Double-Mode Thermometer Based on Photoluminescence of YbGd₂Al₂Ga₃O₁₂: Cr³⁺ Operating in the Biological Windows

A Near-Infrared (NIR) ratiometric luminescence thermometer with the composition of Yb₁Gd₂Al_1.98Cr_0.02Ga₃O₁₂ 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 ²F_5/2 → ²F_7/2 transitions of Yb³⁺ via the energy transfer process from Cr³⁺. Due to the Boltzmann distribution, the Yb³⁺ 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 Yb³⁺ in shorter and longer wavelength ranges works as a luminescence thermometer with a relative sensitivity of 0.55% K⁻¹ at 310 K. In addition, YbGd₂Al_1.98Cr_0.02Ga₃O₁₂ can also be employed for temperature sensing based on the LIR of Cr³⁺ (²E → ⁴A₂) at around 700 nm and Yb³⁺ (²F_5/2 → ²F_7/2) at around 1000 nm, achieving a remarkable relative sensitivity of 2.69% at 100 K. This study confirms that the YbGd₂Al_1.98Cr_0.02Ga₃O₁₂ thermometer fulfills the requirements for biological temperature measurements.