Mechanistic aspects of biosynthesis of silver nanoparticles by several Fusarium oxysporum strains

The dissimilatory ferric reductase, which are found in bacteria are an essential part of the iron cycles [1] and are essentially intracellular, but one extracellular one was isolated from Mycobacterium paratuberculosis [2]. Another possible mechanism could be active in this process since it was discovered that some bacteria reduce Fe3+ oxides by producing and secreting small, diffusible redox compounds that can serve as electron shuttle between the microbe and the insoluble iron substrate [3]. The role of excreted compounds in extracellular electron transfer was recently reviewed [4].The presence of hydrogenase in fungus as Fusarium oxysporum was demonstrated with washed cell suspensions that had been grown aerobically and anaerobically in a medium with glucose and salts amended with nitrate [5]. The nitrate reductase was apparently essential for ferric iron reduction [6]. Many fungi that exhibit these characteristic properties, in general, are capable of reducing Au (III) or Ag (I) [7]. Besides these extracellular enzymes, several naphthoquinones [8-10] and anthraquinones [11] with excellent redox properties, were reported in F. oxysporum that could be act as electron shuttle in metal reductions [3].Although it is known that microorganisms such as bacteria, yeast and now fungi play an important role in remediation of toxic metals through reduction of the metal ions, this was considered interesting as nanofactories very recently [12]. Using these dissimilatory properties of fungi, the biosynthesis of inorganic nanomaterials using eukaryotic organisms such as fungi may be used to grow nanoparticles of gold [13] and silver [14] intracellularly in Verticillium fungal cells [15]. Recently, it was found that aqueous chloroaurate ions may be reduced extracellularly using the fungus F. oxysporum, to generate extremely stable gold [16] or silver nanoparticles in water [17]. Other process, which was described in the literature, was related to produce silver nanoparticles thr