Nanosensors - Nanotechnologies for Water Environment Applications - page 429

A number of companies (e.g., BioVeris, Response Biomedical Corporation, and
QTL) have launched automated devices for the detection of pathogens of interest to
biodefense. These include Bacillus anthracis, Yersinia pestis, Francisella tularensis,
Brucella melitensis, Clostridium botulinum , and vaccinia virus (Gooding, 2006). Many
of these tools are based on sandwich immunoassay by immobilizing antibodies on
magnetic beads and can also be used to develop fast and rugged assays for waterborne
pathogens. QTL bioassay, for example, employs highly fluorescent polymers (tags) and
antibody immobilized magnetic particles to capture, concentrate, and detect in a
sequential manner (Figure 13.7). In the absence of any magnetic field, the spores
(targets) attach themselves to magnetic particles and tags and form a sandwich structure
(Figure 13.7a-b). When the magnetic field is turned on, the unbound and bound
magnetic particles are pulled towards the electrode surface (Figure 13.7c). This step
effectively concentrates the sample; the non-targets are then removed by a washing
during this step. Finally in the presence of a magnetic field, the fluorescent tag is
illuminated, which emits a signal if the target is present (Figure 13.7d). In one study, the
QTL bioassay detected 10,000 anthrax spores within 10 minutes (Chen et al., 1999).

a

MAGNETIC FIELD OFF MAGNETIC FIELD OFF

b

Sample
preparation

MAG

FLUORESCENCE TAG-SPORE-MAG

FLUORESCENCE
TAG

SPORE

Washing

c

MAGNETIC FIELD ON

d

MAGNETIC FIELD ON

Illumination Detection

Detection

Figure 13.7 (a) The QTL bioassay with magnetic particles, spores (targets), and
polymeric fluorescent tags in the absence of magnetic field, (b) formation of sandwich
immunoassay in the absence of magnetic field, (c) interaction of sandwich structures
towards the electrode surface in the presence of magnetic field, (d) detection of target by
the illumination of sandwich structures by light.