Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Sulfidation of Silver Nanoparticles: Natural Antidote to Their Toxicity”
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n Levard et al.,1 the toxicity of Ag0 nanoparticles (NPs) is compared to the toxicity of partially sulfidized Ag0/Ag2S core−shell NPs for four different test organisms in eight exposure media, each having different chemical constituents. The lower observed toxicity of Ag0/Ag2S core−shell NPs compared to Ag0 NPs is attributed to the lower concentration of dissolved Ag species in the exposure medium from sulfidation and/or the presence of chloride in the exposure medium. This correlation between toxicity and the dissolved Ag concentration was consistent across all organisms studied. Liu et al.,2 suggest that the importance of dissolved Ag as the species responsible for the toxicity of Ag0 and Ag0/Ag2S coreshell NPs is overstated. They use an extrapolation to estimate solubility of the Ag0 NPs at the exposure concentration for Caenorhabditis elegans from the solubility measured in the same exposure medium or in DI water but at a much higher Ag0 NP concentration. They conclude that the reported toxicity to C. elegans must be NP specific because there should be no effects or lethality at the estimated Ag+ exposure concentration based on previously reported EC50 lethality for C. elegans exposed to the same particles.3 While the Levard et al.1 study was not designed to prove or disprove a specific toxicity mechanism, there are several reasons why the arguments of Lui et al.2 do not change our assertion about the primary importance of dissolved Ag species in the exertion of toxicity. One argument is physicochemical, while the other arguments are related to the specific conditions of the exposure of C. elegans in Levard et al.1 compared to Yang et al.3
et al. due to a higher percentage of the particles being dissolved at the lower exposure concentration.
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BIOLOGICAL FACTORS Liu et al.2 state that the estimated amount of dissolved silver in our exposure media would not be expected to cause the toxicity that we reported (Figure 4 in Levard et al.,1 2013), based on our own previous dose−response data.3 The actual dissolved silver concentration is likely higher, as discussed above. However, even if the dissolved Ag concentration were as low as estimated by Liu et al.,2 there are differences in the exposure conditions between the two studies that may also account for the apparently higher-than-expected toxicity. The dose− response reported in Yang et al.3 (with LC50 between 0.1 and 0.15 mg/L) was for fed L1 larvae, whereas the toxicity data in Figure 4 in Levard et al., 20131 is for unfed L1 larvae. This is important because the presence of the bacterial food source is highly protective against both Ag0 NP and AgNO3 toxicity.7 Thus, the toxicity that we report in Figure 4 in Levard et al., 20131 is expected to result from a much lower concentration of silver ion than would be predicted based on Yang et al.3 To permit a more direct comparison, we report here (Figure 1) the
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PHYSICOCHEMICAL ARGUMENT Liu et al.2 estimated the maximum dissolved silver concentration at 0.04 mg/L in the C. elegans exposure medium. This is calculated assuming 2% solubility of the 2 mg/L Ag0 NP exposure concentration as indicated in Figure 4 with almost 100% lethality in C. elegans. However, the 2% solubility was measured at high concentrations of Ag0 NPs (1000 mg/L and 144 mg/L respectively). Unfortunately, it is not possible to scale the dissolved Ag concentration measured at high Ag0 NP concentration to a low Ag0 NP concentration. Especially considering that at low concentration, the system is undersaturated with respect to Ag2O as observed by Liu et al.4 who found 100% dissolution for an initial Ag0 NP concentration of up to 2 mg/L in DI water. Similarly, Ostermeyer et al.,5 measured 25 to 100% dissolution of 1 mg/L Ag0 NPs in HEPES buffer and Cunningham et al.6 measured between 12% and 40% dissolution at 50 mg/L Ag0 NPs in zebrafish embryo medium. More importantly, the Ag0 NPs in the presence of organisms like C. elegans are not likely to be at equilibrium with respect to their solubility. Indeed, the worms will likely act as a sink for dissolved Ag, driving further dissolution of the Ag0 NPs. While the dissolved silver concentration was not directly measured at the 0.5−2 mg/L exposure concentration, it was likely significantly higher than the 0.04 mg/L estimated by Liu © 2014 American Chemical Society
Figure 1. Dose−response curve for L1 stage C. elegans exposed to AgNO3 in EPA water for 24 h without food. This data was collected during the Levard et al.1 study, but not previously reported. The experiment was performed twice, each with five replicate wells per dose, containing 10 individuals in 100 μL in 96 well plates as described in Levard et al.1 Error bars are standard errors of the mean.
