16 Biotechnology
➠
antiferromagnetically exchange-coupled layers, separated fluidic force discrimination (FFD) assay. In a FFD assay, a
by a thin non-magnetic conducting layer, can be altered chip has arrays of receptor molecules such as antibodies
by changing the moments of the ferromagnetic layers that capture toxins or other target molecules that have
from anti-parallel to parallel. This change decreases the been labelled with micrometre-sized beads.
spin-dependent interfacial scattering of charge carriers By encapsulating the chip in a microflow chamber,
resulting in a decrease in the resistance of the GMR the fluid flow can be controlled to apply just enough force
material. to remove beads that are resting on the array but not
He realised this very sensitive phenomenon could have truly labelling a toxin. “In this way,” explains lead author
potential in the development of sensors for biological Shawn Mulvaney, “very few molecules can be detected,
materials which are naturally biochemically specific, but because there is almost no background signal. And
are not usually magnetic. By attaching tiny paramagnetic because we can get the background so low, FFD assays
particles to biomolecules, such as proteins or are very specific, with very few false positives.”
single-stranded DNA, scientists could then perform The NRL researchers have adapted FFD assays to
standard sandwich-type immuno or nucleic acid detect a protein toxin at concentrations as low as 35
hybridization assays over the GMR sensors. The GMR attomolar - over 1000 times more sensitive than existing
sensors, each covered with complementary protein or commercial tests for proteins.
single-stranded DNA (the probe), could then detect the In the new assay, dubbed ‘semi-homogeneous fluidic
magnetically labelled biomolecules (the target) the assays force discrimination,’ the antibody-coated microbeads
were designed to identify. are mixed directly with the sample and rapidly collect
In a second development, NRL researchers have the dilute toxin molecules. The toxin-coated beads are
developed what they describe as a forceful new method then injected into the microflow chamber where they are
to sensitively detect proteins. The researchers report the captured by the receptor designed for that target. Finally,
detection of toxins with unprecedented speed, sensitivity, beads that don’t belong are removed with fluid forces.
and simplicity. The approach can sense as few as a few The remaining beads are all attached by the toxin
hundred molecules in a drop of blood in less than ten to the surface and may be counted to indicate the toxin
minutes, with only four simple steps from sample to concentration. NRL has developed both electronic and
answer. optical systems to count the beads, along with reusable
NEW S-Live 1/2
The sensitiv
page 11/20/06
e new test builds on NRL
12:27 PM Page
’s patent-pending
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plastic test cartridges. u
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