Toxicity of various silver nanoparticles compared to silver ions in Daphnia magna

Abstract

Background: To better understand the potential ecotoxicological impacts of silver nanoparticles released into freshwater environments, the Daphnia magna 48-hour immobilization test was used.

Methods: The toxicities of silver nitrate, two types of colloidal silver nanoparticles, and a suspension of silver nanoparticles were assessed and compared using standard OECD guidelines. Also, the swimming behavior and visible uptake of the nanoparticles by Daphnia were investigated and compared. The particle suspension and colloids used in the toxicity tests were well-characterized.

Results: The results obtained from the exposure studies showed that the toxicity of all the silver species tested was dose and composition dependent. Plus, the silver nanoparticle powders subsequently suspended in the exposure water were much less toxic than the previously prepared silver nanoparticle colloids, whereas the colloidal silver nanoparticles and AgNO(3) were almost similar in terms of mortality. The silver nanoparticles were ingested by the Daphnia and accumulated under the carapace, on the external body surface, and connected to the appendages. All the silver species in this study caused abnormal swimming by the D. magna.

Conclusion: According to the present results, silver nanoparticles should be classified according to GHS (Globally Harmonized System of classification and labeling of chemicals) as "category acute 1" to Daphnia neonates, suggesting that the release of nanosilver into the environment should be carefully considered.

Figure 1

TEM micrographs of different nanoparticles: (A) nAg1 colloid, (B) nAg2 colloid, (C) dry powder of nAg3, and (D) suspension of nAg3.

Figure 2

Size distribution of particles based on number frequency determined from transmission electron microscope data in: (A) nAg1 colloid, (B) nAg2 colloid, (C) and dry powder of nAg3.

Figure 3

Size distribution of particles based on cumulative frequency determined from transmission electron microscope data in: (A) nAg1 colloid, (B) nAg2 colloid, (C) and dry powder of nAg3. Statistically significant differences were found among particle size distributions (P < 0.001, nAg1 vs nAg2; P < 0.001, nAg1 vs nAg3; P < 0.001, nAg2 vs nAg3).

Figure 4

EDX spectrometer patterns of nAg2 and nAg3; (Ni signals in EDX spectrometer are from TEM grid).

Figure 5

UV-VIS absorption spectra for colloids of nAg1 and nAg2, nAg3 suspension, and AgNO₃ solution.

Figure 6

Photograph comparing appearance of aqueous stocks of nanoparticles (400 mg/L) used for toxicity tests. A: nAg1 colloid; B: nAg2 colloid; C: suspensions of nAg3 before (left) and after (right) 24 hours.

Figure 7

Light microscope images of daphnia exposed to nAg1 and nAg2 colloids for 24 hours. A: control; B: live daphnia exposed to 0.002 mg/L nAg1, pigmentation can been seen under the brood chamber (circles); C: dead daphnia exposed to 0.01 mg/L nAg2; D: live daphnia exposed to 0.004 mg/L nAg1; E: live daphnia exposed to 0.002 mg/L nAg2. In images C, D, and E, small bubbles can be seen under the carapace; plus, nanoparticle aggregates can be seen on the antennae, body surface, and also in the brood chamber.

Figure 8

D. magna after 24-hour exposure to 0.15 mg/L aqueous suspension of nAg3. A: black color of digestive tract shows uptake of nAg3. B and C: nanoparticle aggregates are attached to antennae and also seen in brood chamber.