402
A. P. Ingle et al.
and thrombolytic activities in another study (Lateef et al. 2016b), while its keratinase
facilitated the synthesis of AgNPs with antibacterial property (Lateef et al. 2015b).
Furthermore, Singh et al. (2016) carried out investigation to evaluate the catalytic
activity of gold nanoparticles. Remarkably, it was found to reduce 4-nitrophenol to
4-aminophenol with AuNPs acting as a catalyst.
A similar co-production of AuNPs and AgNPs was carried out in an investigation
by Sowani and colleagues (Sowani et al. 2016a, b). Interestingly, in this case, an actinomycete characterized as Gordonia amicalis HS-11 was cultivated in n-hexadecane
in order to produce glycolipid which was attributed and confirmed to be the reducing
agent for the conversion of HAuCl 4 and AgNO 3 into Au
0 and Ag
0 , respectively.
Thereafter, these synthesized AuNPs and AgNPs were individually evaluated by their
potential antioxidant scavenging activity, with approximately 90 and 95% inhibitory
effect toward hydroxyl radicals by using, respectively, AuNPs and AgNPs. In this
context, it is of great importance to emphasize the fact that structural properties
such as size, shape, and mono-dispersity of nanoparticles strongly influence their
antioxidant activity (Ramamurthy et al. 2013).
Another strategy was employed by Otari et al. (2012) to synthesize AgNPs by
subjecting a culture of Rhodococcus sp. to a batch flask fermentation along with
AgNO 3 solution at final concentration of 3 × 10
–3 M. Product generation was monitored by UV-vis spectrophotometer at 420 nm, and the change in color from colorless
to brownish indicated the synthesis of AgNPs. It was reported that nitrate reductase
and set of enzymes found in Rhodococcus sp. were responsible for the synthesis
under anaerobic condition rather than respiratory type. Additionally, free-cell broth
did not show any reduction potential in AgNO 3 solution, whereby no change in color
was visualized. This fact supports the assumption that synthesis of nanoparticles may
selectively occur intracellularly.
Moreover, Fayaz et al. (2011) investigated the use of extremophilic bacterium,
namely Geobacillus stearothermophilus to mediate the biosynthesis of AuNPs and
AgNPs, to elucidate the set and types of proteins secreted extracellularly through
a polyacrylamide gel electrophoresis. The results comprised a set of not less than
seven different enzymes of molecular weight ranging between 12 and 98 kDa which
suggestively participate in the reduction reaction of metallic ions and play a crucial
role in providing electrons. Attractively, it was found that amide residual linkages
bound and coated the produced nanoparticles, to enhance their stability and also
serve possibly as a support for activating the nanoparticles for several profitable
applications, e.g., immobilization of enzymes (Ashtari et al. 2012; Ingle et al. 2017),
by substituting the usual chemically activation methods in a sustainable and costeffective strategy.
2.2 Fungal Synthesis
Like bacteria, fungi have emerged as an efficient system for the microbial synthesis
of nanoparticles due to distinctive characteristics such as high wall binding capacity,
A. P. Ingle et al.
and thrombolytic activities in another study (Lateef et al. 2016b), while its keratinase
facilitated the synthesis of AgNPs with antibacterial property (Lateef et al. 2015b).
Furthermore, Singh et al. (2016) carried out investigation to evaluate the catalytic
activity of gold nanoparticles. Remarkably, it was found to reduce 4-nitrophenol to
4-aminophenol with AuNPs acting as a catalyst.
A similar co-production of AuNPs and AgNPs was carried out in an investigation
by Sowani and colleagues (Sowani et al. 2016a, b). Interestingly, in this case, an actinomycete characterized as Gordonia amicalis HS-11 was cultivated in n-hexadecane
in order to produce glycolipid which was attributed and confirmed to be the reducing
agent for the conversion of HAuCl 4 and AgNO 3 into Au
0 and Ag
0 , respectively.
Thereafter, these synthesized AuNPs and AgNPs were individually evaluated by their
potential antioxidant scavenging activity, with approximately 90 and 95% inhibitory
effect toward hydroxyl radicals by using, respectively, AuNPs and AgNPs. In this
context, it is of great importance to emphasize the fact that structural properties
such as size, shape, and mono-dispersity of nanoparticles strongly influence their
antioxidant activity (Ramamurthy et al. 2013).
Another strategy was employed by Otari et al. (2012) to synthesize AgNPs by
subjecting a culture of Rhodococcus sp. to a batch flask fermentation along with
AgNO 3 solution at final concentration of 3 × 10
–3 M. Product generation was monitored by UV-vis spectrophotometer at 420 nm, and the change in color from colorless
to brownish indicated the synthesis of AgNPs. It was reported that nitrate reductase
and set of enzymes found in Rhodococcus sp. were responsible for the synthesis
under anaerobic condition rather than respiratory type. Additionally, free-cell broth
did not show any reduction potential in AgNO 3 solution, whereby no change in color
was visualized. This fact supports the assumption that synthesis of nanoparticles may
selectively occur intracellularly.
Moreover, Fayaz et al. (2011) investigated the use of extremophilic bacterium,
namely Geobacillus stearothermophilus to mediate the biosynthesis of AuNPs and
AgNPs, to elucidate the set and types of proteins secreted extracellularly through
a polyacrylamide gel electrophoresis. The results comprised a set of not less than
seven different enzymes of molecular weight ranging between 12 and 98 kDa which
suggestively participate in the reduction reaction of metallic ions and play a crucial
role in providing electrons. Attractively, it was found that amide residual linkages
bound and coated the produced nanoparticles, to enhance their stability and also
serve possibly as a support for activating the nanoparticles for several profitable
applications, e.g., immobilization of enzymes (Ashtari et al. 2012; Ingle et al. 2017),
by substituting the usual chemically activation methods in a sustainable and costeffective strategy.
2.2 Fungal Synthesis
Like bacteria, fungi have emerged as an efficient system for the microbial synthesis
of nanoparticles due to distinctive characteristics such as high wall binding capacity,