Development of a quasi-distributed optical fibre pH sensor using a covalently bound indicator

A quasi-distributed pH sensing system was developed to utilize information derived from evanescently excited fluorescence signals originating from sen sor sites fabricated on an optical fibre. The sensor sites comprise a secti on of fibre core exposed by polishing upon which is laid down a sensing fil m. The film comprises a fluorescent indicator dye immobilized covalently wi thin a hydrogel matrix which is then photopolymerized and attached covalent ly to the exposed fibre core. Position information is determined from the p ropagation delay of the returning signals. A system comprising eight sensor s spaced at 10 in intervals along an optical fibre has been constructed and characterized. The sensors operate in the region pH 6-pH 8 with a response time of 500 s. The properties of the immobilized indicator dye are found t o be similar to those of the dye in aqueous solution. Furthermore, the sens ing films so created are found to be resistant to dye leaching.