Out-of-plane faradaic ion concentration polarization: stable focusing of charged analytes at a three-dimensional porous electrode

04 January 2022, Version 2
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Ion concentration polarization (ICP) accomplishes preconcentration for bioanalysis by localized depletion of electrolyte ions, thereby generating a gradient in electric field strength that facilitates electrokinetic focusing of charged analytes by their electromigration against opposing fluid flow. Such ICP focusing has been shown to accomplish up to a million-fold enrichment of nucleic acids and proteins in single-stage preconcentrators. However, the rate at which the sample volume is swept is limited, requiring several hours to achieve these high enrichment factors. This limitation is caused by two factors. First, an ion depleted zone (IDZ) formed at a planar membrane or electrode may not extend across the full channel cross section under the flow rate employed for focusing, thereby allowing the analyte “leak” past the IDZ. Second, within the IDZ, large fluid vortices lead to mixing, which decreases the efficiency of analyte enrichment and worsens with increased channel dimensions. Here, we address these challenges with faradaic ICP (fICP) at a three-dimensional (3D) electrode comprising metallic microbeads. This 3D-electrode distributes the IDZ, and therefore, the electric field gradient utilized for counter-flow focusing across the full height of the fluidic channel, and its large area, microstructured surface supports smaller vortices. An additional bed of insulating microbeads restricts flow patterns and supplies a large area for surface conduction of ions through the IDZ. Finally, the resistance of this secondary bed enhances focusing by locally strengthening sequestering forces. This easy-to-build platform lays a foundation for the integration of enrichment with user-defined packed bed and electrode materials.

Keywords

ion concentration polarization
electrokinetic
focusing
preconcentration
separation
faradaic

Supplementary materials

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Description
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Title
Supporting Information: Out-of-plane faradaic ion concentration polarization: stable focusing of charged analytes at a three-dimensional porous electrode
Description
Device designs and fabrication; verification of the mechanism of fICP; collection of current transients and current-voltage curves (CVCs); measurement of the onset potential of water reduction on several distinct electrode materials; electrokinetic enrichment of charged species; scalability of the device; computational method for charged species transport simulation.
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