Copper–Sodium Lignosulfonate Antifouling-Coated Ion-Selective Electrodes for Reliable Long-Term Ammonium Monitoring toward Data-Driven Wastewater Control

Abstract

Ammonium (NH₄⁺) monitoring is essential for process control and effluent compliance in wastewater treatment, but conventional grab sampling provides delayed, discrete measurements that cannot capture rapid concentration fluctuations. Solid-state ion-selective electrodes (S-ISEs) are promising for continuous real-time monitoring, but long-term deployment is limited by membrane biofouling. Here, we developed a Cu²⁺-sodium lignosulfonate (Cu-SLS) antifouling coating for NH₄⁺ S-ISEs that simultaneously reduces biofouling and improves interfacial stability. By combining the hydrophilic hydration effect of SLS with the antimicrobial activity of coordinated Cu²⁺, the coating provided a more stable sensing interface under wastewater-relevant conditions. A systematic 3 × 3 formulation screening identified Cu0.10-SLS0.25 coating as the optimal configuration, retaining a near-Nernstian sensitivity of 56.5 mV/dec, a response time of 5.5 s, and a detection limit of 3.2 × 10^–6 M. The enhanced Cu retention of Cu0.10-SLS0.25 during aqueous exposure was consistent with stronger Cu(II)-sulfonate interactions and its improved long-term sensing stability. In a 14-day real wastewater deployment, Cu0.10-SLS0.25 achieved a mean absolute error of 1.24 mg/L and successfully captured a simulated NH₄⁺ shock event on Day 11, with minimal surface biofouling observed by posttest SEM. These results demonstrate a practical, mechanism-guided strategy for improving the reliability of long-term NH₄⁺ monitoring in complex wastewater environments.