
The so-called Randles equivalent circuit is commonly used to model interfacial electrochemical
reactions under semi-infinite linear diffusion control. A constant phase element (CPE) is
commonly used instead of a real capacitor due to frequency capacitance dispersion (Figure 5.9).
The CPE consists of two elements, Q0 that models the capacitance and n that represents the
degree of frequency dispersion with values from 0 to 1, where 1 represents pure capacitive
behavior and 0 pure ohmic resistance.
Figure 5.9. Randles equivalent circuit and corresponding Nyquist plot.
5.12.2 Chronocoulometry for the determination of DNA coverage
Determination of DNA coverage by means of chronocoulometry is based on the determination
of the charge corresponding to the amount of a cationic redox marker ([Ru(NH3)6] 3+ ) noncovalently
bound to DNA strands 75 that is later used to calculate the amount of immobilized
DNA on the electrode surface. Since measurements are performed at low ionic strength (10 mM
PB with 20 mM K2SO4), trivalent [Ru(NH3)6] 3+ can exchange native counterions surrounding
the DNA and non-covalently bind to phosphate residues on DNA strands. To ensure accurate
determination of the amount of DNA strands on the electrode surface, the measurements need
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