César de Julián Fernández

Circular magnetoplasmonic modes in gold nanoparticles.

The quest for efficient ways of modulating localized surface plasmon resonance is one of the frontiers in current research in plasmonics; the use of a magnetic field as a source of modulation is among the most promising candidates for active plasmonics. Here we report the observation of magnetoplasmonic modes on colloidal gold nanoparticles detected by means of magnetic circular dichroism (MCD) spectroscopy and provide a model that is able to rationalize and reproduce the experiment with unprecedented qualitative and quantitative accuracy. We believe that the steep slope observed at the plasmon resonance in the MCD spectrum can be very efficient in detecting changes in the refractive index of the surrounding medium, and we give a simple proof of principle of its possible implementation for magnetoplasmonic refractometric sensing.

Coexistence of plasmonic and magnetic properties in Au89Fe11 nanoalloys

We describe an environmentally friendly, top-down approach to the synthesis of Au89Fe11 nanoparticles (NPs). The plasmonic response of the gold moiety and the magnetism of the iron moiety coexist in the Au89Fe11 nanoalloy with strong modification compared to single element NPs, revealing a non-linear surface plasmon resonance dependence on the iron fraction and a transition from paramagnetic to a spin-glass state at low temperature. These nanoalloys are accessible to conjugation with thiolated molecules and they are promising contrast agents for magnetic resonance imaging.

Spin-Polarization Transfer in Colloidal Magnetic-Plasmonic Au/Iron Oxide Hetero-nanocrystals

We report on the unprecedented direct observation of spin-polarization transfer across colloidal magneto-plasmonic Au@Fe-oxide core@shell nanocrystal heterostructures. A magnetic moment is induced into the Au domain when the magnetic shell contains a reduced Fe-oxide phase in direct contact with the noble metal. An increased hole density in the Au states suggested occurrence of a charge-transfer process concomitant to the magnetization transfer. The angular to spin magnetic moment ratio, m(orb)/m(spin), for the Au 5d states, which was found to be equal to 0.38, appeared to be unusually large when compared to previous findings. A mechanism relying on direct hybridization between the Au and Fe states at the core/shell interface is proposed to account for the observed transfer of the magnetic moment.

Supported ε and β iron oxide nanomaterials by chemical vapor deposition: structure, morphology and magnetic properties

Supported e- and β-Fe2O3 are synthesized by a chemical vapor deposition (CVD) strategy, yielding systems with controllable morphologies from nanorods (e) to square-like pyramids (β). The hard magnetic properties of e-Fe2O3 and the antiferro/paramagnetic behavior of β-Fe2O3 are directly influenced by the system morphological organization and structural orientations.

Photocoercivity of Nano-Stabilized Au:Fe Superparamagnetic Nanoparticles

Generating, controlling, and monitoring spin effects in conducting nanostructures by using light is a highly important scientifi c and technological challenge. [ 1 , 2 ] Moreover the possibility of coupling the optical and magnetic properties in nanostructured materials can lead to the creation of novel devices with photonic and magnetic properties. Control by using light is a necessary ingredient for the next generation of photonic and/or spintronics devices for information-storage, [ 3 ] optoelectronic systems [ 4 ] and energy conversion. [ 5 ] For some tasks (e.g., energy conversion) only the electronic degrees of freedom are of interest, [ 5 ] whereas for others (e.g., memory-devices or spintronics) the magnetic ones are also important. [ 2 , 3 ]

Active Targeting of Sorafenib: Preparation, Characterization, and In Vitro Testing of Drug-Loaded Magnetic Solid Lipid Nanoparticles.

Sorafenib is an anticancer drug approved by the Food and Drug Administration for the treatment of hepatocellular and advanced renal carcinoma. The clinical application of sorafenib is promising, yet limited by its severe toxic side effects. The aim of this study is to develop sorafenib-loaded magnetic nanovectors able to enhance the drug delivery to the disease site with the help of a remote magnetic field, thus enabling cancer treatment while limiting negative effects on healthy tissues. Sorafenib and superparamagnetic iron oxide nanoparticles are encapsulated in solid lipid nanoparticles by a hot homogenization technique using cetyl palmitate as lipid matrix. The obtained nanoparticles (Sor-Mag-SLNs) have a sorafenib loading efficiency of about 90% and are found to be very stable in an aqueous environment. Plain Mag-SLNs exhibit good cytocompatibility, whereas an antiproliferative effect against tumor cells (human hepatocarcinoma HepG2) is observed for drug-loaded Sor-Mag-SLNs. The obtained results show that it is possible to prepare stable Sor-Mag-SLNs able to inhibit cancer cell proliferation through the sorafenib cytotoxic action, and to enhance/localize this effect in a desired area thanks to a magnetically driven accumulation of the drug. Moreover, the relaxivity properties observed in water suspensions hold promise for Sor-Mag-SLN tracking through clinical magnetic resonance imaging.

