Green Nanotechnology: The Solution to Sustainable Development of Environment - Environmental Sustainability: Role of Green Technologies - page 301

Nanoparticles that produced the “green”
way include gold, silver, platinum, palladium,
metal oxide, metal sulfi de, nonmetal oxide,
nanocomposites, magnetic, and alloy (Popescu
et al. 2010 ; Song et al. 2010 ; Li et al. 2011 ;
Sundrarajan and Gowri 2011 ; Hasna et al.
2012 ; Velayutham et al. 2012 ; Soundarrajan
et al. 2012 ).
Microorganisms and plants have different
mechanisms for nanobiosynthesis. Mechanism
for nanoparticle formations varies for different
microorganisms. However, they have a common
path as metal ions are fi rst trapped on the surface
or inside of the microbial cells, which are then
reduced to nanoparticles in the presence of
enzymes. Generally, microorganisms impact
mineral formation in the following two ways
(Benzerara et al. 2011 ):
1. They modify the composition of the solution
so that it becomes supersaturated or more
supersaturated than it previously was with
respect to a specifi c phase.
2. They impact mineral formation via produc-
tion of organic polymers that are capable of
having an impact on nucleation by favoring or
inhibiting the stabilization of the very fi rst
mineral seeds.
Various mechanisms exist for nanoparticle
formation by plants. Phytomining involves the
use of hyperaccumulating plants to extract a
metal from soil with recovery of the metal from
biomass to return an economic profi t (Lamb
et al. 2001 ). Hyperaccumulator species have
physiological mechanism that regulates the soil
solution concentration of metals. Exudates of
metal chelates from root system, for example,
will allow increased fl ux of soluble metal com-
plexes throughout the root membranes (Arya
2010 ). It has been observed that stress-tolerant
plants have more capacity to reduce metal ions
to the metal nanoparticles (Ankamwar et al.
2005a ). Mechanism of nanobiosynthesis in
plants may be associated with phytoremediation
concept in plants (Huang and Cunningham
1996 ; Anderson et al. 1998 ; Haverkamp et al.
2007 ). Biosilicifi cation also results in nanopar-
ticles in cases of some higher plants as shown in
Fig. 2 (Lopez et al. 2005 ).

Silicic acid taken up through plant roots

Transport through xylem as silicon complex

Complex reaches stems/leaves

Mineral deposition

Breakdown triggered by change in pH

Release of silicic acid induced

Condensation results in silica

Fig. 2 Flow chart for biosilicifi cation process

6 Role of Green Nanoparticles

for Environmental
Applications

Nanoparticles with antimicrobial potential like
gold, silver, magnesium oxide, copper oxide, alu-
minum, titanium dioxide, and zinc oxide are
widely used in water purifi cation systems, in
wastewater treatment, as self-cleaning and self-
disinfecting agents, and as antimicrobial coatings
in the wallpapers in hospitals (Ravishankar Rai
and Jamuna Bai 2011 ). The categories of nanopar-
ticles studied for environmental applications also
include iron, bimetallics, catalytic particles, clays,
carbon nanotubes, fullerenes, dendrimers, and
magnetic nanoparticles (Mansoori et al. 2008 ).

6.1 Gold Nanoparticles

They have been precipitated within bacterial
cells by incubation of cells with Au 3+ ions
(Beveridge and Murray 1980 ). Extracellular syn-
thesis was reported in Fusarium oxysporum and
Thermomonospora sp. and intracellular in