"These gold nanoparticles are mainly between about 9 and 12 nanometres in diameter, with a few larger particles," said Murali Sastry of the National Chemical Laboratory. "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."

To date, researchers have used both bacteria and fungi, as well as alfalfa plants, to synthesize nanoparticles. These techniques have the advantage that they do not use toxic chemicals. This study is the first use of the alkalotolerant actinomycete Rhodococcus sp. to grow nanoparticles intracellularly. Actinomycetes share characteristics of both fungi and bacteria, and can also produce secondary metabolites such as antibiotics.

Exposing the Rhodococcus sp. to a solution of aqueous AuCl4- ions resulted in gold nanoparticles forming on the cell wall and cytoplasmic membrane. The cells continued to multiply normally after nanoparticle synthesis, and what's more, the Rhodococcus sp. produced nanoparticles that were more uniform in size than those synthesized using the fungus Verticillium.

Now, the group plans to investigate growing the nanoparticles on a large scale, perhaps by genetically modifying actinomycetes to produce more of the enzymes that cause the gold to form. The researchers reported their work in Nanotechnology.