Defect-Engineering by Solvent Mediated Mild Oxidation as a Tool to Induce Exchange Bias in Metal Doped Ferrites.

The crystal site occupancy of different divalent ions and the induction of lattice defects represent an additional tool for modifying the intrinsic magnetic properties of spinel ferrites nanoparticles. Here, the relevance of the lattice defects is demonstrated in the appearance of exchange-bias and in the improvement of the magnetic properties of doped ferrites of 20 nm, obtained from the mild oxidation of core@shell (wüstite@ferrite) nanoparticles. Three types of nanoparticles (Fe0.95 O@Fe3 O4 , Co0.3 Fe0.7 O@Co0.8 Fe2.2 O4 and Ni0.17 Co0.21 Fe0.62 O@Ni0.4 Co0.3 Fe2.3 O4 ) are oxidized. As a result, the core@shell morphology is removed and transformed in a spinel-like nanoparticle, through a topotactic transformation. This study shows that most of the induced defects in these nanoparticles and their magnetic properties are driven by the inability of the Co(II) ions at the octahedral sites to migrate to tetrahedral sites, at the chosen mild oxidation temperature. In addition, the appearance of crystal defects and antiphase boundaries improves the magnetic properties of the starting compounds and leads to the appearance of exchange bias at room temperature. These results highlight the validity of the proposed method to impose novel magnetic characteristics in the technologically relevant class of nanomaterials such as spinel ferrites, expanding their potential exploitation in several application fields.

Remanence Increase in SrFe12O19/Fe Exchange-Decoupled Hard-Soft Composite Magnets Owing to Dipolar Interactions.

In the search for improved permanent magnets, fueled by the geostrategic and environmental issues associated with rare-earth-based magnets, magnetically hard (high anisotropy)-soft (high magnetization) composite magnets hold promise as alternative magnets that could replace modern permanent magnets, such as rare-earth-based and ceramic magnets, in certain applications. However, so far, the magnetic properties reported for hard-soft composites have been underwhelming. Here, an attempt to further understand the correlation between magnetic and microstructural properties in strontium ferrite-based composites, hard SrFe12O19 (SFO) ceramics with different contents of Fe particles as soft phase, both in powder and in dense injection molded magnets, is presented. In addition, the influence of soft phase particle dimension, in the nano- and micron-sized regimes, on these properties is studied. While Fe and SFO are not exchange-coupled in our magnets, a remanence that is higher than expected is measured. In fact, in composite injection molded anisotropic (magnetically oriented) magnets, remanence is improved by 2.4% with respect to a pure ferrite identical magnet. The analysis of the experimental results in combination with micromagnetic simulations allows us to establish that the type of interaction between hard and soft phases is of a dipolar nature, and is responsible for the alignment of a fraction of the soft spins with the magnetization of the hard. The mechanism unraveled in this work has implications for the development of novel hard-soft permanent magnets.

Hardening of Cobalt Ferrite Nanoparticles by Local Crystal Strain Release: Implications for Rare Earth Free Magnets.

In this work, we demonstrate that the reduction of the local internal stress by a low-temperature solvent-mediated thermal treatment is an effective post-treatment tool for magnetic hardening of chemically synthesized nanoparticles. As a case study, we used nonstoichiometric cobalt ferrite particles of an average size of 32(8) nm synthesized by thermal decomposition, which were further subjected to solvent-mediated annealing at variable temperatures between 150 and 320 °C in an inert atmosphere. The postsynthesis treatment produces a 50% increase of the coercive field, without affecting neither the remanence ratio nor the spontaneous magnetization. As a consequence, the energy product and the magnetic energy storage capability, key features for applications as permanent magnets and magnetic hyperthermia, can be increased by ca. 70%. A deep structural, morphological, chemical, and magnetic characterization reveals that the mechanism governing the coercive field improvement is the reduction of the concomitant internal stresses induced by the low-temperature annealing postsynthesis treatment. Furthermore, we show that the medium where the mild annealing process occurs is essential to control the final properties of the nanoparticles because the classical annealing procedure (T > 350 °C) performed on a dried powder does not allow the release of the lattice stress, leading to the reduction of the initial coercive field. The strategy here proposed, therefore, constitutes a method to improve the magnetic properties of nanoparticles, which can be particularly appealing for those materials, as is the case of cobalt ferrite, currently investigated as building blocks for the development of rare-earth free permanent magnets.

Dielectric Effects in FeO x -Coated Au Nanoparticles Boost the Magnetoplasmonic Response: Implications for Active Plasmonic Devices.

Plasmon resonance modulation with an external magnetic field (magnetoplasmonics) represents a promising route for the improvement of the sensitivity of plasmon-based refractometric sensing. To this purpose, an accurate material choice is needed to realize hybrid nanostructures with an improved magnetoplasmonic response. In this work, we prepared core@shell nanostructures made of an 8 nm Au core surrounded by an ultrathin iron oxide shell (≤1 nm). The presence of the iron oxide shell was found to significantly enhance the magneto-optical response of the noble metal in the localized surface plasmon region, compared with uncoated Au nanoparticles. With the support of an analytical model, we ascribed the origin of the enhancement to the shell-induced increase in the dielectric permittivity around the Au core. The experiment points out the importance of the spectral position of the plasmonic resonance in determining the magnitude of the magnetoplasmonic response. Moreover, the analytical model proposed here represents a powerful predictive tool for the quantification of the magnetoplasmonic effect based on resonance position engineering, which has significant implications for the design of active magnetoplasmonic devices.

OBP-functionalized/hybrid superparamagnetic nanoparticles for Candida albicans treatment.