24 h dose−response mortality for exposure of unfed L1 larvae in the exposure medium used in Levard et al.1 (i.e., in EPA water; LC50 ∼0.04 mg/L)). Based on this data, the concentration of dissolved silver estimated by Liu et al.2 (0.04 mg/L) is very similar to the concentration that causes high levels of lethality. In addition, the dose−response is very steep, such that very small changes in the amount of dissolution that actually occurred under our experimental conditions could explain a significant shift in mortality. Published: May 8, 2014 6051
dx.doi.org/10.1021/es500991r | Environ. Sci. Technol. 2014, 48, 6051−6052
Environmental Science & Technology
Correspondence/Rebuttal
In conclusion, we agree with Liu et al.2 that the mechanism of toxicity reduction of Ag0 NPs upon sulfidation remains to be proven. The route of exposure and other factors may also influence bioavailability of Ag. We also agree that the release of dissolved Ag species might not be the only process explaining the observed toxicity in our study. However, our results in Levard et al.1 and in the present analysis support our assertion that it is the dominant one. This finding is also consistent with other recent reports demonstrating that dissolved silver is the dominant species responsible for Ag0 NP toxicity.8,9
(8) Newton, K. M.; Puppala, H. L.; Kitchens, C. L.; Colvin, V. L.; Klaine, S. J. Silver nanoparticle toxicity to Daphnia magna is a function of dissolved silver concentration. Environ. Toxicol. Chem. 2013, 32 (10), 2356−2364, DOI: 10.1002/etc.2300. (9) Xiu, Z.-m.; Zhang, Q.-b.; Puppala, H. L.; Colvin, V. L.; Alvarez, P. J. J. Negligible particle-specific antibacterial activity of silver nanoparticles. Nano Lett. 2012, 12 (8), 4271−4275, DOI: 10.1021/ nl301934w.
Clément Levard§,⊥ Xinyu Yang‡,⊥ Joel N. Meyer‡,⊥ Gregory V. Lowry*,†,⊥ †
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Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, 15213, United States of America ‡ Nicholas School of the Environment, Duke University, Durham, North Carolina, 27708, United States of America § Aix-Marseille Université, CNRS, IRD, CEREGE UM34, 13545, Aix en Provence, France ⊥ Center for Environmental Implications of NanoTechnology (CEINT), Duke University, P. O. Box 90287, Durham, North Carolina 27708-0287, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Levard, C.; Hotze, E. M.; Colman, B. P.; Dale, A. L.; Truong, L.; Yang, X. Y.; Bone, A. J.; Brown, G. E.; Tanguay, R. L.; Di Giulio, R. T.; Bernhardt, E. S.; Meyer, J. N.; Wiesner, M. R.; Lowry, G. V. Sulfidation of silver nanoparticles: natural antidote to their toxicity. Environ. Sci. Technol. 2013, 47 (23), 13440−13448, DOI: 10.1021/es403527n. (2) Liu, Z.; Y, H.; Zhi, D. Comment on “Sulfidation of silver nanoparticles: natural antidote to their toxicity”. Environ. Sci. Technol. 2014, DOI: 10.1021/es405384p. (3) Yang, X.; Gondikas, A. P.; Marinakos, S. M.; Auffan, M.; Liu, J.; Hsu-Kim, H.; Meyer, J. N. Mechanism of silver nanoparticle toxicity is dependent on dissolved silver and surface coating in Caenorhabditis elegans. Environ. Sci. Technol. 2012, 46 (2), 1119−1127, DOI: 10.1021/es202417t. (4) Liu, J.; Hurt, R. H. Ion release kinetics and particle persistence in aqueous nano-silver colloids. Environ. Sci. Technol. 2010, 44 (6), 2169−2175, DOI: 10.1021/es9035557. (5) Ostermeyer, A.-K.; Kostigen Mumuper, C.; Semprini, L.; Radniecki, T. Influence of bovine serum albumin and alginate on silver nanoparticle dissolution and toxicity to Nitrosomonas europaea. Environ. Sci. Technol. 2013, 47 (24), 14403−14410, DOI: 10.1021/ es4033106. (6) Cunningham, S.; Brennan-Fournet, M. E.; Ledwith, D.; Byrnes, L.; Joshi, L. Effect of nanoparticle stabilization and physicochemical properties on exposure outcome: Acute toxicity of silver nanoparticle preparations in zebrafish (Danio rerio). Environ. Sci. Technol. 2013, 47 (8), 3883−3892, DOI: 10.1021/es303695f. (7) Yang, X.; Jiang, C.; Hsu-Kim, H.; Raju Badireddy, A.; Dyksra, M.; Wiesner, M.; Hinton, D. E.; Meyer, J. N. Silver nanoparticle behavior, uptake, and toxicity in Caenorhabditis elegans: Effects of natural organic matter. Environ. Sci. Technol. 2014, 48 (6), 3486−3495, DOI: 10.1021/ es404444n. 6052
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