We use Monte Carlo simulations to study the influence of dipolar interaction and polydispersity on the magnetic properties of single-domain ultrafine ferromagnetic particles. From the zero field cooling (ZFC)/field cooling (FC) simulations we observe that the blocking temperature T(B) clearly increases with increasing strength of interaction, but it is almost not effected by a broadening of the distribution of particle sizes. While the dependence of the ZFC/FC curves on interaction and cooling rate are reminiscent of a spin glass transition at T(B), the relaxational behavior of the magnetic moments below T(B) is not in accordance with the picture of cooperative freezing.
Nanotechnology involves the study of nature at a very small scale, searching new properties and applications. The development of this area of knowledge affects greatly both biotechnology and medicine disciplines. The use of materials at the nanoscale, in particular magnetic nanoparticles, is currently a prominent topic in healthcare and life science. Due to their size-tunable physical and chemical properties, magnetic nanoparticles have demonstrated a wide range of applications ranging from medical diagnosis to treatment. Combining a high saturation magnetization with a properly functionalized surface, magnetic nanoparticles are provided with enhanced functionality that allows them to selectively attach to target cells or tissues and play their therapeutic role in them. In particular, iron oxide nanoparticles are being actively investigated to achieve highly efficient carcinogenic cell destruction through magnetic hyperthermia treatments. Hyperthermia in different approaches has been used combined with radiotherapy during the last decades, however, serious harmful secondary effects have been found in healthy tissues to be associated with these treatments. In this framework, nanotechnology provides a novel and original solution with magnetic hyperthermia, which is based on the use of magnetic nanoparticles to remotely induce local heat when a radiofrequency magnetic field is applied, provoking a temperature increase in those tissues and organs where the tumoral cells are present. Therefore, one important factor that determines the efficiency of this technique is the ability of magnetic nanoparticles to be driven and accumulated in the desired area inside the body. With this aim, magnetic nanoparticles must be strategically surface functionalized to selectively target the injured cells and tissues.
Exceptional
magnetic properties of magnetite, Fe3O4, nanoparticles
make them one of the most intensively studied inorganic nanomaterials
for biomedical applications. We report successful gram-scale syntheses,
via hydrothermal route or controlled coprecipitation in an automated
reactor, of colloidal Fe3O4 nanoparticles with
sizes of 12.9 ± 5.9, 17.9 ± 4.4, and 19.8 ± 3.2 nm.
To investigate structure–property relationships as a function
of the synthetic procedure, we used multiple techniques to characterize
the structure, phase composition, and magnetic behavior of these nanoparticles.
For the iron oxide cores of these nanoparticles, powder X-ray diffraction
and electron microscopy both confirm single-phase Fe3O4 composition. In addition to the core composition, the magnetic
performance of nanoparticles in the 13–20 nm size range can
be strongly influenced by the surface properties, which we analyzed
by three complementary techniques. Raman scattering and X-ray photoelectron
spectroscopy (XPS) measurements indicate overoxidation of nanoparticle
surfaces, while transmission electron microscopy (TEM) shows no distinct
core–shell structure. Considered together, Raman, XPS, and
TEM observations suggest that our nanoparticles have a gradually varying
nonstoichiometric Fe3O4+δ composition,
which could be attributed to the formation of Fe3O4–γ-Fe2O3 solid solutions
at their outermost surface. Detailed analyses by TEM reveal that the
hydrothermally produced samples include single-domain nanocrystals
coexisting with defective twinned and dimer nanoparticles, which form
as a result of oriented-attachment crystal growth. All our nanoparticles
exhibit superparamagnetic-like behavior with a characteristic blocking
temperature above room temperature. We attribute the estimated saturation
magnetization values up to 84.01 ± 0.25 emu/g at 300 K to the
relatively large size of the nanoparticles (13–20 nm) coupled
with the syntheses under elevated temperature; alternative explanations,
such as surface-mediated effects, are not supported by our spectroscopy
or microscopy measurements. For these colloids, the heating efficiency
in magnetic hyperthermia correlates with their saturation magnetization,
making them appealing for therapeutic and other biomedical applications
that rely on high-performance nanoparticle-mediated hyperthermia.
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