Aggregation kinetics of diesel soot nanoparticles in lung fluids: Effects of exposure conditions, fluid properties, and pulmonary surfactant lipids.

Soot nanoparticles (SNPs) are carbonaceous particulate matter with significant environmental and health impacts. Once inhaled, their aggregation in the respiratory system can influence their migration patterns and health hazards. This study investigated the effects of exposure conditions (interaction time, particle concentration, and activity state), fluid properties (pH and composition), and pulmonary surfactant lipids [micro-sized (m-DPPC) and nano-sized dipalmitoylphosphatidylcholine (n-DPPC)] on aggregation kinetics of SNPs in five lung fluids. Early-stage aggregation rates ranked artificial lysosomal fluid (0.64 nm/s) > simulated alveolar fluid (0.20 nm/s) > simulated lung fluid (0.17 nm/s) > simulated serum (0.11 nm/s) > Gamble's solution (0.03 nm/s), indicating potential SNP migration into the lower respiratory tract and alveolar interstitial spaces. Increasing particle concentration and reducing pH both promoted aggregation. Under static conditions, SNPs formed larger aggregates (397.8-5441 nm) than dynamic conditions (209.7-2461 nm) across all lung fluids over 24 h. Aggregation was driven by Ca, Mg, citric acid, sodium lactate, sodium citrate, and glycine. Among two lipids, m-DPPC facilitated aggregation through charge neutralization and bridging adsorption, while n-DPPC inhibited aggregation via steric hindrance, consistent with the modified Derjaguin-Landau-Verwey-Overbeek (MDLVO) theory. These findings underscore the significant impact of lung fluids on migration and risks of SNPs in respiratory systems.