A sensing device tailored for mass production of highly
sensitive and stable nerve-gas detectors has been
developed by a research group led by a mechanical
engineer at The University of Texas at Austin.
The new sensor technology, which was more sensitive
and much more stable than its predecessors, was featured
on this week’s cover of Applied Physics Letters. The
researchers’ highlighted study demonstrated the sensor’s
potential ability to detect a single molecule of the
nerve gas, sarin, the most toxic of biological warfare
The researchers, led by Dr. Li Shi, designed and tested
a nanometer-thin crystal of tin oxide sandwiched between
two electrodes. When a built-in micro-heater heated
the super-thin device, the tin oxide reacted with
exquisite sensitivity to gases.
Shi’s group experimented with a non-toxic gas, dimethly
methylphosphonate (DMMP) widely used to accurately
mimic sarin and other nerve agents. The sensor element
responded to as few as about 50 molecules of the DMMP
in a billion air molecules.
Both the nano-sizing of the metal-oxide and the unique
micro-heater element of the sensor gave the detector
its high sensitivity, stability and low power consumption,
said Shi, assistant professor of mechanical engineering.
The thinner a metal-oxide sensor becomes, the more
sensitive it becomes to molecules that react with
it. In addition to improved sensitivity, the group
found its single-crystal metal-oxide nanomaterials
allowed the detector to quickly dispose of previously
detected toxins and accurately warn of new toxins’
Shi’s engineering collaborator, Zhong Lin Wang from
Georgia Institute of Technology, provided single crystals
of tin oxide as thin as 10 nanometers, and with the
ability to rapidly recover from chemical exposure.
The researchers found that the sensor was refreshed
immediately after the DMMP molecules were purged from
the small flow-through chamber where the sensor element
was tested in. By contrast, previous polycrystalline
metal oxide thin film sensors could not recover automatically
after being exposed to toxic or flammable gases, an
effect known as sensor poisoning.
Co-author and collaborator Wang is the first to grow
the ribbon-like, single crystals of tin oxide used
for sensing DMMP. Other sensors of this type consist
of crystals with many imperfections, and recover slowly
because molecules previously detected can become trapped
in these imperfections.
Shi constructed the accompanying sensor components
using traditional computer chip design and fabrication
techniques. Specifically, he used microelectromechanical
systems (MEMS) fabrication methods.
For instance, MEMS was used to fabricate the platinum
electrodes, one of which links to a microfabricated
heating element and thermometer to elevate the nano
sensor’s temperature to a constant 932 degrees Fahrenheit
(500 degrees Celsius) with a power consumption of
only 3-4 milliwatts. These components allow the sensor
to be operated using a battery so that it can be used
as a wearable device. To minimize heat loss, Shi’s
group isolated the silicon nitride membranes attaching
the electrodes using trapeze-like strands of microfabricated
The sensor requires the high temperature to activate
the reaction between DMMP molecules and the tin-oxide
sensor element. That reaction changes the electrical
current across the crystal, which indicates a nerve
agent is present.
The paper was based on the dissertation research of
the lead author, Choongho Yu. Yu received a doctor’s
degree in mechanical engineering from The University
of Texas at Austin last year, and is a post-doctoral
fellow at Lawrence Berkeley National Laboratory.
Shi’s group is continuing to develop methods to integrate
nanomaterials with MEMS devices more efficiently in
order to microfabricate better, lower-cost sensors.
Multiple sensor elements would then be packaged together
to produce a commercial sensing device that acts as
an electronic nose for detecting different toxic and
UT's College of Engineering:
The University of Texas College of Engineering
ranks among the top ten engineering schools in the
With the nation's third highest percentage of faculty
elected members of the National Academy of Engineering,
the College's 6,500 students gain exposure to the
nation's finest engineering practitioners.
Appropriately, the College's logo, an embellished
checkmark used by the first UT engineering dean to
denote high quality student work, is the nation's
oldest quality symbol. The College maintains a web
site at http://www.engr.utexas.edu