High-resolution analytical electron microscopy reveals cell culture media-induced changes to the chemistry of silver nanowires

Environmental behavior of silver nanomaterials in aquatic environments: An updated review

The increasing applications of silver nanomaterials (nano-Ag) and their inevitable release posed great potential risks to aquatic organisms and ecosystems. Considerable attention has been attracted on their behaviors and transformations, which were critically important for their subsequent biological toxicities and ecological effects. Therefore, the summary of the recent efforts on the environmental behavior of nano-Ag would be beneficial for understanding the environmental fate and accurate risk assessment. This review summarized the studies on various physical, chemical and biological transformations of nano-Ag, meanwhile, the influencing factors (including the intrinsic properties and environmental conditions) and related mechanisms were highlighted. Surface structure and facets of nano-Ag, abiotic conditions and natural freeze-thaw cycle processes could affect the transformations of nano-Ag under different environmental scenarios (including freshwater, seawater and wastewater). The interactions with co-present components, such as chemicals and other particles, impacted the multiple processes of nano-Ag. Besides, the contradictory effects and mechanisms by several environmental factors were summarized. Lastly, the key knowledge gaps and some aspects that deserve further investigation were also addressed. Therefore, the current review aimed to provide an overall analysis of transformation processes of nano-Ag, which will provide more available information and pave the way for the future research areas.

Uptake, intracellular dissolution, and cytotoxicity of silver nanowires in cell models

The uptake, intracellular dissolution, and cytotoxicity of silver nanowires (AgNWs) in two cell models (human keratinocytes - HaCaT cells and murine macrophages) were systemically investigated for the first time. Cellular uptake of AgNWs occurred mainly via pathways of clathrin-dependent endocytosis, caveolae-dependent endocytosis, and phagocytosis. AgNWs could be internalized by two types of cells with numerous lysosomal vesicles detected in close vicinity to AgNWs. Meanwhile, AgNWs exposure caused lysosomal permeabilization and release of cathepsisn B into cytoplasm. Furthermore, for the first time, this study found that AgNWs exposure inhibited the transmembrane ATP binding cassette (ABC) efflux transporter activity, which could make AgNWs as chemosensitizers to increase the toxicity of other xenobiotic pollutants. Toxicity assays evaluating reactive oxygen species production and mitochondrial activity indicated that cytotoxicity differed for different cell types and particles. The intracellular presence of AgNWs with different diameters induced similar toxic events but to different extents. AgNWs were absorbed by macrophages more efficiently than HaCaT cells, while AgNWs exhibited only marginal cytotoxicity towards macrophages compared to HaCaT cells. Using an Ag⁺ fluorescence probe, it was found that a fraction of AgNWs was dissolved inside the lysosomes. A higher amount of released Ag⁺ was detected in HaCaT cells than in macrophages, which might partially contribute to their higher cytotoxicity in HaCaT cells. The toxicity of AgNWs in HaCaT cells and macrophages is due to the high-aspect nature of the nanowires rather than the extracellular release of Ag⁺. This study may be useful for risk assessments of AgNWs in their practical applications in the biomedical field.

Characterization of Chinese hamster ovary cell culture feed media precipitate

Process intensification of monoclonal antibody production is leading to more concentrated feed media causing issues with precipitation of solids from the media solution. This results in processing problems since components in the precipitate are no longer in solution, changing the media composition and leading to variability in cell culture performance. The goal of this work is to characterize the feed media precipitate, and in particular to identify the precipitated components so that mitigation strategies can be developed. From the conducted analysis, the precipitate was predominately found to be organic and was analyzed with liquid chromatography-mass spectrometry and inductively coupled plasma-optical emission spectroscopy (ICP-OES) to identify the constituent components. Up to ten amino acids were identified with tyrosine (approximately 77 wt.%) and phenylalanine (approximately 4 wt.%) being the most prevalent amino acids. Elemental analysis with ICP-OES revealed that inorganic components were accounted for less than one weight percentage of the solid precipitate with metal sulfates being the predominant inorganic components.

