Phosphor-Assisted Temperature Sensing and Imaging Using

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Phosphor-Assisted Temperature Sensing and Imaging Using Resonant and Nonresonant Photoexcitation Scheme Artur Bednarkiewicz,*,†,‡ Karolina Trejgis,† Joanna Drabik,† Agnieszka Kowalczyk,‡ and Lukasz Marciniak*,† †

Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, 50-422 Wroclaw, Poland Wroclaw Research Centre EIT+, Stabłowicka 147, 54-066 Wrocław, Poland

‡

S Supporting Information *

ABSTRACT: Phosphor-assisted luminescent thermometry relies on studying, often subtle, temperature-dependent spectral properties, such as luminescence spectra, bands shifts, or luminescence lifetimes. Although this is feasible with highresolution spectrometers or time-resolved detectors, technical implementation of such temperature mapping or wide-field imaging is complex and cumbersome. Therefore, a new approach for noncontact ratiometric temperature detection has been proposed based on comparison of emission properties of bright Cr3+-doped phosphors at single emission band upon two, resonant and nonresonant, optical excitation bands. The proposed method of temperature readout was examined for three different host materials: YAlO3, Y3Al5O12, and LiLaP4O12 nanocrystals. The highest relative sensitivity in physiological temperature range was found for YAlO3 nanocrystals reaching 0.35%/K, which is related to the highest crystal field found for this phosphor. The proposed methodology and the obtained materials enabled to not only reliably measure temperature in the range of −150 to 300 °C but also significantly simplify the technical detection scheme. In consequence, lamp-photoexcited, wide-field, micron-resolution microscopy imaging became possible, which is of special interest for many remote temperature studies in technology and biomedical applications. KEYWORDS: luminescent thermometry, thermal imaging, PATI, temperature sensing, nanocrystals

1. INTRODUCTION 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 μm 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,1,2 whose spectral properties (luminescence intensity or lifetime, interband ratio, spectral shift, etc.) are proportional to local temperature.2,3 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 © XXXX American Chemical Society

been performed with different T-sensitive molecules (e.g., (i) dried layers of rhodamine B using their T-dependent quantum yield,4 (ii) perylene and N-allyl-N-methylaniline, which form Tdependent exciplexes and enable colorimetric T imaging under UV excitation,5 (iii) [Tb0.99Eu0.01(hfa)3(dpbp)]n complexes showing T-dependent Tb−Eu energy transfer and thus occurring color variablility,6 (iv) biocompatible green fluorescent protein and fluorescence polarization spectroscopy,7 (v) FLIM-based fluorescently labeled hydrogel poly(NIPAM-coDBD-AE),8 and (vi) polymeric nanogels to image subcellular spatial distribution9), the reports on T imaging with lanthanide or metal-doped phosphors are scarce. For example, Er3+ and Yb3+ co-doped fluoride glass microparticles were placed at the scanning tip of an atomic force microscope to obtain a scanning thermal microscope with ∼1%/K sensitivity and T readout based on the ratio between 520 and 540 nm emission bands (T ∼ I(2H11/2 → 4I15/2)/I(4S3/2 → 4I15/2)).10,11 A similar T detection phenomenon was used in confocal scanning-based T mapping12 or quasi-scanning-like T mapping using Yb/Er NaYF4 nanoparticles embedded into yeast template and Received: September 8, 2017 Accepted: November 22, 2017 Published: November 22, 2017 A

DOI: 10.1021/acsami.7b13649 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 1. Representative Examples of Luminescent Thermometer17,23−49

multicolor optical tweezers, which enabled point-by-point temperature determination.13 Although numerous studies exhibited promising features in spot measurements, due to photophysics of these optical thermometers, they cannot be easy translated to fast, whole-field (i.e., non-raster-scanning) imaging mode. The issues come from the following facts: the ratiometric emission bands overlapped too closely, spectral shifts or luminescence lifetimes changes were too small in response to T changes, and the probes were too dim for largearea simultaneous wide-field detection. Nevertheless, the use of luminescent nanoparticles is a promising alternative to organic TPM because they exhibit excellent photostability, narrowband absorption, and (antiStokes) 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 different, spectrally distinct multiplets,14 which simplifies 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 filters 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 inefficient

