Influence of Natural Organic Matter and Surface Charge on the Toxicity and Bioaccumulation of Functionalized Ceria Nanoparticles in <i>Caenorhabditis elegans</i>

The objective of this study was to investigate the role of the CeO₂ nanoparticle (NP) surface charge and the presence of natural organic matter (NOM) in determining bioavailability and toxicity to the model soil organism <i>Caenorhabditis elegans</i>. We synthesized CeO₂-NPs functionalized with positively charged, negatively charged, and neutral coatings. The positively charged CeO₂-NPs were significantly more toxic to <i>C. elegans</i> and bioaccumulated to a greater extent than the neutral and negatively charged CeO₂-NPs. Surface charge also affected the oxidation state of Ce in <i>C. elegans</i> tissues after uptake. Greater reduction of Ce from Ce (IV) to Ce (III) was found in <i>C. elegans</i>, when exposed to the neutral and negatively charged relative to positively charged CeO₂-NPs. The addition of humic acid (HA) to the exposure media significantly decreased the toxicity of CeO₂-NPs, and the ratio of CeO₂-NPs to HA influenced Ce bioaccumulation. When the concentration of HA was higher than the CeO₂-NP concentration, Ce bioaccumulation decreased. These results suggest that the nature of the pristine coatings as a determinant of hazard may be greatly reduced once CeO₂-NPs enter the environment and are coated with NOM

Bioaccumulation of Gold Nanomaterials by <i>Manduca sexta</i> through Dietary Uptake of Surface Contaminated Plant Tissue

We investigated the potential for bioaccumulation of engineered nanomaterials (ENMs) by tobacco hornworm (<i>Manduca sexta</i>) caterpillars resulting from the ingestion of plant tissue surface contaminated with ENMs. Caterpillars were fed tomato leaf tissue that had been surface contaminated with 12 nm tannate coated Au ENMs. After dosing was complete, bulk Au concentrations in individual caterpillars were measured after 0, 1, 4, and 7 days of elimination. Growth, mortality, and ingestion rate were monitored. This experiment revealed (1) no evidence that caterpillars were affected by ingestion of ENM contaminated plant tissue; (2) low bioaccumulation factors (BAF = 0.16) compared to a previous study where hornworm caterpillars were fed plants that had previously bioaccumulated Au ENMs (BAF = 6.2–11.6); (3) inefficient elimination of accumulated Au ENMs not associated with hornworm gut contents; and (4) regional differences in translocation of Au ENMs into tissues surrounding the hornworm gut, possibly the result of the interaction between ENM surface chemistry and regional differences in hornworm gut chemistry. These data, along with previous findings, indicate that although ENMs resuspended from soil onto plant surfaces by wind, water, biota, and/or mechanical disturbances are bioavailable to terrestrial consumers, bioaccumulation efficiency may be much lower via this pathway than through direct trophic exposure

Bioavailability of Gold Nanomaterials to Plants: Importance of Particle Size and Surface Coating

We used the model organisms <i>Nicotiana tabacum</i> L. cv <i>Xanthi</i> (tobacco) and <i>Triticum aestivum</i> (wheat) to investigate plant uptake of 10-, 30-, and 50-nm diameter Au manufactured nanomaterials (MNMs) coated with either tannate (T-MNMs) or citrate (C-MNMs). Primary particle size, hydrodynamic size, and zeta potential were characterized using transmission electron microscopy (TEM), dynamic light scattering (DLS), and electrophoretic mobility measurements, respectively. Plants were exposed to NPs hydroponically for 3 or 7 days for wheat and tobacco, respectively. Volume averaged Au concentrations were determined using inductively coupled plasma mass spectrometry (ICP-MS). Spatial distribution of Au in tissue samples was determined using laser ablation ICP-MS (LA-ICP-MS) and scanning X-ray fluorescence microscopy (μXRF). Both C-MNMs and T-MNMs of each size treatment bioaccumulated in tobacco, but no bioaccumulation of MNMs was observed for any treatment in wheat. These results indicate that MNMs of a wide range of size and with different surface chemistries are bioavailable to plants, provide mechanistic information regarding the role of cell wall pores in plant uptake of MNMs, and raise questions about the importance of plant species to MNM bioaccumulation

Trophic Transfer of Au Nanoparticles from Soil along a Simulated Terrestrial Food Chain.

