J. Phys. Chem. C 2007, 111, 7879-7882
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Monodisperse Binary Nanocomposite in Silica with Enhanced Magnetization for Magnetic Separation Chih Hao Yu,† Chester C. H. Lo,‡ Kin Tam,§ and Shik Chi Tsang*,† The Surface and Catalysis Research Centre, School of Chemistry, UniVersity of Reading, Reading RG6 6AD, U.K., Center for NondestructiVe EValuation and Ames Laboratory, Iowa State UniVersity, Ames, Iowa 50011, and AstraZeneca, Mereside, Alderley Park, Macclesfield, Cheshire SK10 4TG, U.K. ReceiVed: February 14, 2007; In Final Form: April 1, 2007
The majority of research on magnetic nanoparticles has focused on optical, electrical, and magnetic storage areas. Recently, the application of magnetic nanoparticles as magnetically separable nanovehicles for chemical or biological species has become an area of intensive research but with rather different challenging criteria that are yet to be addressed. For example, the enhancement of intrinsically weak magnetic properties, avoidance of magnetic interactions among particles, and improvement of the stability of the nanoparticles remain key issues. Here, it is demonstrated using sequential nanochemistry preparation techniques that exchange-coupled nanomagnets, such as FePt-Fe3Pt or FePt-Fe3O4 with dramatically enhanced magnetization, can be placed inside a silica nanosphere. The advantages of enhanced magnetization and the provision of protective coating and anchored sites on the silica shell surface render these new coated particles suitable for use in magnetic separation.
1. Introduction Magnetic nanoparticles have been intensively studied, not only for their electrical, optical, and magnetic properties but also for other new technological applications including magnetically assisted bioseparation and biocatalysis1-6 that pose radically different requirements of magnetic properties. Whereas the majority of current research is focused on the synthesis of magnetic nanoparticles with high magnetocrystalline anisotropies and coercivities for future magnetic recording applications,7,8 superparamagnetic or ferromagnetic/ferrimagnetic nanoparticles with low coercivities are required in biotechnology areas.1 The latter can be used in biomedicine as labeling or imaging reagents when tagged with biological entities of compatible size. Such approaches are very attractive to industry as magnetically tagged biomolecules with superparamagnetic properties can be isolated and recycled easily from solution using magnetic gradient fields, thus minimizing waste production through regeneration.3,4 The magnetophoretic mobility of a magnetic particle, which describes how easily the particle can be displaced in a medium under an applied gradient field, is higher for particles with higher saturation magnetization. However, the traditional single-phase magnetic materials of “nanometric” dimensions tend to show weak magnetic properties with a low level of magnetization on an individual particle basis, which adversely affects the separation efficiency. Single-phase FePt nanoparticles have been identified as a promising candidate for biotechnology applications because of their high magnetic moment (∼2.4 times larger than that of Fe3O4, which is widely used as a biomagnetic carrier) and good chemical stability. Chemical routes using high-temperature solution-phase conditions are the most widely chosen fabrication method for the * Corresponding author. E-mail:
[email protected]. Tel.: 44(0)1189316346. Fax:44(0) 1189316632. † University of Reading. ‡ Iowa State University. § AstraZeneca.
preparation of monodisperse FePt nanoparticles. The so-called “polyol process” involves co-reduction of platinum and iron precursors in the presence of polyol reducing agents. The size and composition of the nanoparticles prepared from the polyol method can be controlled with standard deviations usually within 10%.9-11 It is believed that Pt plays a critical role in accelerating the co-reduction of Fe species, which are considered to be difficult to reduce on their own through the polyol process.8 Most as-synthesized FePt nanoparticles are reported to have a chemically disordered face-centered cubic (fcc) structure11-13 and are superparamagnetic.14 Many studies of FePt nanoparticles employ postsynthesis annealing at temperatures above 500 °C to convert the material to a face-centered tetragonal (fct) structure (the L10 phase), which shows a higher magnetocrystalline anisotropy associated with a higher coercivity.15,16 A high coercivity is undesirable for bioseparation applications, as the magnetic interactions among the nanomagnets in a small confined space could cause agglomeration or suspension of the particles in solution after removal of the applied magnetic field.5,6 This raises the need for a new approach to the production of magnetic nanoparticles with high magnetic moments but zero or low coercivities for biotechnology applications. It has recently been demonstrated that placing FePt in the close vicinity of antiferromagnetic Fe3O4 could dramatically enhance the magnetization of the composite material through exchange coupling between the components.17 It has also been shown that, by reductive annealing of the FePt-Fe3O4, an FePtFe3Pt magnetic nanocomposite can be fabricated with effective exchange coupling between the FePt hard phase and the Fe3Pt soft phase, giving rise to a higher-energy product and higher magnetization compared to traditional single-phase nanomagnets.17-20 However, phase segregation and sintering were observed in previous work, which was manifested as large islands or domains of FePt and Fe3Pt during the reduction process.21 Despite the enhancement in magnetization, the sample was associated with a significantly high coercivity (>20 kOe)
10.1021/jp071289a CCC: $37.00 © 2007 American Chemical Society Published on Web 05/15/2007
7880 J. Phys. Chem. C, Vol. 111, No. 22, 2007 because of the coupling between the domains and was therefore unsuitable for use as a biomolecule carrier.21 An alternative to controlling the dimensions of different phases and achieving the needed intermixing and exchange coupling is therefore required for the fabrication of magnetic nanoparticles with high magnetization but low coercivity. Here, we report that, using sequential nanochemistry preparation techniques, FePt-Fe3O4 and FePt-Fe3Pt nanocrystallites of tailored size can be fabricated and then encapsulated in silica. The silica coating offers the composite nanoparticle a physical barrier against sintering by modifying its diffusion rate and surface energy during the high-temperature reduction required for the formation of Fe3Pt without severe particle agglomeration. Also, the silica encapsulation provides anchored sites for biomolecule immobilization and isolates the nanomagnets from magnetic interference with each other. In this short article, we demonstrate the successful fabrication of silica-encapsulated FePt-Fe3O4 and FePt-Fe3Pt composite nanoparticles with enhanced magnetization and a low coercivity (
