Ion concentration polarization focusing at a millimeter-scale microbead junction: towards higher volumetric throughput†

Abstract

Ion concentration polarization focusing (ICPF) is an electrokinetic technique that has shown promise in achieving even billion-fold preconcentration factors. However, increasing the volumetric throughput of ICPF is challenging because disruptive processes that reduce preconcentration efficiency worsen as the channel cross-section extends beyond the microscale. We previously introduced an approach for mitigating the above challenges in a microfluidic regime. However, in that system, the flow rate was limited to less than 1.0 μL min⁻¹. Herein, we report a high throughput and scalable ICPF of charged analytes in a millimeter-scale channel. Using 3D-printed channels of 4.0 mm² cross-section, we achieve preconcentration factors above 200-fold within 10 min at a flow rate of 30 μL min⁻¹. In this system, ICP is accomplished by ion permselective transport through a packed bed of commercially available cation exchange microbeads (30 μm and 200 μm). We investigate the scalability of the approach by comparing the ICPF performance of channels with four distinct cross-sectional areas. While ICPF occurs in all four cases, the degree of preconcentration drops below 100-fold (per 10 min) in channels with cross-sections beyond 4.0 mm². This drop in efficiency is attributed to dispersion associated with Joule heating. Therefore, by improving the device design to dissipate heat more effectively, we anticipate that this approach can be scaled up further for applications that demand high volumetric throughput electrokinetic focusing.