To determine if nanoparticles (NPs) could be transferred from soil media to invertebrates and then to secondary consumers, we examined the trophic transfer of Au NPs along a simulated terrestrial food chain. Earthworms (<i>Eisenia fetida</i>) were exposed to Au NPs in artificial soil media and fed to juvenile bullfrogs (<i>Rana catesbeina</i>). Earthworm Au concentrations were continuously monitored so that the cumulative dose to bullfrogs could be accurately estimated throughout the experiment. We exposed a second group of bullfrogs to equivalent doses of Au NPs by oral gavage to compare the bioavailability of NPs through direct exposure to trophic exposure. We observed accumulation of Au in liver, kidney, spleen, muscle, stomach, and intestine in both treatment groups. Tissue concentrations decreased on average of approximately 100-fold with each trophic-step. The total assimilated dose averaged only 0.09% of the administered dose for direct exposure (oral gavage), but 0.12% for the trophic exposure. The results suggest that manufactured NPs present in soil may be taken up into food chains and transferred to higher order consumers. They also suggest that Au NPs may be more bioavailable through trophic exposure than direct exposure and that trophic transfer may influence the biodistribution of particles once absorbed

Multitechnique Investigation of the pH Dependence of Phosphate Induced Transformations of ZnO Nanoparticles

In order to properly evaluate the ecological and human health risks of ZnO manufactured nanomaterials (MNMs) released to the environment, it is critical to understand the likely transformation products in various environments, such as soils, surface and ground waters, and wastewater treatment processes. To address this knowledge gap, we examined the transformation of 30 nm ZnO MNMs in the presence of different concentrations of phosphate as a function of time and pH using a variety of orthogonal analytical techniques. The data reveal that ZnO MNMs react with phosphate at various concentrations and transform into two distinct morphological/structural phases: a micrometer scale crystalline zinc phosphate phase (hopeite-like) and a nanoscale phase that likely consists of a ZnO core with an amorphous Zn₃(PO₄)₂ shell. The P species composition was also pH dependent, with 82% occurring as hopeite-like P at pH 6 while only 15% occurred as hopeite-like P at pH 8. These results highlight how reactions of ZnO MNMs with phosphate are influenced by environmental variables, including pH, and may ultimately result in structurally and morphologically heterogeneous end products

Biotic and Abiotic Interactions in Aquatic Microcosms Determine Fate and Toxicity of Ag Nanoparticles. Part 1. Aggregation and Dissolution

To better understand their fate and toxicity in aquatic environments, we compared the aggregation and dissolution behavior of gum arabic (GA) and polyvinylpyrrolidone (PVP) coated Ag nanoparticles (NPs) in aquatic microcosms. There were four microcosm types: surface water; water and sediment; water and aquatic plants; or water, sediment, and aquatic plants. Dissolution and aggregation behavior of AgNPs were examined using ultracentrifugation, ultrafiltration, and asymmetrical flow field flow fractionation coupled to ultraviolet–visible spectroscopy, dynamic and static laser light scattering, and inductively coupled plasma mass spectrometry. Plants released dissolved organic matter (DOM) into the water column either through active or passive processes in response to Ag exposure. This organic matter fraction readily bound Ag ions. The plant-derived DOM had the effect of stabilizing PVP-AgNPs as primary particles, but caused GA-AgNPs to be removed from the water column, likely by dissolution and binding of released Ag ions on sediment and plant surfaces. The destabilization of the GA-AgNPs also corresponded with X-ray absorption near edge spectroscopy results which suggest that 22–28% of the particulate Ag was associated with thiols and 5–14% was present as oxides. The results highlight the potential complexities of nanomaterial behavior in response to biotic and abiotic modifications in ecosystems, and may help to explain differences in toxicity of Ag observed in realistic exposure media compared to simplified laboratory exposures

Impact of Surface Charge on Cerium Oxide Nanoparticle Uptake and Translocation by Wheat (<i>Triticum aestivum</i>)

Nanoparticle (NP) physiochemical properties, including surface charge, affect cellular uptake, translocation, and tissue localization. To evaluate the influence of surface charge on NP uptake by plants, wheat seedlings were hydroponically exposed to 20 mg/L of ∼4 nm CeO₂ NPs functionalized with positively charged, negatively charged, and neutral dextran coatings. Fresh, hydrated roots and leaves were analyzed at various time points over 34 h using fluorescence X-ray absorption near-edge spectroscopy to provide laterally resolved spatial distribution and speciation of Ce. A 15–20% reduction from Ce­(IV) to Ce­(III) was observed in both roots and leaves, independent of NP surface charge. Because of its higher affinity with negatively charged cell walls, CeO₂(+) NPs adhered to the plant roots the strongest. After 34 h, CeO₂(−), and CeO₂(0) NP exposed plants had higher Ce leaf concentrations than the plants exposed to CeO₂(+) NPs. Whereas Ce was found mostly in the leaf veins of the CeO₂(−) NP exposed plant, Ce was found in clusters in the nonvascular leaf tissue of the CeO₂(0) NP exposed plant. These results provide important information for understanding mechanisms responsible for plant uptake, transformation, and translocation of NPs, and suggest that NP coatings can be designed to target NPs to specific parts of plants

