Rare-earth (Gd(3+),Yb(3+)/Tm(3+), Eu(3+)) co-doped hydroxyapatite as magnetic, up-conversion and down-conversion materials for multimodal imaging

Taking advantage of the flexibility of the apatite structure, nano- and micro-particles of hydroxyapatite (HAp) were doped with different combinations of rare earth ions (RE(3+) = Gd, Eu, Yb, Tm) to achieve a synergy among their magnetic and optical properties and to enable their application in preventive medicine, particularly diagnostics based on multimodal imaging. All powders were synthesized through hydrothermal processing at T ≤ 200 °C. An X-ray powder diffraction analysis showed that all powders crystallized in P6(3)/m space group of the hexagonal crystal structure. The refined unit-cell parameters reflected a decrease in the unit cell volume as a result of the partial substitution of Ca(2+) with smaller RE(3+) ions at both cation positions. The FTIR analysis additionally suggested that a synergy may exist solely in the triply doped system, where the lattice symmetry and vibration modes become more coherent than in the singly or doubly doped systems. HAp:RE(3+) optical characterization revealed a change in the energy band gap and the appearance of a weak blue luminescence (λ(ex) = 370 nm) due to an increased concentration of defects. The “up”- and the “down”-conversion spectra of HAp:Gd/Yb/Tm and HAp:Gd/Eu powders showed characteristic transitions of Tm(3+) and Eu(3+), respectively. Furthermore, in contrast to diamagnetic HAp, all HAp:RE(3+) powders exhibited paramagnetic behavior. Cell viability tests of HAp:Gd/Yb/Tm and HAp:Gd/Eu powders in human dental pulp stem cell cultures indicated their good biocompatibility.