Metallic Nanoparticles for Biomedical Applications
39
three ways; (1) formation of colloids of metal (oxide) and mix up with the sol consists
of matrix-forming species, followed by the formation of a gel, (2) direct mixing of
metal and metal oxide within a prehydrolysed silica sol, and (3) the formation of
a complex compound of the metal with silone and subsequent reduction of metal
before hydrolysis. Colloids are generally formed from the ions of metal alkoxides
and alkoxysilanes. For example, silica gel is formed most commonly from tetramethoxysilane and tetraethoxysilane. The overall formation of sol-gel involves the
following steps; hydrolysis, condensation, particle growth, and agglomeration of
particles into NPs. It is possible to control the particle size and morphology by systematically monitoring the reaction entities. The method is generally used to synthesize
thin films, zinc peroxide nanostructures, and metallic oxide NPs like nickel oxide,
iron oxide, etc. [34, 36].
The hydrothermal method is due to the reaction between a solid material and
aqueous solution vapour at high temperature and pressure. Cations precipitate in
the form of polymeric hydroxide get dihydrate and accelerate the formation of
metal oxide crystals. When a base is added to the salt, another cation is formed,
which is essential in controlling particle formation by preventing complex hydroxide
formation. Even though particles with high crystallinity can be synthesized with
this method, processes are complicated to control. This method is suitable for
synthesizing nanoparticles in the powdered form [34].
Solvothermal based synthesis involves the formation of NPs in liquid phase
solvents such as water or organic compounds like methanol, polyol, and ethanol.
These solvents are allowed to heat above their boiling temperature in a pressure
vessel. It uses a metallic precursor, a reducing agent (reductant), and a solvent medium
as essential reagents and sometimes ligands and facets specific capping agents for
controlled synthesis. Narrow sized distribution of monodispersed crystals of NPs
can be synthesized using this method. With the help of a microwave, kinetics of the
crystallization step can be increased by the orders of one or two. MNPs such as Pt,
Pd, Au, Ag, Rh, Ru, Co, and Ni NPs can be rapidly synthesized by this method with
the help of methanol or polyethylene glycol (PEG) as reducing agent [34, 37].
Research analysis showed that the physicochemical properties of MNPs could
change with morphology and size. Hence, controlling the synthesis to achieve the
desired size and morphology is required, and is very challenging. The key parameters to tune the shape, size and crystallinity of MNPs by the solvothermal method
are temperature and reaction time, type of solvent, ligand type and its concentration, facet specific capping agent, and reductant. Temperature and reaction time have
a huge impact on controlling the synthesis of MNPs. Since the synthesis carries
out at a high temperature above the boiling temperature of solvents, its properties change; viscosity decreases, mobility increases, and dielectric constant reduces.
Metal precursor decomposes itself at high temperatures, or some can be carbonized.
Further, at high temperature, the reactivity of reactant increases, hence the reduction
rate of metal precursor increases, results in the formation of a lot of nuclei and a
decrease in the size of particles, as shown in Fig. 4a–b. Here Pd/Pt NPs synthesized
at 180 °C is smaller in size compared to those prepared at 120 °C [37, 38, 41]. High
39
three ways; (1) formation of colloids of metal (oxide) and mix up with the sol consists
of matrix-forming species, followed by the formation of a gel, (2) direct mixing of
metal and metal oxide within a prehydrolysed silica sol, and (3) the formation of
a complex compound of the metal with silone and subsequent reduction of metal
before hydrolysis. Colloids are generally formed from the ions of metal alkoxides
and alkoxysilanes. For example, silica gel is formed most commonly from tetramethoxysilane and tetraethoxysilane. The overall formation of sol-gel involves the
following steps; hydrolysis, condensation, particle growth, and agglomeration of
particles into NPs. It is possible to control the particle size and morphology by systematically monitoring the reaction entities. The method is generally used to synthesize
thin films, zinc peroxide nanostructures, and metallic oxide NPs like nickel oxide,
iron oxide, etc. [34, 36].
The hydrothermal method is due to the reaction between a solid material and
aqueous solution vapour at high temperature and pressure. Cations precipitate in
the form of polymeric hydroxide get dihydrate and accelerate the formation of
metal oxide crystals. When a base is added to the salt, another cation is formed,
which is essential in controlling particle formation by preventing complex hydroxide
formation. Even though particles with high crystallinity can be synthesized with
this method, processes are complicated to control. This method is suitable for
synthesizing nanoparticles in the powdered form [34].
Solvothermal based synthesis involves the formation of NPs in liquid phase
solvents such as water or organic compounds like methanol, polyol, and ethanol.
These solvents are allowed to heat above their boiling temperature in a pressure
vessel. It uses a metallic precursor, a reducing agent (reductant), and a solvent medium
as essential reagents and sometimes ligands and facets specific capping agents for
controlled synthesis. Narrow sized distribution of monodispersed crystals of NPs
can be synthesized using this method. With the help of a microwave, kinetics of the
crystallization step can be increased by the orders of one or two. MNPs such as Pt,
Pd, Au, Ag, Rh, Ru, Co, and Ni NPs can be rapidly synthesized by this method with
the help of methanol or polyethylene glycol (PEG) as reducing agent [34, 37].
Research analysis showed that the physicochemical properties of MNPs could
change with morphology and size. Hence, controlling the synthesis to achieve the
desired size and morphology is required, and is very challenging. The key parameters to tune the shape, size and crystallinity of MNPs by the solvothermal method
are temperature and reaction time, type of solvent, ligand type and its concentration, facet specific capping agent, and reductant. Temperature and reaction time have
a huge impact on controlling the synthesis of MNPs. Since the synthesis carries
out at a high temperature above the boiling temperature of solvents, its properties change; viscosity decreases, mobility increases, and dielectric constant reduces.
Metal precursor decomposes itself at high temperatures, or some can be carbonized.
Further, at high temperature, the reactivity of reactant increases, hence the reduction
rate of metal precursor increases, results in the formation of a lot of nuclei and a
decrease in the size of particles, as shown in Fig. 4a–b. Here Pd/Pt NPs synthesized
at 180 °C is smaller in size compared to those prepared at 120 °C [37, 38, 41]. High