Beyond Membrane Fluidity: Lipid Unsaturation and Hofmeister Cations Govern Nanoplastic Dynamics at Membrane Interfaces.

Interactions between plastic nanoparticles (PNPs) or nanoplastics and lipid membranes are governed by a coupled interplay of membrane composition and ionic environment, yet how lipid unsaturation and ion specificity regulate PNP diffusion remains poorly understood. Here, we investigate the effects of acyl-chain saturation and Hofmeister cations on the interactions of carboxylated polystyrene (PS) nanoparticles with phosphatidylcholine membranes composed of dipalmitoylphosphatidylcholine (DPPC), palmitoyloleoylphosphatidylcholine (POPC), and dioleoylphosphatidylcholine (DOPC) lipids. Langmuir isotherms show that lipid packing decreases with increasing unsaturation and that divalent cations, particularly Ca2+, induce pronounced membrane condensation across all lipid types, while monovalent cations produce weaker effects. To relate membrane structure to nanoparticle diffusion, we combine single-particle tracking (SPT) with fluorescence correlation spectroscopy super-resolution optical fluctuation imaging (fcsSOFI) as an analytical method to quantify nanoparticle confinement and diffusion on supported lipid bilayers. Salt addition modulates diffusion and confinement independently, in a manner that depends strongly on lipid identity. On POPC, confinement follows a divalent/monovalent distinction consistent with the Hofmeister series, while diffusion remains unchanged. On DPPC, diffusion is reduced by all salts without systematic confinement changes. On DOPC, both properties are largely insensitive to ion identity. These results show that nanoplastic dynamics at membrane interfaces cannot be predicted from membrane fluidity alone and are governed by the interplay between membrane phase state, mechanical compliance, and ion-specific headgroup interactions.