Terbium-doped gadolinium oxysulfide (Gd2O2S:Tb3+) nanoparticles were synthesized by using a hydrothermal method followed with a sulfur reaction. We studied parameters such as precursor concentration, stock solutions temperature, reaction time, and reaction temperature to obtain well defined homogeneous particles with specific shape and size. The chemical evolution of the formed particles was followed by the Infrared Spectroscopy (FTIR), thus showing the removal of the hydroxycarbonate precursors at different stages of synthesis. X-ray analysis (WAXD) confirmed that the calcination and sulfidation processes produced cubic (Gd2O2S:Tb3+) and hexagonal (Gd2O2S:Tb3+) crystalline structures. The photoluminescent (PL) properties were evaluated as a response to UV light excitation in both solid phase and colloidal suspension; we determine the maximum emission intensity as a function of the Tb3+ ion dopant concentration and determine the limiting amount in the material before quenching occurs. In colloidal suspensions, the emission spectra of both Gd2O3S:Tb3+ and Gd2O2S:Tb3+ were compared, and we observed that the sulfidation process increased the integrated emission intensity seventyfold with respect to the former. To evaluate the particles behavior in aqueous media for possible uses in medical applications, an electrophoretic mobility study was carried out, thus observing that the fluorescence emission depends on the pH of the solution, which in turn correlates with the electrophoretic mobility of the particles. (C) 2014 Elsevier B.V. All rights reserved.