Simple Sensors

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. With inexpensive sensors, the researchers detected ammonia, hydrogen peroxide, and cyclohexanone, among other gases.

“The beauty of these sensors is that they are really cheap. There’s no wiring involved. There’s no power,” says chemistry professor Timothy Swager, whose group makes the sensors using near-field communication (NFC) tags, wirelessly addressable bar codes typically used to track products. The tags receive the little power they need from NFC-capable smartphones, which send out short pulses of magnetic fields at the radio frequency of 13.56 megahertz, inducing an electric current in the circuit on the tag.

Swager and his team modified NFC tags by replacing part of the circuit with carbon nanotubes that bind to a target gas. That turned them into chemiresistors—simple electrical circuits whose resistance changes when they’re exposed to a particular chemical, thus shifting the radio frequency at which power can be transferred to the device. When a smartphone pings the tag, the tag responds only if it can receive sufficient power at the smartphone-transmitted radio frequency, allowing the phone to determine whether the target gas is present. Current sensors can each detect only one type of gas, but a phone can read multiple sensors to get input on many different gases, down to concentrations of parts per million.