Distributed fiber optic pH sensors using sol-gel silica based sensitive materials

Abstract

• Fiber optic pH sensors demonstrated repeatable and reversible pH sensing responses in the pH range of ∼8–13. • 1.6 um thick SiO 2 and 600 nm thick Au-SiO 2 coatings possessed good pH sensitivity with fast responses. • The developed fiber optic sensors are highly selective to pH compared to ionic strength and refractive index of solutions. • Distributed pH sensing was demonstrated using SiO 2 and Au-SiO 2 coated fiber optic sensors. Fiber optic pH sensors using either silica (SiO 2 ) or gold nanoparticle incorporated silica (Au-SiO 2 ) as the sensitive layers for pH monitoring are presented. A facile sol-gel dip-coating process was utilized to immobilize the SiO 2 based sensitive layers on the coreless fiber. In the high pH range of ∼8−12 simulating the wellbore cement conditions, the transmission spectra at room temperature demonstrated notable sensitivity using the SiO 2 based coating. The sensitivity was enhanced via either increasing the coating thickness or incorporation of Au nanoparticles. The sensitivity was 19.9 T%/pH for the 1.6 μm thick SiO 2 coating and 13.4 T%/pH for the 600 nm thick Au-SiO 2 coating, respectively. And these two sensors revealed good reversibility, and the response times were in the order of 10 s of seconds. No obvious chemical structure changes in SiO 2 thin film were observed through Fourier-transform infrared spectroscopy (FTIR) analysis after the film was exposed to the alkaline media for 30 min. However, the sensing layers were found to become thinner due to corrosion when tested in strong alkaline solutions with the pH up to 14 for 3 days, especially for the pure SiO 2 coating. The incorporation of Au nanoparticles was found to stabilize the pH sensing signals and prolong the lifetime of the pH sensitive layer. Importantly, with the coating of SiO 2 based sensitive materials, spatially distributed pH sensing was enabled. The intensity of the backscattered light increased with the increasing pH values along the coated segments on the fiber optic sensors.