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.
Keywords: LiAl5O8 nanocrystals; iron; luminescent thermometry.