Study of a new evanescent wave optical fibre sensor for methane detection based on cryptophane molecules

An evanescent fibre sensor for detection of methane was fabricated by the inclusion of two types of cryptophane molecules in a transparent polymeric cladding (polysiloxane), deposited on a PCS fibre. Synthesis of crytophanes A and E are described as well as the preparation of the specific polymeric cladding, its deposition on the uncladded part of the optical fibre, the optimal thickness being 5 μm and the optimal length of uncladded part being 5 cm. By modelling the curves P(α)—normalised light power transmitted versus injection angle—it was shown that the specific absorption of methane in cryptophane A led to an increase of the refractive index of the specific cladding. Detection limit for methane is 2% (v/v) with cryptophane A and 6% (v/v) with cryptophane E. When alkane concentrations (methane, ethane, butane) are less than 8% (v/v), only methane leads to an optical response. For concentrations of alkanes higher than 15% (v/v), a strong effect of condensation of butane in the polymer itself is observed.