An overview of methods of fine and ultrafine particle collection for physicochemical characterisation and toxicity assessments

Particulate matter (PM) is a crucial health risk factor for respiratory and cardiovascular diseases. The smaller size fractions, ≤2.5 μm (PM_2.5; fine particles) and ≤0.1 μm (PM_0.1; ultrafine particles), show the highest bioactivity but acquiring sufficient mass for in vitro and in vivo toxicological studies is challenging. We review the suitability of available instrumentation to collect the PM mass required for these assessments. Five different microenvironments representing the diverse exposure conditions in urban environments are considered in order to establish the typical PM concentrations present. The highest concentrations of PM_2.5 and PM_0.1 were found near traffic (i.e. roadsides and traffic intersections), followed by indoor environments, parks and behind roadside vegetation. We identify key factors to consider when selecting sampling instrumentation. These include PM concentration on-site (low concentrations increase sampling time), nature of sampling sites (e.g. indoors; noise and space will be an issue), equipment handling and power supply. Physicochemical characterisation requires micro- to milli-gram quantities of PM and it may increase according to the processing methods (e.g. digestion or sonication). Toxicological assessments of PM involve numerous mechanisms (e.g. inflammatory processes and oxidative stress) requiring significant amounts of PM to obtain accurate results. Optimising air sampling techniques are therefore important for the appropriate collection medium/filter which have innate physical properties and the potential to interact with samples. An evaluation of methods and instrumentation used for airborne virus collection concludes that samplers operating cyclone sampling techniques (using centrifugal forces) are effective in collecting airborne viruses. We highlight that predictive modelling can help to identify pollution hotspots in an urban environment for the efficient collection of PM mass. This review provides guidance to prepare and plan efficient sampling campaigns to collect sufficient PM mass for various purposes in a reasonable timeframe.

Effect of silver nanospheres and nanowires on human airway smooth muscle cells: role of sulfidation

Background: The toxicity of inhaled silver nanoparticles on contractile and pro-inflammatory airway smooth muscle cells (ASMCs) that control airway calibre is unknown. We explored the oxidative activities and sulfidation processes of the toxic-inflammatory response. Method: Silver nanospheres (AgNSs) of 20 nm and 50 nm diameter and silver nanowires (AgNWs), short S-AgNWs, 1.5 μm and long L-AgNWs, 10 μm, both 72 nm in diameter were manufactured. We measured their effects on cell proliferation, mitochondrial reactive oxygen species (ROS) release and membrane potential, and also performed electron microscopic studies. Main results and findings: The greatest effects were observed for the smallest particles with the highest specific surface area and greatest solubility that were avidly internalised. ASMCs exposed to 20 nm AgNSs (25 μg mL^-1) for 72 hours exhibited a significant decrease in DNA incorporation (-72.4%; p < 0.05), whereas neither the 50 nm AgNSs nor the s-AgNWs altered DNA synthesis or viability. There was a small reduction in ASMC proliferation for the smaller AgNS, although Ag⁺ at 25 μL mL^-1 reduced DNA synthesis by 93.3% (p < 0.001). Mitochondrial potential was reduced by both Ag⁺ (25 μg mL^-1) by 47.1% and 20 nm Ag NSs (25 μg mL^-1) by 40.1% (*both at p < 0.05), but was not affected by 50 nm AgNSs and the AgNWs. None of the samples showed a change in ROS toxicity. However, malondialdehyde release, associated with greater total ROS, was observed for all AgNPs, to an extent following the geometric size (20 nm AgNS: 213%, p < 0.01; 50 nm AgNS: 179.5%, p < 0.01 and L-AgNWs by 156.2%, p < 0.05). The antioxidant, N-acetylcysteine, prevented the reduction in mitochondrial potential caused by 20 nm AgNSs. The smaller nanostructures were internalised and dissolved within the ASMCs with the formation of non-reactive silver sulphide (Ag₂S) on their surface, but with very little uptake of L-AgNWs. When ASMCs were incubated with H₂S-producing enzyme inhibitors, the spatial extent of Ag₂S formation was much greater. Conclusion: The intracellular toxicity of AgNPs in ASMCs is determined by the solubility of Ag⁺ released and the sulfidation process, effects related to particle size and geometry. Passivation through sulfidation driven by biogenic H₂S can outcompete dissolution, thus reducing the toxicity of the smaller intracellular Ag nanostructures.

Impacts of Proteins on Dissolution and Sulfidation of Silver Nanowires in an Aquatic Environment: Importance of Surface Charges

With increasing utilization of silver nanomaterials, growing concerns are raised on their deleterious effects to the environment. Once discharged in an aquatic environment, the interactions between silver nanowires (AgNWs) and proteins may significantly affect the environmental behaviors, fate, and toxicities of AgNWs. In the present study, three representative model proteins, including ovalbumin (OVA), bovine serum albumin (BSA), and lysozyme (LYZ), were applied to investigate the impacts of the interactions between proteins and AgNWs on the transformations (oxidative dissolution and sulfidation) of AgNWs in an aquatic environment. Fluorescence spectroscopy and isothermal titration calorimetry analyses indicated that there was very weak interaction between OVA or BSA and AgNWs, but there was a strong interaction between the positively charged LYZ and the negatively charged AgNWs. The presence of LYZ not only reversed the surface charge of AgNWs but also resulted in the breakup of the nanowire structure and increased the reactive surface area. The positively charged surface of AgNWs in the presence of LYZ favored the access of sulfide ions. As a consequence, the kinetics of oxidative dissolution and sulfidation of AgNWs were not affected by OVA and BSA but were significantly facilitated by LYZ. The results shed light on the important roles of electrostatic interactions between AgNWs and proteins, which may have important implications for evaluating the fate and effects of silver nanomaterials in complicated environments.

