In this study, the ZnS:Mn nanocrystals (NCs) were prepared by capping the NC surface with a conventional amino acid, L-cysteine (Cys) molecules, at an acidic (pH 5) aqueous solution. The optical and physical characterizations of the ZnS:Mn-Cys-pH5 NCs were performed using various spectroscopic methods. For instance, the UV-visible and PL spectra of the ZnS:Mn-Cys-pH5 NCs showed broad peaks at 296 and 586 nm, respectively. The obtained HR-TEM image of the ZnS:Mn- Cys-pH5 NCs product showed spherical particle images with an average size of 6.15 nm in the solid state. In addition, measured surface charge of the colloidal ZnS:Mn-Cys-pH5 NCs using a zeta-PSA spectroscopy was -57.9 mV even at the acidic preparation condition. Therefore, the ZnS:Mn-Cys-pH5 NCs were applied as a photosensor to detect specific transition metal cations. As a result, the ZnS:Mn-Cys-pH5 NCs showed exclusive luminescence quenching effect for Fe(II) ions, which suggested that the ZnS:Mn-Cys-pH5 NCs can be applied as a photo-chemical sensor for Fe2+ ion detection in a practical water sample. The sensing ion selectivity of the ZnS:Mn-Cys-pH5 NCs was completely different comparing to ZnS:Mn NCs surface capped with other amino acids at the same condition. In addition, the catalytic activity of the ZnS:Mn-Cys-pH5 NCs was studied in the degradation reaction of an organic dye (methylene blue) molecule under UV light irradiation.