46
K. Illath et al.
formation of bigger triangular plates through Ostwald ripening. The resulted triangular plates are bounded by two {111} planes (top and bottom) and three {100} planes
as side facets. Further increase in MR results in truncated cubic shape (not shown).
When the MR becomes relatively high, NPs are of quasi-spherical polyhedrons in
excess with few nanorods, triangular, and rhombic plates [48].
Synthesizing Cu NPs in aqueous media is very challenging compared to Au
and Ag NPs, due to its oxidative nature during and after the synthesis. Hence, it
always prepares in non-aqueous media under an inert atmosphere. The solvothermal
based synthesis uses hydrazine, ascorbate, NaBH 4 , polyol, and isopropyl alcohol as
reducing agents with a surfactant like a hexadecyl trimethyl bromide (CTAB). The
generally used metallic salt is cupric chloride [28]. Solvothermal based synthesis
is usually preferred for preparing particles (Pt, Pd, Ag, and Au) with controlled
facets which have applications mainly in catalysis. Few studies were reported that
solvothermal based particles could be used in the biomedical application by tuning
their plasmonic behaviour. Particles synthesized in the organic phase has to be transferred to aqueous media, followed by bio-functionalization to achieve biostability
[55]. Let’s see another important liquid state synthesis method, with more control over
the particles properties compared to solvothermal, known as the chemical reduction
method.
In the chemical reduction method, ionic salt is reduced with the help of a reducing
agent in the presence of a surfactant. This is the simplest method to synthesize
MNPs. The commonly employed reducing agent for the synthesis of MNPs is NaBH 4 .
Synthesized NPs are capped with the help of appropriate capping agents such as
trisodium citrate or sodium lauryl sulphate, and sometimes they are stabilized using
the stabilizing agents also [34].
Synthesizing monodispersed particles demand reasonable control of kinetics of
nucleation and growth steps [56]. It, in turn, requires successful dissolution among
these steps. Nucleation and growth steps happen almost simultaneously, hence
reducing the concentration of reagents can limit the nucleation. Otherwise, the growth
step has to be favoured by adjusting the temperature to limit nucleation. This problem
can be solved with the help of weak reducing agents like trisodium citrate [57] or
adopting a seed-mediated growth process. In the seed-mediated process, seed NPs
prepared in advance form the seed for the synthesis of various-shaped particles. It was
introduced first by Murphy et al. to synthesize bigger Au NPs [56]. The ratio of seed
to the metal salt concentration can control the size of NPs (will discuss later) [56].
Compared to seeded growth synthesis, the unseeded method produces low aspect
ratio NPs since it is sensitive to synthesis parameters [57]. By adding polymers and
surfactants, the growth of MNPs can be confined [58]. It has been found that facecentred cubic (fcc) metals with different shapes can be synthesized easily with the
seed-mediated growth process.
Evolution to seed from nuclei decides the final formation of nanocrystals. Seed
can be single crystal, singly twinned, multiply twinned, and plate with stacking
faults. Single crystal seed can synthesize polyhedron shapes such as octahedron,
cuboctahedron, and cubes. Surface activation of these structures can yield anisotropic
nanocrystals like octagonal rod and bar. Right bipyramids can be obtained from
K. Illath et al.
formation of bigger triangular plates through Ostwald ripening. The resulted triangular plates are bounded by two {111} planes (top and bottom) and three {100} planes
as side facets. Further increase in MR results in truncated cubic shape (not shown).
When the MR becomes relatively high, NPs are of quasi-spherical polyhedrons in
excess with few nanorods, triangular, and rhombic plates [48].
Synthesizing Cu NPs in aqueous media is very challenging compared to Au
and Ag NPs, due to its oxidative nature during and after the synthesis. Hence, it
always prepares in non-aqueous media under an inert atmosphere. The solvothermal
based synthesis uses hydrazine, ascorbate, NaBH 4 , polyol, and isopropyl alcohol as
reducing agents with a surfactant like a hexadecyl trimethyl bromide (CTAB). The
generally used metallic salt is cupric chloride [28]. Solvothermal based synthesis
is usually preferred for preparing particles (Pt, Pd, Ag, and Au) with controlled
facets which have applications mainly in catalysis. Few studies were reported that
solvothermal based particles could be used in the biomedical application by tuning
their plasmonic behaviour. Particles synthesized in the organic phase has to be transferred to aqueous media, followed by bio-functionalization to achieve biostability
[55]. Let’s see another important liquid state synthesis method, with more control over
the particles properties compared to solvothermal, known as the chemical reduction
method.
In the chemical reduction method, ionic salt is reduced with the help of a reducing
agent in the presence of a surfactant. This is the simplest method to synthesize
MNPs. The commonly employed reducing agent for the synthesis of MNPs is NaBH 4 .
Synthesized NPs are capped with the help of appropriate capping agents such as
trisodium citrate or sodium lauryl sulphate, and sometimes they are stabilized using
the stabilizing agents also [34].
Synthesizing monodispersed particles demand reasonable control of kinetics of
nucleation and growth steps [56]. It, in turn, requires successful dissolution among
these steps. Nucleation and growth steps happen almost simultaneously, hence
reducing the concentration of reagents can limit the nucleation. Otherwise, the growth
step has to be favoured by adjusting the temperature to limit nucleation. This problem
can be solved with the help of weak reducing agents like trisodium citrate [57] or
adopting a seed-mediated growth process. In the seed-mediated process, seed NPs
prepared in advance form the seed for the synthesis of various-shaped particles. It was
introduced first by Murphy et al. to synthesize bigger Au NPs [56]. The ratio of seed
to the metal salt concentration can control the size of NPs (will discuss later) [56].
Compared to seeded growth synthesis, the unseeded method produces low aspect
ratio NPs since it is sensitive to synthesis parameters [57]. By adding polymers and
surfactants, the growth of MNPs can be confined [58]. It has been found that facecentred cubic (fcc) metals with different shapes can be synthesized easily with the
seed-mediated growth process.
Evolution to seed from nuclei decides the final formation of nanocrystals. Seed
can be single crystal, singly twinned, multiply twinned, and plate with stacking
faults. Single crystal seed can synthesize polyhedron shapes such as octahedron,
cuboctahedron, and cubes. Surface activation of these structures can yield anisotropic
nanocrystals like octagonal rod and bar. Right bipyramids can be obtained from