Design strategies for shape-controlled magnetic iron oxide nanoparticles

Ferrimagnetic iron oxide nanoparticles (magnetite or maghemite) have been the subject of an intense research, not only for fundamental research but also for their potentiality in a widespread number of practical applications. Most of these studies were focused on nanoparticles with spherical morphology but recently there is an emerging interest on anisometric nanoparticles. This review is focused on the synthesis routes for the production of uniform anisometric magnetite/maghemite nanoparticles with different morphologies like cubes, rods, disks, flowers and many others, such as hollow spheres, worms, stars or tetrapods. We critically analyzed those procedures, detected the key parameters governing the production of these nanoparticles with particular emphasis in the role of the ligands in the final nanoparticle morphology. The main structural and magnetic features as well as the nanotoxicity as a function of the nanoparticle morphology are also described. Finally, the impact of each morphology on the different biomedical applications (hyperthermia, magnetic resonance imaging and drug delivery) are analysed in detail. We would like to dedicate this work to Professor Carlos J. Serna, Instituto de Ciencia de Materiales de Madrid, ICMM/CSIC, for his outstanding contribution in the field of monodispersed colloids and iron oxide nanoparticles. We would like to express our gratitude for all these years of support and inspiration on the occasion of his retirement

Enantioselective Crystallization of Sodium Chlorate in the Presence of Racemic Hydrophobic Amino Acids and Static Magnetic Fields

We study the bias induced by a weak (200 mT) external magnetic field on the preferred handedness of sodium chlorate crystals obtained by slow evaporation at ambient conditions of its saturated saline solution with 20 ppm of added racemic (dl) hydrophobic amino acids. By applying the Fisher test to pairs of experiments with opposing magnetic field orientation we conclude, with a confidence level of 99.7%, that at the water-air interface of this saline solution there is an enantioselective magnetic interaction that acts upon racemic mixtures of hydrophobic chiral amino acids. This interaction has been observed with the three tested racemic hydrophobic amino acids: dl-Phe, dl-Try and dl-Trp, at ambient conditions and in spite of the ubiquitous chiral organic contamination. This enantioselective magnetic dependence is not observed when there is only one handedness of added chiral amino-acid, if the added amino acid is not chiral or if there is no additive. This effect has been confirmed with a double blind test. This novel experimental observation may have implications for our view of plausible initial prebiotic scenarios and of the roles of the geomagnetic field in homochirality in the biosphere.This research was supported by the Instituto Nacional de Técnica Aerospacial (INTA) and by the grant AYA2006-15648-C02-02 of the Ministerio de Educación y Ciencia (Spain) and by the project grants AYA2011-25720 and AYA2012-38707 of MINECO (Spain).We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).Peer reviewe

Metastability in supersaturated solution and transition towards chirality in the crystallization of NaClO3

Analysis of the chiral composition of the crystal mixture obtained from samples of boiling solutions of NaClO3 (see picture) indicates that symmetry breaking towards homochiral compositions may begin in the metastable stage preceding crystallization, that is, at the level of subcritical clusters

Doped-iron oxide nanocrystals synthesized by one-step aqueous route for multi-imaging purposes

New doped inorganic nanocrystals (NC) consisting on iron oxide and other metal integrated into the structure have been synthesized in one-step by adapting the oxidant precipitation synthesis route for magnetite. Different metals have been chosen to confer extra and unique properties to the resulting magnetic hetero-nanostructure: Co and Gd for enhancing transversal and longitudinal relaxivities for magnetic resonance imaging and Bi and Au for achieving X-ray absorption for computed tomography imaging. Apart of that, gold optical properties are interesting for photothermal therapy and iron oxides for magnetic hyperthermia. All metals have been incorporated into the magnetite structure in different ways during the synthesis: by forming a solid solution, by modifying the surface of the NCs, or by co-crystallization with the magnetite. The nanostructure formed in each case depends on the ionic radius of the secondary metal ion and the solubility of its hydroxide that control the co-precipitation in the initial steps of the reaction. Magnetic properties and imaging capabilities of the hetero-nanostructures have been analyzed as a function of the element distribution. Due to the synergistic combination of the different element properties, these magnetic hetero-nanostructures have great potential for biomedical applications

Cu-Doped Extremely Small Iron Oxide Nanoparticles with Large Longitudinal Relaxivity: One-Pot Synthesis and in Vivo Targeted Molecular Imaging

Synthesizing iron oxide nanoparticles for positive contrast in magnetic resonance imaging is the most promising approach to bring this nanomaterial back to the clinical field. The success of this approach depends on several aspects: the longitudinal relaxivity values, the complexity of the synthetic protocol, and the reproducibility of the synthesis. Here, we show our latest results on this goal. We have studied the effect of Cu doping on the physicochemical, magnetic, and relaxometric properties of iron oxide nanoparticles designed to provide positive contrast in magnetic resonance imaging. We have used a one-step, 10 min synthesis to produce nanoparticles with excellent colloidal stability. We have synthesized three different Cu-doped iron oxide nanoparticles showing modest to very large longitudinal relaxivity values. Finally, we have demonstrated the in vivo use of these kinds of nanoparticles both in angiography and targeted molecular imaging

