Biology Reference
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strategies such as the detection of redox markers, the detection
based on enzymatic labels, the detection based on electrochemical
labels intercalated within ds-DNA, and the use of Au-NPsas carriers
for other nanoparticles or other electrochemical labels which are
responsible for thegeneration of the analyticalsignals.
4.3.1.1 Direct detection of redox markers
Figure 4.3 showed an example involving DNA immobilization
through thiol binding onto gold nanoparticles electrodeposited on
screen-printed electrodes. This sensor architecture allowed the
development of an electrochemical sensor for the detection of
E.
coli
O157 based on competition between the target gene (com-
plementary to the capture probe DNA) and reporter DNA-tagged,
hexaammineruthenium (III) chloride-encapsulated liposomes. The
current signal of the released liposomal [Ru(NH
3
)
6
]
3
+
was mea-
sured using square wave voltammetry (SWV), yielding a sigmoidal-
shaped dose-response curve whose linear portion was over the
rangefrom1to10
6
fmol.Thisliposomalcompetitiveassayprovided
an amplification route for the detection of the rfbE gene (specific to
E. coli
O157) at ultratrace levels, with a detection limit of 0.75 amol
[12].
Anotherinterestingexampleofdirectdetectionofredoxmarkers
is a ferrocene catalyzed aptamer-based thrombin sensor [29].
Figure 4.6 displays the sensor architecture and functioning. A
thrombin binding aptamer was covalently immobilized onto three-
dimensional Au-NP-doped conducting polymer nanorod electrodes
(Au-NPs/3D-CPNEs). Ferrocene was attached with anti-thrombin
through streptavidin-biotin interactions and it electrochemically
catalyzed the oxidation of ascorbic acid. Since thrombin was
sandwiched between thrombin aptamer and anti-thrombin anti-
body attached with ferrocene, the catalytic current response was
proportional to the thrombin concentration. The aptamer sensor
showed a dynamic range from 5 to 2000 ng L
−
1
with a detection
limit of 5 ng L
−
1
(0.14 pM) and it was applied to the determination
of spiked concentrations ofthrombin inreal human serum samples.
Another interesting approach involves the use of quantum dot
tracers. For example, Huang
et al.
[30] have described a DNA
