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. The inexpensive sensors could be widely deployed, making it easier to monitor public spaces or detect food spoilage in warehouses. The researchers have demonstrated that they can detect gaseous ammonia, hydrogen peroxide, and cyclohexanone, among other gases. For several years, the lab of professor Timothy Swager has been developing gas-detecting sensors based on chemiresistors, which consist of 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, these sensors require almost no energy and can function at ambient temperatures. 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. To adapt these tags for their own purposes, the MIT team disrupted the electronic circuit and then reconnected the circuit with a linker made of carbon nanotubes that are specialized to detect a particular gas. The team refers to the modified tags as CARDs: chemically actuated resonant devices.