Fluorescently Labelled Polypropylene as a Model Microplastic for Cellular Imaging.

Polypropylene (PP) is the second most produced plastic globally and is environmentally pervasive in the form of microplastics (MP). The cellular localisation and fate of MPs within biological systems remain poorly understood. We present a method for the covalent functionalisation of PP with the fluorescent label, rhodamine B (Rb), and its subsequent MP evaluation in a human embryonic kidney model cell line (HEK293T). Rb was successfully coupled into PP (PP Rb ) under Steglich esterification conditions. PP Rb is stabile under physiological pH and so can be utilised in vitro, offering a significantly improved MP model to conventional staining methods that can definitively locate MP in human cells. Cellular uptake of PP Rb was rapid (≤ 1 h), non-toxic up to 70 µM, and was not observed to localise within the nucleus. Increased mean lysosomal fluorescence intensity suggested an endosomal uptake mechanism followed by significant clearance over a 48 h period. This approach provides a robust platform for definitive microplastic tracking, enabling reliable, standardised studies of polymer-cell interactions across biological systems.