Comprehensive Multiphase NMR Examination of Amino Acids Binding to the Dynamic Shell of Polystyrene Nanoparticles to Understand Environmental Hazards Associated with Nanoscale Plastic

Plastic pollution is a growing environmental concern. In addition to posing hazards to wildlife, micro- and nano-scale plastic particles can adsorb toxic small molecules from polluted waterways, which may be released when these plastic particles are ingested by animals or humans. Previous NMR studies have examined the binding between surface-modified polystyrene nanoparticles and amino acids as models for small molecules with a variety of functional groups. These previous studies, however, could only examine the liquid phase and therefore focused on the small molecules. In the current study, we use comprehensive multiphase NMR (CMP-NMR) to examine both the small molecules and polystyrene nanoparticles in all phases including liquid, solid, and gel-like phases. Through proton spectral editing techniques and 13C solid-state NMR experiments, we find that the polystyrene nanoparticles contain both solid and gel-like fractions. The bound amino acid exists primarily in the gel-like phase, with very little amino acid existing in the true solid phase. This suggests that the bound amino acid interacts with the nanoparticle shell rather than the solid-like core. These experiments relied on the ability of CMP-NMR to separately observe the solid, liquid, and gel-like phases of the same sample and demonstrate the complementary nature of this approach for understanding complex multiphase systems.