Polarization digital holography for advanced classification of microplastic particles

Microplastic pollution is a global issue that threatens ecosystems, wildlife, and human health-indeed, they have been found everywhere, from the deepest ocean seabed to the highest mountain peaks. For this reason, microplastic identification has become a main topic in the environmental and the human health world. We propose a non-destructive method based on Polarized Light Microscopy (PLM), which requires minimal sample preparation. Our approach is based on the literature already known behaviour of MPs: such materials modify each electric-field vector component differently-introducing a modulation in the amplitude or phase (birefringence)-and causing alterations in the light polarization states. Using PLM, it is possible to discriminate between microplastics and biological organisms like algae because these latter do not exhibit modifications in light polarization state. Our study focuses on the differences introduced by the aging of microplastics, a process simulated in simplified laboratory conditions. The most frequently used degradation approaches are ultraviolet (UV) irradiation, which simulates the photoinduced oxidation of plastic polymers due to sunlight exposure and mechanical stress of MPs in water solution due to longitudinal transport and contact with other solids.