Growing green gold

A new way to make gold form inside the cells of a micro-organism is published today in the Institute of Physics journal Nanotechnology. Researchers from the National Chemical Laboratory and the Armed Forces Medical College, both in Pune, India, have been using “green chemistry” to develop an eco-friendly way to make tiny gold particles without using toxic chemicals.

Such gold nanoparticles of uniform size can be used in labelling proteins, nucleic acids and other biomolecules, which could lead to new ways of detecting disease, controlling genes and enzymes, and delivering therapeutic drugs directly to the nucleus of the cell. The technology can also be used in developing nanomaterials and nanoelectronics.

The research group took a micro-organism called Rhodococcus from a fig tree, and exposed it to a liquid containing gold ions (which are electrically charged gold particles, rather than neutral ones). They found that the micro-organism caused the gold ions to gain electrons, thereby forming gold nanoparticles within the micro-organism’s cells. These nanoparticles are more concentrated and more uniform in size than particles biosynthesised by previous methods that used a fungus. Although the exact reaction that causes the gold to form is not yet fully known, the group believe that the Rhodococcus species gives better results because it is a certain type of micro-organism (an actinomycete) that shows characteristics of both bacteria and fungi, rather than just being a fungus.

“I am extremely pleased with the formation of these gold nanoparticles. They are mainly between about nine and twelve nanometres in diameter, with a few larger particles. That’s about eight thousand times smaller than a human hair,” said Dr. Murali Sastry from the National Chemical Laboratory, India. “This is much more uniform than the particles formed using other biological methods. Having uniformly sized particles will be needed if we are to use this method in biodiagnosis using gold nanoparticles or to deliver therapeutic drugs.”

Following the biosynthesis of gold nanoparticles in Rhodococcus species, its cells continued to multiply normally, as the ions used were not toxic to the cells – which is important as more gold would be formed as the cells multiplied.

The group will soon be looking into making the nanoparticles on a large scale, which could be attained by genetically modifying actinomycetes to produce more of the enzymes which cause the gold to form.