1 3
Topics in Current Chemistry (2020) 378:40
even more, if the trend described holds to be true. Other parameters that have been
identified to modulate the size, shape, crystallinity and saturation magnetization are
temperature, time, precursor concentration, and surfactant. The role of water was
studied by Hemery et al. [93], when its importance was revealed by inability of the
anhydrous iron chloride to produce magnetic particles [93]. The impact of stoichiometry in polyol synthesis has been studied by Wetegrove et al. [94], showing that
the increase in Fe
3+
concentration forms larger crystallites and the increase in Fe
2+
content promotes nucleation [94].
As stated above, hydrophilicity is important, which is generally lacking in particles synthesized using the polyol method. However, there are reports of the synthesis
of hydrophilic nanoparticles using this method [88, 95] but their limitations include
the lack of a surface functionality for bioconjugation. This problem has further been
remedied by the use of polyamines [96], polyimine with polyol [97], polyamine with
polyol [98] and PAA [99]. The research done by Babić-Stojić et al. [100], wherein
they esterified 3 nm IONPs in situ, implied the importance of the surface layer in the
properties of nanoparticles.
The morphology of the particles is of equal importance as size in in vivo applications and has been shown to be altered by the addition of halide ions [101]. There
have also been advancements in solvents, such as the thermostable ionic solvent
[P6,6,6,14][Tf2N], which has been shown to be capable of synthesizing quasi spherical magnetite nanoparticles of around 14 nm [102].
In conclusion, the method described herein has the advantage of being environment friendly, scalable, and good for synthesizing both single and multicore particles. However, it has the drawback that the particles thus formed lack homogeneity.
2.5 Sol–Gel Method
This is a two-step chemical method, with the first step being the synthesis of the sol
(particles in a solution) via hydroxylation of the precursors, and the second step,
the formation of a gel by condensation and polymerization. Eventually, heat treatments are used to achieve a proper crystalline state. Costa et al. [19] were among
the first to synthesize magnetic nanoparticles using this method, but they failed to
identify the correct mechanism. Subsequently, the work of Portugal et al. [103],
made the mechanism a bit clearer upon finding signatures of iron hydroxide, but the
exact mechanism is still unknown. Like in the polyol method, the solvent is shown
to affect the ferrite grain as well, but changes in grain size have been attributed to a
different growth model with two different solvents [104]. Water concentration is also
shown to improve hardness and structural defects [105].
Size and shape are also affected by other parameters such as solvent ratio, time,
pH, stirring, gelating agent and, temperature. Liu et al. [106] used different calcination temperatures to synthesize different phases of IONPs, and this transformation
has been attributed to two separate mechanisms, crystal regrowth and chemisorption, depending on the temperature. Akbar et al. claimed to have synthesized three
different phases of iron oxide (α-Fe 2 O 3 , γ-Fe 2 O 3 , and Fe 3 O 4 ) simply by varying the
precursor to solvent ratio, thus suggesting the importance of that ratio [107]. The
57
Reprinted from the journal
Topics in Current Chemistry (2020) 378:40
even more, if the trend described holds to be true. Other parameters that have been
identified to modulate the size, shape, crystallinity and saturation magnetization are
temperature, time, precursor concentration, and surfactant. The role of water was
studied by Hemery et al. [93], when its importance was revealed by inability of the
anhydrous iron chloride to produce magnetic particles [93]. The impact of stoichiometry in polyol synthesis has been studied by Wetegrove et al. [94], showing that
the increase in Fe
3+
concentration forms larger crystallites and the increase in Fe
2+
content promotes nucleation [94].
As stated above, hydrophilicity is important, which is generally lacking in particles synthesized using the polyol method. However, there are reports of the synthesis
of hydrophilic nanoparticles using this method [88, 95] but their limitations include
the lack of a surface functionality for bioconjugation. This problem has further been
remedied by the use of polyamines [96], polyimine with polyol [97], polyamine with
polyol [98] and PAA [99]. The research done by Babić-Stojić et al. [100], wherein
they esterified 3 nm IONPs in situ, implied the importance of the surface layer in the
properties of nanoparticles.
The morphology of the particles is of equal importance as size in in vivo applications and has been shown to be altered by the addition of halide ions [101]. There
have also been advancements in solvents, such as the thermostable ionic solvent
[P6,6,6,14][Tf2N], which has been shown to be capable of synthesizing quasi spherical magnetite nanoparticles of around 14 nm [102].
In conclusion, the method described herein has the advantage of being environment friendly, scalable, and good for synthesizing both single and multicore particles. However, it has the drawback that the particles thus formed lack homogeneity.
2.5 Sol–Gel Method
This is a two-step chemical method, with the first step being the synthesis of the sol
(particles in a solution) via hydroxylation of the precursors, and the second step,
the formation of a gel by condensation and polymerization. Eventually, heat treatments are used to achieve a proper crystalline state. Costa et al. [19] were among
the first to synthesize magnetic nanoparticles using this method, but they failed to
identify the correct mechanism. Subsequently, the work of Portugal et al. [103],
made the mechanism a bit clearer upon finding signatures of iron hydroxide, but the
exact mechanism is still unknown. Like in the polyol method, the solvent is shown
to affect the ferrite grain as well, but changes in grain size have been attributed to a
different growth model with two different solvents [104]. Water concentration is also
shown to improve hardness and structural defects [105].
Size and shape are also affected by other parameters such as solvent ratio, time,
pH, stirring, gelating agent and, temperature. Liu et al. [106] used different calcination temperatures to synthesize different phases of IONPs, and this transformation
has been attributed to two separate mechanisms, crystal regrowth and chemisorption, depending on the temperature. Akbar et al. claimed to have synthesized three
different phases of iron oxide (α-Fe 2 O 3 , γ-Fe 2 O 3 , and Fe 3 O 4 ) simply by varying the
precursor to solvent ratio, thus suggesting the importance of that ratio [107]. The
57
Reprinted from the journal