Fouling of Ion-Selective Electrodes (ISEs) in Aquatic Environments: Detection, Cleaning, and Antifouling Strategies

1. Introduction

The monitoring of water quality is vital for maintaining healthy ecosystems, ensuring safe aquaculture production, and protecting natural water resources. Among the available sensing technologies, ion-selective electrodes (ISEs) have become highly relevant tools for real-time monitoring of ionic species such as pH, nitrate, ammonium, and chloride.
ISEs combine several advantages: they are fast, cost-efficient, reagent-free, and suitable for on-site applications even in complex or turbid aquatic systems. Their electrochemical detection principle makes them particularly useful for continuous operation and integration into automated sensor networks. The global demand for sustainable aquaculture and wastewater treatment has increased the need for reliable, low-maintenance sensors. In such environments, parameters like nutrient concentration, salinity, and pH must be continuously tracked to ensure biological balance and process efficiency. However, in long-term use, their performance is severely affected by fouling, which leads to signal drift, loss of sensitivity, and increased maintenance needs. Understanding and controlling fouling is therefore essential for the stable operation of monitoring systems in both marine and freshwater applications.

2. Principles and Function of Ion-Selective Electrodes

ISEs are electrochemical sensors that measure the activity of specific ions in solution by converting ionic concentration into an electrical potential. A typical setup consists of a measuring electrode, equipped with an ion-selective membrane, and a reference electrode, commonly based on an Ag/AgCl system^[1].

The potential difference between both electrodes follows the Nernstian relationship, which links the measured voltage to ion activity and temperature. This potential is independent of sample volume, making ISEs suitable for both laboratory and in situ measurements. By selecting appropriate ionophores and membrane materials, ISEs can be tailored for a wide variety of ions such as Na⁺, K⁺, NH₄⁺, Cl⁻, NO₃⁻, and Ca²⁺. This flexibility, together with their miniaturization potential, enables integration into multiparameter probes and automated control systems for environmental monitoring, industrial water treatment, and precision aquaculture. Their robustness, low energy demand, and compatibility with wireless data systems have made ISEs key components in the growing field of “smart water management” ^[2][3].

3. The Challenge of Fouling

A major limitation of ISEs is their vulnerability to fouling—the accumulation of unwanted material on the sensor surface. Fouling can be organic (e.g., proteins or humic acids), inorganic (e.g., salt precipitation), or biological (biofilm growth) in nature ^[4][5]. In aquatic environments, all three types often occur simultaneously, creating a complex fouling layer. In aquaculture systems, for instance, high nutrient content and microbial activity accelerate fouling formation. Particularly biofouling plays a dominant role, as it leads to microbial colonization and subsequent biofilm growth on the membrane surface. The process typically begins with the adsorption of a conditioning film, followed by bacterial adhesion, colony formation, and biofilm maturation.
These layers alter ion transport across the membrane, increase impedance, and lead to gradual potential drift. Over time, the accuracy of measurements declines significantly, and frequent cleaning or recalibration becomes necessary. Additionally, the presence of biofilms can influence local redox conditions and cause partial deactivation of ionophores. Therefore, controlling fouling is not only a matter of sensor longevity but also of maintaining data integrity over extended deployment periods.

4. Detection and Characterization of Fouling

Since ISEs are electrochemical devices, fouling can be detected using a range of electrochemical techniques. Among these, electrochemical impedance spectroscopy (EIS) is the most powerful, as it provides non-invasive insight into interfacial changes, such as increased resistance or altered capacitance ^[5]. EIS data can indicate the onset of fouling long before visible degradation occurs, making it a valuable tool for predictive maintenance. Other methods, including chronoamperometry and voltammetry, are used to monitor short-term current or potential changes that occur during the early stages of fouling. In addition to electrochemical methods, microscopic techniques such as scanning electron microscopy (SEM) or atomic force microscopy (AFM) visualize the morphology of fouled surfaces, while spectroscopic analyses like FTIR or XPS reveal the chemical composition of the fouling layer. These complementary approaches enable a comprehensive understanding of fouling dynamics and guide the design of more resilient electrode materials.

5. Cleaning and Regeneration Approaches

Several regeneration methods have been developed to restore ISE functionality after fouling. Mechanical cleaning (e.g., brushing or ultrasound) removes loose deposits, while chemical cleaning (e.g., acids, bases, or oxidants) dissolves organic and inorganic residues ^[6]. Electrochemical cleaning via potential pulsing can desorb charged contaminants directly from the membrane surface, allowing rapid in situ regeneration without disassembly. Physical techniques, such as UV irradiation or low-pressure plasma exposure, have also shown promising results by breaking down biofilms and restoring hydrophilic surface properties. However, repeated cleaning can gradually degrade the polymeric membrane, alter ionophore functionality, and shorten sensor lifetime. Consequently, modern research increasingly emphasizes preventive antifouling strategies that inhibit the initial adhesion and growth of foulants, thus reducing the need for aggressive post-fouling treatment.

6. Antifouling Strategies

Current research focuses on developing surfaces that inherently resist fouling.
Passive approaches use hydrophilic or zwitterionic coatings to prevent adhesion of organic or biological materials.
Active antifouling uses photocatalytic or antimicrobial nanoparticles such as TiO₂, ZnO, or Ag to degrade biofilms under light or electrochemical activation ^[7][8].
Bioinspired methods—for example, coatings with natural antifoulants like capsaicin or zosteric acid—offer environmentally friendly protection without toxic leaching ^[9][10].
These material-based strategies have been shown to extend the operational stability of ISEs in long-term aquatic applications.

7. Current Developments and Future Perspectives

Modern ISE research increasingly integrates solid-contact electrode designs with conductive polymers or nanocomposites, improving mechanical stability and reproducibility ^[11][12]. Furthermore, combining these electrodes with real-time impedance monitoring enables early fouling detection and autonomous self-cleaning protocols, allowing the system to operate without human intervention. The integration of machine learning and data analytics for trend recognition and anomaly detection further enhances the reliability of such systems in real-world conditions. The next generation of ISE-based platforms aims for self-sustaining, energy-efficient, and environmentally compatible operation. These sensors will play a key role in circular water systems, precision aquaculture, and decentralized wastewater management, where accurate and continuous monitoring is indispensable.

8. Conclusion

ISEs are indispensable tools for continuous water quality monitoring due to their simplicity, selectivity, and adaptability. Fouling, however, remains a critical challenge that limits their long-term performance and measurement reliability. Advances in material science, surface modification, and integrated electrochemical diagnostics are paving the way for robust, low-maintenance, and environmentally compatible ISE systems. Future efforts will focus on integrating antifouling materials with smart control algorithms and eco-friendly coatings to achieve stable, accurate, and sustainable ISE performance in complex aquatic environments.