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.