The disclosure features methods that include: administering to a subject a composition that includes particles, where each one of the particles features at least one targeting group that binds to a structural entity in the subject and at least one reacting group that reacts chemically with a reactive oxygen species in the subject, and where the particle emits luminescence when the reaction occurs； detecting the luminescence emission from the particles； and displaying an image of the subject showing locations of at least some reactive oxygen species in the subject based on the detected luminescence.
