Discerning pristine and aged true-to-life microplastics through polarization-resolved digital holography and machine learning

The classification and characterization of microplastic fragments have become increasingly central topics in scientific research, given their widespread presence in both natural and urban environments. To support controlled and reproducible experiments, laboratory polymeric microbeads are often synthesized to reproduce the behavior of those found in the environment. However, environmental microplastics are rarely perfect, monodisperse fragments and typically undergo aging processes driven by UV radiation, oxidation, and mechanical abrasion. Replicating and analyzing aged microplastics is therefore essential for accurately assessing their environmental persistence, interactions, and potential impacts. In this study, we focus on the generation of environmentally relevant (“true-to-life”) microplastics, obtained by mechanical fragmentation of every-day plastic products, and we further enhance their relevance by comparing pristine and photo-oxidized (aged) particles. Among the polymers most frequently detected in marine and urban waste—owing to their extensive use in everyday products—are polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET). These were selected not only for their environmental prevalence but also for their contrasting photoaging behavior: PS is widely employed in laboratory studies, PVC undergoes pronounced aging upon UV exposure, whereas PET is comparatively resistant to photooxidation. To investigate these materials, we employed polarization-resolved digital holographic microscopy combined with machine learning to analyse true-to-life PS, PVC, and PET microplastic fragments, along with their aged counterparts. The method detects fragment-induced changes in the polarization state of the incident light, providing meaningful data to classify the three polymers and to distinguish each of them from its aged form. Discerning between aged and pristine microplastics represents an important step toward reconstructing the transformation history and fluxes of plastic fragments in aquatic environments.