The process for producing CNT sensors.A team from MIT has produced a cheap, lightweight wireless sensor that can detect toxic chemical agents. The new sensors, which are made from chemically altered carbon nanotubes (CNTs), can be easily integrated into wireless devices such as light radio-frequency identification (RFID) badges or smartphones to identify trace amounts of hazardous gases or chemical weapons on the battlefield or protect those working with dangerous chemicals.
The sensor comprises a circuit containing many thousand single-walled CNTs covered in an insulating material to maintain them in a highly resistive state. On being exposed to certain toxic gases, the insulating material comes apart allowing the CNTs to become much more conductive, transmitting a readable signal that can be picked up by near-field communication technology.
As reported in the Journal of the American Chemical Society [Ishihara et al. J. Am. Chem. Soc. (2016) DOI: 10.1021/jacs.6b03869], the sensors are highly sensitive to “electrophilic” chemical substances, which are often toxic and used for chemical weapons, aided by a new metallo-supramolecular polymer made of metals binding to polymer chains. The polymer insulates, wrapping each sensor's CNTs, keeping them apart and highly resistant to electricity. However, such electrophilic substances provoke the polymer into disassembling so that the CNTs re-unite and improve conductivity.
“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”Timothy Swager
The team drop-cast the nanotube/polymer material onto gold electrodes, before exposing the electrodes to diethyl chlorophosphate, a reactive simulant of nerve gas, observing a 2,000% increase in electrical conductivity after only five seconds of exposure. Similar increases in conductivity were shown for trace amounts of a range of electrophilic substances. Although the polymer material has keep the CNTs apart, as it disassembles the individual monomers have to interact in a weaker way to allow the nanotubes to regroup.
The team created an NFC tag that can turn on when its electrical resistance goes below a certain threshold. With smartphones emitting short pulses of electromagnetic fields that resonate with an NFC tag at radio frequency, this induces an electric current that transmits information to the phone. As lead author Timothy Swager points out, “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”.
This type of wireless system could also be used to detect leaks in lithium thionyl chloride batteries, which are used in medical instruments, fire alarms and military systems, and the team are now looking to test the sensors on live chemical agents outside of the lab, as they are more dispersed and difficult to detect.