Structural and magnetic properties of mesoporous SiO2 nanoparticles impregnated with iron oxide or cobalt-iron oxide nanocrystals

Magnetic nanocomposites of FeOx@SiO2 and CoFe2O4@SiO2 were prepared via a wet-impregnation route using mesoporous silica nanoparticles as a support matrix. The small pores in the matrix were exploited as nanocavities for controlled growth of the embedded oxide phase, initially examined by introducing different wt% loadings of FeOx in four different samples and sequentially treating them under oxidising and reducing conditions. Comparative examination of the morphological and structural properties of the FeOx@SiO2 compositions shows that a 17 wt% (nominal) loading of the oxide phase, a mixture of Fe3O4 (magnetite) and γ-Fe2O3 (maghemite), is fully embedded within the pores. The 60–70 nm dimensions of the SiO2 nanoparticles are visible in TEM micrographs which reveal a spheroidal shape. TEM also shows a ca. 3 nm size for the crystalline oxide particles embedded within, which agrees with the pore sizes estimated through porosimetric analysis. The measurements for field-cooled (FC), zero-field-cooled (ZFC) magnetizations, and hysteresis loops in the temperature range of 3 K to 300 K reveal that an enhancement in the density of magnetization is obtained for the 17 wt% FeOx@SiO2 sample following reductive thermal treatment. A CoFe2O4@SiO2 nanocomposite prepared with a nominal 14 wt% oxide shows comparable structure and morphology to the 17 wt% FeOx@SiO2 sample, yet superior magnetic properties. The higher density of magnetization in CoFe2O4@SiO2 is attributed to its 40% content of magnetic material in the crystalline phase, versus 6–8% in FeOx@SiO2. Efficient surface functionalisation with APTES, monitored by DRIFT-IR, implies that the magnetic nanocomposites could be used in bio-labelling applications. Data derived from Raman spectroscopy, N2 adsorption/desorption measurements, and TGA are also used to characterise the nanocomposite materials.

Electrochemical characterization of core@shell CoFe2O4/Au composite

In this paper, we address the synthesis and characterization of the core@shell composite magneto-plasmonic cobalt ferrite–gold (Co-ferrite/Au) nanosystem. The synthesis Co-ferrite/Au nanocomposite is not obvious, hence it was of interest to generate it in a simple straightforward method. Co-ferrite/Au nanocomposite was generated by synthesizing first by thermal decomposition Co-ferrite nanoparticles (NPs). On a second step, ionic gold (Au3+) was reduced at the surface of Co-ferrite NPs by ultrasound, to obtain the metallic Au shell. The characterization of the nanomaterial was achieved by microscopy, spectroscopy, and performing magnetic measurements. However, what is attractive about our work is the use of electrochemical techniques as analytical tools. The key technique was cyclic voltammetry, which provided information about the nature and structure of the nanocomposite, allowing us to confirm the core@shell structure.

Tuning morphology and magnetism of magnetite nanoparticles by calix[8]arene-induced oriented aggregation

Magnetite nanoparticles have been prepared by oriented aggregation exploiting the action of calix[8]arene, an organic macrocycle capable of complexing Fe ions, during the synthesis. Control over the degree of aggregation enables tuning of the morphology of the product, which can vary from multicore aggregated nanoparticles to nano-octahedra, with a dramatic change in the magnetic properties. Octahedral magnetite nanoparticles display ferrimagnetic behavior, which is typical of magnetite above 40 nm in size. In contrast, multicore nanostructures exhibit a narrower hysteresis loop and remarkable heating capacity under an alternating magnetic field. With the aim of producing a material useful for biomedical applications, all samples were made to be dispersible in water and biocompatible by ligand exchange with 2,3-dimercaptosuccinic acid. Their morphology and magnetic properties were maintained after functionalization, as well as their good colloidal properties, which were characterized by dynamic light scattering.

Colloidal Au/Iron Oxide Nanocrystal Heterostructures: Magnetic, Plasmonic and Magnetic Hyperthermia Properties

Colloidal magneto-plasmonic nanostructures are multifunctional materials with huge potential for applications in magnetism, optoelectronics, biomedicine and catalysis. Currently it is considered that their optical and magnetic properties are a combination of the modified properties associated with the material constituents. Herein we have investigated the morphological, magnetic and plasmonic properties of Au@magnetite core@shell heterostructured nanocrystals (HNCs) with eccentric topology. We shed light on their behavior as heat mediators for magnetic fluid hyperthermia, a promising approach to cancer therapy. A red-shift and damping of the plasmon resonance was observed, which correlated with the optical contribution and the dielectric screening of the asymmetrically distributed iron oxide shell. The magnetic properties of the Au@magnetite HNCs were investigated by comparison with those of the corresponding carved magnetite nanocrystals (NCs), obtained by selective etching of the Au domain with iodine. The iron oxide NCs featured higher magnetization and coercive field than their parent HNCs, which showed a superparamagnetic behavior instead. In addition, the carved NCs exhibited better hyperthermia performances than the HNCs, being the Specific Absorption Rate (SAR) of heat one order of magnitude higher. On the basis of the peculiar magnetic properties of the HNCs, we hypothesized that a minority wustite phase was stabilized at the Au/iron-oxide interface, which could be eliminated upon oxidation to magnetite during the Au etching process. Our study opens a new scenario in the understanding of the physico-chemical behavior of this class of magneto-plasmonic heterostructures, whereby the asymmetric spatial distribution of the component materials, their complex multiphase composition and hetero-interface structure determine their ultimate plasmonic, magnetic and hyperthermia properties.