Fluorescent activatable probes for imaging biological processes and disease detection in vitro and in vivo. There is also described a method for detecting the presence of an enzyme. A sample is contacted with a nanoparticle including a monomer, at least one cleavable spacer comprising at least one fluorescence activation site, and at least two dyes of at least two types. The cleavable spacer is attached at one end to the nanoparticle. The at least two types comprise an energy donor type and an energy acceptor type. At least one dye of one type is embedded in the nanoparticle and the cleavable spacer has at least one dye of the other type attached. The at least two dyes comprise at least one matched pair capable of fluorescence resonance energy transfer. Fluorescence is detected by exposing the sample to a light source, and detecting emitted light with a detector.