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 profile) laser beam in the field of view of the microscope is another challenging task for 2D imaging.15 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 insufficient temperature sensitivity), and low imaging rates make the whole-field phosphor-assisted temperature imaging (PATI) a serious challenge.16 Recently, we have shown a new approach, which exploited temperature-dependent emission of Cr3+ ions normalized to temperature-independent emission of Nd3+ or Yb3+ ions.17−19 Although high sensitivity (up to 4.89%/K) and high brightness were achieved, one may note some technical difficulties to adopt these phosphors and TLP to be read in wide-field imaging mode. This is because, Cr intensity is a few times brighter than that of Nd3+ or Yb3+ as well as RE3+ emission either overlaps partially with Cr3+ emission (i.e., Nd3+, Yb3+, Er3+, etc.) or falls into the NIR spectral region, where conventional CCD or CMOS cameras lose their sensitivity. Thus, the TLPs defined as Cr3+/Nd3+ LIR or Cr3+/Yb3+ LIR are difficult to quantify by imaging, despite that prerequisite conditions of large Stokes shift and sufficient brightness of the phosphors are satisfied.20−22 Table 1 compares the examples of different thermometers according to TLP definition, sensitivities, mechanism, and feasibility for wide-field temperature imaging. Here, we show a radically new idea of temperature visualization within the concept of phosphor-assisted temperB


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Figure 1. Concept behind the phosphor-assisted temperature imaging, which relies on resonant to nonresonant excitation intensity ratio. Energy diagram of Cr3+ ions presenting resonant and nonresonant excitation (a); spectral characterization of the Cr3+-doped phosphor under resonant and nonresonant excitation (b); and optical setup of optical microscope for broad-band lamp excitation, wide-field, optical micron-resolution temperature imaging (c). Y3Al5O12:Cr3+ (YAG:Cr3+) and YAlO3:Cr3+ (YAP:Cr3+) were prepared using the previously described modified Pechini method.50,51 Yttrium oxide (Y2O3, purity 99.999%, Stanford Materials Corporation), neodymium oxide (Nd2O3, purity 99.998%, Stanford Materials Corporation), chromium nitrate nonahydrate (Cr(NO3)3·9H2O, purity 99.999%, Fluka), aluminum nitrate nonahydrate (Al(NO3)· 9H2O, purity 99.9995%, Alfa Aeser), citric acid (C6H8O7, purity 99.5%, Avantor), and ethylene glycol (C2H6O2, Avantor) were used as starting compounds. Appropriate amounts of oxides were diluted in ultrapure nitric acid to produce nitrates. Afterward, aqueous solutions of nitrates were mixed together with nonhydrate citric acid and ethylene glycol for 1 h. Next, the obtained solution was heated for 1 week at 90 °C until the resin was formed. Finally, fabricated resin was annealed for 16 h in air at 850, 900, 950, 1000, and 1100 °C temperatures. 2.3. Characterization. Powder diffraction studies were carried out on a PANalytical X’Pert Pro diffractometer equipped with Anton Paar TCU 1000 N temperature control unit using Ni-filtered Cu Kα radiation (V = 40 kV, I = 30 mA). The XRD patterns of obtained phosphors are presented in Figure S1. Transmission electron microscopy images were taken using a Philips CM-20 SuperTwin microscope with 160 kV voltage and 0.25 nm spectral resolution. The representative TEM images of the obtained phosphors are presented in Figure S2. The excitation and emission spectra were measured using a FLS980 fluorescence spectrometer from Edinburgh Instruments with 450 W Xe lamp. Both the excitation and emission 300 mm focal length monochromators were in Czerny−Turner configuration. Emission arm was supplied with ruled grating, 1800 lines/mm blazed at 500 nm. The spectral resolution was 0.1 nm. R5509-72 photomultiplier tube from Hamamatsu in nitrogen-flow-cooled housing was used as a detector. Presented excitation spectra were corrected in respect to the spectral characteristic of the lamp. Temperature of the sample was controlled using heating stage from Linkam (0.1 °C temperature stability and 0.1 °C set-point resolution). Thermovision images were recorded using thermovision camera A40 M from Flir Systems. The temperature maps were obtained using an upright fully automated fluorescence microscope (Carl Zeiss

ature imaging, which relies 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-field-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 fluorescence 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 different excitation bands (realized with optical filter 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.

