High-sensitivity FBG humidity sensors enabled by electrohydrodynamically printed carbon-nanofiber functionalized gratings

Fiber Bragg grating (FBG) humidity sensors face manufacturing challenges in depositing sensitizing films, which are typically complex and costly. Herein, we propose a near-field electrohydrodynamic direct-write printing (NEHDP) strategy to fabricate flexible FBG humidity sensors functionalized with carbon nanofiber@multi-walled carbon nanotube nanocomposites. This approach enables rapid, low-cost, and precision patterning of sensing films while maintaining high performance. The optimized sensor exhibits ultrahigh sensitivity (2.0 pm/%RH) with near-perfect linearity (R2 = 0.997) in the 11–54 %RH range, alongside rapid response/recovery times (1.3 s/2.0 s) at 20 °C. Although signal saturation occurs at high humidity (75–95 %RH), the sensor demonstrates exceptional reversibility (>98% signal retention) and stability (<±0.8% drift over 30 cycles) in the 20–30 °C and 10–50 %RH operational window. Temperature cross-sensitivity analysis confirms stable operation within 25–35 °C but reveals performance degradation above 40 °C due to nanofiber swelling. This work establishes NEHDP as a scalable platform for fabricating optical fiber sensors with tunable functional interfaces.