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