Irregularly Shaped True-To-Life Microplastics with Embedded Optical Labels

Each year, several thousand tons of microplastics (MPs) end up in the environment. While available techniques for the identification and quantitative characterization of microplastics are getting more elaborate, studies investigating the fate and ecotoxicological impact of MPs face a major challenge: differentiating between naturally occurring and artificial MPs once they are released into the environment. This issue can be addressed by labeling the artificial MPs; however, there is currently a lack of surrogates that combine labeling with a close resemblance to MPs found in the environment. Most studies use labeled polystyrene microspheres as surrogates, but these differ considerably from environmental MPs in terms of shape, chemical composition, and surface charge. 

In this study, we aim to address this challenge by introducing electrospun microfibers as a precursor for irregularly shaped, optically labeled MPs. The labels were directly embedded into the microfibers, which were then broken down by shear-force exfoliation and ball milling, yielding irregularly shaped fibers and fragments. The resulting MPs exhibited a heterogeneous morphology much closer to that of environmental MPs than commercially available spherical MP surrogates commonly used. In addition to organic fluorophores, we introduced lanthanide-doped upconversion nanoparticles (UCNPs) as optical labels. This special class of luminophores combines excitation in the near infrared (NIR) with high photostability, multiple sharp emissions in the UV/visible and NIR ranges, and versatility in doping composition. The low abundance of lanthanides in the environment also enables the quantitative detection of UCNP-doped MPs using element-specific analytical methods. Overall, this new type of artificial MP offers exciting opportunities for biological and environmental studies.