U-shaped Sagnac optical microfiber coupler biosensor for dual-matrix glucose detection

A high-performance glucose biosensor based on a U-shaped Sagnac optical microfiber coupler (OMC) functionalized with a polyethylene glycol/chitosan–glucose oxidase (PEG/CS-GOD) composite coating is presented. The sensor is designed by integrating the Sagnac interferometer's common-mode noise rejection with the U-shaped microfiber's strong evanescent field, effectively addressing environmental instability in micro-structured optical sensors, while the multifunctional coating simultaneously provides antifouling properties, biocompatibility, and enzymatic specificity. The sensor exhibits a high refractive index sensitivity of ∼1717 nm/RIU, glucose sensitivities of 3.17 and 3.09 nm mL·mg−1 for two interference dips with excellent linearity (R2 > 0.99), a detection limit of 0.224 mg/mL, and a response time of 13 s in simulated urine. Operational stability is demonstrated by minimal hysteresis error (<2%), approximately 81% response retention over 14 days, and satisfactory selectivity against key interferents at physiologically relevant concentrations. Consistent performance in simulated urine (3.03 nm mL·mg−1) and artificial serum (3.03 nm mL·mg−1) confirms dual-matrix detection capability. A dual-dip demodulation matrix enables real-time temperature compensation. With its balanced combination of high sensitivity, rapid response, long-term stability, and dual-matrix compatibility, this biosensor represents a promising platform for future point-of-care glucose detection.