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.