Environmental Engineering Reference
In-Depth Information
morphologies such as triangles, hexagons, and spheres [79]. Furthermore, as an extension of this work, Joerger and co-work-
ers showed the ability to directly synthesize cermet materials by heat treating the Ag nanoparticles containing bacterial bio-
mass [81]. The hard coatings of cermet materials were found to be resistant to mechanical scratching and their optical
properties could be tailored by varying the silver loading factor. This ability to synthesize biofilms of metal nanoparticles
embedded in a biological matrix has important applications in the synthesis of cermet materials for optically functional thin
film coatings. Although the ability of
P. stutzeri
to survive in high silver ion concentration regions was poorly understood at
that time, with the advent of sophisticated techniques several groups have now postulated the presence of a silver ion binding
protein and other associated proteins on the cell surface as potentially responsible factors for the reduction of silver ions to
form silver nanoparticles [80, 82-84].
A more recent study established a direct correlation between the silver resistance machinery (Sil gene cluster) of
bacterial
Morganella
spp. and their Ag nanoparticles biosynthesis ability [80, 83, 84]. Additional studies further defined
the possible presence of a silver ion reductase enzyme in biological systems, especially in silver resistant species.
Furthermore, a
M. psychrotolerans
, psychrotolerant (cold-tolerant) species from the same genus, was demonstrated for
shape controlled biosynthesis of Ag nanoplates by controlling the bacterial growth kinetics by growing them at different
temperatures (Fig. 20.3) [80].
Morganella
spp. is the only example so far, wherein all the biogroups belonging to a
particular genus have been shown for their ability toward Ag ion reduction, thus establishing this nanoparticle synthesis
ability as a genus-wide phenotypic attribute of genus
Morganella
[83]. It was also established that this phenotypic feature
of
Morganella
is independent of environmental variable and is strongly associated with the silver resistance machinery
of these silver resistant bacterial. Due to the well-known similarities between the silver and copper resistance machineries
in different bacterial systems,
M. morganii
was further reported for its ability to synthesize pure metallic copper
nanoparticles (without any passivating oxide layer) in an aqueous phase (Fig. 20.4) [85]. This was the first time that the
important role of the similarities in the bacterial heavy metal resistance machinery was outlined toward the synthesis of
(a)
(b)
100 nm
100 nm
(d)
(c)
500 nm
200 nm
100 nm
fiGUre 20.3
TeM images of Ag nanoparticles biosynthesized using Morganella psychrotolerans at (a) 25°c, (b) 20°c, (c) 15°c, and
(d) 4°c. Images reprinted with permission from Ref. [80]. © 2011, American chemical Society.
