Ultra‐small, Uniform, and Single bcc‐Phased Fe x Co 1‐x /Graphitic Shell Nanocrystals for T 1 Magnetic Resonance Imaging Contrast Agents

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 precursors. 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 functionalized with phospholipid–poly(ethylene glycol) (PL–PEG) in phosphate buffered saline (PBS) are demonstrated to be excellent T 1 MRI contrast agents.