Hydrophobic Mn-doped ZnS quantum dots (Mn:ZnS d-dots) were made to be water soluble by coating a monolayer of mercaptopropionic acid (MPA). It is found that the PL efficiency of Mn: ZnS d-dots with various shell thicknesses were all improved after thiol (MPA) ligand exchange. We attribute their PL enhancement to the effective surface passivation induced by the thiol ligand and significantly decrease of holes transfer from d-dots to ligands. The PL QY of as-synthesized thiol-coated d-dots can reach as high as 50%. These efficient, stable, and water soluble d-dots are confirmed to be suitable for biomedical applications. (C) 2011 Elsevier B.V. All rights reserved.