Environmental Engineering Reference
In-Depth Information
20.5
metal SUlfide nanoparticleS
There is significant interest in developing protocols for efficient and environmental friendly synthesis of semiconductor mate-
rials such as cdS for cell labeling applications due to their unique size-dependent chemical and physical properties. The differ-
ent biological entities employed for the synthesis of metal sulfide nanoparticles are summarized in Table 20.3. Of the different
biological entities, bacteria have shown considerable success in the synthesis of cdS nanoparticles with several species including
Escherichia coli
[121],
Klebsiella
sp. [122, 123], and
Clostridium thermoaceticum
[124]. Some of these bacterial strains
belonging to
Klebsiella pneumoniae
were specific toward the synthesis of cdS nanoparticles as other metal sulfides were not
detected on exposure to Pb and Zn ions [122]. The sulfide source in the case of
Klebsiella
sp. and
C. thermoaceticum
was pri-
marily from the amino acid cysteine, which is either added to the bacterial growth medium or via the activity of cysteine desulf-
hydrase enzyme. Of the aforementioned bacteria, when
E. coli
was incubated with cadmium chloride and sodium sulfide, it
intracellularly synthesized 2-5 nm cdS nanocrystals of wurtzite crystal phase [121]. In contrast to bacterial systems wherein
the quantum dot nanocrystals are found intracellularly, the fungus
F. oxysporum
was shown for its ability to synthesize cdS
quantum dots extracellularly [125]. Further analysis using polyacrylamide gel electrophoresis (SDS-PAGe) indicated the
presence of at least four protein bands in the aqueous extract of the fungal biomass that were proposed to be potentially respon-
sible for the formation of cdS nanocrystals.
The employment of bacteria for the synthesis of cdS nanoparticles was fascinating in itself, but remarkably Dameron et al.
showed the unique ability of yeast toward intracellular synthesis of quantum semiconductor crystallites. In these studies, two
different yeast species, namely,
Candida glabrata
and
Schizosaccharomyces pombe
, were exposed to aqueous cadmium ions
following which 1-3 nm nanocrystallites of cdS nanoparticles were found deposited intracellularly [30, 126]. Phytochelatins
with the general structure (γ-Glu-cys)
n
-Gly, where “
n
” ranged from 2 to 6, were found to control the nucleation and growth of
cdS nanoparticles. Furthermore, the synthesis efficiency for cdS nanocrystals was improved by exposing
S. pombe
cells to
aqueous cadmium ions in their mid-log phase of growth, suggesting the importance of the growth phase on the synthesis of
nanomaterials. In addition to improving the efficiency of nanoparticle synthesis, the authors also demonstrated the ability of
Torulopsis
sp. (yeast) for the synthesis of spherical PbS nanocrystals within the quantum confinement regime [127]. The property
of yeast for the synthesis of metal sulfides was postulated to be the result of metal-chelating peptides that nullify the stress gen-
erated by these metal ions. A classic bioremediation pathway is followed by yeast cells wherein the exposure of metal ions ini-
tially results in a metal ion-γ glutamyl complex with an increase in the intracellular sulfide ion level. This is followed by the
complexation of sulfide ions with cd or Pb ions to form cdS or PbS nanocrystals that further accumulate in the vacuoles within
yeast cells. In an interesting study, Banfield and co-workers employed natural biofilms of sulfate-reducing bacteria from the
taBle 20.3
list of biological entities employed for metal sulfide nanoparticle synthesis
Microorganism
Sulfides
References
Bacteria
Escherichia coli
cdS
[121]
Clostridium thermoaceticum
cdS
[124]
Klebsiella pneumoniae
cdS
[122]
Klebsiella planticola
cdS
[123]
Sulfate reducing bacteria
ZnS
[128]
Magnetotactic bacteria
Fe
3
S
4
[129, 130]
Fungus
Fusarium oxysporum
cdS
[125]
Actinomycete
Actinobacter
sp.
Fe
3
S
4
, FeS
2
[113]
Yeasts
Candida glabrata
cdS
[30]
Schizosaccharomyces pombe
cdS
[30, 126]
Torulopsis
sp.
PbS
[127]
