Development of Doped Hybrid Xerogels as Optical Fibre Sensors for Environmental pH Monitoring

A set of optical fibre sensors (OFSs) for pH detection have been prepared by doping hybrid xerogel (XG) matrices with pH indicators for environmental monitoring. The hydrophobic sol-gel siliceous materials were synthesised from propyltriethoxysilane (pTEOS) and tetraethoxysilane (TEOS), and subsequently doped with phenolphthalein, bromophenol blue and cyanidin chloride, resulting in three photonic pH devices (OFS5pTEOSPH, OFS5pTEOSBP and OFS5pTEOSCY). Among them, OFS5pTEOSBP exhibited the best sensing performance, showing a reversible optical response in the pH range from 2 to 6 with an intensity ratio variation from 0.92 to 0.52. The developed sensors exhibit negligible dye leaching, good stability, reproducibility, reversible response, and low hysteresis, with response time from 5 to 190 s depending on the pH. Moreover, their applicability was validated through pH measurements in real soil samples.