In operando spatiotemporal analysis of ion concentration profile using ion-selective membrane probes in electrokinetic systems

Abstract

Accurate in operando measurement of electric potential in electrokinetic systems is critical yet challenging due to complications arising from electrochemical reactions and electrical double layer capacitance at the metal electrode-electrolyte interface. These challenges are further compounded under in operando conditions requiring simultaneous voltage application and measurement, where conventional probes often induce leakage currents and distort measurements. To overcome these limitations, we developed an ion-selective membrane (ISM) probe with high input impedance, capable of transferring charge as a form of ion, thereby suppressing redox reactions and eliminating leakage currents. Integrated into a microfluidic platform, the ISM probe demonstrated robust performance across a wide resistance range, with reliable operation even in giga-ohm environments, a suitable cutoff frequency for tracking dynamic potentials, and significantly reduced noise and measurement offset compared to traditional metal-based probes. Leveraging these advantages, we employed the ISM probe to perform in operando dynamic analysis of ion concentration, particularly focusing on electrokinetic phenomena such as ion concentration polarization (ICP). The probe enabled precise spatial and temporal measurement of concentration gradients formed by ICP, offering new insights into ion distribution dynamics; (1) issues in visualizing ion concentrations using fluorescence and (2) the second plateau of ion concentration near nanoporous membranes. This work establishes the ISM probe as a versatile tool for advancing the understanding of ion profiles in complex electrokinetic systems, addressing key challenges in both fundamental research and practical applications.