Highly aqueous soluble CaF2:Ce/Tb nanocrystals: effect of surface functionalization on structural, optical band gap, and photoluminescence properties

The design of nanostructured materials with highly stable water-dispersion and luminescence efficiency is an important concern in nanotechnology and nanomedicine. In this paper, we described the synthesis and distinct surface modification on the morphological structure and optical (optical absorption, band gap energy, excitation, emission, decay time, etc.) properties of highly crystalline water-dispersible CaF2:Ce/Tb nanocrystals (core-nanocrystals). The epitaxial growth of inert CaF2 and silica shell, respectively, on their surface forming as CaF2:Ce/Tb@ CaF2 (core/shell) and CaF2: Ce/Tb@CaF2@SiO2 (core/shell/SiO2) nanoarchitecture. X-ray diffraction and transmission electron microscope image shows that the nanocrystals were in irregular spherical phase, highly crystalline (similar to 20 nm) with narrow size distribution. The core/shell nanocrystals confirm that the surface coating is responsible in the change of symmetrical nanostructure, which was determined from the band gap energy and luminescent properties. It was found that an inert inorganic shell formation effectively enhances the luminescence efficiency and silica shell makes the nanocrystals highly water-dispersible. In addition, Ce3+/Tb3+-co-doped CaF2 nanocrystals show efficient energy transfer from Ce3+ to Tb3+ ion and strong green luminescence of Tb3+ ion at 541 nm(5D4 -> 7F5). Luminescence decay curves of core and core/shell nanocrystals were fitted using mono and biexponential equations, and R-2 regression coefficient criteria were used to discriminate the goodness of the fitted model. The lifetime values for the core/shell nanocrystals are higher than core-nanocrystals. Considering the high stable water-dispersion and intensive luminescence emission in the visible region, these luminescent core/shell nanocrystals could be potential candidates for luminescent bio-imaging, optical bio-probe, displays, staining, and multianalyte optical sensing. Graphical Abstract A newly designed CaF2:Ce/Tb nanoparticles via metal complex decomposition rout shows high dispersibility in aqueous solvents with enhanced photoluminescence. The epitaxial growth of inert CaF2 shell and further amorphous silica, respectively, enhanced their optical and luminescence properties, which is highly usable for luminescent biolabeling, and optical bioprobe etc. [GRAPHICS] .