Ion Altered Fluorescence Imaging (IAFI): A Non-invasive, Visualization Method Which Simultaneously Images Scalar Fields and Quantifies Local Ion Concentration

Electrokinetic flows are leveraged for a wide range of microfluidic and lab-on-a-chip systems, and are often used to mix, preconcentrate, and/or separate analytes. Traditionally, temperature, conductivity, electrochemical, and UV absorbance detectors have been used to indirectly estimate analyte concentration profiles in these flows. However, these typically are point detectors and thus do not permit dynamic, full-field visualization of unsteady scalar fields. To address this, we propose a novel visualization and quantitation method we term ion altered fluorescence imaging (IAFI). IAFI leverages florescence quenching or enhancement of electrically neutral dyes by ions. IAFI therefore provides a non-intrusive quantitation of full-field concentration of non-fluorescent ions endogenous to the flow and its application. We demonstrate this method in visualization of two non-linear electrokinetic flows: isotachophoresis (ITP) and electrokinetic instability (EKI) in an electrokinetic focusing flow. We have quantified shock propagation and ion concentrations upstream and downstream of shocks in cationic and anionic ITP. We quantified and visualized chaotic EKI flow, including complex secondary flows and local ion densities as the flow develops downstream.