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PSA (prostate-specific antigen) is one of the most widely used proteins for the diagnosis of breast and prostate cancer. Of note, PSA displays enzymatic activity for the specific peptide sequence HSSKLQ, which it recognizes and cleaves. In this study, we developed a site-specific enzymatic-cleavage-reaction-based biosensor for the detection of PSA using fluorescein isothiocyanate (FITC)/peptide-conjugated gold (Au) nanoparticle complexes (FPANs). The FPANs do not initially fluoresce in the spectral region associated with the fluorophore, due to the quenching effect of the Au nanoparticles. When PSA was added to a solution containing the FPANs, PSA recognized and cleaved the specific sequence of the peptides attached to the Au nanoparticles. As a result, FITCs were separated from the Au nanoparticles and emitted strong fluorescence in their spectral region. Using this detection method, PSA was …