Role of manganese in red long-lasting phosphorescence of manganese-doped diopside for in vivo imaging

Materials with red long-lasting phosphorescence, such as MnII-doped diopsides, can be used for small animal in vivo imaging. CaMgSi2O6:Mn powders with various amounts of Mn were prepared by sol–gel to investigate their long-lasting phosphorescence mechanism. X-ray diffraction, X-ray absorption fine and near-edge structure and electron paramagnetic resonance showed that manganese is quantitatively introduced in the structure as MnII. Most of the Mn doping ions substitute Mg and possess a highly elongated octahedral environment. While photoluminescence and X-ray excited optical luminescence spectra show both orange (585 nm) and red (685 nm) 4T1 (4G) → 6A1 (6S) emission of MnIICa and MnIIMg, respectively, MnIIMg red emission dominates long-lasting phosphorescence and thermally stimulated luminescence spectra. These results point to MnIIMg as the preferential hole trap and recombination center in the long-lasting phosphorescence mechanism. An intense persistent red emission suitable for in vivo imaging probes is obtained for the highest nominal Mn content (7.5%).