Biology Reference
In-Depth Information
carbon electrode using potentiometric stripping voltammetry after
acid dissolution. However, excess silver ions are of major concern
in this method, as they can affect the reliability of the stripping-
based detection. This is because the polyanionic DNA backbone
itself can act as a nucleation site for silver deposition following
cation exchange with sodium for ion-pair complexation to the DNA
bases,whichcanleadtoahighbackground.Toobviatethisproblem,
sodium thiosulfate can be used as a fixer [it transfers the silver
cations to [Ag(S
2
O
3
)]
5
−
]. Control of the silver precipitation time
is also needed. Silver-enhanced colloidal gold stripping led to a
dramatic (
>
100 fold) signal amplification. Instead of dissolving the
silver for stripping analysis, a direct assay of the silver metal can
alsobeperformedbyeitherconstant-currentchronopotentiometric
detection after magnetic collection of the duplex-linked particle
assembly [37], or a differential pulse voltammetry measurement
of the large number of silver atoms anchored on the duplexes,
using a glassy carbon electrode [38]. Lee
et al.
[39] reported the
catalytic effects of various gold nanoparticles for silver deposition
on indiumtin oxide(ITO)basedelectrodes.
The use of silver enhancement may cause a significant back-
ground signal due to non-specific silver deposition on the DNA
support (i.e., the magnetic bead or electrode surface) and/or on
the negatively charged DNA (asmentioned above). Hence, Rochelet-
Dequaire
et al.
[40] insteadusedgoldionsforthecatalyticenhance-
ment,sincethegoldautocatalyticprocessoffersalowerbackground.
This is because there is minimal autonucleation from AuCl
4
−
and
less interaction between the anionic AuCl
4
−
and the negatively
charged DNA. Their work showed that classical gold enhancement
procedures based on incubation in a mixture of chloroauric acid
and hydroxylamine could not provide effective amplification, due to
lossoftheenhancedgoldlabelsduringthepost-enlargementrinsing
step. Therefore, the authors modified the enhancement procedure
to use polyethylene glycol and NaCl in the growth media, to act as
an aggregating agent during the catalytic process. This resulted in
retention of the enlarged labels on the bottom of the microwell,
providing a detection limit of 0.6 fM. Liao
et al.
[41] reported
a similar scheme by using a square wave stripping voltammetry
and were able to detect a mutated
BRAF
gene associated with
