Oscar Bomati-Miguel

Comparative study of ferrofluids based on dextran-coated iron oxide and metal nanoparticles for contrast agents in magnetic resonance imaging

Colloidal suspensions of iron oxide and metal iron nanoparticles prepared by laser pyrolysis have been obtained by coating the particles with dextran in an aqueous media giving rise to biocompatible ferrofluids. The structural characteristics of the powders and the size of the particles and the aggregates in the colloidal suspensions have been analysed and correlated with the magnetic properties of both solids and fluids. For the first time, to our knowledge, a stable ferrofluid based on metal particles (<10?nm) has been obtained with aggregate sizes of ?nm. In comparison to iron oxide based products, this material exhibits higher saturation magnetization (45?emu?g?1) and susceptibilities (4000?emu/g?T). In addition, the nuclear magnetic resonance response of the ferrofluids has been measured in order to gain information about the influence of the crystallochemical and magnetic properties on their relaxation behaviour. The main parameter affected by the presence of the magnetic nanoparticles is the transversal relaxation time T2 and the corresponding relaxivity R2 value that is of the order of 400?(mmol/l)?1?s?1. It has been shown that R2 value increases not only by using iron metal instead of iron oxide but also by increasing the crystal size of the particles. From this study an evaluation of the possibilities of these materials as contrast agents for magnetic resonance imaging has been made.

Effect of the process conditions on the structural and magnetic properties of γ-Fe2O3 nanoparticles produced by laser pyrolysis

Pure and uniform γ-Fe2O3 nanoparticles between 4 and 7 nm, have been prepared by a continuous process based on ethylene sensitised laser pyrolysis of iron pentacarbonyl vapours in an oxidant atmosphere. In this paper the effect of the laser power and oxygen proportion on the properties of the maghemite nanoparticles has been investigated.

Colloidal dispersions of maghemite nanoparticles produced by laser pyrolysis with application as NMR contrast agents

Biocompatible magnetic dispersions have been prepared from γ-Fe2O3 nanoparticles (5 nm) synthesized by continuous laser pyrolysis of Fe(CO)5 vapours. The feasibility of using these dispersions as magnetic resonance imaging (MRI) contrast agents has been analysed in terms of chemical structure, magnetic properties, 1H NMR relaxation times and biokinetics. The magnetic nanoparticles were dispersed in a strong alkaline solution in the presence of dextran, yielding stable colloids in a single step. The dispersions consist of particle-aggregates 25 nm in diameter measured using transmission electron microscope and a hydrodynamic diameter of 42 nm measured using photon correlation spectroscopy. The magnetic and relaxometric properties of the dispersions were of the same order of magnitude as those of commercial contrast agents produced using coprecipitation. However, these dispersions, when injected intravenously in rats at standard doses showed a mono-exponential blood clearance instead of a biexponential one, with a blood half-life of 7 ± 1 min. Furthermore, an important enhancement of the image contrast was observed after the injection, mainly located at the liver and the spleen of the rat. In conclusion, the laser pyrolysis technique seems to be a good alternative to the coprecipitation method for producing MRI contrast agents, with the advantage of being a continuous synthesis method that leads to very uniform particles capable of being dispersed and therefore transformed in a biocompatible magnetic liquid.

Surface functionalization for tailoring the aggregation and magnetic behaviour of silica-coated iron oxide nanostructures

