Toward Reliable Environmental Monitoring with Potentiometric Ion Sensors: Unveiling the Effects of Humic Acids and Developing Antifouling Sensors Based on a Poly(vinylidene fluoride) Coating

Although potentiometric ion sensors have attracted growing interest in the environmental analysis of water quality, reliable long-term continuous monitoring with these sensors remains a great challenge. Fouling of the sensor membranes can seriously impair analytical performance. As a major fraction of natural organic matter, humic acids (HAs) are widespread in natural waters. Here, for the first time, we investigated the influences of HAs as foulants on polymeric membrane ion-selective electrodes (ISEs). It was found that HAs could adsorb on the sensor surface and penetrate the membranes, thus causing potential drifts and reduced sensitivity. To mitigate these effects, we modified the ISEs with a poly(vinylidene fluoride) coating. Owing to its low surface energy and high electronegativity, the functional layer could significantly enhance the resistance of the sensors to HAs. The practical applications of the proposed environ-compatible sensors were validated via long-term (30 days) in situ monitoring of CO₃^2- (pCO₂) in the coastal area of the Yellow Sea of China. These findings may provide new insights into reliable environmental monitoring with potentiometric ion sensors.