Chemical vapour sensing by surface plasmon resonance optical fibre sensor coated with fluoropolymer

A surface plasmon resonance fibre optic sensor has been developed allowing sensitive remote detection. A 50 nm thick silver film is deposited by thermal evaporation onto the silica core of the optical fibre. To protect silver from oxidation, self-assembled monolayers (SAM) of long-chain alkanethiols were used. For detecting gas and vapours, the surrounding dielectric medium consists of a polymeric material deposited on the modified metallic film. Chlorinated and aromatic compounds have been tested with detection limits of 0.3%, 0.7%, 1% and 2% for trichloroethylene, carbon tetrachloride, chloroform and methylene chloride, respectively, and 0.13%, 0.19% and 0.95% for xylene, toluene and benzene, respectively. For aromatic compounds, the response of the sensor depends on the boiling temperature, whereas for chlorinated compounds it depends on the difference between the refractive index of the analyte and specific polymeric cladding. Moreover, both the response time (about 2 min) and the desorption time (about 2.5 min) have been obtained with good reproducibility.