DOI: 10.1002/asia.201200950 Ultra-small, Uniform, and Single bcc-Phased Fe x Co 1-x /Graphitic Shell Nanocrystals for T 1 Magnetic Resonance Imaging Contrast Agents In Ae Choi, [a] Yan Li, [a, b] Da Jeong Kim, [a] Mou Pal, [a] Jee-Hyun Cho, [c] Kyujoon Lee, [d] Myung-Hwa Jung, [d] Chulhyun Lee, [c] and Won Seok Seo* [a] Introduction Inorganic nanocrystals are of great interest for biomedical applications due to their unique optical, magnetic, and elec- tronic properties. [1] In particular, magnetic nanocrystals have been widely used for in vivo applications such as magnetic resonance imaging (MRI) contrast enhancement, drug deliv- ery, cancer therapy, hyperthermia, and in vitro cell separa- tion. [2] For these biomedical applications, nanocrystals are required to be uniform so that each individual nanocrystal has nearly identical physical and chemical properties for controlled biodistribution and contrast enhancement ef- fects. [3] Especially, small-size magnetic nanocrystals are ad- vantageous for T 1 MRI contrast enhancement because they can enhance the T 1 effect by their large surface area and reduce the T 2 effect by their small magnetic moment. [4] However, only a few methods including colloidal synthesis have been reported for the synthesis of uniform and small- size magnetic nanocrystals. [5] Moreover, current investiga- tions have focused mainly on iron oxide-based magnetic ma- terials. Among all known magnetic nanocrystals, FeCo alloys have superior magnetic properties including the highest sat- uration magnetization and a high superparamagnetic limit of about 20 nm. [6] In spite of their superior magnetic proper- ties, the poor oxidation resistance and the potential cytotox- icity of bare FeCo nanocrystals have prevented their wide- spread biomedical applications. To circumvent these prob- lems, FeCo/graphitic carbon shell (FeCo/GC) nanocrystals were recently produced by a simple chemical vapor deposi- tion (CVD) method. [7] A single-layered graphitic carbon coverage over the nanocrystalline FeCo surface has been proven to prevent the rapid environmental degradation of core metals and to improve the biocompatibility. [7] More- over, it can provide the potential of using the nanocrystals as a photothermal agent based on the near-infrared optical absorbance of carbon. [7, 8] FeCo/GC nanocrystals functional- ized with phospholipid–poly(ethylene glycol) (PL–PEG) molecules were demonstrated to be the most powerful non- toxic agents for MRI contrast enhancement, drug delivery, and cancer therapies. [7, 9] Despite their important applica- tions, ultra-small, uniform, discrete, and single body-cen- tered-cubic (bcc)-phased FeCo/GC nanocrystals suitable for T 1 MRI contrast enhancement have not yet been report- ed. [10] Furthermore, a systematic study on the relationship between composition and magnetic resonance (MR) proper- ties of FeCo/GC nanocrystals has not been conducted so far. Abstract: We have synthesized ultra- small and uniform Fe x Co 1-x /graphitic carbon shell (Fe x Co 1-x /GC) nanocrystals (x = 0.13, 0.36, 0.42, 0.50, 0.56, and 0.62, respectively) with average diameters of < 4 nm by thermal decomposition of metal precursors in approximately 60 nm MCM-41 and methane CVD. The composition of the Fe x Co 1-x /GC nanocrystals can be tuned by changing the Fe:Co ratios of the metal precur- sors. The Fe x Co 1-x /GC nanocrystals show superparamagnetic properties at room temperature. The Fe 0.50 Co 0.50 /GC, Fe 0.56 Co 0.44 /GC, and Fe 0.62 Co 0.38 /GC nanocrystals have a single bcc FeCo structure, whereas the Fe 0.13 Co 0.87 /GC, Fe 0.36 Co 0.64 /GC, and Fe 0.42 Co 0.58 /GC nanocrystals have a mixed structure of bcc FeCo and fcc Co. The single bcc- phased Fe x Co 1-x /GC nanocrystals func- tionalized with phospholipid–poly(eth- ylene glycol) (PL–PEG) in phosphate buffered saline (PBS) are demonstrat- ed to be excellent T 1 MRI contrast agents. Keywords: alloys · graphitic shell · magnetic resonance · mesoporous materials · nanoparticles [a] I. A. Choi, Y. Li, D. J. Kim, Dr. M. Pal, Prof. W. S. Seo Department of Chemistry and Inorganic and Bio-Materials Center of BK21 Sogang University Seoul, 121-742 (Korea) E-mail : wsseo@sogang.ac.kr [b] Y. Li Interdisciplinary Program of Integrated Biotechnology Sogang University Seoul, 121-742 (Korea) [c] J.-H. Cho, Dr. C. Lee Division of Magnetic Resonance Research Korea Basic Science Institute Ochang, Cheongwon, Chungbuk 363-883 (Korea) [d] K. Lee, Prof. M.-H. Jung Department of Physics Sogang University Seoul, 121-742 (Korea) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201200950. Chem. Asian J. 2013, 8, 290 – 295  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 290 FULL PAPER