DISSERTATION

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Results and Discussion
Taking into account Equation (3.2), it is assumed that the investigated ionic strengths are not
low enough to result in the desired capacitance minimum from the Cd-E dependence alone.
Namely, for the highest ionic strength (10 mM PB, 20 mM K2SO4) Ci dominates the overall
capacitance response, however, at the lowest investigated ionic strength (0.1 mM PB, 0.2 mM
K2SO4) obviously contributions from both Ci and Cddl are significant. Therefore, by subtraction
of these two values Cddl can be obtained:
1
C ddl =
1
C
− 1 (3.5)
d C i
Finally, by plotting the Cddl-E dependence (Figure 3.15), the pzc can be determined from the
capacitance minimum at a potential of 0.5 V (vs. Ag/AgCl/3 M KCl), where the most
pronounced minimum is observed for a frequency of 100 Hz. The non-ideal U-shape of the
obtained curves is probably a consequence of the polycrystalline nature of the gold surfaces and
the contribution of all crystallographic facets.
Figure 3.15. Determination of the pzc of the bare gold electrode. Cddl values were
calculated using the data from Figure 3.14 for different frequencies.
To investigate the effect of the modification of the surface with DNA on the pzc value,
determination of the pzc (pztc) of the DNA-modified electrode (pzc (DNA)) was performed.
This was done in the same manner as for the pzc determination of bare gold, except that Ci was
3.2 Importance of knowing the surface 47