Case Study

Polymer-Coated Optical Sensor Offers Improved Explosives Detection

Published: April 20, 2010

Click for article gallery (3 images).

Detecting explosives is important for countering terrorism, locating buried land mines and aiding environmental protection efforts. The most common explosive is TNT - technically known as 2,4,6-trinitrotoluene - and detecting trace amounts of TNT requires a very sensitive sensor.

Researchers at Georgia Tech and the College of Wooster in Ohio have designed and tested a polymer film-coated optical sensing device that is capable of selectively and sensitively detecting the presence of TNT in the parts-per-trillion range.

"This sensor has great potential for use in real-world applications because of its selectivity, low limits of detection, and insensitivity to water vapor," said Kenneth Johnson, a GTRI senior research scientist.

Details on the polymer film and sensor used to detect TNT were published in January 2010 in the journal Sensors and Actuators B: Chemical. This research is supported by the National Science Foundation.

The new explosives detection device is composed of an inorganic polymer film deposited on the surface of an optical sensor. The sensor utilizes the interference of light waves, a concept called interferometry, to precisely determine how much TNT is present in a sample.

In operation, light from a laser diode is coupled into an optical waveguide through a grating and travels under two channels that comprise the interferometer. One channel interacts with the sensing film and the other serves as a reference.

The reference channel minimizes the response of any non-specific interactions, such as temperature and mechanical motion. In addition, since chemicals other than TNT interact with both channels of the interferometer equally, their presence is cancelled out. This allows researchers to select TNT out of a mixture of chemicals that may be present in a sample.

At the end of the waveguide, the light beams from the sensing and reference channels are combined to create an interference pattern. The pattern of alternating dark and light vertical stripes, or fringes, is imaged on a simple CCD detector. By doing a mathematical Fourier transform, the researchers determine the degree to which the fringe patterns are in or out of step with each other, known as phase shift. This phase shift indicates the amount of TNT adsorbed to the film.

The sensor technology is currently licensed to NuWave Sensors, LLC, a Georgia Tech Advanced Technology Development Center (ATDC) company.

Paul Edmiston, an associate professor of chemistry at the College of Wooster, created a wide range of films designed around the chemical structure of the TNT molecule, a process called molecular imprinting. A TNT analogue was introduced during the film preparation process, but was removed after the film was deposited on the surface of the sensor. That created a pocket in the film capable of recognizing and accepting a TNT molecule.

A series of imprinted films were tested for their ability to adsorb and distinguish TNT from other chemicals. The films were evaluated for their sensitivity, reversibility and response to environmental changes such as humidity. Experiments showed that the film able to most sensitively detect TNT was a film derived from bis(triethoxysilyl)benzene (BTB). It was able to detect TNT vapor in the low parts-per-trillion range.

The BTB-derived films were responsive to TNT over a wide range of humidity and showed good selectivity for TNT even when chemicals similar to TNT were present. However, the response to TNT by BTB films was only 70 percent reversible.

"It's a real challenge to develop film surface chemistry that is reversible, meaning that once the chemical of interest goes away, the system recovers and is ready for the next detection event," said Daniel P. Campbell, a GTRI principal research scientist. "We will continue to work on creating a 100 percent reversible chemistry that provides a continuous real-time direct measurement with no additional steps or consumable reagents."

The research team is also working to add acoustic agitation to the sensor for complete explosives detection.

"Explosives like TNT need to be dislodged from whatever surface they are located on so that they can be detected," explained Jayme Caspall, a research engineer in the Woodruff School of Mechanical Engineering at Georgia Tech. "We are hitting objects with an ultrasound beam to generate a vibration on the surface of the object that will dislodge the particles and allow us to detect them with our sensor."

This device is not limited to detecting TNT. By altering the chemistry of the polymer film, the optical sensor can detect a wide variety of chemical and biological entities including organic and inorganic compounds, proteins, toxins, viruses, and whole organisms. Fabricating multiple interferometers on a single sensor chip allows multiple contaminants to be detected simultaneously. A single laser and detector can be used to illuminate and detect the signals from a dozen or more sensing channels.

The research team is currently developing polymer films capable of detecting explosives, chemical warfare toxins, biological agents, narcotics, chemical compounds, and groundwater contaminants. They are designing systems for mobile, on-site field analysis with instant results and monitoring with automatic data logging and communication to a base monitoring station.

Nanotechnology Research Center senior research scientist David Gottfried also contributed to this work.

This work was funded by grant number CBET-0804261 from the National Science Foundation (NSF). The content is solely the responsibility of the principal investigator and does not necessarily represent the official views of the NSF.