Grantee Research Project Results

Hypothesis 1:  We conditionally rejected this hypothesis. We have observed this effect only for smaller particles (4 versus 18 nm Au); however, when comparing 15 nm Ag to 30 nm Ag, we observed increasing uptake with increasing particle size on a mass basis. There are no consistent differences when comparing 20 nm to 55 nm Au, Cu, and Ag particles. There may be an optimal diameter for uptake by cells so that sometimes larger particles can be more bioavailable than smaller particles. It also is important to note that on a particle number basis, we always observe greater uptake for smaller particles, even though on a particle mass concentration basis we did not.

 

Hypothesis 2:  We conditionally failed to reject this hypothesis. Our studies with trophic transfer of Au nanoparticles from earthworms to frogs showed that there was trophic dilution rather than biomagnification and that the particles were indeed more bioaccumulative through trophic exposure than direct exposure. In another study, however, we observed biomagnification when particles were transferred from plants to insect larvae.

 

Hypothesis 3:  We reject this hypothesis. We saw little evidence of dramatic differences in toxicity based on particle size; however, particle composition seemed to be the driving factor in determining toxicity.

 

Hypothesis 4:  We failed to reject this hypothesis. We have observed some evidence of reproductive toxicity in Eisenia fetida from Au particles and there is evidence from gene expression studies and x-ray absorption microspectroscopy that no free Au ions are released. Additionally, we have demonstrated that behavioral avoidance of Ag nanoparticle-containing soils is related to intact particles and not free ions.

We have published six peer-reviewed journal articles, one book chapter, and two peer-reviewed conference proceedings; we also have three additional manuscripts in preparation. We expect a total of 11 peer-reviewed publications and one book chapter resulting from this award. We also have given numerous invited talks on our research and published numerous abstracts. We provided the first solid evidence that nanomaterials are bioavailable from soils and can be transmitted from one trophic level to the next. Our studies were considered groundbreaking and received considerable attention in the scientific and popular media. One study also provided the first evidence for biomagnification of nanoparticles in a terrestrial food chain. Given our findings that Ag nanoparticles were the most toxic, we focused on additional endpoints for Ag particles as well as comparing different particle sizes (15 and 30 nm) and surface functionalities (citrate, PVP and oleic acid). We found that reproductive toxicity of Ag nanoparticles in earthworms is far less than for AgNO₃ and that reproductive responses are related to the release of the free Ag ion rather than direct particle effects. On the other hand, we have found that the most sensitive endpoint is behavioral avoidance of soils containing Ag nanoparticles, which occurs at concentrations 100X lower than for reproductive toxicity. Based on X-ray absorption spectroscopy data, we concluded that the responses were due to intact particles and not Ag ions released as a result of dissolution. We also confirmed that the avoidance was not an indirect effect of changes in microbial composition, thus providing the first evidence that organisms can actually sense nanomaterials. We also demonstrated that Ag nanoparticles cause changes in gene expression and cause oxidative damage to proteins in earthworms exposed at relatively low concentrations in soil.