<i>In Situ</i> Measurement of CuO and Cu(OH)₂ Nanoparticle Dissolution Rates in Quiescent Freshwater Mesocosms

Recent studies have characterized copper-based nanoparticles (CBNPs) as relatively insoluble, raising potential persistence, accumulation, and toxicological concerns about their long-term application as agricultural pesticides. The dissolution rates of two CBNPs were measured in natural and artificial waters under both saturated and unsaturated conditions with respect to CuO_(s) (total Cu, <1 mg/kg). Kocide 3000, an agricultural pesticide formulation with nanoscale Cu­(OH)₂ particles, rapidly dissolved with an experimental half-life of <8 h in natural water. Copper oxide nanoparticles were longer-lived, with an experimental half-life of 73 h in natural water. In contrast to prior reports of CuONP dissolution, our results suggest that even in moderately alkaline waters, CuO and Cu­(OH)₂ NPs may persist as particles for days to weeks under quiescent conditions in a freshwater environment

Toxicogenomic Responses of the Model Organism Caenorhabditis elegans to Gold Nanoparticles

We used Au nanoparticles (Au-NPs) as a model for studying particle-specific effects of manufactured nanomaterials (MNMs) by examining the toxicogenomic responses in a model soil organism, Caenorhabditis elegans. Global genome expression for nematodes exposed to 4-nm citrate-coated Au-NPs at the LC₁₀ level (5.9 mg·L^–1) revealed significant differential expression of 797 genes. The levels of expression for five genes (<i>apl-1</i>, <i>dyn-1</i>, <i>act-5</i>, <i>abu-11</i>, and <i>hsp-4</i>) were confirmed independently with qRT-PCR. Seven common biological pathways associated with 38 of these genes were identified. Up-regulation of 26 <i>pqn</i>/<i>abu</i> genes from noncanonical unfolded protein response (UPR) pathway and molecular chaperones (<i>hsp-16.1</i>, <i>hsp-70</i>, <i>hsp-3</i>, and <i>hsp-4</i>) were observed and are likely indicative of endoplasmic reticulum stress. Significant increase in sensitivity to Au-NPs in a mutant from noncanonical UPR (<i>pqn-5</i>) suggests possible involvement of the genes from this pathway in a protective mechanism against Au-NPs. Significant responses to Au-NPs in endocytosis mutants (<i>chc-1</i> and <i>rme-2</i>) provide evidence for endocytosis pathway being induced by Au-NPs. These results demonstrate that Au-NPs are bioavailable and cause adverse effects to C. elegans by activating both general and specific biological pathways. The experiments with mutants further support involvement of several of these pathways in Au-NP toxicity and/or detoxification

Toxicogenomic Responses of the Model Legume <i>Medicago truncatula</i> to Aged Biosolids Containing a Mixture of Nanomaterials (TiO₂, Ag, and ZnO) from a Pilot Wastewater Treatment Plant

Toxicogenomic responses in Medicago truncatula A17 were monitored following exposure to biosolids-amended soils. Treatments included biosolids produced using a pilot wastewater treatment plant with either no metal introduced into the influent (control); bulk/ionic TiO₂, ZnO, and AgNO₃ added to influent (bulk/dissolved treatment); or Ag, ZnO, and TiO₂ engineered nanomaterials added to influent (ENM treatment) and then added to soil, which was aged in the field for 6 months. In our companion study, we found inhibition of nodulation in the ENM but not in the bulk/dissolved treatment. Gene expression profiling revealed highly distinct profiles with more than 10-fold down-regulation in 239 genes in M. truncatula roots from the ENM treatment, while gene expression patterns were similar between bulk/dissolved and control treatments. In response to ENM exposure, many of the identified biological pathways, gene ontologies, and individual genes are associated with nitrogen metabolism, nodulation, metal homeostasis, and stress responses. Expression levels of nine genes were independently confirmed with qRT-PCR. Exposure to ENMs induced unique shifts in expression profiles and biological pathways compared with bulk/dissolved treatment, despite the lack of difference in bioavailable metal fractions, metal oxidation state, and coordination environment between ENM and bulk/dissolved biosolids. As populations of Sinorhizobium meliloti Rm2011 were similar in bulk/dissolved and ENM treatments, our results suggest that inhibition of nodulation in the ENM treatment was primarily due to phytotoxicity, likely caused by enhanced bioavailability of Zn ions