Surfactant-free Gd3+-ion-containing carbon nanotube MRI contrast agents for stem cell labelingElectronic supplementary information (ESI) available: NMRD profiles, the Fourier transforms of the EXAFS data, EXAFS curve fitting data, cell viability data. See DOI: 10.1039/c5nr02078f

There is an ever increasing interest in developing new stem cell therapies. However, imaging and tracking stem cells in vivo after transplantation remains a serious challenge. In this work, we report new, functionalized and high-performance Gd 3+ -ion-containing ultra-short carbon nanotube (US-tube) MRI contrast agent (CA) materials which are highly-water-dispersible ( ca . 35 mg ml −1 ) without the need of a surfactant. The new materials have extremely high T 1 -weighted relaxivities of 90 (mM s) −1 per Gd 3+ ion at 1.5 T at room temperature and have been used to safely label porcine bone-marrow-derived mesenchymal stem cells for MR imaging. The labeled cells display excellent image contrast in phantom imaging experiments, and TEM images of the labeled cells, in general, reveal small clusters of the CA material located within the cytoplasm with 10 9 Gd 3+ ions per cell. Porcine MSCs were safely label with a novel carbon nanotube-base contrast agent for MR imaging.