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

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, 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.

Direct numerical simulation of electrokinetic transport phenomena: variational multi-scale stabilization and octree-based mesh refinement

“…We see that the presence of the microbeads causes the local enhancement of the electric field to extend throughout the channel cross section rather than being concentrated only in the bottom region. We note that these observations are qualitatively consistent with the ones reported in the experimental study of Berzina et al [62].…”

“…The device consists of a packed bed of metallic beads that act as a set of electrodes embedded within a microfluidic device. This device was recently studied experimentally by Berzina et al [62] to circumvent two challenges faced by conventional ICP occurring at a planar electrode, detailed in the next paragraph. The device configuration offers a challenging canonical simulation example due to (a) the complex geometries of the electrodes involved, (b) the steep gradients (especially near these complex geometries), and small EDL's produced by the operating conditions, (c) the ensuing channel scale vortical structures that have to be accurately resolved, and (d) the long time horizon over which the current measurements stabilize.…”

“…A promising approach has been to move away from a planar electrode (along the bottom of the channel) to using engineered, porous electrodes across the channel cross-section. Recently, Berzina et al [62] introduced 3-D packed bed electrodes that successfully control unwanted instabilities and extend high electric field area for enrichment of bioanalytes prior to detection. In a follow-up study by Berzina et al [66], an additional bed of packed bioconjugated beads was incorporated for DNA capture.…”