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Luminescent lanthanide-containing biocompatible nanosystems represent promising candidates as nanoplatforms for bioimaging applications. Herein, citrate-functionalized calcium-doped terbium phosphate hydrate nanophosphors of the rhabdophane type were prepared at different synthesis times and different Ca²⁺/Tb³⁺ ratios by a bioinspired crystallization method consisting of thermal decomplexing of Ca²⁺/Tb³⁺/citrate/phosphate/carbonate solutions. Nanoparticles were characterized by XRD, TEM, SEM, HR-TEM, FTIR, Raman, Thermogravimetry, inductively coupled plasma spectroscopy, thermoanalysis, dynamic light scattering, electrophoretic mobility, and fluorescence spectroscopy. They displayed ill-defined isometric morphologies with sizes ≤50 nm, hydration number n ~ 0.9, tailored Ca²⁺ content (0.42–8.11 wt%), and long luminescent lifetimes (800–2600 µs). Their relative luminescence intensities in solid state are neither affected by Ca²⁺, citrate content, nor by maturation time for Ca²⁺ doping concentration in solution below 0.07 M Ca²⁺. Only at this doping concentration does the maturation time strongly affect this property, decreasing it. In aqueous suspensions, neither pH nor ionic strength nor temperature affect their luminescence properties. All the nanoparticles displayed high cytocompatibility on two human carcinoma cell lines and cell viability correlated positively with the amount of doping Ca²⁺. Thus, these nanocrystals represent promising new luminescent nanoprobes for potential biomedical applications and, if coupled with targeting and therapeutic moieties, they could be effective tools for theranostics.