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How Does Bioavailability of Metal-Based Engineered Nanomaterials Influence Their Ecological and Health Implications?

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Written by Hope Sisley

Posted on 19 January 2014

Dr. Sam Luoma, a fellow of the John Muir Institute of the Environment and a professor at the University of California (Davis) spoke this Friday to the Mines chemistry department about metallic nanoparticle toxicity. By definition, a nanoparticle is a particle 100 nanometers in size or smaller. Nanoparticles (NPs) behave differently from larger particles because of their exceptionally high surface-to-volume ratio.

While NPs are made of many different materials, Luoma's talk focused on silver (Ag). Since Roman times, silver's antibacterial properties have led it to be used for drinking vessels. More recently, silver NPs have been used in catheters and surgical tools to prevent hospital infections; someday soon Ag NPs may coat internal human prosthetics, such as heart valves or replacement joints, to ensure their sterility. As with most exciting new technologies, however, this one has been somewhat overused. Everything from socks to teddy bears have incorporated silver NPs in hopes of preventing bacterial growth. It was even proposed that the Hong Kong subway system be coated with silver.

All of these applications increase the amount of Ag in the enviroment. Even silver dishes, every time you wash them, bequeath some Ag particles to the waste stream; the more silver is used, the more of it will escape. Silver is a known toxin for phytoplankton and some marine species; some Ag complexes can be quite toxic to humans as well. Thus, regulations are already in place to control Ag contamination. Unfortunately, because nanoparticles behave so uniquely, the regulations, which concern normal-size silver particles, must be reevaluated to account specifically for NPs. In order to do this, many factors must be considered. In general, the risk posed by a substance is the combination of hazard with exposure. Hazard includes things like the persistence of the substance in the environment, its toxicity, and the rate of bioaccumulation (that is, how quickly it builds up in organisms); exposure deals with the bioavailability of the substance (ie. how easily it can be taken up into the body) and its environmental frequency. The sources of the substance, its behavior in the environment, and what organism is being dealt with all must be taken into account. Some substances are toxic to fish but not humans, for instance; some are found so rarely as to not pose a threat; some complex with natural elements to form harmless compounds; and so on. Determining these characteristics for a nanoparticle is difficult - even determining a dosage measure - because of their very newness. The ultimate fate of an NP depends on its shape, its crystal structure, the coating agent used - things that are of little concern for a normal metal.

Metals do not occur on the nano scale in nature - but some substances do, including particles that a living organism's cells are structured to take in. Furthermore, while most creatures have evolved some defense against naturally-occurring toxic substances, there is no such defense for NPs. Depending on the nature of the particle, it may not be recognized as dangerous by the cells - may even be treated as a nutrient, to catastrophic result. Cells can take in outside substances through ion channels and through endocytosis, where the cell membrane folds around the particle and pulls it inside. Thus, a particle can be taken in whole or dissolved. Laboratory experiments have proven that this process occurs in worm cells: nematodes exposed to Ag NPs incorporated the silver within their tissues.

Luoma paused here to explain how exactly an organism's interactions with a metal are characterized. First, the bioavailability of the metal is the sum of the organism's uptake of the substance across all avenues: ingestion, respiration, transport through skin or mucous membranes, and so on. Bioaccumulation is quantified as the total concentration of the metal in the creature, or its total uptake minus its loss of the metal through excretion or natural detoxification processes. Bioaccumulation depends on the physiology of the animal (loss rates, for instance, as well as its diet and behavior), the ecology of the animal's habitat, and the properties and behavior of the particle. When a particle is released into the environment, it may dissolve, aggregate with other like particles, form ligands with other substances, or remain unchanged; the rate and mechanisms of its dispersal may differ depending on the nature of both the particle and its behavior.

