Biosynthesis of Monodisperse Gold Nanoparticles by Rhodobacter sphaeroides

Gold nanoparticles exhibit unique electronic, optical, and catalytic properties that are different from those of bulk metal and have several applications in optoelectronics, imaging technology, catalysis, and drug delivery. Currently, there is a growing need to develop eco-friendly nanoparticle synthesis processes using living organisms, such as bacteria, fungi and algae. In particular, microorganisms are well known to protect themselves from metal ion stress either by intracellular-segregation mechanism or by secreting them into the external medium. This defensive behaviour can be exploited to obtain a more efficient fabrication of advanced functional nanomaterials than chemical synthesis routes: biological syntheses do not require hazardous organic solvents and surfactants , and can work at environmental temperature and pressure, preserving high selectivity and reproducibility. Rhodobacter sphaeroides is a facultative phototrophic anoxygenic proteobacterium known for its capacity to grow under a wide range of environmental conditions, with promising applications in bioremediation [1, 2]. The response of the photosynthetic bacterium Rhodobacter sphaeroides to gold exposure and its reducing capability of Au(III) to produce stable Au(0) nanoparticles is reported in this study. The properties of prepared nanoparticles were characterized by UV-Visible (UV-Vis) spectroscopy, Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy, Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Fluorescence Spectrometry (XRF) and X-ray Absorption Spectroscopy (XAS) measurements. Gold nanoparticles (AuNPs) were spherical in shape with an average size of 10±3 nm. Based on our experiments, the particles were likely fabricated by the aid of reducing sugars present in the bacterial cell membrane and were capped by a protein/peptide coat. The nanoparticles were hydrophilic and resisted to aggregation for several months. Gold nanoparticles were also positively tested for their catalytic activity in nitroaromatic compounds degradation.