DISSERTATION

________________________________________________________________ Experimental Work
5.9 Characterization of DNA sensors
5.9.1 Electrochemical impedance spectroscopy
EIS measurements were performed after each assay preparation step in 10 mM PB with 20 mM
K2SO4 (pH 7.4) containing equimolar concentrations of K3[Fe(CN)6] and K4[Fe(CN)6] (5 mM
each). Experiments were conducted at the equilibrium potential of the redox couple (DC
potential, +220 mV vs. Ag/AgCl/3 M KCl) superimposed by an AC perturbation of 10 mVpp
amplitude. The frequency range from 30 kHz to 10 mHz was scanned unless stated otherwise.
The charge transfer resistance was determined by fitting the Nyquist plots to an [R(Q[RW])]
Randles equivalent circuit.
5.9.2 Cyclic voltammetry
Characterization of the surface before and during the assay preparation was performed also via
cyclic voltammetry in 10 mM PB with 20 mM K2SO4 (pH 7.4) containing equimolar
concentrations of K3[Fe(CN)6] and K4[Fe(CN)6] (5 mM). Measurements were performed at 100
mV/s scan rate in the potential window from -0.1 V to 0.5 V (vs. Ag/AgCl/3 M KCl).
5.9.3 Chronocoulometry for determination of DNA coverage
Determination of ssDNA coverage was done using the chronocoulometric method developed
by Steel and coworkers 75 . A potential step from 0 V to -0.4 V (vs. Ag/AgCl/3 M KCl) was
applied for 500 ms at ssDNA/MCH modified electrodes immersed in two different solutions,
initially in 10 mM PB solution containing 20 mM K2SO4 and subsequently in the same buffer
containing additionally 100 µM [Ru(NH3)6] 3+ . The resulting charge was measured in both cases.
Prior to measurements solutions were purged with argon for at least 30 min.
5.10 Hybridization and dehybridization
DNA hybridization was performed via incubation with the target strand after characterization
of ssDNA/thiol modified electrodes. The electrode was initially rinsed with the hybridization
buffer (10 mM PB, 450 mM K2SO4) and then placed in an Eppendorf tube containing 200 μL
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