Surface modification of hydroxyapatite nanocrystallite by a small amount of terbium provides a biocompatible fluorescent probe

As the main inorganic component of biological bone and tooth enamel, hydroxyapatite (HAP) is highly biocompatible and bioactive. Our recent work has shown that 20 nm HAP can be readily internalized by living cells. However, HAP nanoparticles are not luminescent so that they are difficult to measure in living cells during in vitro experiments. A novel inorganic biological probe was suggested by doping 20 nm HAP with terbium. The calcium ions on the HAP particle surface could be partially replaced by Tb, which resulted in the 20 nm Tb-HAP particles. The Tb-doped HAP became luminescent and could be observed under a visible excitation of 488 nm. Compared with the usual probes, the modified HAP nanoparticles were more photostable and were almost nontoxic. It was important that the HAP characteristics remained after the surface modification with a small amount of Tb. After incubating with rabbit bone marrow mesenchymal stem cells (MSCs) in culture, the luminescence of the internalized HAP in the living cells was clearly observed under a fluorescent microscope. Transmission electron microscopy analysis also confirmed the uptake of the particles by MSCs. It was demonstrated that the modified HAP was a stable biological probe for cellular research. We suggested that the Tb-doped HAP should have a great potential to trace the evolvement of nanoparticles, which is a key topic in nanobiotechnology.