Design and Study of a Plastic Optical Fibre-Based Evanescent-Field Biosensor

Abstract: A refractive-index (RI) sensor based on the evanescent-field principle of a declad plastic optical fibre (POF) was designed and experimentally characterized. A section of the polymer cladding of a step-index POF (core RI 1.49, numerical aperture 0.5, 650 μm core diameter, 20 μm cladding thickness) was removed by an acetone/distilled-water de cladding procedure to expose the fibre core to the surrounding medium, converting the medium itself into an effective cladding. Aqueous sodium chloride (NaCl) solutions of increasing weight/volume concentration (0–0.20 %, RI 1.332– 1.361) were used as surrogate analytes, chosen because they reproduce the intensity response obtained when a biofilm of Pseudomonas aeruginosa is grown on the exposed core. White light was launched into the fibre and the transmitted spectrum was recorded on a spectrograph interfaced with Avantes acquisition software. The transmitted light intensity was found to increase monotonically with the refractive index of the surrounding solution, and the sensitivity, defined as the change in intensity per unit change in refractive index, reached a maximum of 3.4 × 10⁵ counts/RIU between RI 1.350 and 1.360. The results confirm that a simply prepared, low-cost declad-POF probe can operate as a sensitive intrinsic evanescent-field transducer and provide a viable low-cost platform for optical bio-sensing applications.

Keywords: Plastic optical fibre; Evanescent-field sensor; Refractive-index sensing; Biosensor; Total internal reflection; Decladding.