Remote sensing of tetrachloroethene with a micro-fibre optical gas sensor based on surface plasmon resonance spectroscopy

A miniaturized fibre optical sensor based on surface plasmon resonance spectroscopy is investigated in view of the detection of organic solvent vapours, particularly tetrachloroethene. Surface plasmons are excited on a silver coated multimode fibre by polychromatic light, and the resonant excitation is detected as a resonant absorption band in the measured output spectrum. When the analyte is absorbed in a thin gas-sensitive polysiloxane film deposited on the silver layer the polymer film changes its thickness and its refractive index. These changes result in a wavelength shift of the resonant curve depending on the analyte gas concentration. Theoretical considerations about the sensing effect are made and resonance curves were computer-simulated. Based on these simulations the layout of all sensor parameters was optimized. The sensor shows an excellent response to tetrachloroethene with a response time of two seconds and high reproducibility. When using self-assembling monolayers on the silver surface a long-term stability of more than 3 months can be obtained. Very low cross sensitivities of less than 1% to other solvent vapours like acetone and ethanol are obtained, furthermore, the influence of humidity is very low. This miniaturized fibre optical sensor in combination with an easy-to-handle and non-sophisticated measuring and evaluation unit is excellently suitable for the remote sensing of special organic solvent vapours.