Physical Background for Luminescence Thermometry Sensors Based on Pr3+:LaF3 Crystalline Particles
The main goal of this study was creating multifunctional nanoparticles based on rare-earth doped LaF3 nanocrystals, which can be used as fluorescence thermal sensors operating over the 80–320 K temperature range including physiological temperature range (10–50°C). The Pr3+:LaF3 ( = 1%) microcrystalline powder and the Pr3+:LaF3 ( = 12%, 20%) nanoparticles were studied. It was proved that all the samples were capable of thermal sensing into the temperature range from 80 to 320 K. It was revealed that the mechanisms of temperature sensitivity for the microcrystalline powder and the nanoparticles are different. In the powder, the 3P1 and 3P0 states of Pr3+ ion share their electronic populations according to the Boltzmann and thermalization of the 3P1 state takes place. In the nanoparticles, two temperature dependent mechanisms were suggested: energy migration within 3P0 state in the temperature range from 80 K to 200 K followed by quenching of 3P0 state by OH groups at higher temperatures. The values of the relative sensitivities for the Pr3+:LaF3 ( = 1%) microcrystalline powder and the Pr3+:LaF3 ( = 12%, 20%) nanoparticles into the physiological temperature range (at 45°C) were 1, 0.5, and 0.3% °C−1, respectively.
Nanothermometry aims to measure the local temperature of a studied system with submicrometric spatial resolution . Among different thermometers such as nanoscale thermocouples  and coulomb blockade nanothermometers , the luminescent nanothermometers hold a special place because of its high spatial and temporal resolution, accuracy, and contactlessness [1, 4–6]. The luminescent nanothermometry is based on the luminescence features which are analyzed and which are used for the temperature measurement [1, 4]. Luminescent nanothermometers are widely applied in the thermal imaging of integrated circuits , in vitro and in vivo imagining of biological objects , the subtissue thermal sensing [9–11], and temperature detecting for harsh environments or fast moving objects . Generally, there are six parameters that define the luminescence emission of a material: intensity, lifetime, band shape, band width, polarization, and spectral position . Therefore, the luminescent nanothermometers can be grouped into six corresponding subclasses. The most widespread is the subclass of luminescent nanothermometers based on the analysis of relative fluorescence intensity between the different emission bands corresponding to the suitable transitions [1, 9, 13]. Among the most usable materials for luminescent nanothermometers such as quantum dots (QD) and dye-based luminescent nanothermometers, the rare-earth based materials have a special place mainly because of their excellent photostability, long luminescent lifetimes, sharp emission bands, and low toxicity which is highly important for biomedical application, for example, in photothermal and photodynamic therapies . Indeed, in biomedicine, the luminescent nanothermometers should be nontoxic and stay chemically stable under light irradiation so that toxic components of reaction product are not delivered to the cells . Generally, toxicity of inorganic substances generally depends on their solubility in the water. Also rare-earth doped fluorides demonstrate the lowest solubility among other rare-earth based materials (oxides et al.). For instance, the CeF3 and other trifluorides with tysonite crystal structure demonstrate solubility around 10−5–10−6 mol/L. and as a consequence low toxicity . The lanthanides such as Dy3+, Er3+, Nd3+, Yb3+, Sm3+, and Pr3+ demonstrate temperature sensitivity in different temperature ranges providing different absolute (K−1 or °C−1) and relative (%K−1 or %°C−1) sensitivities. In general, for the majority of lanthanides, temperature sensing technique is based on the ratio of the fluorescence intensity of the emission bands corresponding to the suitable transitions . For example, Dy3+:LaF3 glass ceramics under 352 nm excitation (6P7/2 of Dy3+) demonstrate sensitivity K−1 obtained at = 294 K  and the temperature is extracted from the ratio of the fluorescence intensities of the 480 and 572 nm emission bands corresponding to 4F9/2-6H15/2 and 4F9/2-6H13/2 transitions of Dy3+ . For such ions as Pr3+ and Er3+ the same approach is used and these ions operate in overlapping temperature ranges of 300–600 K and 300–2000 K, respectively [12, 16]. For the biomedical applications and in vitro and in vivo experiments, the luminescent thermometers which can operate in physiological temperature range (10–60°C) are highly relevant . Sm3+:TiO2 NPs operating in a temperature range from room temperature to 110°C demonstrate that maximum value of relative sensitivity is 10.54% °C−1 at 57.5°C . The Nd3+ based luminescent nanothermometers such as Nd3+:LaF3 NPs or active-core/active-shell Nd3+- and Yb3+-doped LaF3 NPs are exited at 808 nm wavelength which lays in the biological window (650–1350 nm) what paves the way toward subcutaneous thermal sensing [9–11, 18]. For example, the active-core/active-shell Nd3+/Yb3+-doped LaF3 NPs demonstrate relative thermal sensitivity around %·°C−1, at 10°C [9, 11]. The systems based on Pr3+ are studied in [19–21]. In case of Pr3+ ions, the energy gap between 3P0 and the next-highest state, 3P1, is only a few hundred cm−1 for most host matrixes (for example, for LaF3  and CsCdBr3 it is around 500 cm−1), 3P1 becomes thermally populated following 3P0 excitation, and the luminescence spectrum contains both 3P0 and 3P1 emission, with intensity ratios consistent with the predicted Boltzmann distribution among these states . In particular, in , the temperature sensing technique is based on the ratio of the fluorescence intensity for 3P1→ 3H5 and 3P0→ 3H5 transitions in temperature range 273–453 K providing sensitivity within ~1.15%K−1. Also, Pr3+ demonstrates strong luminescence lifetime dependence on temperature as it is shown in . For the system Pr3+:YAG the measured lifetime of 1D2 decreases linearly from ~190 μs at 0°C to ~110 μs at 1000°C. From another side, the thermal sensing materials based on Pr3+ ions attract great attention of the scientific community because of its thermal sensitivity in broad temperature range [12, 23] and biomedical potential .
