Study of stability of luminescence intensity of $\beta$-NaGdF$_4$:Yb:Er nanoparticle colloids in aqueous solution

Hexagonal modification $\beta$-NaGdF$_4$:Yb:Er with a particle size of 24 nm was synthesized by the solvothermal technique. Concentrated aqueous colloids of nanoparticles were prepared using polyvinylpyrrolidone as the surfactant. The study of the luminescence characteristics for 25 days revealed that the luminescence intensity did not significantly change and hydrolysis of nanoparticles was not observed.