The only way to get thermal images of living organisms without perturbing them is to use luminescent probes with temperature-dependent spectral properties. The acquisition of such thermal images become essential to distinguish various states of cells, to monitor thermogenesis, to study cellular activity, and to control hyperthermia therapy. Current efforts are focused on the development and optimization of luminescent reporters such as small molecules, proteins, quantum dots, and lanthanide-doped nanoparticles. However, much less attention is devoted to the methods and technologies that are required to image temperature distribution at both in-vitro or in-vivo levels. Indeed, rare examples can be found in the scientific literature showing technologies and materials capable of providing reliable 2D thermal images of living organisms. In this review article, examples of 2D luminescence thermometry are presented alongside new possibilities and directions that should be followed to achieve the required level of simplicity and reliability that ensure their future implementation at the clinical level. This review will inspire specialists in chemistry, physics, biology, medicine and engineering to collaborate with materials scientists to jointly develop novel more accurate temperature probes and enable mapping of temperature with simplified technical means.

Luminescent and temperature sensitive properties of YAG:Cr,Nd nanocrystals were analyzed as a function of temperature, nanoparticle size, and excitation wavelength. Due to numerous temperature-dependent phenomena (e.g. Boltzmann population, thermal quenching, and inter-ion energy transfer) occurring in this phosphor, four different thermometer definitions were evaluated with the target to achieve a high sensitivity and broad temperature sensitivity range. Using a Cr to Nd emission intensity ratio, the highest 3.48% K sensitivity was obtained in the physiological temperature range. However, high sensitivity was compromised by a narrow sensitivity range or vice versa. The knowledge of the excitation and temperature susceptibility mechanisms enabled wise selection of the spectral features found in luminescence spectra for a temperature readout, which enabled the preservation of relatively high temperature sensitivity (>1.2% K max) and extended the temperature sensitivity range from 100 K to 850 K. The size of the nanophosphors had negligible impact on the performance of the studied materials.

One of the challenges currently facing luminescent thermometry is providing an in-depth analysis of thermally dependent processes occurring in the studied type of nanothermometer. By understanding all possible thermal phenomena, the properties of a given kind of nanosensor can be intentionally improved. Verification of the existing theories on a nanometric scale is particularly important. In this work, a comprehensive characterization of the structural and optical properties of nanocrystalline KLaP4O12 doped with different concentrations of Tb3+ ions was performed. It was shown that Tb3+ ions excited using two distinct wavelengths matched to the ground- and excited-state absorption show luminescences of opposite thermal dependencies. This enabled the single-band-ratiometric (SBR) thermometric approach to be implemented. The experimentally obtained results turned out to be consistent with the proposed form of state equations describing the dynamics of populating Tb3+ energy levels as a function of the temperature. As reported here, the first SBR nanothermometer based on Tb3+ ion emission stands out because of its nanometric size and high relative sensitivity, reaching 5%/°C at 0 °C. Finally, on the basis of modeling changes in the individual parameters in the state equations, their impact on the results obtained for SBR thermometers based on Tb3+ ions was widely discussed.