Silver nanoparticles (5-50 nm) could be synthesized extracellularly using Fusarium oxysporum, with no evidence of flocculation of the particles even a month after the reaction (Ahmad et al. 2003a). The long-term stability of the nanoparticle solution might be due to the stabilization of the silver particles by proteins. The morphology of nanoparticles was highly variable, with generally spherical and occasionally triangular shapes observed in the micrographs. Silver nanoparticles have been reported to interact strongly with proteins including cytochrome c (Cc). This protein could be self-assembled on citrate-reduced silver colloid surface (Macdonald and Smith 1996). Interestingly, adsorption of (Cc)-coated colloidal Au nanoparticles onto aggregated colloidal Ag resulted Ag: Cc: Au nanoparticle conjugate (Keating et al. 1998). 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 (Ahmad et al. 2003b). The exposure of silver ions to F. oxysporum, resulted in release of nitrate reductase and subsequent formation of highly stable silver nanoparticles in solution (Kumar et al. 2007). The secreted enzyme was found to be dependent on NADH cofactor. They mentioned high stability of nanoparticles in solution was due to capping of particles by www.intechopen.com 12 The Delivery of Nanoparticles release of capping proteins by F. oxysporum. Stability of the capping protein was found to be pH dependent. At higher pH values (>12), the nanoparticles in solution remained stable, while they aggregated at lower pH values (<2) as the protein was denatured. Kumar et al. (Kumar et al. 2007) have demonstrated enzymatic synthesis of silver nanoparticles with different chemical compositions, sizes and morphologies, using ┙-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 in vitro synthesis of nanoparticles showed interesting advantages. This would eliminate the downstream processing required for the use of these nanoparticles 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. Ingle et al. (Ingle et al. 2008) demonstrated the potential ability of Fusarium acuminatum Ell. and Ev. (USM-3793) cell extracts in biosynthesis of silver nanoparticles. The nanoparticles produced within 15-20 minutes 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 (Vigneshwaran et al. 2006a). The most dominant morphology was pyramidal shape, in different sizes, but hexagonal structures were also observed. Possible involvement of proteins in synthesizing silver nanoparticles was observed in Plectonema boryanum UTEX 485 (a filamentous cyanobacterium) (Lengke et al. 2007). Stable silver nanoparticles could be achieved by using Aspergillus flavus (Vigneshwaran et al. 2007). These nanoparticles were found to be stable in water for more than 3 months with no significant aggregation because of surface binding of stabilizing materials secreted by the fungus (Vigneshwaran et al. 2007). Extracellular biosynthesis of silver nanoparticles using Aspergillus fumigatus (a ubiquitous saprophytic mold) has also been investigated (Bhainsa and D’Souza 2006). The resulted TEM micrograph showed well-dispersed silver nanoparticles (5-25 nm) with variable shapes. Most of them were spherical in nature with some others having occasionally triangular shapes (Bhainsa and D’Souza 2006). Compared to intracellular biosynthesis of nanoparticles; extracellular synthesis could be developed as a simple and cheap method because of uncomplicated downstream processing and handling of biomasses. The extracellular filtrate of Cladosporium cladosporioides biomass was used to synthesize silver nanoparticles (Balaji et al. 2009). It was suggested that proteins, organic acids and polysaccharides released by C. cladosporioides were responsible for formation of spherical crystalline silver nanoparticles. Kathiresan et al. (Kathiresan et al. 2009) have shown that when the culture filtrate of Penicillium fellutanum was incubated with silver ions and maintained under dark conditions, spherical silver nanoparticles could be produced. They also changed crucial factors such as pH, incubation time, temperature, silver nitrate concentration and sodium chloride to achieve the maximum nanoparticle production. The highest optical density at 430 nm was found at 24 h after the start of incubation time, 1 mM concentration of silver nitrate, pH 6.0, 5˚C temperature and 0.3% sodium chloride. Fungi of Penicillium genus were used for green synthesis of silver nanoparticles (Sadowski et al. www.intechopen.com Silver Nanoparticles 13 2008). Penicillium sp. J3 isolated from soil was able to produce silver nanoparticles (Maliszewska et al. 2009). The bioreduction of silver ions occurred on the surface of the cells and proteins might have critical role in formation and stabilization of the synthesized nanoparticles. Sanghi et al. (2009) have investigated the ability of Coriolus versicolor in formation of monodisperse spherical silver nanoparticles. Under alkaline conditions (pH 10) the time taken for production of silver nanoparticles was reduced from 72 h to 1 h. It was indicated that alkaline conditions might be involved in bioreduction of silver ions, water hydrolysis and interaction with protein functionalities. Findings of this study have shown that glucose was necessary for the reduction of silver nanoparticles, and S-H of the protein played an important role in the bioreduction.
2.Synthesis of silver nanoparticles by fungi
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