The effect of crystallization kinetics on the fluorescence behavior of label-free PET nanoplastics: ecotoxicological relevance for freshwater zooplankton

• Nanoplastics from PET bottles were characterized using multiple analytical techniques • Crystallization kinetics helped to restore polymer crystallinity mimicking aging • Crystallinity level directly influenced the fluorescence signal of nanoplastics • Daphnia Magna exposure confirmed label-free PET NPs suited biological applications • High temperatures increased toxicity and sublethal effects after exposure removal Nanoplastics (NPs) are pervasive pollutants whose detection and collection from the environment is challenging due to their extremely small size, which limits the effectiveness of characterization techniques. This study analyses the impact of crystallization kinetics on the autofluorescence behavior of unlabeled polyethylene terephthalate (PET) NPs produced by mechanically fragmenting plastic waste in conditions that simulate natural degradation. This process yields particles with properties similar to those found in nature. By studying crystallization kinetics, it is possible to restore polymer crystallinity, effectively replicating the increased crystallinity observed in environmentally degraded polymers. Additionally, the enhanced crystallinity leads to a stronger fluorescence signal allowing the resulting nanoparticles to be used without additional fluorescent staining, a novel feature that overcomes the limitations of conventional dye labelling. The ecotoxicological relevance of the obtained label-free PET nanoplastics is validated through in vivo exposure on the freshwater zooplanctonic model species Daphnia magna at two different temperatures. Confocal microscopy indicates a prevalent accumulation in the gastrointestinal tract. Recovery results reveal a clear decline in organism survival that is dependent on both dose and temperature, indicating that higher temperatures amplify the toxic effects of PET nanoparticles. This highlights the potential for global warming to exacerbate the ecological impacts of microplastics.