Microbial synthesis of noble metal nanoparticles using the Fe(III)-reducing bacterium shewanella algae

Summary form only given. Noble metals nanoparticles can be applied to a wide range of functions such as catalysis, optics and biosensing. Although chemical and physical synthetic routes to noble metal nanoparticles have been extensively developed, another possibility is synthesis by bioreduction of noble metal ions using microorganism. For microbial synthesis of noble metal nanoparticles, we focused on the Fe(III)-reducing bacterium Shewanella algae, which is able to transfer electrons to Fe³⁺ ions. Intracellular synthesis of gold nanoparticles was achieved at 25degC and pH 7 using S. algae (ATCC 51181) with H₂ as the electron donor. The reductive deposition of gold by S. algae was a fast process: 1 mol/m³ AuCl₄ ions were completely reduced to insoluble gold within 30 min. Speciation of gold in bacterial cultures of S. algae by X-ray Absorption Near-Edge Spectroscopy (XANES) showed that the trivalent gold ions were rapidly reduced to gold metal in the bacterial cells. Transmission electron microscopy (TEM) of thin sections of S. algae cells revealed that the biogenic gold nanoparticles of 10-20 nm were located in the periplasm. When the pH of the HAuCl₄ solution was decreased from 7 to 1, many of the biogenic gold particles were synthesized extracellularly. The gold particles synthesized at pH 1 were typically 50 nm and 500 nm, and some were angular in shape and single crystal. This suggests that at pH 1, a gold-reducing enzyme is released from the periplasmic space into the aqueous solution, and the enzyme catalyzes the reduction of gold ions in the aqueous solution. We can therefore conclude that the solution pH is an important factor in controlling the morphology of biogenic gold particles and location of particle formation