Electrokinetic Paper-Based Ion Pre-concentrator for On-Site Detection of Ionic Water Contaminants

Abstract

Effective water quality monitoring is critical for safeguarding public health and protecting the environment. However, conventional analytical methods are either precise but too complex for on-site applications or portable yet limited by relatively high limits of detection (LOD). Here, we propose an electrokinetic pre-concentration platform based on ion concentration polarization (ICP) integrated directly with a commercially available paper-based colorimetric detection kit, enabling simultaneous ion enrichment and immediate visual detection. In a microfluidic device featuring narrowed microchannels confined by same-polarity ion-exchange membranes, the merging of depletion and enrichment zones and focused electric fields facilitate rapid ion accumulation. Using representative ionic contaminants such as heavy metal ions, sulfate, and nitrate, the device achieved pre-concentration factors of 48-fold, 156-fold, and sevenfold, respectively, within 120 s, successfully detecting ions at initial concentrations of 0.025 ppm, 2.5 ppm, and 2.5 ppm and substantially reducing their LOD by 75%, 97%, and 75%. Furthermore, by integrating smartphone imaging with machine learning-based analysis, the enriched detection kit allowed automated, on-site estimation of original ion concentrations without complex instrumentation. This study demonstrates an efficient solution for rapid, sensitive, and field-deployable monitoring of ionic water contaminants.