Boron nitride nanotubes for boron neutron capture therapy as contrast agents in magnetic resonance imaging at 3 T L. Menichetti a,n , D. De Marchi b , L. Calucci c , G. Ciofani d , A. Menciassi d , C. Forte c a CNR—National Research Council of Italy, Institute of Clinical Physiology, via G. Moruzzi 1, 56124 Pisa, Italy b Fondazione Toscana Gabriele Monasterio per la Ricerca Medica e la Sanit  a Pubblica, CNR—Regione Toscana, via Trieste 41, 56126 Pisa, Italy c CNR—National Research Council of Italy, ICCOM—Institute of OrganoMetallic Chemistry, via G. Moruzzi 1, 56124 Pisa, Italy d Italian Institute of Technology c/o Scuola Superiore Sant’Anna, viale R. Piaggio 34, 56025 Pontedera, Italy article info Available online 25 February 2011 Keywords: Magnetic nanoparticles Relaxivity Multifunctional nanomaterials Longitudinal and transverse proton relaxation Contrast agents abstract The applicability of boron nitride nanotubes (BNNTs) containing Fe paramagnetic impurities as contrast agents in magnetic resonance imaging (MRI) was investigated. The measurement of longitudinal and transverse relaxation times of water protons in homogeneous aqueous dispersions of BNNTs wrapped with poly(L-lysine) at different concentrations allowed longitudinal (r 1 ) and transverse (r 2 ) relaxivities to be determined at 3 T. The r 2 value was comparable to those of commercial superparamagnetic iron oxide nanoparticles, indicating that Fe-containing BNNTs have the potential to be used as T 2 contrast- enhancement agents in MRI at 3 T. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Boron nitride nanotubes (BNNTs) are nanostructured com- pounds having a graphite-like sheet structure constituted by alternating B and N atoms, very recently investigated for a wide range of biomedical applications for their chemical and physical properties (Golberg et al., 2007; Zhi et al., 2010). Very recently BNNTs have been proposed as delivery agents able to target high boron concentration in neoplastic cells for boron neutron capture therapy (BNCT) (Ciofani et al., 2010). A recent publication by some of us (Ciofani et al., 2009b) demonstrated that the cellular uptake of BNNTs can be modulated with an external magnetic field exploiting the superparamagnetic properties imparted to these materials by Fe nanoparticles present as impurities deriving from the solid-state preparation (Chen et al., 1999, 2002; Yu et al., 2005). The same BNNTs have been shown to be cytocompatible and well tolerated by several cell lines (Ciofani et al., 2008a, 2008b). The presence of paramagnetic impurities renders these materials also suitable as magnetic resonance imaging (MRI) contrast agents, as already done in the case of carbon nanotubes (Al Faraj et al., 2009; Ananta et al., 2009; Choi et al., 2007), or as magnetic-field-guided drug delivery vehicles (Cherukuri et al., 2010; Frey et al., 2009; Laurent et al., 2008; Na et al., 2009; Veiseh et al., 2010; Villaraza et al., 2010). The performance of Fe-containing BNNTs as contrast agents has indeed been already assessed by us by determining the longitudinal (r 1 ) and transverse (r 2 ) water proton relaxivities of aqueous dispersions of BNNTs at 1.5 and 7.05 T from measure- ments of longitudinal (T 1 ) and transverse (T 2 ) relaxation times (Calucci et al., 2010). In the present paper, the same relaxation properties were measured at 3 T, a field which is now becoming more common for clinical MRI investigations thanks to the higher signal-to-noise ratio, spatial resolution, and speed with respect to the fields which have been commonly used up to now. 2. Materials and methods 2.1. Sample preparation BNNTs were provided by the Australian National University (Canberra, Australia). Sample purity and composition were as follows: overall BNNTs yield 80%, boron nitride 97 wt%, Fe and Cr derived from the milling process  1.5 wt% and absorbed O 2  1.5 wt%. Pristine BNNTs were stabilized in water using a polymeric wrapping with poly-L-lysine (PLL, 81339 from Fluka, MW 70,000–150,000). The dispersion was prepared with phos- phate buffer solution, mixing 5 mg of BNNTs with 10 mL of a 0.1 w/v% PLL solution in a polystyrene tube. The sample was sonicated for 12 h with a Bransonic sonicator 2510 using an output power of 20 W. After sonication, it was centrifuged at 1100g for 10 min to remove nondispersed residuals and impu- rities. Excess PLL was removed by ultracentrifugating three times at 30,000g for 30 min at 4 1C (Allegra 64R, Beckman). The concen- tration of BNNTs was quantified by spectrophotometric analysis using a LIBRA S12 spectrophotometer UV/vis/NIR (Biochrom), as Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/apradiso Applied Radiation and Isotopes 0969-8043/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2011.02.032 n Corresponding author. Tel.: + 39 050 315 2137; fax: + 39 050 315 2166. E-mail address: luca.menichetti@ifc.cnr.it (L. Menichetti). Applied Radiation and Isotopes 69 (2011) 1725–1727