Ratiometric Optical Thermometer Based on Dual Near-Infrared Emission in Cr3+-Doped Bismuth-Based Gallate Host

Detailed spectroscopic analysis of the electronic configuration of Cr3+ in Bi2Ga4O9 is reported. The material exhibits unique luminescent properties arising from the crystal field experienced by Cr3+, with simultaneous strong sharp and broadband near-infrared emissions from the 2E and 4T2 excited states, in a wide range of temperature. The system displays dual near-infrared emission characterized by a remarkable thermal sensitivity over the whole explored range of temperatures, reaching a value of 0.7%·K–1 in the physiological range. Moreover, the possibility to absorb and emit in the first biological window, allows one to consider the system as a new promising candidate for ratiometric fluorescent thermal sensing in biotechnological applications.