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H. Khurshid et al.
6.1 Introduction
Magnetic nanoparticles (MNPs) have attracted a lot of attention in the scientific
community during the last decades due to their novel magnetic properties and their
promising applications [1–4]. These nanoparticles can exhibit a series of interesting
magnetic phenomena, such as superparamagnetism, exchange bias, and surface
disorder [6, 7], and they have been used for a wide variety of applications from
data storage [2, 8] to biomedicine [9, 10]. Many of these magnetic nanoparticles
are usually iron oxide based, due in part to their relatively large magnetic moment,
biocompatibility, and well-established synthetic methods that afford good reproducibility, narrow size distributions, and a high degree of control over their characteristics. Since the magnetic properties of these nanoparticles strongly depend on
their structural and morphological characteristics, advances in the ability to synthesize these nanoparticles have motivated different groups to develop MNPs with novel
properties and applications [11, 12].
In order to further extend the possibilities of these MNPs, researchers have tried
to tune their magnetic properties by changing their shape, size, and morphology. In
particular, recently, MNPs with a cavity inside making them hollow have been fabricated. From a magnetic point of view, this “hollow” morphology is especially interesting because the presence of both inner and outer surfaces contributes to increase of
the total surface area of the MNPs, and this can lead to enhanced surface disorder and
therefore to higher surface anisotropy and exchange bias [13–19]. Several different
kinds of hollow MNPs with enhanced properties have recently been reported in
the literature. Many of them are based on ferrites and magnetic oxides materials.
For example, Jaffari et al. [20] have described an enhancement of the surface spin
disorder in hollow NiFe 2 O 4 nanoparticles. And Shin et al. [21] have recently synthesized 20 nm hollow manganese oxide nanoparticles with a paramagnetic behavior
around room temperature.
In addition, hollow MNPs with tunable shell thickness and composition are
promising building blocks for new advanced materials with lots of potential applications. In principle, hollow morphology allows encapsulating different contents inside
the MNPs for various applications [1, 2, 22–24]. For example, anticancer drugs can
be encapsulated inside the hollow MNPs used for drug delivery applications. The
anticancer drug is camouflaged and protected inside the MNPs on its way toward the
target once introduced into the body. These hollow MNPs can be guided toward the
tumor area with an external magnetic field and make them release the anticancer drug
in a localized and controlled way without affecting other regions of the body [22, 25,
26]. In the same way, hollow MNPs have also been proposed for other applications
such as lithium-ion batteries. Hollow MNPs can store Li-ions inside, and they offer
fast diffusion for Li-ions uptake/removal [27]. Likewise, the void space in hollow
particles has been used to modulate refractive index, lower density, increase active
area for catalysis, etc. [28]. Therefore, hollow MNPs have potential applications
in a wide variety of areas, including catalysis, memristors, batteries, targeted drug
delivery, and environmental treatment.
H. Khurshid et al.
6.1 Introduction
Magnetic nanoparticles (MNPs) have attracted a lot of attention in the scientific
community during the last decades due to their novel magnetic properties and their
promising applications [1–4]. These nanoparticles can exhibit a series of interesting
magnetic phenomena, such as superparamagnetism, exchange bias, and surface
disorder [6, 7], and they have been used for a wide variety of applications from
data storage [2, 8] to biomedicine [9, 10]. Many of these magnetic nanoparticles
are usually iron oxide based, due in part to their relatively large magnetic moment,
biocompatibility, and well-established synthetic methods that afford good reproducibility, narrow size distributions, and a high degree of control over their characteristics. Since the magnetic properties of these nanoparticles strongly depend on
their structural and morphological characteristics, advances in the ability to synthesize these nanoparticles have motivated different groups to develop MNPs with novel
properties and applications [11, 12].
In order to further extend the possibilities of these MNPs, researchers have tried
to tune their magnetic properties by changing their shape, size, and morphology. In
particular, recently, MNPs with a cavity inside making them hollow have been fabricated. From a magnetic point of view, this “hollow” morphology is especially interesting because the presence of both inner and outer surfaces contributes to increase of
the total surface area of the MNPs, and this can lead to enhanced surface disorder and
therefore to higher surface anisotropy and exchange bias [13–19]. Several different
kinds of hollow MNPs with enhanced properties have recently been reported in
the literature. Many of them are based on ferrites and magnetic oxides materials.
For example, Jaffari et al. [20] have described an enhancement of the surface spin
disorder in hollow NiFe 2 O 4 nanoparticles. And Shin et al. [21] have recently synthesized 20 nm hollow manganese oxide nanoparticles with a paramagnetic behavior
around room temperature.
In addition, hollow MNPs with tunable shell thickness and composition are
promising building blocks for new advanced materials with lots of potential applications. In principle, hollow morphology allows encapsulating different contents inside
the MNPs for various applications [1, 2, 22–24]. For example, anticancer drugs can
be encapsulated inside the hollow MNPs used for drug delivery applications. The
anticancer drug is camouflaged and protected inside the MNPs on its way toward the
target once introduced into the body. These hollow MNPs can be guided toward the
tumor area with an external magnetic field and make them release the anticancer drug
in a localized and controlled way without affecting other regions of the body [22, 25,
26]. In the same way, hollow MNPs have also been proposed for other applications
such as lithium-ion batteries. Hollow MNPs can store Li-ions inside, and they offer
fast diffusion for Li-ions uptake/removal [27]. Likewise, the void space in hollow
particles has been used to modulate refractive index, lower density, increase active
area for catalysis, etc. [28]. Therefore, hollow MNPs have potential applications
in a wide variety of areas, including catalysis, memristors, batteries, targeted drug
delivery, and environmental treatment.