CTSB and cancer: In the work, researchers developed one kind of amine-rich CDs and further covalently assembled the nucleolin-targeting recognition nucleic acid aptamer AS1411 and a CTSB-cleavable peptide substrate that tethered with chlorin e6 (Ce6), enabling a cancer-targeting and CTSB stimulus-responsive ratiometric nanoprobe of AS1411-Ce6-CQDs (Figure 6A).