Luminescent thermometry

Phosphor Assisted Temperature Sensing and Imaging

Temperature distribution (raster scanned mapping or imaging) is typically performed by either cooled narrow-band-gap semiconductor (i.e., InAt, InAs, HgCdTe, PbS, PbSe) or microbolometric (i.e., amorphous Si or vanadium oxide) cameras - the arrays of near-infrared (NIR)-sensitive pixels. Such cameras work in ca. 3 − 14 μ m spectral range and quantify the intensity, which is directly related to the blackbody emission. Both the detector itself and the appropriate optical components (e.g., germanium or sapphire crystals) make the system relatively expensive and, because of operation in NIR spectral range, not easily adopted for high-spatial-resolution imaging or ultimately make it unsuitable for microscopy-based bioimaging. The solution to the latter issue is the application of temperature-dependent photoluminescent materials (TPM), e.g., polymers, proteins, quantum dots, complexes, phosphors, whose spectral properties (luminescence intensity or lifetime, interband ratio, spectral shift, etc.) are proportional to local temperature. Although the necessity to supply  exogenous luminescent particles may be somehow invasive for cells or tissues, the nanoparticle-based bioimaging has been proven effective and safe.

Although temperature imaging, to be distinguished from more straightforward point or homogenous measurements, has been performed with different T-sensitive molecules the reports on T imaging with lanthanide or metal-doped phosphors are scarce. Although numerous studies exhibited promising features in spot measurements, due to photophysics of these optical thermometers, they cannot be easy translated to fast, whole- fi eld (i.e., non-raster-scanning) imaging mode. The issues come from the following facts: the ratiometric emission bands overlap too closely, spectral shifts or  luminescence lifetimes changes were too small in response to T changes, and the probes were too dim for large-area simultaneous wide- field detection. the use of luminescent nanoparticles is a promising alternative to organic TPM because they exhibit excellent photostability, narrowband absorption, and (anti-Stokes) emission, as well as long luminescent lifetimes  the features that are all highly desirable for ultrasensitive bioimaging and biodetection.

Unfortunately, their luminescence intensity is relatively low, temperature sensitivity does not typically go beyond 2%/K, and the temperature readout is rather technically cumbersome because of closely located and overlapping spectral bands, which are used to determine the temperature-dependent luminescent parameter (TLP). Such approach typically requires high-resolution emission spectra to enable the trustworthy conversion of TLP, after respective calibration, to temperature units. Although some attempts have been made to exploit the emission intensity ratio of di ff erent, spectrally distinct multiplets, which simpli fi es the TLP determination in 2D, their temperature sensitivity or brightness are still not satisfactory. In combination with another requirement, i.e., high-resolution spectra, the 2D mapping of temperature becomes cumbersome because neither optical  bandpass fi lters with sensitive cameras nor 32-multichannel PMT detectors in raster-scanning imaging mode does not provide satisfactory spectral resolution for reliable temperature determination. Moreover, the consequence of using ine ffi cient and narrowband emissive lanthanide-doped phosphors is the requirement to use high-intensity lasers, which not only are expensive, but may also undesirably overheat the sample or disturb in reliable T determination. Moreover, the formation of a large-area and homogenous (e.g., speckle-free, top-hat beam pro fi le) beam in the fi eld of view of the microscope is another challenging task for 2D imaging.

Although the beam homogeneity problems do not count for raster-scanning imaging, the cost of the laser, long luminescence lifetimes causing signal “ bleeding ” to neighbor pixels, often too low spectral resolution (which is then translated to insu ffi cient temperature sensitivity), and low imaging rates make the whole- fi eld phosphor-assisted temperature imaging (PATI) a serious challenge. radically new idea of temperature visualization within the concept of phosphor-assisted temper- ature imaging, which relays on resonant to nonresonant excitation intensity ratio (RNR-EIR), where a single emission band is observed under two spectrally distinct, resonant and nonresonant, excitation wavelengths. This approach shall satisfy all of the requirements for practical adoption of luminescent metal (i.e., RE or TM) ions for wide- fi eld-sensitive temperature mapping. This concerns both materials ’ performance (e.g., high photostability, high brightness, and temperature sensitivity in physiological temperature range) and technical readout simplicity and feasibility. The great advantage of the proposed approach is the capability to use conventional fl uorescence microscope (e.g., halide) lamp to excite and a conventional CMOS-Vis camera to acquire the spectrally integrated images at single emission band under two di ff erent excitation bands (realized with optical fi lter cubes). The new approach exploits the intensities at the same emission band, under two, resonant and nonresonant, excitation bands. The ratio of the two obtained images has been demonstrated to be proportional to temperature.

We have proposed a radically novel approach to temperature visualization. The proposed phosphor-assisted temperature imaging (PATI) relay on resonant to nonresonant excitation intensity ratio (RNR-EIR), where a single emission band is observed under two spectrally distinct excitation wavelengths. This stands in stark contrast to the conventional luminescence intensity ratio (LIR) methodology and challenges the existing phosphor-based temperature mapping techniques with higher phosphor brightness (i.e., conventional microscope halide lamp was su ffi cient as photoexcitation source) and most of all with technical simplicity of wide- fi eld temperature 2D mapping with optical resolution of single micrometers. Three di ff erent host materials, namely, YAP, YAG, and LiLaP 4 O 12 nanocrystals doped with Cr 3+ ions, were examined for luminescent thermometry application based on RNR-EIR, using single emission band upon resonant and nonresonant excitations. By taking advantage from the fact that higher temperature facilitates absorption from higher laying vibrational components of the ground 4A 2 state, the nonresonantly excited luminescence can be promoted at higher temperatures. On the other hand, the temperature a ff ects the emission intensity upon both resonant and nonresonant excitations. Therefore, a substantial di ff erence between thermal quenching of Cr3+luminescence 482 upon resonant and nonresonant excitations was expected and 483 used here as a temperature-dependent parameter of noncontact temperature sensing. The highest sensitivity was found for YAP nanocrystals and reached 0.25%/K in physiological temperature range and up to 0.37%/K at − 150 ° C. This good ability of YAP nanocrystals for temperature sensing is related to high crystal fi eld of the corresponding host materials. The obtained results con fi rm the high applicative potential of the presented technique for noncontact temperature readout and can be a fast and a ff ordable alternative to luminescent 2D thermometry based on the relative emission intensity changes. Moreover, the proof-of-concept experiment of thermal imaging using RNR-EIR luminescent thermometer reveals not only higher spatial resolution of the proposed technique in respect to thermovision cameras but also capability to be adopted for fluorescence optical microscopes, which is of great importance for many biological studies. Due to the brightness of phosphors, conventional halide lamp was used as the excitation source with two excitation fi lter cubes and single sensitive Vis camera. Therefore, the proposed materials and method promise also much higher response rates and o ff er prospects for the determination of temperature of fast-moving/rotating mechanical elements.

The results have been published in a paper:

Phosphor Assisted Temperature Sensing and Imaging using resonant and non-resonant photoexcitation scheme, Bednarkiewicz, Artur; Trejgis, Karolina; Drabik, Joanna; Kowalczyk, Agnieszka; Marciniak, Lukasz, ACS Applied Materials & Interfaces, 2017 Dec 13;9(49):43081-43089. doi: 10.1021/acsami.7b13649