Near-Infrared Luminescent Lifetime-Based Thermometry with Mn5+-Activated Sr3(PO4)2 and Ba3(PO4)2 Phosphors

This paper describes Mn5+-activated Sr-3(PO4)(2) and Ba-3(PO4)(2) phosphors as near-infrared lifetime-based luminescence thermometry probes. Materials were prepared by a solid-state method, and their rhombohedral structures were confirmed by X-ray diffraction analysis. Diffuse reflectance measurements showed broad and strong absorption between 650 and 950 nm covering the first biological transparency window and having an absorption maximum at similar to 660 nm. By switching Sr with Ba, the following changes in the photoluminescent properties were observed: (i) a red shift of the emission maximum (1173 nm -> 1191 nm) and (ii) a decrease in the excited-state lifetime. Thermometric properties of the phosphors were assessed by measuring and analyzing the temperature dependence of the Mn5+ excited-state lifetime. Lifetime-based luminescence thermometry revealed a relative sensitivity of 0.5% K-1 at 310 K (physiologically relevant range) and a maximal value of similar to 1% K-1 at temperatures between 400 and 500 K.