Materials, surfaces, and interfacial phenomena in nanoplastics toxicology research

Abstract

In response to the growing worldwide plastic pollution problem, the field of nanoplastics research is attempting to determine the risk of exposure to nanoparticles amidst their ever-increasing presence in the environment. Since little is known about the attributes of environmental nanoplastics (concentration, composition, morphology, and size) due to fundamental limitations in detection and quantification of smaller plastic particles, researchers often improvise by engineering nanoplastic particles with various surface modifications as models for laboratory toxicological testing. Polystyrene and other commercially available or easily synthesized polymer materials functionalized with surfactants or fluorophores are typically used for these studies. How surfactants, additives, fluorophores, the addition of surface functional groups for conjugation, or other changes to surface attributes alter toxicological profiles remains unclear. Additionally, the limited polymers used in laboratory models do not mimic the vast range of polymer types comprising environmental pollutants. Nanomaterials are tricky materials to investigate due to their high surface area, high surface energies, and their propensity to interact with molecules, proteins, and biological probes. These unique properties can often invalidate common laboratory assays. Extreme care must be taken to ensure that results are not artefactual. We have gathered zeta potential values for various polystyrene nanoparticles with different functionalization, in different solvents, from the reported literature. We also discuss the effects of surface engineering and solvent properties on interparticle interactions, agglomeration, particle-protein interactions, corona formation, nano-bio interfaces, and contemplate how these parameters might confound results. Various toxicological exemplars are critically reviewed, and the relevance and shortfalls of the most popular models used in nanoplastics toxicity studies published in the current literature are considered.

Keywords: Nanoplastics; Pollution; Polymeric nanoparticles; Polystyrene; Protein corona; Toxicity.

Fig. 1.

Reported relative abundance of (a) microplastics and (b) nanoplastics in the envrionment, compared to the published toxicological studies on nanoplastics by polymer type (c).

Fig. 2.

Zeta potential measurements and electrical double layer on surfaces of negatively charged (left) and positively charged (right) nanoparticles.

Fig. 3.

Reported zeta potential values for PS beads used in nanoplastics toxicological studies. Values are shown for aminated polystyrene (APS), carboxylated polystyrene (CPS), fluorescent polystyrene (FPS), or “unfunctionalized” polystyrene (PS) in freshwater (FW), saltwater (SW), or media.