Department of Oriental Medicine Biotechnology, ..
Silver NPs (5-50 nm) could be synthesized extracellularly using Fusarium oxysporum, with no evidence of flocculation of the particles even a month after the reaction (). The long-term stability of the nanoparticle solution might be due to the stabilization of the silver particles by proteins. The morphology of NPs was highly variable, with generally spherical and occasionally triangular shapes observed in the micrographs. Silver NPs have been reported to interact strongly with proteins including cytochrome c (Cc). This protein could be self-assembled on citrate-reduced silver colloid surface (). Interestingly, adsorption of (Cc)-coated colloidal gold NPs onto aggregated colloidal Ag resulted Ag: Cc: Au nanoparticle conjugate (). In UV-vis spectra from the reaction mixture after 72 h, the presence of an absorption band at ca. 270 nm might be due to electronic excitations in tryptophan and tyrosine residues in the proteins. In F. oxysporum, the bioreduction of silver ions was attributed to an enzymatic process involving NADH-dependent reductase (). The exposure of silver ions to F. oxysporum, resulted in the release of nitrate reductase and subsequent formation of highly stable silver NPs in solution (). The secreted enzyme was found to be dependent on NADH cofactor. They mentioned high stability of NPs in solution was due to capping of particles by release of capping proteins by F. oxysporum. Stability of the capping protein was found to be pH dependent. At higher pH values (>12), the NPs in solution remained stable, while they aggregated at lower pH values () have demonstrated enzymatic synthesis of silver NPs with different chemical compositions, sizes and morphologies, using α-NADPH-dependent nitrate reductase purified from F. oxysporum and phytochelatin, in vitro. Silver ions were reduced in the presence of nitrate reductase, leading to formation of a stable silver hydrosol 10-25 nm in diameter and stabilized by the capping peptide. Use of a specific enzyme in vitro synthesis of NPs showed interesting advantages. This would eliminate the downstream processing required for the use of these NPs in homogeneous catalysis and other applications such as non-linear optics. The biggest advantage of this protocol based on purified enzyme was the development of a new approach for green synthesis of nanomaterials over a range of chemical compositions and shapes without possible aggregation. Korbekandi and colleagues () demonstrated the bioreductive synthesis of silver NPs using F. oxysporum. Previous researchers reported qualitative production of silver NPs by F. oxysporum, but they did not optimize the reaction mixture. In SEM micrographs, silver NPs were almost spherical, single (25-50 nm) or in aggregates (100 nm), attached to the surface of biomass. The reduction of metal ions and stabilization of the silver NPs was confirmed to occur by an enzymatic process. It seems that the first step involves trapping of the Ag+ ions by F. oxysporum cells. More details of the location of NPs production by this fungus were revealed, and the previous theories were corrected. In contrast with the previous studies, it is claimed that the nanoparticle production in F. oxysporum is intracellular by engulfing the NPs in vesicles, transporting, and excreting of them through exocytosis outside of the cells (). Ingle and coworkers () demonstrated the potential ability of Fusarium acuminatum Ell. and Ev. (USM-3793) cell extracts in biosynthesis of silver NPs. The NPs produced within 15-20 min and were spherical with a broad size distribution in the range of 5-40 nm with the average diameter of 13 nm. A nitrate-dependent reductase enzyme might act as the reducing agent. The white rot fungus, Phanerochaete chrysosporium, also reduced silver ions to form nano-silver particles (). The most dominant morphology was pyramidal shape, in different sizes, but hexagonal structures were also observed.