As one of the most promising
biomarkers for numerous malignant
tumors, accurate and reliable reporting of
Cathepsin B (CTSB) activity is of great significance to achieve efficient diagnosis of
cancers at an early stage and predicting
metastasis. Here, we report a vigorous ratiometric fluorescent method integrating a
cancer-targeting recognition moiety with a remarkably large emission wavelength shift into a single matrix to report CTSB activity sensitively and specifically. As a proof of concept, we synthesized
amine-rich
carbon quantum dots (CQDs) with a blue fluorescence, which offered an efficient scaffolding to covalently assemble the
nucleolin-targeting recognition
nucleic acid aptamer
AS1411 and a CTSB-cleavable
peptide substrate Gly-
Arg-Arg-
Gly-Lys-
Gly-Gly-Cys-COOH that tethered with a near-infrared (NIR) fluorophore
chlorin e6 (Ce6-GRRGKGGC, Ce6-Pep), enabling a
cancer-targeting and CTSB stimulus-responsive ratiometric nanoprobe AS1411-Ce6-CQDs. Owing to the efficient fluorescence resonance energy transfer (FRET) process from the CQDs to Ce6 inside the assembly of nanoprobe, the blue fluorescence of CQDs at ∼450 nm was remarkably quenched, along with an obvious NIR fluorescence enhancement of Ce6 at ∼650 nm. After selective entry into
cancer cells via
nucleolin-mediated endocytosis, the overexpressed CTSB in lysosome could cleave Ce6-Pep and trigger the Ce6 moiety dissociation from AS1411-Ce6-CQDs, thus leading to the termination of FRET process, achieving the efficient ratiometric fluorescence response toward endogenous CTSB with a remarkably large emission wavelength shift of ∼200 nm from NIR to blue emission region. Notably, the nanoprobe AS1411-Ce6-CQDs exhibited an excellent specificity for ratiometric fluorescent sensing of CTSB activity with an ultralow detection limit of 0.096 ng/mL, demonstrating its promising use for early precise
cancer diagnosis in the near future.