Detection of charged macromolecules by means of fi eld-effect devices (FEDs): possibilities and limitations - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 205

and hybridization signal between the C-V curves measured in the same buffer solution
before and after immobilization, and before and after hybridization yielded biosensor
signals of 35-55 mV (immobilization) and 24-33 mV (hybridization), respectively.
The results obtained with PE multilayers as well as DNA on top of the capacitive EIS
sensor could verify their feasibility as transducer for a label-free detection of adsorption,
binding, and interactions of charged macromolecules. Nevertheless, our experiments do
not enable us to clearly distinguish between the contributions in the signal generation
from each of the mechanisms discussed in sections 7.3 and 7.4. Probably, both basic
mechanisms, namely, the intrinsic charge of molecules and the ion-concentration redis-
tribution in the intermolecular spaces or in the multilayer, affect the sensor signal by
superposition.
7.6 CONCLUSIONS AND FUTURE PERSPECTIVES
Field-effect-based DNA chips can be considered as a new tool for the simultaneous
analysis of a multitude of nucleic acids, representing a wide fi eld of possible applica-
tions, like biomedical research, clinical diagnostics, drug screening, genomics, envi-
ronmental monitoring and food analysis. The main advantage of FEDs is that they
combine the possibility of a direct electrical detection of the DNA hybridization with-
out labels, on the one hand, and the large integration ability as active microelectronic
devices, on the other hand (e.g. high-density arrays of active areas up to 10 5 cm 2 are
achievable).
At the same time, however, it must be concluded that the practical development of
FEDs for a label-free detection of DNA and other charged macromolecules by their
intrinsic molecular charge seems to be more complicated than originally expected.
Although the discussed results are highly exciting, they are rather diverse and even
sometimes inconsistent. Factors infl uencing the DNA immobilization and hybridiza-
tion detection by FEDs are:
● the gate material of the FED (metal, insulator, polymer) and its surface charge or
surface modifi cation,
● the immobilization method of the ssDNA (passive or electrochemical adsorption,
covalent binding, binding via cross-linker),
● the immobilization, hybridization as well as measuring conditions (pH, ionic
strength, temperature), and
● the packing density and length of the immobilized ss-DNA, the length of the
spacer or linker molecule, etc.
Therefore, a deep understanding of the adsorption and interaction of charged macro-
molecules such as DNA onto (charged) surfaces of FEDs is of great interest not only
for biosensor applications but also for the fundamental characterization of many key
physiological processes. Further experiments are required to fi gure out the achieved
results with DNA-based FEDs and to develop accompanying correct theoretical mod-
els. In this context, utilizing new devices and transducer strategies might be a promising