"The work has demonstrated that using alfalfa is a cost-effective and environmentally friendly method of producing gold nanoparticles," said Jorge Gardea-Torresdey of the University of Texas-El Paso, US. "Future work will involve the full physical characterization of the nanoparticles and the development of methods to extract them from the plants."

The alfalfa plants were grown in an artificial, gold-rich "soil" at the University of Texas-El Paso. Plants typically use their roots to extract nutrients and even heavy metals from the soil.

The researchers examined the resulting alfalfa using transmission electron microscopy at the University of Texas-Austin, US. This confirmed that gold nanoparticles were present in the roots and along the entire shoot of the plants. X-ray absorption spectroscopy performed at the Stanford Synchrotron Radiation Laboratory, US, showed that the physical properties of the particles were similar those of gold nanoparticles formed by conventional chemical techniques.

The researchers, who reported their work in Nano Letters, reckon they may be able to extract the gold nanoparticles from the plants by centrifuge.

The work was funded by the US Environmental Protection Agency, the Department of Energy and the National Institutes of Health.