Quantifying the adsorption of ionic silver and functionalized nanoparticles during ecotoxicity testing: Test container effects and recommendations

Toxic Effects of Different Coating-Related Functionalized Nanoparticles on Aquatic Organisms

The peculiar physico-chemical characteristics of nanomaterials (NMs) and the use of different coatings to improve their expected properties result in a huge amount of nanoforms, which vary in chemical composition, size, shape and surface characteristics. This makes it almost impossible to test all the nanoforms available, and efforts have been made to establish grouping or read-across strategies. The aim of this work was to find a behavior pattern of effect among nanoforms of different metallic core nanoparticles (NPs) (TiO2, CeO2 and Ag NP) with the same coatings (sodium citrate, poly (ethylene glycol), dodecylphosphonic acid or oleylamine). Daphnia magna, rainbow trout and two fish cell lines (PLHC-1 and RTH-149) were exposed to a range of concentrations (up to 100 mg/L) of the uncoated or coated NPs. Ag NPs were the most toxic, followed by CeO2 NPs and finally by TiO2 NPs. The results show that a clear pattern of toxicity in the studied species could not be established related to the coatings. However, it was possible to confirm different inter-species sensitivities. RTH-149 was the most sensitive cell line, and Daphnia magna was more sensitive than fish. Moreover, some differences in coating-core interactions were found between the metal oxide and the metal NPs in Daphnia magna.

Comparative in vivo toxicokinetics of silver powder, nanosilver and soluble silver compounds after oral administration to rats

Silver (Ag; massive, powder and nanoform) and Ag compounds are used in industrial, medical and consumer applications, with potential for human exposure. Uncertainties exist about their comparative mammalian toxicokinetic ('TK') profiles, including their relative oral route bioavailability, especially for Ag massive and powder forms. This knowledge gap impedes concluding on the grouping of Ag and Ag compounds for hazard assessment purposes. Therefore, an in vivo TK study was performed in a rat model. Sprague-Dawley rats were exposed via oral gavage for up to 28 days to silver acetate (AgAc; 5, 55, 175 mg/kg(bw)/d), silver nitrate (AgNO3; 5, 55, 125 mg/kg(bw)/d), nanosilver (AgNP; 15 nm diameter; 3.6, 36, 360 mg/kg(bw)/d) or silver powder (AgMP; 0.35 µm diameter; 36, 180, 1000 mg/kg(bw)/d). Total Ag concentrations were determined in blood and tissues to provide data on comparative systemic exposure to Ag and differentials in achieved tissue Ag levels. AgAc and AgNO3 were the most bioavailable forms with comparable and linear TK profiles (achieved systemic exposures and tissue concentrations). AgMP administration led to systemic exposures of about an order of magnitude less, with tissue Ag concentrations 2-3 orders of magnitude lower and demonstrating non-linear kinetics. The apparent oral bioavailability of AgNP was intermediate between AgAc/AgNO3 and AgMP. For all test items, highest tissue Ag concentrations were in the gastrointestinal tract and reticuloendothelial organs, whereas brain and testis were minor sites of distribution. It was concluded that the oral bioavailability of AgMP was very limited. These findings provide hazard assessment context for various Ag test items and support the prediction that Ag in massive and powder forms exhibit low toxicity potential.

Availability and effects of n-TiO2 and PCB77 in fish in vitro models of the intestinal barrier and liver under single- and/or co-exposure scenarios

Titanium dioxide nanoparticles (n-TiO₂) and polychlorinated biphenyls (PCBs) can be present in the food of fish, leading to intestinal exposure uptake, and accumulation in inner organs. This study examined combination effects of n-TiO₂ and PCB77 in vitro models of the fish intestinal epithelium and liver, i.e., RTgut-GC cell cultures grown in ThinCerts™ and RTL-W1 cell cultures grown in standard tissue culture plates. Mass spectrometry and microscopy techniques were used to obtain information on nanoparticle translocation across the intestinal barrier model. In addition, the substances' effect on intestinal barrier permeability, cell viability, expression of dioxin - and antioxidant response element -controlled genes, and induction of cytochrome P450 1a (Cyp1a)-dependent ethoxyresorufin-O-deethylase (EROD) activity were assessed. TiO₂ nanoparticles were taken up by RTgut-GC cells and detected in the bottom compartment of the intestinal epithelial barrier model. It was not possible to conclude definitively if n-TiO₂ translocation occurred via transcytosis or paracellular migration but observations of nanoparticles in the lateral space between adjacent epithelial cells were rare. PCB77 (1 and 10 µM, 24 h) did not affect barrier permeability, i.e., n-TiO₂ translocation is probably not facilitated in case of co-exposure. Furthermore, previous and simultaneous exposure to n-TiO₂ (1 and 10 mg/L, 24 h) did not have any influence on PCB77-induced Cyp1a mRNA and enzyme activity levels in RTL-W1 cells. Furthermore, there were no significant differences in expression of antioxidant response element-controlled genes comparing control, single substance, and mixture treatments, not even following long-term exposure (0.01-1 mg/L n-TiO₂ + 1 nM PCB77, 4 weeks). While an underestimation of the effects of n-TiO₂ and PCB77 cannot be fully excluded as concentration losses due to sorption to cell culture plastics were not measured, the results suggest that the test substances probably have a low potential to exhibit combination effects on the assessed endpoints when co-existing in fish tissues.

