1 3
Topics in Current Chemistry (2020) 378:40
precursor as the reaction proceeds differently depending on the way the precursor
is broken down [48].
There are several other factors that could affect the size and morphology of particles, such as temperature, nature of the solvent, reactants ratio, reflux time, and seed
concentration [45, 49, 50]. Thus, Hyeon found that the heating rate of the reaction,
along with the boiling point of the solvent used, is also a crucial factor to adjust
the size of the nanoparticles [45], and Pellegrino’s group concluded that there is
an inverse relationship between the size of the nanoparticles and the heating rate
[49]. However, controversy still exists regarding the role of the temperature ramp in
the synthesis of IONPs, and, therefore, comprehensive and deeper studies are still
needed to properly elucidate the mechanism involved.
Kovalenko et al. [51] showed the importance of surfactants, not only to prevent
aggregation, but also to modulate shape and size. They displayed the use of fatty
acids, such as oleic acid (OA) or salts of OA, to synthesize spheres and cubic nanoparticles, respectively. Later, several groups have tried to shed light on the role of
OA as well as other fatty acids regarding the size and shape of IONPs, but up to
now, a fully elucidated theory is still lacking [52–59]. Quality of particles can be
further improved by the controlled addition of water and oxygen in the inert environment to decrease crystal defects, and improve magnetic properties and homogeneity
[60, 61].
In summary, thermal decomposition, albeit a bit complex and time-consuming,
yields very homogenous and monodisperse nanoparticles, making it one of the most
used methods to synthesize nanoparticles for biological applications. The shape and
size of nanoparticles can be controlled by tuning the parameters described above.
Major drawbacks of this method include the inability to properly scale up and the
lack of dispersibility of the particles in aqueous solvents, although this can be remedied by surface modifications in situ, as described by Li et al. [56, 62], or using post
preparative methods, as explained in greater detail in later sections of this review.
2.3 Hydrothermal and Solvothermal Synthesis
In this method, the hydrolysis and oxidation (or neutralization) reaction takes place
in a reactor or autoclave at high temperature and pressure. Depending on the reaction solvent, it is either referred to as hydrothermal (if the solvent is water) or solvothermal (any other solvent or combination). Both reactions follow the aforementioned model of nucleation and growth [63, 64]. There have been several reports [4,
37, 65–68] on the use of this method to synthesize magnetic nanoparticles as well as
its comparison with other methods [69].
The reaction parameters, such as temperature, reactor size, time, concentration of
the reactants, and the nature of the solvent and capping agents, affect the size, shape
and other properties of the final product. Out of all these parameters, the effect of the
solvent has been studied the most [70, 71], closely followed by that of the surfactant
[72, 73]. The particles show a preferential surface binding towards the carboxylate
from the OA rather than the amine from the oleylamine [72], which very likely is the
case for every method described in this article, although it still needs verification.
55
Reprinted from the journal
Topics in Current Chemistry (2020) 378:40
precursor as the reaction proceeds differently depending on the way the precursor
is broken down [48].
There are several other factors that could affect the size and morphology of particles, such as temperature, nature of the solvent, reactants ratio, reflux time, and seed
concentration [45, 49, 50]. Thus, Hyeon found that the heating rate of the reaction,
along with the boiling point of the solvent used, is also a crucial factor to adjust
the size of the nanoparticles [45], and Pellegrino’s group concluded that there is
an inverse relationship between the size of the nanoparticles and the heating rate
[49]. However, controversy still exists regarding the role of the temperature ramp in
the synthesis of IONPs, and, therefore, comprehensive and deeper studies are still
needed to properly elucidate the mechanism involved.
Kovalenko et al. [51] showed the importance of surfactants, not only to prevent
aggregation, but also to modulate shape and size. They displayed the use of fatty
acids, such as oleic acid (OA) or salts of OA, to synthesize spheres and cubic nanoparticles, respectively. Later, several groups have tried to shed light on the role of
OA as well as other fatty acids regarding the size and shape of IONPs, but up to
now, a fully elucidated theory is still lacking [52–59]. Quality of particles can be
further improved by the controlled addition of water and oxygen in the inert environment to decrease crystal defects, and improve magnetic properties and homogeneity
[60, 61].
In summary, thermal decomposition, albeit a bit complex and time-consuming,
yields very homogenous and monodisperse nanoparticles, making it one of the most
used methods to synthesize nanoparticles for biological applications. The shape and
size of nanoparticles can be controlled by tuning the parameters described above.
Major drawbacks of this method include the inability to properly scale up and the
lack of dispersibility of the particles in aqueous solvents, although this can be remedied by surface modifications in situ, as described by Li et al. [56, 62], or using post
preparative methods, as explained in greater detail in later sections of this review.
2.3 Hydrothermal and Solvothermal Synthesis
In this method, the hydrolysis and oxidation (or neutralization) reaction takes place
in a reactor or autoclave at high temperature and pressure. Depending on the reaction solvent, it is either referred to as hydrothermal (if the solvent is water) or solvothermal (any other solvent or combination). Both reactions follow the aforementioned model of nucleation and growth [63, 64]. There have been several reports [4,
37, 65–68] on the use of this method to synthesize magnetic nanoparticles as well as
its comparison with other methods [69].
The reaction parameters, such as temperature, reactor size, time, concentration of
the reactants, and the nature of the solvent and capping agents, affect the size, shape
and other properties of the final product. Out of all these parameters, the effect of the
solvent has been studied the most [70, 71], closely followed by that of the surfactant
[72, 73]. The particles show a preferential surface binding towards the carboxylate
from the OA rather than the amine from the oleylamine [72], which very likely is the
case for every method described in this article, although it still needs verification.
55
Reprinted from the journal