In this study, water-dispersible manganese (II) ion-doped ZnS nanocrystals (ZnS:Mn NCs) were synthesized by capping the surface with L-glycine molecules at various pH conditions. The UV-Visible and solution photoluminescence (PL) spectra showed broad peaks approximately at 330 and 590 nm, respectively. The calculated relative quantum efficiencies were in the range of 4.9∼5.8%, and the average particle sizes measured from the HR-TEM images were about 8 nm. Moreover, the surface charges of the corresponding ZnS:Mn-Gly NCs were determined by the electrophoretic method as: (-)14.8 mV (pH 2), (-)9.5 mV (pH 5), and (-)6.4 mV (pH 10) respectively. The most negatively charged ZnS:Mn-Gly-pH 2 NCs were applied as a fluorescence sensor to detect copper (II) ions in water by an exclusive luminescence quenching effect. In addition, photocatalytic activity of the ZnS:Mn-Gly-pH 2 NCs was also evaluated by measuring the degradation rate of an organic dye (methylene blue, MB) molecule under the UV light irradiation. The ZnS:Mn-Gly-pH 2 NCs showed the photodegradation efficiency of the MB molecule with a pseudo-first-order reaction constant (kobs) of 4.3 × 10-4 min-1, which is higher than those of other amino acids-capped ZnS:Mn NCs at the same conditions.