Small micro- and nanoplastic test and reference materials for research: Current status and future needs

Environmental plastic pollution comprises partially degraded particles representing a continuum of sizes, shapes, polymer types and chemical compositions. Owing to their potential for biological uptake, small microplastic particles (sMP; <100 μm) and nanoplastics (NPs) are considered to be a potential risk to organisms. Understanding how sMPs and NPs behave in the environment, and how environmental matrices affect their detection, is fundamental to quantifying exposures, assessing hazards and understanding these risks. For this purpose, highquality, well-characterised and environmentally relevant test and reference materials are crucial. The current lack of environmentally relevant sMP and NP reference materials has resulted in many studies applying commercially available spherical, homogenous and monodisperse particles, typically produced for specific purposes and without environmental relevance. There is a need for sMP and NP test/reference materials for fate and effects assessments and analytical protocol validation that more accurately represent the sMP and NP present in the environment. To date, feasible methods for producing relevant sMP and NP test materials in sufficient quantities for environmental fate and effects studies remain lacking. The current review provides an overview and comparison of the available methods, highlighting those that show the most promise for producing environmentally relevant sMP and NP with further development and optimisation.