Environmental Engineering Reference
In-Depth Information
the morphology of these nanoparticles were unique in each case that span from spherical 300 nm particles to nanorods of
approximately 10 nm in diameter by 200 nm length, which clustered together in time to form large rosettes (~1000 nm) con-
taining individual shards of 100 nm width by 1000 nm length. Persistent research by these research groups have unraveled
some of the mysteries (metal reductases) and elucidated the biochemical pathways involved in the reduction, but a large void
in the available literature continues to persist, particularly in the context of exact biochemical mechanism involved in metal
nanoparticle biosynthesis.
In addition to the synthesis of metal nanoparticles by bacteria and fungi, plants and plant extracts have also shown the
ability to reduce and synthesize metal nanoparticles. A few examples of plant extract-based metal nanoparticle synthesis like
gold and copper have been discussed earlier. Additionally, plant-based routes have led to the formation of silver and gold-
silver alloy nanoparticles. For example,
Azadirachta indica
plant extract was earlier reported for its ability to reduce gold ions
[66], and it also showed the ability to reduce silver ions [66]. This plant species was further employed to reduce gold and silver
together to form a bi-metallic nanoparticulate system thus enabling this route to compete with chemical synthesis fabrication
protocols.
20.4
metal oxide nanoparticleS
Similar to metal nanoparticles, metal oxides are an important class of inorganic materials that are extensively used in a wide
range of applications including catalyst support, semiconductors, separation media, and biology [4, 95-97]. In biological sys-
tems, several organisms have already been reported for their ability to process metal ions to their oxide form (Table 20.2). One
such important process that has been widely studied is “biosilicification” wherein marine organisms like diatoms, sponges, and
radiolarians take up and process soluble silicon to form ornate hierarchical patterns of biosilica [18, 98-101]. Although several
biological and bio-inspired methods for the synthesis of silica provide an environmentally benign and energy-conserving
process [18, 98-106], there have hitherto only been limited efforts toward the exploration of microorganisms like bacteria and
fungi for the biosynthesis of oxide nanoparticles.
Bansal and co-workers explored the possibility of employing eukaryotic organisms such as fungi for the synthesis of
metal oxide nanoparticles. To achieve metal oxide nanoparticle biosynthesis, these researchers chose plant pathogenic
fungi such as
Fusarium oxysporum
and
Verticillium
sp. as potential candidates based on the rationale that plant pathogenic
fungi produce a large amount of extracellular hydrolases. These hydrolytic enzymes might also be able to hydrolyze metal
oxide precursors into their respective metal oxide nanoparticle form, when these precursors are exposed to plant patho-
genic fungi. Based on this rationale, the biological synthesis of a range of simple binary and complex ternary oxide
nanoparticles including silica (Fig. 20.5a) [107], titania (Fig. 20.5b) [107], zirconia (Fig. 20.5c) [108], magnetite [109],
and barium titanate [110] could be achieved. Detailed investigations revealed that the fungus
F. oxysporum
releases at least
two low molecular weight cationic proteins that were capable of hydrolyzing the aqueous anionic complex precursors
taBle 20.2
list of biological entities employed for metal oxide nanoparticle synthesis
Microorganism
Oxides
References
Fungi
Fusarium oxysporum
SiO
2
, TiO
2
, ZrO
2
, BaTiO
3
, Bi
2
O
3
, Fe
3
O
4
[107-110, 115]
Verticillium
sp.
Fe
3
O
4
[109]
Humicola
sp.
cuAlO
2
[119]
Bacteria
Geobacter metallireducens
Fe
3
O
4
[114]
Magnetotactic bacteria
Fe
3
O
4
[111]
Actinomycete
Actinobacter
sp.
Fe
3
O
4
, γ-Fe
2
O
3
[112, 113]
Plant and plant extract
Glycine Max (Soy bean)
Fe
3
O
4
[116, 117]
Salix viminalis
ZnO and laMnO
3
[118]
