New wireless, wearable sensors can detect toxic gases

MIT researchers have developed new low-cost chemical sensors that can enable smartphones or other wireless devices to detect trace amounts of toxic gases.
Using the sensors, researchers hope to design lightweight, inexpensive radio-frequency identification (RFID) badges to be used for personal safety and security.
Such badges could be worn by soldiers on the battlefield to rapidly detect the presence of chemical weapons - such as nerve gas or choking agents - and by people who work around hazardous chemicals prone to leakage, researchers said.
"Soldiers have all this extra equipment that ends up weighing way too much and they cannot sustain it," said Timothy Swager from Massachusetts Institute of Technology (MIT) in the US.

"We have something that would weigh less than a credit card. And soldiers already have wireless technologies with them, so it is something that can be readily integrated into a soldier's uniform that can give them a protective capacity," said Swager.
The sensor is a circuit loaded with carbon nanotubes, which are normally highly conductive but have been wrapped in an insulating material that keeps them in a highly resistive state, researchers said.

When exposed to certain toxic gases, the insulating material breaks apart, and the nanotubes become significantly more conductive, they said.
This sends a signal that is readable by a smartphone with near-field communication (NFC) technology, which allows devices to transmit data over short distances.

The sensors are sensitive enough to detect less than 10 parts per million of target toxic gases in about five seconds.
"We are matching what you could do with benchtop laboratory equipment, such as gas chromatographs and spectrometers, that is far more expensive and requires skilled operators to use," said Swager.
"You really cannot make anything cheaper. That is a way of getting distributed sensing into many people's hands," he said.
To build their wireless system, researchers created an NFC tag that turns on when its electrical resistance dips below a certain threshold.
Smartphones send out short pulses of electromagnetic fields that resonate with an NFC tag at radio frequency, inducing an electric current, which relays information to the phone, researchers said.

When exposed to 10 parts per million of thionyl chloride (SOCl2) for five seconds, the material's resistance dropped to the point that the smartphone could ping the tag, they said.
Basically, it is an "on/off indicator" to determine if toxic gas is present, said Swager.
According to researchers, such a wireless system could be used to detect leaks in Li-SOCl2 (lithium thionyl chloride) batteries, which are used in medical instruments, fire alarms, and military systems.
The findings were published in the Journal of the American Chemical Society.