Dynamic nano-Ag colloids cytotoxicity to and accumulation by Escherichia coli: Effects of Fe3+, ionic strength and humic acid

Released Ag ions or/and Ag particles are believed to contribute to the cytotoxicity of Ag nanomaterials, and thus, the cytotoxicity and mechanism of Ag nanomaterials should be dynamic in water due to unfixed Ag particle:Ag⁺ ratios. Our recent research found that the cytotoxicity of PVP-Ag nanoparticles is attributable to Ag particles alone in 3 hr bioassays, and shifts to both Ag particles and released Ag⁺ in 48 hr bioassays. Herein, as a continued study, the cytotoxicity and accumulation of 50 and 100 nm Ag colloids in Escherichia coli were determined dynamically. The cytotoxicity and mechanisms of nano-Ag colloids are dynamic throughout exposure and are derived from both Ag ions and particles. Ag accumulation by E. coli is derived mainly from extracellular Ag particles during the initial 12 hr of exposure, and thereafter mainly from intracellular Ag ions. Fe³⁺ accelerates the oxidative dissolution of nano-Ag colloids, which results in decreasing amounts of Ag particles and particle-related toxicity. Na⁺ stabilizes nano-Ag colloids, thereby decreasing the bioavailability of Ag particles and particle-related toxicity. Humic acid (HA) binds Ag⁺ to form Ag⁺-HA, decreasing ion-related toxicity and binding to the E. coli surface, decreasing particle-related toxicity. HA in complex conditions showed a stronger relative contribution to toxicity and accumulation than Na⁺ or Fe³⁺. The results highlighted the cytotoxicity and mechanism of nano-Ag colloids are dynamic and affected by environmental factors, and therefore exposure duration and water chemistry should be seriously considered in environmental and health risk assessments.

Advancing the Understanding of Environmental Transformations, Bioavailability and Effects of Nanomaterials, an International US Environmental Protection Agency-UK Environmental Nanoscience Initiative Joint Program

Nanotechnology has significant economic, health, and environmental benefits, including renewable energy and innovative environmental solutions. Manufactured nanoparticles have been incorporated into new materials and products because of their novel or enhanced properties. These very same properties also have prompted concerns about the potential environmental and human health hazard and risk posed by the manufactured nanomaterials. Appropriate risk management responses require the development of models capable of predicting the environmental and human health effects of the nanomaterials. Development of predictive models has been hampered by a lack of information concerning the environmental fate, behavior and effects of manufactured nanoparticles. The United Kingdom (UK) Environmental Nanoscience Initiative and the United States (US) Environmental Protection Agency have developed an international research program to enhance the knowledgebase and develop risk-predicting models for manufactured nanoparticles. Here we report selected highlights of the program as it sought to maximize the complementary strengths of the transatlantic scientific communities by funding three integrated US-UK consortia to investigate the transformation of these nanoparticles in terrestrial, aquatic, and atmospheric environment. Research results demonstrate there is a functional relationship between the physicochemical properties of environmentally transformed nanomaterials and their effects and that this relationship is amenable to modeling. In addition, the joint transatlantic program has allowed the leveraging of additional funding, promoting transboundary scientific collaboration.

Silver Nanowire Particle Reactivity with Human Monocyte-Derived Macrophage Cells: Intracellular Availability of Silver Governs Their Cytotoxicity

