Biopolymer Optical Fibers for High-Sensitivity Quantitative Humidity Monitoring

Because of their tunable refractive index and surface functionalities, biopolymers have emerged as excellent candidates for the fabrication of sustainable optical fibers. To date, the focus has been on identifying suitable biopolymers for optical fiber fabrication and their associated waveguiding properties. Despite a few studies showing their potential for short-distance applications and humidity sensing, the quantitative sensing of environmental parameters using biopolymer optical fibers has not been reported. Herein, for the first time, we report the quantitative determination of relative humidity (RH) across the visible and near-infrared region of the electromagnetic spectra using fully biopolymer optical fibers. Specifically, we demonstrate that methylcellulose and methylcellulose-alginate composite optical fibers exhibit sensitivity of up to 0.33 dB/%RH. Notably, the sensitivity of our fibers is equal to or greater than most of the optical sensors reported in recent literature and exceeds the reported values for capacitance-based humidity sensors. The sensitivity, response time, and dynamic range can be readily tuned by changing the fiber composition and coagulation methods, with ethanol coagulated composite fibers displaying a 6-fold increase in sensitivity compared to ionically coagulated composite fibers. Our results suggest that the humidity sensing properties of fiber sensors are insensitive to temperature changes. Dynamic Vapor Sorption analysis reveals that the fiber composition regulates the sorption-desorption kinetics, thereby affecting the humidity sensing behavior. The fully biopolymer optical fibers open a new avenue for sustainable sensors that enable the quantitative sensing of environmental parameters for condition monitoring applications.