Article pubs.acs.org/cm
Core/Shell Magnetite/Bismuth Oxide Nanocrystals with Tunable Size, Colloidal, and Magnetic Properties Manuel Andrés-Vergés,† María del Puerto Morales,*,‡ Sabino Veintemillas-Verdaguer,‡ F. Javier Palomares,‡ and Carlos J. Serna*,‡ †
Departamento de Química Orgánica e Inorgánica, Universidad de Extremadura, Avda Elvas s/n, 06071 Badajoz, Spain Instituto de Ciencia de Materiales de Madrid (CSIC), C/Sor Juana Inés de la Cruz 3, 28049, Madrid, Spain
‡
S Supporting Information *
ABSTRACT: New inorganic hybrid nanoparticles integrating different materials in a core/shell structure of iron and bismuth oxides have been synthesized by a simple aqueous route. The method is based on the precipitation of an Fe(II) salt in the presence of bismuth(III) cations in a mild oxidant and leads to highly uniform and crystalline magnetic nanoparticles with sizes in the range between 8 and 30 nm. Bismuth in proportions between 1 and 20% atomic ratio of Bi to Fe acts as surfactant being accumulated at the nanoparticle surface, controlling particle size and colloidal properties. Evidence of a core/shell structure is revealed by X-ray photoelectron spectroscopy analysis with bismuth enrichment in an outer shell. This robust material with very long chemical stability and resistance to degradation, good magnetic properties, and high density due to the presence of bismuth ions is expected to have important applications in diagnosis as a new double contrast agent for both magnetic resonance imaging and computed tomography. KEYWORDS: magnetic nanoparticles, core/shell nanoparticles, NMR contrast agent, hybrid inorganic nanosystems, size control
I. INTRODUCTION Magnetic nanocrystals have potential to bring dramatic improvements in biomedical sciences because of their size and magnetic properties. Among them, iron oxide particles ranging from nanometer to micrometer diameters are the most promising material for almost all medical applications because of their chemical stability, biocompatibility, natural routes for biodegradation, good magnetic response, and low price.1−3 Different inorganic hybrid nanostructures based on iron oxide have been synthesized to improve their magnetic performance and to obtain more efficient systems.4 The main interest in these nanocrystals is that each of the components should be able in principle to accomplish a specific task, which can be performed simultaneously. This has been achieved by first synthesizing nanoparticles with different properties, such as magnetic and luminescent, and the use of polymers as a “glue” for the whole.5−7 Additionally, interesting features might arise from the interfaces between components, which might be translated into new applications. It has been recently shown that a significant increase in the magnetic thermal induction by nanoparticles can be achieved by core/shell particles growth by a multistep seed-mediated particle process.8 However, it should be noted that most of these methods leading to these multipurpose nanostructures convey a high level of complexity. Here, we present an aqueous route for the synthesis of bismuth oxide-coated magnetite nanoparticles with sizes around the monodomain diameter (8−30 nm). The method is based on the Matijevic’s procedure9 carried out in the presence of bismuth(III) nitrate. Bismuth is expected to act as © 2011 American Chemical Society
surfactant, as it has been previously observed in semiconductors thin film,10,11 being accumulated at the nanoparticle surface, yielding hybrid inorganic nanoparticles in a single step. The bioutility of bismuth and its compounds has a 250 year history.12 Some compounds have been approved for humans for more than 30 years but others are currently under development. The diversity of bismuth compounds in medicine extends to the treatment of digestive disorders and tumors, in radioisotope therapies, in addition to its antimicrobial action. Bismuth-based nanoparticles have been employed also as contrast agents for standard X-ray computed tomography (CT) because of their high X-ray attenuation13 and gained particular relevance because of their use in a recently developed spectral scanner able to specifically detect and image bismuth.14 It would be highly