Plants with the Midas Touch: Formation of Gold Nanoparticles by Alfalfa Plants

In the well-known Greek legend the touch of King Midas would convert anything to metallic gold. Recently, a team working at SSRL lead by Professor Jorge Gardea-Torresdey from the University of Texas at El Paso have shown that ordinary alfalfa plants can accumulate very small particles (nanoparticles) of metallic gold (1). The best-known materials that contain nanoparticles of metallic gold are gold colloids. These lack the familiar metallic luster, but show bright colors which range from red, violet or blue, depending upon the size of the nanoparticles (2,3). Colloidal gold has traditionally been used to color materials such as glass (e.g. gold ruby glass and cranberry glass) and enamels (e.g. famille rose enamels) since the 16th century. The earliest report of a colloidal gold preparation may be in the Bible. The book of Exodus reports that Moses destroyed the golden calf in a manner that may have resulted in an aqueous (water-based) gold colloid, which he then forced the Israelites to drink. In modern times gold colloids are imbibed for a variety of ailments (despite little or no evidence of any health-related benefits), but the most important applications may be in the field of nano-technology (see 1, and refs therein). Gold salts are readily reduced to elemental gold, and chemical methods for making gold nanoparticles have been known for a long time, but only in recent years has accurate control of nanoparticle size been achieved (4, see also discussion in 1). Unfortunately, many of the new methods have proven to be cumbersome or give rise to toxic side-products, and a need to develop better methods is indicated. The use of biomaterials provides a possible alternative, and Professor Gardea-Torresdey and co-workers chose alfalfa as a model system. Plants were germinated and grown on gold-rich agar media and plant tissue samples were analyzed using Au L-edge X-ray absorption spectroscopy (XAS) on SSRL’s Beam Line 7-3, and at UT-Austin using high resolution transmission electron microscopy lead b