Electrokinetic stacking on paper-based analytical device by ion concentration polarization with ion exchange membrane interface

Ion concentration polarization (ICP) effect is an effective preconcentration method for charged species in microfluidic systems. In recent years, ICP has also been introduced in paper-based analytical device (PAD) to improve the detection sensitivity. In this paper, a simple and cheap ICP interface was introduced in PAD by using commercial available ion exchange membrane (IEM) that is commonly used in industry. Significant ICP effect was observed by placing the IEM directly on a wetted paper fluidic channel, and electrokinetic stacking of both anion and cation probes was demonstrated with cation exchange membrane (CEM) and anion exchange membrane (AEM), respectively. Simply based on LED illumination and smart phone fluorescent imaging, about 10-fold signal improvement, was achieved for negatively charged fluorescent probes of fluorescein sodium and FITC-BSA with CEM, and 50-fold signal improvement was achieved for positively charged fluorescent probe of rhodamine 6G with AEM. The performance of this PAD was investigated with fluorescein sodium and rhodamine 6G, and the limits of detection of 0.34 and 0.011 mu mol L-1 were achieved, respectively. This method provides a cost-effective route for improving the detection sensitivity of charged species on PAD taking advantage of ICP effect.