desirable to have nanoparticles with both good magnetic properties and high X-ray attenuation, leading to a new generation of contrast agents for NMR and CT imaging techniques. Up to now, most of the work on the synthesis of magnetic materials containing bismuth has been focused on bismuth ferrites (BiFeO3) that belong to the group of multiferroic materials with numerous applications in spintronic devices.15 Some synthetic procedures have been described to obtain these materials in nanoparticulated form by solvothermal16 and sol− gel17 methods. However, magnetic properties of these Received: September 30, 2011 Revised: December 15, 2011 Published: December 15, 2011 319
dx.doi.org/10.1021/cm202949q | Chem. Mater. 2012, 24, 319−324
Chemistry of Materials
Article
Table 1. Structural and Magnetic Properties of the Core/Shell Magnetite/Bismuth Oxide Samplesa Sample
Biadded, at %
Bitotal, at %; ICP
Bisurface, at %; XPS
Particle size, nm; TEM
Crystal size, nm; XRD
Ms emu/g
Hc Oe
Mr emu/g
Bi0 Bi1 Bi5 Bi10 Bi20
0 1 5 10 20
0.06 0.9 4.9 9 19.2
0 8 35 60 84
35 30 19 10 8
37 32 16 10 7
80 80 69 55 43
69 47.5 21 25 25
6.3 4.6 2.7 2.6 0.7
Bismuth concentration is given in Bi/Fe atomic ratio ((Bi/Fe)at × 100); Bitotal is the Bi by ICP; Bisurface is the Bi by XPS (Bi4f/Fe2p); Magnetization values are normalized to gram of sample. a
The microstructure of the obtained nanoparticles was analyzed by X-ray diffraction (XRD) using Cu Kα radiation. The crystallite size was determined from the full-width at halfmaximum of the reflection (311) of magnetite by using the Scherrer equation. Cell parameters of samples were determined by a least-squares fit of the XRD data using silicon as internal reference standard. The infrared transmission spectra of the powders diluted in KBr (2 wt %) were recorded in a Nicolet 510 FT-IR spectrometer between 250 and 4000 cm−1. The bismuth content in the magnetite samples was determined by plasma emission (ICP, Perkin Elmer Optima 2100 DV). For this analysis, 25 mg of powder was first dissolved with concentrated HCl and then diluted with doubly distilled water. Bismuth was also analyzed in the supernatants after repeated washing to analyze possible leaching. The bismuth concentration at the particle surface was analyzed by X-ray photoelectron spectroscopy (XPS). XPS spectra were recorded with a SPECS Phoibos150 electron spectrometer using nonmonochromatic Al Kα radiation (1486.61 eV) at a pressure of 6 × 10−10 mbar. Fe2p, O1s, Bi4d5/2, and Bi4f core level XPS spectra were acquired with 20 eV pass energy and 0.1 eV energy step. Spectra have been acquired at takeoff angles from normal (0°) to 75° to significantly enhance emission from the nanoparticle outer surface region. Data analysis was performed using the CasaXPS processing software (Casa Software Ltd., Cheshire, UK). The integral peak areas after background subtraction and normalization using sensitivity factors were used to calculate the atomic concentration of each element. Magnetic characterization of the samples was carried out in a vibrating sample magnetometer (MLVSM9MagLab 9 T, Oxford Instrument). Magnetization curves were recorded at room temperature and 5 K by first saturating the sample in a field of 3 T. Saturation magnetization values (Ms) were evaluated by extrapolating to infinite field the experimental results obtained in the high field range where the magnetization linearly increases with 1/H. The squareness (Mr/Ms, where Mr is the remanent magnetization) and the coercive field (Hc) were also determined for each sample. The hysteresis loop of one sample containing a high bismuth proportion was taken at 5 K after cooling in the presence of a 5 T field to check the presence of extra anisotropy sources. Hydrodynamic size and surface charge of the “as prepared” nanoparticle aqueous dispersions as a function of pH were evaluated in a ZETASIZER NANO-ZS device (Malvern Instruments). Intensity data were analyzed to obtain the average size that is the most appropriate number produced by this technique to measure the hydrodynamic diameter of nanoparticles in solution. Zeta potential (ζ) was measured with a 0.01 M concentration of KNO3 at different pH values between 2 and 12, adjusted by adding KOH or HNO3.
