Luminescent lanthanide-doped calcium phosphate from oyster shell waste: an example of bright recycling

This research explores the transformation of biogenic CaCO₃ microparticles (Ø < 45 μm) from oyster shell waste into luminescent Eu- or Tb-doped calcium phosphate (apatites), using a sustainable “one-step” and eco-friendly method. The full transformation was achieved at 200 °C via a dissolution-precipitation mechanism. Precipitates were composed of Eu- or Tb-doped apatite particles, with average sizes L = 163 ± 7 nm and anisometric shapes for the former, and 41 ± 8 nm and more isometric shapes for the latter. Alongside these, particles of either EuPO₄·nH₂O or TbPO₄·nH₂O (rhabdophane) were present. The physicochemical and electrokinetic analysis revealed the A- and B-carbonate substitutions and labile CO₃²⁻ species in the apatite particles, and ζ-potentials approaching zero in the aqueous suspensions at physiological pH levels, indicating a tendency for particle aggregation. The luminescence properties, such as relative luminescent intensity and luminescence lifetimes, were dependent on the lanthanide content and the presence of the rhabdophane phase. The Ap-Ln samples demonstrated cytocompatibility, with cell viability exceeding 85% when incubated with murine pancreatic endothelial cells (MS1) and murine mesenchymal stem cells (m17.ASC), regardless of the lanthanide type or the particle dosage used (ranging from 0.1 to 100 μg mL⁻¹).