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

This paper describes Mn5+-activated Sr3(PO4)2 and Ba3(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 ∼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 ∼1% K–1 at temperatures between 400 and 500 K.