Potential Artifacts and Control Experiments in Toxicity Tests of Nanoplastic and Microplastic Particles

To fully understand the potential ecological and human health risks from nanoplastics and microplastics (NMPs) in the environment, it is critical to make accurate measurements. Similar to past research on the toxicology of engineered nanomaterials, a broad range of measurement artifacts and biases are possible when testing their potential toxicity. For example, antimicrobials and surfactants may be present in commercially available NMP dispersions, and these compounds may account for toxicity observed instead of being caused by exposure to the NMP particles. Therefore, control measurements are needed to assess potential artifacts, and revisions to the protocol may be needed to eliminate or reduce the artifacts. In this paper, we comprehensively review and suggest a next generation of control experiments to identify measurement artifacts and biases that can occur while performing NMP toxicity experiments. This review covers the broad range of potential NMP toxicological experiments, such as in vitro studies with a single cell type or complex 3-D tissue constructs, in vivo mammalian studies, and ecotoxicity experiments testing pelagic, sediment, and soil organisms. Incorporation of these control experiments can reduce the likelihood of false positive and false negative results and more accurately elucidate the potential ecological and human health risks of NMPs.

Bioaccumulation but no biomagnification of silver sulfide nanoparticles in freshwater snails and planarians

Bioaccumulation studies are critical in regulatory decision making on the potential environmental risks of engineered nanoparticles (NPs). The present study evaluated the toxicokinetics of silver, taken up from sulfide nanoparticles (Ag₂S NPs; simulating an aged Ag NP form) and AgNO₃ (ionic counterpart), in the pulmonate snail Physa acuta and the planarian Girardia tigrina. The snails were first exposed for 7 days to Ag-spiked water, along with the microalgae Raphidocelis subcapitata upon which they fed setting up a double route exposure, and subsequently provided as pre-exposed food to the planarians. Ag toxicokinetics and bioaccumulation were assessed in planarians and snails, and potential biomagnification from snail to planarian was evaluated. Gut depuration was also explored to understand whether it constitutes a factor likely to influence Ag toxicokinetics and internal concentrations in the test species. Both species revealed Ag uptake in Ag₂S NP and AgNO₃ treatments, with higher uptake from the latter. Uptake by the snails was probably via a combination of water exposure and ingested algae provided as food, but ingestion of algae possibly had higher relevance for Ag uptake from the Ag₂S NPs compared to AgNO₃. For planarians, diet probably was the most important exposure route since no Ag uptake was observed in previous waterborne exposures to Ag₂S NPs. Kinetics and internal Ag concentrations did not significantly differ between depurated and non-depurated snails or planarians. The planarians fed on snails revealed no biomagnification. To the best of our knowledge this is the first study investigating the toxicokinetics and biomagnification of NPs in planarians, and with that providing important data on the kinetics and bioaccumulation of NPs in a relevant benthic species.

Incubation media modify silver nanoparticle toxicity for whitefish (Coregonus lavaretus) and roach (Rutilus rutilus) embryos

Toxicological studies were performed to examine silver nanoparticle (AgNP, size: 14.4 ± 2.5 nm) transformation within three different test media and consequent effects on embryos of whitefish (Coregonus lavaretus) and roach (Rutilus rutilus). The test media, namely ASTM very hard water, ISO standard dilution medium, and natural lake water differed predominantly in ionic strength. Total silver was determined using inductively coupled plasma mass spectrometry (ICP-MS), while AgNPs were characterized by transmission electron microscopy and single particle ICP-MS. Silver species distributions were estimated via thermodynamic speciation calculations. Data demonstrated that increased AgNP dissolution accompanied by decreasing ionic strength of the test medium did not occur as noted in other studies. Further, other physicochemical parameters including AgNP size and metallic species distribution did not markedly affect AgNP-induced toxicity. Irrespective of the test medium, C. lavaretus were more sensitive to AgNP exposure (median lethal concentration after 8 weeks: 0.51-0.73 mg/L) compared to R. rutilus, where adverse effects were only observed at 5 mg/L in natural lake water. In addition, AgNP-induced toxicity was lower in the two standard test media compared to natural lake water. Currently, there are no apparent studies assessing simultaneously the sensitivity of C. lavaretus and R. rutilus to AgNP exposure. Therefore, the aim of this study was to (1) investigate AgNP-induced toxicity in C. lavaretus and R. rutilus cohabiting in the same aquatic environment and (2) the role played by test media in the observed effects of AgNPs on these aquatic species.

Silver nanomaterials released from commercial textiles have minimal impacts on soil microbial communities at environmentally relevant concentrations

Silver nanomaterials (Ag NMs) have been used in a variety of commercial products to take advantage of their antimicrobial properties. However, there are concerns that these AgNMs can be released during/after use and enter wastewater streams, potentially impacting aquatic systems or accumulating in wastewater biosolids. Biosolids, which are a residual of wastewater treatment processes, have been found to contain AgNMs and are frequently used as agricultural fertilizer. Since the function of soil microbial communities is imperative to nutrient cycling and agricultural productivity, it is important to characterize and assess the effects that silver nanomaterials could have in agricultural soils. In this study agricultural soil was amended with pristine engineered (PVP-coated or uncoated AgNMs), aged silver (sulphidized or released from textiles) nanomaterials, and ionic silver to determine the fate and toxicity over the course of three months. Exposures were carried out at various environmentally relevant concentrations (1 and 10 mg Ag/kg soil) representing between 30 to over 800 years of equivalent biosolid loadings. Over thirteen different methodologies and measures were used throughout this study to assess for potential effects of the silver nanomaterials on soil, including microbial community composition, average well colour development (AWCD) and enzymatic activity. Overall, the AgNM exposures did not exhibit significant toxic effects to the soil microbial communities in terms of density, activity, function and diversity. However, the positive ionic silver treatment (100 mg Ag/kg soil) resulted in suppression to microbial activity while also resulting in significantly higher populations of Frankia alni (nitrogen-fixer) and Arenimonas malthae (phytopathogen) as compared to the negative control (p < 0.05, Tukey HSD) which warrants further investigation.

