Cellulose Fibers Extraction from Cotton Stalk via One-Step Acid Bleachable Pretreatment for Fiber-Optic Humidity Sensor Fabrication

In this study, cellulose fibers was extracted from cotton stalks via one-step acid bleachable pretreatment under mild conditions (≤ 80 °C). 75.24% of hemicellulose and 83.02% of lignin were dissolved out at a p -TsOH concentration of 80 wt%, a pretreatment temperature of 80 °C, and a pretreatment time of 45 min. Subsequently, cellulose fibers (pulp) were obtained from the pretreated cotton stalks matrix via subsequent 10 s ultrasonic treatment. The obtained fibers displayed a high whiteness of 47.5% ISO and a length range between 0.20 and 0.84 mm. After dissolving the fibers in lithium bromide solution (LiBr·3H 2 O) and coating the cellulose hydrogel on the end-face of optical fiber, a fiber-optic relative humidity sensor (FORHS) was prepared. The fabricated FORHS displayed a relatively high humidity response (0.15 dB/RH%), small humidity hysteresis error ( r H = ± 0.07%), and a high stability ( R 2 = 0.98) in a humidity monitoring range from RH 40% to RH 80% and a temperature range between 25 and 40 °C. The novelty of this study is to successfully fabricate a FORHS using cotton stalks cellulose hydrogel as moisture-sensitive material. Thus, the research provided a sustainable strategy for preparation of FORHS and value-added utilization of cotton stalk. Graphical Abstract