pH sensor based on a detection sol–gel layer onto optical fiber

A pH sensor was made via the deposition of detection layer onto an optical fiber. This layer contains a pH-sensitive dye [thymol blue (TB)] trapped within a sol–gel silica matrix. The light was launched directly at the front of the fiber with an angle chosen in order to increase the power transmitted by the evanescent wave. This technique was used to induce the absorption of the dye. TB can be used both as an acid or a base titration indicator. However, the response of the optical device in acid conditions is rather poor and only the alkaline transition in the pH range 8–12 has been exploited. The dye appears to be firmly trapped within the silica matrix and no leaching is observed during repetitive experiments.