Highly Sensitive Physiological Sensor Based on Tapered Fiber-Optic Interferometer for Sweat pH Detection

The pH value of human sweat correlates with metabolic activities and diseases. Precise sweat pH sensing can serve as a health-profiling strategy. This study demonstrates a compact fiber-optic sweat pH sensor that contains an optimized conical structure coated with polyelectrolyte membrane as the pH sensitive layer. To functionalize the tapered sensing area for pH monitoring, chitosan/polyacrylic acid (PAA) polyelectrolytes are deposited as pH-sensitive membranes. As the swelling degree of the sensing film depends on the pH value, the tapered fiber-optic sensor based on the Mach–Zehnder interferometer (MZI) demonstrates the corresponding wavelength shift to various pH values in sweat. The fiber-optic sensor presents a high pH sensitivity of −27.06 nm/pH in the physiological sweat pH value ranging from 5.0 to 7.5 with a low temperature cross-sensitivity of 0.012 nm/°C in the normal physiological range of 25 °C–45 °C. Further, the pH sensing range can be extended to pH 10 in aqueous solutions, and the sensitivity is 28.44 nm/pH. Considering the advantages including wide sensing range, simple fabrication technique, and high sensitivity, the sensor can be potentially applied in many other liquid physiological detections for healthcare monitoring.