Photonic Materials Laboratory of Kyoto University, Japan

Dr. Michele Back obtained both his B.Sc and M.Sc in Materials Science and he completed his Ph.D. degree cum laude in Chemical Sciences at Ca’ Foscari University of Venice, Italy (January 2017) in the field of optical materials. Currently, he is Post-doc (JSPS Fellow) at the Photonic Materials Laboratory of Kyoto University, Japan (Professor S. Tanabe).

His research interests include the design of bulk and nanoscale materials with advanced optical properties for applications in the fields of bioimaging, telecommunication and phosphors. At present his research is focused on the synthesis and characterization of bismuth-based luminescent materials and optical thermometers. He is author of 20 peer-reviewed publications and 2 international patents.

Speech Title: Cr³⁺-activated Phosphors: Advanced Ratiometric Luminescent Thermometers for Biological Applications

Aims: Sensing temperature variations with high thermal sensitivity is one of the hot topics for the study of crucial mechanisms in the biological field. In the last decade, ratiometric optical thermometry based on fluorescence intensity ratio (FIR) method has been recognized as the most reliable technique. However, the design of single-doped ratiometric luminescent thermal sensors with high sensitivity and reliable performances is still a challenge.

Methods: As a case study to show the potential of Cr³⁺-activated phosphors as thermal biosensor, Bi₂Ga₄O₉:Cr³⁺ and Bi₂Al₄O₉:Cr³⁺ systems were selected. A detailed spectroscopic investigation aimed at spreading light on the crystal field experienced by Cr³⁺ ion was performed in a wide temperature range (20-600 K). The potential of the materials as optical thermometers was accessed by combining experimental analysis with theoretical modeling.

Results: A detailed investigation on the electronic configuration of Cr³⁺ allows to estimate parameters such as the Debye temperature, the phonon energy and the Huang-Rhys parameter. The simultaneous emissions from the ²E_g → ⁴A_2g and ⁴T_2g → ⁴A_2g transitions in a wide range of temperatures, within the first biological optical window, is demonstrated. Key parameters such as the relative and absolute sensitivity (S_r and S_a), the temperature uncertainty (δT) and the repeatability (R) are calculated and discussed respect to the state of the art. Finally, the comparison between the theoretical calculation of the absolute sensitivity for Cr³⁺ and Nd³⁺-doped phosphors demonstrates the potential of the Cr³⁺-activated phosphors.

Conclusions: By considering Cr³⁺ in Bi₂M_4O9 (M=Ga, Al) systems as prototype, the key parameters describing the optical properties of Cr³⁺ and the potential of the systems as optical bio-thermometers are summarized. The superior performances of Cr³⁺-activated phosphors are experimentally and theoretically demonstrated.