The utility of the single-molecule magnet Fe 8 as a magnetic resonance imaging contrast agent over a broad range of concentration Brant Cage a, * , Stephen E. Russek a , Richard Shoemaker b , Alex J. Barker c , Conrad Stoldt c , Vasanth Ramachandaran d , Naresh S. Dalal d,1,2 a National Institute of Standards and Technology, Electromagnetics Division, 325 Broadway, ms 818.03, Boulder, CO 80305, United States b Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80390, United States c Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309, United States d Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306, United States Received 24 August 2006; accepted 10 December 2006 Available online 14 December 2006 Abstract Recent reports have compared Fe 8(aq) versus Magnevist with seemingly conflicting conclusions: one claims a much greater efficiency of Fe 8 for magnetic resonance imaging contrast, and the other claims a much lower efficiency. Our study shows that at concentrations below 1.5 mM Fe 8(aq) had a T 1 relaxivity, r 1 , of 5.4 s 1 mM 1 which is comparable to Magnevist. Above 1.5 mM Fe 8(aq) had an r 1 of 1.1 s 1 mM 1 , significantly lower than Magnevist. These results agree with the previous literature over the concentrations they examined. The results for the T 2 relaxivity, r 2 , were similar. Here, we show that the concentration dependence of the relaxivity accounts for these discrepancies. Further, the relaxivity data are correlated with frequency-dependent maxima in v 00 ac of frozen solutions of Fe 8 dissolved in deionized water over the temperature range of 1.8–4 K and the frequency range of 200–1400 Hz. The magnetic properties of the single- molecule magnet Fe 8 in room temperature and frozen aqueous solution were found to be highly non-linear when examined over a wide concentration range. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Fe 8 ; MRI contrast agent; SMM; ac susceptibility; NMR; Relaxivity 1. Introduction The design and testing of magnetic resonance imaging (MRI) contrast agents is important for medical diagnostics. MRI contrast agents function by changing the nuclear reso- nant frequency, the longitudinal energy relaxation time T 1 , and the transverse dephasing time T 2 , of nearby aqueous hydrogen nuclei in biological systems. These changes allow the medical community to selectively enhance the contrast of structures of interest such as tumors, the vasculatory sys- tem, and lesions. The most common MRI contrast agents in current use are the gadolinium chelates and the superpara- magnetic iron oxides (SPIO), each of which have their advantages in terms of contrast applications [1]. Recently, there has been interest in the literature for using materials that bridge the gap between the constrained chemical and magnetic structure of the gadolinium chelates with the superparamagnetism of the SPIOs [2–4]. One class of mate- rials that may meet this need are the single-molecule mag- nets (SMM). These nanometer-scale magnetic structures possess large adjustable moments with anisotropy energies that allow remnant moment and controllable magnetic fluctuations. Among these, the SMM [Fe 8 O 2 (OH) 12 (1,4, 7-triazacyclononane) 6 ]Br 8 Æ 9H 2 O, henceforth Fe 8 , has 0277-5387/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.12.009 * Corresponding author. Tel.: +1 303 497 4224; fax: +1 303 497 7364. E-mail address: bcage@boulder.nist.gov (B. Cage). 1 Contribution of the National Institute of Standards and Technology; not subject to US copyright. 2 Certain commercial contrast agents are identified to specify the experimental study adequately. This does not imply endorsement by NIST or that the contrast agents are the best available for the purpose. www.elsevier.com/locate/poly Polyhedron 26 (2007) 2413–2419