Potentiometric Sensors in the Wild: Long-Term Stability, Biofouling, and Calibration Strategies for Environmental Applications

Potentiometric ion-selective electrodes (ISEs) are attractive tools for environmental monitoring, enabling simple, rapid, and continuous measurements directly at the sampling site. However, their long-term operation under real environmental conditions remains challenging. While laboratory studies focus on sensitivity, selectivity, and detection limits, prolonged deployment introduces potential drift, water-layer formation, membrane-component leaching, fouling, biofouling, and instability of the reference electrode. These effects alter sensor response and compromise measurement reliability without causing immediate failure. This review discusses mechanisms responsible for sensor degradation during in situ operation and summarizes material- and system-level strategies for improving long-term stability. Particular attention is given to antifouling approaches, calibration strategies, drift compensation, and methods for evaluating sensor condition, including electrochemical impedance spectroscopy, water-layer tests, and surface characterization. Recent developments toward self-calibrating, calibration-free, and autonomous potentiometric platforms are also discussed. Finally, current limitations and future directions are considered, emphasizing the need for standardized long-term testing, validation in real environmental matrices, integrated diagnostics, and reliable operation of the complete cell to maintain measurement reliability throughout deployment.