Applications of Microbe-Based Nanoparticles …
347
1. Mechanism of NPs synthesis is linked to cell wall polymers, sugars (responsible
for attachment and reduction of metal ions).
2. In the bioreduction step, cell wall surface traps metal ions due to the electrostatic
interaction among the metal ions and carboxylate group of the enzymes in the
cell wall.
3. This step is followed with metal ions reduction by the enzymes such as NADPHdependent reductases or nitrate-dependent nitrate reductase, oxidoreductase to
form nanoparticles. In addition, quinine derivatives of naphthoquinones and
anthraquinones also play roles in the reduction process.
The synthesis of NPs in fungi has been shown to be intracellular, extracellular
or take place on the cell surface (Kashyap et al. 2013; Alghuthaymi et al. 2015;
Moghaddam et al. 2015; Yadav et al. 2015). Extracellular method is rapid, economical and easy to produce, because during this process, the cell synthesizes enormous
amount of enzymes and/or compounds, compared to intracellular synthesis (Nayak
et al. 2011; Kashyap et al. 2013). Extracellular AgNPs synthesis in F. oxysporum was
achieved by reduction of NADPH to NADP
+ (Kumar et al. 2007b), enzyme nitratedependent reductase and shuttle quinone (Devi and Joshi 2015), in Aspergillus terreus
using NADH-dependent reductase enzyme (Li et al. 2012). Chan and Mashitah
(2012) synthesized AgNPs using different macrofungi and reported a possible role
of a diketone compound for silver ion reduction. Sastry et al. (2003) proposed the
intracellular AgNPs synthesis in Verticillium sp. and involve the following steps:
1. Absorption: Binding of Ag
+ on the surface of the fungal cell wall. This could be
possibly due to electrostatic interaction between the Ag
+ and negatively charged
carboxylate groups on the enzymes on the cell wall.
2. Reduction of Ag
+ : Cell wall enzymes reduce Ag
+ and form a silver nuclei.
3. Synthesis of AgNPs: It involves the growth of the silver nuclei to Ag
0 , thus
forming AgNPs.
4. They are finally secreted out through the membrane, by a process known as
exocytosis (Korbekandia et al. 2013).
The synthesis of AuNPs using fungi comprises two steps. The synthesis of intracellular AuNPs in Rhizopus oryzae was reported by Das et al. (2012). Rhizopus oryzae
mycelia was treated with HAuCl 4 solution and incubated for 48 h. This resulted in
gradual color change from pale white to purple. The authors proposed the following
mechanisms: (1) Adsorption: In this step, Au
3+ binds to the cell wall through electrostatic interaction. Gold ions bind to the fungal mycelia by electrostatic interaction
and get reduced to AuNPs through the action of enzymes on the cell wall surface,
(2) Transportation: In this step, Au
3+ is transported to the cytoplasm and is reduced
by cytoplasmic proteins such as oxidoreductases in a three step process to form Au
nanoparticles. Chemical reactions involved in the synthesis of gold nanoparticles are
1. AuCl
−
4 → H
+
+ AuCl
−
4
2. AuCl
−
4 → AuCl 3 + Cl
−
3. AuCl 3 → Au
3+
+ 3Cl
−
347
1. Mechanism of NPs synthesis is linked to cell wall polymers, sugars (responsible
for attachment and reduction of metal ions).
2. In the bioreduction step, cell wall surface traps metal ions due to the electrostatic
interaction among the metal ions and carboxylate group of the enzymes in the
cell wall.
3. This step is followed with metal ions reduction by the enzymes such as NADPHdependent reductases or nitrate-dependent nitrate reductase, oxidoreductase to
form nanoparticles. In addition, quinine derivatives of naphthoquinones and
anthraquinones also play roles in the reduction process.
The synthesis of NPs in fungi has been shown to be intracellular, extracellular
or take place on the cell surface (Kashyap et al. 2013; Alghuthaymi et al. 2015;
Moghaddam et al. 2015; Yadav et al. 2015). Extracellular method is rapid, economical and easy to produce, because during this process, the cell synthesizes enormous
amount of enzymes and/or compounds, compared to intracellular synthesis (Nayak
et al. 2011; Kashyap et al. 2013). Extracellular AgNPs synthesis in F. oxysporum was
achieved by reduction of NADPH to NADP
+ (Kumar et al. 2007b), enzyme nitratedependent reductase and shuttle quinone (Devi and Joshi 2015), in Aspergillus terreus
using NADH-dependent reductase enzyme (Li et al. 2012). Chan and Mashitah
(2012) synthesized AgNPs using different macrofungi and reported a possible role
of a diketone compound for silver ion reduction. Sastry et al. (2003) proposed the
intracellular AgNPs synthesis in Verticillium sp. and involve the following steps:
1. Absorption: Binding of Ag
+ on the surface of the fungal cell wall. This could be
possibly due to electrostatic interaction between the Ag
+ and negatively charged
carboxylate groups on the enzymes on the cell wall.
2. Reduction of Ag
+ : Cell wall enzymes reduce Ag
+ and form a silver nuclei.
3. Synthesis of AgNPs: It involves the growth of the silver nuclei to Ag
0 , thus
forming AgNPs.
4. They are finally secreted out through the membrane, by a process known as
exocytosis (Korbekandia et al. 2013).
The synthesis of AuNPs using fungi comprises two steps. The synthesis of intracellular AuNPs in Rhizopus oryzae was reported by Das et al. (2012). Rhizopus oryzae
mycelia was treated with HAuCl 4 solution and incubated for 48 h. This resulted in
gradual color change from pale white to purple. The authors proposed the following
mechanisms: (1) Adsorption: In this step, Au
3+ binds to the cell wall through electrostatic interaction. Gold ions bind to the fungal mycelia by electrostatic interaction
and get reduced to AuNPs through the action of enzymes on the cell wall surface,
(2) Transportation: In this step, Au
3+ is transported to the cytoplasm and is reduced
by cytoplasmic proteins such as oxidoreductases in a three step process to form Au
nanoparticles. Chemical reactions involved in the synthesis of gold nanoparticles are
1. AuCl
−
4 → H
+
+ AuCl
−
4
2. AuCl
−
4 → AuCl 3 + Cl
−
3. AuCl 3 → Au
3+
+ 3Cl
−