The surface charge density in semiconductor metal oxide quantum dots dispersed in alkaline aqueous colloids was obtained by solving the three-dimensional Schrodinger and Poisson＇s equations. The calculation was carried out self-consistently within the frame of the finite difference method. Surface charge density, in the range of 0.1-0.3 C/m(2), was obtained for spherical ZnO nanoparticles in an aqueous medium at pH around 11. The calculated surface charge density, in very good agreement with the average value reported in the literature (0.2 C/m(2)), is obtained as long as a proton-transfer mechanism through the semiconductor-electrolyte interface takes place. The calculated band energy profile for water-based semiconductor colloidal quantum dots, is very much similar to the band energy profile found in symmetric modulation-doped semiconductor quantum wells.