Experimental evolution of Pseudomonas putida under silver ion versus nanoparticle stress

Whether the antibacterial properties of silver nanoparticles (AgNPs) are simply due to the release of silver ions (Ag⁺ ) or, additionally, nanoparticle-specific effects, is not clear. We used experimental evolution of the model environmental bacterium Pseudomonas putida to ask whether bacteria respond differently to Ag⁺ or AgNP treatment. We pre-evolved five cultures of strain KT2440 for 70 days without Ag to reduce confounding adaptations before dividing the fittest pre-evolved culture into five cultures each, evolving in the presence of low concentrations of Ag⁺ , well-defined AgNPs or Ag-free controls for a further 75 days. The mutations in the Ag⁺ or AgNP evolved populations displayed different patterns that were statistically significant. The non-synonymous mutations in AgNP-treated populations were mostly associated with cell surface proteins, including cytoskeletal membrane protein (FtsZ), membrane sensor and regulator (EnvZ and GacS) and periplasmic protein (PP_2758). In contrast, Ag⁺ treatment was selected for mutations linked to cytoplasmic proteins, including metal ion transporter (TauB) and those with metal-binding domains (ThiL and PP_2397). These results suggest the existence of AgNP-specific effects, either caused by sustained delivery of Ag⁺ from AgNP dissolution, more proximate delivery from cell-surface bound AgNPs, or by direct AgNP action on the cell's outer membrane.

Setting a Protective Threshold Value for Silver Toward Freshwater Organisms

Driven by Regulation (EC) No. 1272/2008 and the European Water Framework Directive 2000/60/EC, we have re-evaluated the available chronic freshwater ecotoxicity data for ionic silver (Ag) using strict data quality criteria. In addition, we generated new chronic ecotoxicity data for species potentially sensitive to Ag (the rotifer Brachionus calyciflorus, the cyanobacteria Anabaena flos-aquae, and the aquatic plant Lemna minor) using Ag nitrate as the test substance. The 10% effect concentrations for the most sensitive endpoint per test species were 0.31 µg dissolved Ag/L for B. calyciflorus (population size), 0.41 µg dissolved Ag/L for A. flos-aquae (growth rate), and 1.40 µg dissolved Ag/L for L. minor (root length). We included these values in the set of reliable chronic freshwater data, subsequently covering a total of 12 taxonomic groups and 15 species. Finally, we applied a species sensitivity distribution approach to the data set using various models. The best-fitting model (Rayleigh distribution) resulted in a threshold value protective for 95% of the species of 0.116 µg dissolved Ag/L. This value is considered reliable and conservative in terms of species protection and can be used as a solid basis for setting thresholds for Ag in freshwater after application of an appropriate assessment factor. Furthermore, this value represents reasonable worst-case conditions for bioavailability in European Union surface waters (low hardness and low dissolved organic carbon). Environ Toxicol Chem 2021;40:1678-1693. © 2021 European Precious Metals Federation. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

A Sensitive Single Particle-ICP-MS Method for CeO2 Nanoparticles Analysis in Soil during Aging Process

The increasing prevalence of products that incorporate engineered nanoparticles (ENPs) has prompted efforts to investigate the potential release, environmental fate, and exposure of the ENPs. However, the investigation of cerium dioxide nanoparticles (CeO₂ NPs) in soil has remained limited, owing to the analytical challenge from the soil's complex nature. In this study, this challenge was overcome by applying a novel single particle-inductively coupled plasma-mass spectrometry (SP-ICP-MS) methodology to detect CeO₂ NPs extracted from soil, utilizing tetrasodium pyrophosphate (TSPP) aqueous solution as an extractant. This method is highly sensitive for determining CeO₂ NPs in soil, with detection limits of size and concentration of 15 nm and 194 NPs mL^-1, respectively. Extraction efficiency was sufficient in the tested TSPP concentration range from 1 mM to 10 mM at a soil-to-extractant ratio 1:100 (g mL^-1) for the extraction of CeO₂ NPs from the soil spiked with CeO₂ NPs. The aging study demonstrated that particle size, size distribution, and particle concentration underwent no significant change in the aged soils for a short period of one month. This study showed an efficient method capable of extracting and accurately determining CeO₂ NPs in soil matrices. The method can serve as a useful tool for nanoparticle analysis in routine soil tests and soil research.

A novel method to estimate cellular internalization of nanoparticles into gram-negative bacteria: Non-lytic removal of outer membrane and cell wall

Bacteria efficiently take up small organic molecules and ions. However, the internalization of particulate forms, specifically nanoparticles (NPs) has been understudied and is a newly-emerging area of interest. However, determination of true cellular internalization is challenging owing to the difficulty of separating the aqueous phase from bacteria-associated NPs and, more importantly, of differentiating between internalized and NPs sorbed on bacteria surfaces. In this work, we developed and validated an extraction method which can operationally estimate internalization of metal NPs into Gram-negative bacteria. The outer cell membrane and cell wall, collectively called the periplasm, was successfully removed from bacteria using ethylenediaminetetraacetic acid (EDTA) at an optimized exposure period and concentration, without lysis of bacteria. This was followed by standard digestion and metal measurements. Verification of each step of the methodology was conducted by assessing both cellular and metal behavior. Specifically, the combined approaches of live/dead staining of bacteria, optical density measurements, transmission electron microscopy (TEM) and metal analyses of the supernatant indicated that the method operationally separated externally-sorbed NPs from those internalized actually localized within the bacterial cytoplasm. However, this new method is ideally used alongside other methods in a multi-method approach, to provide improved data quality. Therefore, it should be used with CSLM, FACS, TEM and other available methods.

