Synthesis of colloidal magnetite nanocrystals using high molecular weight solvent† Ricardo H. Gonc ¸alves, * a Claudio A. Cardoso b and Edson R. Leite * a Received 18th August 2009, Accepted 31st October 2009 First published as an Advance Article on the web 15th December 2009 DOI: 10.1039/b917030h In this work, we describe a single-step synthetic route to obtain magnetic nanocrystals (MN), Fe 3 O 4 nanocrystals, soluble in different solvents. To synthesize grafted MN in a single step, we used a high molecular weight solvent (a polyol) that can be attached to the particle surface, which then transfers its solubility to the particle. Using polyols with different molecular weights and polarity enabled us to control the size, aggregation and morphology. The solubility behavior of the MN confirms our success in transferring the solubility of the adsorbed polymer to the nanocrystal. Introduction Magnetic nanocrystals (MN) have attracted increasing attention in recent years because of their unique physical properties such as superparamagnetism. This inherent property makes this class of nanocrystals desirable for medical imaging, magnetic field- assisted transport, drug targeting, agents for magnetothermal therapy and other applications of magnetic particle materials. 1–5 Advances in synthetic methods have allowed for the preparation of a wide range of highly crystalline and uniformly sized MN. 6–10 However, most technological applications require stabilized nanocrystals to prevent agglomeration and induce solubility in a suitable solvent. 11 Current applications require tunable surface chemistry. In biomedical applications, for instance, the ability to solubilize nanocrystals in water (at different pH values) is a crucial step toward their widespread use. In addition, appli- cations of MN materials will require solubility in different solvents. In general, the solubility and chemical functionality of MN is achieved by adding specific stabilizing compounds in the reaction system 12–14 or post-synthesis surface modifications, using methods such as molecular exchange, amphiphilic molecules and encapsulation. 15–22 The critical issues involved in the post- synthesis surface modification approach are the time and reagent-consuming process, since it requires a treatment after the MN synthesis and the selective chemical functionality induced by this treatment, resulting in nanocrystals with selective solu- bility. 23–25 For instance, the post-synthetic treatment that induces MN solubility in water is not suitable for promoting its solubility in a solvent with a lower solubility parameter (d) and different degrees of hydrogen bonding strength. A route that allows for the synthesis grafted MN in a single step has so far not been devised. The development of such a route is not simple, since control must be maintained of the MN’s critical parameters, such as particle size and shape, and magnetic behavior (saturation magnetization). In this work, we describe a novel single-step synthetic route to obtain functionalized MN (Fe 3 O 4 nanocrystals) soluble in different solvents. To develop this synthetic route, we modified the thermal decomposition method based on non-aqueous processes. 26–30 In general, non-aqueous processes allow for greater control of the reaction pathways on a molecular level, enabling the synthesis of nanomaterials with high crystallinity and well defined, uniform particle morphologies. It is important to keep in mind that the organic components used during the synthesis strongly influence the composition, size, shape, and surface properties of the inorganic product, and hence, the nanocrystal’s solubility behavior. Thus, we developed a solvent- controlled synthesis, without the addition of surfactant. To synthesize and control the solubility of MN in a single step, we used a high molecular weight solvent that can be attached to the particle surface, which then transfers its solubility to the particle. The solvents we selected were polyols with different molecular weights. Another important parameter of the process is the iron precursor. In this work, we selected the iron acetylacetonate complex Fe(acac) 3 , because this compound is highly soluble in the polyols used here. Scheme 1 summarizes the synthetic route Scheme 1 Schematic representation of the synthetic route developed in this work and representation of different self-assembly and partially oxidized polyol attachment to the MN surface. a Department of Chemistry, Federal University of Sa˜o Carlos, Sa˜o Carlos, SP, 13565-905, Brazil. E-mail: derl@power.ufscar.br; ricardohg.ufscar@ gmail.com; Fax: +55 16 3351 8214; Tel: +55 16 3351 8214 b Department of Physics, Federal University of Sa˜o Carlos, Sa˜o Carlos, SP, 13565-905, Brazil † Electronic supplementary information (ESI) available: Supplementary characterization including: particle size distribution, XRD, BF-STEM, photograph, infrared spectrum and magnetization curve. See DOI: 10.1039/b917030h This journal is ª The Royal Society of Chemistry 2010 J. Mater. Chem., 2010, 20, 1167–1172 | 1167 PAPER www.rsc.org/materials | Journal of Materials Chemistry