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In the present study, a series of aqueous-based ZnSe (S) nanocrystals (NCs) was prepared at different solution pH ranging from 8 to 11.9, and using N-acetyl-L-cysteine (NAC) as capping agent. In addition to zinc blende structure, the X-ray diffraction studies demonstrated the quantum size regime of the ZnSe (S) NCs. To gain further insight toward the influence of the quantum confinement and pH values on optical properties of the as-prepared NCs, their UV-visible absorption and photoluminescence spectra were systematically analyzed. The absorption spectra experienced a red shift from~ 340 to~ 382 nm as the pH increased from 8.0 to 11.9, indicating the growth of the as-prepared ZnSe (S) NCs. The emission spectra also show the obvious red shift and the relative area of excitonic to trap emission, firstly increases from pH= 8.0 to 10.7, and then decreases by further increasing of the solution pH. The initial …