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Researchers Spin Straw Into Gold

Peter N. Spotts
The Christian Science Monitor
August 29, 2002
 
Rumpelstiltskin, the fairy-tale rogue who spun straw into gold, has
nothing on Miguel Yacaman and Jorge Gardea-Torresdey.

The two University of Texas researchers have developed a way to draw gold from wheat, alfalfa, or—best of all—oats.

No spinning wheel required. In this day and age, a simple solvent will suffice to turn homely vegetation into a source of precious metals.

But if you're thinking of quitting the day job and buying an alfalfa farm, don't be too hasty. The quantities of gold at stake won't quickly cover the cost of a harvesting combine.

The yields, in fact, are microscopic. The gold appears as particles mere billionths of a meter wide.



But the duo holds that their approach—which takes advantage of plants' metal-absorbing abilities—could provide a cheap way to "mine" gold from soils, with the plants supplying the gold in forms tailor-made for use in the burgeoning field of nanotechnology.

The work represents the first time researchers have reported that living plants form these gold micro-nuggets, opening "exciting new ways to fabricate nanoparticles," according to Gardea-Torresdey, who heads the chemistry department at the University of Texas at El Paso.

He notes that current approaches to making gold nanoparticles, now used as tags for studying cellular processes in biology and coveted for use as electrical contacts in nanoelectronic circuits, are expensive and involve chemical processes that generate pollution.

The use of plants, he holds, "is both cost-effective and environmentally friendly."

Researchers have known for years that plants take up metals. Plants' abilities to absorb all sorts of toxic compounds have led to their use as biological vacuum cleaners on sites tainted with pollutants ranging from arsenic, TNT, and zinc to radioactive cesium. By some estimates, phytoremediation could become a U.S. $214 million to $370 million business within the next three years.

Indeed, the key piece in the gold-from-alfalfa puzzle fell into place during a hazardous-waste clean-up effort outside Mexico City, according to Yacaman, a chemical engineering professor who came to Austin from Mexico two years ago. While in Mexico serving as director of the physics institute at the National Autonomous University of Mexico, Yacaman teamed up with Gardea-Torresdey to use plants to clean up the site, heavily contaminated with chromium.

When the two analyzed the plants, "the tremendous surprise was that the metal was not dispersed in the plant as we assumed, but was precipitated in the plant as clusters of nanoparticles, exactly the same ones called quantum dots in the electronics industry," Yacaman says.

What started out as a plant-based pollution clean-up project quickly turned into a nanotechnology research project, he adds. The two scientists and their colleagues also knew that plants had been used to prospect for gold.

In the tropics, for example, researchers from Australia, Canada, and Papua New Guinea found that gold concentrations in plants could serve as effective stand-ins for direct soil samples in efforts to find new gold deposits. Plants were particularly effective where soils had been covered by dust and ash from volcanic eruptions and so couldn't be tested directly.

The question was whether easily grown crop plants could also sequester gold, and in nanoparticle form.

The team started with alfalfa, germinating seeds in an artificial, gold-rich medium. Using powerful x-ray and electron microscopes, they not only struck gold in the alfalfa shoots, but found that they formed the nanoparticles they were looking for.

Extracting the metals presents no problem, Yacaman says. In essence, "you can easily dissolve the organic material," leaving the gold intact.

Although initial experiments showed that the gold particles formed in random shapes, Yacaman says it appears that by changing the acidity of the growing medium, the shapes become more uniform.

Since it first reported its work in the American Chemical Society's Nano Letters in January, the team has worked with other metals, using plants to manufacture nanoparticles of silver, Europium, palladium, and iron.

"We are now…fabricating a platinum ion that could be used for magnetic recording," Yacaman says.

For industrial-scale production, the team holds that the plants can be grown indoors in gold-enriched soils, or they can be "farmed" at abandoned gold mines.

In addition, they've tested the approach on wheat and oats, finding that oats are much more efficient at taking up gold than alfalfa.

Copyright 2002 The Christian Science Monitor
 

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