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