Extracellular Saccharide-Mediated Reduction of Au³⁺ to Gold Nanoparticles: New Insights for Heavy Metals Biomineralization on Microbial Surfaces

Abstract

Biomineralization is a critical process controlling the biogeochemical cycling, fate, and potential environmental impacts of heavy metals. Despite the indispensability of extracellular polymeric substances (EPS) to microbial life and their ubiquity in soil and aquatic environments, the role played by EPS in the transformation and biomineralization of heavy metals is not well understood. Here, we used gold ion (Au) as a model heavy metal ion to quantitatively assess the role of EPS in biomineralization and discern the responsible functional groups. Integrated spectroscopic analyses showed that Auwas readily reduced to zerovalent gold nanoparticles (AuNPs, 2-15 nm in size) in aqueous suspension of Escherichia coli or dissolved EPS extracted from microbes. The majority of AuNPs (95.2%) was formed outside Escherichia coli cells, and the removal of EPS attached to cells pronouncedly suppressed Au reduction, reflecting the predominance of the extracellular matrix in Au reduction. XPS, UV-vis, and FTIR analyses corroborated that Au reduction was mediated by the hemiacetal groups (aldehyde equivalents) of reducing saccharides of EPS. Consistently, the kinetics of AuNP formation obeyed pseudo-second-order reaction kinetics with respect to the concentrations of Au and the hemiacetal groups in EPS, with minimal dependency on the source of microbial EPS. Our findings indicate a previously overlooked, universally significant contribution of EPS to the reduction, mineralization, and potential detoxification of metal species with high oxidation state.

Spherical Gold Nanoparticles (AuNPs) Formation by Selected Intracellular Protein Fraction of Fungi

“…No spurious diffractions due to crystallographic impurities were found. In addition, the intensity of the (111) diffraction is much stronger than the other planes so that the (111) plane was the predominant orientation [26]. According to Scheller's formula (1), the average size of AuNPs under optimized conditions was 16.76 nm, which was consistent with the AuNPs size distribution above ( Figure 4B).…”

Insight into eco-friendly fabrication of silver nanoparticles by Pseudomonas aeruginosa and its potential impacts

“…A better understanding of microbial transformation pathway at genetic level will leads to develop new genetic tools for accelerating the bioremediation. 44 . Moreover, metal reducing microbes and extracellular electron transfer mechanism may have implications in electro-microbiolog ical applications for renewable energy …”

Alleviating the Cadmium Toxicity and Growth‐Promotion in Paddy Rice by Photosynthetic Bacteria

“…However, the immobilization effectiveness of Cd in the CB treatment was 3.60–15.51% at seedling stage and 3.62–10.80% at mature stage which is lower compared to the RP treatment. It might be related to the reason that under the certain soil capacity, too much mixing of bacteria can cause microbial survival competition for space and nutrition, leading to microbial death, therefore, weakening the immobilization of Cd …”

“…Among all potentially toxic metals, Cd is considered as main pollutant of paddy soils or rice plants. Cd accumulation in plants may disturb the physiological and biochemical activities and restrict the growth of plants . Moreover, Cd from rice grains will be transferred to the food chain, where it poses serious health risks associated with Cd toxicity.…”