Influence of organic pH dyes on the structural and optical characteristics of silica nanostructured matrix for fiber optic sensing

Owing to the fast responsive fiber optical pH sensing characteristics, silica porous matrix is synthesized through a low temperature sol-gel technique. Three different pH dyes namely bromophenol blue (BPB), creosol red (CR) and phenol red (PR) are encapsulated in silica matrix to observe their behavior on structural and optical characteristics of host matrix. FESEM/EDS and AFM analyses reveal that CTAB assisted silica matrix before and after dyes encapsulation has crack-free and porous morphology. The creosol red encapsulated silica matrix (SiO2-CR) has small average surface roughness (Ra) ∼1.05 nm with ultra-thin texture ∼4.06 nm compared to other dyes. FTIR analysis shows that higher content of SiOH group in SiO2-CR matrix shows more hydrophilic features than others matrices. BET analysis indicates that nano-porous structures possess extremely large surface area ∼450 m2/g, and they display specific selectivity towards each dye. UV-vis analysis shows that SiO2-CR matrix has low refractive index 1.36 and high transparency 81% at 632.8 nm with a certain extent of absorption in the visible region. The pH-responsive matrices are deposited onto the PCS (Plastic Clad Silica) optical fibers. The performance of the prepared devices is assessed by measuring their response in the pH range of 1-12. SiO2-PR matrix coated fiber sensor exhibited rapid response of time 0.51 s at pH 12 and 0.17 s at pH 1. The prepared pH sensor has linear response with coefficient of determination (R2) ∼ 0.90 at 440 nm within pH 1-12. Experimental findings suggest that SiO2-PR matrix has more sensing potential at dynamic pH range 1-12 than SiO2-CR matrix and SiO2-BPB matrix.