Fall 2008.
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Illumination magazine.
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Photo illustration by Rob Hill and Blake Dinsdale.

Nanoparticle silver has become a high-tech superstar, a marketing sensation that promises to free an ever-growing variety of household items from the scourges of odor and bacteria. But a growing number of scientists fear these tiny nanosilvers, while inarguably hygienic, could be less than completely benign.

MU's Zhiqiang Hu, an assistant professor of civil and environmental engineering, is among the most prominent. Hu says nanosilver first appeared on his investigative radar screen when he learned that public utilities in California were nervous about the tiny element's effect on wastewater. Hu specializes in wastewater treatment research, and he immediately recognized why California officials had reason to be worried: Nanosilvers can kill good bacteria along with the bad.

"If these nanoparticles go to our wastewater facilities, they may cause some problems because we rely on healthy bacteria [to remove contaminants from wastewater]," he says. Armed with a grant from the National Science Foundation, Hu set about to test his concerns. And sure enough, it wasn't long before he determined that nanosilver did, in fact, have a strong toxic effect, stronger even than that of other toxic silvers.

"The occurrence of toxicity is not a novel finding, necessarily, but Dr. Hu's observation that nanosilver is more toxic than the silver ion by two-fold is, indeed, a new finding," says Samuel Luoma, a researcher with the John Muir Institute of the Environment and editor-in-chief of San Francisco Estuary & Watershed Science. The concentration at which the Bacteria toxicity occurs is also lower than conventional knowledge suggests, he says, and would be problematic for sewage treatment.

Sewage treatment plant operators worry about silver when concentrations are above one milligram per liter; Hu shows effects of nanosilver at 0.14 milligram per liter.

Nanosilver is a tiny form of silver, so small it almost defies comprehension. One nanometer is one billionth of a meter. To put that into perspective, consider this: You would have to magnify a one nanometer-sized object a million times before it reached the size of a red ant. Enlarge that ant to the same degree, and you'd have an ant that's 5 kilometers, or 3.1 miles, long.

Just being able to see materials at the nanoscale seems incredible. But the 1981 invention of the scanning tunneling microscope allowed scientists to go a step further and actually begin manipulating nanoscale materials. This advancement made possible what we today call nanotechnology, defined by the federal National Nanotechnology Initiative as "the understanding and control of matter at dimensions of roughly 1 to 100 nanometers, where unique phenomena enable novel applications." The tunneling microscope's inventors won the 1986 Nobel Prize in physics.

Novel applications are possible at the nanoscale because nanomaterials often have -- or can be engineered to have -- physical and chemical properties that differ from their "bulk" species counterparts. At the bulk scale, for example, gold is an excellent conductor of heat and electricity but not of light. Gold nanoparticles, on the other hand, can be structured to absorb light and turn that light into heat. This heat, according to the NNI website, can "act like miniature thermal scalpels that can kill unwanted cells in the body, such as cancer cells."

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Published by the Office of Research.

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