A Biocompatible and High-Sensitivity Lanthanide Coordination Complex for Luminescent Temperature Sensor Constructed...

Temperature is a crucial thermodynamic parameter for sustaining life, driving industrial development, and regulating environmental processes. In this context, luminescence thermometry has emerged as an effective, rapid, and noninvasive method for temperature measurement. In this work, we synthesized a new complex containing Eu³⁺ and Tb³⁺ ions coordinated by 2,2'-bipyridyl (bpy) and the nonsteroidal anti-inflammatory drug ketoprofen (keto), with the empirical formula [Eu_0.4Tb_1.6(keto)₆(bpy)₂]. The ligand's T₁ energy of 23364.48 cm^-1 makes it effective for sensitizing lanthanide ions. The complex was fully characterized by infrared spectroscopy, elemental (CHN) analysis, powder X-ray diffraction, and thermal analysis. Photophysical studies revealed temperature-dependent emission, with the emission color shifting from yellow at cryogenic temperatures to red at 393 K. This behavior is consistent with a temperature-activated Tb³⁺ → Eu³⁺ energy-transfer contribution, as supported by the decrease in τ-(⁵D₄) from 1.278 to 0.789 ms between 283 and 343 K. The compound was evaluated as a luminescent thermometer using the intensity ratio between the ⁵D₄ → ⁷F₅ (Tb³⁺) and ⁵D₀ → ⁷F₂ (Eu³⁺) transitions as the thermometric parameter. The complex exhibited a maximum relative thermal sensitivity of 4.72% K^-1 at 343 K, a minimum temperature uncertainty of 0.07 K, and reproducibility of 91.7% over the range 283-343 K. Its high sensitivity, together with favorable cell-viability results, indicates that [Eu_0.4Tb_1.6(keto)₆(bpy)₂] is a promising candidate for use as an efficient luminescent thermometer, including potential applications as a temperature sensor in biological systems.