The last thing anyone wants nearby when there’s a hydrogen gas leak is a hot object with a lot of electrical connections. But the electronic H2 sensors widely used in the chemical industry typically operate at temperatures between 100 and 300 °C and are hard-wired into electronic displays. Bernard Dam and Peter Ngene of Delft University of Technology, in the Netherlands, and coworkers have developed an optical sensor that more safely indicates the presence of H2 with a simple color change that’s visible to the naked eye (Adv. Funct. Mater. 2013, DOI: 10.1002/adfm.201303065). The device is built with thin films of yttrium and palladium. Upon absorption of H2, the nanocrystalline yttrium layer becomes transparent while the underlying palladium layer remains metallic and reflective. Refraction of light by the different yttrium-hydrogen adducts that form changes the sensor color to indicate the amount of H2 present. Because the sensor is selective and sensitive even in the presence of water and oxygen, which bedevil conventional H2 sensors, it could also be used to detect H2 in human breath, which can indicate malabsorption of lactose or sucrose in the intestines during digestion.