Detection of charged macromolecules by means of fi eld-effect devices (FEDs): possibilities and limitations - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 202

“Rod-like” dsDNA

Ions

Ions

“Coil-like”
ssDNA

Hybridization

Ion-sensitive layer Ion-sensitive layer

FED FED

(a) (b)

FIGURE 7.9 Model for the DNA detection based on an alteration of the ion sensitivity of an FED
induced upon the DNA hybridization. If fl exible ssDNA lie preferentially fl at on the ion-sensitive gate of the
FED, they can partially cover the surface active sites for ion-binding or ion-exchange processes as well as
prevent or hinder potential-determining ions to reach the transducer surface (a). In contrast, after hybridiza-
tion a rigid rod-like dsDNA is formed. Now, the surface of the FED is opened for ion interaction (b), result-
ing in the hybridization-induced signal generation.

7.5 MEASUREMENT RESULTS UTILIZING
POLYELECTROLYTE LAYERS AND SYNTHETIC DNA

In this chapter, exemplary experiments with capacitive EIS sensors that have been uti-
lized for DNA immobilization and hybridization detection as well as for monitoring the
layer-by-layer adsorption of polyelectrolytes anionic poly(sodium 4-styrene sulfonate):
PSS and cationic poly(allylamine hydrochloride: PAH) are presented. Polyelectrolyte
(PE) multilayers represent a very useful model system for studying effects induced in
FEDs by the adsorption of charged macromolecules [41, 46-50]. They are linear mac-
romolecule chains bearing a large number of charged or chargeable groups when dis-
solved in a suitable polar solvent, e.g. in water. The formation of PE multilayers is
based on the consecutive adsorption of polyions with alternating charge. Each adsorp-
tion step of a charged PE layer leads to a charge inversion of the surface, the so-called
charge overcompensation effect [66-68]. The subsequent deposition, fi nally, results in
a PE multilayer stabilized by strong electrostatic forces. Figure 7.10 sketches the FED
build-up with a PE multilayer arrangement.
The preparation of the FEDs, the experimental set-up and measuring conditions for
the detection of DNA immobilization and hybridization as well as for the monitor-
ing of the layer-by-layer adsorption of the polyelectrolyte multilayers are described
in detail elsewhere [46-50]. The attachment of these charged macromolecules to the
FED surfaces has been systematically characterized by means of capacitance-voltage,