Highly efficient sample stacking by enhanced field amplification on a simple paper device

We present a novel electrokinetic stacking (ES) method based on field amplification on a simple paper device for sample preconcentration. With voltage application, charged probe ions in a solution of lower conductivity stack and form a narrow band at the boundary between the sample and the background electrolyte of higher conductivity. The stacking band appears quickly and stabilizes in a few minutes. With this ES method, three orders of magnitude signal improvement was successfully achieved for both a fluorescein probe and a double-stranded DNA within 300 s. This enhanced stacking efficiency is attributed to a focusing effect due to the balance between electromigration and counter electroosmotic flow. We also applied this ES method to other low-cost fiber substrates such as cloth and thread. Such a simple and highly efficient ES method will find wide applications in the development of sensitive paper-based analytical devices (PADs), especially for low-cost point-of-care testing (POCT).