Tailoring the Retention of Charged Model Compounds in Polymer Functionalized Paper-Based Microfluidic Devices

Studies on the retention of model compounds in tailor-made microfluidic paper devices using laboratory-made paper sheets and lithographic polymer deposition for the design of microfluidic paper substrate are presented. The capillary-driven transport of two differently charged dyes (tartrazine, methylene blue) as model molecules is studied under fully wetted conditions. By quantitative retention factor analysis, it is shown that the dye retention can be controlled within the paper channel by the pH and the conductivity of the fluid phase. The retention is further strongly influenced by the application of solutions of just one model dye compared to a mixture of multiple dyes, and the presence of in-channel defined polymer patches (i.e., areas where the paper fibers have been modified with a functional polymer carrying charges prior to the fully wetted flow). Such covalently attached polymer patches can be used easily to concentrate oppositely charged molecules at spatially defined positions within the microfluidic paper.