The influence of the crystal field strength and host material composition on the spectral position of the emission band and thermal stability of Fe³⁺luminescence was investigated in order to develop a highly sensitive luminescence thermometer.

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 S_max 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 S_max = 0.56%/°C at −80 °C, upon separate excitation of Fe³⁺ and Nd³⁺ ions using 266 nm and 808 nm light, respectively.

In this work the influence of the Ga³⁺ concentration on the luminescent properties and the abilities of the Y₃Al_5−xGa_xO₁₂: V nanocrystals to noncontact temperature sensing were investigated. It was shown that the increase of the Ga³⁺ amount enables enhancement of V⁴⁺ emission intensity in respect to the V³⁺ and V⁵⁺ and thus modify the color of emission. The introduction of Ga³⁺ ions provides the appearance of the crystallographic sites, suitable for V⁴⁺ occupation. Consequently, the increase of V⁴⁺ amount facilitates V⁵⁺ → V⁴⁺ interionic energy transfer throughout the shortening of the distance between interacting ions. The opposite thermal dependence of V⁴⁺ and V⁵⁺ emission intensities enables to create the bandshape luminescent thermometr of the highest relative sensitivity of V-based luminescent thermometers reported up to date (S_max, 2.64%/°C, for Y₃Al₂Ga₃O₁₂ at 0 °C). An approach of tuning the performance of Y₃Al_5−xGa_xO₁₂: V nanocrystals to luminescent temperature sensing, including the spectral response, maximal relative sensitivity and usable temperature range, by the Ga³⁺ doping was presented and discussed.

The relative sensitivity and usable temperature range of Fe³⁺,Cr³⁺-based luminescent thermometers can be tuned by modification of the crystal field strength.

The growing interest in luminescence thermometry and the desire to improve the thermometric properties of luminescent thermometers prompt the search for new solutions including new materials and types of luminescent ions. In this work, we present for the first time a systematic study verifying the application potential of materials doped with Co³⁺ ions in luminescence thermometry. It was shown that the luminescence intensity of the emission band localized at 800 nm associated with a spin-forbidden ³T_1(g)(H) → ¹A_1(g)(I) electronic transition of Co³⁺ ions exhibits strong temperature quenching accompanied by a strong shortening of the lifetime of the ³T_1(g)(H) level upon temperature increase. This has enabled the development of a ratiometric luminescent thermometer in CaAl₂O₄:Co³⁺,Nd³⁺, particularly effective at temperatures around room temperature (250 K < T < 350 K). It is characterized by a relative sensitivity of S_R of 1.86% K⁻¹ at 263 K and a thermometer based on a luminescence lifetime of S_R of 2.36% K⁻¹ at 349 K. It is shown that changing the structure from hexagonal to monoclinic CaAl₂O₄ does not change the spectral position of the emission band but increases the temperature quenching behavior of the Co³⁺ luminescence.