Aminopeptidase N (APN/CD13) plays an important role in the growth and metastasis, of tumor, and is a potential biomarker for the post-treatment surveillance of cancer reoccurrence and progression of various malignancies. Thus, we have designed and prepared a convenient and ultrasensitive APN-targeting activity-based ratiometric electrochemical molecular substrate (Ala-AFC) for direct real-time monitoring of APN activity in biosamples. The APN in our experiment was used to hydrolyze the alanine moiety of the Ala-AFC probe and, as a result of this hydrolysis, realize concomitantly a cascade reaction to unmask the electrochemical reporter N-alkylated amino ferrocene (AAF). The Ala-AFC probe exhibited high sensitivity with a wide detection range of 0.05–110 ng mL-1 and a low limit of detection of 23.18 pg mL-1. The electrochemical signals were found to be distinctly specific for APN and free of interference from other electroactive biological species. Furthermore, the Ala-AFC probe was employed to monitor and quantify, in real-time, the activity of APN in tumor cells, whole blood, and urine. In addition, the results of our direct electrochemical quantifications of the amount of APN in whole blood and urine were found to be consistent with the results of the use of commercially available fluorometric assay kits to sense APN in serum and urine. Thus our approach shows promise as a point-of-care tool for cancer diagnostics and post-treatment surveillance of cancer reoccurrence.
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