2. MATERIALS AND METHODS 2.1. Synthesis of the LiLaP4O12:Cr3+ Nanocrystals. LiLaP4O12:Cr3+ nanocrystals were successfully synthesized using previously described co-precipitation method.22 Li2CO3 (potassium carbonate, Aventor, pure), La2O3 (purity 99.999%, Stanford Materials Corporation), Cr(NO3)3·9H2O (purity 99.999%, Fluka), and (NH4)2HPO4 (ammonium phosphate dibasic, Sigma Aldrich, purity ≥ 99.99%) were used as starting materials. Lithium carbonate and all of lanthanide oxides (Ln2O3) were dissolved in deionized water with addition of HNO3 in separate glasses. After triple recrystallization, nitrates of potassium and lanthanides were mixed together with chromium nitrate and added to the water solution of ammonium phosphate dibasic. In result, a white suspension was obtained and dried for 3 days at 90 °C. Finally, the obtained powders were annealed for 6 h at 450 °C. 2.2. Synthesis of the Y3Al5O12:Cr3+ (YAG:Cr3+) and YAlO3:Cr3+ (YAP:Cr 3+ ) Nanocrystals. The nanocrystalline powders of C


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Figure 2. Modeling of a system behavior. Theoretical thermal evolution of Δ (a) and calculated relative sensitivity as a function of temperature (b).

⎛ ΔE ⎞ 1 Inonresonant(T ) = A exp⎜ − 1 ⎟ · ⎝ kBT ⎠ 1 + exp

AxioObserver Z1) equipped with definite focus for the Z drift elimination and operated by the ZEN software. Image detection was performed by a high-sensitivity and high-speed EMCCD (Rolera EMC2) camera. The bright-field images were taken using a halogen lamp source, whereas the fluorescence images were taken using halide lamp and a set of two custom-made filter cubes. The mosaic images (25 × 16 images, thus the visualized area is 8.5 mm × 6.4 mm, with optical resolution going down to single ∼2 μm/pixel) were collected in tiles mode using the Zeiss ND Plan-NEOFLUAR 20×/0.4 PH2 objective. The heating element was made using tungsten wire (0.15 mm diameter) using up to 1.2 A DC laboratory power supply (Matrix MP3-3005D) (Figure S1). 2.4. Temperature Sensing Model. As it is well known, due to the distortion caused by crystal field, the excited-state parabola of Cr3+ is shifted in respect to the ground-state one (Figure 1a). One of the consequences of this fact is Stokes shift of emission band observed for transition-metal ion. The magnitude of this shift is related to the electron−phonon coupling interaction. In case of Cr3+ ions, the 4T1 and 4T2 excited-state parabolas are shifted in respect to 4A2 ground state (E2-state parabola was not presented in the figure), which make them transect (in our case, more important is the transection of 4A2 and 4T2 parabolas) at energy ΔE1. The value of this energy difference affects the luminescence temperature quenching due to the fact that it provides thermal energy, which enables overcoming this barrier energy by electrons, which then responds to nonradiative depopulation of the excited states. Therefore, the temperature-dependent luminescence intensity I(T) is described by eq 1.

I(T )thermal_dep =

ΔE1 kT

( )

Δ=

Ires(T ) − Inres(T ) Inres(T )

(4)

As can be seen in Figure 2b, obviously, the higher the ΔE2 value, the lower the intensity of nonresonantly excited luminescence (NREL); however, its high value leads to the increase of Δ values and more rapid changes of Δ with temperature (Figure 2a). One of the most important factors that describes ability of such luminescent thermometers for accurate temperature sensing is their relative sensitivity, which in our case is described as follows

(1)

where I0 and k are the initial intensity at low temperature and the Boltzmann constant, respectively. On the other hand, the increase of temperature increases the population of higher vibrational states within the ground state (eq 2). The energy state E2 of the ground-state parabola becomes populated at a given temperature T, which is well described by the Boltzmann model ⎛ ΔE ⎞ I(T )Boltzmann = A exp⎜ − 2 ⎟ ⎝ kT ⎠

(3)

