Environmental Engineering Reference
In-Depth Information
noble metal ions [62]. Typically, these
Lactobacilli
strains were able to form gold, silver, and gold-silver alloy nanoparticles
intracellularly. comparable to the earlier case, the UV-visible spectra obtained from the nanoparticles synthesized by simulta-
neous exposure of Au and Ag ions showed a plasmon band at 537 nm, which is centered between the plasmon bands observed
for pure Ag and Au nanoparticles, respectively, indicating the formation of an Au-Ag alloy and not a core shell structure.
Interestingly, the intracellular manifestation of these nanoparticles created a metal environment within the bacterial cell, which
was exploited by Nair and Pradeep toward mapping the bacterial cell using surface-enhanced Raman scattering (SeRS).
considering the sensitivity of SeRS-based sensors, it may be of utmost importance to explore this avenue of research to further
enhance the Raman signals from within the bacterial cell. This could lead to the identification of infectious agents in humans
and animals by looking for fingerprint molecules in these microbes [62].
In addition to employing biological entities like bacteria, fungi, and cyanobacterial species that offer a unique ability to
transform metal ions to nanoparticles, plant and plant extracts offer an exciting and unique fabrication strategy, as this technique
is fast, generally extracellular and with faster rate of reduction. Moreover, several plant species have been reported to have resis-
tance machineries to manage the metal ion stress similar to that found in the bacterial system. For instance, it was recently
reported that
Iris pseudocorus
(yellow iris) could tackle copper stress environment by transforming ionic copper to metallic
copper nanoparticles in and near its roots with some degree of assistance from the endomycorrhizal fungi present in association
with these plants [63]. Although this ability of plants to synthesize nanoparticles in a high stress environment is fascinating, the
ability to use leaf extracts of different plants for rapid synthesis of nanoparticles appears more exciting from a practical perspec-
tive. Sastry et al. used a range of plant extracts including germanium (
Pelargonium graveolens
) [58, 64], lemongrass
(
Cymbopogon flexuosus
) [65], and neem (
Azadirachta indica
) [66] for the synthesis of gold nanoparticles. Interestingly, these
particles showed high degree of morphological control by displaying spherical, triangular, decahedral, and icosahedral shapes.
The presence of alcohol, terpenoids, flavonoids, and aldehyde/ketone groups present in these extracts were believed to facilitate
the reduction of chloroaurate ions.
Furthermore, as an application of biosynthesized metal nanoparticles, the ability of gold nanotriangles biosynthesized by
lemongrass extract was elucidated toward the detection of aqueous mercuric ions at ultra-low femtomolar concentrations [67].
This is an important application of biosynthesized nanomaterials, as mercury is one of the most serious environmental pollut-
ants that are known to have serious detrimental effects on living organisms. Particularly, in humans, mercury causes severe
aliments to the kidney, heart, nervous system, respiratory system, and muscles. In addition, mercury is known to amalgamate
with gold thereby enabling the potential use of gold nanoparticles for environmental sensing applications [38, 68]. The ability
of these nanotriangles (thin triangular nanoplates) to detect ultra-low concentrations of mercury was attributed to their thin
nature and the presence of thermodynamically unstable high energy sites at the nanotriangle edges, which facilitates a high
level of amalgamation of these nanotriangles with mercury. It was observed that during the process, mercury did not get incor-
porated into the lattice structure of Au nanotriangles; however, mercury caused deconstruction of the high energy tips and
edges, resulting in damaging nanostructure morphology. Further experiments revealed that these nanotriangles showed high
specificity toward the detection of mercuric ions [67]. In addition, Katti and co-workers reasoned that phytochemicals present
within plants possess a plethora of antioxidant properties that could serve as chemical reducing agents for the reduction of
metal salts into their corresponding nanoparticulate forms. They hypothesized that the polyphenols, phytoestrogens, and
related chemicals in plants could provide a robust coating of tumor-specific phytochemicals on the surface of gold nanopar-
ticles that provides a basis for green nanotechnological processes for the production of cancer-specific gold nanoparticles. The
results reported by them provided experimental evidence toward the validation of these hypotheses [69-74]. This group
further exploited the reduction capabilities of antioxidant phytochemicals present in green tea, soybean, cumin, and cinnamon
for reducing gold salts into nanoparticles with consequent coatings of these polyphenols, phytoestrogens, and a host of other
phytochemicals present in these phytoextracts [69, 71, 73]. Interestingly, due to robust phytochemicals surface modification,
these nanoparticles showed remarkable stability under a range of pH conditions and did not aggregate in high salt concentra-
tions and other biologically relevant media [69, 71, 73]. Some of these biosynthesized gold nanoparticles were also employed
for electrochemistry applications including the study of electron transfer rates in molecules [75, 76], biomedical applications
including drug delivery [71, 77], imaging [72, 73], targeted therapy [74], and for catalytic reactions to reduce common envi-
ronmental pollutants [78].
Unlike gold, ionic forms of silver, copper, selenium, and tellurium are known to be highly toxic to biological cells [79, 80].
Nonetheless, several strains of biological organisms have mechanisms in place for efficient remediation of these toxic metal
ions. Incidentally, several silver resistant bacterial species are reported to accumulate metallic silver as much as 25% of the
dry weight biomass [79, 81]. The first report on the biosynthesis of silver nanoparticles was on using a silver resistant
bacterium
Pseudomonas stutzeri
AG259 isolated from silver mines. In a pioneering study, Klaus et al. exposed
P. stutzeri
AG259 to high concentrations of silver ions that resulted in the intracellular (periplasmic space) accumulation of silver and
silver sulfide nanoparticles. These particles typically ranged in diameter from a few nanometers to 200 nm with different