Silver nanowires (AgNWs) are increasingly being used in the production of optoelectronic devices, with manufacturing processes posing a risk for occupational exposures via inhalation. Although some studies have explored the environmental effects of AgNWs, few data exist on human health effects. Alveolar macrophages are central in the clearance of inhaled fibers from the lungs, with frustrated phagocytosis often stated as a key determinant for the onset of inflammatory reactions. However, the mechanisms through which fully ingested AgNWs interact with, degrade, and transform within primary macrophages over time, and whether the reactivity of the AgNWs arises due to ionic or particulate effects, or both, are poorly understood. Here, a combination of elemental quantification, 3D tomography, analytical transmission electron microscopy (TEM), and confocal microscopy were employed to monitor the uptake, intracellular Ag⁺ availability, and processing of AgNWs of two different lengths (1 and 10 μm) inside human monocyte-derived macrophages (HMMs). Using AgNO₃ and spherical silver nanoparticles (AgNPs) as a comparison, the amount of total bioavailable/intracellular Ag highly correlated to the cytotoxicity of AgNWs. The 10 μm AgNWs were completely internalized in HMMs, with numerous lysosomal vesicles observed in close vicinity to the AgNWs. Following cellular uptake, AgNWs dissolved and transformed intracellularly, with precipitation of AgCl as well as Ag₂S. These transformation processes were likely due to AgNW degradation in the acidic environment of lysosomes, leading to the release of Ag⁺ ions that rapidly react with Cl^- and SH^- species of the cell microenvironment. Our data suggest that, in HMMs, not only frustrated phagocytosis but also the extent of intracellular uptake and dissolution of AgNWs dictates their cytotoxicity.

Inactivation, Clearance, and Functional Effects of Lung-Instilled Short and Long Silver Nanowires in Rats

There is a potential for silver nanowires (AgNWs) to be inhaled, but there is little information on their health effects and their chemical transformation inside the lungs in vivo. We studied the effects of short (S-AgNWs; 1.5 μm) and long (L-AgNWs; 10 μm) nanowires instilled into the lungs of Sprague-Dawley rats. S- and L-AgNWs were phagocytosed and degraded by macrophages; there was no frustrated phagocytosis. Interestingly, both AgNWs were internalized in alveolar epithelial cells, with precipitation of Ag₂S on their surface as secondary Ag₂S nanoparticles. Quantitative serial block face three-dimensional scanning electron microscopy showed a small, but significant, reduction of NW lengths inside alveolar epithelial cells. AgNWs were also present in the lung subpleural space where L-AgNWs exposure resulted in more Ag+ve macrophages situated within the pleura and subpleural alveoli, compared with the S-AgNWs exposure. For both AgNWs, there was lung inflammation at day 1, disappearing by day 21, but in bronchoalveolar lavage fluid (BALF), L-AgNWs caused a delayed neutrophilic and macrophagic inflammation, while S-AgNWs caused only acute transient neutrophilia. Surfactant protein D (SP-D) levels in BALF increased after S- and L-AgNWs exposure at day 7. L-AgNWs induced MIP-1α and S-AgNWs induced IL-18 at day 1. Large airway bronchial responsiveness to acetylcholine increased following L-AgNWs, but not S-AgNWs, exposure. The attenuated response to AgNW instillation may be due to silver inactivation after precipitation of Ag₂S with limited dissolution. Our findings have important consequences for the safety of silver-based technologies to human health.

Silver nanoparticles reduce brain inflammation and related neurotoxicity through induction of H2S-synthesizing enzymes

Silver nanoparticles (AgNP) are known to penetrate into the brain and cause neuronal death. However, there is a paucity in studies examining the effect of AgNP on the resident immune cells of the brain, microglia. Given microglia are implicated in neurodegenerative disorders such as Parkinson's disease (PD), it is important to examine how AgNPs affect microglial inflammation to fully assess AgNP neurotoxicity. In addition, understanding AgNP processing by microglia will allow better prediction of their long term bioreactivity. In the present study, the in vitro uptake and intracellular transformation of citrate-capped AgNPs by microglia, as well as their effects on microglial inflammation and related neurotoxicity were examined. Analytical microscopy demonstrated internalization and dissolution of AgNPs within microglia and formation of non-reactive silver sulphide (Ag2S) on the surface of AgNPs. Furthermore, AgNP-treatment up-regulated microglial expression of the hydrogen sulphide (H2S)-synthesizing enzyme cystathionine-γ-lyase (CSE). In addition, AgNPs showed significant anti-inflammatory effects, reducing lipopolysaccharide (LPS)-stimulated ROS, nitric oxide and TNFα production, which translated into reduced microglial toxicity towards dopaminergic neurons. Hence, the present results indicate that intracellular Ag2S formation, resulting from CSE-mediated H2S production in microglia, sequesters Ag+ ions released from AgNPs, significantly limiting their toxicity, concomitantly reducing microglial inflammation and related neurotoxicity.