We report here a detailed structural and magnetic study of different silica nanocapsules containing uniform and highly crystalline maghemite nanoparticles. The magnetic phase consists of 5 nm triethylene glycol (TREG)- or dimercaptosuccinic acid (DMSA)-coated maghemite particles. TREG-coated nanoparticles were synthesized by thermal decomposition. In a second step, TREG ligands were exchanged by DMSA. After the ligand exchange, the ζ potential of the particles changed from -10 to -40 mV, whereas the hydrodynamic size remained constant at around 15 nm. Particles coated by TREG and DMSA were encapsulated in silica following a sol-gel procedure. The encapsulation of TREG-coated nanoparticles led to large magnetic aggregates, which were embedded in coalesced silica structures. However, DMSA-coated nanoparticles led to small magnetic clusters inserted in silica spheres of around 100 nm. The final nanostructures can be described as the result of several competing factors at play. Magnetic measurements indicate that in the TREG-coated nanoparticles the interparticle magnetic interaction scenario has not dramatically changed after the silica encapsulation, whereas in the DMSA-coated nanoparticles, the magnetic interactions were screened due to the function of the silica template. Moreover, the analysis of the AC susceptibility suggests that our systems essentially behave as cluster spin glass systems.

Microstructural characterization of ellipsoidal iron metal nanoparticles

Ellipsoidal metal nanoparticles about 200 nm in length and with different axial ratios were obtained by reduction with hydrogen of an iron oxide. These metal particles were stabilized without the presence of an antisintering and protecting layer of aluminium or yttrium oxide, giving rise to a significant improvement of the magnetic properties. The precursors were uniform ellipsoidal haematite particles synthesized by forced hydrolysis of iron perchlorate in the presence of urea and phosphate ions. A detailed characterization of the nanoparticles was carried out to correlate the microstructure of the haematite precursors with the structural and magnetic properties of the final metal particles. It was observed that the single-crystal character of the particles is preserved during the transformation of iron oxide to metal. The resulting metal particles consist of a metal core of α-Fe and an oxide layer about 5 nm thick, with a spinel structure. The magnetic properties of this material showed very high saturation magnetization () and coercivity values increasing from 1000 to 1200 Oe as the particle axial ratio increases. Measurements of the time dependence of the magnetization yielded activation volumes eight and five times smaller than the particle physical volumes, suggesting a mechanism of incoherent reversal of the magnetization.

Use of a polyol liquid collection medium to obtain ultrasmall magnetic nanoparticles by laser pyrolysis

The present work addresses the main bottleneck in the synthesis of magnetic nanoparticles by laser pyrolysis. Since the introduction of laser pyrolysis for the production of nanoparticles nearly three decades ago, this method has been repeatedly presented as a highly promising alternative, on account of two main characteristics: (i) its flexibility, since nanoparticles can be formed from a wide variety of precursors in both gas and liquid phase, and (ii) its continuous nature, avoiding the intrinsic variability of batch processing. However, the results reported to date invariably show considerable aggregation of the obtained nanoparticles, which strongly limits their application in most fields. In this work, we have been able to circumvent this problem by collecting the particles in a polyol liquid medium. This method prevents the formation of aggregates and renders a uniform distribution of well dispersed ultrasmall nanoparticles (<4 nm) in a water-compatible solvent. We consider that the effectiveness of this novel collection method for the production of well-dispersed magnetic nanoparticles will be of high interest to a wide range of scientists working in the nanoparticle synthesis field and may enable new applications wherever there is a strict requirement for non-agglomerated nanoparticles.

Controlled release of precipitating agents through solvothermal destabilization of microemulsions: one-pot synthesis of monoclinic zirconia nanostructures

We report a versatile one-step multiphase approach to control size, crystallinity and assembly at the nanoscale. The method takes advantage of the controlled release of precipitating agents achieved through the solvothermal destabilization of reverse microemulsions. This method permits the preparation of nanoparticles (as an example we prepare zirconia materials) self-assembled in open (palmate leaf-like) and compact nanostructures that could be easily processed using existing solution routes. The mechanisms of formation of these nanostructures are presented on the basis of the structural information and surface charge data provided by various techniques. The morphology of the nanostructures is found to be the combined result of several growth features, among which self-assembly driven by electrostatic forces plays a critical role. In addition, we have found that more open nanostructures lead to monoclinic zirconia samples with a higher degree of crystallinity, which supports the idea that surface energy must play a critical role in the stabilization of tetragonal polymorphs or amorphous zirconia phases.

