The three-dimensional Schrodinger and Poisson's equations are used to calculate the conduction band profile, energy levels, and Fermi energy of negatively charged semiconductor quantum dots. The calculation is carried out self-consistently within the frame of the finite-difference method. Assuming the effective mass of the proton at the semiconductor-electrolyte interface, we found the conduction band profile for the spherical ZnO quantum dots dispersed as aqueous colloids very similar to the conduction band profile of symmetric modulation-doped semiconductor quantum wells. The energy levels and Fermi energy of the spherical ZnO quantum dots are obtained as a function of the band offset at the semiconductor-electrolyte interface. A comparison of the energy levels for negatively charged and uncharged quantum dots is used as an alternative explanation of the observed reversible blue shift in the absorption spectrum of semiconductor colloids under illumination.