ARTICLE pubs.acs.org/JPCC
Stabilization in Water of Large Hydrophobic Uniform Magnetite Cubes by Silica Coating Gema Marcelo,* Ernesto Perez, Teresa Corrales, and Carmen Peinado† Instituto de Ciencia y Tecnología de Polímeros (ICTP-CSIC), Juan de la Cierva 3, 28006 Madrid, Spain
bS Supporting Information ABSTRACT: Novel water dispersible large cubic nanocapsules consisting of uniform hydrophobic magnetite core (79 and 126 nm) and amorphous silica shell were prepared. Silica modification of magnetite particles and functionalization of silica surface by introducing methylphosphonate groups were performed at the same time. These nanoparticles were characterized by both transmission and scanning electron microscopy, dynamic light scattering, ζ potential determinations, X-ray techniques, Fourier-transform infrared spectroscopy, and magnetization measurements. The results show that the methodology employed to coat the hydrophobic magnetite with a silica shell provides uniform coreshell magnetitesilica nanoparticles with a good colloidal stability in water and control of the morphology and of the silica thickness. Thus, the high hydrophobicity and strong magnetic dipole interaction of these magnetite nanoparticles are tailored by silica modification, therefore opening a new range of applications in water for these magnetic nanoparticles.
’ INTRODUCTION Magnetic nanoparticles, especially magnetite and maghemite, are of great interest for a broad range of applications, due to their biocompatibility, the Food and Drug Administration (FDA) approval (as magnetic resonance imaging contrast agents), and absence of toxicity. These applications include magnetic fluids, data storage, catalysis, and bioapplications. Examples of applications in the study of biology and biomedicine are magnetic bioseparation, cell sorting, detection of biological entities, clinical diagnosis and therapy (such as magnetic resonance imaging, MRI, and magnetic fluid hyperthermia, MFH), targeted drug delivery, immunoassays, and bio-macromolecules purification.1 In recent years, much attention has been focused on the synthesis of uniformly sized magnetite nanoparticles.2 Although ferri- or ferromagnetic nanoparticles are desirable for many of these applications, super-paramagnetic nanoparticles of 60 nm), with high hydrophobicity and strong magnetic dipole interaction, can be easily dispersed in water by surface silica modification. That methodology leads to well-defined and independent magnetite silica structures with a uniform silica shell thickness. There is not background in the literature about silica modification of this kind of nanoparticles. The controlled synthesis of magnetitesilica nanocomposites was performed via a seeded solgel approach using electrostatically stabilized magnetite nanoparticles with citrate groups as seeds under ultrasonic vibrations. The ligand exchange on the magnetite surface, from oleic acid (synthesis capping ligand) to citrate, plays a very important role, because it affords that these nanoparticles can also be transferred to water and kept welldispersed during silica modification. Moreover, modification and functionalization of silica shell by introducing methylphosphonate groups was performed at the same time. Functionalization could be deduced from ζ potential measurements at acid pH. Besides, as it is shown by TEM and SEM images, functionalization leads to more defined and independent coreshell nanoparticles. Silica modification of the hydrophobic magnetite nanoparticles has a double effect on their physical behavior: (1) a change of the hydrophobicity of the magnetite, as shown by DLS and ζ potential measurements and (2) a decrease of the magnetic dipole coupling interaction among particles, as deduced from coercivity values. Therefore, the nanoparticles are easier to disperse in water, and a large range of applications (MRI, drug delivery, magnetic fluid hyperthermia, and catalysis, etc.) is open, which was unexplored before due to the high hydrophobicity and strong magnetic dipole interaction of the unmodified magnetite nanoparticles. The modified nanoparticles are appropriate for biomedical applications, such as contrast agents for magnetic resonance imaging and drug delivery. ’ ASSOCIATED CONTENT
bS
Supporting Information. Figures showing TEM images of magnetite nanoparticales, of the 126 nm Fe3O4 nanocubes, and reaction products prior to the isolation of Fe3O4@silica nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes †
Deceased August 2011.
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