Ultra‐small, Uniform, and Single bcc‐Phased FexCo1‐x/Graphitic Shell Nanocrystals for T1Magnetic 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.