How to induce red persistent luminescence in biocompatible Ca3(PO4)2

Abstract

Highly biocompatible β-TriCalcium Phosphate (β-TCP) β-Ca₃(PO₄)₂ was tailored to emit intense red persistent luminescence suitable for small animal in vivo imaging. Mn²⁺ substituting the octahedral Ca5 site was selected as the luminescent dopant showing ⁴T₁ (⁴G) → ⁶A₁ (⁶S) emission at ∼660 nm. TCP:Mn²⁺ annealed in Ar–H₂ showed only low temperature thermally stimulated luminescence (TSL) and therefore persistent luminescence at room temperature (RT) was weak. Ln³⁺ (Ln = Tb, Dy) co-doping strongly enhanced Mn²⁺ X-ray excited optical luminescence while totally annihilating TSL. A subsequent Ar–H₂ annealing of (Ln³⁺, Mn²⁺) codoped TCPs restored intense TSL. The TSL mechanism was described in terms of deep hole trapping at Mn²⁺ and shallow electron trapping at defects created by reduction, i.e. most probably diphosphates. While Tb³⁺ co-doping promoted below RT TSL peaks, Dy³⁺ co-doping induced intense TSL between 320 and 450 K. TCP:Mn²⁺,Dy³⁺ annealed in Ar–H₂ therefore showed highly enhanced X-ray excited persistent luminescence relative to the silicate reference used for in vivo imaging.