The toxicity of non-aged and aged coated silver nanoparticles to the freshwater shrimp Paratya australiensis

Nanoparticles (NPs) transform in the environment which result in alterations to their physicochemical properties. However, the effects of aging on the toxicity of NPs to aquatic organisms remain to be determined. Further the reports that have been published present contradictory results. The aim of this study was to examine the stability of differently coated silver nanoparticles (AgNPs) in media and the influence of aging of these NP on potential toxicity to freshwater shrimp Paratya australiensis. Coating-dependent changes in the stability of AgNP were observed with aging. Curcumin (C) coated AgNPs were stable, while tyrosine (T) coated AgNPs and epigallocatechin gallate (E) coated AgNPs aggregated in the P. australiensis medium. Increased lipid peroxidation and catalase activity was noted in P. australiensis exposed to AgNPs, suggesting oxidative stress was associated with NP exposure. The enhanced oxidative stress initiated by aged C-AgNPs suggests that aging of these NPs produced different toxicological responses. In summary, data suggest that coating-dependent alterations in NPs, together with aging affect both persistence and subsequent toxicity of NPs to freshwater organisms. Thus, the coating-dependent fate and toxicity of AgNPs together with the effect of their aging need to be considered in assessing the environmental risk of AgNPs to aquatic organisms.

The Toxicity of Nonaged and Aged Coated Silver Nanoparticles to Freshwater Alga Raphidocelis subcapitata

The transformation of coated silver nanoparticles (AgNPs) and their impacts on aquatic organisms require further study. The present study investigated the role of aging on the transformation of differently coated AgNPs and their sublethal effects on the freshwater alga Raphidocelis subcapitata. The stability of AgNPs was evaluated over 32 d, and the results indicated that transformation of AgNPs occurred during the incubation; however, coating-specific effects were observed. Fresh AgNPs increased reactive oxygen species (ROS) formation, whereas aged AgNPs induced excessive ROS generation compared with their fresh counterparts. Increased ROS levels caused increased lipid peroxidation (LPO) in treatment groups exposed to both fresh and aged NPs, although LPO was comparatively higher in algae exposed to aged AgNPs. The observed increase in catalase (CAT) activity of algal cells was attributed to early stress responses induced by excessive intracellular ROS generation, and CAT levels were higher in the aged NP treatment groups. In conclusion, AgNPs increased ROS levels and LPO in algae and caused the activation of antioxidant enzymes such as CAT. Overall, the results suggest that aging and coating of AgNPs have major impacts on AgNP transformation in media and their effects on algae. Environ Toxicol Chem 2019;38:2371-2382. © 2019 SETAC.

Nanoengineered Antibacterial Coatings and Materials: A Perspective

This feature article begins by outlining the problem of infection and its implication on healthcare. The initial introductory section is followed by a description of the four distinct classes of antibacterial coatings and materials, i.e., bacteria repealing, contact killing, releasing and responsive, that were developed over the years by our team and others. Specific examples of each individual class of antibacterial materials and a discussion on the pros and cons of each strategy are provided. The article contains a dedicated section focused on silver nanoparticle based coatings and materials, which have attracted tremendous interest from the scientific and medical communities. The article concludes with the author’s view regarding the future of the field.

Biocompatible functionalisation of nanoclays for improved environmental remediation

Among the wide range of materials used for remediating environmental contaminants, modified and functionalised nanoclays show particular promise as advanced sorbents, improved dispersants, or biodegradation enhancers. However, many chemically modified nanoclay materials are incompatible with living organisms when they are used in natural systems with detrimental implications for ecosystem recovery. Here we critically review the pros and cons of functionalised nanoclays and provide new perspectives on the synthesis of environmentally friendly varieties. Particular focus is given to finding alternatives to conventional surfactants used in modified nanoclay products, and to exploring strategies in synthesising nanoclay-supported metal and metal oxide nanoparticles. A large number of promising nanoclay-based sorbents are yet to satisfy environmental biocompatibility in situ but opportunities are there to tailor them to produce "biocompatible" or regenerative/reusable materials.

A One Health approach to managing the applications and implications of nanotechnologies in agriculture

The need for appropriate science and regulation to underpin nanosafety is greater than ever as ongoing advances in nanotechnology are rapidly translated into new industrial applications and nano-enabled commercial products. Nevertheless, a disconnect persists between those examining risks to human and environmental health from nanomaterials. This disconnect is not atypical in research and risk assessment and has been perpetuated in the case of engineered nanomaterials by the relatively limited overlap in human and environmental exposure pathways. The advent of agri-nanotechnologies brings both increased need and opportunity to change this status quo as it introduces significant issues of intersectionality that cannot adequately be addressed by current discipline-specific approaches alone. Here, focusing on the specific case of nanoparticles, we propose that a transdisciplinary approach, underpinned by the One Health concept, is needed to support the sustainable development of these technologies.

Using 3D gastrointestinal tract in vitro models with microfold cells and mucus secreting ability to assess the hazard of copper oxide nanomaterials

BACKGROUND

Copper oxide nanomaterials (CuO NMs) are exploited in many products including inks, cosmetics, textiles, wood preservatives and food contact materials. Their incorporation into these products may enhance oral exposure in consumer, environmental and occupational settings. Undifferentiated and differentiated monocultures of Caco-2 cells are commonly used to assess NM toxicity to the intestine in vitro. However, the integration of other cell types into Caco-2 in vitro models increases their physiological relevance. Therefore, the aim of this study is to evaluate the toxicity of CuO NMs and copper sulphate (CuSO4) to intestinal microfold (M) cell (Caco-2/Raji B) and mucus secreting (Caco-2/HT29-MTX) co-culture in vitro models via assessment of their impact on barrier integrity, viability and interleukin (IL)-8 secretion. The translocation of CuO NMs and CuSO4 across the intestinal barrier was also investigated in vitro.

