Stable and Online Potentiometric Phosphate Monitoring via Dynamic Interface Regeneration on an Iron Electrode

Traditional potentiometric sensors depend on establishing a stable thermodynamic equilibrium at the electrode-solution interface to yield a Nernstian response. However, this approach fails for anions like inorganic phosphate (Pi), which undergo spontaneous and irreversible reactions on metallic electrodes, leading to surface passivation and signal drift. To overcome this limitation, we present a dynamic interfacial sensing strategy that replaces static equilibrium with controlled interface renewal. The method couples periodic cathodic regeneration of an iron electrode (restoring it to Fe⁰) with open-circuit potential (OCP) relaxation. Upon renewal, a brief OCP phase captures the rapid potential shift resulting from Pi's immediate interaction with the fresh surface, serving as the analytical signal. This kinetically controlled process ensures reproducibility by avoiding slower precipitation pathways. Relaxation time analysis and X-ray photoelectron spectroscopy confirm that signal generation occurs within the first second due to fast, specific Pi adsorption. The resulting sensor offers a wide linear detection range (1.0 × 10^-7-2.5 × 10^-4 mol L^-1; 2.5 × 10^-4-1.7 × 10^-3 mol L^-1), an ultralow detection limit (3.0 × 10^-8 mol L^-1), high selectivity against interfering ions, and excellent long-term stability. Validation in spiked environmental water samples showed recoveries of 95.0%-101.8%. Beyond providing a robust and low-maintenance Pi sensing platform, this methodology establishes a broadly applicable framework for detecting analytes that defy conventional potentiometric principles─replacing reliance on thermodynamic equilibrium with deliberate exploitation of kinetically controlled interfacial processes.