Silver nanowire exposure results in internalization and toxicity to Daphnia magna

Abstract

Nanowires (NWs), high-aspect-ratio nanomaterials, are increasingly used in technological materials and consumer products and may have toxicological characteristics distinct from nanoparticles. We carried out a comprehensive evaluation of the physicochemical stability of four silver nanowires (AgNWs) of two sizes and coatings and their toxicity to Daphnia magna . Inorganic aluminum-doped silica coatings were less effective than organic poly(vinyl pyrrolidone) coatings at preventing silver oxidation or Ag(+) release and underwent a significant morphological transformation within 1 h following addition to low ionic strength Daphnia growth media. All AgNWs were highly toxic to D. magna but less toxic than ionic silver. Toxicity varied as a function of AgNW dimension, coating, and solution chemistry. Ag(+) release in the media could not account for observed AgNW toxicity. Single-particle inductively coupled plasma mass spectrometry distinguished and quantified dissolved and nanoparticulate silver in microliter-scale volumes of Daphnia magna hemolymph with a limit of detection of approximately 10 ppb. The silver levels within the hemolymph of Daphnia exposed to both Ag(+) and AgNW met or exceeded the initial concentration in the growth medium, indicating effective accumulation during filter feeding. Silver-rich particles were the predominant form of silver in hemolymph following exposure to both AgNWs and Ag(+). Scanning electron microscopy imaging of dried hemolymph found both AgNWs and silver precipitates that were not present in the AgNW stock or the growth medium. Both organic and inorganic coatings on the AgNW were transformed during ingestion or absorption. Pathway, gene ontology, and clustering analyses of gene expression response indicated effects of AgNWs distinct from ionic silver on Daphnia magna .

Figure 1

TEM study of the morphology of the aluminosilicate coating on (a) as-synthesized S-SiO₂-NW, or following a two-hour exposure to (b) COMBO or (c) EPA media. Other representative micrographs showing morphological changes in the S-SiO₂-NW exposed to EPA media are given in Figure 2 and Figure S4.

Figure 2

STEM study of S-SiO2-NW following exposure to EPA medium. Left panel shows the STEM image; other panels show corresponding elemental maps depicting the distribution of silver (Ag), silicon (Si) and sodium (Na).

Figure 3

Determination of relative rates of release of Ag⁺ by long (L) and short (S) AgNWs at a silver concentration of 111 μM in water and in Daphnia media. Silver was undetectable for S-PVP-NW in COMBO and water and for all L-NW samples. Ag⁺ was defined as the fraction passing through a 0.2-μm filter. The main source of error is Ag⁺ retention in the filter. To represent the magnitude of this uncertainty, the plot contains error bars that are the standard deviations on individual measurements of the second, third and fourth 1-mL portions of the filtrate passed through the same filter.

Figure 4

Acute 24-hour LC₅₀ values for AgNW and Ag⁺ on Daphnia magna in (a) COMBO and (b) EPA media. LC₅₀ values are measured in micrograms silver per liter (μg/L). LC₅₀ values and 95% confidence intervals were determined with probit analysis. A table of LC₅₀ values is found in Table S4. Statistical analysis of LC₅₀ values showed the LC₅₀ values for each AgNW in both media were significantly different. LC₅₀ values for Ag⁺ were not significantly different between the two media.

Figure 5

Hierarchical Ordered Partitioning and Collapsing Hybrid (HOPACH) analysis of gene expression data from AgNW-exposed versus media-only control daphnids. HOPACH clustered data into three groups based on similarity of gene expression profile. Groups are labeled on the x-axis as 1 (L-PVP-NW), 2 (L-SiO₂-NW) or 3 (S-SiO₂-NW, Ag⁺ and S-PVP-NW).

Figure 6

Single-particle inductively-coupled plasma mass spectrometry (spICPMS) quantification of dissolved and particulate silver in the hemolymph of a daphnid exposed to ~2-μm-long silica-coated silver nanowires (AgNWs) at the LC₅₀ concentration (155 μg Ag/L). a) Raw data. b) Histogram of detector response. c) Calculated AgNW length distribution. Blue lines in a) and b) represent the cut-off in spICPMS response used to distinguish the signal from dissolved and particulate silver.

Figure 7

Scanning electron microscope images of comparing AgNWs taken from stock suspension and located in Daphnia hemolymph. (a) PVP-coated AgNW. (b) silica-coated AgNW.