CTSB and cancer: Amine-rich CDs with blue fluorescence were synthesized from CA and PEI and were then covalently assembled with nucleolin-targeting recognition nucleic acid aptamer AS1411 and a CTSB-cleavable peptide substrate that tethered with near-infrared (NIR) fluorophore chlorin e6 (Ce6-Pep), forming the nanoprobe AS1411–Ce6–CDs for cancer targeting and the imaging of cathepsin B (CTSB, a biomarker for malignant tumors), in living cancer cells (Figure 4(c)).