DISSERTATION
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_____________________________________________________________ Results and Discussion<br />
electrostatically, a maximal hybridization efficiency of ideally 100 % is obtained providing a<br />
sufficient concentration of tDNA. Therefore, an increase of the ssDNA coverage leads to an<br />
increase in the hybridization yield and in the current signal obtained from labelled tDNA<br />
(observed for DNA immobilization up to 2 min). However, further increase of the ssDNA<br />
concentration causes a steric and electrostatic hindrance depending on the ionic strength<br />
towards the hybridization process and the efficiency of hybridization decreases leading to a<br />
lower signal. Therefore, for the employed detection scheme (hybridization with Fc-tDNA, 20-<br />
mer probe DNA) the optimal ssDNA coverage is around 2-3 × 10 12 molecules/cm 2 , achieved<br />
by immobilization of ssDNA for 2 min using the developed potential-assisted immobilization<br />
method.<br />
Figure 3.47. a) CV characterization of the surface during sensor preparation, and b)<br />
FSCV measurements before and after hybridization of the ssDNA/MCU-modified electrode<br />
with Fc-tDNA. ssDNA immobilization: 10 mM PB, 450 mM K2SO4, 1 µM DNA; p.a.<br />
immobilization (0.5/-0.2 V vs. Ag/AgCl/3 M KCl, 10 ms pulse duration), 2 min. MCU<br />
passivation: 10 mM PB, 20 mM K2SO4, 1 mM MCU (30 % ethanol), 0.5/-0.2 V vs.<br />
Ag/AgCl/3 M KCl (10 ms), 1 min. CV and FSCV measurements were performed as<br />
explained in Figure 3.46.<br />
This chapter demonstrates that using the developed potential-pulse assisted immobilization<br />
technique DNA sensor preparation can be done in as little as 3 min depending on the desired<br />
DNA coverage. Compared to the standard sensor preparation praxis where surface modification<br />
3.4 Potential-assisted preparation of DNA sensors 89