Multi-CEM-embedded microfluidic system for simultaneous molecular enrichment and separation by multi-stage ion concentration polarization

The ion concentration polarization (ICP) effect is widely utilized in low-abundance particle preconcentration with a high enrichment factor. However, it is still challenging to realize the locational molecular separation based on their mobilities in traditional single-cation-exchange-membrane (CEM) microsystems. In this study, we developed a multi-CEM-embedded molecular enrichment and separation system leveraging the ICP effect, where analytes could be selectively enriched at distinct membrane interfaces. The enrichment and separation mechanism and the coupling effect of two membranes are studied, and the results show that an insufficient depletion effect before the first cation exchange membrane (1st-CEM) would decline the separation efficiency before the second cation exchange membrane (2nd-CEM). Conversely, an intensified depletion effect at the 2nd-CEM nearly has no influence on the enrichment and separation performance of the 1st-CEM. To validate these findings, fluorescein sodium and sulforhodamine B are selected to demonstrate the behavior of analytes along the multi-stage ICP microsystem. The results show that sodium fluorescein and sulforhodamine B could be successfully enriched at the two membrane interfaces, achieving enrichment factors of 5600 and 6200, respectively, at a flow rate of Q₁ = 6 μL h⁻¹ and applied voltages of V_L = 100 V and V_M = 400 V. This device could provide a novel strategy and theoretical framework for the simultaneous enrichment and separation of multiple analytes like nucleic acids and proteins, as well as for the design of multi-stage ion concentration polarization systems.