Magnetite/Polypyrrole Hybrid Nanocomposites as a Promising Magnetic Resonance Imaging Contrast Material Kleber G. B. Alves, 1 Cesar A. S. Andrade, 2 Sergio L. Campello, 3 Ricardo E. de Souza, 3,4 Celso P. de Melo 3,4 1 Departamento de Engenharia Meca ˆnica, Universidade Federal de Pernambuco, Recife 50670-901, Pernambuco, Brazil 2 Departamento de Bioquı ´mica, Universidade Federal de Pernambuco, Recife 50670-901, Pernambuco, Brazil 3 P os-Graduaça ˜o em Cie ˆncia de Materiais, Universidade Federal de Pernambuco, Recife 50670-901, Pernambuco, Brazil 4 Departamento de Fı ´sica, Universidade Federal de Pernambuco, Recife 50670-901, Pernambuco, Brazil Correspondence to: C. P. de Melo (E-mail: celso@df.ufpe.br) ABSTRACT: We have prepared magnetite nanoparticles (Fe 3 O 4 _NPs) almost spherical in shape with average particle size of 10 nm and successfully encapsulated them in an envelope of polypyrrole (PPY) chains via an emulsion polymerization route using sodium dodecyl sulfate as surfactant. The resulting PPY-coated Fe 3 O 4 _NPs (Fe 3 O 4 _NPs/PPY) suspensions were stable with particles exhibiting a trian- gular prismatic morphology and an average diameter below 100 nm. In fact, all colloidal solutions were stable in aqueous media with typical f-surface potential values of 33.9 mV (Fe 3 O 4 _NPs) and 20.0 mV (Fe 3 O 4 _NPs/PPY). Although X-ray diffraction studies revealed the presence of a magnetic phase Fe 3 O 4 , the identified diffraction peaks are consistent with the presence of a spinel structure of magnetite. A ferromagnetic behavior, such as lower coercive force (H c ¼ 0.065 T), was observed for all magnetic nanoparticles examined. The 1 H NMR relaxation times T 1 and T 2 of selected Fe 3 O 4 _NPs/PPY samples were also measured and their relaxivities r 1 (1.1 s 1 mM 1 ) and r 2 (61.9 s 1 mM 1 ) compare favorably to those of contrast agents commercially used in human examinations. We suggest that the present results indicate that these hybrid nanocomposites are promising materials for the development of a plat- form of specialized contrast agents for 1 H Magnetic Resonance Imaging. V C 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 000: 000–000, 2012 KEYWORDS: nanocomposites; surfactants; conducting polymers; colloids Received 12 June 2012; accepted 16 August 2012; published online DOI: 10.1002/app.38481 INTRODUCTION Polypyrrole (PPY), one of the best-studied conducting poly- mers, has received large attention as active component of nano- composites owing to the high environmental stability of its conducting oxidized form. Although biological and health appli- cations of this polymer include its use for the immobilization of enzymes, antibodies, and nucleic acids, 1 there has been an increased interest in exploiting the excellent in vivo biocompati- bility of this material. 2–4 At the same time, Fe 3 O 4 particles, named magnetite, have been widely studied because of their large range of possible applications in the preparation of ferro- fluids and catalytic materials, and as active elements in biologi- cal assays and chemical sensors. 5,6 However, magnetite micropar- ticles do not seem appropriate for use in magnetic resonance imaging (MRI) owing to the lack of sufficient contrast-to-noise ratio associated to the limited signal intensity. Things are differ- ent for iron oxide nanoparticles as substantial changes in the magnetic properties occur in this domain size as consequence of quantum confinement effects related to their large surface to volume area. In fact, iron oxide nanoparticles have been shown to play an important role as MRI contrast agents in many in vivo applications owing to the capability of allowing a better differentiation between healthy and pathological tissues. 7 As the parameters T 1 (spin–lattice or longitudinal relaxation time) and T 2 (spin–spin or transverse relaxation time) may differ from one tissue to the next, they can be used as a source of contrast in MRI images. The enhanced image definition obtained with the use of contrast agents of different T 2 /T 1 ratios enables par- ticular tissues to be visualized by increasing or decreasing the signal level of the particular area of interest relative to that of its surroundings. Contrast agents increase both longitudinal T 1 1 and transverse T 1 2 relaxation rates; those that raise the signal level of the target site relative to that of its surroundings are termed positive contrast agents, whereas those lowering the sig- nal level relative to their immediate vicinity are termed negative contrast agents. 8 Although the former causes a reduction in the T 1 relaxation time, resulting in brighter images, negative V C 2012 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2012, DOI: 10.1002/APP.38481 1