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 semi-permeable 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.

The mechanism of trenching and pitting at intermetallic particles in AA1050 aluminum was investigated by open circuit potential measurements and microscopic polarization measurements, and the role played by the two types of intermetallic particles, Al-Fe and Al-Fe-Si, in this process was determined. Trenching was observed only around the Al-Fe-Si particles in the naturally aerated 0.1 M NaCl solution. Under the anodic polarization of AA1050 in naturally aerated 0.1 M NaCl, a crystallographic pit was initiated in the trench. However, neither trenching nor pitting occurred in the citric-citrate buffer with 0.1 M NaCl under naturally aerated conditions. Trenching was confirmed to be the result of the local alkalization induced by the oxygen reduction reaction on the particles. Microscale polarization showed that trenching occurred around the Al-Fe-Si particle from −0.9 to −0.6 V and did not occur above −0.5 V. While the surface of pure Al dissolved at pH 14 in 1 M NaCl, the corrosion morphology was different from pitting. Crystallographic pits occurred when the pure Al was immersed in 1 M NaCl at pH 0.0 after preimmersion in 1 M NaCl (pH 14). It was concluded that the change in local pH from alkaline to acidic triggers the morphological change from trenching to pitting.

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