Abstract
New types of contactless luminescence nanothermometers, namely, LiAl5O8:Fe3+ and LiAl5O8:Fe3+, Nd3+ are presented for the first time, revealing the potential for applications in biological systems. The temperature-sensing capability of the nanocrystals was analyzed in wide range of temperature (−150 to 300 °C). The emission intensity of the Fe3+ ions is affected by the change in temperature, which induces quenching of the 4T1 (4G) → 6A1 (6S) Fe3+ transition situated in the 1st biological window. The highest relative sensitivity in the temperature range (0 to 50 °C) was found to be 0.82% °C (at 26 °C) for LiAl5O8: 0.05% Fe3+ nanoparticles that are characterized by long luminescent lifetime of 5.64 ms. In the range of low and high temperatures the Smax was calculated for LiAl5O8:0.5% Fe3+ to be 0.92% °C at −100 °C and for LiAl5O8:0.01% Fe3+ to be 0.79% °C at 150 °C. The cytotoxicity assessment carried out on the LiAl5O8:Fe3+ nanocrystals, demonstrated that they are biocompatible and may be utilized for in vivo temperature sensing. The ratiometric luminescent nanothermometer, LiAl5O8:Fe3+, Nd3+, which was used as a reference, possesses an Smax = 0.56%/°C at −80 °C, upon separate excitation of Fe3+ and Nd3+ ions using 266 nm and 808 nm light, respectively.
Highlights
Temperature measurement and accurate control of its value play a key role in many different fields of science, medicine and technology
Several approaches have focused on the enhancement of the relative sensitivity of luminescent thermometers, which includes the optimization of the stoichiometry of the host material as well as the appropriate selection of the optically active ions
The LiAl5O8:Fe3+ nanoparticles exhibit no significant effect on cell viability, at the highest concentration of 50 μg/mL
Summary
Temperature measurement and accurate control of its value play a key role in many different fields of science, medicine and technology. Accurate and reliable real time temperature measurement with submicron-scale spatial resolution during in vitro and in vivo experiments is of paramount importance All these requirements are provided by luminescent thermometry, an experimental technique that enables temperature determination based on the analysis of the luminescent properties of the phosphor [1–5]. Because of the shielding of the 4f electrons by the 5s2 and 5p6 electrons, their interaction with the crystal field of ligands (102 cm−1) is much less than the interaction of the d electrons with the surrounding ligands (104 cm−1) [6,13,14] This produces the strong electron-phonon coupling for TM ions (3dn configuration) inducing a relatively large displacement (∆R) between the ground and excited states potential parabolas resulting in the intersection of the two parabolas, which facilitates nonradiative decay to the ground state. The optimization process, which includes the size of the nanoparticles as well as Fe3+ dopant concentration, was used in order to enhance the relative sensitivity of LT in the physiological temperature range
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