Influence of Shell Formation on Morphological Structure,Optical and Emission Intensity on Aqueous Dispersible NaYF4:Ce/Tb Nanoparticles

A highly water-dispersible NaYF₄:Ce/Tb (core), NaYF₄:Ce/Tb@NaYF₄(core/shell) and NaYF₄:Ce/Tb@NaYF₄@SiO₂ (core/shell/SiO₂) nanoparticles (NPs) were synthesized via a general synthesis approach. The growth of an inert NaYF₄ and silica shell (~14 nm) around the core-NPs resulted in an increase of the average size of the nanopaticles as well as broadening of their size distribution. The optical band-gap energy slightly decreases after shell formation due to the increase the crystalline size. To optimize the influence of shell formation a comparative analysis of photoluminescence properties (excitation, emission, and luminescence decay time) of the core, core/shell, and core/shell/SiO₂ NPs were measured. The emission intensity was significantly enhanced after inert shell formation around the surface of the core NPs. The Commission International de l’Eclairage chromaticity coordinates of the emission spectrum of core, core/shell, core/shell/SiO₂ NPs lie closest to the standard green color emission at 545 nm. By quantitative spectroscopic measurements of surface-modified core-NPs, it was suggested that encapsulation with inert and silica layers was found to be effective in retaining both luminescence intensity and dispersibility in aqueous environment. Considering the high aqueous dispersion and enhanced luminescence efficiency of the core-NPs make them an ideal luminescent material for luminescence bioimaging and optical biosensors.