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

Highly biocompatible beta-TriCalcium Phosphate (beta-TCP) beta-Ca-3(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 T-4(1) ((4)G) -> (6)A(1) (S-6) emission at similar to 660 nm. TCP:Mn2+ annealed in Ar-H-2 showed only low temperature thermally stimulated luminescence (TSL) and therefore persistent luminescence at room temperature (RT) was weak. Ln(3+) (Ln = Tb, Dy) co-doping strongly enhanced Mn2+ X-ray excited optical luminescence while totally annihilating TSL. A subsequent Ar-H-2 annealing of (Ln(3+), 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-H-2 therefore showed highly enhanced X-ray excited persistent luminescence relative to the silicate reference used for in vivo imaging.