Nanoparticles are viewed as the fundamental building blocks of nanotechnology [G. A. Mansoori, Principles of Nanotechnology—Molecular-Based Study of Condensed Matter in Small Systems, World Scientific Pub. Co., Hackensack, N.J., (2005); Mansoori et al., Molecular Building Blocks for Nanotechnology, Springer, New York, (2007)]. They are the starting points for preparing many nanostructured materials and devices. Their synthesis is an important component of the rapidly growing research efforts in nanoscience and nanoengineering. Nanoparticles from a wide range of materials can be prepared by a number of methods. Precursors from liquids, solid or gas phase are used for synthesis and assembly of nanoparticles or nanomaterials.
Moreover, nanoparticles themselves have useful applications in areas such as medicine and molecular biology research. In particular, silver nanoparticles may be used as antimicrobial agents against bacteria, viruses, and fungi, including drug-resistant strains of these microorganisms. Typically, bacteria have diameters in the micron range, while viruses have diameters less than a micron in size. The fact that the silver nanoparticles are so small allows them to interact readily with such microorganisms. The antimicrobial action occurs because the silver nanoparticles interfere with the enzymatic metabolism of oxygen by the microbes, which effectively “suffocates” and kills the particular microorganism. The nanoscale size of silver nanoparticles means that the particles have a very large surface area, therefore only a small volume of silver nanoparticles is required to act as an effective antimicrobial agent.
Metal nanoparticles are typically produced on a small laboratory scale using methods such as chemical vapor deposition, irradiation or chemical reduction of metal salts. However, there is a growing need to prepare environmentally friendly nanoparticles that do not produce toxic wastes in their process synthesis protocol. To achieve this, scientists in the field of synthesis and assembly of nanoparticles are inclined to shift to benign synthesis processes, which happen to be mostly of a biological nature [G. A. Mansoori, Principles of Nanotechnology—Molecular-Based Study of Condensed Matter in Small Systems, World Scientific Pub. Co., Hackensack, N.J., (2005)].
Biological entities like microorganisms and living cells possess operating parts at the nanoscale level and may perform a number of jobs ranging from generation of energy to extraction of targeted materials at a very high efficiency [D. S. Goodsell et al., Bionanotechnology: Lessons from Nature, Wiley-Liss, Hoboken, N.Y., (2004)].
Recently, the utilization of biological entities has emerged as a novel method for the synthesis of nanoparticles. Biotechnology approaches toward the synthesis of nanoparticles can have many advantages, such as a greater ease with which the process can be scaled up, economic viability, possibility of readily covering large surface areas by suitable growth of the mycelia, and its green chemistry nature. Some examples of the use of microbes and other biological entities in the synthesis of nanoparticles of different chemical compositions include the following:                i. Ribosomes for biosynthesis of gold nanoparticles [I. S. Pavel, “Assembly of Gold Nanoparticles by Ribosomal Molecular Machines” Ph.D. Dissertation, The University of Texas at Austin, (May 2005)];        ii. Bacteria for production of cadmium sulfide, zinc sulfide, magnetite, iron sulfide and silver nanoparticles [D. Mandal, et al. “The use of microorganisms for the formation of metal nanoparticles and their application”—Applied Microbiology and Biotechnology 69(5) 485-492 (2006)];        iii. Yeast for production of lead sulfide and cadmium sulfide nanoparticles [A. Ahmad, et al. —“Enzyme mediated extracellular synthesis of CdS nanoparticles by the fungus, Fusarium” J. Am. Chem. Soc. 124 (41), pp 12108-12109 (2002)];        iv. Production of silver nanoparticles using Emblica Officinalis herbal fruit extract [B. Ankamwar, C. Damle, A. Ahmad, M. Sastry “Biosynthesis of gold and silver nanoparticles using Emblica officinalis fruit extract” J, Nanoscience and Nanotechnology 5(10):1665-1671 (October 2005)]; and        v. Production of gold nanoparticles using lemongrass extract and synthesis of nanoparticles of variable morphology using leaves of different plants, sprouts, roots and stems of live alfalfa plant [S. S. Shankar, et al “Biological synthesis of triangular gold nanoprisms”—Nature Materials 3, 482-488 (2004)].        
