Topics in Current Chemistry (2020) 378:40
1 3
as compared to the reaction in its absence. When the reaction is performed in the
presence of PEG, it tends to favor the formation of magnetite instead of maghemite.
This happens due to PEG being sacrificial in nature and thus preventing oxidation.
High microwave power and low synthesis time also favors the formation of maghemite [176]. Other studies have shown that the presence of OA during the synthesis,
along with OLA, reduces aggregation among particles [177]. Temperature has also
been shown to transform phases in IONPs [178]. Blanco-Andujar et al. proposed
a facile method to synthesize citric acid coated IONPs and potentially scale them
up [179]. The importance of aging temperature on crystallinity can be seen when
Fe 2 O 3 nanocubes are synthesized by decomposing iron oleate in a microwave and
aging it in an autoclave at 180 ℃ for different time intervals [180]. Particles aged for
20 h showed cubic shape and higher saturation magnetization. Hu et al. [181] argued
that the precursor is the most important parameter by synthesizing three phases of
iron oxide, hematite, magnetite and maghemite, using FeCl 3 alone or in combination
with FeCl 2 .
Literature suggests that the morphology and composition of the particles
can also be controlled using this method. Different morphologies, such as lamellar sheets [182], octahedrons [182] and hexagonal plates [183] are synthesized by
slight changes in salts. Cu-doped IONPs with good colloidal stability are obtained
in 10 min [184] using the microwave method. In fact, even using a domestic microwave, sizes of 8–10 nm can be easily achieved [185].
This method has been shown to be better than hydrothermal [186] or thermal
decomposition [187] in terms of size, crystallinity and saturation magnetization.
However, particles thus synthesized display lower surface reactivity than those synthesized using the thermal decomposition method, although with more ease of stabilization. The versatility of this method is acknowledged by its association with
different methods: coprecipitation [179], thermal decomposition, [177] polyol [188]
and sol–gel methods [189]. The particle size can be varied by modulating the power
and hence the temperature, the time spent in the reactor, the cooling rate, etc. This
method has become more popular recently due to its multiple advantages.
2.10 Biosynthesis
This is an eco-friendly method as most of the constituents needed are available from
nature directly or indirectly. It generally involves the use of microbes [190] or plant
extracts [191] to synthesize nanoparticles. Lovely et al. [192] were the first to use a
microbe, GS-15, to form magnetite nanoparticles. Thereafter, many different magnetic
bacterial strains were found and studied in order to produce IONPs [193–197].These
nanoparticles are formed by the reduction/hydrolyzing capabilities of these biological
entities. However, when a bacterium is used, its nature as well as its incubation time
becomes an important parameter since it allows changes in size and morphology [198,
199]. Even fungi such as Fusarium oxysporum and Verticillium sp., have been shown
to possess hydrolyzing capabilities to form different sizes and shapes of nanoparticles
[200]. Viruses such as tobacco mosaic virus (TMV) have also been used as templates
to synthesize nanotubes [201]. Iron oxides formed by microbial reduction have been
62
Reprinted from the journal
1 3
as compared to the reaction in its absence. When the reaction is performed in the
presence of PEG, it tends to favor the formation of magnetite instead of maghemite.
This happens due to PEG being sacrificial in nature and thus preventing oxidation.
High microwave power and low synthesis time also favors the formation of maghemite [176]. Other studies have shown that the presence of OA during the synthesis,
along with OLA, reduces aggregation among particles [177]. Temperature has also
been shown to transform phases in IONPs [178]. Blanco-Andujar et al. proposed
a facile method to synthesize citric acid coated IONPs and potentially scale them
up [179]. The importance of aging temperature on crystallinity can be seen when
Fe 2 O 3 nanocubes are synthesized by decomposing iron oleate in a microwave and
aging it in an autoclave at 180 ℃ for different time intervals [180]. Particles aged for
20 h showed cubic shape and higher saturation magnetization. Hu et al. [181] argued
that the precursor is the most important parameter by synthesizing three phases of
iron oxide, hematite, magnetite and maghemite, using FeCl 3 alone or in combination
with FeCl 2 .
Literature suggests that the morphology and composition of the particles
can also be controlled using this method. Different morphologies, such as lamellar sheets [182], octahedrons [182] and hexagonal plates [183] are synthesized by
slight changes in salts. Cu-doped IONPs with good colloidal stability are obtained
in 10 min [184] using the microwave method. In fact, even using a domestic microwave, sizes of 8–10 nm can be easily achieved [185].
This method has been shown to be better than hydrothermal [186] or thermal
decomposition [187] in terms of size, crystallinity and saturation magnetization.
However, particles thus synthesized display lower surface reactivity than those synthesized using the thermal decomposition method, although with more ease of stabilization. The versatility of this method is acknowledged by its association with
different methods: coprecipitation [179], thermal decomposition, [177] polyol [188]
and sol–gel methods [189]. The particle size can be varied by modulating the power
and hence the temperature, the time spent in the reactor, the cooling rate, etc. This
method has become more popular recently due to its multiple advantages.
2.10 Biosynthesis
This is an eco-friendly method as most of the constituents needed are available from
nature directly or indirectly. It generally involves the use of microbes [190] or plant
extracts [191] to synthesize nanoparticles. Lovely et al. [192] were the first to use a
microbe, GS-15, to form magnetite nanoparticles. Thereafter, many different magnetic
bacterial strains were found and studied in order to produce IONPs [193–197].These
nanoparticles are formed by the reduction/hydrolyzing capabilities of these biological
entities. However, when a bacterium is used, its nature as well as its incubation time
becomes an important parameter since it allows changes in size and morphology [198,
199]. Even fungi such as Fusarium oxysporum and Verticillium sp., have been shown
to possess hydrolyzing capabilities to form different sizes and shapes of nanoparticles
[200]. Viruses such as tobacco mosaic virus (TMV) have also been used as templates
to synthesize nanotubes [201]. Iron oxides formed by microbial reduction have been
62
Reprinted from the journal