Synthesis, characterization and kinetic study of silver and gold nanoparticles produced by the archaeon Haloferax volcanii.

AIMS: To evaluate the ability of the haloarchaeon Haloferax volcanii to produce Ag and Au nanoparticles (NPs) and to characterize the obtained material in order to find relevant properties for future potential applications. METHODS AND RESULTS: Nanoparticles were produced by incubating H. volcanii cells with the corresponding metal salt. In the presence of precursor salts, cultures evidenced a colour change associated to the formation of metallic nanostructures with plasmonic bands located in the visible range of the spectrum. X-ray fluorescence analysis confirmed the presence of Ag and Au in the NPs which were spherical, with average sizes of 25 nmol l-1 (Ag) and 10 nmol l-1 (Au), as determined by electronic microscopy. Fourier transformed infrared spectroscopy indicated that both types of NPs showed a stable protein capping. Ag NPs evidenced antibacterial activity and Au NPs improved the specificity of polymerase chain reaction reactions. Au and Ag NPs were able to reduce 4-nitrophenol when incubated with NaBH4 . CONCLUSIONS: Haloferax volcanii is able to synthesize metallic NPs with interesting properties for technological applications. SIGNIFICANCE AND IMPACT OF THE STUDY: Our data demonstrate the ability of H. volcanii to synthesize metal NPs and constitutes a solid starting point to deepen the study and explore novel applications.

Magnetic relaxation of gamma-Fe2O3 nanoparticles arrangements and electronic phase-segregated systems.

Gamma-Fe2O3 nanoparticles have been synthesized and dispersed in a polymeric matrix, forming a series of composites with different concentrations of magnetic particles. The effect of volume polydispersity and dipolar interactions on the relaxation behavior is discussed. We have paid special attention to the dynamic approach to discuss a possible true superspin-glass transition in highly concentrated composites. To avoid the practical limitations that appear in highly concentrated systems of particles, like the formation of aggregates, etc., we have studied the glassy phase that appears spontaneously in certain strongly electronic correlated materials close to a metal-insulator transition. It must be emphasized that from a theoretical point of view these inhomogenous magnetic states could present important advantages over classical dispersions of particles, like field-control of the effective particle size. The results are compared with other recently obtained for classical systems of particles.