Water at the Interface: How Membrane Charge Controls Nanoplastic–Lipid Interactions

Nanoplastics are pervasive environmental contaminants capable of crossing biological barriers and interacting with cell membranes. However, the molecular mechanisms by which they perturb lipid interfaces remain poorly understood. Here, we combine Fourier-transform infrared spectroscopy (FTIR), ultrafast two-dimensional infrared spectroscopy (2D IR), and surface-enhanced infrared absorption (SEIRA) spectroscopy to investigate how anionic polystyrene nanoplastic (NP) particles modulate the environment at the lipid–water interface in model lipid membranes by varying composition and ionic environment. Neutral phosphatidylcholine (POPC) and anionic phosphatidylcholine/phosphatidylserine (POPC/POPS) membranes were used to probe the role of membrane charge on NP-induced interfacial perturbations. FTIR measurements show that NP-induced dehydration of the lipid headgroup region depends strongly on membrane composition, with partial dehydration observed in neutral POPC membranes but negligible changes in anionic POPC/POPS membranes. 2D IR spectra show that NPs induce faster H-bond fluctuations, as water molecules become more labile at the interface. SEIRA measurements further show that these interfacial perturbations are accompanied by localized changes in the acyl-chain ordering. Addition of Ca2+ enhances NP-induced changes to the negatively charged membranes, resulting in hydration dynamics and acyl-chain ordering that resemble those observed in neutral POPC membranes. These results are consistent with the hypothesis that interfacial dehydration is an early event associated with NP-membrane interactions and highlight the role of membrane charge and ionic environment in modulating these interactions.