A new type of detector invented by researchers at MIT visually signals the presence of a target chemical by emitting a fluorescent glow. |
Researchers
at MIT have developed a new way of revealing the presence of specific
chemicals—whether toxins, disease markers, pathogens or explosives. The
system visually signals the presence of a target chemical by emitting a
fluorescent glow.
The
approach combines fluorescent molecules with an open scaffolding called
a metal-organic framework (MOF). This structure provides lots of open
space for target molecules to occupy, bringing them into close proximity
with fluorescent molecules that react to their presence.
The findings were reported in the Journal of the American Chemical Society
in a paper by assistant professor of chemistry Mircea Dinc?, with
postdoc Natalia Shustova and undergraduate student Brian McCarthy,
published online in November and to appear in a forthcoming print issue.
The
work could have significant applications in sensors attuned to specific
compounds whose detection could be read at a glance simply by watching
for the material to glow. “A lot of known sensors work in reverse,”
Dinc? says, meaning they “turn off” in the presence of the target
compound. “Turn-on sensors are better,” he says, because “they’re easier
to detect, the contrast is better.”
Mark
Allendorf, a research scientist at Sandia National Laboratory, who was
not involved in this work, agrees. “Present materials generally function
via luminescence quenching,” and thus “suffer from reduced detection
sensitivity and selectivity,” he says. “Turn-on detection would address
these limitations and be a considerable advance.”
For
example, if the material is tuned to detect carbon dioxide, “the more
gas you have, the more intensity in the response,” making the device’s
readout more obvious. And it’s not just the presence or absence of a
specific type of molecule: The system can also respond to changes in the
viscosity of a fluid, such as blood, which can be an important
indicator in diseases such as diabetes. In such applications, the
material could provide two different indications at once — for example,
changing in color depending on the presence of a specific compound, such
as glucose in the blood, while changing in intensity depending on the
viscosity.
MOF
materials were first produced about 15 years ago, but their amazing
porosity has made them a very active area of research. Although they
simply look like little rocks, the sponge-like structures have so much
internal surface area that one gram of the material, if unfolded, would
cover a football field, Dinc? says.
The
material’s inner pores are about one nanometer (one billionth of a
meter) across, making them “about the size of a small molecule” and well
suited as molecular detectors, he says.
The
new material is based on the MIT team’s discovery of a way to bind a
certain type of fluorescent molecules, also known as chromophores, onto
the MOF’s metal atoms. While these particular chromophores cannot emit
light by themselves, they become fluorescent when bunched together. When
in bunches or clumps, however, target molecules cannot reach them and
therefore cannot be detected. Attaching the chromophores to nodes of the
MOF’s open framework keeps them from clumping, while also keeping them
close to the empty pores so they can easily respond to the arrival of a
target molecule.
Ben
Zhong Tang, a professor of chemistry at the Hong Kong University of
Science and Technology, who was not involved in this work, says the MIT
researchers have taken “an eleTurn-On Fluorescence in Tetraphenylethylene-Based Metal–Organic Frameworks: An Alternative to Aggregation-Induced Emissiongant approach” to producing functional
MOFs, and “have already demonstrated the utility of their MOFs for
detection and differentiation of normally difficult-to-distinguish”
molecules called volatile organic compounds.
Tang
says the new system still needs further refinement to improve the
efficiency of production, which he says should be easily accomplished.
Once that is achieved, he says, it could find many uses. “Many more
applications may be envisioned: For example, the MOFs may serve as smart
vehicles and monitors for controlled drug deliveries,” with the
additional benefit that “the fluorescence should be gradually weakened
in intensity along with progressive release of the drugs, thus enabling
in situ real-time monitoring of the drug release profiles.” But for now,
he says, “the work is excellent in terms of proof of concept.”
The
work was supported by MIT’s Center for Excitonics, an Energy Frontier
Research Center funded by the U.S. Department of Energy, and by the
National Science Foundation.
