Magnetic resonance imaging has become a critical technology in medical diagnostics, and chemists continue to devise new types of MRI contrast agents. In one of the latest examples, a team led by Eric T. Ahrens, Roger Y. Tsien, and Alexander A. Kislukhin of the University of California, San Diego, has created a paramagnetic fluorinated nanoemulsion with embedded metal ions that provides up to a fivefold boost in sensitivity over previously reported 19F MRI cellular detection systems (Nat. Mater. 2016, DOI: 10.1038/nmat4585). The researchers first crafted a metal-binding fluorinated compound by introducing β-diketone groups to the ends of a perfluoropolyether precursor. They then emulsified this fluorinated oil (yellow) in an aqueous solution containing a commercial surfactant. The researchers next added various transition and lanthanide metals, which were bound by the diketonate groups. In a surprise finding, the team observed that iron(III) provided the best MRI sensitivity, surpassing that of gadolinium(III), which typically provides the best image contrast for conventional 1H MRI. Kislukhin says the intended use of the new nanoemulsion is for labeling therapeutic stem cells and immune cells to track their movement via 19F MRI once the cells are implanted in a patient, which the team demonstrated in mice.