Luminescent lanthanide nanocomposites in thermometry: Chemistry of dopant ions and host matrices

Recently, there has been observed huge progress in the advancement of remote optical nanothermometry, as it plays a vital role in remote, noninvasive, and remote temperature sensing, which is especially important for various bio-applications, e.g., in theranostics and photodynamic therapy. This is because the luminescence thermometry technique can accurately identify the temperature distribution in the molecular objectives or the biological body. Especially, the use of temperature-induced change in the luminescence intensity ratio (LIR) of two thermally coupled emission bands of lanthanide (Ln3+) ions, embedded in some inorganic nanoparticles (NPs), allows temperature representing of a single living cell. In this review, we comprehensively summarized the concept of luminescence optical thermometry, photophysical properties of Ln3+ ions, synthesis process, and various hosts doped with different Ln3+ ions, in which they act as either sensitizers or activators. Selecting a proper host matrix is an efficient route to achieve the high performance of optical nanothermometers since it takes an important role in determining the luminescent efficiency. Initially, we defined and compared various classes of optical thermometers based on diverse spectroscopic parameters, such as emission intensity, band intensity ratio, bandwidth, band shape, polarization, spectral shift, and luminescence lifetime. Furthermore, we emphasized the most common approach for temperature monitoring, which is based on the thermally coupled levels (TCLs) of Ln3+ ions, by exploiting the LIR technique, since it can provide a fast response with high accuracy, precision, and resolution. Additionally, this review also discusses the recent progress in diverse strategies to boost thermometric performances, such as changing dopants and utilizing various energy transfer mechanisms. Ultimately, we summed up the recent research achievements through analyzing the current research strategies, discussing future guidelines, as well as exploring the major difficulties for further advancement in the area.