Quenching of the Eu³⁺ Luminescence by Cu²⁺ Ions in the Nanosized Hydroxyapatite Designed for Future Bio-Detection

The hydroxyapatite nanopowders of the Eu³⁺-doped, Cu²⁺-doped, and Eu³⁺/Cu²⁺-co-doped Ca₁₀(PO₄)₆(OH)₂ were prepared by a microwave-assisted hydrothermal method. The structural and morphological properties of the products were investigated by X-ray powder diffraction (XRD), transmission electron microscopy techniques (TEM), and infrared spectroscopy (FT-IR). The average crystal size and the unit cell parameters were calculated by a Rietveld refinement tool. The absorption, emission excitation, emission, and luminescence decay time were recorded and studied in detail. The ⁵D₀ → ⁷F₂ transition is the most intense transition. The Eu³⁺ ions occupied two independent crystallographic sites in these materials exhibited in emission spectra: one Ca(1) site with C₃ symmetry and one Ca(2) sites with C_s symmetry. The Eu³⁺ emission is strongly quenched by Cu²⁺ ions, and the luminescence decay time is much shorter in the case of Eu³⁺/Cu²⁺ co-doped materials than in Eu³⁺-doped materials. The luminescence quenching mechanism as well as the schematic energy level diagram showing the Eu³⁺ emission quenching mechanism using Cu²⁺ ions are proposed. The electron paramagnetic resonance (EPR) technique revealed the existence of at least two different coordination environments for copper(II) ion.