Detection of charged macromolecules by means of fi eld-effect devices (FEDs): possibilities and limitations - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 198

unspecifi c binding of DNA molecules at the reference electrode used in the experi-
mental set-up, can reduce or even mask the expected biosensor signal. Moreover, sen-
sor drift as well as leakage currents can also yield a falsifi ed sensor signal, or at least
interfere with it. It is always favorable to use differential measuring set-ups [15, 28,
43] to at least exclude some of these disturbing and interfering effects. Nonetheless,
much more theoretical modeling and experimental research has to be done in order to
understand and correctly interpret the DNA-detection experiments by means of FEDs.

7.4 NEW METHOD FOR LABEL-FREE ELECTRICAL DNA
DETECTION

As a new mechanism for a direct label-free DNA detection using an FED, the ion-
concentration redistribution in the intermolecular spaces of the immobilized ssDNA
(due to the DNA-hybridization event) and the alteration of the ion sensitivity has been
proposed recently [51]. In this approach, the top surface of the ion-sensitive FED is
modifi ed with immobilized ssDNA probe molecules arranged normal to the surface
with a center-to-center average interprobe distance a s . The remaining surface of the
ion-sensitive layer between the immobilized molecules is in contact with the electrolyte
solution. In such a DNA brush-like model (see also Fig. 7.3b), the mobile ions pass
freely between the DNA layer and the external electrolyte. The probe ssDNA molecules
should be arranged on the surface with enough interstitial space to allow a rapid hybrid-
ization and to provide a high hybridization effi ciency. A preferable average center-
to-center separation distance could be in the range from 2.5 nm to 10 nm which

10 13 to 1.3 10 12 molecules cm 2 ,

corresponds to a probe density from about 2

typically reported in the literature (see, e.g., [60-63]).
Since ssDNA and dsDNA molecules are negatively charged via their phosphate
groups, such negatively charged molecules will attract positively charged coun-
ter-ions (including protons) from the solution and repel the co-ions. As a result, the
DNA charge is effectively compensated by the surrounding small counter-ions. This
may result in a local ion-concentration redistribution within the intermolecular spaces
(increasing the cation concentration and decreasing the anion concentration) which can
substantially differ from the concentration in the bulk electrolyte, n 0 . After hybridiza-
tion, because the charge of the dsDNA is nearly doubled, a new distribution of the
electrostatic potential and of the ions within the intermolecular spaces will be reached.
The hybridization-induced ion-concentration (including proton concentration) redis-
tribution in the intermolecular spaces (or in the DNA layer) can be detected by the
underlying ion-sensitive FED. Thus, in contrast to the above-discussed FEDs for the
DNA-hybridization detection by the intrinsic molecular charge, where the screening of
the molecular charge by small counter-ions is considered as a major obstacle, here the
counter-ion condensation effect is used to detect the DNA-hybridization event.
For sensor applications, a more interesting parameter is the degree of change in the
average ion concentration in the intermolecular spaces upon the hybridization event.
The model for the theoretical calculations of the average concentration of cations and