RESULTS

CuO NMs and CuSO4 impaired the function of the intestinal barrier in the co-culture models [as indicated by a reduction in transepithelial electrical resistance (TEER) and Zonular occludens (ZO-1) staining intensity]. Cu translocation was observed in both models but was greatest in the Caco-2/Raji B co-culture. CuO NMs and CuSO4 stimulated an increase in IL-8 secretion, which was greatest in the Caco-2/HT29-MTX co-culture model. CuO NMs and CuSO4 did not stimulate a loss of cell viability, when assessed using light microscopy, nuclei counts and scanning electron microscopy. CuO NMs demonstrated a relatively similar level of toxicity to CuO4 in both Caco-2/Raji B and Caco-2/HT29-MTX co- culture models.

CONCLUSIONS

The Caco-2/Raji B co-culture model was more sensitive to CuO NM and CuSO4 toxicity than the Caco-2/HT29-MTX co-culture model. However, both co-culture models were less sensitive to CuO NM and CuSO4 toxicity than simple monocultures of undifferentiated and differentiated Caco-2 cells, which are more routinely used to investigate NM toxicity to the intestine. Obtained data can therefore feed into the design of future studies which assess the toxicity of substances (e.g. NMs) and pathogens to the intestine (e.g. by informing model and endpoint selection). However, more testing with a wider panel of NMs would be beneficial in order to help select which in vitro models and endpoints to prioritise when screening the safety of ingested NMs. Comparisons with in vivo findings will also be essential to identify the most suitable in vitro model to screen the safety of ingested NMs.

Sorption of PFOA onto different laboratory materials: Filter membranes and centrifuge tubes

Measurement and reporting of concentrations of contaminants of emerging concern such as per- and polyfluoroalkyl substances (PFASs), including perfluorooctanoic acid (PFOA), is an integral part of most investigations. Occurrence of sorption losses of PFAS analytes onto particular laboratory-ware (e.g. glass containers) has been suggested in the published literature but has not been investigated in detail. We examined sorption losses from aqueous PFOA solutions in contact with different commonly-used materials in filter units and centrifuge tubes (glass and plastics). Sorption of PFOA onto different filter membrane types ranged from 21-79% indicating that filtration can introduce a major source of error in PFOA analysis; pre-treatment of filter membranes with phosphate or methanol solutions did not improve PFOA recovery. Substantial adsorption of PFOA was also observed on tubes made from polypropylene (PP), polystyrene (PS), polycarbonate (PC), and glass where losses observed were between 32-45%, 27-35%, 16-31% and 14-24%, respectively. Contrary to suggestions in the literature, our results indicated that the greatest sorption losses for PFOA occurred on PP, whereas losses on glass tubes were much lower. Variations in ionic strength and pH did not greatly influence PFOA recovery. When PFOA concentrations were increased, the percent recovery of PFOA increased, indicating that binding sites on tube-walls were saturable. This study draws attention towards analytical bias that can occur due to sorption losses during routine procedures, and highlights the importance of testing the suitability of chosen laboratory-ware for specific PFAS analytes of interest prior to experimental use.

Silver Toxicity Thresholds for Multiple Soil Microbial Biomarkers

Material flow analysis shows that soil is a key repository for silver (Ag) from (nano)silver-functionalized consumer products, but the potential effects of Ag toxicity, via Ag⁺ release, on soil microbial communities and their ecosystem services remains largely unknown. We examined the responses of multiple microbial biomarkers to increasing Ag⁺ doses (nine concentrations, 0-2000 mg kg^-1) in nine different soils representing a wide range of soil properties. Analyses included substrate-induced microbial respiration, nine different soil enzyme activities, and quantification of bacterial 16S-rRNA (SSU) and fungal intergenic spacer (ITS) copies. The resulting half-maximal effective concentrations (EC₅₀) for Ag ranged from ∼1 to >500 mg kg ^-1 and showed soil-specific responses, including some hormesis-type responses. Carbon cycle-associated enzyme activities (e.g., cellobiohydrolase, xylosidase, and α/β-glucosidase) responded similarly to Ag. Sulfatase and leucine-aminopeptidase activities (linked to the sulfur and nitrogen cycles) were the most sensitive to Ag. Total organic carbon, and to a lesser extent pH, were identified as potentially useful response predictors, but only for some biomarkers; this reflects the complexity of soil Ag chemistry. Our results show Ag toxicity is highly dependent on soil characteristics and the specific microbial parameter under investigation, but end point redundancies also indicated that representative parameters for key microbial functions can be identified for risk assessment purposes. Sulfatase activity may be an important Ag toxicity biomarker; its response was highly sensitive and not correlated with that of other biomarkers.

Complementary Imaging of Silver Nanoparticle Interactions with Green Algae: Dark-Field Microscopy, Electron Microscopy, and Nanoscale Secondary Ion Mass Spectrometry

Increasing consumer use of engineered nanomaterials has led to significantly increased efforts to understand their potential impact on the environment and living organisms. Currently, no individual technique can provide all the necessary information such as their size, distribution, and chemistry in complex biological systems. Consequently, there is a need to develop complementary instrumental imaging approaches that provide enhanced understanding of these "bio-nano" interactions to overcome the limitations of individual techniques. Here we used a multimodal imaging approach incorporating dark-field light microscopy, high-resolution electron microscopy, and nanoscale secondary ion mass spectrometry (NanoSIMS). The aim was to gain insight into the bio-nano interactions of surface-functionalized silver nanoparticles (Ag-NPs) with the green algae Raphidocelis subcapitata, by combining the fidelity, spatial resolution, and elemental identification offered by the three techniques, respectively. Each technique revealed that Ag-NPs interact with the green algae with a dependence on the size (10 nm vs 60 nm) and surface functionality (tannic acid vs branched polyethylenimine, bPEI) of the NPs. Dark-field light microscopy revealed the presence of strong light scatterers on the algal cell surface, and SEM imaging confirmed their nanoparticulate nature and localization at nanoscale resolution. NanoSIMS imaging confirmed their chemical identity as Ag, with the majority of signal concentrated at the cell surface. Furthermore, SEM and NanoSIMS provided evidence of 10 nm bPEI Ag-NP internalization at higher concentrations (40 μg/L), correlating with the highest toxicity observed from these NPs. This multimodal approach thus demonstrated an effective approach to complement dose-response studies in nano-(eco)-toxicological investigations.

