These inexpensive sensors could be widely deployed, making it easier to monitor public spaces or detect food spoilage in warehouses. Using this system, the researchers have demonstrated that they can detect gaseous ammonia, hydrogen peroxide, and cyclohexanone, among other gases.
“The beauty of these sensors is that they are really cheap," says MIT's Professor Timothy Swager. "You put them up, they sit there, and then you come around and read them. There’s no wiring involved. There’s no power. You can get quite imaginative as to what you might want to do with a technology like this.”
For several years, Swager’s lab has been developing gas-detecting sensors based on devices known as chemiresistors, which consist of simple electrical circuits modified so that their resistance changes when exposed to a particular chemical. Measuring that change in resistance reveals whether the target gas is present.
Unlike commercially available chemiresistors, the sensors developed in Swager’s lab require almost no energy and can function at ambient temperatures. They are made from modified near-field communication (NFC) tags which receive the little power they need from the device reading them.
NFC tags can be read by any smartphone that has near-field communication capability, which is included in many newer smartphone models. These phones can send out short pulses of magnetic fields at radio frequency (13.56MHz), inducing an electric current in the circuit on the tag, which relays information to the phone.
To adapt these tags for their own purposes, the MIT team first disrupted the electronic circuit by punching a hole in it. Then, they reconnected the circuit with a linker made of carbon nanotubes that are specialised to detect a particular gas. In this case, the researchers added the carbon nanotubes by 'drawing' them on the tag with a mechanical pencil they first created in 2012, in which the usual pencil lead is replaced with a compressed powder of carbon nanotubes. The team refers to the modified tags as CARDs (chemically actuated resonant devices).
When carbon nanotubes bind to the target gas, their ability to conduct electricity changes, which shifts the radio frequencies at which power can be transferred to the device. When a smartphone pings the CARD, the CARD responds only if it can receive sufficient power at the smartphone-transmitted radio frequencies, allowing the phone to determine whether the circuit has been altered and that, therefore, the gas is present.
Current versions of the CARDs can each detect only one type of gas, but a phone can read multiple CARDs to get input on many different gases, down to concentrations of parts per million. With the current version of the technology, the phone must be within 5cm of the CARD to get a reading, but the researchers are currently working with Bluetooth technology to expand the range.