| Summary: | Three novel luminescent Eu(III) complexes, <b>Eu1</b>–<b>Eu3</b>, have been synthesized and characterized with CHN analysis, mass-spectrometry and <sup>1</sup>H NMR spectroscopy. The complexes display strong emission in dichloromethane solution upon excitation at 405 and 800 nm with a quantum yield from 18.3 to 31.6%, excited-state lifetimes in the range of 243–1016 ms at 20 °C, and lifetime temperature sensitivity of 0.9%/K (<b>Eu1</b>), 1.9%/K (<b>Eu2</b>), and 1.7%/K (<b>Eu3</b>). The chromophores were embedded into biocompatible latex nanoparticles (<b>NPs_Eu1</b>–<b>NPs_Eu3</b>) that prevented emission quenching and kept the photophysical characteristics of emitters unchanged with the highest temperature sensitivity of 1.3%/K (<b>NPs_Eu2</b>). For this probe cytotoxicity, internalization dynamics and localization in CHO-K1 cells were studied together with lifetime vs. temperature calibration in aqueous solution, phosphate buffer, and in a mixture of growth media and fetal bovine serum. The obtained data were then averaged to give the calibration curve, which was further used for temperature estimation in biological samples. The probe was stable in physiological media and displayed good reproducibility in cycling experiments between 20 and 40 °C. PLIM experiments with thermostated CHO-K1 cells incubated with <b>NPs_Eu2</b> indicated that the probe could be used for temperature estimation in cells including the assessment of temperature variations upon chemical shock (sample treatment with mitochondrial uncoupling reagent).
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