For developing metal nanoparticles of which surface chemical structures could be altered by enzymes in the cells, functional linkers caged by coumaric acids have been synthesized. Synthesized gold nanorod (GNR) conjugates possessing coumaric acid precursors underwent (porcine liver) esterase-catalyzed hydrolysis on their surface to afford GNRs coated with amino-functionalized polyethylene glycol and fluorescent coumarins as reporter molecules for monitoring the conversion. The chemical structural conversion on the GNR surfaces was successfully observed inside cells by fluorescence microscopy when GNR conjugates were incubated with tumor cells.