DISSERTATION

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Results and Discussion
coiling on the surface due to their persistence length and flexible behavior. This leads to a steric
hindrance towards the hybridization with target DNA. Thus, in the next step of the DNA sensor
the build-up the ssDNA-modified surface is passivated using self-assembling properties of thiol
molecules (Figure 3.6, c), as originally proposed by Tarlov et al. 74,75 . The formed thiol
monolayer removes unspecifically adsorbed ssDNA from the electrode surface and forces
grafted DNA to obtain an orientation that is more favorable for hybridization. It should be noted,
that the DNA still does not obtain a perpendicular position with respect to the surface but rather
lies down on the thiol layer due to its flexibility. Consequently, since the flux of the redox
mediator to the electrode surface is facilitated, a decrease in Rct is commonly observed in EIS
measurements upon the passivation step (Figure 3.8). On the other hand, tDNA can also
unspecifically bind on the bare electrode surface (data not shown) and therefore this step is very
important for preventing the unspecific adsorption of the target DNA and with this a false
signal.
Figure 3.7. Typical Nyquist plots representing the Rct increase upon ssDNA immobilization.
ssDNA immobilization was performed in 10 mM PB with 450 mM K2SO4
containing 1 µM ssDNA by incubation at 37 °C. EIS was measured in 10 mM PB with 20
mM K2SO4 containing equimolar concentrations (5 mM) of K3[Fe(CN)6] and K4[Fe(CN)6].
A DC potential of 220 mV was applied superimposed with a 5 mV AC perturbation. The
frequency range was from 30 kHz to 10 mHz.
3.2 Importance of knowing the surface 37