Nanoparticle-based biosensors and bioassays - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 447

on-chip PCR amplifi cation [42]. The hybridization of probe coated magnetic beads
with the gold-tagged targets results in three-dimensional network structures of “large”
(micrometer) magnetic beads, cross-linked together through the DNA and gold nanopar-
ticles. In these aggregates, the DNA duplex “bridges” the magnetic beads with the metal
nanoparticles. No such aggregation was observed in the presence of non-complementary
or mismatched oligonucleotides. Similar DNA-induced aggregation has been exploited by
Mirkin and coworkers to detect the hybridization in connection with distance-dependent
color changes [11]. Very recently, we described an electrochemical protocol for detecting
DNA hybridization based on preparing the metal marker along the DNA backbone (instead
of capturing it at the end of the duplex) [43]. The new protocol relies on DNA template-
induced generation of conducting nanowires as a mode of capturing the metal tag. The
use of DNA as a metallization template has evoked substantial research activity directed at
the generation of conductive nanowires and the construction of functional circuits [44-46].
Such an approach was applied to grow silver [44], palladium [45], or platinum [46] clus-
ters on DNA templates. Yet, the DNA-templated assembly of metal wires has not been
exploited for detecting DNA hybridization. The new detection scheme consists of the vec-
torial electrostatic “collection” of silver ions along the captured DNA target, followed by
the hydroquinone-catalyzed reductive formation of silver aggregates along the DNA skel-
eton, along with dissolution and stripping detection of the nanoscale silver cluster.
Mirkin and coworkers have developed an array-based electrical detection using
oligonucleotide-functionalized gold nanoparticles and closely spaced interdigitated
microelectrodes [47]. The oligonucloetide probe was immobilized in the gap between
the two microelectrodes. The hybridization event thus localizes gold nanoparticles in
the electrode gap and, along with subsequent silver deposition, leads to measurable
conductivity signals. Such hybridization-induced conductivity signals, associated with
resistance changes across the electrode gap, offer high sensitivity with a 0.5 pM detec-
tion limit. Controlling the salt concentration allowed high point-mutation selectivity
(with a factor of 100 000:1) without thermal stringency.
Changes in the resistance across a microelectrode gap, resulting from the hybridi-
zation of nanoparticle-labeled DNA, have been exploited also by Urban et al. [48] for
a paralleled array-readout system. A self-contained microanalyzer, allowing such par-
allel readout of the entire array, indicates great promise for point-of-care applications.
Colloidal gold was employed also for improving the immobilization of DNA on elec-
trode surfaces and hence for increasing the hybridization capacity of the surface [49].
Such use of nanoparticle supporting fi lms relied on self-assembly on 16 nm diameter
colloidal gold onto a cystamine-modifi ed gold electrode and resulted in surface densities
of oligonucleotides as high as 4

10 14 molecules cm 2 . Detecting the ferrocenecar-
boxaldehyde tag (conjugated to the target DNA) resulted in a detection limit of 500 pM.
Owing to their unique (tunable-electronic) properties, semiconductor (quantum dots)
nanocrystals have generated considerable interest for optical DNA detection [12]. Recent
activity has demonstrated the utility of quantum dot nanoparticles for enhanced electri-
cal DNA detection [33, 34, 50]. Willner et al. reported on a photoelectrochemical trans-
duction of DNA sensing events in connection with DNA cross-linked CdS nanoparticle
arrays [50]. The electrostatic binding of the Ru(NH 3 ) 6 3 electron acceptor to the dsDNA