DISSERTATION

_____________________________________________________________ Results and Discussion
The typical approach for SAM desorption is the polarization of SAM-modified Au surfaces to
potentials that are either too positive or too negative, leading to the cleavage of the Au-S bond,
that is, oxidative or reductive desorption, respectively. However, the exact potential values
needed in order to provoke SAM desorption are under discussion for decades and a substantial
variety of values is reported. According to our knowledge, applying both positive and negative
potentials within the same desorption process by pulsing between these potentials was not yet
reported. Even though fairly fast desorption times are reported in the literature 18 (down to 30 s)
the aim of the study was to further improve the efficiency of desorption.
Applying very high potentials seems a reasonably simple approach to achieve SAM desorption,
however, another important condition for the development of a desorption process is to leave
the electrode surface undamaged after polarization, especially in the case of multi-electrode
chips where the Au layer thickness is in the order of nm. Therefore, the potential pulse-assisted
desorption approach was developed by exposing the electrode to very short potential pulses
within the range of ms. The modified electrode was exposed to potential pulsing between
potential values of 0.9 V and -0.9 V (vs. Ag/AgCl/3 M KCl) with a pulse time of 10 ms. The
influence of the applied pulse profile on a DNA-modified electrode is presented in Figure 3.41.
Prior to desorption, a bare electrode was modified with ssDNA by incubation for 2 min to mimic
the conditions that would be observed on the chip. Namely, during potential-assisted DNA
immobilization on a selected chip electrode, the rest of the electrodes would be exposed to the
same solution and the DNA molecules would immobilize at OCP. In order to prevent later
cross-talk between electrodes, these surfaces need to be cleaned prior to further modification
with a desired probe DNA. Even though short DNA immobilization at OCP leads to a minor
Rct increase (Figure 3.41, a) and consequently to a small DNA coverage, it can behave as an
impurity significantly affecting the surface modification. Subsequent modification with a DNA
probe of interest is then leading to a less efficient hybridization since the probe DNA coverage
is higher than the optimal one. Even though the optimized immobilization procedure is
employed the hybridization will be hindered due to the contribution from the leftover DNA.
After the treatment of the DNA-modified electrode with the developed desorption method for
30 s Rct decreases to the value of the bare electrode and the voltammogram obtains the same
shape as for the bare electrode, showing that all DNA was efficiently removed from the surface.
3.3 Importance of controlling the surface 82