Rhodamine B dye staining for visualizing microplastics in laboratory-based studies

In the recent years, microplastics have attracted much attention as new emerging environmental pollutants. Previously, several studies were performed to understand the source and fate of microplastics in the environment, organisms, and food webs. To track microplastics and improve their legibility, labeling them is a very effective method during laboratory experiments. This study presents an effective Rhodamine B dye (RhB) staining method for microplastics. The method is crucial for the visual observation of white or transparent plastics by dyeing them in purple or pink, as well as makes the microplastics to fluoresce under common microscope fluorescence filter ranges. Five types of microplastic polymers, namely polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polyurethane were used as the test materials. The efficiencies of ethanol, acetone, and distilled water as possible solvents for dissolving RhB were investigated. Next, the fluorescence stability in various conditions was assessed. The results indicated that ethanol was the most appropriate solvent in dissolving RhB used in staining the microplastics. RhB was fluorescently stable under varying conditions (light and gut fluid) or different solutions (KOH, nitric acid, and saturated NaCl). Additionally, RhB staining exhibited an insignificant effect on the Raman spectra of the microplastics. Our proposed method is simple and robust and helps to visualize the different types of microplastic polymers tested in laboratory experiments, particularly the transparent, white, and small size microplastics.