Polymer terminated by 2-(2,4-dinitrophenylsulfanyl) ethanol was synthesized by ring opening bulk polymerization of epsilon-Caprolactone initiated by the reaction product of aluminum isopropoxide and 2-(2,4-dinitrophenylsulfenyl) ethanol. The corresponding thiolyated poly(epsilon-Caprolactone) (PCL) was obtained after removal of protecting group under slightly basic condition. Both of PCL end capped by protected thiol and thiolyated PCL (PCLS-H) were characterized by H-1 NMR and the molecular weights were determined by gel permeation chromatography (GPC). Decrease in polydispersity after deprotection was observed. The PCLS-H can be grafted to the gold nanoparticles either by direct incorporation or ligand exchange with the existing undecanethiol on the gold nanoparticles. The resulting nanoparticles were characterized by Raman spectroscopy and transmission electron microscope (TEM). Mixed ligands stabilized gold nanoparticles were further used for the labeling of PCL biocarrier by nanoprecipitation. The sizes of these nanoparticles-labeled biocarrier were determined by TEM. This is an easy and convenient way to label biocarrier by the gold nanoparticles based contrasting agent in order to study its consequences in vivo. (c) 2005 Elsevier B.V All rights reserved.