Exploring the concentration-dependent transport and the loss of rhodamine B, tartrazine, methylene blue, and amaranth dyes in common paperfluidic substrates

Abstract

Understanding the fluid flow on paperfluidic substrates is crucial due to an increasing interest in affordable sensors for diagnosis, environmental testing, food testing, etc. Charged dyes are the most commonly used model analytes for characterizing the fluid flow in paperfluidic devices. Here, we propose a framework to quantify the concentration-dependent loss during transport of four charged dyes (e.g., rhodamine B, methylene blue, tartrazine and amaranth). At concentrations lower than a critical value (Cc), electrostatic interaction of the dye molecules with the filter paper retards the flow of cationic dyes rhodamine B and methylene blue, resulting in 75%–99% sample loss. We determined Cc to be 20 μM for rhodamine B and 5 mM for methylene blue. The large difference in their Cc values is explained by the molecular structures of these dyes. While anionic dyes (e.g., tartrazine and amaranth) are not retarded during flow on filter paper due to negligible electrostatic interaction with the substrate, they form strong coffee ring patterns in the detection zone when the dye concentration is below Cc. We introduced a new parameter called the ‘coffee ring index’ (CRI) to quantify the non-uniform sample distribution in the detection zone, and from this, determined Cc to be 250 μM for amaranth and 1.25 mM for tartrazine. Finally, we demonstrated that an increase in the dye viscosity by adding 30% glycerol can lower Cc of tartrazine from 1.25 mM to 100 μM. These results provide useful guidelines for the paperfluidics community to design more effective sensors.