Biology Reference
In-Depth Information
aromatic heterocyclic ligands) or bind to dsDNA grooves (e.g.,
Hoechst 33258). All together, cationic indicators, intercalators, and
groove binders accumulate at the immobilized dsDNA layer (e.g.,
after hybridization or prior to damage to the DNA duplex), thus
increasing their measured voltammetric response. The biosensor
can be used repeatedly after its renewal using the sequence
of steps: indicator accumulation, voltammetric measurement and
chemical removal or desorption of the accumulated indicator
from the DNA layer. Then, for one and the same biosensor a
mean indicator response and its standard deviation are calculated
[21].
Indicators associating preferentially with ssDNA have been
advantageously used with electrochemical DNA hybridization sen-
sors.Forinstance,thephenothiazinedyemethyleneblue(MB)asso-
ciates with unpaired guanine moieties. In dsDNA this interaction
is hampered, which results in decrease in the current response
due to MB reduction [22]. On the other hand, there are also ds-
specific electroactive indicators such as the intercalator ferrocenyl
naphthalenediimide,whichresultsinadetectionlimitof10zmolat
the differential pulse voltammetric mode [23, 24].
Finally, electrochemically active DNA labels (tracers), which are
covalently bound to DNA, can be used for detection. The DNA labels
considerably improve analytical selectivity/specificity, for instance,
at DNA hybridization as the labeled DNA can be distinguished
from the unlabeled one [17, 25]. Among such labels, ferrocene,
daunomycin, anthraquinone, thionine, bipyridine complexes of Ru
andOs,nitrophenyl,andaminophenylgroupshavetobementioned.
Osmiumtetroxidecomplexeswithnitrogenligands(Os
VIII
,L)[26,27]
or analogous osmate complexes (Os
VI
,L) [28] represent examples
of electroactive tags. Nanoparticles or nanocrystals of gold, indium,
zinc, cadmium, or lead chalcogenides and other materials have
been used as labels covalently (often via thiol linkage) attached
to DNA probes applied in amplifying the response. By combining
various nanoparticles such as ZnS, CdS, and PbS, electrochemical
“multicolor” DNA coding has been attained [29]. Carbon nanotubes
as DNA tags can also be loaded with multiple nanoparticles or
enzyme molecules, thus offering considerable signal enhancement
[22, 29, 30].
