Biology Reference
In-Depth Information
magnetic bead-DNA conjugate was collected by positioning a
magnet behind a screen-printed carbon electrode. The attraction of
theconjugatedirectlyontotheworkingareaoftheelectrodeallowed
the stripping detection step to take place without dissolution of the
metal obviating the needfor the caustic acidic medium.
Inorganic semiconductor nanocrystals have also found their
use as electrochemical labels for DNA detection. Cadmium sulfide,
for instance, was reported to be a viable alternative for gold
nanoparticles. After dissolution in nitric acid Cd
+
2
ions can be
detected at a mercury or bismuth film electrode [25]. Taking
advantage of the wide potential window and the fact that multiple
group II and III metals can be detected simultaneously at mercury
and bismuth film electrodes, a multitarget DNA hybridization
assay was developed using three different inorganic nanocrystals
(ZnS, CdS, and PbS) to simultaneously detect three different DNA
targets in the same solution [26]. A general scheme of Wang's
nanoparticlemagneticbead-basedprotocolforelectrochemicalDNA
detection consisting of gold nanoparticles (A), silver enhancement
(B), magnetic collection and solid state detection (C), the use of CdS
(D), and multiple inorganic semiconducting encoding nanoparticles
(E) ispresented in Fig. 14.4.
Merkoci and coworkers [27] have reported several works
describing DNA electrochemical biosensors based on the direct
determination of gold nanoparticles which have been adsorbed
onto the rough surface of graphite-epoxy composite electrodes,
their electrochemical oxidation at
1.25 V, and the detection of the
resulting tetrachloroaurate ions by differential pulse voltammetry.
The use of 1.4 nm Au
67
particles allowed the 1:1 conjugation of
nanoparticle to magnetic bead-DNA probe and prevented cross-
linking effects resulting in lower detection limits over previous
assays [28]. A magnet placed into the graphite-epoxy electrode
transducer collected the hybridized DNA after magnetic separation
and allowed for the direct detection of the gold nanoparticle label.
Two other gold nanoparticle assays were described based on this
method using larger gold nanoparticles conjugated to DNA using
biotin/streptavidin interactions with the first being a two-strand
detection technique to detect the BRCA1 breast cancer gene, and
the second a sandwich assay to detect a DNA sequence related to
+
