Web Date: May 9, 2012
A Flexible Nerve-Gas Sensor
A chemical sensor based on polymer nanostructures can detect nerve gas at concentrations as low as 10 parts per trillion (Nano Lett., DOI: 10.1021/nl204587t). With further development, the flexible sensor materials might enable wearable systems for detecting chemical weapons, the researchers say.
Currently, soldiers and police use mass spectroscopy-based devices to detect organophospates, the group of compounds that includes the nerve gas sarin. Jyongsik Jang, a polymer scientist at Seoul National University, says sensors based on nanostructured polymers would be less expensive and more sensitive, while also being lightweight and flexible enough to make a wearable device built on plastic or even fabric.
Jang’s sensors use the inexpensive conductive polymer poly(3,4-ethylenedioxythiophene). When chemists add hydroxyl groups to PEDOT’s sidechains, the polymer can interact with organophosphates via hydrogen bonds. This interaction changes the polymer’s electrical resistance, which simple electronics can easily measure. The more surface area a PEDOT sensor has to interact with gases in the environment, the stronger the response, Jang’s team reasoned. Based on that idea, they wanted to make hydroxylated PEDOT nanostructures to maximize surface area, and in turn produce ultrasensitive sensors.
They started by electrospinning mats of the polymer to make nanotubes. Then the scientists used a vapor-deposition process to coat the tubes’ surfaces with nanosized nodules or rods. The nanorod-coated tubes have twice the surface area of untreated tubes. The team made resistors out of mats of these tubes and then placed them between two wires on a plastic sheet to make a flexible sensing device.
To test the sensors in the lab, the researchers used dimethyl methylphosphonate, a standard gas used in the laboratory as a stand-in for sarin, which is classified by the United Nations as a weapon of mass destruction. The tubes coated with nanorods performed the best, demonstrating measurable changes in resistance at concentrations as low as 10 parts per trillion. This detection limit is two to three orders of magnitude more sensitive than previously reported sensors, says Jang. His team is now developing a wearable device that contains the sensor, its power source, and all the other necessary parts.
One advantage of these sensors, says Paul Rhodes, a team manager at chemical-sensor company Nanosense, is that the sensors can be used continuously, because the gas molecules don’t stay bound to the polymer for long. Other proposed detection methods involve the gas binding irreversibly to a detector. However, he says he would like to see more evidence that the sensors are specific for organophosphate gases. “You can’t freak out that you’ve got nerve gas if someone has mopped the floor with ammonia,” he says.
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