Time-resolved toxicity study reveals the dynamic interactions between uncoated silver nanoparticles and bacteria

It is still unclear whether the toxicity of silver nanoparticles (AgNPs) can be attributed solely to the release of Ag⁺ or whether dissolved and nanoparticulate Ag act in parallel; this is due to the difficulty in distinguishing Ag⁺- from AgNP-effects. Also, AgNPs undergo changes during toxicity tests. This is the first study to investigate the influence of AgNP dissolution over time on viable counts at high time resolution and low cell density, avoiding the apparently reduced toxicity at higher cell densities identified in our study. Uncapped AgNPs were synthesized to avoid any interference from surface coatings. The transformations of AgNPs during storage were reduced. Lowering the concentration of AgNPs reduced their aggregation in Davis minimal medium (DMM). Also, AgNPs dissolved more slowly in DMM than in water. The minimum inhibitory concentrations (MICs) of Ag⁺ and AgNPs increased with cell density according to a power law, suggesting that binding to cells decreased effective concentrations. However, AgNPs acted as a reservoir of Ag, releasing new Ag⁺ to maintain the Ag stress. The toxicity of AgNPs was dominated by dissolved Ag. Combining controlled conditions, high time-resolution and low cell density, we could demonstrate different roles of ionic and nano Ag in bacterial death caused by AgNPs.

Natural marine bacteria as model organisms for the hazard-assessment of consumer products containing silver nanoparticles

Scarce information is available regarding the fate and toxicology of engineered silver nanoparticles (AgNPs) in the marine environment, especially when compared to other environmental compartments. Hence, the antibacterial activity of the NM-300 AgNPs (OECD programme) and a household product containing colloidal AgNPs (Mesosilver) was investigated using marine bacteria, pure cultures and natural mixed populations (microcosm approach). Bacterial susceptibility to AgNPs was species-specific, with Gram negative bacteria being more resistant than the Gram positive species (NM-300 concentration used ranged between 0.062 and 1.5 mg L^-1), and the Mesosilver product was more toxic than the NM-300. Bacterial viability and the physiological status (O₂ uptake measured by respirometry) of the microbial community in the microcosm was negatively affected at an initial concentration of 1 mg L^-1 NM-300. The high chloride concentrations in the media/seawater led to the formation of silver-chloro complexes thus enhancing AgNP toxicity. We recommend the use of natural marine bacteria as models when assessing the environmental relevant antibacterial properties of products containing nanosilver.

Impact of copper oxide nanomaterials on differentiated and undifferentiated Caco-2 intestinal epithelial cells; assessment of cytotoxicity, barrier integrity, cytokine production and nanomaterial penetration

BACKGROUND

Copper oxide nanomaterials (CuO NMs) are exploited in a diverse array of products including antimicrobials, inks, cosmetics, textiles and food contact materials. There is therefore a need to assess the toxicity of CuO NMs to the gastrointestinal (GI) tract since exposure could occur via direct oral ingestion, mucocillary clearance (following inhalation) or hand to mouth contact.

METHODS

Undifferentiated Caco-2 intestinal cells were exposed to CuO NMs (10 nm) at concentrations ranging from 0.37 to 78.13 μg/cm2 Cu (equivalent to 1.95 to 250 μg/ml) and cell viability assessed 24 h post exposure using the alamar blue assay. The benchmark dose (BMD 20), determined using PROAST software, was identified as 4.44 μg/cm2 for CuO NMs, and 4.25 μg/cm2 for copper sulphate (CuSO4), which informed the selection of concentrations for further studies. The differentiation status of cells and the impact of CuO NMs and CuSO4 on the integrity of the differentiated Caco-2 cell monolayer were assessed by measurement of trans-epithelial electrical resistance (TEER), staining for Zonula occludens-1 (ZO-1) and imaging of cell morphology using scanning electron microscopy (SEM). The impact of CuO NMs and CuSO4 on the viability of differentiated cells was performed via assessment of cell number (DAPI staining), and visualisation of cell morphology (light microscopy). Interleukin-8 (IL-8) production by undifferentiated and differentiated Caco-2 cells following exposure to CuO NMs and CuSO4 was determined using an ELISA. The copper concentration in the cell lysate, apical and basolateral compartments were measured with Inductive Coupled Plasma Optical Emission Spectrometry (ICP-OES) and used to calculate the apparent permeability coefficient (Papp); a measure of barrier permeability to CuO NMs. For all experiments, CuSO4 was used as an ionic control.

RESULTS

CuO NMs and CuSO4 caused a concentration dependent decrease in cell viability in undifferentiated cells. CuO NMs and CuSO4 translocated across the differentiated Caco-2 cell monolayer. CuO NM mediated IL-8 production was over 2-fold higher in undifferentiated cells. A reduction in cell viability in differentiated cells was not responsible for the lower level of cytokine production observed. Both CuO NMs and CuSO4 decreased TEER values to a similar extent, and caused tight junction dysfunction (ZO-1 staining), suggesting that barrier integrity was disrupted.

CONCLUSIONS

CuO NMs and CuSO4 stimulated IL-8 production by Caco-2 cells, decreased barrier integrity and thereby increased the Papp and translocation of Cu. There was no significant enhancement in potency of the CuO NMs compared to CuSO4. Differentiated Caco-2 cells were identified as a powerful model to assess the impacts of ingested NMs on the GI tract.

