Biology Reference
In-Depth Information
attached to a graphite pencil electrode surface was used by Ozsoz
et al.
[32]. Fang and coworkers [33] have reported the use of silver
nanoparticles (AgNP) as the oligonucleotide label. By oxidative
metal dissolution and the indirect determination of the solubilized
Ag
+
by ASV at a carbon fiber microelectrode, detection down
to 0.5 pM DNA was reported. The same group has also labeled
oligonucleotide probes with an alloy of gold-coated copper core-
shell nanoparticles for a DNA sensing assay [34]. Hybridization
events between probe and target were monitored by the release
of the copper metal atoms anchored on the hybrids by oxidative
metaldissolution,andthenindirectdeterminationofthesolubilized
Cu
2
+
ions by ASV. Despite the good sensitivity of all the above
reports, detection limits remained in the range of nanomolar to
subpicomolar (see Table 8.1). Further improvements are needed
to meet the challenge of detecting as low as hundreds of copies
of target DNA—required to avoid using pre-amplification schemes
such as thepolymerase chain reaction.
Since the analytical signal in ASV comes from consumption
of the metal film deposited on the electrode (see Sec. 8.4), the
signal can be increased by increasing the size of the nanoparticle.
However, large diameters (e.g., for gold greater than about 20 nm)
are seldom used as electrochemical labels due to reasons such as
poor control of size distribution and poor stability in a solution
of the resulting bioconjugates, causing lower hybridization rates.
A preferable method has been to use smaller nanoparticles, and
then, after hybridization, increase the quantity of the metal by
forming shells of gold or silver on the original nanoparticle through
autocatalytic reduction. Silver deposition has been commonly used
in histochemical microscopy to visualize DNA-conjugated gold
nanoparticles. Based on this concept, Mirkin and coworkers [35]
developed a scanometric DNA array based on silver amplification
of the hybridization event. Wang and coworkers [36] extended this
formofamplificationtoelectrochemicaldetectionbymeasuringthe
deposited silver by stripping analysis. Basically, after hybridization
gold nanoparticles function as catalytic sites for chemical reduction
of silver ions (from silver lactate or silver nitrate) in the presence of
the reducing agent, hydroquinone. Hence, metallic silver is formed
on the gold nanoparticles. This was detected at a screen-printed
