6 Hollow Magnetic Nanoparticles
153
MNPs for drug delivery [2, 21, 22, 29, 67]. For example, Xing et al. have shown
that human serum albumin (HSA)-coated iron oxide hollow MNPs can encapsulate
more anticancer drug (doxorubicin, DOX) than solid MNPs, being more effectively
uptaken by drug resistant cells than the free DOX, and consequently, becoming
more effective in killing the cancer cells [26]. In addition, hollow MNPs can exhibit
additional capacities in biomedicine, such as contrast agents for magnetic resonance
imaging or heating agents for hyperthermia treatment of cancer [22]. For example,
recently Li et al. [25] showed that hollow ferrite nanoparticles exhibited good heating
capacities under the actuation of an alternating magnetic field and therefore could be
used for both infrared and thermal imaging. These hollow ferrite nanoparticles also
can provide temperature activated drug release and hyperthermia, demonstrating
great potential for in vivo cancer therapy. Despite the growing interest of hollow
MNPs in biomedical application, there are relatively few in vitro and in vivo studies.
In addition, the hollowing process can create great strain and fragmentation of the
shell that eventually can lead to a deterioration of their properties [68]. Therefore,
further research in this area is still needed.
Another field of interest for the application of these hollow MNPs is water remediation and treatment. Unfortunately, there is a rising amount of water pollution and
waste. This has led the scientific community to develop materials that can adsorb
and remove organic and inorganic contaminants from water. In particular, in the last
years, nanomaterials have been investigated for this task [69]. Magnetic nanoparticles, displaying large surface area and functionalized with an appropriate coating,
can bind to specific pollutants in water, and thanks to their magnetic properties,
they can be easily separated from water, removing this way the contaminants [70,
71]. In particular, iron-oxide-based MNPs have been investigated for the removal of
heavy metals (Cr, Pb, Co, As …) from water, but there have been only a few reports
on the use of hollow MNPs in this area. The large effective specific surface area of
hollow MNPs, double if we compare them with conventional solid MNPs, is of strong
interest for increasing the adsorption capacity of these nanomaterials. In particular,
Balcells et al. have developed very stable iron oxide hollow nanocuboids that exhibit
high efficiency in the absorption of both As(III) and As(V) (326 and 192 mg/g) [72].
As an alternative, some studies have focused on the combination of carbon-based
and iron-oxide-based nanomaterials in order to develop magnetic “nanocages” with
high surface area, capable of binding heavy metals on both carbon and iron oxide
surfaces [73]. Along these lines, hollow MNPs can also be used for other tasks that
require capture and removal of specific elements, such as bioseparation [29].
Another interesting application of hollow MNPs is related to Li-ion batteries.
Batteries are considered essential to maximize the efficiency in energy use. In
battery applications, ideal electrodes should be cheap, have high capacity and rate
performances, and last long. For these reasons, iron-oxide-based nanomaterials have
become attractive candidates as electrodes in Li-ion-based batteries [74, 75]. In
particular, hollow iron oxide MNPs have great potential for these applications.
The larger surface area of these hollow MNPs enables an increased electrode–electrolyte contact area as well as more Li-ion storage sites, the hollow morphology
allows faster diffusion for Li-ions uptake/removal as compared to solid MNPs; and
153
MNPs for drug delivery [2, 21, 22, 29, 67]. For example, Xing et al. have shown
that human serum albumin (HSA)-coated iron oxide hollow MNPs can encapsulate
more anticancer drug (doxorubicin, DOX) than solid MNPs, being more effectively
uptaken by drug resistant cells than the free DOX, and consequently, becoming
more effective in killing the cancer cells [26]. In addition, hollow MNPs can exhibit
additional capacities in biomedicine, such as contrast agents for magnetic resonance
imaging or heating agents for hyperthermia treatment of cancer [22]. For example,
recently Li et al. [25] showed that hollow ferrite nanoparticles exhibited good heating
capacities under the actuation of an alternating magnetic field and therefore could be
used for both infrared and thermal imaging. These hollow ferrite nanoparticles also
can provide temperature activated drug release and hyperthermia, demonstrating
great potential for in vivo cancer therapy. Despite the growing interest of hollow
MNPs in biomedical application, there are relatively few in vitro and in vivo studies.
In addition, the hollowing process can create great strain and fragmentation of the
shell that eventually can lead to a deterioration of their properties [68]. Therefore,
further research in this area is still needed.
Another field of interest for the application of these hollow MNPs is water remediation and treatment. Unfortunately, there is a rising amount of water pollution and
waste. This has led the scientific community to develop materials that can adsorb
and remove organic and inorganic contaminants from water. In particular, in the last
years, nanomaterials have been investigated for this task [69]. Magnetic nanoparticles, displaying large surface area and functionalized with an appropriate coating,
can bind to specific pollutants in water, and thanks to their magnetic properties,
they can be easily separated from water, removing this way the contaminants [70,
71]. In particular, iron-oxide-based MNPs have been investigated for the removal of
heavy metals (Cr, Pb, Co, As …) from water, but there have been only a few reports
on the use of hollow MNPs in this area. The large effective specific surface area of
hollow MNPs, double if we compare them with conventional solid MNPs, is of strong
interest for increasing the adsorption capacity of these nanomaterials. In particular,
Balcells et al. have developed very stable iron oxide hollow nanocuboids that exhibit
high efficiency in the absorption of both As(III) and As(V) (326 and 192 mg/g) [72].
As an alternative, some studies have focused on the combination of carbon-based
and iron-oxide-based nanomaterials in order to develop magnetic “nanocages” with
high surface area, capable of binding heavy metals on both carbon and iron oxide
surfaces [73]. Along these lines, hollow MNPs can also be used for other tasks that
require capture and removal of specific elements, such as bioseparation [29].
Another interesting application of hollow MNPs is related to Li-ion batteries.
Batteries are considered essential to maximize the efficiency in energy use. In
battery applications, ideal electrodes should be cheap, have high capacity and rate
performances, and last long. For these reasons, iron-oxide-based nanomaterials have
become attractive candidates as electrodes in Li-ion-based batteries [74, 75]. In
particular, hollow iron oxide MNPs have great potential for these applications.
The larger surface area of these hollow MNPs enables an increased electrode–electrolyte contact area as well as more Li-ion storage sites, the hollow morphology
allows faster diffusion for Li-ions uptake/removal as compared to solid MNPs; and