88
A. Antony and J.-H. Boo
and growth mechanism dealing with metal oxides need further developments in the
above theory.
4.2 Green Methods-Bacteria-Mediated and Plant Extracts
In green methods, MONMs are prepared with reactions from the bacterial supernatant
solution. The bacterial supernatant contains variety of energetic sources such as
electrons, radicals and chemical molecules. The precursor for the required MONMs
are made to react with such biological moieties and they are reduced to metallic or
MONMs (in the presence of oxygen or hydroxyls) which are formed after certain
incubation time. These are generally based on enzymatic reductions. For example,
probable mechanism for the preparation of metallic nanoparticles using fungi extracts
is due to an enzymatic reduction (reductase). It has been shown that two key aspects in
B. licheniformis bacteria mediated biosynthesis of silver nanoparticles are based on
NADH (nicotinamide adenine dinucleotide) and NADH-dependent nitrate reductase
[20]. The intracellular formation of NMs by microorganisms are due to genetic and
proteomic responses.
For
synthesizing
metal/metal
oxide
NMs
using
plant
extr
acts [21–23], organic molecules and phytochemicals such as ketones, flavones,
aldehydes, amides, carboxylic acids, terpenoids, phenols, sugars, and ascorbic acid
are believed to play vital roles in reducing metal salts into metal or metal oxide
nanoparticles. For example, flavonoids show different functional groups that release
reactive hydrogen atom due to tautomeric transformations from which enol-form
is transformed into keto-form. This process is achieved by the reduction of metal
ions into metal nanomaterials [24] using sweet basil (Ocimum basilicum) extracts
for preparing silver NMs. The functional groups (for example, –C–O–C–, –C–O–,
–C=C–, and –C=O–) act as capping ligands and stabilize the NMs during growth
and control further aggregation. Alcoholic compounds such as hydroquinone and
quinol act as reducing agents. At the same time, exact mechanisms for MONMs
preparation using plant extracts is not fully understood which can be conceived as
the presence of oxygen-containing compounds would facilitate the formation. It is
stated that there are three steps of nanoparticle synthesis from plant extracts; (i)
activation phase or nucleation process involving bio-reduction of metal ions/salts,
(ii) growth phase as guided by the Ostwald ripening, and (iii) termination phase
which defines the final shape of NMs [24].
4.3 Hydrothermal Method of Synthesis
Hydrothermal methods of MONMs synthesis are considered effective to prepare high
uniformity bulk synthesis with an oriented growth. This method relies based on the
chemical reactions and changes in solubility of a substance inside a sealed chamber
(the heated solution kept above ambient temperature and pressure). It is perfectly
A. Antony and J.-H. Boo
and growth mechanism dealing with metal oxides need further developments in the
above theory.
4.2 Green Methods-Bacteria-Mediated and Plant Extracts
In green methods, MONMs are prepared with reactions from the bacterial supernatant
solution. The bacterial supernatant contains variety of energetic sources such as
electrons, radicals and chemical molecules. The precursor for the required MONMs
are made to react with such biological moieties and they are reduced to metallic or
MONMs (in the presence of oxygen or hydroxyls) which are formed after certain
incubation time. These are generally based on enzymatic reductions. For example,
probable mechanism for the preparation of metallic nanoparticles using fungi extracts
is due to an enzymatic reduction (reductase). It has been shown that two key aspects in
B. licheniformis bacteria mediated biosynthesis of silver nanoparticles are based on
NADH (nicotinamide adenine dinucleotide) and NADH-dependent nitrate reductase
[20]. The intracellular formation of NMs by microorganisms are due to genetic and
proteomic responses.
For
synthesizing
metal/metal
oxide
NMs
using
plant
extr
acts [21–23], organic molecules and phytochemicals such as ketones, flavones,
aldehydes, amides, carboxylic acids, terpenoids, phenols, sugars, and ascorbic acid
are believed to play vital roles in reducing metal salts into metal or metal oxide
nanoparticles. For example, flavonoids show different functional groups that release
reactive hydrogen atom due to tautomeric transformations from which enol-form
is transformed into keto-form. This process is achieved by the reduction of metal
ions into metal nanomaterials [24] using sweet basil (Ocimum basilicum) extracts
for preparing silver NMs. The functional groups (for example, –C–O–C–, –C–O–,
–C=C–, and –C=O–) act as capping ligands and stabilize the NMs during growth
and control further aggregation. Alcoholic compounds such as hydroquinone and
quinol act as reducing agents. At the same time, exact mechanisms for MONMs
preparation using plant extracts is not fully understood which can be conceived as
the presence of oxygen-containing compounds would facilitate the formation. It is
stated that there are three steps of nanoparticle synthesis from plant extracts; (i)
activation phase or nucleation process involving bio-reduction of metal ions/salts,
(ii) growth phase as guided by the Ostwald ripening, and (iii) termination phase
which defines the final shape of NMs [24].
4.3 Hydrothermal Method of Synthesis
Hydrothermal methods of MONMs synthesis are considered effective to prepare high
uniformity bulk synthesis with an oriented growth. This method relies based on the
chemical reactions and changes in solubility of a substance inside a sealed chamber
(the heated solution kept above ambient temperature and pressure). It is perfectly