compounds are very poor in comparison to those for magnetite and they would hardly be used in biothechnology. BiFeO3 is antiferromagnetic but the spins are in fact not perfectly antiparallel and there is a weak canting moment (ca. 10−6 μB per Fe).18 In summary, the synthesis and physicochemical properties of core/shell magnetic nanoparticles based on magnetite/bismuth oxide as a function of the bismuth proportion have been carried out. Mayor drawbacks of dual contrast agents based on fluorescent dyes or semiconductor and magnetic encapsulated particles19 such as stability or degradation in biological media, partial release, large sizes, or toxicity could be overcome here. These particles could show great potential in cancer therapy since they combine the ability to image a specific tumor area by two different techniques, NMR and tomography, and the ability to target it under the influence of a magnetic field and destroy the tumor with heat. Magnetic particles in this size range have shown the maximum heat efficiency for magnetic hyperthermia.20
II. EXPERIMENTAL SECTION Magnetite nanoparticles containing different amounts of bismuth were synthesized following a procedure previously described for pure magnetite.20 Iron(II) sulfate heptahydrate and bismuth(III) nitrate pentahydrate coprecipitation and the subsequent aging were carried out in a three-necked 500 cm3 round-bottom flask placed in an oil bath with mechanical stirring using a dual glass blade and under nitrogen flow. In a typical experiment, two solutions 2.4 × 10−1 M FeSO4 or a mixture of iron and bismuth salts (20 mL of water 2.0 × 10−2 M H2SO4), and 7.0 × 10−2 M NaOH and 0.1 M KNO3 (180 mL water−ethanol 80:100), were degasified for 1 h by nitrogen bubbling and mixed at room temperature. When the precipitation was completed, the system was heated to 90 °C and left undisturbed for 24 h. Then the suspension was cooled at room temperature with an ice bath, and the solid was separated by magnetic decantation and washed several times with distilled water. The final water dispersion obtained is fairly stable. This synthetic procedure only works for Bi/Fe atomic ratios ((Bi/Fe)at × 100) equal to or smaller than 20%. Higher bismuth proportions lead to an inhomogeneous system formed by mixtures of nonmagnetic iron oxides and bismuth oxides (Figure S1). Morphology, particle size, and distribution were investigated using a transmission electron microscope (JEOL-2000 FXII) operating at 200 keV. HRTEM images were obtained by means of a Philips TECNAI 200 T. A drop of the suspensions was deposited onto a carbon-coated copper grid and left to evaporate at room temperature. Mean size (D) and the standard deviation (SD) associated with it were evaluated from the electron micrographs by counting around 500 particles. 320
dx.doi.org/10.1021/cm202949q | Chem. Mater. 2012, 24, 319−324
Chemistry of Materials
Article
III. RESULTS AND DISCUSSION The main characteristics of the samples obtained in the present study as a function of the bismuth proportion are summarized in Table 1. The proportion of bismuth determined by ICP in the samples closely follows the preexistent in the reactant mixture, indicating that both iron and bismuth are precipitated during the reaction process. As the concentration of Bi increases from 0 to 20% (Bi/Fe)at × 100, nanoparticles size decreases from 35 nm down to 8 nm with no significant effect on the particle size distribution (SD = 1 nm). Larger amounts of bismuth (30%) lead to the precipitation of nonmagnetic iron oxide nanoparticles with an amorphous structure similar to ferrihydrite (Supporting Information). Previous studies on this synthesis method showed that mean size of magnetite nanoparticles is strongly dependent on the pH ranging from 50 nm up to 1 μm, being maximum at around 7 (the isoelectric point of magnetite) and lowering dramatically on either side of this pH.9 Further size reduction was achieved down to 30 nm by introducing ethanol in the media, hampering the nanoparticle growth process.20 An extra reduction in size has now been found down to 8 nm by introducing bismuth(III) cations in the reaction media, which is a clear indication of the interference of this element on the nanoparticle growth process as well as on the reaction kinetics. Particle shape is octahedral (Figure 1A,B) for low bismuth concentrations (
