Spectroscopic Investigation of Increased Fluorescent Intensity of Fluorescent Dyes When Adsorbed onto Polystyrene Microparticles

Microplastics as environmental pollutants are increasingly a source of alarm. The characterization of microplastics will be necessary to discriminate microplastics from other types of particles. To discriminate specific microplastics, plastic-adsorbable fluorescent dyes are used, the stained microplastics are separated from the dye-microplastic mixture by filtration, and the type of fluorescent staining of the microplastics is analyzed by fluorescent microscopy. In this study, to realize the in situ analysis of fluorescent staining, i.e., to discriminate microplastics without any separation or filtration processes, we studied the change in the fluorescent properties after adsorption of the fluorescent dyes to the microplastic particle surfaces using a 3D excitation emission matrix fluorescence spectroscopy (the excitation wavelength-dependent emission spectrum). We used three fluorescent dyes: Fluorescein, Rhodamine 6G, and Methylene Blue, and polystyrene microparticles as our model microplastic. Fluorescein and Methylene Blue showed increases in the fluorescent intensity, while Rhodamine 6G showed negligible intensity changes. This is likely due to the degree of affinity of the dyes to the polystyrene particle surface, the structural stability of the dyes on the surface, and the changes in the environment around the dyes after the adsorption of each dye to the surface. We conclude that we have demonstrated the potential to look for appropriate fluorescent dyes using the method studied here to identify and estimate individual plastic materials.