1 3
Topics in Current Chemistry (2020) 378:40
[161]. The synthesis of particles in the presence of different ligands has also been
performed, giving rise to particles between 5 and 16 nm [162].
Vijayakumar et al. [163] used a similar route to synthesize IONPs. They proposed
a mechanism stating that ultrasonic waves produce the vaporization of water and
further pyrolyzation into H and OH radicals due to prolonged temperature and pressure, which leads to the formation of hydrogen (H 2 ) and hydrogen peroxide (H 2 O 2 )
from the reaction between H 2 and hydroxyl radicals, respectively. Meanwhile, the
same energy also breaks down iron acetate into Fe(II) ions. These Fe(II) ions are
later oxidized to Fe(III) using H 2 O 2 as oxidant and forming Fe 3 O 4 by using OH radicals [163]. There are several studies showing the effect of surfactants on the particles. Mukh-Qasim et al. [164] used SDS as stabilizer to get around 8.5 nm amorphous but water dispersible Fe 3 O 4 particles, while Rahamwati et al. used iron sands
along with different concentrations of PEG-6000. This latter group showed that, as
PEG concentrations increased, the crystallite size of the particles increased [165].
They also showed that the morphology of the particles shifted from flower-like
to cubes to spheres with increasing PEG concentrations. Kim et al. [166] synthesized OA-capped IONPs, which form a ferrofluid when dispersed in chitosan, with
a hydrodynamic diameter of 65 nm, thus being potential MRI CAs. This method
can also be used to synthesize composite nanoparticles [167] or other ferrites [168,
169].This method has also been used for surface functionalization in very short time
[170].
The main advantage of this method is its accelerated nature to produce nanoparticles with good yield, but it falls short when it comes to phase homogeneity.
2.9 Microwave Synthesis
This is a modern-day hydrothermal method of synthesizing nanoparticles and one of
the most used in recent days due to its much-improved kinetics of crystallization. It
requires as low as 10 s and yields small and monodisperse particles due to homogenous heating [171].
Palchik et al. were among the first to use this method in a domestic microwave
oven and suggested that the synthesis of particles was happening due to thermal
breakdown of Fe(CO) 5 , which in turn was taking place due to heating of chlorobenzene, since Fe(CO) 5 is a microwave resistant compound [172]. This indirectly marks
the importance of the solvent. On the other hand, Liu et al. demonstrated the importance of water in maintaining a stable heating environment, along with the role of
stoichiometry [173]. Another important parameter that have been studied extensively
is the nature of the surfactant, with studies reporting the use of different concentrations of OA [174], amino acids [175], polyethylene glycol (PEG) [176], and different
ratios of OA and oleylamine (OLA) [177]. OA is shown to increase saturation magnetization with increasing concentration, with no definite trend in size. However,
concentrations beyond 0.35 mmol/dm
3
led to agglomeration and the product became
difficult to isolate. Recently, amino acids such as glycine have been shown to reduce
the crystallite size of IONPs, opening the path to explore other amino acids [175].
The presence of PEG in the reaction has also been shown to lead to smaller IONPs,
61
Reprinted from the journal
Topics in Current Chemistry (2020) 378:40
[161]. The synthesis of particles in the presence of different ligands has also been
performed, giving rise to particles between 5 and 16 nm [162].
Vijayakumar et al. [163] used a similar route to synthesize IONPs. They proposed
a mechanism stating that ultrasonic waves produce the vaporization of water and
further pyrolyzation into H and OH radicals due to prolonged temperature and pressure, which leads to the formation of hydrogen (H 2 ) and hydrogen peroxide (H 2 O 2 )
from the reaction between H 2 and hydroxyl radicals, respectively. Meanwhile, the
same energy also breaks down iron acetate into Fe(II) ions. These Fe(II) ions are
later oxidized to Fe(III) using H 2 O 2 as oxidant and forming Fe 3 O 4 by using OH radicals [163]. There are several studies showing the effect of surfactants on the particles. Mukh-Qasim et al. [164] used SDS as stabilizer to get around 8.5 nm amorphous but water dispersible Fe 3 O 4 particles, while Rahamwati et al. used iron sands
along with different concentrations of PEG-6000. This latter group showed that, as
PEG concentrations increased, the crystallite size of the particles increased [165].
They also showed that the morphology of the particles shifted from flower-like
to cubes to spheres with increasing PEG concentrations. Kim et al. [166] synthesized OA-capped IONPs, which form a ferrofluid when dispersed in chitosan, with
a hydrodynamic diameter of 65 nm, thus being potential MRI CAs. This method
can also be used to synthesize composite nanoparticles [167] or other ferrites [168,
169].This method has also been used for surface functionalization in very short time
[170].
The main advantage of this method is its accelerated nature to produce nanoparticles with good yield, but it falls short when it comes to phase homogeneity.
2.9 Microwave Synthesis
This is a modern-day hydrothermal method of synthesizing nanoparticles and one of
the most used in recent days due to its much-improved kinetics of crystallization. It
requires as low as 10 s and yields small and monodisperse particles due to homogenous heating [171].
Palchik et al. were among the first to use this method in a domestic microwave
oven and suggested that the synthesis of particles was happening due to thermal
breakdown of Fe(CO) 5 , which in turn was taking place due to heating of chlorobenzene, since Fe(CO) 5 is a microwave resistant compound [172]. This indirectly marks
the importance of the solvent. On the other hand, Liu et al. demonstrated the importance of water in maintaining a stable heating environment, along with the role of
stoichiometry [173]. Another important parameter that have been studied extensively
is the nature of the surfactant, with studies reporting the use of different concentrations of OA [174], amino acids [175], polyethylene glycol (PEG) [176], and different
ratios of OA and oleylamine (OLA) [177]. OA is shown to increase saturation magnetization with increasing concentration, with no definite trend in size. However,
concentrations beyond 0.35 mmol/dm
3
led to agglomeration and the product became
difficult to isolate. Recently, amino acids such as glycine have been shown to reduce
the crystallite size of IONPs, opening the path to explore other amino acids [175].
The presence of PEG in the reaction has also been shown to lead to smaller IONPs,
61
Reprinted from the journal