Environmental Engineering Reference
In-Depth Information
Silver ion
Peptide molecule
Silver cluster
Silver crystal
Reduced silver cluster
FiGurE 19.2
Peptide aided the growth of the silver crystal. Modified from Naik et al. [32].
of the tyrosine. Finally, the phenolic group turns out to become a semi-quinone structure. Then, the formed silver NPs are
separated [33]. It is also found to have reducing action for gold NP synthesis [34] (Fig. 19.2).
19.8
EnzymES
As stated earlier, the role of enzymes in the biosynthesis of silver NPs was well established by the work of Anil Kumar et al. [31].
They had taken purified nitrate reductase from
F. oxysporum
along with NAdPH to synthesize silver NPs. The brown color
indicated the formation of silver NPs [31]. Another experiment with
B. subtilis
was performed by Saifuddin et al. [35]. They
performed extracellular synthesis of silver NPs in the culture supernatant of
B. subtilis
. The silver ion (Ag
+
) when subjected to
microwave irradiation was shown to synthesize silver NPs. The synthesized NPs were found to be stable without aggregation
mainly because of the protein capping over the silver NPs. Moreover, the culture supernatant also had a considerable amount of
nitrate reductase activity. They proposed that the reductase enzyme along with electron shuttling compounds and other peptides
could be the reason for silver ion reduction, leading to the formation of silver NPs, as in the case of fungi [36, 37].
An extended view on the function of nitrate reductase enzyme for the synthesis of silver NPs using bacteria was proposed
by Kalimuthu et al. [22] who studied silver NP synthesis in
B. licheniformis
. Nitrate reductase helps in the conversion of nitrate
to nitrite, and it is mainly involved in the nitrogen cycle [37]. They proposed a mechanism in which electron shuttle enzymatic
metal reduction occurs. Earlier, Ahmed et al. [25] had highlighted the importance of NAdH- and NAdH-dependent nitrate
reductase in the synthesis of metal NPs.
In vitro
analysis provided the need for nitrate reductase in the synthesis of silver NPs
[31].
B. licheniformis
is also found to express NAdH- and NAdH-dependent reductases as an electron carrier. Thus silver ion
(Ag
+
) is reduced to free silver metal (Ag
0
) [22] (Fig. 19.3).
Primarily metal NPs are produced only if the microorganisms are resistant against metal ions. Parikh et al. [4] demonstrated
that extracellular synthesis of silver NPs using silver-resistant Morganella species and suggested that the microorganisms would
have separate unique mechanisms for synthesis. In that case, the silver crystals were formed in the extracellular matrix. Sintubin
et al. [38] suggested a mechanism for synthesizing metal NPs in lactic acid bacteria in which the effect of pH was studied. At
higher levels of pH, monosaccharides like glucose are converted to open chain aldehydes. These aldehydes are responsible for
reducing metal ions as they are oxidized to their corresponding carboxylic acid.
The influence of visible light on NP synthesis was studied in the culture supernatant of
Klebsiella pneumoniae
. Silver NPs
were formed, and the size of the silver NPs was found to be around 3 nm. The reduction of silver ions in the presence of light
was made by reducing agents/electron shuttles of Enterobacteriaceae. Even if the bacterial cells were absent, the silver ions
were reduced, which confirmed the release of reducing agents from the Entero bacteria. This suggested that reduction of silver
ions occurred after conjugation of the photosensitive electron shuttles with nitrate reductase [39].
