Yb³⁺/Ho³⁺ Co-Doped Apatite Upconversion Nanoparticles to Distinguish Implanted Material from Bone Tissue

The exploration of bone reconstruction with time requires the combination of a biological method and a chemical technique. Lanthanide Yb³⁺ and Ho³⁺ co-doped fluorapatite (FA:Yb³⁺/Ho³⁺) and hydroxyapatite (HA:Yb³⁺/Ho³⁺) particles with varying dopant concentrations were prepared by hydrothermal synthesis and thermal activation. Controllable green and red upconversion emissions were generated under 980 nm near-infrared excitation; the FA:Yb³⁺/Ho³⁺ particles resulted in superior green luminescence, while HA:Yb³⁺/Ho³⁺ dominated in red emission. The difference in the green and red emission behavior was dependent on the lattice structure and composition. Two possible lattice models were proposed for Yb³⁺/Ho³⁺ co-doped HA and FA along the hydroxyl channel and fluorine channel of the apatite crystal structure. We first reported the use of the upconversion apatite particles to clearly distinguish implanted material from bone tissue on stained histological sections of harvested in vivo samples. The superposition of the tissue image and material image is a creative method to show the material-tissue distribution and interrelation. The upconversion apatite particles and image superposition method provide a novel strategy for long-term discriminable fluorescence tracking of implanted material or scaffold during bone regeneration.