High-quality ZnS nanocrystrals are synthesized with a coordinating solvent (i.e. high boiling-point long chain amine) using zinc stearate and elemental sulfur as the precursors. Spherical ZnS nanoparticles (~2nm) were obtained and characterized using several different techniques (XRD, SEM, TEM and UV-Vis). Fluorescence intensity of ZnS was acquired using a fluorometer. The long chain amines were exchanged with 11-mercaptoundecanoic acid, making the ZnS nanoparticles water soluble. In water, ZnS nanoparticles have three distinct fluorescence peaks from 330 nm to 470 nm. The fluorescence intensity ratio of the three peaks is proportional to the dissolved oxygen concentration. Calibrating this relationship allows one to determine the concentration of dissolved oxygen in solution.