Lanthanide-based nanothermometers for bioapplications: excitation and temperature sensing in optical transparency windows

Nanoparticles containing lanthanide (Ln³⁺) ions in their structure have become one of the most important tools in nanomedicine, mainly due to their appealing spectroscopic properties. The unique energy level structure of Ln³⁺ allows for the generation of characteristic luminescence, which depends highly on the temperature. It is possible to use the intensity ratio between two emission lines of a single Ln³⁺ ion or the emission of two different ions to monitor the system's temperature. This approach often leads to the high sensitivity of such thermometers; however, the most important is the possibility of remote temperature sensing. That property allows for monitoring various physiological processes in living organisms and is helpful in theranostics. What is essential for bioapplications is that the excitation and emission wavelengths of Ln³⁺ ions can occur within three spectral ranges, known as optical transparency windows (biological windows). The biological materials, such as tissues, are transparent to radiation with wavelengths in the ranges of 750–950 nm, 1000–1350 nm and 1500–1800 nm. In this article, we review the state of the art regarding nanoparticles doped with Ln³⁺ ions for applications in temperature sensing within optical transparency windows regarding both excitation and emission wavelengths. The information provided in our review article will enable the selection of the type of nanothermometer for specific applications, help in selection of the emission or excitation wavelength, understanding the differences between systems based on down-shifting and upconversion phenomena, recognizing differences in the thermosensitive properties of various lanthanide ions, such as Nd³⁺, Tm³⁺, or Er³⁺, as well as the matrices and chemical compounds that form the basis for nanoparticles.