Spectroscopic and microscopic characterization of gold nanoparticles formed by cells and S-layer sheets of Bacillus sphaericus JG-A12

The strain Bacillus sphaericus JG-A12 was isolated from a uranium mining waste pile near the town of Johanngeorgenstadt, Germany. The cells of this strain are capable of selective and reversible accumulation of U, Cu, Pb, Al, and Cd from the U waste waters. It was demonstrated that this strain is enveloped by a surface layer protein (S-layer) which differs significantly in its primary structure from the other B. sphaericus S-layers studied up to date. The highly regular structure of the S-layers with many pores of identical size offers good binding sites for different kind of molecules and also nucleation sites for formation of metal nanoclusters like Pd, for example. In this study we demonstrate that the cells of B. sphaericus JG-A12 and their purified and recrystallized S-layer are capable to bind effectively Au(III) from salt solutions. Moreover, in the presence of reducing agents such as molecular H2, Au(III) is reduced to metallic nanoclusters. Reduced gold nanoclusters were regularly distributed and sized according to the pores of the protein lattice. The metallic nature of the clusters was confirmed by different techniques such as EXAFS, XANES, UV-Vis spectroscopy and by X-ray powder diffraction. The size of the gold nanoparticles was estimated to be about 10 Å. Changes in functional groups in the cells and S-layer due to gold binding were observed by FT-IR microscopy. The cellular localization of the deposited Au(III) and of the gold nanoclusters was defined using transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM). The results demonstrated that B. sphaericus JG-A12 might be used to prepare gold nanoparticles that can be tailor-made for particular applications.