Biology Reference
In-Depth Information
protein) or class selectivity to DNA as the recognition element itself
(e.g., damage to DNA) [8]. With respect to this characteristic, DNA-
based biosensors represent irreplaceable testing (bio)analytical
devices.
Working procedures with these biosensors utilize special detec-
tion principles. In the first place, label-free techniques utilizing
electrochemical and/or surface activity of DNA have to be men-
tioned [17]. Electrochemical activity of DNA is based on the
presence of redox changes in nucleobases and sugar residues.
All common nucleobases are known to undergo electrochemical
oxidation at carbon electrodes. At neutral and weakly acidic pH,
adenine, cytosine, and guanine residues in DNA produce reduction
signals at mercury-based electrodes at highly negative potentials,
while guanine residues yield anodic signals due to oxidation of
their reduction product back to guanine. Protonation of base
residuesisinvolvedintheelectrodeprocess.Mercuryelectrodesare
particularly sensitive to minor conformational changes in DNA such
as those induced by nucleases and chemical and physical agents,
including ionizingradiation [17].
Nucleic acids are usually strongly adsorbed on electrodes,
particularly on mercury and carbon ones. For mercury electrodes,
the adsorption/desorption behavior of DNA strongly depends on
the structure of the DNA molecules. DNA electrochemical surface
activity depends on what DNA components take part in adsorption
at the electrode surface. The height of the tensammetric peak
increases with the chain length. Adsorption of DNA on mercury
electrodes proceeds only in one layer, and the formation of further
layers does not influence the intensity of electrochemical signals.
The polyanionic nature of nucleic acids leads to characteristic
adsorption/desorption (reorientation) processes at mercury-based
electrodes upon application of negative electrode potentials due
to interplay between electrostatic repulsion and relatively strong
adsorption via hydrophobic parts of the polynucleotide chains
(particularly bases) [13, 17]. Electrochemical analysis of the DNA
can, thus, in principle, be performed without introducing labels
into the DNA recognition element (label-free techniques) and even
without introducing any additional reagent into the measuring
system (reagent-less techniques).
