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

An optical fiber surface plasmon\nresonance (SPR) sensor, leveraging\nhyperbolic metamaterials (HMMs) and pH-sensitive hydrogels, has been\ndevised for pH detection in perspiration. Dispersion-tunable HMMs\nenable the sensor to transcend the inherent structural constraints\nof an optical fiber and enhance its refractive index (RI) sensitivity.\npH-sensitive hydrogels exhibit diverse swelling behaviors due to varying\nionization degrees of carboxyl groups under different solution pH\nconditions, leading to a notable RI change. The sensor achieves a\nhigh RI sensitivity of 6963.64 nm RIU<sup>–1</sup> and remarkable\npH sensitivity of −64.04 and −30.63 nm pH<sup>–1</sup> within the pH ranges of 2.7 to 4.7 and 4.7 to 7.5, respectively.\nCompared to the sensitivity of three other constituents in perspiration,\nnamely, urea, sodium chloride, and glucose, the sensor demonstrates\nexceptional pH selectivity. Additionally, it maintains good stability\nduring operation and after prolonged storage. It is believed that\nthe sensor has potential in health monitoring, medical diagnosis,\ndisease treatment, etc.