This paper described a facile aqueous route to prepare CdS:Eu quantum dots (QDs) with strong yellow emission using L-cysteine (L-Cys) as the capping ligands. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) confirm that the CdS:Eu QDs have a cubic structure with good crystallinity, the average diameter of the CdS:Eu QDs is about 8 nm. The traces of groups adsorbed on CdS:Eu QDs were confirmed via FT-IR spectra. In addition, the synthesis parameters including the concentration of doped Eu3+ ions, the pH value and the reaction time have influence on the optical properties of CdS:Eu QDs. The as-prepared L-Cys capped CdS: Eu QDs were used as a fluorescence probe to detect Hg2+ ions in aqueous media based on fluorescence quenching. The response of L-Cys capped CdS: Eu QDs as a fluorescence probe was linearly proportional to the concentration of Hg2+ ions in the range of 5 x 10(-6) to 1 x 10(-3) mol L-1 with a limit of detection of 2 x 10(-6) mol L-1. (C) 2013 Elsevier B.V. All rights reserved.