Biomedical Engineering Reference
In-Depth Information
units provided a tunneling route for the electron-band electrons and thus led to increased
photocurrents. We reported on the detection of DNA hybridization in connection with
cadmium sulfi de nanoparticle tracers and electrochemical stripping measurements of the
cadmium [33]. A nanoparticle-promoted cadmium precipitation was used to enlarge the
nanoparticle tag and amplify the stripping DNA hybridization signal (Fig. 14.3, see Plate
15 for color version). In addition to measurements of the dissolved cadmium ion, we
demonstrated solid-state measurements following a “magnetic” collection of the mag-
netic bead/DNA hybrid/CdS tracer assembly onto a thick fi lm electrode transducer. Such
a protocol combines the amplifi cation features of nanoparticle/polynucleotide assem-
blies and highly sensitive potentiometric stripping detection of cadmium with an effec-
tive magnetic isolation of the duplex. The low detection limit (100 fmol) was coupled to
good reproducibility (RSD
6%). Such a protocol was recently extended to other inor-
ganic colloids (e.g. ZnS or PbS) which can be similarly synthesized in reversed micelles.
Such extension has paved the way for an electrochemical coding technology for the
simultaneous detection of multiple DNA targets based on nanocrystal tags with diverse
redox potentials [34]. Functionalizing the nanocrystal tags with thiolated oligonucleotide
probes thus offered a voltammetric signature with distinct electrical hybridization signals
for the corresponding DNA targets (Fig. 14.3e, see Plate 15 for color version). The posi-
tion and size of the resulting stripping peaks provided the desired identifi cation and quan-
titative information, respectively, on a given target DNA. The multi-target DNA detection
capability was coupled to the amplifi cation feature of stripping voltammetry (to yield
femtomolar detection limits) and with an effi cient magnetic removal of non-hybridized
nucleic acids to offer high sensitivity and selectivity. Up to fi ve to six targets can thus be
measured simultaneously in a single run in connection with ZnS, PbS, CdS, InAs, and
GaAs semiconductor particles. Conducting massively parallel assays (in microwells of
microtiter plates or using multi-channel microchips, with each microwell or channel car-
rying out multiple measurements) could thus lead to a high throughput operation.
14.4.2 Nanoparticle-based electrochemical immunosensors and
immunoassays
On the basis of a specifi c reaction of the antibody and antigen, immunosensors provide
a sensitive and selective tool for determining immunoreagents. Here, the immunologic
material is immobilized on a transducer; the analyte is measured through a label spe-
cies conjugated with one of the immunoreagents. Quantifi cation is generally achieved
by measuring the specifi c activity of a label, i.e. its radioactivity, enzyme activity, fl uo-
rescence, chemiluminescence or bioluminescence. There is no ideal label, and each of
them has its own advantages and disadvantages. Metalloimmunoassays, i.e. immu-
noassays involving metal-based labels, were developed in the 1970s [51] to overcome
problems associated with the common radioisotopic, fl uorescent or enzyme labels.
Metalloimmunoassays with electrochemical detection can offer several advantages;
e.g. measurement can be performed in very low volumes of liquid (a few microliters),
eventually in turbid media, with the possibility of having good sensitivity for a relatively
inexpensive instrumentation (fi eld portable). Although the electrochemical techniques
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