The origin and stability of photoluminescence (PL) are critical issues for silicon nanoparticles to be used as biological probes. Optical and electronic properties of propionic-acid (PA) -terminated silicon quantum dots (SiQDs) were studied using the density-functional tight-binding method. We find that the adsorbed PA molecules slightly affect the structure of silicon core. The PA adsorption does not change the optical properties of SiQDs, while it substantially decreases the ionization potentials in the excited state and results in some new active orbitals with adjacent energies around the Fermi energy level. Accordingly, the modified surface of SiQDs can serve as a reaction substrate to oxygen and solvent molecules, which is responsible for the increase in both PL stability and water solubility.