A new ex vivo method to evaluate the performance of candidate MRI contrast agents: a proof-of-concept study

BackgroundMagnetic resonance imaging (MRI) plays an important role in tumor detection/diagnosis. The use of exogenous contrast agents (CAs) helps to improve the discrimination between lesion and neighbouring tissue, but most of the currently available CAs are non-specific. Assessing the performance of new, selective CAs requires exhaustive assays and large amounts of material. Accordingly, in a preliminary screening of new CAs, it is important to choose candidate compounds with good potential for in vivo efficiency. This screening method should reproduce as close as possible the in vivo environment. In this sense, a fast and reliable method to select the best candidate CAs for in vivo studies would minimize time and investment cost, and would benefit the development of better CAs.ResultsThe post-mortem ex vivo relative contrast enhancement (RCE) was evaluated as a method to screen different types of CAs, including paramagnetic and superparamagnetic agents. In detail, sugar/gadolinium-loaded gold nanoparticles (Gd-GNPs) and iron nanoparticles (SPIONs) were tested. Our results indicate that the post-mortem ex vivo RCE of evaluated CAs, did not correlate well with their respective in vitro relaxivities. The results obtained with different Gd-GNPs suggest that the linker length of the sugar conjugate could modulate the interactions with cellular receptors and therefore the relaxivity value. A paramagnetic CA (GNP (E_2)), which performed best among a series of Gd-GNPs, was evaluated both ex vivo and in vivo. The ex vivo RCE was slightly worst than gadoterate meglumine (201.9 ± 9.3% versus 237 ± 14%, respectively), while the in vivo RCE, measured at the time-to-maximum enhancement for both compounds, pointed to GNP E_2 being a better CA in vivo than gadoterate meglumine. This is suggested to be related to the nanoparticule characteristics of the evaluated GNP.ConclusionWe have developed a simple, cost-effective relatively high-throughput method for selecting CAs for in vivo experiments. This method requires approximately 800 times less quantity of material than the amount used for in vivo administrations.

Laser-Assisted Synthesis of Colloidal Ni/NiOx Core/Shell Nanoparticles in Water and Alcoholic Solvents

The nanosecond-pulse laser-assisted generation of Ni/NiOx core/shell nanoparticles (NPs) in water and alcoholic fluids can yield colloidal solutions without surfactants. The size distribution can be controlled by the nature of the alcohol, the number of laser pulses and the laser fluence. The incubation of the nickel target ablation in liquid contact shows a dependence on the carbon number of the respective alcohol. The laser-generated NPs consist of crystalline nickel cores with face-centred cubic patterns and stacking fault defects surrounded by nickel oxide shells. The solvent butanol, in contrast to ethanol and isopropanol, yields a narrow, nearly unimodal, size distribution. The majority of NPs have low size distributions, with medians in the range of 10-20 nm. These can be related to a metal ablation plume interacting with a supercritical liquid that decelerates the ejected material in a low-density metal-water mixing region. NPs in the range above 30 nm result in a minority distribution tail that strongly depends on the fluid nature, the pulse number and the fluence. This coarse NP set may be correlated with the rupture of a superheated molten-metal layer into larger entities.

Modeling of the laser pyrolysis process by means of the aerosol theory: Case of iron nanoparticles

Laser pyrolysis is a technique in which the interaction between a laser and a gaseous flow of precursors is used to obtain homogeneous nanoparticles. One of the main advantages of using this method is that it generates ultrafine powders in a continuous way with narrow particle-size distribution. The absence of surfactants of potential toxicity makes the product ideal for the preparation of colloidal dispersions for use in biomedical applications. It is of particular interest in the case of the iron nanoparticles due to their high magnetic response. In this paper, a simple coagulation model adapted from the theory of aerosol formation is successfully used in the modeling of the production of iron nanoparticles. The experimental conditions needed to maximize the productivity were obtained as a function of particle size. The main conclusion is that for the production of “large” particle-size nanomaterials (>20 nm), the ruling factors are the pressure and the carrier gas flux. However, the production of small...