The sensing system is composed of a rationally designed peptide nucleic acid (PNA)–peptide–PNA triblock copolymer, which can be divided by tumor-overexpressed cathepsin B. The cathepsin B-activatable probe is in double-stranded form, which is the Cy5-labeled DNA aptamer strand and the quencher (BHQ2)-labeled short complementary DNA strand. The gene discussed is CTSB; the disease is neoplasm.