New insights into microbial-mediated synthesis of Au@biolayer nanoparticles

Abstract

Gold nanoparticles (Au NPs) synthesized by bacteria have received increasing attention due to their unique properties and great applications. Bacteria have developed extra- and intra-cellular mechanisms to reduce Au(III) to Au NPs as a resistant strategy. However, the whole dynamic process of Au NP biotransformation and the fine structure and composition of the biosynthesized product remain unknown. Here we describe the process and mechanism of Au@biolayer core–shell NPs biosynthesized by Pantoea sp. IMH. Our multiple complementary characterization results provided the first direct evidence of a biolayer on biogenic Au NPs. This 3 nm thick biolayer was composed of membrane proteins, lipoproteins, and phospholipids, as determined by liquid chromatography-mass spectrometry/mass spectrometry. The Au NPs occurred first in the culture medium, then on the cell wall, and finally in the cytoplasmic space. Au(I) was detected as an intermediate in the bioreduction process by X-ray absorption near-edge structure spectroscopy. Pantoea sp. IMH utilizes several Au(III) reducing mechanisms, including reduction of Au(III) by acetal groups of exopolysaccharides in aqueous extracellular polymeric substances (EPS) outside the cell, reduction of Au(III) on cell walls by EPS adhering to cell surfaces, and protein/enzymatic reduction in the cytoplasm involving fucO, glutathione, and metal resistance proteins. Our results shed new light on the structure and dynamic process of biosynthesizing Au NPs.