Hyperbolic-Metamaterial-Based Optical Fiber SPR Sensor Enhanced by a Smart Hydrogel for Perspiration pH Measurements

An optical fiber surface plasmon resonance (SPR) sensor, leveraging hyperbolic metamaterials (HMMs) and pH-sensitive hydrogels, has been devised for pH detection in perspiration. Dispersion-tunable HMMs enable the sensor to transcend the inherent structural constraints of an optical fiber and enhance its refractive index (RI) sensitivity. pH-sensitive hydrogels exhibit diverse swelling behaviors due to varying ionization degrees of carboxyl groups under different solution pH conditions, leading to a notable RI change. The sensor achieves a high RI sensitivity of 6963.64 nm RIU–1 and remarkable pH sensitivity of −64.04 and −30.63 nm pH–1 within the pH ranges of 2.7 to 4.7 and 4.7 to 7.5, respectively. Compared to the sensitivity of three other constituents in perspiration, namely, urea, sodium chloride, and glucose, the sensor demonstrates exceptional pH selectivity. Additionally, it maintains good stability during operation and after prolonged storage. It is believed that the sensor has potential in health monitoring, medical diagnosis, disease treatment, etc.