A device-independent approach to evaluate the heating performance during magnetic hyperthermia experiments: peak analysis and zigzag protocol

Accurate knowledge of the heating performance of magnetic nanoparticles (MNPs) under AC fields is critical for the development of hyperthermia-mediated applications. Usually reported in terms of the specific loss power (SLP) obtained from the temperature variation ($\Delta{T}$) vs. time (t) curve, such estimate is subjected to a huge uncertainty. Thus, very different SLP values are reported for the same particles when measured on different equipment/laboratories. This lack of control clearly hampers the further development of MNP-mediated heat-triggered technologies. Here we report a device-independent approach to calculate the SLP value of a suspension of MNPs: the SLP is obtained from the analysis of the peak at the field on/off switch of the $\Delta{T}(t)$ curve. The measurement procedure, which itself constitutes a change of paradigm within the field, is based on fundamental physics considerations: specifically to guarantee the applicability of Newton's law of cooling, as i) it corresponds to the ideal scenario in which the temperature profiles of the system during heating and cooling are the same; and ii) it diminishes the role of coexistence of various heat dissipation channels. Such an approach is supported by theoretical and computational calculations to increase the reliability and reproducibility of SLP determination. This is experimentally confirmed, demonstrating a reduction in SLP variation across 3 different devices located in 3 different laboratories. Furthermore, the application of this peak analysis method (PAM) to a rapid succession of field on/off switches that result in a zigzag-like $\Delta{T}(t)$, which we term the zigzag protocol, allows evaluating possible variations of the SLP values with time or temperature.Comment: main text: 30 pages, 9 figure

Formation Mechanism of Maghemite Nanoflowers Synthesized by a Polyol-Mediated Process

Magnetic nanoparticles are being developed as structural and functional materials for use in diverse areas, including biomedical applications. Here, we report the synthesis of maghemite (¿-Fe2O3) nanoparticles with distinct morphologies: single-core and multicore, including hollow spheres and nanoflowers, prepared by the polyol process. We have used sodium acetate to control the nucleation and assembly process to obtain the different particle morphologies. Moreover, from samples obtained at different time steps during the synthesis, we have elucidated the formation mechanism of the nanoflowers: the initial phases of the reaction present a lepidocrocite (¿-FeOOH) structure, which suffers a fast dehydroxylation, transforming to an intermediate "undescribed" phase, possibly a partly dehydroxylated lepidocrocite, which after some incubation time evolves to maghemite nanoflowers. Once the nanoflowers have been formed, a crystallization process takes place, where the ¿-Fe2O3 crystallites within the nanoflowers grow in size (from ~11 to 23 nm), but the particle size of the flower remains essentially unchanged (~60 nm). Samples with different morphologies were coated with citric acid and their heating capacity in an alternating magnetic field was evaluated. We observe that nanoflowers with large cores (23 nm, controlled by annealing) densely packed (tuned by low NaAc concentration) offer 5 times enhanced heating capacity compared to that of the nanoflowers with smaller core sizes (15 nm), 4 times enhanced heating effect compared to that of the hollow spheres, and 1.5 times enhanced heating effect compared to that of single-core nanoparticles (36 nm) used in this work

Structural determination of Bi-doped magnetite multifunctional nanoparticles for contrast imaging

To determine with precision how Bi atoms are distributed in Bi-doped iron oxide nanoparticles their structural characterization has been carried out by X-ray absorption spectroscopy (XAS) recorded at the K edge of Fe and at the L edge of Bi. The inorganic nanoparticles are nominally hybrid structures integrating an iron oxide core and a bismuth oxide shell. Fe K-edge XAS indicates the formation of a structurally ordered, non-stoichiometric magnetite (FeO) phase for all the nanoparticles. The XAS spectra show that, in the samples synthesized by precipitation in aqueous media and laser pyrolysis, the Bi atoms neither enter into the iron oxide spinel lattice nor form any other mixed Bi-Fe oxides. No modification of the local structure around the Fe atoms induced by the Bi atoms is observed at the Fe K edge. In addition, contrary to expectations, our results indicate that the Bi atoms do not form a well-defined Bi oxide structure. The XAS study at the Bi L edge indicates that the environment around Bi atoms is highly disordered and only a first oxygen coordination shell is observed. Indefinite [BiO(OH)] units (isolated or aggregated forming tiny amorphous clusters) bonded through hydroxyl bridges to the nanoparticle, rather than a well defined BiO shell, surround the nanoparticle. On the other hand, the XAS study indicates that, in the samples synthesized by thermal decomposition, the Bi atoms are embedded in a longer range ordered structure showing the first and second neighbors