Infections caused by the opportunistic yeast Candida albicans are one of the major life threats for hospitalized and immunocompromised patients, as a result of antibiotic and long-term antifungal treatment abuse. Odorant binding proteins can be considered interesting candidates to develop systems able to reduce the proliferation and virulence of this yeast, because of their intrinsic antimicrobial properties and complexation capabilities toward farnesol, the major quorum sensing molecule of Candida albicans. In the present study, a hybrid system characterized by a superparamagnetic iron oxide core functionalized with bovine odorant binding protein (bOBP) was successfully developed. The nanoparticles were designed to be suitable for magnetic protein delivery to inflamed areas of the body. The inorganic superparamagnetic core was characterized by an average diameter of 6.5 ± 1.1 nm and a spherical shape. Nanoparticles were functionalized by using 11-phosphonoundecanoic acid as spacer and linked to bOBP via amide bonds, resulting in a concentration level of 26.0 ± 1.2 mg bOBP/g SPIONs. Finally, both the biocompatibility of the developed hybrid system and the fungistatic activity against Candida albicans by submicromolar OBP levels were demonstrated by in vitro experiments.

Nutlin-loaded magnetic solid lipid nanoparticles for targeted glioblastoma treatment.

AIM: Glioblastoma multiforme is one of the deadliest forms of cancer, and current treatments are limited to palliative cares. The present study proposes a nanotechnology-based solution able to improve both drug efficacy and its delivery efficiency. MATERIALS & METHODS: Nutlin-3a and superparamagnetic nanoparticles were encapsulated in solid lipid nanoparticles, and the obtained nanovectors (nutlin-loaded magnetic solid lipid nanoparticle [Nut-Mag-SLNs]) were characterized by analyzing both their physicochemical properties and their effects on U-87 MG glioblastoma cells. RESULTS: Nut-Mag-SLNs showed good colloidal stability, the ability to cross an in vitro blood-brain barrier model, and a superior pro-apoptotic activity toward glioblastoma cells with respect to the free drug. CONCLUSION: Nut-Mag-SLNs represent a promising multifunctional nanoplatform for the treatment of glioblastoma multiforme.

Stimuli-responsive lipid-based magnetic nanovectors increase apoptosis in glioblastoma cells through synergic intracellular hyperthermia and chemotherapy.

In this study, taking into consideration the limitations of current treatments of glioblastoma multiforme, we fabricated a biomimetic lipid-based magnetic nanovector with a good loading capacity and a sustained release profile of the encapsulated chemotherapeutic drug, temozolomide. These nanostructures demonstrated an enhanced release after exposure to an alternating magnetic field, and a complete release of the encapsulated drug after the synergic effect of low pH (4.5), increased concentration of hydrogen peroxide (50 µM), and increased temperature due to the applied magnetic field. In addition, these nanovectors presented excellent specific absorption rate values (up to 1345 W g-1) considering the size of the magnetic component, rendering them suitable as potential hyperthermia agents. The presented nanovectors were progressively internalized in U-87 MG cells and in their acidic compartments (i.e., lysosomes and late endosomes) without affecting the viability of the cells, and their ability to cross the blood-brain barrier was preliminarily investigated using an in vitro brain endothelial cell-model. When stimulated with alternating magnetic fields (20 mT, 750 kHz), the nanovectors demonstrated their ability to induce mild hyperthermia (43 °C) and strong anticancer effects against U-87 MG cells (scarce survival of cells characterized by low proliferation rates and high apoptosis levels). The optimal anticancer effects resulted from the synergic combination of hyperthermia chronic stimulation and the controlled temozolomide release, highlighting the potential of the proposed drug-loaded lipid magnetic nanovectors for treatment of glioblastoma multiforme.

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.

Developing functionalized Fe3O4-Au nanoparticles: a physico-chemical insight.

Nanotechnology for biomedicine has recently attracted increasing interest from the scientific community. In particular, among the different nanodevices suitable for this application, multifunctionalizable hybrid nanoparticles are one of the most investigated research topics. Here we present a detailed physico-chemical characterization of hybrid magneto-plasmonic iron oxide-gold nanoparticles (NPs) with core-shell structure. In particular, we underline all the synthetic difficulties concerning the preparation of these systems. Based on all our results, after different tests of a commonly reported protocol for the synthesis of the core-shell system, we believe that several issues are still open in the synthetic preparation of these particular NPs. Indeed, at least for the conditions that we adopted, core-shell morphology nanoparticles cannot be produced. However, independent of the core structure, we describe here an optimized and efficient functionalization protocol to obtain stable nanoparticle aqueous suspensions, which can be easily exported to other kinds of metal and metal-oxide NPs and used to develop biocompatible systems. Furthermore, reliable information that could be useful for researchers working in this field is extensively discussed.

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.

At the frontier between heterogeneous and homogeneous catalysis: hydrogenation of olefins and alkynes with soluble iron nanoparticles.

The use of non-supported Fe nanoparticles in the hydrogenation of unsaturated C-C bonds is a green catalytic concept at the frontier between homogeneous and heterogeneous catalysis. Iron nanoparticles can be obtained by reducing Fe salts with strong reductants in various solvents. FeCl(3) reduced by 3 equivalents of EtMgCl forms an active catalyst for the hydrogenation of a range of olefins and alkynes. Olefin hydrogenation is relatively fast at 5 bar using 5 mol% of catalyst. The catalyst is also active for terminal olefins and 1,1' and 1,2-cis disubstituted olefins while trans-olefins react much slower. 1-Octyne is hydrogenated to mixtures of 1-octene and octane. Kinetic studies led us to propose a mechanism for this latter transformation where octane is obtained by two different pathways. Characterization of the nanoparticles via TEM, magnetic measurements and poisoning experiments were undertaken to understand the true nature of our catalyst.