Ion adsorption in the porous sol–gel silica layer in the fibre optic pH sensor

Abstract

In the paper, the pH optical fibre sensor without any pH-sensitive dye is described. Porous silica film made by means of the sol–gel method was cladded onto optical fibre core and was exploited as the optical transducer. Acid–base properties of silica surface cause that the surface charge of silica changes with pH of solution. Negative surface charge (at pH higher than point of zero charge) of the silica surface is compensated by adsorption of cations. Saturation of the film with cations leads to an increase of the electron density of the film, hence, the refractive index of the film. Since the surface charge of silica depends on pH, the refractive index of the silica film varies also with pH. Thus, changes of the refractive index of the film, which plays role of the fibre cladding, modifies conditions of light propagation within the optical fibre. These phenomena are exploited in the U-shaped plastic clad silica fibre (core diameter 200 μm), in which 10 cm of original cladding was replaced with the sol–gel film. It is shown that the sol–gel film can be successfully used as the optical transducer with sensitivity up to 0.66 dB/pH for pH range 7–10.5. Moreover, the influence of the film preparation conditions on the sensor sensitivity and response time is also investigated.