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Terbium doped calcium fluoride (CaF 2 :Tb3+) and cerium co-doped CaF 2 :Tb3+ (CaF 2 :Ce3+, Tb3+) nanoparticles were synthesized by the hydrothermal method by using citric acid (Cit) as surfactant. The as-synthesized nanoparticles have good dispersibility in water to form an aqueous transparent dispersion at a concentration of 20 mg/mL. The nanoparticles have the CaF 2 -type cubic structures. Photoluminescence (PL) investigation showed that energy transfer can be occurred effectively from Ce3+ ions to Tb3+ ions in Cit/CaF 2 : Ce3+, Tb3+ nanoparticles. And the PL intensity of Cit/CaF 2 : Ce3+, Tb3+ nanoparticles is very strong at a low concentration of 1 mg/mL, which is 16-fold as high as that of Cit/CaF 2 : Tb3+ nanoparticles. The strong green emission from Cit/CaF 2 : Ce3+, Tb3+ nanoparticles can selectively be quenched by Cu2+ ions in comparison to other metal ions and its detection limit is 1.02 × 10−5 M in an aqueous solution. The PL quenching is reversible by the addition of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) with the recovery of almost 80% of the original PL. The results revealed that Cit/CaF 2 : Ce3+, Tb3+ nanoparticles could find a promising application for the detection of Cu2+ ions in an aqueous solution. • CaF 2 :Tb3+ and CaF 2 :Ce3+, Tb3+ nanoparticles were synthesized by using Citrate as the surface modification. • The as-prepared nanoparticles formed a transparent aqueous dispersion with concentration up to 20 mg/mL. • Cit/CaF 2 : Ce3+, Tb3+ nanoparticles have strong green emission in an aqueous dispersion at a low concentration of 1 mg/mL. • Cit/CaF 2 : Ce3+, Tb3+ nanoparticles can selectively detect Cu2+ ions in water and detection limit is 1.02 × 10−5 M. [ABSTRACT FROM AUTHOR]