Journal of the Korean Physical Society, Vol. 62, No. 10, May 2013, pp. 1376∼1381 Air-stable Fe@Au Nanoparticles Synthesized by the Microemulsion’s Methods Jos´ e Rivas ∗ INL- International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal Yolanda Pi˜ neiro Redondo Department of Applied Physics, University of Santiago de Compostela University, Santiago de Compostela, E15782, Spain Esther Iglesias-Silva, J. M. Vilas-Vilela and L. M. Le´ on Physical-Chemistry Department, University of the Basque Country, Faculty of Science and Technology, Leioa, E-48940, Bizkaia, Spain Manuel Arturo L´ opez-Quintela Department of Chemistry Physics, University of Santiago de Compostela University, Santiago de Compostela, E15782, Spain (Received 31 May 2012, in final form 8 September 2012) Magnetic particles covered by gold are very important in many biological applications. However, there are not simple methods to produce small (< 5-10 nm) nanoparticles. One of the main reasons for that is the general use of iron oxides as magnetic cores, which have a large crystalline mismatch with gold. The use of Fe would be more appropriate, but its high tendency to oxidation has largely precluded it from being used as a core. Here, we will show that using a simple “one-pot” successive reaction method in microemulsions, can avoid such problems and is able to produce very stable core-shell Fe@Au nanoparticles. With this procedure, nanoparticles of ∼ 6 nm with a Fe core of 3 nm can easily be obtained. These Fe@Au nanoparticles, with a saturation magnetization of 1.13 emu/g, are very stable even in air after magnetic separation from the solution, which shows the good covering of the Fe core by the Au shell. In this contribution we will report the key parameters, which have to be taken into account, to prepare such stable Fe@Au dispersions and analyze their optical and magnetic properties, as well as their possible applications as biosensors, targeted magnetic separation, etc. PACS numbers: 75.75.Cd, 78.40.-q, 81.07.Bc, 81.16.Be Keywords: Chemical synthesis method, Fabrication of magnetic crystalline nanostructures, Absorbance spectra UV-Vis, XRD, HRTEM, Magnetometry DOI: 10.3938/jkps.62.1376 I. INTRODUCTION The interest in core-shell nanoparticles (NP) is driven by the fact that substantially different functionalities arising from the core and shell can be joined to obtain in a single nano-device a complete set of physical poten- tialities [1]. Gold reassembles a set qualities, such as it is non- toxic and biocompatible, it is chemically easy to engi- neer due to its ability for surface functionalization (via thiol groups bonding) and it has remarkable optical prop- erties with its surface plasmonic response that can be tuned from the visible to the near IR, making it the best choice as coating material for contrast agent in X-Ray to- mography, drug nanocarriers, enhanced imaging of cells, * E-mail: jose.rivas@inl.int bio-sensing, among others [2]. Iron presents a very large magnetic response arriving to a saturation magnetization M s = 215 emu/g at room temperature, more than twice that of magnetite, which is the biocompatible magnetic material most used in bio- applications. In this sense, the use of iron as magnetic core would allow to highly reduce the size of the nano- device or the external exciting magnetic source and in that, still being of magnetic interest. However, iron is non biocompatible and, from the chemical point of view, largely reactive on air conditions, therefore a protective coating has to be performed to as- sure stability against oxidation and assure non toxicity for bio-applications. Conveniently coated, magnetic NPs are used as contrast agents for MRI, magnetic heaters combined with therapeutic agents in magnetic hyper- thermia, magnetic driving agents for collecting target -1376-