6 Hollow Magnetic Nanoparticles
139
However, these pure magnetic materials can present some limitations, especially
in biomedical applications, and recently other strategies have been proposed in order
to create composite hollow MNPS. These have mainly focused on the use of a nonmagnetic shell made of biocompatible materials, such as silica or carbon, and a
magnetic phase in the interior of the shell in order to grant the composite magnetic
response. For example, Son et al. [29] have synthesized silica nanotubes with a layer
of magnetite (Fe 3 O 4 ) on the inner surface for magnetic-field-assisted bioseparation,
biointeraction, and drug delivery. Magnetic carbon nanocages have been introduced
by Quin et al. [30] as efficient and recycled adsorbents in the removal of dye staff from
textile wastewater. And Zhou et al. [31] have fabricated hollow silica nanospheres
with Fe 3 O 4 nanoparticles in the core. In addition, in the last years, a great progress has
been made in the fabrication of multishelled hollow MNPs, with two or more shells,
such as metal oxides and metal ferrites, which would provide additional tunability
and improved applicability, as has been reported by Qi et al. [32], although the
synthesis procedure becomes more complex.
In this book chapter, we focus on iron-oxide-based hollow MNPs, presenting
a comprehensive report about their interest, properties, and applications. First, we
analyze the synthesis procedures and describe the most relevant magnetic properties of these nanoparticles, including surface anisotropy, shell thickness dependence,
exchange bias, etc. Then, we show how the structure and magnetism of the nanoparticles evolves as they become hollow through Kirkendall effect. The experimental
findings are complemented with Monte Carlo simulations. Finally, we describe in
detail some of the most relevant applications for these hollow MNPs, ending the
chapter with a summary and future outlook.
6.2 Synthesis of Hollow MNPS
During the past years, there have been several reports on various physical and chemical methods to synthesize hollow nanoparticles, both magnetic and non-magnetic
(CuO, C, NiO, SiO 2 , CoSe, Al 2 O 3 , etc.) [19, 28, 33, 34]. Hollow nanoparticles can
be essentially prepared by using two different fabrication methods, i.e., templateassisted method and template-free method. The template-mediated growth of hollow
nanoparticles usually gives particles of microscale size and is more favorable
to synthesize silicates, polymer-latex colloids, and polystyrene-magnetite hollow
composites [35]. The template-free methods are more versatile to produce hollow
MNPs and can be either hydrothermal or solvothermal reactions [36]. In the case of
hollow MNPs, most of the fabrication methods reported follow chemical routes,
such as templating approach, nanoscale etching, Ostwald ripening, or layer-bylayer growth [28, 37–43]. For example, monodisperse magnetite hollow MNPs with
400 nm average diameter and 60 nm shell thickness have been prepared through a
one-pot solvothermal process based on Ostwald ripening [44].
The most facile method to produce monodisperse hollow MNPs is based on the
Kirkendall effect. This method is a two-step procedure; at first, core/shell MNPs
139
However, these pure magnetic materials can present some limitations, especially
in biomedical applications, and recently other strategies have been proposed in order
to create composite hollow MNPS. These have mainly focused on the use of a nonmagnetic shell made of biocompatible materials, such as silica or carbon, and a
magnetic phase in the interior of the shell in order to grant the composite magnetic
response. For example, Son et al. [29] have synthesized silica nanotubes with a layer
of magnetite (Fe 3 O 4 ) on the inner surface for magnetic-field-assisted bioseparation,
biointeraction, and drug delivery. Magnetic carbon nanocages have been introduced
by Quin et al. [30] as efficient and recycled adsorbents in the removal of dye staff from
textile wastewater. And Zhou et al. [31] have fabricated hollow silica nanospheres
with Fe 3 O 4 nanoparticles in the core. In addition, in the last years, a great progress has
been made in the fabrication of multishelled hollow MNPs, with two or more shells,
such as metal oxides and metal ferrites, which would provide additional tunability
and improved applicability, as has been reported by Qi et al. [32], although the
synthesis procedure becomes more complex.
In this book chapter, we focus on iron-oxide-based hollow MNPs, presenting
a comprehensive report about their interest, properties, and applications. First, we
analyze the synthesis procedures and describe the most relevant magnetic properties of these nanoparticles, including surface anisotropy, shell thickness dependence,
exchange bias, etc. Then, we show how the structure and magnetism of the nanoparticles evolves as they become hollow through Kirkendall effect. The experimental
findings are complemented with Monte Carlo simulations. Finally, we describe in
detail some of the most relevant applications for these hollow MNPs, ending the
chapter with a summary and future outlook.
6.2 Synthesis of Hollow MNPS
During the past years, there have been several reports on various physical and chemical methods to synthesize hollow nanoparticles, both magnetic and non-magnetic
(CuO, C, NiO, SiO 2 , CoSe, Al 2 O 3 , etc.) [19, 28, 33, 34]. Hollow nanoparticles can
be essentially prepared by using two different fabrication methods, i.e., templateassisted method and template-free method. The template-mediated growth of hollow
nanoparticles usually gives particles of microscale size and is more favorable
to synthesize silicates, polymer-latex colloids, and polystyrene-magnetite hollow
composites [35]. The template-free methods are more versatile to produce hollow
MNPs and can be either hydrothermal or solvothermal reactions [36]. In the case of
hollow MNPs, most of the fabrication methods reported follow chemical routes,
such as templating approach, nanoscale etching, Ostwald ripening, or layer-bylayer growth [28, 37–43]. For example, monodisperse magnetite hollow MNPs with
400 nm average diameter and 60 nm shell thickness have been prepared through a
one-pot solvothermal process based on Ostwald ripening [44].
The most facile method to produce monodisperse hollow MNPs is based on the
Kirkendall effect. This method is a two-step procedure; at first, core/shell MNPs