Impacts of Morphology, Natural Organic Matter, Cations, and Ionic Strength on Sulfidation of Silver Nanowires

Silver nanowires (AgNWs) are being widely utilized in an increasing number of consumer products, which could release silver to aquatic environments during the use or washing process, and have received growing concerns on their potential risks to bio-organisms and humans. The present study demonstrated that AgNWs mainly experienced direct oxysulfidation by reacting with dissolved sulfide species (initial S^2- concentration at 1.6 mg/L) to produce silver sulfide nanostructures under environmentally relevant conditions. Granular Ag₂S nanoparticles were formed on the surface of the nanowires. The sulfidation rate constant (k_Ag) of AgNWs was compared with those of silver nanoparticles (AgNPs) at different particle sizes. It was found that the k_Ag positively correlated with the specific surface areas of the silver nanomaterials. Natural organic matter (NOM) suppressed the sulfidation of AgNWs to different extents depending on its concentration. Divalent cations (Mg²⁺ and Ca²⁺ ions) substantially accelerated the sulfidation rates of AgNWs compared to monovalent cations (Na⁺ and K⁺ ions). At the same ionic strengths, Ca²⁺ ions displayed the highest promoting effect among the four metallic ions.

Dietary silver nanoparticles can disturb the gut microbiota in mice

Background

Humans are increasingly exposed via the diet to Ag nanoparticles (NP) used in the food industry. Because of their anti-bacterial activity, ingested Ag NP might disturb the gut microbiota that is essential for local and systemic homeostasis. We explored here the possible impact of dietary Ag NP on the gut microbiota in mice at doses relevant for currently estimated human intake.

Methods

Mice were orally exposed to food (pellets) supplemented with increasing doses of Ag NP (0, 46, 460 or 4600 ppb) during 28 d. Body weight, systemic inflammation and gut integrity were investigated to determine overall toxicity, and feces DNA collected from the gut were analyzed by Next Generation Sequencing (NGS) to assess the effect of Ag NP on the bacterial population. Ag NP were characterized alone and in the supplemented pellets by scanning transmission electron microscopy (STEM) and energy dispersive X-ray analysis (EDX).

Results

No overall toxicity was recorded in mice exposed to Ag NP. Ag NP disturbed bacterial evenness (α-diversity) and populations (β-diversity) in a dose-dependent manner. Ag NP increased the ratio between Firmicutes (F) and Bacteroidetes (B) phyla. At the family level, Lachnospiraceae and the S24-7 family mainly accounted for the increase in Firmicutes and decrease in Bacteroidetes, respectively. Similar effects were not observed in mice identically exposed to the same batch of Ag NP-supplemented pellets aged during 4 or 8 months and the F/B ratio was less or not modified. Analysis of Ag NP-supplemented pellets showed that freshly prepared pellets released Ag ions faster than aged pellets. STEM-EDX analysis also showed that Ag sulfidation occurred in aged Ag NP-supplemented pellets.

Conclusions

Our data indicate that oral exposure to human relevant doses of Ag NP can induce microbial alterations in the gut. The bacterial disturbances recorded after Ag NP are similar to those reported in metabolic and inflammatory diseases, such as obesity. It also highlights that Ag NP aging in food, and more specifically sulfidation, can reduce the effects of Ag NP on the microbiota by limiting the release of toxic Ag ions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12989-016-0149-1) contains supplementary material, which is available to authorized users.

Dynamic protein coronas revealed as a modulator of silver nanoparticle sulphidation in vitro

Proteins adsorbing at nanoparticles have been proposed as critical toxicity mediators and are included in ongoing efforts to develop predictive tools for safety assessment. Strongly attached proteins can be isolated, identified and correlated to changes in nanoparticle state, cellular association or toxicity. Weakly attached, rapidly exchanging proteins are also present at nanoparticles, but are difficult to isolate and have hardly been examined. Here we study rapidly exchanging proteins and show for the first time that they have a strong modulatory effect on the biotransformation of silver nanoparticles. Released silver ions, known for their role in particle toxicity, are found to be trapped as silver sulphide nanocrystals within the protein corona at silver nanoparticles in serum-containing cell culture media. The strongly attached corona acts as a site for sulphidation, while the weakly attached proteins reduce nanocrystal formation in a serum-concentration-dependent manner. Sulphidation results in decreased toxicity of Ag NPs.

The biomolecule layer adsorbed at the nanoparticle surface and defined as protein corona affects the nanoparticle biophysical properties and functions. Here, the authors suggest that rapidly-exchanging proteins on the outermost layer of the corona modulate sulphidation of silver nanoparticles in vitro.

