Toxic Effects of Silver Ions on Early Developing Zebrafish Embryos Distinguished from Silver Nanoparticles

Currently, effects of nanomaterials and their ions, such as silver nanoparticles (Ag NPs) and silver ions (Ag⁺), on living organisms are not yet fully understood. One of the vital questions is whether nanomaterials have distinctive effects on living organisms from any other conventional chemicals (e.g., their ions), owing to their unique physicochemical properties. Due to various experimental protocols, studies of this crucial question have been inconclusive, which hinders rational design of effective regulatory guidelines for safely handling NPs. In this study, we chronically exposed early developing zebrafish embryos (cleavage-stage, 2 hours post-fertilization, hpf) to a dilution series of Ag⁺ (0-1.2 μM) in egg water (1 mM NaCl, solubility of Ag⁺ = 0.18 μM) until 120 hpf. We systematically investigated effects of Ag⁺ on developing embryos and compared them with our previous studies of effects of purified Ag NPs on developing embryos. We found the concentration- and time-dependent effects of Ag⁺ on embryonic development, and only half of the embryos developed normally after being exposed to 0.25 μM (27 μg/L) Ag⁺ until 120 hpf. As the Ag⁺ concentration increases, the number of embryos that developed normally decreases, while the number of embryos that became dead increases. The number of abnormally developing embryos increases as the Ag⁺ concentration increases from 0 to 0.3 μM and then decreases as the concentration increases from 0.3 to 1.2 μM because the number of embryos that became dead increases. The concentration-dependent phenotypes were observed, showing fin fold abnormality, tail and spinal cord flexure, and yolk sac edema at low Ag⁺ concentrations (≤0.2 μM) and head and eye abnormalities along with fin fold abnormality, tail and spinal cord flexure, and yolk sac edema at high concentrations (≥0.3 μM). Severities of phenotypes and the number of abnormally developing embryos were far less than those observed in Ag NPs. The results also show concentration-dependent effects on heart rates and hatching rates of developing embryos, attributing to the dose-dependent abnormally developing embryos. In summary, the results show that Ag⁺ and Ag NPs have distinctive toxic effects on early developing embryos, and toxic effects of Ag⁺ are far less severe than those of Ag NPs, which further demonstrates that the toxicity of Ag NPs toward embryonic development is attributed to the NPs themselves and their unique physicochemical properties but not the release of Ag⁺ from the Ag NPs.