Thymol blue immobilized on tapered fibres as an optical transducer for pH sensing

The present work is concerned with the optical characterization of an evanescent wave sensor for pH detection. First, the interaction between the solution containing the acid-base indicator and the fibre core was investigated. Then, the acid-base indicator, thymol blue, was covalently immobilized on the core of a 200/380 itm fibre by means of a silylation process of the glass surface. The fibre core surface was modified along a section of 8 mm. A comparison was made using both bare and tapered fibres, with a tapering ratio (fibre diameter/waist diameter) of 2.3. An enhancement in sensitivity of a factor 6 was observed with tapered fibres in thel÷2.5 range, and a sensitivity of 0.05 pH units was attained.