No fairy tale: Researchers spin straw into gold

Grains contain gold in forms that seem tailor-made for industrial use.

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.

By Peter N. Spotts | Staff writer of The Christian Science Monitor

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 Dr. 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
estimated, phytoremediation could become a $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 Dr. 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.

http://www.csmonitor.com/2002/0829/p02s02-usgn.html