Overcoming challenges in single particle inductively coupled plasma mass spectrometry measurement of silver nanoparticles

Single particle ICP-MS has evolved rapidly as a quantitative method for determining nanoparticle size and number concentration at environmentally relevant exposure levels. Central to the application of spICP-MS is a commonly used, but not rigorously validated, calibration approach based on the measured transport efficiency and the response of ionic standards. In this work, we present a comprehensive and systematic study of the accuracy, precision and robustness of spICP-MS using the rigorously characterized reference material (RM) 8017 (Polyvinylpyrrolidone Coated Nominal 75 nm Silver Nanoparticles), recently issued by the National Institute of Standards and Technology (NIST). We report for the first time, statistically significant differences in frequency-based and size-based measures of transport efficiency with NIST RM 8013 Gold Nanoparticles and demonstrate that the size-based measure of transport efficiency is more robust and yields accurate results for the silver nanoparticle RM relative to TEM-based reference values. This finding is significant, because the frequency-based method is more widely applied. Furthermore, we demonstrate that the use of acidified ionic standards improves measurement of ICP-MS Ag response, but does not degrade the accuracy of the results for AgNP suspensions in water or various other diluents. Approaches for controlling AgNP dissolution were investigated and are shown to effectively improve particle stability in dilute suspensions required for spICP-MS analysis, while minimally affecting the measured intensity and allowing for more robust analysis. This study is an important and necessary advancement toward full validation and adoption of spICP-MS by the broader research community. Graphical abstract Measurement challenges in spICP-MS analysis.

How test vessel properties affect the fate of silver nitrate and sterically stabilized silver nanoparticles in two different test designs used for acute tests with Daphnia magna

The relation between test conditions such as medium composition or pH on silver nanoparticle (AgNP) behavior and its link to toxicity is one of the major topics in nanoecotoxicological research in the last years. In addition, the adaptation of the ecotoxicological standard tests for nanomaterials is intensely discussed to increase comparability and reliability of results. Due to the limitation of test material production volumes and the need for high-throughput screening, miniaturization has been proposed for several test designs. In the present study, the effect of a miniaturization of the acute Daphnia immobilization test on AgNP behavior was investigated. For this purpose, available, adsorbed, and dissolved silver fractions were measured using AgNP and silver nitrate in the following two test designs: a standard test (ST) design and a miniaturized test (MT) design with reduced test volume and less animals. Despite the increase in surface area in relation to the test volume in MT, more AgNP attached to the ST vessel surface, so that in this case, exposure concentrations were significantly lower compared to the MT assessment. Ionic silver concentrations resulting from AgNP dissolution were similar in both test designs. The same was observed for ionic silver concentrations in silver nitrate (AgNO₃) treatments, but adsorbed silver was also higher in ST treatments. Assessing the structure-activity relationships revealed that surface properties such as hydrophobicity, potential binding sites, or surface roughness were of higher importance than surface:volume ratios for both test substances.

Natural water as the test medium for Ag and CuO nanoparticle hazard evaluation: An interlaboratory case study

Engineered nanoparticles (NPs) have realistic potential of reaching natural waterbodies and of exerting toxicity to freshwater organisms. The toxicity may be influenced by the composition of natural waters as crucial NP properties are influenced by water constituents. To tackle this issue, a case study was set up in the framework of EU FP7 NanoValid project, performing an interlaboratory hazard evaluation of NPs in natural freshwater. Ag and CuO NPs were selected as model NPs because of their potentially high toxicity in the freshwater. Daphnia magna (OECD202) and Danio rerio embryo (OECD236) assays were used to evaluate NP toxicity in natural water, sampled from Lake Greifen and Lake Lucerne (Switzerland). Dissolution of the NPs was evaluated by ultrafiltration, ultracentrifugation and metal specific sensor bacteria. Ag NP size was stable in natural water while CuO NPs agglomerated and settled rapidly. Ag NP suspensions contained a large fraction of Ag(+) ions and CuO NP suspensions had low concentration of Cu(2+) ions. Ag NPs were very toxic (48 h EC50 1-5.5 μg Ag/L) to D. magna as well as to D. rerio embryos (96 h EC50 8.8-61 μg Ag/L) in both standard media and natural waters with results in good agreement between laboratories. CuO NP toxicity to D. magna differed significantly between the laboratories with 48 h EC50 0.9-11 mg Cu/L in standard media, 5.7-75 mg Cu/L in Lake Greifen and 5.5-26 mg Cu/L in Lake Lucerne. No toxicity of CuO NP to zebrafish embryos was detected up to 100 mg/L independent of the medium used. The results show that Ag and CuO NP toxicity may be higher in natural water than in the standard media due to differences in composition. NP environmental hazard evaluation can and should be carried out in natural water to obtain more realistic estimates on the toxicity.

Behavior and chronic toxicity of two differently stabilized silver nanoparticles to Daphnia magna

While differences in silver nanoparticle (AgNP) colloidal stability, surface potential, or acute aquatic toxicity for differently stabilized AgNP have often been reported, these have rarely been studied in long-term ecotoxicity tests. In the current study, we investigated the chronic toxicity of AgNP to Daphnia magna over a 21-day period with two different stabilizers (citrate and detergent), representative for charge and sterical stabilizers, respectively. This was coupled with a series of short-term experiments, such as mass balance and uptake/depuration testing, to investigate the behavior of both types of AgNP during a typical media exchange period in the D. magna test for chronic toxicity. As expected, the sterically stabilized AgNP was more stable in the test medium, also in the presence of food; however, a higher uptake of silver after 24h exposure of the charge stabilized AgNP was found compared to the detergent-stabilized AgNP (0.046±0.006μgAgμgDW(-1) and 0.023±0.005μgAgμgDW(-1), respectively). In accordance with this, the higher reproductive effects and mortality were found for the charge-stabilized than for the sterically-stabilized silver nanoparticles in 21-d tests for chronic toxicity. LOEC was 19.2μgAgL(-1) for both endpoints for citrate-coated AgNP and >27.5μgAgL(-1) (highest tested concentration for detergent-stabilized AgNP). This indicates a link between uptake and toxicity. The inclusion of additional short-term experiments on uptake and depuration is recommended when longer-term chronic experiments with nanoparticles are conducted.

