Monodispersed silica-coated ZnO nanoparticles were successfully synthesized and modified with hydrophilic amino groups on the surfaces. The emission colour of the as-prepared ZnO nanoparticles could be tuned by adjusting the reactant ratio and the reaction time. The resulting particles were stable in water, phosphate buffer saline and cell culture medium. Their aqueous solutions had high quantum yields of 53.8%, 48.7% and 33.5%, corresponding to blue, green and yellow emitting ZnO@silica nanoparticles respectively. And the cytotoxicity (24 hours LC50 for Hela cells) of these three samples was 315, 374 and 241 μg mL⁻¹ respectively. The cytotoxicity data and the cellular uptake kinetics experiments confirmed the safety of all these ZnO@silica nanoparticles under visible light, while under UV light of 365 nm the yellow emitting ZnO@silica was fatal to Hela cells because it released plenty of reactive oxygen species.