Metallic Nanoparticles for Biomedical Applications
41
progress, it evolves into a regular shape, and with the help of shape-controlled capping
agents, NPs with specific facets can be formed. If the temperature is high enough, it
can reconstruct the MNPs and promote the Ostwald ripening process. Hence, the final
products are usually morphologically evolved, and longer time promotes thermodynamic growth [37, 41]. Figure 4c–g shows the effect of reaction time on synthesized
Pd nanocrystals. It can be easily observed the morphological evolution on synthesized particles; the crystals produced within 1 h has a side length of 16 nm, and it
starts to increase to 24 nm at 2 h, 38 nm at 4 h and 45 nm at 8 h. Hence, the trend is
increasing in the length of NPs as reaction time progresses. Further, it is noted that
concave morphology is shaping as the size increases. At 2 h, concave morphology is
seen, and it continues to maintain even after increasing the reaction time. After 8 h
of reaction time, there is not much significant change on morphology is observed.
Figure 4h shows the dependence of side length with reaction time. It is observed that
beyond 8 h, there is no increment in length of NPs. Hence, in solvothermal based
synthesis, reaction time can be adjusted to get a particle with specific size [39]. Similar
to size, the shape also depends on reaction time in the solvothermal treatment of Ag
NPs. In a typical solvothermal treatment in ethanol at 80 °C was performed with Ag
nanocubes at different reaction times. Initial nanocubes and other structures evolved
after 6 and 12 h of reaction time are shown in Fig. 5. From Fig. 5a–c it is clear that Ag
nanocubes are transformed into nanocubes with concave facets and then into wavey
edged NPs with 6 and 12 h of reaction time, respectively. Nanocubes obtained after
6 h have dark contrast in their {100} faces, and with 12 h of reaction time, NP yield
is increased. The dramatic change in shape might be due to oxidative etching or
Ostwald ripening. As reaction time increases to 6 h, oxidative etching happens due
to the distribution of different surface energies of Ag NPs and sufficient availability
of etching pair. But as reaction time is increased to 12 h, Ostwald ripening happens
between in situ generated multiply twinned particles and plate-like seeds [40].
The effects of solvents on the synthesized NPs were studied in detail for various
metals [43–45]. Reactants may react with solvents and form products that can significantly affect the reaction kinetics, equilibrium, and final products. Some solvents
act as structure modulator, and some can influence synthesis if surfactants are used.
Solvent properties such as viscosity, dielectric constant, and polarities are responsible for changes in particle synthesis. Further, mixtures of solvents can influence the
synthesis greatly by reacting with the other reactants used. As an example, consider
the synthesis of Ag NPs synthesized in various mixed solvent solutions by Rosa et al.
They studied the effect of Ag nanograins synthesized in solvents including a mixture
of ethanol + water, hexane + water, toluene + water, acetone + water, and ethanol
alone. It has been found that grain size increases in the order of hexane + water,
ethanol + water, toluene + water, followed by pure ethanol and acetone + water.
With acetone + water, particles were present separately in a bigger size, whereas
with hexane + water, grains are small in size present together in groups. Hence, it can
be concluded that particles synthesized in immiscible solvents are bigger compared
to the other solvents [37, 44].
The ligand, facet specific capping agents, reductant, and reactant molar ratio
can also act as tuning parameters in controlling the MNP synthesis based on the
41
progress, it evolves into a regular shape, and with the help of shape-controlled capping
agents, NPs with specific facets can be formed. If the temperature is high enough, it
can reconstruct the MNPs and promote the Ostwald ripening process. Hence, the final
products are usually morphologically evolved, and longer time promotes thermodynamic growth [37, 41]. Figure 4c–g shows the effect of reaction time on synthesized
Pd nanocrystals. It can be easily observed the morphological evolution on synthesized particles; the crystals produced within 1 h has a side length of 16 nm, and it
starts to increase to 24 nm at 2 h, 38 nm at 4 h and 45 nm at 8 h. Hence, the trend is
increasing in the length of NPs as reaction time progresses. Further, it is noted that
concave morphology is shaping as the size increases. At 2 h, concave morphology is
seen, and it continues to maintain even after increasing the reaction time. After 8 h
of reaction time, there is not much significant change on morphology is observed.
Figure 4h shows the dependence of side length with reaction time. It is observed that
beyond 8 h, there is no increment in length of NPs. Hence, in solvothermal based
synthesis, reaction time can be adjusted to get a particle with specific size [39]. Similar
to size, the shape also depends on reaction time in the solvothermal treatment of Ag
NPs. In a typical solvothermal treatment in ethanol at 80 °C was performed with Ag
nanocubes at different reaction times. Initial nanocubes and other structures evolved
after 6 and 12 h of reaction time are shown in Fig. 5. From Fig. 5a–c it is clear that Ag
nanocubes are transformed into nanocubes with concave facets and then into wavey
edged NPs with 6 and 12 h of reaction time, respectively. Nanocubes obtained after
6 h have dark contrast in their {100} faces, and with 12 h of reaction time, NP yield
is increased. The dramatic change in shape might be due to oxidative etching or
Ostwald ripening. As reaction time increases to 6 h, oxidative etching happens due
to the distribution of different surface energies of Ag NPs and sufficient availability
of etching pair. But as reaction time is increased to 12 h, Ostwald ripening happens
between in situ generated multiply twinned particles and plate-like seeds [40].
The effects of solvents on the synthesized NPs were studied in detail for various
metals [43–45]. Reactants may react with solvents and form products that can significantly affect the reaction kinetics, equilibrium, and final products. Some solvents
act as structure modulator, and some can influence synthesis if surfactants are used.
Solvent properties such as viscosity, dielectric constant, and polarities are responsible for changes in particle synthesis. Further, mixtures of solvents can influence the
synthesis greatly by reacting with the other reactants used. As an example, consider
the synthesis of Ag NPs synthesized in various mixed solvent solutions by Rosa et al.
They studied the effect of Ag nanograins synthesized in solvents including a mixture
of ethanol + water, hexane + water, toluene + water, acetone + water, and ethanol
alone. It has been found that grain size increases in the order of hexane + water,
ethanol + water, toluene + water, followed by pure ethanol and acetone + water.
With acetone + water, particles were present separately in a bigger size, whereas
with hexane + water, grains are small in size present together in groups. Hence, it can
be concluded that particles synthesized in immiscible solvents are bigger compared
to the other solvents [37, 44].
The ligand, facet specific capping agents, reductant, and reactant molar ratio
can also act as tuning parameters in controlling the MNP synthesis based on the