The use of energy looping between Tm$^{3+}$ and Er$^{3+}$ ions to obtain an intense upconversion under the 1208 nm radiation and its use in temperature sensing

The upconversion phenomenon allows for the emission of nanoparticles (NPs) under excitation with near-infrared (NIR) light. Such property is demanded in biology and medicine to detect or treat diseases such as tumours. The transparency of biological systems for NIR light is limited to three spectral ranges, called biological windows. However, the most frequently used excitation laser to obtain upconversion is out of these ranges, with a wavelength of around 975 nm. In this article, we show an alternative – Tm$^{3+}$/Er$^{3+}$-doped NPs that can convert 1208 nm excitation radiation, which is in the range of the 2$^{nd}$ biological window, to visible light within the 1$^{st}$ biological window. The spectroscopic properties of the core@shell NaYF$_4$:Tm$^{3+}$@NaYF$_4$ and NaYF$_4$:Er$^{3+}$,Tm$^{3+}$@NaYF$_4$ NPs revealed a complex mechanism responsible for the observed upconversion. To explain emission in the studied NPs, we propose an energy looping mechanism: a sequence of ground state absorption, energy transfers and cross-relaxation (CR) processes between Tm$^{3+}$ ions. Next, the excited Tm$^{3+}$ ions transfer the absorbed energy to Er3+ ions, which results in green, red and NIR emission at 526, 546, 660, 698, 802 and 982 nm. ... mehrThe ratio between these bands is temperature-dependent and can be used in remote optical thermometers with high relative temperature sensitivity, up to 2.37%/°C at 57 °C. The excitation and emission properties of the studied NPs fall within 1$^{st}$ and 2$^{nd}$ biological windows, making them promising candidates for studies in biological systems.