Double-Mode Thermometer Based on Photoluminescence of YbGd<sub>2</sub>Al<sub>2</sub>Ga<sub>3</sub>O<sub>12</sub>: Cr<sup>3+</sup> Operating in the Biological Windows
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A<b> </b>Near-Infrared (NIR) ratiometric luminescence thermometer with the composition of Yb<sub>1</sub>Gd<sub>2</sub>Al<sub>1.98</sub>Cr<sub>0.02</sub>Ga<sub>3</sub>O<sub>12</sub> was prepared and studied. When excited by 660 nm in the first biological transparent window (BTW), the sample shows a peak structure of around 1000 nm due to the <sup>2</sup>F<sub>5/2</sub> → <sup>2</sup>F<sub>7/2</sub> transitions of Yb<sup>3+</sup> via the energy transfer process from Cr<sup>3+</sup>. Due to the Boltzmann distribution, the Yb<sup>3+</sup> PL intensities in the shorter wavelength side (i.e., 1st BTW) and longer wavelength side (i.e., 2nd BTW) exhibit opposite temperature dependencies. The luminescence intensity ratio (LIR) of Yb<sup>3+</sup> in shorter and longer wavelength ranges works as a luminescence thermometer with a relative sensitivity of 0.55% K<sup>−1</sup> at 310 K. In addition, YbGd<sub>2</sub>Al<sub>1.98</sub>Cr<sub>0.02</sub>Ga<sub>3</sub>O<sub>12</sub> can also be employed for temperature sensing based on the LIR of Cr<sup>3+</sup> (<sup>2</sup>E → <sup>4</sup>A<sub>2</sub>) at around 700 nm and Yb<sup>3+</sup> (<sup>2</sup>F<sub>5/2</sub> → <sup>2</sup>F<sub>7/2</sub>) at around 1000 nm, achieving a remarkable relative sensitivity of 2.69% at 100 K. This study confirms that the YbGd<sub>2</sub>Al<sub>1.98</sub>Cr<sub>0.02</sub>Ga<sub>3</sub>O<sub>12</sub> thermometer fulfills the requirements for biological temperature measurements.
