Biology Reference
In-Depth Information
As only guanine moieties in the close vicinity of the electrode
surface can undergo direct electrooxidation, soluble redox media-
tors such as rhodium or ruthenium complexes are sometimes used
to shuttle electrons from guanine residues in distant parts of DNA
chains to the electrode [20]. In such a case, we cannot speak more
about the reagent-less technique. Nevertheless, the electrochemical
reduction and oxidation of nucleobases are irreversible and thus do
notallow reusability ofbiosensors.
An alternative approach to the intrinsic DNA electrochemical
activity utilizes electroactive species as redox indicators of the
presence of immobilized DNA as well as its interaction events such
as hybridization, damage, and association with another substance
[14]. This mode was also used in a pioneering work on the DNA
biosensor used for sequence detection [7]. In this case, it is still a
label-free method in the sense that DNA probes or targets are not
chemicallymodifiedbyaspeciallabel;however,astheindicatorhas
to be added to a test system as an additional reagent, we cannot
speak more about the reagent-less technique. Redox indicators
typically possess electrochemical responses at a “safe” electrode
potential and often reversibly. The terms
redox probe
and
redox
marker
are sometimes used in the literature to mean the redox
indicator, which is confusable with the DNA capture probe used as
a recognition element at hybridization and with markers used in
medical diagnostics [8].
DNA redox indicators bind to DNA or are present in the
solution phase. Some of them interact with DNA on the basis of
electrostatic forces [21]. Cationic indicators such as metal complex
cations can be attracted to the DNA by the negative charge of
the DNA backbone. On the other hand, anionic indicators, for
instance hexacyanoferrate (III/II) [Fe(CN)
6
]
3
-
/
4
-
,workonthe
principle of repulsion by the negatively charged DNA backbone. As
a consequence, its voltammetric current response is lower than and
anodic to the cathodic peak potential separation, higher than that
observed at bare electrodes without DNA. Electrostatic indicators
can also respond to differences in negative charge density between
ssDNA and dsDNA [14].
OtherDNAredox indicatorsintercalate intothedsDNAstructure
(e.g., daunomycin, phenoxazines, metal complexes with condensed
