Methods and materials for the imaging of cells containing active proteases such as cathepsin are disclosed. The present materials include activity based probes that bind to an enzyme and are subsequently cleaved. Cleavage results in a fluorescent signal due to removal of a quenching group which, when present on the probe causes altered or no fluorescence. The probes employ an acyloxymethyl ketone reactive group, one or more amino acids for determining specificity, a fluorophore and a quencher. The probes are cell permeable and may use, for example, a QSY7 (diarylyrhodamine) quencher and a BODIPY (bora-diaza-indecene) dye.