Electrokinetic Enrichment and Purification of Proteins and Anions from High-salt Samples on Liquid Plasticine-Based Open Chip

Abstract Microfluidic chips are promising for on-site analysis in medical diagnostics and food safety because they enable miniaturized, integrated sample processing. However, the analysis of complex, salt-rich matrices of biological and food samples remain challenging for conventional closed chips and electrokinetic pretreatment methods. Here, we developed an open, reconfigurable electrophoresis chip based on liquid plasticine, in which a moving neutralization reaction electrophoresis system (MNRES) was implemented for high-salt sample pretreatment. Liquid plasticine enabled facile construction of pump-free, valve-free, optically transparent, and cuttable open reservoirs and channels. In the MNRES, OH– and H+ generated by water electrolysis reacted with NH4+ and CH3COO–, producing moving neutralization reaction boundaries accompanied by dynamic electric field and pH gradients. These two coupled gradients enabled salt removal, field amplified stacking, and meanwhile the pH gradient induced protein focusing, thereby allowing simultaneous enrichment and purification of proteins and anionic species. It is demonstrated that proteins and anionic species in 0.9% NaCl solution can be effectively separated and enriched by approximately 10-fold. Moreover, the MNRES-pretreated analyte zones can be directly monitored by smartphone-based online colorimetry or transferred to liquid-plasticine microreactors for offline chromogenic assays. This work provides a flexible open-fluidic strategy for integrated electrokinetic pretreatment and visual detection of complex high-salt samples.