Low-cost fiber optic H2S gas sensor

In this paper the realization and characterization of a new plastic optical fiber sensor for the detection of H₂S and other sulfide compounds is described. The sensor is specifically tailored for applications in cultural heritage preservation, where the capability to evaluate the total exposure even to small quantities of pollutants is required. The substitution of plastic for glass optical fibers accounts for the possibility of having all the advantages of a fiber-based device, while lowering the cost of the final device and furthermore improving the sensitivity for evanescent field sensing. The transducer exploits the variation of the optical absorption of Ag when exposed to the sulfide atmosphere and is fabricated by a preliminary chemical etching of the cladding in ethyl-acetate, followed by deposition of the Ag thin film by plasma sputtering. Some sensors prototypes have been developed and tested in a climatic chamber to assess their response in the presence of sulfide vapors. In addition chemical and morphological characterization of the plastic fibers have been carried out by means of infrared spectroscopy and scanning electron microscopy. The prototypes exposed to H₂S have shown a very fast response achieving a 90% decay in the transmitted power in 900 seconds upon exposure to a 0.1 M H₂S solution; such a decay has been found to be nearly wavelength independent, and is correlated to the transformation of the metallic Ag into Ag₂S.