Credit: © 2008 Wiley

Reactive oxygen species (ROS) such as the hydroxyl radical or superoxide anion O2− are toxic and have a role in several diseases. Fluorescent probes that can detect these species would be extremely useful as tools for medical diagnoses and research. Ones developed so far, such as dihydroethidium, suffer from several problems that limit their wider application. Now, Niren Murthy and colleagues at the Georgia Institute of Technology and Emory University, Atlanta, have made a new class of sensors that can image ROS in cells.

The hydrocynanines are synthesized by reducing cyanine dye molecules. Although negligibly fluorescent themselves, when they encounter ROS their fluorescence intensity increases 100-fold, making them ideal sensing molecules. Furthermore, some of the hydrocyanines are membrane-permeable to begin with and are able to pass into the cells, before oxidation makes them membrane-impermeable. This means they will accumulate in and identify cells that overproduce ROS.

Although they have a similar reaction mechanism to the previous probes, hydrocynanines have a much improved sensitivity. They can detect nanomolar amounts of hydrogen peroxide and have a linear response. A further advantage is that hydrocynanines are far more stable against autoxidation in aqueous solution, and were still 80% intact after 24 hours, compared with dihydroethidium falling to around 20% after an hour or two. The probes worked in cell cultures, tissue explants and even in vivo, offering great promise for their eventual use in real applications.