Rare-earth doped inorganic upconversion nanoparticles(UCNPs)can convert near-infrared excitation into visible emission.Their unique physical and chemical properties,such as superior photochemical stability,low toxicity,and large penetration depth,make them attractive in various biomedical applications.One of the most common methods for the synthesis of UCNPs is organic phase synthesis method,which can be utilized to obtain high-quality UCNPs with a narrow size distribution,good crystallinity and excellent optical properties.In order to meet the requirements of biomedical applications,the UCNPs need to be water-dispersible.It is inevitable to replace organic solvent with water.Unfortunately,the change of solvents might affect the upconversion luminescence of the nanoparticles.In this work,hydrophilic a-NaYF4:Yb3+,Er3+nanoparticles with mean size of 46.1 ± 2.3 nm were synthesized by solvothermal method.The effect of water on the upconversion luminescence of NaYF4:Yb3+,Er3+nanoparticles was investigated systematically.It was found that OH-in water played an important role in the decrease of the upconversion emission intensity and the increase of the red/green upconversion luminescence intensity ratio.It is attributed to the strong vibration of OH-in water,which accelerates the nonradiative relaxation process.The results are useful not only for further understanding the quenching mechanism of the upconversion luminescence but also for the biomedical applications of the UCNPs.