Temperature Sensing in the Short-Wave Infrared Spectral Region Using Core-Shell NaGdF4:Yb3+, Ho3+, Er3+@NaYF4 Nanothermometers

Abstract

The short-wave infrared region (SWIR) is promising for deep-tissue visualization and temperature sensing due to higher penetration depth and reduced scattering of radiation. However, the strong quenching of luminescence in biological media and low thermal sensitivity of nanothermometers in this region are major drawbacks that limit their practical application. Nanoparticles doped with rare-earth ions are widely used as thermal sensors operating in the SWIR region through the luminescence intensity ratio (LIR) approach. In this study, the effect of the shell on the sensitivity of temperature determination using NaGdF4 nanoparticles doped with rare-earth ions (REI) Yb3+, Ho3+, and Er3+ coated with an inert NaYF4 shell was investigated. We found that coating the nanoparticles with a shell significantly increases the intensity of luminescence in the SWIR range, prevents water from quenching luminescence, and decreases the temperature of laser-induced heating. Thermometry in the SWIR spectral region was demonstrated using synthesized nanoparticles in dry powder and in water. The core-shell nanoparticles obtained had intense luminescence and made it possible to determine temperatures in the range of 20–40 °C. The relative thermal sensitivity of core-shell NPs was 0.68% °C−1 in water and 4.2% °C−1 in dry powder.

Impact of sensitizer Yb and activator Tm on luminescence intensity of beta-NaYF4:Yb/Tm nanoluminophores

“…Two series of β-NaYF 4 :Yb 3+ /Tm 3+ samples were synthesised with constant concentration of the sensitizer and varied concentration of activator and vice versa in the other series. Nanoparticles β-NaYF 4 :Yb 3+ /Tm 3+ were synthesized by the solvothermal synthesis technique in a high-boiling solvent according to the procedure described elsewhere [12,39]. Ytterbium, thulium and yttrium acetates of purity 99.99 (Lanhit, Russia) were added to oleic acid (pure, Chimmed, Russia) and 90 % octadecene-1 (Sigma Aldrich).…”

“…In such nanoparticles, it is possible to implement up-conversion luminescence in which two or more photons with low energy turn into one photon with higher energy [1][2][3]. Such phosphors are used for optical imaging in biology and medicine [4][5][6], as optical coding elements [7], photovoltaic devices and anticounterfeiting [8], thermometry [9][10][11][12], and increasing the efficiency of solar panels [13][14][15].…”

“…Currently, the most effective up-conversion nanoluminophores have been developed on the basis of substances with gagarinite type crystal structure -NaYF 4 [9,16,17] or NaGdF 4 [5,12,18], which are doped with two types of ions. One (sensitizer ion) has a large absorption cross-section, and the other (activator ion) has high luminescence efficiency.…”

“…The sensitizer ion absorbs the excitation energy and non-radiatively transfers it to the activator ion, which then emits up-conversion luminescence. Ytterbium ions often act as sensitizer while erbium, thulium, and holmium ions are used as activators [5,8,9,12,[16][17][18].…”

“…The luminescent properties of nanoluminophores with REE ions are influenced by a number of factors such as intensity of exciting radiation [19][20][21], temperature [9][10][11][12], the ratio of concentrations of activator and sensitizer ions [5,8,17,[22][23][24][25][26][27][28][29][30][31][32], and environment of the emitting nanoparticles [33].…”