Material-Specific Determination Based on Microscopic Observation of Single Microplastic Particles Stained with Fluorescent Dyes

Microplastics are increasingly suspected of having serious negative effects on ecological systems and living organisms. These effects are different based on the materials of the microplastics, leading to the importance of the determination of the materials. For material determination, spectral fingerprints based on FT-IR and Raman microspectroscopy are previously and commonly used, though they require patience and special skills. In this study, we have developed a novel technique for microscopic observation of single microplastic particles stained with fluorescent dyes to enable fluorescence-based determination of materials of these particles as a first screening of material determination. Commercially available and popular microplastic particles and fluorescent dyes were used. Fluorescence microscopy was carried out to observe the degree of fluorescent intensity for various combinations of microplastics and dyes based on the difference in fluorescent intensity of microplastics before and after staining with the dyes. We have found a dependence of the fluorescent intensity on the combination of the microplastics and the dye. Fluorescein gave the highest increase in intensity for PS (polystyrene), showing a statistically significant difference between fluorescent intensity for PS and that for PP (polypropylene) or PE (polyethylene). The use of Fluorescein thus enables specific detection of PS. On the other hand, Nile Red gave the highest increase in fluorescence for PP, indicating that the combination of Nile Red and PP gives a significantly greater interaction than with other combinations. The use of Nile Red thus enables the specific detection of PP. These results indicate the possibility of the material determination of microplastics by using fluorescent dyes. This is the first demonstration of the differential determination of the materials of single-particle microplastics based on a material-specific increase in fluorescent intensity by staining microplastics with fluorescent dyes.