Environmental Engineering Reference
In-Depth Information
er
Nitrate reductase
Ag
+
Ag
0
NADH
NAD
+
FiGurE 19.3
The possible mechanism by which nanoparticles (NPs) are produced in a bacterial cell. The enzyme nitrate reductase present
in the bacterial cell mediates the reduction of silver ions to silver NPs. Modified from Kalimuthu et al. [22].
Fungi are also found to show a similar kind of reductase-mediated mechanism found in bacteria for silver NP synthesis.
The involvement of NAdH-dependent reductases for silver NP synthesis was assayed by Ahmed et al. [25] in the fungi
F. oxysporum
. They suggested that fungi that lack NAdH-dependent reductases might not be involved in silver NP production.
duran et al. [37] verified that with nitrate-dependent reductase and quinine, an extracellular electron shuttle is responsible for
the reduction of silver ions in the fungi
F. oxysporum
. The NPs were synthesized extracellularly, and their size ranged from 20
to 50 nm [37]. It was also found that the NPs produced were stabilized by fungal proteins [40]. Ingle et al. [41] confirmed the
existence of nitrate reductase by nitrate reductase disks in the fungal filtrate, thereby confirming that NAdH-dependent nitrate
reductases are important in the synthesis of NPs as they reduce silver ions to silver NPs.
19.9
ExtErnAl FActorS AFFEctinG nps production
It has been reported that factors like pH, temperature, and even silver nitrate concentration have an effect on the production of
silver NPs [24]. The effect of silver nitrate concentration on the synthesis of silver NPs was studied, and the results showed that
the synthesis of NPs increased with the increase in silver nitrate concentration. The synthesis of NPs was confirmed by
measuring the absorbance at 420 nm. The maximum optimum concentration at which the synthesis of silver NPs occurred the
maximum was found to be 5 nM in case of
E. coli
[24]. By increasing the concentration over 5 nM, absorbance was reduced,
which indicated the reduction in production of silver NPs. Also, the size of NPs decreased with increase in concentration up to
5 nM after which the size increased. There is no clear explanation as to why the size of NPs varied with varying concentrations
of silver nitrate.
Increase in temperature caused an increase in the rate of synthesis of silver NPs up to a certain threshold temperature after
which the rate reduced. In
E.
that threshold temperature is found to be 60°C after which the rate of synthesis of silver NPs is
reduced. Even as the temperature was increased, NPs were produced in smaller sizes after that threshold temperature the size
of the synthesized NPs was found to be increasing. The higher rate of reduction of silver ions might be because enzymes are
induced more at that temperature.
The pH is found to have a profound impact on the synthesis of silver NPs. Usually NP production increases with an increase
in pH. This is mainly because at higher pH, the hydroxide ions increase the reduction in capacity of the enzymes involved in
the synthesis of silver NPs, leading to increased silver NP production. For example, in
E. coli
, maximum production occurred
at pH 10.0, and above this pH the reduction decreased. This was observed by a decrease in the absorbance value at 420 nm [24].
19.10
puriFicAtion oF nps
NPs have amazing properties, most of which are due to their unique size and shape. Synthesized NPs are mostly of different
sizes; hence it is necessary to separate and select them according to their size and shape, so that certain functions of the particles
can be determined. The size and shape of NPs have an effect on the catalytic activity and cytotoxicity [42, 43]. They are also
