DISSERTATION

_____________________________________________________________ Results and Discussion
DNA chips were prepared employing potential-assisted immobilization and desorption methods
by initially immobilizing a DNA sequence 1 (20-mer, “FRIZ sequence” in the further text) on
the first two columns of the chip for 30 s with a subsequent passivation with MCU for 1 min
(Figure 3.49, a-c). Since the whole chip was immersed in the same solution the other electrodes
were subjected to immobilization at OCP. In order to prevent any undesired signal on these
columns, prior to the immobilization with the sequence 2 (42-mer, “E. coli sequence”) columns
5 and 6 were cleaned by the potential-assisted desorption for 5 s. Afterwards, they were
modified with an E. coli sequence for 30 s (Figure 3.49, d). Subsequently, columns 3 and 4
were cleaned with the potential-assisted desorption (5 s) and finally columns 3-8 were
passivated with MCU (Figure 3.49, e). It should be noted that the aim of the experiment was
not the optimization of the assay parameters but rather a proof of concept. Therefore, the
employed immobilization and passivation times are not optimized. Independently it was
observed that an immobilization time of only 5 s and a passivation time of 3 s are sufficient to
achieve optimal DNA coverage for the employed system and the desired passivation of the
electrode surface, respectively (data not shown). Compared to macro electrodes, the optimal
modification time is much shorter, which can be explained by the improved diffusion profile of
microelectrodes.
By designing the experiment in the explained manner the obtained DNA chip is supposed to
consist of 4 different regions: region a modified with FRIZ/MCU, region b modified only with
MCU, region c modified with E. coli/MCU and region d where both FRIZ and E. coli
sequences are immobilized by incubation at OCP and a subsequent passivation is done by the
potential-assisted method (Figure 3.49, f). The whole chip was exposed to each solution leaving
the possibility of contamination between different regions. Therefore, to verify the quality of
preparation, the chip was subjected to hybridization using both target DNA sequences, FRIZ
and E. coli tDNA, labelled with ferrocene. The results obtained by FSCV are shown in Figure
3.50. In general, it can be observed that each region that was exposed to the same conditions
shows reproducible results among individual electrodes (Figure 3.50, a).
Figure 3.50, b and c compare representative electrodes from each region upon hybridization
with FRIZ and E. coli tDNA sequences, respectively. In order to understand what these figures
show, each region is discussed for both cases. Region a is the only region where FRIZ ssDNA
immobilization was performed via the potential-assisted method and therefore should show the
hybridization signal from Fc-labelled FRIZ-tDNA. Indeed, this is observed on each electrode
3.4 Potential-assisted preparation of DNA sensors 92