DISSERTATION

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Results and Discussion
replaced by the capacitance of the DNA monolayer (CDNA). Cd is represented by the following
equation:
1
= 1 + 1
(3.6)
C d C DNA C ddl
Upon DNA immobilization, Cd (Figure 3.16) and subsequently Cddl values (Figure 3.17) were
obtained in relation to the applied potential using PDEIS. In this case, the investigated potential
range was shifted to -0.4 V to 0.5 V (vs. Ag/AgCl/3 M KCl) to avoid the cleavage of the Au-S
bond at higher potentials. The pzc (DNA) was determined to be around 0.1 V (vs. Ag/AgCl/3
M KCl) showing that the pzc shifts to more negative values due to the surface modification
with DNA. Unlike in the case of the bare polycrystalline gold electrode, for the DNA-modified
electrode a well-defined minimum was observed. A possible explanation is a more
homogeneous surface after the modification with the DNA layer. Furthermore, the broad
minimum is in agreement with the GC double layer model that assumes peak broadening with
decreasing ionic strength 43 .
Figure 3.16. Determination of Cd at varying applied potentials at the DNA-modified
electrode. Impedance was measured for two ionic strengths: 10 mM PB with 20 mM
K2SO4 and 0.1 mM PB with 0.2 mM K2SO4. Cd was calculated at a frequency of 10 Hz.
DNA immobilization: 0.5/-0.2 V pulse profile (10 ms pulse duration, vs. Ag/AgCl/3 M
KCl), 5 min in 1 µM DNA solution prepared in 10 mM PB with 450 mM K2SO4. Procedure
explained in the Section 3.3.1.
3.2 Importance of knowing the surface 48