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spherical nanoparticles observed at 60°c, while a mixture of spherical and octahedral crystal platelets of 5-200 nm dimensions
were observed when the reaction was carried out at 100°c [50].
P. boryanum
also showed the ability to reduce the platinum
group of salts when reacted with aqueous Pt (IV) and Pd (II) ions for 28 days, respectively. The precipitation of an amorphous
Pt (II)-organic spherical complex was promoted initially, which was characteristically connected into long bead-like chains by
the continuous coating of organic material. On increasing the reaction temperature and reaction time, these bead-like structures
were converted to crystalline Pt nanoparticles of 30-300 nm diameter [48]. In contrast, crystalline spherical and elongated Pd
nanoparticles of 30 nm diameter were directly precipitated at lower temperatures. Interestingly, the reactions at higher temper-
atures only yielded palladium hydride with a small amount of Pd metal nanoparticles [49]. Although this organism was found
to have the ability to reduce a range of noble metal ions, the exact biochemical mechanism involved in the formation of metal
nanoparticles is yet to be elucidated and still remains a mystery.
A recent study also highlighted the ability of bacterium
Ralstonia metallidurans
to precipitate colloidal gold nanoparticles
and the ability of this bacterium to contribute toward the mineralization of metallic gold was highlighted [51]. However, the
exact mechanism involved in the biomineralization of metallic gold by
R. metallidurans
is also not yet clear. On the other hand,
the presence of several mechanisms for heavy metal resistance in this bacterium including cation efflux, cation reduction, cyto-
plasmic accumulation, and organic compound formation leads us to believe the importance and possible involvement of these
biochemical detoxifying mechanisms toward nanoparticle synthesis [52].
In a series of reports, Sastry's group explored different biological entities with more emphasis on fungus, actinomycetes,
and plant extracts for the biosynthesis of metal nanoparticles with significant control over morphologies and size distribu-
tion. less control over the size and shape of nanoparticles and the relatively large synthesis time (from several hours to days)
were some of the well-known challenges in employing a biological approach for nanoparticle synthesis. To address some of
these challenges, an alkali thermophilic actinomycete
Thermomonospora
sp. was shown for the first time for its ability to
not only synthesize gold nanoparticles but also obtain a narrow size distribution (Fig. 20.1) [53]. The extracellular manifes-
tation of these particles further overcomes the limitations associated with harvesting of intracellular nanoparticles from
microbial cells, as observed in previous reports, thereby reducing yet another step involved in the post-synthesis processing
and harvesting of nanoparticles. Further enhancement in the monodispersity was obtained by employing
Rhodococcus
sp.,
wherein 10 nm Au nanoparticles were observed, however only intracellularly [54]. Further control over size and shape was
observed by tuning reaction parameters, wherein Sastry et al. showed the ability of an
Actinobacter
sp. to synthesize
triangular and hexagonal Au particles with an edge length of 30-50 nm when the reaction was carried out in the absence of
molecular oxygen [55]. Interestingly, at lower temperatures, the reaction was comparatively slow and resulted in smaller
triangular Au nanoparticles with an edge length of approximately 10 nm, while the reaction in the presence of atmospheric
oxygen was found slower and yielded distinct triangular and hexagonal gold particles with edge lengths of 50-500 nm. The
biochemical analysis recognized the important role of the protease enzyme secreted by
Actinobacter
sp. toward the efficient
reduction of Au (III) ions. Further research suggested that molecular oxygen slows down the reduction of gold possibly by
inhibiting the protease activity [55]. These studies point toward the important role of controlling simple reaction parameters
for controlling the size and shape of nanoparticles during biosynthesis. Additional experiments further revealed the probable
involvement of cytochrome oxidase as the primarily responsible component for the reduction of aqueous Au (III) ions.
(a)
(b)
311
220
200
111
100 nm
fiGUre 20.1
(a) TeM image and (b) electron diffraction of Au nanoparticles synthesized using
Thermomonospora
sp. Images reprinted
with permission from Ref. [53]. © 2003, American chemical Society.
