In the last few years, quantum confinement effects in semiconductor nanocrystals (quantum dots - QDs) have attracted a significant amount of interest due to their new optical properties and also because of their potential applications in biological systems. In this work, cadmium sulphide (CdS) nanoparticles were synthesized in aqueous medium and passivated with Cd(OH)2. Polyphosphate ions were used in order to avoid particle aggregation. After the passivation step, CdS/Cd(OH)2 quantum dots were coated with silica. Silica coating has been extensively investigated concerning its properties in biocompatibilizing QDs to biological systems. Silica coated core-shell CdS/Cd(OH)2 water soluble QDs optical properties were studied by absorption, excitation and emission spectroscopies, while their morphological characterization was carried out by transmission electron microscopy.