This invention provides a novel probe, which contains a prostate peptide substrate. The probe can be used as a prostate diagnostic agent for optical imaging. An 11 amino acid peptide substrate is synthesized for specific prostate-specific antigen (PSA). This protected graft copolymer (PGC) consists of a poly-L-lysine (PL) backbone and methoxypoly(ethylene glycol) (MPEG). Two terminals of peptide substrate are bound to poly-L-lysine and Cy5.5 fluorochrome. This probe will be used as a NIR (near-infrared fluorescence) probe for optical imaging.