NIST
Chemical Sensor Technology
Today,
portable devices for detecting toxic airborne chemicals are largely
limited to specialized equipment
designed
for use by the military or by first responders
to chemical spills. In the event of an attack involving toxic
chemical agents—such
as the recent use of sarin gas in a Tokyo subway station—such
portable detectors typically would not arrive on the scene until
after victims already have been harmed by the gas.
Infrared
Image of Microheater
Heating Up
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NIST Chemical Sensor Semiconductor Circuit with Four Microheaters
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NIST
is currently conducting research on a class of microsensors that
have the potential to serve as a cost-effective early warning
system for the presence of toxic gases. This same basic technological
approach also may be applicable to detection of vapors from explosive
materials. The NIST devices use an array of tiny microhotplates
in conjunction with thin metallized films such as tin, titanium,
or zinc oxides. Both the hotplates and sensing films are incorporated
into an integrated circuit device that can be fabricated with
standard CMOS methods. This means the miniature detectors
can be designed
and produced inexpensively with digital electronic processing
circuits built in. A key advantage of this technology is that an array of hotplates
with various types of films can be programmed to cycle through
specific temperatures. This creates a sensitive surface for
detecting ambient chemicals. If a specific chemical of
interest is present,
the resistance of the device changes in a reproducible way.
This change in resistance with different temperatures produces
a type
of “signature” for specific chemicals that can be matched
up against a library of chemical signatures to identify both the
type and concentration of the gas in the ambient air. NIST already
has demonstrated this ability for a variety of oxygenated hydrocarbons,
such as solvents, methanol, and ethanol.
By
changing the way the metal oxide films are grown, NIST researchers
have produced films with different grain sizes that react
in different ways to specific chemicals. This matrix of
responses
to an array
of sensors programmed to cycle through specific temperatures
with different microstructures is what allows the devices
to produce
a unique chemical signature.
Research and Development
With funding from the Defense Threat Reduction Agency, NIST
researchers have demonstrated that simulates of sulfur-mustard
compounds
and nerve agents (sarin, VX, etc.) can be detected at
or significantly below the 1-part-per-million level in
laboratory
testing. Preliminary
testing at the Army’s Edgewood Arsenal has confirmed
that this sensitivity is feasible with actual chemical
warfare agents.
Further research is under way to demonstrate that the
presence of each agent will produce unique temperature-dependent
response
signatures.
Recent experiments with integrated circuits containing
the microhotplate sensors have shown sensitivity to
test chemicals—methanol,
ethanol, and acetone—at the 100-parts-per-billion level.
These chips were made at a chip “foundry” with CMOS-compatible
technology, demonstrating that the current research design, at
least, can be inexpensively manufactured. By integrating the sensors
onto the same semiconductor substrate as the electronic circuitry,
the signal-to-noise ratio for the sensors is enhanced significantly.
Such integration also makes it possible to include self-calibrating
circuits and chemical recognition processors on the same chip.
In
the explosives detection area, following initial feasibility
experiments at NIST, a research
license was issued to the University of Massachusetts/Lowell.
The UMASS team
now is investigating whether
minute quantities of vapors from explosive materials can be detected
reliably using microhotplate platforms.
In addition, NIST chemists have extensive experience
in both the forensic detection of gunpowder and
other explosive residues and in the development
of “lab
on a chip” techniques using microfluidic samples. These capabilities have
the potential to be combined in a single integrated circuit with the microhotplate
sensors in the future for multipurpose detection of both chemical and biological
threats.
The micromachined platforms used in the microsensors are patented
and have been refined over the last decade. Prototype sensors
are now produced
on
a regular
basis.
Further research is needed to determine selectivity for a variety
of toxic agents. In addition, issues such as determining
the rate of false
positives
and robustness
of the sensors with repeated use need further study.
NIST is interested in partnering with a manufacturer to continue
the needed scientific and engineering work required to
move this technology
from the
research lab to
the marketplace. Ultimately, the goal is to license the
technology for production of inexpensive, multi-array detectors
that
could be deployed
in a wide range
of settings from subway stations to concert halls, from
factories to office buildings. Just as a network of heat
or smoke detectors
warns
occupants
of a potential fire,
these chemical sensors then could warn occupants of potentially
toxic agents in the air.
The ability to tune the sensors to specific chemicals of interest
also has attracted interest from the National Aeronautics and Space
Administration for monitoring the safety of the air inside spacecraft
and for planetary environmental monitoring. The Department of Energy
has expressed interest in the technology for monitoring levels of
hazardous organic chemicals used in the disposal of radioactive
waste.
Partial list of relevant patents:
Temperature-controlled Micromachined Arrays for Chemical
Sensor Fabrication and Operation, Patent # 5,345,213.
Micro-hotplate Devices and Methods for Their Fabrication,
Patent # 5,464,966.
Micron-scale Differential Calorimeter on a Chip,
Patent # 6,079,873.
Method for Operating a Sensor to Differentiate
Between Analytes in a Sample, Patent # 6,095,681.
For further information contact the NIST Office
of Technology Partnerships, (301) 975-3084.
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