- Hydrocyanine dyes are sensitive "turn-on" type optical probes that can detect reactive oxygen species (ROS). We have developed a method to prepare an 18 F-labeled hydrocyanine dye as a multi-modal PET and optical "turn-on" probe. A commercially available near infrared (NIR) dye was modified with a fluorinated prosthetic group that did not alter its ROS sensing properties in the presence of superoxide and hydroxyl radicals. The 18 F-labeled analogue was produced using a single-step terminal fluorination procedure. Positron emission tomography (PET) imaging and quantitative in vivo biodistribution studies indicated this novel probe had remarkably different pharmacokinetics compared to the oxidized cyanine analogue. The chemistry reported enables the use of quantitative and dynamic PET imaging for the in vivo study of hydrocyanine dyes as ROS probes.