Those who work with hazardous substances could find an extra measure of protection in an optical sensor that puffs up in the presence of even trace amounts of toxic gases. Made from a 400-nm-thick film of polymethyl methacrylate (PMMA), the sensors are infused with fluorescent dye and patterned with 1-D photonic crystal cavities suspended in air. High-sensitivity detection of dilute gases is demonstrated by monitoring the resonance of a suspended polymer nanocavity. The inset shows the target gas molecules (darker) interacting with the polymer material (lighter). This interaction causes the nanocavity to swell, resulting in a shift of its resonance. Courtesy of Hannah Clevenson, MIT. Certain gases cause the PMMA to swell, changing the optical resonance of the cavities and producing minute color changes in the polymer detectable by a spectral filter. Researchers at MIT said they achieved an experimental sensitivity of 10 ppm and predict detection levels in the parts-per-billion range for a variety of gases. “Because of their deformation in response to biochemical substances, cavity sensors made entirely of this polymer lead to a sensor with faster response and much higher sensitivity," said Hannah Clevenson, an MIT doctoral student who led the study. Isopropyl alcohol vapor was used in the laboratory, but the researchers said the sensor can detect any gas that interacts with PMMA. The polymer shrinks back to its original size once the targeted gas has been removed, meaning the sensors can be reused, the researchers said. Optical sensors are suited to detecting trace gas concentrations due to their high signal-to-noise ratio, compact, lightweight nature and immunity to electromagnetic interference, the researchers said. Clevenson said potential applications for the sensor range “from industrial sensing in large chemical plants for safety applications, to environmental sensing out in the field, to homeland security applications for detecting toxic gases, to medical settings, where the polymer could be treated for specific antibodies.” The work was published in Applied Physics Letters (doi: 10.1063/1.4879735).