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voltammogramwasobtained.TheelectrochemicaldepositionofAu-
NP was carried out in the HAuCl 4 solution containing 0.1 M KNO 3
as electrolyte at 400 mV. The freshly prepared nanogold electrode
wasincubatedwiththethiolated15-meraptameranti-thrombinfor
about 16 h to produce an aptamer attached electrode. Then the
modified electrode was immersed in a solution of 6-mercapto-1-
hexanol for 1 h to block the uncovered gold surface. At this point,
the aptamer-modified electrode was interacted with different con-
centrations of thrombin and then with a solution of Au-NP probe.
It consisted of Au-NPs conjugated to the thiolated 15-mer aptamer
anti-thrombin and to CdS-NPs linked with a single-stranded DNA
sequence. The resulting sandwich complex was treated with 1.0 M
of HNO 3 solution for 5 min to dissolve the CdS-NPs and then with
acetate buffer containing Hg 2 + . The DPV measurements of the dis-
solved Cd 2 + were performed using an in situ prepared mercury film
on a glassy carbon electrode with a deposition time of 300 s and
deposition potential of 1.1 V. An anodic stripping peak current at
0.67 V was taken as the analytical response.
A detection limit of 0.55 fM of thrombin was calculated. Authors
attribute the significant improvement of the sensitivity of such
aptasensors with respect to others present in literature to the use
of a nanoelectrode, formed by immobilization of Au-NPs on the sur-
face of a gold electrode, to the use of NPs as labels, and to the use of
DPVtechniqueforthedetectionofthedissolvedCd 2 + inthesolution.
Moreover,theelectrochemicalaptasensorwassuccessfullytestedin
some serum samples.
2.7 Electrochemical Aptasensors Based on Noncovalent
Redox Species Label
These aptasensors are based on the use of a redox probe such
as methylene blue (MB) that undergoes an oxidation and reduc-
tion due to the electron transfer from an electrode surface to a
probe.Theseredoxprobesarenoncovalentlyboundtoaptamersand
intercalate or interact with aptamers mainly by electrostatic inter-
actions. For example, MB, positively charged, interacts with nega-
tively charged proteins or other negatively charged analytes. When
 
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