Are all nanoplastics equally neurotoxic? Influence of size and surface functionalization on the toxicity of polystyrene nanoplastics in human neuronal cells

Nanoplastics (NPs) are an emerging environmental concern, particularly due to their potential interactions with the nervous system. We evaluated the neurotoxic effects of four types of polystyrene nanoplastics (PS-NPs): unmodified (plain) 50 nm and 100 nm particles, and 100 nm particles functionalized with carboxyl (C-NPs) or amine (A-NPs) groups. Human SH-SY5Y neuroblastoma cells were exposed to 1-500 μg/mL for 24 or 48 h. Dynamic light scattering revealed aggregation of plain 50 nm NPs in water, while presence of ≥1 % fetal bovine serum improved colloidal stability across all particle types. Zeta potentials in water were approximately -45 mV for plain and C-NPs and -30 mV for A-NPs. In complete medium, all values shifted toward -20 to -13 mV, consistent with protein corona formation. Integrating colloidal stability, surface chemistry, and organelle-level analyses, we established a unified mechanistic framework for PS-NP neurotoxicity. Toxicity endpoints included cell viability, ROS/RNS generation, particle internalization, and morphological and ultrastructural analysis. Plain PS-NPs showed negligible cytotoxicity, whereas C-NPs and especially A-NPs significantly reduced cell viability. A-NPs caused a 21-41 % reduction at 100-500 μg/mL after 24 h, and 29-66 % at 200-500 μg/mL after 48 h (p = 0.0007 vs. C-NPs). ROS/RNS levels were highest with plain 100 nm and A-NPs at ≥200 μg/mL, increasing over time. Transmission electron microscopy revealed size- and surface chemistry-dependent damage, including endoplasmic reticulum dilation, Golgi fragmentation, and mitochondrial abnormalities. Internalization followed the trend A-NPs > C-NPs ≫ plain PS-NPs, mirroring organelle damage, apoptosis, autophagy, and lysosomal disruption. These findings identify surface chemistry, more than particle size, as the primary driver of PS-NP neurotoxicity, highlighting its importance for risk prioritization and regulatory frameworks addressing NP hazards.