Pulmonary surfactant mitigates silver nanoparticle toxicity in human alveolar type-I-like epithelial cells

Accompanying increased commercial applications and production of silver nanomaterials is an increased probability of human exposure, with inhalation a key route. Nanomaterials that deposit in the pulmonary alveolar region following inhalation will interact firstly with pulmonary surfactant before they interact with the alveolar epithelium. It is therefore critical to understand the effects of human pulmonary surfactant when evaluating the inhalation toxicity of silver nanoparticles. In this study, we evaluated the toxicity of AgNPs on human alveolar type-I-like epithelial (TT1) cells in the absence and presence of Curosurf(®) (a natural pulmonary surfactant substitute), hypothesising that the pulmonary surfactant would act to modify toxicity. We demonstrated that 20nm citrate-capped AgNPs induce toxicity in human alveolar type I-like epithelial cells and, in agreement with our hypothesis, that pulmonary surfactant acts to mitigate this toxicity, possibly through reducing AgNP dissolution into cytotoxic Ag(+) ions. For example, IL-6 and IL-8 release by TT1 cells significantly increased 10.7- and 35-fold, respectively (P<0.01), 24h after treatment with 25μg/ml AgNPs. In contrast, following pre-incubation of AgNPs with Curosurf(®), this effect was almost completely abolished. We further determined that the mechanism of this toxicity is likely associated with Ag(+) ion release and lysosomal disruption, but not with increased reactive oxygen species generation. This study provides a critical understanding of the toxicity of AgNPs in target human alveolar type-I-like epithelial cells and the role of pulmonary surfactant in mitigating this toxicity. The observations reported have important implications for the manufacture and application of AgNPs, in particular for applications involving use of aerosolised AgNPs.

Avoiding artefacts during electron microscopy of silver nanomaterials exposed to biological environments

Electron microscopy has been applied widely to study the interaction of nanomaterials with proteins, cells and tissues at nanometre scale. Biological material is most commonly embedded in thermoset resins to make it compatible with the high vacuum in the electron microscope. Room temperature sample preparation protocols developed over decades provide contrast by staining cell organelles, and aim to preserve the native cell structure. However, the effect of these complex protocols on the nanomaterials in the system is seldom considered. Any artefacts generated during sample preparation may ultimately interfere with the accurate prediction of the stability and reactivity of the nanomaterials. As a case study, we review steps in the room temperature preparation of cells exposed to silver nanomaterials (AgNMs) for transmission electron microscopy imaging and analysis. In particular, embedding and staining protocols, which can alter the physicochemical properties of AgNMs and introduce artefacts thereby leading to a misinterpretation of silver bioreactivity, are scrutinized. Recommendations are given for the application of cryogenic sample preparation protocols, which simultaneously fix both particles and diffusible ions. By being aware of the advantages and limitations of different sample preparation methods, compromises or selection of different correlative techniques can be made to draw more accurate conclusions about the data.

Comparison of 20 nm silver nanoparticles synthesized with and without a gold core: Structure, dissolution in cell culture media, and biological impact on macrophages

Widespread use of silver nanoparticles raises questions of environmental and biological impact. Many synthesis approaches are used to produce pure silver and silver-shell gold-core particles optimized for specific applications. Since both nanoparticles and silver dissolved from the particles may impact the biological response, it is important to understand the physicochemical characteristics along with the biological impact of nanoparticles produced by different processes. The authors have examined the structure, dissolution, and impact of particle exposure to macrophage cells of two 20 nm silver particles synthesized in different ways, which have different internal structures. The structures were examined by electron microscopy and dissolution measured in Rosewell Park Memorial Institute media with 10% fetal bovine serum. Cytotoxicity and oxidative stress were used to measure biological impact on RAW 264.7 macrophage cells. The particles were polycrystalline, but 20 nm particles grown on gold seed particles had smaller crystallite size with many high-energy grain boundaries and defects, and an apparent higher solubility than 20 nm pure silver particles. Greater oxidative stress and cytotoxicity were observed for 20 nm particles containing the Au core than for 20 nm pure silver particles. A simple dissolution model described the time variation of particle size and dissolved silver for particle loadings larger than 9 μg/ml for the 24-h period characteristic of many in-vitro studies.

Chloride-induced shape transformation of silver nanoparticles in a water environment

The effects of chloride on dissolution and toxicity of silver nanoparticles (AgNPs) have been well studied. However, their intermediate shapes during the transition have not been illustrated to-date. Herein, the chloride-induced shape transformation process of AgNPs under long-term, low-concentration conditions is explored. A unique triangular Ag-AgCl heterostructure is observed. The structure then evolves into a symmetric hexapod and finally into a smaller AgNP. This transformation process could be affected by other environmental conditions, such as 0.4 mg/mL humic acid, 5% surfactants and 1 mg/mL bovine serum albumin protein. Our results offer new knowledge regarding the shape transformation process of AgNPs in the presence of chloride, which can be valuable in relevant studies concerning the effect of water chemistry on the behavior of AgNPs.

