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A laboratory test used to detect disease and perform biological
research could be made more than 3 million times more sensitive, according to
researchers who combined standard biological tools with a breakthrough in
nanotechnology.
The increased performance could greatly improve the early
detection of cancer, Alzheimer’s disease, and other disorders by allowing
doctors to detect far lower concentrations of telltale markers than was
previously practical.
The breakthrough involves a common biological test called an
immunoassay, which mimics the action of the immune system to detect the
presence of biomarkers—the chemicals associated with diseases. When biomarkers
are present in samples, such as those taken from humans, the immunoassay test
produces a fluorescent glow (light) that can be measured in a laboratory. The
greater the glow, the more of the biomarker is present. However, if the amount
of biomarker is too small, the fluorescent light is too faint to be detected,
setting the limit of detection. A major goal in immunoassay research is to
improve the detection limit.
The Princeton
University researchers
tackled this limitation by using nanotechnology to greatly amplify the faint
fluorescence from a sample. By fashioning glass and gold structures so small
they could only be seen with a powerful electron microscope, the scientists were
able to drastically increase the fluorescence signal compared to conventional
immunoassays, leading to a 3-million-fold improvement in the limit of
detection. That is, the enhanced immunoassay would require 3 million times
fewer biomarkers to be present compared to a conventional immunoassay. (In
technical terms, the researchers measured an improvement in the detection limit
from 0.9 nanomolars to 300 attomolars.)
“This advance opens many new and exciting opportunities for
immunoassays and other detectors, as well as in disease early detection and
treatment,” said Stephen Chou, the Joseph C. Elgin Professor of
Engineering, who led the research team. “Furthermore, the new assay is
very easy to use, since for the person conducting the test, there will be no
difference from the old one– they do the procedure in exactly the same way.”
Professor Stephen Chou (right) demonstrates how biological samples are added to an immunoassay test. Postdoctoral researcher LiangCheng Zhou (left) and graduate student Fei Ding (middle) were part of a team that used nanotechnology to demonstrate a 3 million-fold improvement in the test. Photo: Frank Wojciechowski |
The researchers published their results in Nanotechnology and Analytical
Chemistry.
The key to the breakthrough lies in a new artificial nanomaterial
called D2PA, which has been under development in Chou’s laboratory for several
years. D2PA is a thin layer of gold nanostructures surrounded glass pillars
just 60 nm in diameter. The pillars are spaced 200 nm apart and capped with a
disk of gold on each pillar. The sides of each pillar are speckled with even
tinier gold dots about 10 to 15 nm in diameter. In previous work, Chou has
shown that this unique structure boosts the collection and transmission of
light in unusual ways—in particular, a 1
billion-fold increase in an effect called surface Raman scattering. The
current work now demonstrates a giant signal enhancement with fluorescence.
In a typical immunoassay, a sample such as blood, saliva, or urine
is taken from a patient and added to small glass vials containing antibodies
that are designed to “capture” or bind to biomarkers of interest in
the sample. Another set of antibodies that have been labeled with a fluorescent
molecule are then added to the mix. If the biomarkers are not present in the
vials, the fluorescent detection antibodies do not attach to anything and are
washed away. The new technology developed at Princeton
allows the fluorescence to be seen when very few antibodies find their mark.
In addition to diagnostic uses, immunoassays are commonly used in
drug discovery and other biological research. More generally, fluorescence
plays a significant role in other areas of chemistry and engineering, from
light-emitting displays to solar energy harvesting, and the D2PA material could
find uses in those fields, Chou said.
As next steps in his research, Chou said he is conducting tests to
compare the sensitivity of the D2PA-enhanced immunoassay to a conventional
immunoassay in detecting breast and prostate cancers. In addition he is
collaborating with researchers at Memorial
Sloan-Kettering Cancer
Center in New York to develop tests to detect proteins
associated with Alzheimer’s disease at a very early stage.
“You can have very early detection with our approach,”
he said.
