The MIT researchers' wireless chemical sensor. (Image Credit: MIT; Photo: Melanie Gonick)
MIT chemists have devised a new way to wirelessly detect hazardous gases and environmental pollutants, using a simple sensor that can be read by a smartphone. 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. You put them up, they sit there, and then you come around and read them. There’s no wiring involved. There’s no power,” says Timothy Swager, the John D. MacArthur Professor of Chemistry at MIT. 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. The new sensors are made from modified near-field communication (NFC) tags. These tags, which receive the little power they need from the device reading them, function as wirelessly addressable barcodes and are mainly used for tracking products such as cars or pharmaceuticals as they move through a supply chain, such as in a manufacturing plant or warehouse.
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 (13.56 megahertz), 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 specialized to detect a particular gas. In this case, the researchers added the carbon nanotubes by “drawing” them onto 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.