Chemists at MIT have developed inexpensive chemical sensors that can be read by devices many people carry around: smartphones. Such sensors could be used to detect explosives, pollutants, or spoiled food.
Professor Timothy M. Swager, grad student Joseph M. Azzarelli, and coworkers adapted near-field communication (NFC) tags—simple integrated circuits on plastic substrates—as chemical sensors for selective gas detection (Proc. Natl. Acad. Sci. USA 2014, DOI: 10.1073/pnas.1415403111). The ultra-low-power requirements of their carbon-nanotube-based sensors enable the NFC tags, which are battery-free, to communicate with and be powered by cell phones via radio-frequency pulses.
To make the sensors, the researchers punch a hole in the conductive aluminum of an NFC tag’s circuit, making the tag unreadable. They then recomplete the circuit with carbon-nanotube-based materials designed to respond to specific gas molecules. In the presence of these molecules, the nanotubes change the resistance of the circuit and the resonant frequency of the tag, thus affecting the tag’s ability to communicate with cell phones. “It’s all about switching the resonant frequency in and out of tune with the cell phone,” Swager says.
The team designed each sensor to turn on or off in the presence of a particular gas. For example, one sensor stopped communicating with the cell phone in the presence of 35 ppm of ammonia. Another sensor turned on in the presence of 225 ppm of hydrogen peroxide.
“One of the cool things about using a cell phone to do this is that now you have access to big data,” Swager says. “The readings may not be as meaningful in isolation as when you get thousands of them uploaded to the Web, all with GPS coordinates.”
“The use of NFC tags as sensors is an economical idea because these tags are already mass-marketed,” says Lindsey Fiddes, a researcher at the University of Toronto who is developing wireless sensors. “The sensing tags do not require a line of sight to be read, so tags can be easily incorporated into packaging. I see it being adopted by commercial smart-packaging manufacturers as well as consumers.”