Fluorescent Dyes for Visualizing Microplastic Particles and Fibers in Laboratory-Based Studies

Observing microplastics in manipulative experiments is of paramount importance for understanding the fate of microplastics in the environment, organisms, and food webs. Labeling microplastics with fluorescent dyes is a useful tool in laboratory experiments for tracking microplastics. However, literature using fluorescence-based detection is heavily biased toward the use of polystyrene and polyethylene microspheres, potentially due to their commercial availability. Consequently, much less is understood about the fate of nonspherical morphologies and other types of plastics common in the environment. Presented here is a heat-mediated microplastic dyeing protocol that facilitates the stable incorporation of inexpensive commercially available fluorescent disperse dyes directly into the polymer structure for use in laboratory-based studies. We demonstrate this microplastic labeling approach is compatible with a wide variety of plastic types [polystyrene (PS), low-density polyethylene (LDPE), high-density polyethylene (HDPE), polyvinyl chloride (PVC), polypropylene (PP), polyethylene terephthalate (PET), and polyacrylonitrile (PAN)] and can be customized with different colors and fluorescent spectra. The stability of the fluorescence intensity of the labeled plastics was measured over 72 h and compared to that of an existing microplastic dying approach using Nile Red. We found the dyeing approach was more fluorescently stable for PS, HDPE, PET, PVC, and PAN than for LDPE and PP. The dyes are also more robust to 4 M KOH digestion and exposure to mineral oil than Nile Red. Finally, the cost of preparing microplastics with the technique shown here is half of the cost of Nile Red. This new plastic dyeing method represents a low-cost, versatile approach enabling laboratory-based experiments with different polymer types and shapes using existing fluorescent microscopy tracking techniques. This will help provide a more representative understanding of the interactions of microplastics with organisms.