Lignin-containing fibers extraction and hydrogel preparation for fiber-optic relative humidity sensor fabrication

Herein, a physicochemical pretreatment method, namely, a combination of recyclable acid hydrotrope ( p -toluenesulfonic acid, p -TsOH) treatment and ultrasonic treatment (10 s), was employed to extract lignin-containing fibers (LFs) from wood material ( poplar ). The LFs were used to produce reinforced polyvinyl alcohol (PVA) hydrogels via a dissolution-regeneration process in LiBr⋅3H 2 O solution. After coating the produced hydrogel on the end face of a single-mode fiber (SMF), a fiber-optic relative humidity sensor (FORHS) was fabricated to monitor environmental humidity. A high sensitivity (4.7 dB) and good repeatability ( R 2 = 0.98, the confidence factor) were found within a humidity range from 45 % to 80 %. Therefore, this study provides a sustainable approach for valorization of lignocellulosic biomass.