Topics in Current Chemistry (2020) 378:12
1 3
reducing and stabilizing agents. Green synthesis of AgNPs has become very trendy
in the past few years [75]. This methodology has gained increasing attention due
to its eco-friendliness and low cost [76]. There is a great variety of plants that have
been reported as agents for AgNPs synthesis under mild conditions [75, 77]. Some
of the most common plant extracts that can behave as reducing agents and stabilizers are from Polyalthia longifolia [78] or Moringa oleifera [79]. Some families
of bacteria, for example Pseudomonas [80], have been used for the preparation of
colloidal silver. Other methodologies for obtaining AgNPs (common to AuNPs) are
based on microemulsion techniques [81], microwave heating [82], and laser ablation
[83] (Fig. 4).
3.3 Synthesis of IONPs
As described for AuNPs and AgNPs, IONPs can be synthesized through a large
diversity of methods, and these can be categorized into physical, chemical and green
syntheses. Chemical methods are most often used, particularly chemical co-precipitation and thermal decomposition.
In 1981, Massart first reported the preparation of magnetite NPs in alkaline
media using FeCl 3 and FeCl 2 as precursors in a molar ratio 1:2 (Fe
2+
/Fe
3+
) under
inert atmosphere (Eq. 4) [84].
This procedure allows a large mass of NPs to be obtained but with broad size
distribution and low crystallinity. Unlike chemical coprecipitation, thermal decomposition of organic complexes produces NPs with narrow size distributions and high
crystallinity, which makes it the most attractive method to prepare IONPs for biomedical applications [85, 86]. This method is based on the thermal decomposition
of organic complexes of iron, such as iron oleate [87], pentacarbonyliron(0) [88],
Fe(acac) 3 [89] or ferrocene [90]. Other chemical strategies are sol–gel [91], oxidation [92], and solvothermal reactions [93]. The laser ablation technique allows preparation of NPs with various sizes and compositions by modifying the wavelength,
pulse duration, and power of the laser [94]. On the other hand, green synthesis of
IONPs has gained much attention recently due to their good reproducibility, low
cost, high yields, mild reactions conditions and biocompatibility [95].
4 Stabilization and Surface Functionalization
4.1 Functional Groups Relevant for NP Modification
In the case of noble metal NPs (e.g., Au and Ag), thiolated ligands are essentially
used to cover the surface due to the strong Au–S and Ag–S interaction (approximately 200 kJ mol
−1
[96]). In the case of IONPs, the most employed coating agents
are carboxylates, phosphates, hydroxyls, and in some cases thiols, although oxygenated groups are preferred for modifying IONPs [97] (Fig. 5). The choice of the
(4)
Fe
2+ (ac) + 2Fe
3+ (ac) + 8OH
− (ac) → Fe 3 O 4 (s) + 4H 2 O.
102
Reprinted from the journal
1 3
reducing and stabilizing agents. Green synthesis of AgNPs has become very trendy
in the past few years [75]. This methodology has gained increasing attention due
to its eco-friendliness and low cost [76]. There is a great variety of plants that have
been reported as agents for AgNPs synthesis under mild conditions [75, 77]. Some
of the most common plant extracts that can behave as reducing agents and stabilizers are from Polyalthia longifolia [78] or Moringa oleifera [79]. Some families
of bacteria, for example Pseudomonas [80], have been used for the preparation of
colloidal silver. Other methodologies for obtaining AgNPs (common to AuNPs) are
based on microemulsion techniques [81], microwave heating [82], and laser ablation
[83] (Fig. 4).
3.3 Synthesis of IONPs
As described for AuNPs and AgNPs, IONPs can be synthesized through a large
diversity of methods, and these can be categorized into physical, chemical and green
syntheses. Chemical methods are most often used, particularly chemical co-precipitation and thermal decomposition.
In 1981, Massart first reported the preparation of magnetite NPs in alkaline
media using FeCl 3 and FeCl 2 as precursors in a molar ratio 1:2 (Fe
2+
/Fe
3+
) under
inert atmosphere (Eq. 4) [84].
This procedure allows a large mass of NPs to be obtained but with broad size
distribution and low crystallinity. Unlike chemical coprecipitation, thermal decomposition of organic complexes produces NPs with narrow size distributions and high
crystallinity, which makes it the most attractive method to prepare IONPs for biomedical applications [85, 86]. This method is based on the thermal decomposition
of organic complexes of iron, such as iron oleate [87], pentacarbonyliron(0) [88],
Fe(acac) 3 [89] or ferrocene [90]. Other chemical strategies are sol–gel [91], oxidation [92], and solvothermal reactions [93]. The laser ablation technique allows preparation of NPs with various sizes and compositions by modifying the wavelength,
pulse duration, and power of the laser [94]. On the other hand, green synthesis of
IONPs has gained much attention recently due to their good reproducibility, low
cost, high yields, mild reactions conditions and biocompatibility [95].
4 Stabilization and Surface Functionalization
4.1 Functional Groups Relevant for NP Modification
In the case of noble metal NPs (e.g., Au and Ag), thiolated ligands are essentially
used to cover the surface due to the strong Au–S and Ag–S interaction (approximately 200 kJ mol
−1
[96]). In the case of IONPs, the most employed coating agents
are carboxylates, phosphates, hydroxyls, and in some cases thiols, although oxygenated groups are preferred for modifying IONPs [97] (Fig. 5). The choice of the
(4)
Fe
2+ (ac) + 2Fe
3+ (ac) + 8OH
− (ac) → Fe 3 O 4 (s) + 4H 2 O.
102
Reprinted from the journal