Directional Alternating Current Electrophoresis on Paper Driven by Spatially Nonuniform Electric Fields

Electrokinetic mechanisms are known to enrich a variety of analytes in paper-based media, yet localized, membrane-free enrichment of small charged analytes has remained an unmet capability. Recently, we reported biased alternating current electrophoresis (b-ACEP), a new electrokinetic mechanism, different from dielectrophoresis, in which spatially nonuniform AC electric fields generate time-averaged, directional electrophoretic migration of small charged analytes. Here, we establish the first paper-based implementation of this mechanism (pb-ACEP) and demonstrate that b-ACEP remains operative in aqueous, porous cellulose substrates. Using screen-printed orthogonal carbon electrodes to generate spatially nonuniform AC electric fields, pb-ACEP achieved up to 410-fold enrichment of the model small-molecule analyte xylene cyanol (XC) in 1× Tris-Borate-EDTA within 15 min and improved the limit of detection by approximately 400-fold compared with field-free conditions. Control experiment results confirmed the electrophoretic origin of pb-ACEP by excluding convective flow, dielectrophoretic effects, and Faradaic reactions, while voltage-frequency sweeps revealed tunable enrichment behavior consistent with theoretical predictions for AC-field-driven electrophoresis. Pb-ACEP also maintained substantial enrichment performance in biological matrices, achieving 280-fold enrichment in artificial urine and 233-fold enrichment in human plasma. Furthermore, pb-ACEP demonstrated broad applicability by enriching representative biomolecules, including up to 240-fold enrichment of hemoglobin, 150-fold enrichment of immunoglobulin G, and 50-fold enrichment of double-stranded DNA. Together, these results establish pb-ACEP as a fundamental and generalizable electrokinetic strategy for high-efficiency, membrane-free, and spatially localized enrichment of charged analytes in porous media, expanding AC-field electrophoresis capabilities and enabling enhanced sensitivity in integrated paper-based assays.