Static and Dynamic Microscopy of the Chemical Stability and Aggregation State of Silver Nanowires in Components of Murine Pulmonary Surfactant

The increase of production volumes of silver nanowires (AgNWs) and of consumer products incorporating them may lead to increased health risks from occupational and public exposures. There is currently limited information about the putative toxicity of AgNWs upon inhalation and incomplete understanding of the properties that control their bioreactivity. The lung lining fluid (LLF), which contains phospholipids and surfactant proteins, represents a first contact site with the respiratory system. In this work, the impact of dipalmitoylphosphatidylcholine (DPPC), Curosurf, and murine LLF on the stability of AgNWs was examined. Both the phospholipid and protein components of the LLF modified the dissolution kinetics of AgNWs, due to the formation of a lipid corona or aggregation of the AgNWs. Moreover, the hydrophilic proteins, but neither the hydrophobic surfactant proteins nor the phospholipids, induced agglomeration of the AgNWs. Finally, the generation of a secondary population of nanosilver was observed and attributed to the reduction of Ag(+) ions by the surface capping of the AgNWs. Our findings highlight that combinations of spatially resolved dynamic and static techniques are required to develop a holistic understanding of which parameters govern AgNW behavior at the point of exposure and to accurately predict their risks on human health and the environment.

Use of Synchrotron Radiation-Analytical Techniques To Reveal Chemical Origin of Silver-Nanoparticle Cytotoxicity

To predict potential medical value or toxicity of nanoparticles (NPs), it is necessary to understand the chemical transformation during intracellular processes of NPs. However, it is a grand challenge to capture a high-resolution image of metallic NPs in a single cell and the chemical information on intracellular NPs. Here, by integrating synchrotron radiation-beam transmission X-ray microscopy (SR-TXM) and SR-X-ray absorption near edge structure (SR-XANES) spectroscopy, we successfully capture the 3D distribution of silver NPs (AgNPs) inside a single human monocyte (THP-1), associated with the chemical transformation of silver. The results reveal that the cytotoxicity of AgNPs is largely due to the chemical transformation of particulate silver from elemental silver (Ag(0))n, to Ag(+) ions and Ag-O-, then Ag-S- species. These results provide direct evidence in the long-lasting debate on whether the nanoscale or the ionic form dominates the cytotoxicity of silver nanoparticles. Further, the present approach provides an integrated strategy capable of exploring the chemical origins of cytotoxicity in metallic nanoparticles.

Silver nanowire interactions with primary human alveolar type-II epithelial cell secretions: contrasting bioreactivity with human alveolar type-I and type-II epithelial cells

Inhaled nanoparticles have a high deposition rate in the alveolar units of the deep lung. The alveolar epithelium is composed of type-I and type-II epithelial cells (ATI and ATII respectively) and is bathed in pulmonary surfactant. The effect of native human ATII cell secretions on nanoparticle toxicity is not known. We investigated the cellular uptake and toxicity of silver nanowires (AgNWs; 70 nm diameter, 1.5 μm length) with human ATI-like cells (TT1), in the absence or presence of Curosurf® (a natural porcine pulmonary surfactant with a low amount of protein) or harvested primary human ATII cell secretions (HAS; containing both the complete lipid as well as the full protein complement of human pulmonary surfactant i.e. SP-A, SP-B, SP-C and SP-D). We hypothesised that Curosurf® or HAS would confer improved protection for TT1 cells, limiting the toxicity of AgNWs. In agreement with our hypothesis, HAS reduced the inflammatory and reactive oxygen species (ROS)-generating potential of AgNWs with exposed TT1 cells. For example, IL-8 release and ROS generation was reduced by 38% and 29%, respectively, resulting in similar levels to that of the non-treated controls. However in contrast to our hypothesis, Curosurf® had no effect. We found a significant reduction in AgNW uptake by TT1 cells in the presence of HAS but not Curosurf. Furthermore, we show that the SP-A and SP-D are likely to be involved in this process as they were found to be specifically bound to the AgNWs. While ATI cells appear to be protected by HAS, evidence suggested that ATII cells, despite no uptake, were vulnerable to AgNW exposure (indicated by increased IL-8 release and ROS generation and decreased intracellular SP-A levels one day post-exposure). This study provides unique findings that may be important for the study of lung epithelial-endothelial translocation of nanoparticles in general and associated toxicity within the alveolar unit.

