A gem of a breathalyzer

Among the dizzying assortment of chemical sensors are those based on photonic crystals—regular arrays of materials with different refractive indexes—with their well-defined physical properties and good sensitivity. Gem-quality opals are naturally occurring photonic crystals made of closely packed, tiny spheres of hydrated silica. An opal’s shimmering range of colors arises from the interference and diffraction of light passing through its microarrays. Synthetic opals made with nanospheres of silica or polystyrene and infiltrated with a chemically active hydrogel make for good, inexpensive chemical sensors; as the hydrogel reacts, it swells or shrinks, which changes the spacing of the crystal and thereby shifts the crystal’s Bragg diffraction peak. A team of researchers at the University of Palermo in Italy has now engineered a hydrogel that swells to a baseline volume when infused with water vapor, and swells significantly more when ethanol vapor is present. What’s more, when the hydrogel infiltrates a synthetic opal film (shown here), the additional swelling shifts the Bragg peak linearly with respect to the concentration of ethanol, producing a calibrated color change. As a bonus, the device can be reused many times just by letting the ethanol evaporate. The downside of the prototype device is that the full response took about 47 minutes. However, the researchers expect that removing the polystyrene will speed things up significantly, because then the spheres won’t impede ethanol’s diffusion. (R. Pernice et al., Opt. Mater. Exp. 3, 1820, 2013, doi:10.1364/OME.3.001820 .)