Environmental Engineering Reference
In-Depth Information
like size control over the NPs synthesized. It was proven that the size of the NPs synthesized varies according to the pH and
temperature of the reaction conditions [24].
19.4
bioGEnErAtorS oF SilvEr nps
Soon after the biosynthesis of silver NPs came into the spotlight, a lot of works were conducted for a wide range of organisms
encompassing both prokaryotes and eukaryotes. Moreover, because this method of synthesis is pollution free, it adds to the
value of the biological reduction method. A wide range of biogenerators have been shown to synthesize silver NPs including a
variety of bacteria, fungi, algae, and plant species. Among them bacteria are shown to be the best platform for synthesis as they
are easy to handle and are easy subjects for genetic modifications [3, 4].
Bacteria like Bacillus licheniformis , E. coli , and Brevibacterium casei have been shown to synthesize silver NPs mostly
around the range of 50 nm [22, 24, 23]. Some of the fungal species also synthesized silver NPs, notably Fusarium oxysporum ,
Aspergillus niger , Fusarium semitectum , and Trichoderma asperellum [25-28] . Silver NPs have also been synthesized in plants
due to the presence of phytochemicals that reduce silver ions. For instance, anthraquinone emodin leads to the synthesis of
silver NPs in Bryophyllum species [29].
19.5
principlE oF np bioSynthESiS
Metal aggregates are reported to be formed over the cellular surfaces of the microorganisms [30]. Metal aggregates are formed
only if metal ions are reduced, and the process of this reduction is undertaken by cellular components. Such reducing components
are proteins, amino acids, polysaccharides, and even vitamins, which are the host components of the microorganism itself. This
reduction is possible only if the microorganisms have the resistant machinery against metal ions. At higher concentrations,
silver ions are found to inhibit the growth of microorganisms by destroying them. This could be the key reason for silver's
antimicrobial activity. Therefore, only those microorganisms possessing the resistant machinery can be used for metal NP syn-
thesis. Moreover, the effectiveness of the microorganisms is decided by parameters like reduction potential and the capacity of
the system. Enzymes are found to be the key source of reduction of metal ions. The role of enzymes in the reduction of metal
ions was studied by Anil Kumar and his colleagues [31]. They carried out the experiment by adding purified enzyme nitrate
reductase from the fungi F. oxysporum with silver nitrate and nicotinamide adenine dinucleotide phosphate-oxidase (NAdPH).
The change of color to brown indicated the formation of silver NPs. This experiment provided direct evidence that enzymes
produced by the microorganisms are responsible for reducing the metal ions to their NPs.
19.6
mEchAniSm oF SilvEr np SynthESiS
Even though a lot of work has been done in the biosynthesis of silver NPs, the exact mechanism by which NPs are synthesized
is not clear. Many mechanisms have been hypothesized for the synthesis of NPs involving biological components, including
those that are used for the reduction of metal ions, and also some electro shuttles found in the microorganisms.
19.7
pEptidES
Naik et al [32] introduced the idea that peptides can be used as stabilizers for the synthesis of silver NPs. They showed that the
peptides would bind over nuclei and enhance crystal growth. Thus, the peptides create a reducing environment that enables easy
reduction of silver ions to form silver NPs. This model was confirmed by adding peptides into a solution of silver ions. Upon
addition, these peptides interacted with the nanoclusters present in the solution, and it produced a reducing environment that
reduced the silver ions at the peptide-metal interface. Thus, the peptides provide a reducing environment over the nanoclusters,
enabling crystal formation.
Certain amino acid moieties are found to be involved in the recognition and reduction of silver ions. Mostly amino acids
like arginine, cysteine, lysine, and methionine are found to be involved in the formation of silver crystals from silver ions, which
gives a better understanding of silver with peptides [32]. In addition, tyrosine is found to reduce silver ions in the alkaline condition.
Thus in an alkaline environment, tyrosine acts as a reducing agent where reduction occurs by ionization of the phenolic group
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