Equation 3 is sufficient to plot theoretical curves as a function of phenomenological ΔE2/ΔE1 ratio. Because ΔE1 is a constant characteristic for the given host material, the presented theoretical considerations were performed keeping ΔE 1 constant and manipulating ΔE2 values. The ΔE2 values can be arbitrarily chosen during the experiment by selection of detuning from the resonance excitation line. This possibility to change ΔE2 opens completely new possibilities, which were however not studied here in more detail. By changing the ΔE2 values, one gets the possibility to tune the temperature sensitivity range (Figure 2a), as described below. One may expect that emission intensities upon resonant and nonresonant excitation should differ and stay proportional to temperature. Therefore, the comparison of their intensities can be used as an actual temperature sensor Δ = fun(T)

I0 1 + exp

ΔE 2 kT

( )

S=

1 ΔΔ × 100% Δ ΔT

(5)

On the basis of the obtained model, theoretical thermal dependence of S for different values of ΔE2/ΔE1 can be determined (Figure 2b). It is evident that the temperaturedependent Δ becomes more significant for higher values of the ΔE2/ΔE1 ratio. It is important to note here that the value of activation energy ΔE1 is dictated by the host material and, in some range, may be tuned by the size of the nanocrystals or by the changes of structural environment of the optically active ions. Interestingly, the value of ΔE2 can be arbitrarily modified on demand by the proper selection of the excitation line. The smaller is the difference between resonant and nonresonant excitation line, the lower value of ΔE2 becomes, which improves the emission intensity of the nonresonantly excited luminescence but reduces the sensitivity of T determination.

(2)

where A is a proportional constant.

3. RESULTS AND DISCUSSION The Cr3+-doped phosphors in the current configuration exhibit two types of phenomena. Besides ground-state absorption (eq 1), excited-state absorption from these thermal equilibrium states (eq 2) can occur as well, at lower and temperaturedependent rates. The thermal dependence of emission intensity upon nonresonant excitation can thus be described by using eqs 1 and 2 in the following equation D


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Figure 3. Thermal evolution of excitation spectra of YAP:Cr (λem = 720 nm) (a), YAG:Cr (λem = 720 nm) (b), and LiLaP4O12:Cr (λem = 875 nm) (c); thermal evolution of Δ (d) and sensitivity (e) for different hosts (YAP, YAG, LiLaP4O12).

3.1. Physical Mechanism. One of the most important differences, from optical point of view, between YAlO3, Y3Al5O12, and LiLaP4O12 host materials doped with Cr3+ ions is related to crystal field splitting (10 Dq), which equals 16 524 cm−1 for YAP, 16 400 cm−1 for YAG, and 15 229 cm−1 for LiLaP4O12 hosts.22,52 The relatively low value of this crystal field parameter (10 Dq) is related to high covalency of the phosphors atoms.52 In the orthorhombic structure of YAP host material, Cr3+ ions occupy Al3+ site of Ci point symmetry and are surrounded by oxygen octahedral. On the other hand, in cubic YAG structure, the Cr3+ ions substitute octahedral (Oh) and tetrahedral (Td) sites of Al3+; however, emission can be obtained only from the ions localized in the octahedral site. In the monoclinic LiLaP4O12 structure, Cr3+ ions substitute La3+ cations. Representative X-ray diffraction patterns of YAP, YAG, and LiLaP4O12:Cr nanocrystals are presented in Figure S1a−c, respectively. The lack of additional diffraction peaks in XRD patterns confirms high crystallographic purities of obtained phosphors. Thermal evolutions of excitation spectra for YAP:Cr, YAG:Cr, and LiLaP4O12:Cr phosphors are presented in Figure 3a−c, respectively. In each of the presented examples, three temperature-dependent factors can be found, namely, band intensity, spectral width (full width at half-maximum, FWHM), and position (λMAX) of the band. Decrease of band intensity with temperature is caused by thermal quenching of emission intensity related to nonradiative depopulation of excited states. Shifting of excitation band toward longer wavelength (smaller energies) can be associated with two different phenomena. First of them is related to electron−phonon coupling and thermal dependence of its strength. Because the electron−phonon coupling also affects the position of emission band in temperature-dependent emission spectra, which was not observed in this case, its influence can be neglected. On the other hand, the increase of population of higher laying vibrational components of the 4A2 ground state facilitates absorption from them, which results in a spectral shifting of the absorption band. The full width at half-maximum (FWHM)

