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ZnS quantum dots (QDs) has received a great attention due to its unique properties and wide applications. The objective of this work is to synthesize ZnS QDs by hydrothermal method and capped with mercaptoacetic acid (MAA) to be used as a chlorine sensor in the range from 1 to 35 mg/L. Optical, structural and morphological properties of MAA capped-ZnS QDs were investigated. MAA capped-ZnS QDs exhibited a cubic structure with an average diameter size of 8.8 nm. Photoluminescence (PL) spectra of the fabricated MAA capped-ZnS QDs revealed three basic emission peaks at 371, 423 and 486 nm due to level transitions involving of zinc vacancy, interstitial sulfur and zinc, respectively. The photostability of the MAA capped-ZnS QDs was examined after 14 months which retains 12% of the original PL intensity without any peak shift. The MAA capped-ZnS QDs PL intensity was changed linearly with the chlorine concentration in the range from 1 to 35 mg/L with correlation coefficient, sensitivity and limit of detection of 0.9782, 6.96×10-2 ppm-1 and 3.6 mg/L, respectively.