Comparative investigation on the crystal structure and cell behavior of rare-earth doped fluorescent apatite nanocrystals

• The doping of Tb3+ ions promotes the preferred growth of apatite crystals along the c-axis direction.
• The doping of Tb3+ ions causes the binding energy to increase for FA:Tb3+ and decrease for HA:Tb3+.
• The FA:Tb3+ crystals show stronger green fluorescence than HA:Tb3+ crystals.
• Four Tb-doping lattice models reveal a necessity of coexistent substitution mechanism.
• The Tb-doped apatite nanocrystals have good cytocompatibility and cell imaging capacity.

Terbium (Tb) doped fluorapatite (FA:Tb) and hydroxyapatite (HA:Tb) crystals are hydrothermally synthesized. Their composition, crystal structure, fluorescence and biological properties are investigated. The Tb-doped crystals are in nanoscale and present a uniform slender morphology. The doping of Tb3+ ions can promote the preferential growth of apatite nanocrystals along the c-axis (002) direction, and cause the binding energy to increase for FA:Tb3+ crystals or decrease for HA:Tb3+ crystals. The FA crystal structure tends to combine more Tb3+ ions than the HA crystal structure, and shows a stronger green fluorescence. Four Tb-doping lattice models along the apatite hexagonal structure are proposed, revealing a necessity for coexistent substitution mechanism of (Ca7Tb2©)(PO4)6(OH)2, (Ca6Tb2Na2)(PO4)6(OH)2, (Ca9Tb)(PO4)6(OH)O, and (Ca8TbNa)(PO4)6(OH)2. The Tb-doped apatite nanocrystals exhibit bright fluorescence, good cytocompatibility and excellent cell imaging capacity, providing feasibility for imaging and tracking of cells with multilineage differentiation.