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Posted: Nov 24, 2011

Natural nanomaterials - they are everywhere

(Nanowerk Spotlight) Last month, when the European Commission offered its definition of "nanomaterial", it recommended to identify a nanomaterial only on the basis of its particle size: "The justification for this choice is that properties or risks posed by a nano-sized material are not determined by the intention of the manufacturer and do not differ depending on whether the nanomaterial is natural, produced incidentally, or the result of a manufacturing process with or without the explicit intention to produce a nanomaterial. There are many naturally occurring nanomaterials and they may exhibit similar properties to those that are manufactured. From a definition point of view it is therefore not logical to omit certain types of materials on the basis of their genesis." (Recommendation on the definition of nanomaterial; pdf)
"There is an inherent recognition in this definition that all sources of nanomaterials are important in evaluating the possible impact of nanoscale materials on human health and the environment; however, perhaps the greatest benefit to studying these materials will be in their ability to inform us about the manner in which nano-sized materials have been a part of our environment from the beginning." So argue Mark R. Wiesner and Michael F. Hochella, Jr. and their team in a thought piece published in the November 22, 2011 online edition of ACS Nano ("Meditations on the Ubiquity and Mutability of Nano-Sized Materials in the Environment").
Naturally occurring nanomaterials can be found everywhere in nature (fullerenes and graphene even have been discovered in space) and only with recent advances in instrumentation and metrology equipment are researchers beginning to locate, isolate, characterize and classify the vast range of their structural and chemical varieties.
"The closer we look, the more it appears that particles at the nanoscale are instrumental in important geochemical and biogeochemical reactions and kinetics such as the availability of elements in the oceans, the partitioning of metals in other aquatic systems, and the bioavailability and toxicity of many elements," the authors write. "An examination of the potential risks to human health and the environment posed by engineered nanoparticles and the ability to construct nanophases with great precision are leading to fundamental discoveries in the ecological and life sciences."
Recent findings by Hutchinson et. al. ("Generation of Metal Nanoparticles from Silver and Copper Objects: Nanoparticle Dynamics on Surfaces and Potential Sources of Nanoparticles in the Environment") for instance challenge conventional thinking about nanoparticle reactivity and imply that the production of new nanoparticles is an intrinsic property of the material that is not strongly size dependent. The discovery that silver and copper nanoparticles are generated spontaneously from man-made objects implies that humans have long been in direct contact with these nanomaterials and that macroscale objects represent a potential source of incidental nanoparticles in the environment.
This work also shows that in some cases, nanomaterials that are virtually identical to those engineered by researchers can form spontaneously in nature, either from ions in solution or from larger non nano-objects.
While almost all research on nanomaterial toxicity and the potential risks involved with nanomaterials has focused on engineered nanoparticles, the dynamic nature of nanoparticles in the environment adds another dimension to this field.
Naturally occurring nanomaterials are continuously forming within and distributed throughout continental soils, ground and surface waters, the oceans, and the atmosphere. According to Hochella, researchers estimate that soils are the most prolific generators of Earth's nanomaterials, and that oceans provide the largest collective reservoir of these materials (Naturally occurring inorganic nanoparticles: General assessment and a global budget for one of Earth's last unexplored geochemical components).
The authors write that all nanoparticles, once released into the environment, undergo dramatic and complex transformations through interactions with various chemicals and other factors – e.g., UV light, interaction with (in)organic ligands, redox reactions, biotransformations, aggregation. Such transformations will in turn affect the nanoparticles' toxicity.
They point out that "even without a unique mechanism of toxicity for nanoparticles, nanophases could contribute to greater-than-expected toxicity via additional routes of exposure or simply through a more efficient delivery system for toxic metal ions (e.g., a Trojan horse effect)."
The team concludes that "fundamental research is needed to adapt existing equilibrium models to account for the generation of nanomaterials from dissolved metals, the formation of nanoparticles from macroscopic objects, and to improve our understanding of how engineered nanomaterials may confound equilibrium modeling."
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