On the basis of the presented considerations, an obvious conclusion can be deduced. To enhance relative sensitivities, the ΔE2/ΔE1 ratio should be increased, which is however compromised by reduced nonresonantly excited emission intensity, which accordingly decreases and hampers the detection accuracy. Therefore, a proper balance between these two processes should be sought. Because the value of ΔE1 is a characteristic parameter of host material, the sensitivity enhancement can be obtained by appropriate selection of ΔE2 ∼ (λREXC)−1 − (λNREXC)−1, which stands for spectral shift between resonant λREXC and nonresonant excitation wavelength λNREXC. In this study, we investigate the abilities for noncontact temperature measurements, which exploit resonant and nonresonant excitation of Cr3+ ions for three different types of host materials: YAlO3 (YAP), Y3Al5O12 (YAG), and LiLaP4O12. The proposed excitation intensity ratio (EIR) approach has also potential important implications for the simplification of temperature mapping. Conventionally, either high-resolution (Δλ < 1 nm) hyperspectral imaging or raster scanning is necessary to distinguish and quantify the LIR parameter. Such approach is troublesome and costly. For large-spectral-shift LIR, single or double light source must be combined with one (in the case of using switchable filter cubes) or two (in the case of dichroic mirror) CCD cameras working in different narrowband spectral windows. Although easier to realize, multiple light sources and two cameras leverage the costs of instrumentation and complicate technical readout. In opposite to conventional approaches, the proposed combination of bright temperature-sensitive phosphors and EIR, a broad-band lab with two filters was used, but alternatively two narrowband light sources (e.g., LED lamps) can be potentially used and switched on alternately to illuminate the phosphors and a single CCD detector equipped with narrow-bandpass filter. The latter technique may thus offer high sensitivity, accuracy, and brightness, as well as technical simplicity, which challenges all currently existing phosphor-based temperature mapping methods. E


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Figure 4. Thermal imaging of the heating elements using luminescent thermometers: Δ image of heated element (a); Δ room-temperature image (b); Δ cross-section profiles for the heated (red line in (a)) and unheated (blue line in (c)) element as well as the cross section of bright-field image (orange line in (d)) along the red, blue, and orange lines, respectively, presented in (a), (c), and (d) are shown in (b); and bright-field image of heating elements (d).

values of the 4A2 → 4T2 absorption band for all of the phosphors studied here are presented as a function of temperature in Figure S3a. As it is shown, relatively small changes of the FWHM can be observed for LiLaP4O12:Cr, whereas around 50% broadening of absorption band can be found for YAP and YAG by increasing the temperature from −150 to 400 °C. Moreover, similar dependence can be found in the case of band position (Figure S3b). Barycenter of absorption bands shifts only by 200 cm−1 for the analyzed temperature range (in the case of LiLaP4O12 above 200 °C, its emission intensity was totally quenched), whereas 813 and 836 cm−1 shifts can be observed for YAG and YAP, respectively. Because the population of higher vibrational components of ground states increases with temperature rise, the photoexcitation upon nonresonant condition (i.e., nonresonant in respect to the ground-state absorption) should be facilitated at rising temperature. In the same time, the thermal quenching should depopulate the emitting levels at higher temperature; thus, both processes contribute, i.e., compete in the observed thermal evolution of the spectra leading to reduction of efficiency of thermal quenching. According to expectations, thermal quenching rate for nonresonantly excited luminescence (NREL) is lower compared to that of resonantly excited luminescence (RE), as presented in Figure 3a, Figure S4a,b for YAP, YAG, and LiLaP4O12 nanocrystals, as well as in Figure S4c−e, where integral emission intensity as a function of temperature is presented. Hence, these features and the observed differences can be further exploited as a temperature sensor. The most evident difference in thermal evolution of the emission intensity upon RNR-EIR can be found for YAP, which shall be associated with the strongest crystal field in this host

