Label-Free, Non-Optical Readout of Bead-Based Immunoassays with and without Electrokinetic Preconcentration

In this article, we report a microfluidic bead-based lateral flow immunoassay (LFIA) with a novel sensing mechanism for label-free, non-optical detection of protein binding. This device comprises two packed beds of microbeads: first, bioconjugated microbeads that serve as a test line, and second, a three-dimensional (3D) electrode for sensing. As the protein target binds the bioconjugated microbeads, a shift in ionic conductivity across the bioconjugated beads is produced and can be directly measured at the surface of the 3D electrode by obtaining current-voltage curves before and after incubation of the analyte. We use a model antigen, rabbit IgG, for quantitative evaluation of this sensor, obtaining a limit of detection (LOD) of 50 nM for the LFIA. We demonstrate that this device can be used to measure binding kinetics, exhibiting a rapid (<3 min) increase in the signal after the introduction of the analyte and an exponential decay in the signal after replacing the sample with buffer only. To improve the LOD of our system, we implement an electrokinetic preconcentration technique, faradaic ion concentration polarization (fICP), to increase the local concentration of antigen available during binding as well as the time the antigen interacts with the test line. Our results indicate that this enrichment-enhanced assay (fICP-LFIA) has an LOD of 370 pM, an 135-fold improvement over the LFIA and a 7-fold improvement in sensitivity. We anticipate that this device can be readily adapted for point-of-care diagnostics and translated to any desired protein target by simply modifying the biorecognition agent on these off-the-shelf microbeads.