Metal oxides based fiber optic pH sensors for elevated temperature sensing applications

Metal oxides based materials are often employed for harsh environment applications since they tend to be stable at high-temperature and high-pressure conditions. In this work, TiO₂ and ZrO₂ based materials coated fiber optic pH sensors were evaluated with respect to pH sensitivity and stability by measuring the pH-dependent transmission at elevated temperature and high pH range, which are relevant to the wellbore cement monitoring. The TiO₂ thin film coated fiber optic sensor showed reversible pH sensitivity during the pH cycling between DI water and pH 12 at room temperature and 80 °C. It demonstrated improved stability and reversibility as compared to the SiO₂ or ZrO₂ based materials at 80 °C. Au-nanoparticles incorporated TiO2 coating was showed to maintain the pH sensing capability for about 30 hours at 80 °C. This finding is beneficial for future development and deployment of robust distributed optical pH sensors for harsh environment applications in the wellbores.