The staircase microfluidic device can be used not only for capturing and size-sorting microparticles from water but also to identify if the microparticles are made of plastic. To demonstrate this, we ground polypropylene and suspended the formed microparticles in water. We then flowed the water solution into the channel, followed by a solution of nile red in acetone. The dyeing solution reacted in the microfluidic chamber for 20 secs. After that, the acetone was flushed out, and the microfluidic device was placed on a fluorescence microscope using a rhodamine filter. The micrographs show that the microparticles have been captured and sorted, from particle sizes of 400 µm down to 25 µm (Figure The differences between plastic microparticles and other microplastics are especially evident in the overlay of the brightfield image and the fluorescence one (Figure bottom). We have also tried a field sample from a nearby pond and managed to discriminate a few tiny microplastics from, most probably, biomass (Figure ). Nile red can be exchanged for other staining agents for detecting specific polymers. Here the limitations lie in the PDMS, as it cannot be used with some organic solvents, and some apolar plastic staining agents may also stain PDMS. For example, leaving the nile red solution in the channel for hours will also turn the PDMS fluorescent. This is usually not the case because of the large surface area of microplastics that makes them stainable in a few seconds compared to the PDMS microfluidic walls.