Controlled Synthesis and Stability of Co@SiO 2 Aqueous Colloids M. Aslam, S. Li, and V. P. Dravid w Department of Materials Science and Engineering, International Institute for Nanotechnology, Northwestern University, Evanston, Illinois 60208 Magnetic nanoparticles have emerged as an important class of functional nanostructures with potential applications of magnet- ic resonance imaging, drug targeting, and bio-conjugation. We have developed a modified sol–gel approach to synthesize stable and well-dispersed magnetic Co@SiO 2 nanoparticles with im- proved control over shell thickness and larger core diameters. These well-defined Co@SiO 2 core–shell nanoparticles exhibit useful magnetic properties, and the protective silica shell allows them to be surface modified for bioconjugation for various bio- medical applications. The core–shell nanoparticles were charac- terized by transmission electron microscopy, energy-dispersive spectroscopy, elemental mapping, and the line compositional analyses to demonstrate that uniform individually isolated core– shell nanoparticles are obtained through the improved synthetic route. I. Introduction M AGNETIC nanomaterials in their various forms offer simple and versatile strategies for promising applications in bio- medical applications. 1–5 In particular, core–shell magnetic na- nostructures offer the interesting possibility of making use of the much-improved properties of an arbitrary magnetic core, and a protective coating that is amenable to surface modification for numerous diagnostic and therapeutic purposes. The surface modification of the shell not only provides a route for func- tionalization but also enhances the stability and biocompatibil- ity of the core particles. For example, bare metal/alloy nanoparticles (e.g., Co, CoSm, FePt, etc.), which have poor sta- bility under ambient conditions but have better magnetic prop- erties compared with oxide magnetic particles, could be ideal candidates in core–shell form for possible in vivo bio-applica- tions. However, most of these high-quality metallic particles (narrow and uniform particle size distribution with standard deviation r5%) are prepared under nonaqueous conditions, and surface functionalization strategies to disperse them in water are very critical, which invariably leads to a poor yield and a limited time stability under ambient conditions. 3,6,7 In principle, the properties could be tailored in a controlled fashion by independently designing the composition, structure, and di- mension of a biocompatible shell on top of the magnetic core. In addition, the shell can function as a unique platform for many bio-conjugation strategies and thus provide a multifunctional colloidal delivery system. 8,9 Apart from biomedical applications of magnetic core–shell structures, the other core–shell structures have also been explored as building blocks to build supracrystals with photonic properties different from the one based on con- ventional particles. 10 Various sol–gel-based precursor strategies have been used for generating core–shell structures of iron oxide; however, reports on core–shell structure formation for metallic particles like Co, Fe, and Ni are scarce. 6,11,12 In the case of Fe 3 O 4 nanoparticles, a dense and homogeneous silica layer could be directly grown from tetraethoxysilane (TEOS)-based high-pH gelation, while in other cases like Ag, Au, and Co, the surfaces of the particles are first modified with a primer silane-based monolayer that acts as a coupling agent for silica growth on the metal particle sur- face. 8,13,14 Both these methods result in a dense coating of ce- ramic material on top of the core nanoparticles. This simple and attractive approach has so far been applied to smaller particles. In this study, we report a homogeneous coating of silica as thick as 100 nm on core nanoparticles exceeding 70 nm in size. We have also investigated whether a larger size offers con- formal coverage, heterogeneous nucleation, and formation of naked silica particles, which have been reported for micron size particles. 15,16 Herein, we report the successful synthesis of a series of strik- ing core–shell nanoparticles with a control over core size as well as shell thickness. We have used a citrate-based approach to stabilize the Co particles and then applied the monolayer func- tionalization chemistry of aminopropoxysilane (APS) mole- cules, which reveal good linkage with Co layers, for further growth of TEOS through slow hydrolysis. We report their mor- phological and chemical identity using transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS) and magnetic properties as a function of various parameters. The magnetic properties are directly correlated to the morphological evolution at different temperatures. Moreover, the magnetic properties show an increment in saturation magnetization coer- civity, reaching a maximum value at 4001C and then decreasing drastically during the annealing process. II. Experimental Procedure (1) Materials Cobalt(II) chloride hydrate (99.999%), sodium borohydride (99%), sodium citrate dehydrate (99%), 3-aminopropyltrieth- oxysilane (99%), and TEOS (99%) were obtained from Sigma- Aldrich (St. Louis, MO), and used as received. Ethyl alcohol (200 proof) was obtained from Pharmco (Brookfield, CT), and used as received. Deionized water was distilled by a Milli-Q water purification system (Millipore, Billerica, MA). All glass- ware and Teflon-coated magnetic stir bars were cleaned with aqua regia, followed by copious rinsing with distilled water before drying in an oven. (2) Synthesis Procedure (A) Co Nanoparticle Synthesis: Citrate-stabilized Co na- noparticles were first prepared from the conventional NaBH 4 reduction of CoCl 2 . In a typical procedure, 02.6 mg of Co(II) chloride hydrate and 6 mg of sodium citrate were added to 100 mL of deoxygenated water. Deoxygenation was achieved by bubbling nitrogen through the solution in a vigorously stirred C. Randall—contributing editor This work was supported by Grant No. U54CA119341 from the NIH National Cancer Institute (NCI) under Center for Cancer Nanotechnology Excellence (CCNE) program at Northwestern and partially supported by NSF-NSEC and NU-IBNAM. This work was performed at the EPIC/NIFTI facility of the NUANCE center (supported by NSF-NSEC, NSF-MRSEC, Keck Foundation, the State of Illinois, and Northwestern University) at Northwestern University. The contents are solely the resoponsibility of the authors and do not necessarily represent the official views of the NIH. w Author to whom correspondence should be addressed. e-mail: v-dravid@northwestern. edu Manuscript No. 21947. Received June 27, 2006; approved November 13, 2006. J ournal J. Am. Ceram. Soc., 90 [3] 950–956 (2007) DOI: 10.1111/j.1551-2916.2007.01509.x r 2007 The American Ceramic Society 950