Topics in Current Chemistry (2020) 378:12
1 3
moment within the magnetic core. In most cases, the heating power is governed by
the fastest regime of relaxation [52]. The ability of IONPs to generate heat in the
presence of an alternating magnetic field [55] has made them the base for the development of hyperthermia applications, employed mainly to induced death in tumor
cells [56]. In order to produce hyperthermia, energy dissipation phenomena must be
involved, according to Néel or Brown relaxation [57], although in most cases is difficult to distinguish which process is actually taking place.
The most studied iron oxides at the nanoscale are hematite (α-Fe 2 O 3 ), magnetite (Fe 3 O 4 ) and maghemite (γ-Fe 2 O 3 ). Hematite presents a hexagonal unit cell with
a = 0.5034 nm and c = 1.375 nm, and six formula units per cell. Hematite may also
appear in the rhombohedral system with a = 0.5427 nm and α = 55.3º, and two formula units per cell. The structure consists in an hcp array of oxygen ions along the
[001] direction and two-thirds of the sites are occupied by Fe(III) arranged regularly,
with two filled sites followed by one vacant site in the (001) plane. At bulk scale,
hematite behaves as a weak ferromagnetic material or antiferromagnetic.
On the other hand, magnetite and maghemite present a cubic unit cell, and, and
at bulk scale behave as a ferrimagnet. The structure of magnetite is typical of an
inverse spinel containing iron(II) and iron(III) distributed in its structure. Fe(III)
ions are located in both tetrahedral and octahedral sites, while Fe(II) ions occupy
only octahedral sites due to the higher ferrous crystal field stabilization energy. The
cubic unit cell has an edge length a = 0.839 nm, presenting eight formula units per
cell. Magnetite is non-stoichiometric, with cation deficiency in the Fe(III) sublattice.
Maghemite is isostructural to Fe 3 O 4 , but the majority of Fe ions are trivalent, and
cation vacancies compensate the oxidation of Fe(II). Maghemite has a cubic unit
cell with a = 0.834 nm. In the cell, eight cations occupy tetrahedral sites and the rest
are distributed randomly in octahedral sites. The vacancies are limited to octahedral
sites [58].
3 Synthetic Routes
3.1 Synthesis of AuNPs
There are numerous reports on the different methodologies for the synthesis
of AuNPs with a wide variety of sizes and morphologies (reviewed in [59, 60]).
Herein, we will describe only the most important.
The chemical synthesis of AuNPs is based on the reduction of a gold salt and
the stabilization of the NPs. The two conventional chemical methods for obtaining AuNPs are the Turkevich and the Brust-Schiffrin methods. In 1951, Turkevich reported an easy pathway to obtain spherical AuNPs by reducing hydrogen
tetrachloroaurate(III) (HAuCl 4 ) using trisodium citrate as both reducing agent and
stabilizer [61]. This method involves the use of water as a solvent, and the citrate
is added once the solution is boiling. The NPs obtained are in a range from 10 to
150 mn in size. The Turkevich method is still used widely due to its simplicity and
reproducibility. Recent studies show that citrate can be further substituted for α and
100
Reprinted from the journal
1 3
moment within the magnetic core. In most cases, the heating power is governed by
the fastest regime of relaxation [52]. The ability of IONPs to generate heat in the
presence of an alternating magnetic field [55] has made them the base for the development of hyperthermia applications, employed mainly to induced death in tumor
cells [56]. In order to produce hyperthermia, energy dissipation phenomena must be
involved, according to Néel or Brown relaxation [57], although in most cases is difficult to distinguish which process is actually taking place.
The most studied iron oxides at the nanoscale are hematite (α-Fe 2 O 3 ), magnetite (Fe 3 O 4 ) and maghemite (γ-Fe 2 O 3 ). Hematite presents a hexagonal unit cell with
a = 0.5034 nm and c = 1.375 nm, and six formula units per cell. Hematite may also
appear in the rhombohedral system with a = 0.5427 nm and α = 55.3º, and two formula units per cell. The structure consists in an hcp array of oxygen ions along the
[001] direction and two-thirds of the sites are occupied by Fe(III) arranged regularly,
with two filled sites followed by one vacant site in the (001) plane. At bulk scale,
hematite behaves as a weak ferromagnetic material or antiferromagnetic.
On the other hand, magnetite and maghemite present a cubic unit cell, and, and
at bulk scale behave as a ferrimagnet. The structure of magnetite is typical of an
inverse spinel containing iron(II) and iron(III) distributed in its structure. Fe(III)
ions are located in both tetrahedral and octahedral sites, while Fe(II) ions occupy
only octahedral sites due to the higher ferrous crystal field stabilization energy. The
cubic unit cell has an edge length a = 0.839 nm, presenting eight formula units per
cell. Magnetite is non-stoichiometric, with cation deficiency in the Fe(III) sublattice.
Maghemite is isostructural to Fe 3 O 4 , but the majority of Fe ions are trivalent, and
cation vacancies compensate the oxidation of Fe(II). Maghemite has a cubic unit
cell with a = 0.834 nm. In the cell, eight cations occupy tetrahedral sites and the rest
are distributed randomly in octahedral sites. The vacancies are limited to octahedral
sites [58].
3 Synthetic Routes
3.1 Synthesis of AuNPs
There are numerous reports on the different methodologies for the synthesis
of AuNPs with a wide variety of sizes and morphologies (reviewed in [59, 60]).
Herein, we will describe only the most important.
The chemical synthesis of AuNPs is based on the reduction of a gold salt and
the stabilization of the NPs. The two conventional chemical methods for obtaining AuNPs are the Turkevich and the Brust-Schiffrin methods. In 1951, Turkevich reported an easy pathway to obtain spherical AuNPs by reducing hydrogen
tetrachloroaurate(III) (HAuCl 4 ) using trisodium citrate as both reducing agent and
stabilizer [61]. This method involves the use of water as a solvent, and the citrate
is added once the solution is boiling. The NPs obtained are in a range from 10 to
150 mn in size. The Turkevich method is still used widely due to its simplicity and
reproducibility. Recent studies show that citrate can be further substituted for α and
100
Reprinted from the journal