Cosubstitution of Lanthanides (Gd3+/Dy3+/Yb3+) in β-Ca3(PO4)2 for Upconversion Luminescence, CT/MRI Multimodal Imaging.

Multifunctional Gd3+, Dy3+ and Yb3+ cosubstitutions in β-Ca3(PO4)2 were achieved through a facile synthetic technique. Gd3+ and Dy3+ prefer to occupy the 7-fold coordinated Ca2+(1), 6- or 8-fold coordinated Ca2+(2), and 8-fold coordinated Ca2+(3) sites of the β-Ca3(PO4)2 structure. The shortest Ca(5)-O bond length of te Ca2+(5) site with 6-fold co-ordination favors Yb3+ accommodation. The substitution limit of each dopant to attain β-Ca3(PO4)2 solid solution is determined as ∼1.25 mol %. The combined substitutions displayed upconversion emission in blue, yellow and red regions on excitation at 980 nm via the energy transfer mechanism from Yb3+ to Dy3+. Yb3+ and Dy3+ combination induces a high X-ray absorption ability for computed tomography (CT). The materials also displayed paramagnetic behavior and exhibited high longitudinal (r1 = 48.71 mM-1 s-1) and transverse relaxivity (r2 = 61.79 mM-1 s-1) indicating their potential as T1 and T2 contrast agents in magnetic resonance imaging (MRI). The negligible toxicity of Gd3+/Dy3+/Yb3+ cosubstitutions in β-Ca3(PO4)2 is also demonstrated. The overall results justify the potential of the developed material as possible contrast agent for T1 and T2 MRI/CT/upconversion luminescence.