material. The slight difference between thermal evolution of emission intensity RNR-EIR, which was observed for YAG, is related to the different conditions provided for nonresonant excitation (λnonres = 610 nm for YAP and λnonres = 650 nm for YAG), which makes it impossible to directly compare absolute numbers between the host materials. The temperaturedependent parameter Δ defined previously with eq 4, was determined for each of the analyzed phosphors as a function of temperature (Figure 3b). The observed profiles agree well with theoretical predictions (Figure 2a), and the obtained differences can be related to differences between respective ΔE2/ΔE1 (Figure 1a). The analysis of thermal evolution of the relative sensitivity for the analyzed phosphor (Figure 3e) indicates YAP to show the highest potential for the noncontact temperature sensing applications. YAP exhibits the highest values of relative sensitivity (reaching 0.25%/K in physiological temperature range and 0.37%/K at −150 °C) almost in the whole temperature range of −175 to 400 °C. We relate these effects to the strength of the crystal field in the analyzed host material manifested as a difference in the electron−phonon coupling (α). On the basis of the calculation presented in the SI, a straight correlation between sensitivity and α can be found (α = 110 cm−1 for LiLaP4O12, α = 150 cm−1 for YAG, and α = 180 cm−1 for YAP). 3.2. Temperature Imaging. The proof-of-concept thermal imaging using the proposed materials and new T imaging technique of resonantly and nonresonantly excited luminescent (RNR-EIR) thermometer was performed using 0.15 mm tungsten wire placed on the layer of YAP:Cr phosphor (Figure S5) and power supplied with a laboratory current driver. Due to the size of the heater and the field of the view of 20× F


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fast and affordable alternative to luminescent 2D thermometry based on the relative emission intensity changes. Moreover, the proof-of-concept experiment of thermal imaging using RNREIR 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 filter cubes and single sensitive Vis camera. Therefore, the proposed materials and method promise also much higher response rates and offer prospects for the determination of temperature of fast-moving/rotating mechanical elements.

microscope objective, the presented images are large mosaic composed of 25 × 16 high-resolution (∼2.1 μm/pixel) images (6.4 mm × 8.5 mm in total), which were subsequently evaluated with eq 4. The obtained false-colored fluorescence images of Δ at 60 °C and room temperature (Figure 4a,b, respectively), as well as the respective cross sections of the Δ images, show evident differences between the two temperatures. Some black points observed in the images result from nonhomogenously distributed phosphor, as it can be seen in the bright-field image (Figure 4d). Darker color represent lower Δ value that corresponds to the higher temperature. As it can be clearly seen, significant differences of Δ confirm high applicative potential of this kind of luminescent thermometer and new method. Moreover, comparing the obtained results with an image from thermovision camera (Figure S5), significantly higher spatial resolution of thermal imaging can be obtained in the case of the proposed RNR-EIR luminescent thermometer, which reaches micrometer resolution typical for fluorescence optical microscopes. It is important to underline that the images have been obtained with a conventional halide UV−vis lamp and not with laser sources, as is typically required with lanthanide-doped (upconverted) nanoparticles.15 This advantage is of great importance for potential studies of dynamic temperature mapping in biology. On the basis of the comparison of the obtained cross sections of Δ for the system heated at 60 and 25 °C, the increase of Δ value at higher temperature can be observed (Figure 4a−c). Moreover, heat diffusion profile can be noted with higher spatial resolution in respect to thermovision cameras.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13649. XRD and TEM data for all three hosts; thermal evolution of FWHM and band position versus temperature; emission−excitation spectra; normalized emission intensity under resonant and nonresonant excitations as a function of temperature; heater setup and thermal images (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.B.). *E-mail: [email protected] (L.M.).

4. CONCLUSIONS We have proposed a radically novel approach to temperature visualization. The proposed phosphor-assisted temperature imaging (PATI) rely 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 sufficient as photoexcitation source) and most of all with technical simplicity of wide-field temperature 2D mapping with optical resolution of single micrometers. Three different host materials, namely, YAP, YAG, and LiLaP4O12 nanocrystals doped with Cr3+ 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 4A2 state, the nonresonantly excited luminescence can be promoted at higher temperatures. On the other hand, the temperature affects the emission intensity upon both resonant and nonresonant excitations. Therefore, a substantial difference between thermal quenching of Cr3+ luminescence upon resonant and nonresonant excitations was expected and 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 field of the corresponding host materials. The obtained results confirm the high applicative potential of the presented technique for noncontact temperature readout and can be a

ORCID

Artur Bednarkiewicz: 0000-0003-4113-0365 Lukasz Marciniak: 0000-0001-5181-5865 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The “High sensitive thermal imaging for biomedical and microelectronic application” project is carried out within the First Team program of the Foundation for Polish Science cofinanced by the European Union under the European Regional Development Fund.

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REFERENCES

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