The stability of silver nanoparticles in a model of pulmonary surfactant

The growing use of silver nanoparticles (AgNPs) in consumer products has raised concerns about their potential impact on the environment and human health. Whether AgNPs dissolve and release Ag⁺ ions, or coarsen to form large aggregates, is critical in determining their potential toxicity. In this work, the stability of AgNPs in dipalmitoylphosphatidylcholine (DPPC), the major component of pulmonary surfactant, was investigated as a function of pH. Spherical, citrate-capped AgNPs with average diameters of 14 ± 1.6 nm (n = 200) were prepared by a chemical bath reduction. The kinetics of Ag ⁺ ion release was strongly pH-dependent. After 14 days of incubation in sodium perchlorate (NaClO₄) or perchloric acid (HClO₄) solutions, the total fraction of AgNPs dissolved varied from ∼10% at pH 3, to ∼2% at pH 5, with negligible dissolution at pH 7. A decrease in pH from 7 to 3 also promoted particle aggregation and coarsening. DPPC (100 mg·L^-1) delayed the release of Ag⁺ ions, but did not significantly alter the total amount of Ag⁺ released after two weeks. In addition, DPPC improved the dispersion of the AgNPs and inhibited aggregation and coarsening. TEM images revealed that the AgNPs were coated with a DPPC layer serving as a semipermeable layer. Hence, lung lining fluid, particularly DPPC, can modify the aggregation state and kinetics of Ag ⁺ ion release of inhaled AgNPs in the lung. These observations have important implications for predicting the potential reactivity of AgNPs in the lung and the environment.