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.

The comparison of the operating schemes and LIR definitions of the conventional ratiometric (A, B) and excited state absorption-based ratiometric (C, D) luminescent thermometry at low (A, C) and high (B, D) temperatures.

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Luminescence thermometry is one of the most promising techniques of temperature sensing which provides fast and accurate readout in the non-contact regime.

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.

A new thermographic nanocrystalline Sr 4 Al 14 O 25 :Mn 4+ ,Tb 3+ phosphor was developed, and the concentrations of both dopants and the synthesis conditions were optimized. The combination of the thermally quenched luminescence from the Mn 4+ ions to the almost temperature-independent emission from Tb 3+ provides a sensitive luminescent thermometer ( S R = 2.8%/°C at 150 °C) with strong emission color variability. In addition, a figure of merit for this luminescence thermochromism was proposed, as the relative sensitivities of the x and y CIE coordinates, which for this phosphor reaches at 150 °C S R ( x ) = 0.6%/°C and S R ( y ) = 0.4%/°C, respectively. Noncontact thermal imaging was demonstrated with this phosphor using a single consumer digital camera and exploiting the ratio of red (R) and green (G) channels of the RGB images, thereby confirming the high application potential of Sr 4 Al 14 O 25 :Mn 4+ ,Tb 3+ nanocrystals for thermal sensing and mapping.

Luminescent thermometry (LT) is a technique that enables contactless temperature determination based on temperature dependent luminescence of phosphors. Among different LTs that have been described in the literature so far, the luminescence intensity ratio (LIR) based thermometers have shown the highest application potential. Nevertheless, to determine accurate temperature, ratiometric method encounters technical restrictions related to the need for spectral separation of temperature dependent emission bands. An alternative ratiometric approach can exploit the intensity of a single emission band being excited in two ways related to ground state absorption (GSA) and excited state absorption (ESA). In this work, this approach, i.e., luminescent thermometry involving ESA process in LaPO4:Nd3+ nanocrystals, was demonstrated. Thermal energy delivered to the system was responsible for partial population of Nd3+:4I11/2 level, which enabled non-GSA-absorption of 1060 nm excitation line and resulted in appearance of strongly temperature dependent emission band at 890 nm. The further temperature increase favored population of higher laying levels, resulting in observation of 810 and 750 nm emission. On the other hand, the intensity of the emission band at 890 nm being excited in a resonant GSA way via the 808 nm line was strong and barely dependent on temperature, thus serving as a reference. Therefore, three luminescence intensity ratio (LIR i ) equations were defined to determine temperature in a contactless way. The subsequent LIRs were calculated as the ratios of emission intensities at 890, 810, and 750 nm being excited in a non-GSA-resonant ESA-resonant way normalized to the band at 890 nm excited with 808 nm line (through GSA). The highest relative sensitivities were unprecedentedly high and reached S 1 = 7.19%/°C at 30 °C, S 2 = 3.04%/°C at 100 °C, and S 3 = 4.35%/°C at 180 °C for the subsequent LIR i ratios.

Luminescent thermometers based on transition metal and lanthanide ion codoped nanocrystals have become a group of non-contact thermometers which are gaining importance due to their high sensitivity upon temperature changes. Here we present two types of luminescent thermometers, namely, bandshape and lifetime temperature sensors based on Y3Al5O12:Mn3+,Mn4+,Nd3+ nanocrystals. Their ability for temperature sensing was investigated as a function of Mn concentration. It was found that both sensitivity and usable temperature range depend on the Mn concentration. The highest sensitivity (S = 2.69%/K) was found for the lifetime luminescent thermometer with 0.01%Mn concentration and its value is gradually reduced with Mn content. Similarly, in the case of the bandshape luminescent thermometer, the sensitivity decreases from 1.69%/K for 0.01%Mn to 0.54%/K for 1%Mn. On the other hand the usable temperature range extends with dopant concentration. The concentration effect on the temperature dependent optical parameters is discussed in terms of interionic interactions facilitated for shorter Mn-Mn distances.