Environmental Engineering Reference
In-Depth Information
Cu
2+
/Ag
+
Cu
0
/Ag
0
Lipid bilayer
CusF/SilE
5
CusR
/SilR
1
Reductase
CusS
/SilS
P-type
/SilP
CusA/SilA
CusB/SilB
2
4
CusC/SilC
3
fiGUre 20.4
Schematic representation of the potential similarities between the copper and silver resistance systems that enables
Morganella morganii
, a silver resistant organism, to synthesize cuNPs. The top and the bottom sides of the schematic show the outer surface
and the inner cytoplasmic end of the bacterium, respectively. Typically, the process involves the uptake of cu
2+
ions by the bacterium (step 1)
followed by the presentation of cu
2+
ions to the silver resistance machinery (step 2). This initiates a number of processes whereby either a
metal ion reductase or similar proteins bind to cu
2+
ions (step 3), thereby reducing them to cu
0
nanoparticles (step 4). The biosynthesized
nanoparticles are thereafter released from the cell using a cellular efflux system (step 5). Images reprinted from Ref. [85]. Reproduced by
permission of The Royal Society of chemistry (RSc).
technologically important nanomaterials. The biosynthesis of pure copper nanoparticles is particularly important as
chemical synthesis approaches or previously employed biosynthesis approaches led to the stabilization of copper parti-
cles in its oxide form due to the propensity of surface oxidation of metallic copper. chemical synthesis approaches,
through which pure metallic copper nanoparticles can be produced, require strong capping agents and laborious synthesis
processes to prevent surface oxidation [86, 87].
Similar to gold and silver nanoparticles, palladium nanoparticles biosynthesis is also of great interest with reports appearing
in the early 2000s using sulfate reducing bacterium
Desulfovibrio desulfuricans
in the presence of molecular hydrogen or
formate as electron donor [88]. Similar to the case in
S. algae
for platinum biosynthesis, the activity of enzymes hydrogenase
and/or cytochrome c3 were postulated to be responsible for the reduction of palladium precursors [43]. Interestingly, the
absence of anaerobic conditions did not alter the production of palladium nanoparticles, as aerobic conditions are known to be
toxic to the bacterium and hydrogenase enzyme. Similarly, other bacterial species belonging to genus
Desulfovibrio
and
Shewanella
also showed ability to reduce palladium precursors [89-91]. These palladium nanoparticles were shown to be effi-
cient catalysts toward dehalogenation reactions with some biologically synthesized Pd catalysts showing better response than
chemically synthesized Pd nanoparticles [88-91].
The use of bacteria toward the synthesis of selenium and tellurium nanoparticles is fascinating due to its significant impor-
tance in the semiconductor industry [92]. Unlike gold, silver, and copper, selenium and tellurium ions are toxic to bacterial
cells at ultra-low concentrations. However, the Mother Nature has devised resistance systems that are unique and intelligent
to counter these toxic metalloid ions. Oremland and co-workers have isolated several bacterial species from metalloid-rich
environments that have the ability to convert these toxic ions to nanoparticles. These organisms include
Sulfurospirillum
barnesii
,
Bacillus selenitireducens
, and
Selenihalanaerobacter shriftii
, wherein the first two species were able to reduce sel-
enate and tellurate oxyanions to selenium and tellurium nanoparticles, respectively, while the latter could only reduce selenate
to selenium nanoparticles [93, 94]. Although some bacterial species showed the ability to reduce both these metalloid ions,
