Noncovalent Functionalization of Carbon Nanotubes with Amphiphilic Gd 3+ Chelates: Toward Powerful T 1 and T 2 MRI Contrast Agents Cyrille Richard,* ,² Bich-Thuy Doan, ‡,§ Jean-Claude Beloeil, ‡,§ Michel Bessodes, ² E Ä va To ´ th, § and Daniel Scherman* ,² Unite ´ de Pharmacologie Chimique et Ge ´ ne ´ tique; CNRS, UMR 8151, Paris, F-75270 cedex France; Inserm, U 640, Paris, F-75270 cedex France; UniVersite ´ Paris Descartes, Faculte ´ des Sciences Pharmaceutiques et Biologiques, Paris, F-75270 cedex France; ENSCP, Paris, F-75231 cedex France; Laboratoire de RMN Biologique, ICSN, CNRS, UPR 2301, 91198 Gif-sur-YVette cedex France, and CBM, CNRS, UPR 4301, Rue Charles Sadron, 45071 Orle ´ ans cedex France Received October 1, 2007 ABSTRACT An amphiphilic gadolinium (III) chelate (GdL) was synthesized from commercially available stearic acid. Aqueous solutions of the complex at different concentrations (from 1 mM to 1 μM) were prepared and adsorbed on multiwalled carbon nanotubes. The resulting suspensions were stable for several days and have been characterized with regard to magnetic resonance imaging (MRI) contrast agent applications. Longitudinal water proton relaxivities, r 1 , have been measured at 20, 300, and 500 MHz. The r 1 values show a strong dependence on the GdL concentration, particularly at low field. The relaxivities decrease with increasing field as it is predicted by the Solomon-Bloembergen-Morgan theory. Transverse water proton relaxation times, T 2 , have also been measured and are practically independent of both the frequency and the GdL concentration. An in vivo feasibility MRI study has been performed at 300 MHz in mice. A negative contrast could be well observed after injection of a suspension of functionalized nanotubes into the muscle of the leg of the mouse. Magnetic resonance imaging (MRI) is one of the most powerful diagnostic techniques in clinical medicine for in vivo assessment of anatomy and biological function. 1,2 MRI is based on the property of mainly water hydrogen nuclei to precess around an applied magnetic field. By applying radio frequency pulses and magnetic field gradients, the relaxation processes through which they return to their original aligned state can be exploited to give an image. The contrast of the image is related to various physical parameters, such as the local differences in spin relaxation kinetics along the longitudinal (spin-lattice relaxation time, T 1 ) and transverse (spin-spin relaxation time, T 2 ) planes of the main magnetic field applied to the specimen. Paramagnetic contrast agents (CA) are frequently used to enhance the image contrast. 3 They reduce T 1 (positive agents) and/or T 2 (negative agents) relaxation times of water protons. Positive contrast agents are Gd 3+ complexes in majority and provide brighter images, whereas negative contrast agents are mainly superparamag- netic iron-oxide nanoparticles and produce darker images. 4 The efficiency of an MRI CA is expressed in terms of its relaxivity (r 1,2 ), defined as the paramagnetic relaxation rate enhancement referred to 1 mM concentration of the agent. 5 The clinically used Gd 3+ complexes have low relaxivities. To increase their efficacy, the number of Gd 3+ ions should be increased. Relaxivity is strongly dependent on the molecular motion, hence on the size and rigidity of the Gd 3+ chelate. In the recent years, various macromolecular carriers have been explored, involving proteins, 6 dendrimers, 7 linear polymers, 8 water-soluble fullerenes, 9 or micellar structures. 10 Carbon nanotubes are ultrasmall cylinders of few mi- crometers in length and several nanometers in diameter, exclusively made of carbon atoms. 11 Recently, we have reported the noncovalent functionalization of carbon nano- tubes via chemical adsorption of various anionic surfactants. 12 The negative charge created by the surfactant adsorbed on the nanotube surface prevents their aggregation and induces stable suspensions in water. We report herein the first example of noncovalent functionalization of the outer surface * Corresponding authors. E-mail: (C.R.) cyrille.richard@univ-paris5.fr. ² Unite ´ de Pharmacologie Chimique et Ge ´ne ´tique; CNRS, UMR 8151, Paris, F-75270 cedex France; Inserm, U 640, Paris, F-75270 cedex France; Universite ´ Paris Descartes, Faculte ´ des Sciences Pharmaceutiques et Biologiques, Paris, F-75270 cedex France; ENSCP, Paris, F-75231 cedex France. ‡ Laboratoire de RMN Biologique, ICSN, CNRS, UPR 2301, 91198 Gif- sur-Yvette cedex France. § CBM, CNRS, UPR 4301, Rue Charles Sadron, 45071 Orle ´ans cedex France. NANO LETTERS 2008 Vol. 8, No. 1 232-236 10.1021/nl072509z CCC: $40.75 © 2008 American Chemical Society Published on Web 12/19/2007