Magnetic resonance imaging (MRI) is one of the most promising techniques for the non-invasive visualization of biomarkers and biologically relevant species, both in vivo and ex vivo. Although 1 H MRI with paramagnetic contrast agents, such as Gd 3 complexes and iron oxide, is widely used, it often suffers from low contrast because of the large background signals caused by the abundant distribution of protons in biological samples. Here we report the use of supramolecular organic nanoparticles to detect specific proteins by 19 F-based MRI in an off/on mode. In NMR spectroscopy these designed probes are silent when aggregated, but in the presence of a target protein they disassemble to produce a sharp signal. This turn-on response allowed us to visualize clearly the protein within live cells by 19 F MRI and construct an in-cell inhibitor assay. This recognition-driven disassembly of nanoprobes for a turn-on 19 F signal is unprecedented and may extend the use of 19 F MRI for specific protein imaging.