Krishna Naishadham, left, and Xiaojuan (Judy) Song display two types of wireless ammonia-sensing prototype devices. Photo: Georgia Tech Photo/Gary Meek |
Researchers at the Georgia Institute of Technology have developed a
prototype wireless sensor capable of detecting trace amounts of a key ingredient
found in many explosives.
The device, which employs carbon nanotubes and is printed on paper or
paper-like material using standard inkjet technology, could be deployed in
large numbers to alert authorities to the presence of explosives, such as improvised
explosive devices (IEDs).
“This prototype represents a significant step toward producing an
integrated wireless system for explosives detection,” says Krishna
Naishadham, a principal research scientist who is leading the work at the
Georgia Tech Research Institute (GTRI). “It incorporates a sensor and a
communications device in a small, low-cost package that could operate almost
anywhere.”
Other types of hazardous gas sensors are based on expensive semiconductor
fabrication and gas chromatography, Naishadham says, and they consume more
power, require human intervention, and typically do not operate at ambient
temperatures. Furthermore, those sensors have not been integrated with communication
devices such as antennas.
The wireless component for communicating the sensor information—a resonant
lightweight antenna—was printed on photographic paper using inkjet techniques
devised by Professor Manos Tentzeris of Georgia Tech’s School of Electrical and
Computer Engineering. Tentzeris is collaborating with Naishadham on development
of the sensing device.
The sensing component, based on functionalized carbon nanotubes (CNTs), has
been fabricated and tested for detection sensitivity by Xiaojuan (Judy) Song, a
GTRI research scientist. The device relies on carbon-nanotube materials
optimized by Song.
A presentation on this sensing technology was given at the IEEE Antennas and
Propagation Symposium (IEEE APS) in Spokane, Wash., by Hoseon Lee, a PhD student in the School of Electrical and Computer Engineering
co-advised by Tentzeris and Naishadham. The paper received the Honorable
Mention Award in the Best Student Paper competition at the symposium.
This is not the first inkjet-printed ammonia sensor that has been integrated
with an antenna on paper, says Tentzeris. His group produced a similar
integrated sensor last year in collaboration with the research group of C.P.
Wong, who is Regents professor and Smithgall Institute Endowed Chair in the School of Materials Science and Engineering at
Georgia Tech.
“The fundamental difference is that this newest CNT sensor possesses
dramatically improved sensitivity to miniscule ammonia concentrations,”
Tentzeris says. “That should enable the first practical applications to
detect trace amounts of hazardous gases in challenging operational environments
using inkjet-printed devices.”
Tentzeris explains that the key to printing components, circuits, and
antennas lies in novel “inks” that contain silver nanoparticles in an
emulsion that can be deposited by the printer at low temperatures—around 100 C.
A process called sonication helps to achieve optimal ink viscosity and
homogeneity, enabling uniform material deposition and permitting maximum
operating effectiveness for paper-based components.
“Ink-jet printing is low-cost and convenient compared to other
technologies such as wet etching,” Tentzeris says. “Using the proper
inks, a printer can be used almost anywhere to produce custom circuits and
components, replacing traditional clean-room approaches.”
Low-cost materials—such as heavy photographic paper or plastics like
polyethylene terephthalate—can be made water resistant to ensure greater
reliability, he adds. Inkjet component printing can also use flexible organic
materials, such as liquid crystal polymer (LCP), which are known for their
robustness and weather resistance. The resulting components are similar in size
to conventional components but can conform and adhere to almost any surface.
Naishadham explains that the same inkjet techniques used to produce RF
components, circuits, and antennas can also be used to deposit the
functionalized carbon nanotubes used for sensing. These nanoscale cylindrical
structures are functionalized by coating them with a conductive polymer that
attracts ammonia, a major ingredient found in many IEDs.
Sonication of the functionalized carbon nanotubes produces a uniform
water-based ink that can be printed side-by-side with RF components and
antennas to produce a compact wireless sensor node.
“The optimized carbon nanotubes are applied as a sensing film, with
specific functionalization designed for a particular gas or analyte,” Song
says. “The GTRI sensor detects trace amounts of ammonia usually found near
explosive devices, and it can also be designed to detect similar gases in
household, health care, and industrial environments at very low concentration
levels.”
The sensor has been designed to detect ammonia in trace amounts—as low as
five parts per million, Naishadham says.
The resulting integrated sensing package can potentially detect the presence
of trace explosive materials at a distance, without endangering human lives.
This approach, called standoff detection, involves the use of RF technology to
identify explosive materials at a relatively safe distance. The GTRI team has
designed the device to send an alert to nearby personnel when it detects
ammonia.
The wireless sensor nodes require relatively low power, which could come
from a number of technologies including thin-film batteries, solar cells, or
power-scavenging and energy-harvesting techniques. In collaboration with
Tentzeris’s and Wong’s groups, GTRI is investigating ways to make the sensor
operate passively, without any power consumption.
“We are focusing on providing standoff detection for those engaged in
military or humanitarian missions and other hazardous situations,”
Naishadham says. “We believe that it will be possible, and cost-effective,
to deploy large numbers of these detectors on vehicles or robots throughout a
military engagement zone.”
