Hydrogen (H₂) can be detected in several ways but the technique chosen must take into account the conditions of its use, other impurities likely to be present and the physical demands upon it. The methods include semiconductor metal oxides, electrochemical methods, pellistors, palladium and optical means. Response time and the threshold limit are important factors. As H₂ becomes more widely used, reliable detectors able to detect hydrogen before it gets to explosive amounts in air (> 4.65 vol.% H₂) are increasingly important.

Now scientists at the University of California, San Diego, U.S.A., have used optical interferometry to detect H₂ using Pd-coated porous Si (H. Lin, T. Gao, J. Fantini and M. J. Sailor, Langmuir, 2004, 20, (12), 5104–5108; doi: 10.1021/1a04974lu). Thin porous Si films were immersion plated with Pd. On exposure to H₂, the wavelength and intensity of their Fabry-Pérot fringes, obtained from the interferometric reflectance spectrum were simultaneously measured. The intensity of the fringes depends on the reflectivity of the Pd/porous Si composite and their wavelength depends on the refractive index of the Pd film. H₂ expands the Pd lattice and this shifts the optical fringes and decreases the intensity of the reflected light.

The set-up used by the researchers gave a detection limit of H₂ at room temperature of ∼ 0.2% (by volume) in nitrogen, with the lowest concentration detected being ∼ 0.17%. The response time was a few seconds. This sensor design reliably detects H₂ concentrations well below the explosive limit. The sensor is safe, sensitive, selective, reproducible and can operate at room temperature. However, as CO impedes the adsorption and desorption of H₂, the response time is longer, if CO is present.