In order to determine just how these many factors affect the interaction of silver NPs with living things, Luoma ran experiments on aquatic snails. He and his fellow researchers exposed the snails to water contaminated with set concentrations of either silver ions (Ag+) or various types of silver NPs. They found that the rate of uptake was faster for the ion than for the NP, both in fresh- and salt-water, but the uptake for freshwater was a full order of magnitude faster than saltwater. This shows that the dissolution of the nanoparticle plays a significant role in its uptake rate. Silver ions tend to aggregate in saltwater, hence the slower rate. Furthermore, the "capping agent" - that is, the coating of the NP - has a major effect on uptake rate. Industrial applications require a multitude of various capping agents, including acids, organic polymers, and more; depending on which capping agent is used, the total uptake of Ag may be drastically increased, or the rate of uptake may be significantly decreased. To add a further layer of complexity, the hardness of the water had no effect on silver uptake for Ag+, but showed a strong inverse relationship with NP uptake rate: the softer the water, the higher the rate. The capping agent caused even more discrepancy between the results.
Though, for many of the NPs tested, dissolved silver seemed to play the largest part, it was clear to Luoma that another uptake mechanism was involved. He tested lower concentrations of Ag using tracer isotopes - that is, artificially concentrated samples with a particular silver isotope not usually found in nature. At low concentrations, he found dissolved Ag was more important than NP silver, but that this relationship changed with higher concentrations. One reason for this is silver's tendency to aggregate on available surfaces, including an organism's food, the sediments where it lives, or even the organism itself. Such deposition favors food uptake over other pathways. Ingestion of metals through normal dietary processes (as opposed to accidental ingestion) isn't considered in existing regulations, even though it could be of great importance. For example, many major soft drink manufacturers use silver NPs to clean their cans before filling and distributing them; this means that humans may be ingesting Ag NPs every time they drink a pop. If this could be harmful to human health, regulations must be changed. With this in mind, Luoma set out to test whether ingestion of Ag NP-contaminated food was harmful to his snails.

First, diatoms were doped with Ag nanoparticles and fed to the snails. Then, the snails' waste was collected and analysed for Ag. The difference between the amount ingested and the amount excreted gave Luoma the amount of silver the snails retained in their bodies. Luoma and his colleagues found that the vast majority of the foodborne metals ended up in the snails' tissues rather than in their waste. Furthermore, while aggregation of the Ag particles decreased the uptake rate in the earlier water experiments, it had no noticeable effect when the Ag was foodborne. When Luoma stopped the Ag exposure, he found the rate at which the silver was removed from the snails' bodies depended on the capping agent: it was either typical of a metal or, shockingly, was almost nonexistent. As the potential toxicity of a metal depends on the rate at which it builds up in an organism, a material with a fast influx rate and a slow efflux (removal) rate is one of high toxicity. To have no efflux at all suggests extreme toxic potential.

Most troubling, however, was the observation that, as the concentration of NPs increased, the rate at which the snails grazed decreased. Even small NP concentrations caused the snails to eat 60% slower than normal. The efficiency of the snails' digestion also decreased, so that at high concentrations of NPs, the diatoms were passing through the snails essentially undigested. Furthermore, though Ag+ was more assimilated - that is, incorporated into the snails' tissues - than the silver NPs, the assimilation of NPs was over 50% for all types. Even after the observed decrease in digestion rate, the assimilation of Ag remained high - in other words, even though the snail was not getting the nutrients from the diatoms, it was still getting the NP contaminants. These experiments were of short duration - about 24 hours each - so the long-term effects of the NPs on the snails were not observed, but presumably, if the trends Luoma saw were to continue, the snails would have starved as a result of exposure to the NPs.

Though this presentation focused on silver, similar experiments have been conducting using other nanoparticles. Some, such as titanium NPs, were found to be mainly nontoxic; but others, such as cadmium and copper, were shown to be uniformly dangerous. Silver had the most variable toxicity of the metals examined. The type of cell being exposed to the NP also factored into the risk posed: zinc NPs were shown to be benign in most circumstances, but when affecting the skin or lungs, zinc NPs were worse than any other particle tested.

The lesson learned from these investigations, Luoma concluded, is that nanoparticles are so unique and so complex in their nature and behavior that new regulations must be created to deal with them. Nanomaterials are predicted to become a $15 billion industry within the next decade; if this prediction comes to pass, NP toxicity will become a very pressing issue indeed. Better to put regulations in place now than to do damage control later.

Colorado School of Mines

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