Turn-on fluorescent mesoporous silica nanoparticle-based system was synthesized and characterized. A dark quencher was attached onto fluorescent silica nanoparticles via disulfide bond. The quencher rendered nanoparticles non-fluorescent through a Förster resonance energy transfer (FRET) mechanism. Nanoparticles regained fluorescence upon reduction of disulfide bonds and dissociation of quencher from the particles' surface. Experiments showed that the increase of fluorescence intensity was associated with a higher concentration of reducing agent and a longer exposure time of the system to the reducing agent. As disulfide-reducing compounds (e.g. glutathione) are attractive stimuli in controlled delivery of drugs, the present system could provide a visual information regarding the onset of the stimulus and, simultaneously, release of the drug, thus acting as a release sensor.