3712 IEEE TRANSACTIONS ON MAGNETICS, VOL. 36, NO. 5, SEPTEMBER 2000 Raman Spectroscopy in Oleoylsarcosine-Coated Magnetic Fluids: A Surface Grafting Investigation P. C. Morais, Member, IEEE, S. W. da Silva, M. A. G. Soler, and N. Buske Abstract—Raman spectroscopy is used to investigate magnetite nanoparticles pre-coated with oleoylsarcosine and dispersed as stable water-based (WB) and hydrocarbon-based (HB) mag- netic fluids (MF’s). The Raman spectra obtained from the MF samples and from liquid water were compared and the data discussed in terms of the suppression of the symmetrical hydroxyl (OH) vibrational modes. The effective grafting coefficient of the oleoylsarcosine in the WB-MF, with respect to the HB-MF, was estimated through the OH-stretching Raman modes of the OH-group chemisorbed at the nanoparticle surface. It was found that the OH-grafting coefficient in the HB-MF sample is about 3 times lower than the grafting coefficient in the WB-MF sample. Index Terms—Ferrites, magnetic fluids, nanocrystalline mate- rials, Raman spectroscopy. I. INTRODUCTION T HERE has been an increasing interest on MF’s based on pre-coated magnetic nanoparticles, for they represent the material basis in the development of specific biocompatible MF’s [1], [2]. The interest on biocompatible MF’s is due to a variety of potential application, as for instance, in drug delivery systems [3], contrast agents in magnetic resonance imaging [4], and hyperthermia of biological tissue with the goal of tumor therapy [5]. In the biocompatible MF applications, the physicochemical properties of the chemisorbed molecular coating layer plays a key role, though little has been done to investigate its structure and interaction with the surrounding medium. In many biological applications one seeks to achieve the highest molecular specificity associated to the ideal grafting coefficient, in order to have the best efficiency in the employed treatment. The biological specificity and the grafting coefficient of a particular bioactive molecule depend upon the efficiency of the link between the nanoparticle surface and the biomolecule. However, before the nanoparticle can be coated with a particular biomolecule, at a specific grafting coefficient, its surface has to be molecular designed using pre-coating species. Therefore, the understanding of the structure and properties of the link between the biomolecule and the nanoparticle surface is of fundamental importance for the engineering of biocompatible MF. Among the experimental techniques used to investigate Manuscript received February 14, 2000. This work was supported in part by the Brazilian agenciesCNPq, FAP-DF, PADCT, and CAPES. P. C. Morais, S. W. da Silva, and M. A. G. Soler are with Universidade de Brasília, Instituto de Física, Núcleo de Física Aplicada, CEP 70919-970, Brasília-DF, Brazil (e-mail: pcmor@fis.unb.br). N. Buske is with Medioport Kardiotechnik GmbH, Weisenweg 10, D-12274 Berlin, Germany. Publisher Item Identifier S 0018-9464(00)07850-X. MF neither the traditional magnetization measurements nor the unconventional magnetic resonance would be of great help to monitor the molecular layer chemisorbed at the nanopar- ticle surface. In contrast, Raman spectroscopy emerges as a promising and powerful technique to investigate qualitatively as well as quantitatively chemisorbed species at the nanoparticle surface. In a previous work we have studied the interaction of the molecular layer chemisorbed at the nanoparticle surface in ionic WB copper and zinc-ferrite MF’s [6]. Single-coated (dodecanoic acid) and double-coated (dodecanoic acid plus ethoxylated polyalcohol) magnetite-based MF’s have been recently investigated using Raman spectroscopy as well [7]. In this work, Raman spectroscopy was used to investigate magnetite nanoparticles pre-coated with oleoylsarcosine and dispersed as WB-MF or as HB-MF. The two functional groups (amino and carboxyl) of the oleoylsarcosine are used to bind to the nanoparticle surface as well as to guarantee the water-based colloid stability. On the other hand, when peptized in hydrocarbon, both functional groups may be used to bind to the nanoparticle surface and the colloidal stability is achieved by the interaction of the nonpolar chain of the coating molecule with the solvent. In particular, the usefulness of the Raman spectroscopy to track the pre-coating grafting in nanomagnetic particle is demonstrated. The Raman probe used to investigate the grafting coefficient is the OH-stretching mode associated to the OH-group chemisorbed at the nanoparticle surface. II. EXPERIMENTAL PROCEDURE The samples used in the present work were obtained by chemical co-precipitation of Fe (II) and Fe (III) ions in alkaline medium, following treatment with oleoylsarcosine and pep- tized either in water or in hydrocarbon to yield stable MF’s, as described elsewhere [8]. Transmission electron microscopy was used to obtain the average nanoparticle diameter (9 nm) and the corresponding logarithmic standard deviation (0.35). The nanoparticle concentration in both samples were of the order of particle/cm . The Raman setup consists of a double 0.85 m 1401 Spex monochromator equipped with the a photomultiplier tube (dark count of about 5 s ). The samples were optically excited with the 514 nm line from an Argon ion laser, at an optical power of the order of 150 mW, outside the sample holder. The laser beam passes vertically through the sample holder, i.e., parallel to the spectrometer entrance slit. The scattered light is collected from a small rectangular area of about 1/2 mm wide 1 mm high. The Raman measurements were performed at room temperature in the typical range of the intra-molecular OH-stretching modes (around 3500 cm ). 0018–9464/00$10.00 © 2000 IEEE