Gold nanoparticles synthesis mediated by fungus isolated from aerobic granular sludge: Process and mechanisms

Due to the low toxicity, biocompatibility and eco-friendliness, microorganisms have received a lot of attention for gold nanoparticles (AuNPs) synthesis. This work isolated a fungal strain capable of efficiently generating AuNPs from aerobic granular sludge, named XY3. Comparison of 18S rDNA sequence results showed that fungus XY3 belongs to Candida rugopelliculosa. AuNPs were synthesized by initiating an Au³⁺-induced stress response that prompted the reduction of Au³⁺ to Au⁰ by the fungus XY3. It is worth noting that the addition of nutritional substrates weakens the stress response induced by Au³⁺, resulting in a decrease in the yield of AuNPs. As evidenced by nystatin inhibition studies, the synthesis of AuNPs is based on biochemical reactions rather than purely physical changes. The XRD results suggested that XY3-secreted biomolecules were involved in the reduction of Au³⁺ and AuNPs synthesis. The results of the three variation patterns of reducing power, biomolecules, and AuNPs absorbance revealed that Au³⁺ reduction was mostly dependent on the reducing polysaccharides. In addition, extracellular proteins were shown to be involved in the synthesis of AuNPs, which is responsible for the uniform distribution of AuNPs. This work provided a wide and cost-effective seed source for AuNPs synthesis, and also offered a resourceful solution for residual sludge treatment of fungal type aerobic granular sludge.