Two novel fluorescein-based OCl^- chemosensors were designed, synthesized and characterized. Both fluorescence probes utilize an irreversible OCl^--promoted oxidation reaction to trigger the activation of fluorescence. These probes were shown to be highly selective for OCl^-, and showed real time responses as well as a positive linear relationship to OCl^- concentrations. The probes were successfully applied for the detection of OCl^- in tap water and for fluorescence imaging of OCl^- in bacterial Rhodobacter ferrooxidans sp. strain SW2 cell–EPS–mineral aggregates.