CTSB and cancer: With the quenching of blue fluorescence from CDs and NIR fluorescence enhancement from the Ce6 by the efficient fluorescence resonance energy transfer (FRET), the overexpressed CTSB in lysosome could cleave Ce6-Pep and trigger the Ce6 moiety dissociation from AS1411-Ce6-CQDs after selective accumulation in cancer cells, thus leading to the termination of FRET and achieving the ratiometric fluorescence response to CTSB (Figure 6B).