Inhalation of silver nanomaterials--seeing the risks

Demand for silver engineered nanomaterials (ENMs) is increasing rapidly in optoelectronic and in health and medical applications due to their antibacterial, thermal, electrical conductive, and other properties. The continued commercial up-scaling of ENM production and application needs to be accompanied by an understanding of the occupational health, public safety and environmental implications of these materials. There have been numerous in vitro studies and some in vivo studies of ENM toxicity but their results are frequently inconclusive. Some of the variability between studies has arisen due to a lack of consistency between experimental models, since small differences between test materials can markedly alter their behaviour. In addition, the propensity for the physicochemistry of silver ENMs to alter, sometimes quite radically, depending on the environment they encounter, can profoundly alter their bioreactivity. Consequently, it is important to accurately characterise the materials before use, at the point of exposure and at the nanomaterial-tissue, or "nanobio", interface, to be able to appreciate their environmental impact. This paper reviews current literature on the pulmonary effects of silver nanomaterials. We focus our review on describing whether, and by which mechanisms, the chemistry and structure of these materials can be linked to their bioreactivity in the respiratory system. In particular, the mechanisms by which the physicochemical properties (e.g., aggregation state, morphology and chemistry) of silver nanomaterials change in various biological milieu (i.e., relevant proteins, lipids and other molecules, and biofluids, such as lung surfactant) and affect subsequent interactions with and within cells will be discussed, in the context not only of what is measured but also of what can be visualized.

Short versus long silver nanowires: a comparison of in vivo pulmonary effects post instillation

Background

Silver nanowires (Ag NWs) are increasingly being used to produce touchscreens for smart phones and computers. When applied in a thin film over a plastic substrate, Ag NWs create a transparent, highly-conductive network of fibers enabling the touch interface between consumers and their electronics. Large-scale application methods utilize techniques whereby Ag NW suspensions are deposited onto substrates via droplets. Aerosolized droplets increase risk of occupational Ag NW exposure. Currently, there are few published studies on Ag NW exposure-related health effects. Concerns have risen about the potential for greater toxicity from exposure to high-aspect ratio nanomaterials compared to their non-fibrous counterparts. This study examines whether Ag NWs of varying lengths affect biological responses and silver distribution within the lungs at different time-points.

Methods

Two different sizes of Ag NWs (2 μm [S-Ag NWs] and 20 μm [L-Ag NWs]) were tested. Male, Sprague-Dawley rats were intratracheally instilled with Ag NWs (0, 0.1, 0.5, or 1.0 mg/kg). Broncho-alveolar lavage fluid (BALF) and lung tissues were obtained at 1, 7, and 21 days post exposure for analysis of BAL total cells, cell differentials, and total protein as well as tissue pathology and silver distribution.

Results and conclusions

The two highest doses produced significant increases in BAL endpoints. At Day 1, Ag NWs increased total cells, inflammatory polymorphonuclear cells (PMNs), and total protein. PMNs persisted for both Ag NW types at Day 7, though not significantly so, and by Day 21, PMNs appeared in line with sham control values. Striking histopathological features associated with Ag NWs included 1) a strong influx of eosinophils at Days 1 and 7; and 2) formation of Langhans and foreign body giant cells at Days 7 and 21. Epithelial sloughing in the terminal bronchioles (TB) and cellular exudate in alveolar regions were also common. By Day 21, Ag NWs were primarily enclosed in granulomas or surrounded by numerous macrophages in the TB-alveolar duct junction. These findings suggest short and long Ag NWs produce pulmonary toxicity; thus, further research into exposure-related health effects and possible exposure scenarios are necessary to ensure human safety as Ag NW demand increases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12989-014-0052-6) contains supplementary material, which is available to authorized users.

Identification and avoidance of potential artifacts and misinterpretations in nanomaterial ecotoxicity measurements

Novel physicochemistries of engineered nanomaterials (ENMs) offer considerable commercial potential for new products and processes, but also the possibility of unforeseen and negative consequences upon ENM release into the environment. Investigations of ENM ecotoxicity have revealed that the unique properties of ENMs and a lack of appropriate test methods can lead to results that are inaccurate or not reproducible. The occurrence of spurious results or misinterpretations of results from ENM toxicity tests that are unique to investigations of ENMs (as opposed to traditional toxicants) have been reported, but have not yet been systemically reviewed. Our objective in this manuscript is to highlight artifacts and misinterpretations that can occur at each step of ecotoxicity testing: procurement or synthesis of the ENMs and assessment of potential toxic impurities such as metals or endotoxins, ENM storage, dispersion of the ENMs in the test medium, direct interference with assay reagents and unacknowledged indirect effects such as nutrient depletion during the assay, and assessment of the ENM biodistribution in organisms. We recommend thorough characterization of initial ENMs including measurement of impurities, implementation of steps to minimize changes to the ENMs during storage, inclusion of a set of experimental controls (e.g., to assess impacts of nutrient depletion, ENM specific effects, impurities in ENM formulation, desorbed surface coatings, the dispersion process, and direct interference of ENM with toxicity assays), and use of orthogonal measurement methods when available to assess ENMs fate and distribution in organisms.