There are about 80,000 known species that belong to the kingdom “Fungi” (Encyclopedia Britannica, 2008. Encyclopedia Britannica Online). Some fungi exhibit a characteristic property of generating reducing enzymes, and are capable of reducing one or more transition metals, such as Vanadium (V) [J. R. Lloyd, FEMS Microbioal. Rev, 27:411-425 (2003); E. M. Bautista and M. Alexander, Soil Sci. Soc. Am. Proc. 36, 918-920 (1972)]. Other fungi, for instance, Fusarium moniliforme, will reduce Fe (III) to Fe (II), but will not reduce Ag (I) [C. J. R. Klittich et al., Genetics 118:417-423 (1998)].
Specifically, the following results towards production of nanoparticles have been achieved using certain fungi or bacteria resembling fungi:
i. Biosynthesis of magnetite using the fungus Fusarium oxysporum and Verticillium species [A. Ahmad, et al—“Enzyme mediated extracellular synthesis of CdS nanoparticles by the fungus, Fusarium” J. Am. Chem. Soc. 124 (41), pp 12108-12109 (2002)];
ii. Production of gold nanotriangles by actinomycete, which is a bacteria resembling fungi [S. S. Shankar, et al “Biological synthesis of triangular gold nanoprisms”—Nature Materials 3, 482-488 (2004)];
iii. Intracellular synthesis of gold and silver nanoparticles in Verticillium fungal cells [A. Ahmad, et al—“Intracellular synthesis of gold nanoparticles by a novel alkalotolerant actinomycete, Rhodococcus species” Nanotechnology 14: 824-828 (2003)];
iv. Extracellular production of gold, silver and bimetallic Au—Ag alloy nanoparticles by the fungus Fusarium oxysporum. It has been observed that the exposure of aqueous solutions of metal salts or a mixture of metal salts to Fusarium oxysporum resulted in extracellular formation of nanoparticles of dimensions 5-50 nm and alloy nanoparticles of dimensions 8-14 nm [D. Mandal, et al. “The use of microorganisms for the formation of metal nanoparticles and their application”—Applied Microbiology and Biotechnology 69(5) 485-492 (2006)]; and
v. Production of silver nanoparticles as a result of the reduced state of pretreated fungus Phoma Species [J. C. Chen, Z. H. Lin, X. X. Ma, “Evidence of the production of silver nanoparticles via pretreatment of Phoma sp. 3.2883 with silver nitrate”—Letters in applied microbiology, 37(2), 105-108 (2003)].
Not all biotechnological processes, however, are environmentally safe. For example, the fungus Fusarium oxysporum produces environmentally harmful toxins such as fumonisins and trichothecenes, as well as mycotoxins, which can negatively affect human or animal health if they enter the food chain. Certain types of Verticillium fungal mycelia, such as Verticillium albo-atrum and Verticillium dahliae, can cause Verticillium wilt, plus Verticillium dahliae is the species that most commonly attacks woody ornamental plants in the United States. The fungus Aspergillus fumigatus is extremely toxic, causing the disease Aspergillus when its spores are inhaled. In addition, Streptomyces sp. is known to be a productive source of secondary cytotoxic and mitochondriotoxic metabolites. The fungus Phoma lingam can cause the blackleg disease on cabbage, as well as on many other vegetables. Consequently, the use of such fungi is potentially dangerous to the environment and human or animal health. Moreover, not all biotechnological processes lend themselves to effective large scale processing. For instance, Pisolithus sp. is a rare, sensitive, and difficult to mass-produce fungus, and Neurospora sp. is a very delicate fungus used for food aroma production in small quantities. Accordingly, their use for large scale production of nanoparticles would not be economical. There remains a need for a biotechnological method for efficiently and safely producing silver nanoparticles on a large scale.
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