1 3
Topics in Current Chemistry (2020) 378:40
the mechanism, or explain why the shape of the CTAB nanodroplets changes upon
varying concentration. Nor did they explain why particle size changed with concentration [128]. Bonachhi et al. [129] achieved ultra-small magnetic nanoparticles by
using γ-cyclodextrin by hydrolyzing Fe
2+
ions in aqueous solution, while Lee et al.
[130] varied the ratios of the precursor and solvent from 3.6 to 8.1, and achieved 2 to
10 nm magnetite particles. Vidal et al. showed the importance of oleylamine as surfactant to prevent aggregation [131], while Pileni et al. explained the importance of
using functionalized surfactants and pH to improve the crystallinity and morphology
of the nanoparticles [132]. Following a similar approach, Han et al. used a nonionic
surfactant, C 16 E 15 , to synthesize nanoparticles with high saturation magnetization
(74.8 emu/g) [133]. It is worth mentioning that if the surfactant described in this
method is replaced by a phospholipidic molecule to form particles within liposomes,
they are termed magnetoliposomes, which show significantly higher blood half-life
[134–136]. However, if the particles are formed within the aqueous compartment,
they are known as magnetovesicles. These special particles can be synthesized using
film hydration and extrusion [137], sonication [66], phase evaporation [138] and
nanoreactor [139], and are very promising for biomedical applications.
Recently, even metallosurfactants have been used as precursors to synthesize particles of around 3 nm [140]. This method has also been utilized in exchanging the
capping of iron oxide nanoparticles to improve solubility [141–143].
Similarly, oil-in-water has a hydrophilic solution with oil droplets used as a reactor. Recently, spinel ferrites have been shown to be synthesized using this method,
with metal ethylhexanoates as precursors and a pseudo ternary solvent system,
which includes oil, surfactant and water in the ratios of 20:20:60 [144]. The oil in
water method has also been used as a strategy to cap nanoparticles [145].
The microemulsion method has several advantages, such as providing a narrow
range of particles with relative ease, good morphology and without the need for
high temperatures. But it also has disadvantages, including scalability, the toxicity
of some surfactants, the amount of surfactant used, as well as the need for ligand
exchange.
2.7 Aerosol Method
This is also a chemical method, which leads to high production of particles. This
method can be subdivided in two categories. The first is spray pyrolysis, in which
precursor salts are sprayed into the reactors, where they are condensed and solvent
is evaporated, which in turn also means that the size of the particles depends on the
droplets [146].
Serna’s group [147] were among the first to synthesize Fe 2 O 3 nanoparticles using
this method. Their study claimed that if small size is the most important feature
for the application, iron acetylacetonate should be used because of its exothermic
decomposition reaction; however, if crystallinity is to be considered, then iron chloride is favored due to solvent elimination at higher temperature. This leaves other
precursor benefits open for exploration. The importance of intraparticle reactions in
controlling the size of particles was established later, along with the solvent, rate
59
Reprinted from the journal
Topics in Current Chemistry (2020) 378:40
the mechanism, or explain why the shape of the CTAB nanodroplets changes upon
varying concentration. Nor did they explain why particle size changed with concentration [128]. Bonachhi et al. [129] achieved ultra-small magnetic nanoparticles by
using γ-cyclodextrin by hydrolyzing Fe
2+
ions in aqueous solution, while Lee et al.
[130] varied the ratios of the precursor and solvent from 3.6 to 8.1, and achieved 2 to
10 nm magnetite particles. Vidal et al. showed the importance of oleylamine as surfactant to prevent aggregation [131], while Pileni et al. explained the importance of
using functionalized surfactants and pH to improve the crystallinity and morphology
of the nanoparticles [132]. Following a similar approach, Han et al. used a nonionic
surfactant, C 16 E 15 , to synthesize nanoparticles with high saturation magnetization
(74.8 emu/g) [133]. It is worth mentioning that if the surfactant described in this
method is replaced by a phospholipidic molecule to form particles within liposomes,
they are termed magnetoliposomes, which show significantly higher blood half-life
[134–136]. However, if the particles are formed within the aqueous compartment,
they are known as magnetovesicles. These special particles can be synthesized using
film hydration and extrusion [137], sonication [66], phase evaporation [138] and
nanoreactor [139], and are very promising for biomedical applications.
Recently, even metallosurfactants have been used as precursors to synthesize particles of around 3 nm [140]. This method has also been utilized in exchanging the
capping of iron oxide nanoparticles to improve solubility [141–143].
Similarly, oil-in-water has a hydrophilic solution with oil droplets used as a reactor. Recently, spinel ferrites have been shown to be synthesized using this method,
with metal ethylhexanoates as precursors and a pseudo ternary solvent system,
which includes oil, surfactant and water in the ratios of 20:20:60 [144]. The oil in
water method has also been used as a strategy to cap nanoparticles [145].
The microemulsion method has several advantages, such as providing a narrow
range of particles with relative ease, good morphology and without the need for
high temperatures. But it also has disadvantages, including scalability, the toxicity
of some surfactants, the amount of surfactant used, as well as the need for ligand
exchange.
2.7 Aerosol Method
This is also a chemical method, which leads to high production of particles. This
method can be subdivided in two categories. The first is spray pyrolysis, in which
precursor salts are sprayed into the reactors, where they are condensed and solvent
is evaporated, which in turn also means that the size of the particles depends on the
droplets [146].
Serna’s group [147] were among the first to synthesize Fe 2 O 3 nanoparticles using
this method. Their study claimed that if small size is the most important feature
for the application, iron acetylacetonate should be used because of its exothermic
decomposition reaction; however, if crystallinity is to be considered, then iron chloride is favored due to solvent elimination at higher temperature. This leaves other
precursor benefits open for exploration. The importance of intraparticle reactions in
controlling the size of particles was established later, along with the solvent, rate
59
Reprinted from the journal