A case study to optimise and validate the brine shrimp Artemia franciscana immobilisation assay with silver nanoparticles: The role of harmonisation

Brine shrimp Artemia sp. has been recognised as an important ecotoxicity and nanotoxicity test model organism for salt-rich aquatic environments, but currently there is still no harmonised testing protocol which would ensure the comparable results for hazard identification. In this paper we aimed to design the harmonised protocol for nanomaterial toxicity testing using Artemia franciscana and present a case study to validate the protocol with silver nanoparticles (AgNPs). We (i) revised the existing nanotoxicity test protocols with Artemia sp. (ii) optimised certain methodological steps based on the experiments with AgNPs and potassium dichromate (K2Cr2O7) as a soluble reference chemical and (iii) tested the optimised protocol in an international inter-laboratory exercise conducted within the EU FP7 NanoValid project. The intra- and inter-laboratory reproducibility of the proposed protocol with a soluble reference chemical K2Cr2O7 was good, which confirms the suitability of this assay for conventional chemicals. However, the variability of AgNPs toxicity results was very high showing again that nanomaterials are inherently challenging for toxicity studies, especially those which toxic effect is linked to shed metal ions. Among the identified sources for this variability were: the hatching conditions, the type of test plate incubation and the illumination regime. The latter induced variations assumingly due to the changes in bioavailable silver species concentrations. Up to our knowledge this is the first inter-laboratory comparison of the Artemia sp. toxicity study involving nanomaterials. Although the inter-laboratory exercise revealed poor repeatability of AgNPs toxicity results, this study provides valuable information regarding the importance of harmonisation of all steps in the test procedure. Also, the presented AgNPs toxicity case study may serve as a platform for further validation steps with other types of NMs.

Influence of polyethylene microplastic beads on the uptake and localization of silver in zebrafish (Danio rerio)

This study aimed to determine whether the uptake and localization of Ag in zebrafish was affected by the presence of polyethylene microplastic beads (PE MPBs). Zebrafish were exposed to 1 μg Ag L(-1) (radiolabelled with (110m)Ag) for 4 and 24 h in the presence or absence of PE MPBs (10, 100 or 1000 MPBs mL(-1)), and one treatment in which MPBs (1000 MPBs mL(-1)) were incubated with Ag to promote adsorption. The presence of MPBs, at any of the tested doses, had no effect on the uptake or localization of Ag. However, exposure to the Ag-incubated MPBs (∽75% of the Ag bound to MPBs) significantly reduced Ag uptake at both time points and also significantly increased the proportion of intestinal Ag. This study demonstrates that microplastics can alter the bioavailability and uptake route of a metal contaminant in a model fish species.

Gaining a Critical Mass: A Dose Metric Conversion Case Study Using Silver Nanoparticles

Mass concentration is the standard convention to express exposure in ecotoxicology for dissolved substances. However, nanotoxicology has challenged the suitability of the mass concentration dose metric. Alternative metrics often discussed in the literature include particle number, surface area, and ion release (kinetics, equilibrium). It is unlikely that any single metric is universally applicable to all types of nanoparticles. However, determining the optimal metric for a specific type of nanoparticle requires novel studies to generate supportive data and employ methods to compensate for current analytical capability gaps. This investigation generated acute toxicity data for two standard species (Ceriodaphnia dubia, Pimephales promelas) exposed to five sizes (10, 20, 30, 60, 100 nm) of monodispersed citrate- and polyvinylpyrrolidone-coated silver nanoparticles. Particles were sized by various techniques to populate available models for expressing the particle number, surface area, and dissolved fraction. Results indicate that the acute toxicity of the tested silver nanoparticles is best expressed by ion release, and is relatable to total exposed surface area. Particle number was not relatable to the observed acute silver nanoparticle effects.

Adapting OECD Aquatic Toxicity Tests for Use with Manufactured Nanomaterials: Key Issues and Consensus Recommendations

The unique or enhanced properties of manufactured nanomaterials (MNs) suggest that their use in nanoenabled products will continue to increase. This will result in increased potential for human and environmental exposure to MNs during manufacturing, use, and disposal of nanoenabled products. Scientifically based risk assessment for MNs necessitates the development of reproducible, standardized hazard testing methods such as those provided by the Organisation of Economic Cooperation and Development (OECD). Currently, there is no comprehensive guidance on how best to address testing issues specific to MN particulate, fibrous, or colloidal properties. This paper summarizes the findings from an expert workshop convened to develop a guidance document that addresses the difficulties encountered when testing MNs using OECD aquatic and sediment test guidelines. Critical components were identified by workshop participants that require specific guidance for MN testing: preparation of dispersions, dose metrics, the importance and challenges associated with maintaining and monitoring exposure levels, and the need for reliable methods to quantify MNs in complex media. To facilitate a scientific advance in the consistency of nanoecotoxicology test results, we identify and discuss critical considerations where expert consensus recommendations were and were not achieved and provide specific research recommendations to resolve issues for which consensus was not reached. This process will enable the development of prescriptive testing guidance for MNs. Critically, we highlight the need to quantify and properly interpret and express exposure during the bioassays used to determine hazard values.