| Summary: | This work explores the preparation of luminescent and biomimetic Tb<sup>3+</sup>-doped citrate-functionalized carbonated apatite nanoparticles. These nanoparticles were synthesized employing a citrate-based thermal decomplexing precipitation method, testing a nominal Tb<sup>3+</sup> doping concentration between 0.001 M to 0.020 M, and a maturation time from 4 h to 7 days. This approach allowed to prepare apatite nanoparticles as a single hydroxyapatite phase when the used Tb<sup>3+</sup> concentrations were (i) ≤ 0.005 M at all maturation times or (ii) = 0.010 M with 4 h of maturation. At higher Tb<sup>3+</sup> concentrations, amorphous TbPO<sub>4</sub>·nH<sub>2</sub>O formed at short maturation times, while materials consisting of a mixture of carbonated apatite prisms, TbPO<sub>4</sub>·H<sub>2</sub>O (rhabdophane) nanocrystals, and an amorphous phase formed at longer times. The Tb<sup>3+</sup> content of the samples reached a maximum of 21.71 wt%. The relative luminescence intensity revealed an almost linear dependence with Tb<sup>3+</sup> up to a maximum of 850 units. Neither pH, nor ionic strength, nor temperature significantly affected the luminescence properties. All precipitates were cytocompatible against A375, MCF7, and HeLa carcinogenic cells, and also against healthy fibroblast cells. Moreover, the luminescence properties of these nanoparticles allowed to visualize their intracellular cytoplasmic uptake at 12 h of treatment through flow cytometry and fluorescence confocal microscopy (green fluorescence) when incubated with A375 cells. This demonstrates for the first time the potential of these materials as nanophosphors for living cell imaging compatible with flow cytometry and fluorescence confocal microscopy without the need to introduce an additional fluorescence dye. Overall, our results demonstrated that Tb<sup>3+</sup>-doped citrate-functionalized apatite nanoparticles are excellent candidates for bioimaging applications.
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