Boron Nitride-coated Evanescent Wave Optical Fiber Sensor for Humidity Monitoring in Diapers: A Comparative Study with commercial DHT11 Sensor

Prolonged moisture exposure due to incontinence in infants and elderly individuals often leads to discomfort, skin irritation, and serious health issues such as diaper dermatitis and urinary tract infections. Real time moisture detection is essential for hygiene maintenance. However, the integration of electronics like DHT11 with diapers are restricted by low sensitivity, limited humidity sensing range, and slow response, frequent removal and reapplication, and battery management, hand inspection still poses practical challenges. To overcome these challenges, this study addresses by developing a sensor utilizing a boron nitride (BN)-coated evanescent wave optical fiber, designed for precise, non-invasive, and real-time measurement of humidity levels. The sensor probe was developed using clad-modification technique. The Optical microscopy analysis revealed the etched and unetched regions of the core, which was subsequently dip-coated with hydrothermally synthesized BN nanosheets. The performance of the sensor was tested experimentally with controlled moisture samples at different distances (0, 1, 3, and 5 cm) from the sensing zone and compared with a commercial DHT11 humidity sensor. Spectral analysis showed a distinct peak at 690 nm and a linear, smooth intensity profile rising with level of moisture. The BN-coated sensor exhibited excellent sensitivity of 190.45 counts/mL when exposed directly and continued to display measurable signals at a 5 cm distance, with an intensity change of 20.82% for 10 mL of moisture, thereby demonstrating excellent spatial selectivity and non-contact sensing performance. In contrast, the DHT11 sensor exhibited rapid saturation and reduced sensitivity at larger distances. The developed sensor showed a high linear correlation (R² = 0.9225) and large dynamic range (0.5–10 mL), making the sensor optimally suited for real-time, non-invasive humidity sensing in smart diapers, elderly care devices, and wearable health systems.