Effect of hydrophobic coating on the magnetic anisotropy and radiofrequency heating of γ-Fe 2 O 3 nanoparticles Mandeep Singh a , Pavel Ulbrich a , Vadym Prokopec a , Pavel Svoboda b , Eva Šantavá c , František Štěpánek a,n a Institute of Chemical Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic b Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 120 00 Prague 2, Czech Republic c Institute of Physics ASCR, Na Slovance 2, 182 21 Prague 8, Czech Republic article info Article history: Received 20 November 2012 Received in revised form 27 February 2013 Available online 13 March 2013 Keywords: Nanoparticle Superparamagnetism Saturation magnetization Blocking temperature Maghemite abstract The effect of a hydrophobic (oleic acid) coating on the magnetic properties of maghemite (γ-Fe 2 O 3 ) nanoparticles was investigated. The nanoparticles were prepared by a novel bi-phasic co-precipitation route and their properties compared with uncoated nanoparticles and nanoparticles prepared by a standard single-phase process. The oleic acid coated nanoparticles had a mean diameter of 6 nm when the two-phase precipitation procedure was used compared to 12 nm for nanoparticles prepared in a single phase under otherwise identical conditions. Super Quantum Interference Device measurements show superparamagnetism of the nanoparticles, with a saturation magnetization at 4 K to be 66.4 emu/g and 89.0 emu/g for the coated nanoparticles obtained by two- and single-phase procedure, respectively. Zero-field-cooled and field-cooled curves reveal a dramatic shift in the blocking temperature of the coated nanoparticles, and a significant change in their anisotropy. The hydrophobic nanoparticles were able to form stable ferrofluids in a range of organic solvents and show good heating rates in a 400 kHz alternating magnetic field. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Due to their stability and biocompatibility, magnetite (Fe 3 O 4 ) and maghemite (γ-Fe 2 O 3 ) nanoparticles find applications in diverse scientific and technological applications such as targeted drug delivery, magnetic resonance imaging (MRI) [1], hyperther- mia [1–3], or water remediation [4–6]. A number of synthesis approaches are known for producing maghemite nanoparticles, including co-precipitation [7], hydrothermal reactions [8], sol–gel reactions [4], or microemulsion methods [9,10]. The magnetic properties of maghemite nanoparticles depend on a number of factors including the particle size and size distribution, the pre- paration method used, and the presence and type of coating on the nanoparticles. In order to form “designer particles” with a priori known magnetic properties, it is important to systematically investigate the relationships between these factors. However, the challenge is to prepare the nanoparticles in such a way that only one factor is changed at a time. A commonly used method for the preparation of iron oxide nanoparticles is co-precipitation, whereby iron oxide particles are formed from water-soluble Fe 2 þ and Fe 3 þ salts by the addition of a precipitating agent such as NH 4 OH. This method has the advantage of being a rapid chemical pathway with a high yield at relatively benign conditions [7]. On the other hand, it is relatively difficult to control the size, size distribution and shape of the formed nanoparticles when co-precipitation is used in bulk. Therefore, microemulsion methods have been proposed to yield nanoparticles with a narrow size distribution [10]. By reducing the volume of the solution from which the nanoparticles are formed, the quantity of the precursors and therefore the maximum particle size can be better controlled. The magnetic properties of the nanoparticles depend not only on the phase and individual particle size obtained during the particle formation process, but also on subsequent interaction between the nanoparticles, namely their aggregation. Aggregation can be controlled by steric (coating the particles with a surfactant layer) or ionic (inducing opposite charge on the particle surface) stabilization. The use of low molecular weight compounds such as citric or oleic acid, polymers such as dextran or polyethylene glycol, or inorganic materials such as silica or gold has been reported to stabilize iron oxide nanoparticle dispersions in both aqueous and non-aqueous media [10–14]. Hydrophobic coatings are necessary in cases where the nanoparticles are to be dispersed in non-aqueous media such as organic ferrofluids, phospholipid bilayers (magnetoliposomes), or various wax or lipid-based com- posite nanoparticles. Additionally, if the nanoparticles are to be used as susceptors for local radiofrequency (RF) heating, e.g. to control drug release or the rate of local reaction–diffusion pro- cesses, then the influence of their hydrophobic coating on the specific absorption rate (SAR) is of interest. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.02.051 n Corresponding author. Tel.: þ420 220 443 236; fax: þ420 220 444 320. E-mail address: Frantisek.Stepanek@vscht.cz (F. Štěpánek). Journal of Magnetism and Magnetic Materials 339 (2013) 106–113