Biological synthesis of gold nanoparticles by yeasts

English: Biological systems are used to build nanoparticles of specific morphology and function (Eustis and EI-Sayed, 2005). The difficulty in synthesizing nanoparticles of the desired size, shape and monodispersity directs research into new and redefined synthetic methods. The microbial interaction with metals might also supply eco-friendly methods for metal nanoparticle production. Various yeast species obtained from UNESCO MIRCEN Yeast culture collection at UFS were evaluated for their ability to reduce Au3 + ions into elemental gold nanoparticles by visual analysis. Colour intensity and time taken for the biomass to turn purple were used to select for those organisms which are able to reduce gold. The ability of whole cells to reduce Au3+ ions was dependent on concentration of gold(III) ions and (contact) time. Transmission electron microscopy (TEM) micrographs of Candida viswanathii, Geotrichum fermentans and Rodotorula graminis showed cell bound nanoparticles. Cell free extracts of the above mentioned three species showed enhanced gold reduction over a shorter period. Physicochemical parameters such as pH, gold concentration and temperature influenced the reduction and particle formation. From sub-cellular fractionation, the cytoplasm showed higher gold reduction activity. Partial purification was achieved in two anion exchange chromatography steps and 15 fold nanoparticle gold reduction activity as well as 20 fold nitrate reductase activity. The partially purified fraction exhibited both nanoparticle formation and nitrate reductase activity. The nanoparticles formed were confirmed to be gold by EDS analysis and electron diffraction patterns were characteristic of polycrystalline material. The average size of the formed gold nanoparticles was 15 nm.