LiAl5O8:Fe3+ and LiAl5O8:Fe3+, Nd3+ as a New Luminescent Nanothermometer Operating in 1st Biological Optical Window

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.

Effect of Dopant Concentrations on the Luminescent Properties of LiAl5O8:Fe Phosphors

“…[25] Figure 4 shows the excitation spectra of pure and Fe-doped LiAlO 8 monitored at 691 nm. The optical properties of Fe 3þ ions are described in the Tanabe-Sugano diagram for the electronic configuration d-d. [12] The spectra of pure and doped samples consist of two regions. The first, around 267 nm, is attributed to the Fe 3þ dopant caused by the charge-transfer mechanism between oxygen and iron O 2À !…”

“…[10] A wide range of compounds are suitable for this purpose, such as the aluminate group, which has been the subject of several studies in recent years. [5,11,12] In this class of materials, lithium aluminate (LiAl 5 O 8 ) is seen as a promising host as reported in several recent works. [5,11,12] LiAl 5 O 8 has a cubic spinel unit cell, belonging to the space group P4 3 32, composed of seven nonequivalent atoms, where the Al 3þ and Li þ cations occur in tetrahedral and octahedral sites, [11] respectively.…”

“…[13] Lanthanide ions (Ln 3þ ) are one of several activators that act as a luminescent center due to their excellent optical properties originating from the f-f and f-d transitions, which are exploited for the generation of high intensity materials and luminescent efficiency for their numerous applications, such as in light-emitting diodes (LEDs). [5,11,12] In addition to these, transition metal ions (TM) are also activators used with the same efficiency, providing an alternative to lanthanides, as they have a lower cost benefit. [12,14] TM ions have optical properties associated with d-d transitions and are strongly affected by the lattice environment.…”

“…[5,11,12] In addition to these, transition metal ions (TM) are also activators used with the same efficiency, providing an alternative to lanthanides, as they have a lower cost benefit. [12,14] TM ions have optical properties associated with d-d transitions and are strongly affected by the lattice environment. The luminescence properties of Mn 2þ ions, for example, exhibit great differences in different host lattices.…”

NIR luminescence lifetime nanothermometry based on phonon assisted Yb3+–Nd3+energy transfer

“…The most commonly exploited spectroscopic feature of the thermometric phosphors that, after calibration, reports about temperature is luminescence spectra shape. Most commonly the intensity ratio of two emission bands are used [15][16][17][18][19][20][21][22][23][24] . However, because the shape of the emission band may be affected by the unknown and sample-to-sample variable absorption of the medium surrounding the phosphor or by the interparticle interactions within the sample, the credibility of the ratiometric LT approach, especially in the in vivo applications, has been recently undermined 25,26 .…”