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3.Synthesis of silver nanoparticles by bacteria

It was reported that highly stable silver nanoparticles (40 nm) could be synthesized by bioreduction of aqueous silver ions with a culture supernatant of nonpathogenic bacterium, Bacillus licheniformis (Kalishwaralal et al. 2008b). Moreover, well-dispersed silver nanocrystals (50 nm) were synthesized using the bacterium Bacillus licheniformis (Kalishwaralal et al. 2008a). Saifuddin et al. (Saifuddin et al. 2009) have described a novel combinational synthesis approach for the formation of silver nanoparticles by using a combination of culture supernatant of B. subtilis and microwave irradiation in water. They reported the extracellular biosynthesis of monodispersed Ag nanoparticles (5-50 nm) using supernatants of B. subtilis, but in order to increase the rate of reaction and reduce the aggregation of the produced nanoparticles, they used microwave radiation which might provide uniform heating around the nanoparticles and could assist the digestive ripening of particles with no aggregation. Silver nanocrystals of different compositions were successfully synthesized by Pseudomonas stutzeri AG259 (Klaus et al. 1999). The silver-resistant bacterial strain, Pseudomonas stutzeri AG259, isolated from a silver mine, accumulated silver nanoparticles intracellularly, along with some silver sulfide, ranging in size from 35 to 46 nm (Slawson et al. 1992). Larger particles were formed when P. stutzeri AG259 challenged with high concentrations of silver ions during culturing, resulted intracellular formation of silver nanoparticles, ranging in size www.intechopen.com Silver Nanoparticles 11 from a few nm to 200 nm (Klaus-Joerger et al. 2001; Klaus et al. 1999). P. stutzeri AG259 detoxificated silver through its precipitation in the periplasmic space and its reduction to elemental silver with a variety of crystal typologies, such as hexagons and equilateral triangles, as well as three different types of particles: elemental crystalline silver, monoclinic silver sulfide acanthite (Ag2S), and a further undetermined structure (Klaus et al. 1999). The periplasmic space limited the thickness of the crystals, but not their width, which could be rather large (100-200 nm) (Klaus-Joerger et al. 2001). In another study, rapid biosynthesis of metallic nanoparticles of silver using the reduction of aqueous Ag+ ions by culture supernatants of Klebsiella pneumonia, E. coli, and Enterobacter cloacae (Enterobacteriacae) was reported (Shahverdi et al. 2007). The synthetic process was quite fast and silver nanoparticles were formed within 5 min of silver ions coming in contact with the cell filtrate. It seems that nitroreductase enzymes might be responsible for bioreduction of silver ions. It was also reported that visible-light emission could significantly increase synthesis of silver nanoparticles (1-6 nm) by culture supernatants of K. pneumoniae (Mokhtari et al. 2009). Monodispersed and stable silver nanoparticles were also successfully synthesized with bioreduction of [Ag (NH3)2] + using Aeromonas sp. SH10 and Corynebacterium sp. SH09 (Mouxing et al. 2006). It was speculated that [Ag (NH3)2] + first reacted with OH− to form Ag2O, which was then metabolized independently and reduced to silver nanoparticles by the biomass. Lactobacillus strains, when exposed to silver ions, resulted in biosynthesis of nanoparticles within the bacterial cells (Nair and Pradeep 2002). It has been reported that exposure of lactic acid bacteria present in the whey of buttermilk to mixtures of silver ions could be used to grow nanoparticles of silver. The nucleation of silver nanoparticles occurred on the cell surface through sugars and enzymes in the cell wall, and then the metal nuclei were transported into the cell where they aggregated and grew to larger-sized particles.

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