DISSERTATION

_____________________________________________________________ Results and Discussion
formed in the electrolyte in order to compensate for this excess charge. A potential profile is
formed and the potential decays with the distance from the electrode as it is explained in Section
1.2.2. Even though the Gouy-Chapman double layer theory does not take into account the linear
potential drop within the Helmholtz plane, it shows the difference in the system response in
relation to parameters such as the ionic strength of the solution or the applied potential. In order
to increase the immobilization kinetics and DNA coverage, DNA immobilization is often
performed in solutions of high ionic strength. However, according to the GC model, the
potential drop is steeper with increasing ionic strength. Therefore, under conditions of high
ionic strength a significant potential drop is observed in the immediate proximity of the
electrode surface. Thus, only a small fraction of a DNA strand in close vicinity of the electrode
can be affected by the applied potential.
Moreover, DNA is a highly negatively charged polyelectrolyte that strongly interacts with
surrounding ions resulting in charge compensation, i.e., DNA screening (described in Section
1.2.1). In the case of monovalent cations, the charge at a DNA strand is screened by counterions
accumulating around the DNA strand in two layers, namely a condensed layer and additional
ions in a second sphere 34 . Therefore, due to the absence of an effective net charge, a DNA strand
cannot be directly affected by the applied potential as it is generally suggested.
These observations imply that the polarized electrode neither attracts nor repels DNA strands
directly, but rather affects the ions in the vicinity of the electrified interface. Namely, during
the charging of the electrochemical double layer, ions have to rearrange in both Helmholtz
planes and the diffuse layer. Thus, when the electrode is polarized to negative values with
respect to the pzc, cations move towards the electrified interface while anions move towards
the bulk of the solution and vice versa. This suggests that switching fast enough between these
two situations creates a “stirring effect” that effectively exceeds the Debye length in front of
the electrified interface. Furthermore, efficient stirring should also move DNA strands present
in close proximity to the electrode surface including their condensed ion cloud. This way the
immobilization will not be diffusion controlled but driven by the migration of ions in front of
the electrode. Based on this hypothesis, we created a potential pulse-assisted immobilization
method that consists of fast switching between potentials more positive and more negative with
respect to the pzc. Figure 3.20 demonstrates the principle of the measurements conducted
during this study.
3.3 Importance of controlling the surface 54