Partial leaky-mode fiber-optic humidity sensor enabled by hydrophilic B,N-GQDs film

This study presents a fiber-optic humidity sensor based on boron and nitrogen co-doped graphene quantum dots (B,N-GQDs). Hydrophilic B,N-GQDs film was fabricated by a hydrothermal method, followed by photo-driven deposition on plastic optical fiber cores. The sensor showed a wide response range (0–85%RH) with high sensitivity (0.0072/%RH), a low detection limit (0.2%RH), high linearity (${R^2} = {0.999}$), and rapid response (2.5 s). The sensor exhibited a low hysteresis and excellent stability. Good stability and promising applications in respiration and skin moisture monitoring were also demonstrated. The sensing mechanism is attributed to humidity-dependent adsorption of water molecules by the abundant hydrophilic groups of the film. This adsorption alters the film’s refractive index and extinction coefficient, modulating the transmitted light intensity via a partial leaky-mode mechanism.