Development of fibre-optic pH sensor based on silver nanoparticle embedded in silica matrix for high temperature/pressure conditions

Quantifying hydrogen ion concentration of a solution (or pH) has important applications in different industries such as cosmetics, pharmaceuticals, textile, food and beverage, etc. However, it is challenging to maintain and detect accurately pH under conditions of high temperature and pressure (HTHP). At the moment, there is no reliable instrument that is capable of measuring pH at temperatures exceeding 100 oC in oil wells, or at elevated pressure and in the presence of aggressive chemical species.

Recently, surface plasmon resonance property of gold nanoparticles (Np) have been coated on optical fibres to determine the pH of solutions at different temperatures and pressures [1], [2]. The use of silver nanoparticles (AgNp) on fibre optic as a pH sensor in the HTHP environment, however, is yet to be reported. This study focuses on developing a coating of AgNp embedded in silica (SiO2) on optical fibres to be used as a pH sensor in the HTHP environment. In order for this system to be useful in harsh environments, it is expected to have excellent optical and adhesive properties, durability and sensitivity.

Silver nanoparticles embedded in silica was prepared by adding tetraethoxysilane (TEOS) to ethanol, water and silver nitrate, and the pH of the silver nitrate solution was adjusted to 2 using nitric acid. The solution was left to evaporate for gel formation. Glass slides were then dipped into the gel and dried in an oven at 300oC where silver ions reduced to form AgNp. Characterization of the coating using UV/Visible spectrophotometer indicated the formation of silver nanoparticles showing an absorbance peak around 400 nm, which is consistent with what is reported in the literature [3], [4].