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Abstract:
The same properties of nanoparticles that make them so appealing to manufacturers may also have negative effects on the environment and human health.

However, little is known which particles may be harmful. Part of the problem is determining exactly what a nanoparticle is.

When Nano May Not Be Nano

Durham, NC | Posted on September 14th, 2009

A new analysis by an international team of researchers from the Center for the Environmental Implications of NanoTechnology (CEINT), based at Duke University, argues for a new look at the way nanoparticles are selected when studying the potential impacts on human health and the environment. They have found that while many small particles are considered to be "nano," these materials often do not meet full definition of having special properties that make them different from conventional materials.

Under the prevailing definition, a particle is deemed nano if its diameter is between 1 and 100 nanometers (nm) - about 1/10,000 the diameter of a human hair - and if it has properties that significantly differ from its naturally occurring, or bulk, counterpart.

The special properties of nanoparticles come from their high surface-area-to-volume ratio. They also have a considerably higher percentage of atoms on their surface compared to bulk particles, which can make them more reactive. These man-made materials can be found in a vast array of consumer products, including paints and sunscreens, as well as in water treatment plants and drug delivery systems.

For most of this decade, discussions of nanoparticles have tended to focus more on their size than their properties. However, after reviewing the scientific literature, the Duke-led team believes that the old definition is not specific enough. A definition that focuses on properties is critical, they say, to help scientists determine which particular nanoparticles are the most likely to represent a threat to the environment or human health.

Generally speaking, it is the very smallest particles (less than 30 nanometers) that should receive the most attention in studying the environmental and human health impacts of nanomaterials, according to Mark Wiesner, a Duke professor of civil and environmental engineering and director of the federally funded CEINT.

"There are an infinite number of potential new man-made nanoparticles, so we need to find a way to narrow our efforts to those that have the greatest likelihood of having the unique properties with unique effects," Wiesner said.

"A key question to be answered is whether or not a particular nanoparticle has toxic or hazardous properties that are truly different from identical particles in their bulk form," Wiesner continued. "This question has not been answered. To do so, we need to be speaking the same language when assessing any unique properties of these novel materials."

The results of Wiesner's analysis were published online in the journal Nature Nanotechnology. The study was supported by CEINT, which is jointly funded by the National Science Foundation and Environmental Protection Agency.

Specifically, the researchers found that nanoparticles approaching the 100 nm end of the size spectrum tend to have fewer special properties when compared to their bulk counterparts. Furthermore, they found that nanoparticles smaller than 30 nm tend to exhibit the unique properties that should command increased scrutiny, Wiesner said.

"Many nanoparticles smaller than 30 nanometers undergo drastic changes in their crystalline structure that enhance how the atoms on their surface interact with the environment," Wiesner said.

For example, because of the increased surface-area-to-volume ratio, nanoparticles can be highly reactive with other chemicals in the environment and can also disrupt certain activities within cells.

"While there have been reports of nanoparticle toxicity increasing as the size decreases, it is still uncertain whether this increase in reactivity is harmful to the environment or human safety," Wiesner said. "To settle this issue, toxicological studies should contrast particles that exhibit novel size-dependant properties, particularly concerning their surface reactivity, and those particles that do not exhibit these properties."

Other members of the research team include Melanie Auffan, Duke; Jerome Rose and Jean-Yves Bottero, Aix-Marseille Universite, France; Gregory Lowry, Carnegie Mellon University; and Jean-Pierre Jolivet, Laboratoire de Chimie de la Matiere Condensee de Paris, France.

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About Duke University
"James B. Duke's founding Indenture of Duke University directed the members of the University to 'provide real leadership in the educational world' by choosing individuals of 'outstanding character, ability, and vision' to serve as its officers, trustees and faculty; by carefully selecting students of 'character, determination and application;' and by pursuing those areas of teaching and scholarship that would 'most help to develop our resources, increase our wisdom, and promote human happiness.'

“To these ends, the mission of Duke University is to provide a superior liberal education to undergraduate students, attending not only to their intellectual growth but also to their development as adults committed to high ethical standards and full participation as leaders in their communities; to prepare future members of the learned professions for lives of skilled and ethical service by providing excellent graduate and professional education; to advance the frontiers of knowledge and contribute boldly to the international community of scholarship; to promote an intellectual environment built on a commitment to free and open inquiry; to help those who suffer, cure disease, and promote health, through sophisticated medical research and thoughtful patient care; to provide wide ranging educational opportunities, on and beyond our campuses, for traditional students, active professionals and life-long learners using the power of information technologies; and to promote a deep appreciation for the range of human difference and potential, a sense of the obligations and rewards of citizenship, and a commitment to learning, freedom and truth.

“By pursuing these objectives with vision and integrity, Duke University seeks to engage the mind, elevate the spirit, and stimulate the best effort of all who are associated with the University; to contribute in diverse ways to the local community, the state, the nation and the world; and to attain and maintain a place of real leadership in all that we do.”

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