Water soluble cubic ZnS nanoparticles (NPs) have been synthesized at room temperature by using 3-mercaptopropionic acid (MPA) as a modifier molecule and characterized by X-ray diffraction (XRD), steady-state absorption, and emission spectroscopy. Electron transfer (ET) dynamics have been carried out in ZnS semiconductor nanoparticles and quinizarin (Qz) molecules as studied by picosecond time-resolved fluorescence spectroscopy. We have proposed that electron injection takes place from photoexcited Qz molecules into the surface states of wide band gap ZnS NPs. We have revealed that the formation of a charge transfer complex between the Qz molecule and ZnS nanoparticles facilitates electron injection into the surface states of nanoparticles. In the present investigation, we have detected charge transfer (CT) emission in the Qz-ZnS system as the injected electrons from surface states return back to the parent Qz cation radical. We have determined back ET rates by monitoring the CT emission.