Biosynthesis of Metallic Nanoparticles and Their Applications - Intracellular Delivery: Fundamentals and Applications - page 376

(Fukuoka et al. 2002 ), and the reverse micelles method (Yadav et al. 2003 ) have
been developed for the preparation of NPs.
There are also several modified chemical methods including seed-mediated
growth where small particles (produced by other techniques like irradiation) are
utilized as seeds and fresh metallic ions are reduced by reducing agent and grow
along the surface of the seed particle (Samanta et al. 2010 ). Reduction agent usage
differs depending on the purpose, required properties, types, sizes. For instance, the
first reduction of gold salt by the so-called Turkevich method was introduced in
1951, when a sodium citrate reduction of HAuCl 4 was used for synthesis of stable
gold nanoparticles (AuNPs) (Nguyen et al. 2010 ). There are also other reducing
agents such as NaBH 4 (Wagner et al. 2008 ), methoxypolyethylene glycol (Mallick
et al. 2004 ), stannous chloride (Vaskelis et al. 2007 ) and ascorbic acid (Wagner and
Köhler 2005 ). Amine or hydroxyl-containing molecules such as branched
poly(ethyleneimine) (Note et al. 2006 ), azacryptand, amino acid (Selvakannan
et al. 2004 ) or chitosan (Shih et al. 2009 ) were also reported as a suitable reducing
agents for metallic NP preparation.
The chemical methods are relatively inexpensive for high volume, usually easy
to perform, and very variable. However, their disadvantages include toxic chemi-
cals usage, likely contamination from precursor chemicals or reducing agents, and
also formation of dangerous and hazardous reaction byproducts.

2.3 Biological Methods

Because of certain aforementioned limitations of physical and chemical method,
there is an increasing need to develop methods, which will be nontoxic, fast, high-
yield, energy saving (occur under normal conditions - normal air pressure and
temperature), and environmentally benign. Consequently, researchers have turned
to biological systems for inspiration. Bioprocesses mediated by living organisms
(employing their cells, enzymes, transport chains etc.) therefore became important
for metallic NP synthesis. For this purpose, we have a vast variety of organisms in
nature such as viruses, bacteria, yeast, fungi, algae, plants and plant products at our
disposal.
The ability to form inorganic materials by many organisms either intra- or extra-
cellularly has been well known for almost 30 years (Wilbur and Simkiss 1979 ; De
Stefano et al. 2008 ). Many biotechnological applications, such as the remediation
of toxic metals, employ microorganisms such as bacteria (Pérez-de-Mora et al.
2006 ). Therefore, many microorganisms (e.g. fungi, bacteria) were found as pos-
sible nanofacilities for NP fabrication. These nature-derived processes contributed
and led to development of relatively new biosynthesis methods for fabrication of
nano- and microscale inorganic materials by microbes and other living organisms
(Ahmad et al. 2003 ). Until now, a large number of both unicellular and multicel-
lular organisms have been known to produce intracellular or extracellular metallic
NPs (Thakkar et al. 2010 ).