Also it should be noted that during the last two decades, an immense progress has been done toward facile synthesis of LnF3 nanoparticles (NPs) with different size and shape [25, 26]. For these reasons, LaF3 host matrix is considered as a very promising material for doping with rare-earth elements for broad range of applications.
Here we analyze the temperature dependent luminescence features of Pr3+ doped LaF3 both microcrystalline powder and crystalline NPs and demonstrate the opportunity of applying them as nanothermometers for the 80–320 K temperature range, especially focusing on the physiological one (10–60°C).
2. Materials and Methods
2.1. Growth of the Pr3+:LaF3 (CPr = 1%) Crystal, Synthesis of the Pr3+:LaF3 (CPr = 12%, 20%) Nanoparticles, and Their Characterization
All the samples were obtained in the MRS laboratory, Institute of Physics, Kazan Federal University.
The NPs Pr3+:LaF3 ( = 20%) were synthesized via coprecipitation method. 0.4 g Pr2O3 and 2 g La2O3 were added to 110 mL of 10% nitric acid in a polypropylene glass. The mixture was heated to 50°С and stirred for 45 min; then a transparent light-green solution appeared. Then the mixture was filtered and placed in an ultrasonic cleaner (ultrasonic power 100 W) and solution of 3 g of NaF in 100 mL of distillated water was added. Then the pH was adjusted to 4 by adding 25% solution of ammonium hydrate. Then the mixture was stirred for 10 minutes under the ultrasonic treatment. The precipitation was purified with distillated water by centrifugation (12,000 revolutions/min, centrifugation time 15 min) for 8 times. The presence of nitrates after each stage of centrifugation was controlled via diphenylamine test. In order to synthesize Pr3+:LaF3 ( = 12%) NPs, the suitable ratio of Pr2O3 and La2O3 was taken. Then nanoparticles were dried in air. The structure of the material was characterized by X-ray diffraction method with Shimadzu XRD-7000S X-ray diffractometer. Analysis of samples was carried out in a transmission electron microscope Hitachi HT7700 Exalens. Sample preparation was followed: 10 microliters of the suspension was placed on a formvar/carbon lacey 3 mm copper grid; drying was performed at room temperature. After drying, grid was placed in a transmission electron microscope using special holder for microanalysis. Analysis was held at an accelerating voltage of 100 kV in TEM mode. The control of amount of nitrates in colloidal solution of the NPs after each stage of centrifugation was performed by identification test using diphenylamine (diphenylamine test).
The Pr3+:LaF3 ( = 1%) crystal was grown by the method of directional solidification in graphite crucibles . The samples were cylinder shape of 6 mm diameter and 20 mm length. The microcrystalline powder was obtained by mealing.
The luminescence spectra were recorded using CCD spectrometer (StellarNet), which detects the emission in 200–1100 nm spectral range with a spectral resolution of 0.5 nm. The optical parametric oscillator laser system (420–1200 nm) from JV LOTIS TII was used for excitation of the luminescence of the samples. The pulse width and the pulse-repetition rate were 10 ns and 10 Hz, respectively. The spectral width of laser radiation was less than 0.15 nm. The pulse duration and the pulse-repetition rate of laser pulses were 7 ns and 10 Hz, respectively. The experiments were carried out at 80–320 K temperature range.
3. Results and Discussion
3.1. NPs Characterization (HR TEM, and X-Ray Diffraction)
Transmission electron microscopy (TEM) data indicate that obtained Pr3+:LaF3 samples consist of nearly monodisperse well-crystallized NPs with an average diameter of 20 nm (Figure 1(a)). Also the sample has regular almost spherical shape. Selected area electron diffraction (SAED) patterns correspond to LaF3 crystal structure (Figure 1(a)). The presence of circular rings in the SAED patterns indicates that the sample consists of disordered monocrystalline NPs. The lattice structures are clearly seen under the higher magnifications, what also confirms that each NP is monocrystalline (Figure 1(b)). According to the X-ray diffraction data (Figure 2), all the Pr:LaF3 NPs were hexagonal structured crystals that correspond to the structure of matrixes of LaF3 and PrF3. Sharp peaks of the patterns also confirm good crystallinity of the NPs. The lack of amorphous phase was also proved. The diphenylamine test did not reveal presence of nitrates in the NPs colloidal solution after 4th stage of centrifugation. Those facts indicate that all the nitrate precursors and reaction products were removed and did not affect the physical properties of the samples.
3.2. Luminescent Nanothermometry of Pr3+:LaF3 (CPr = 1%) Crystal
Room-temperature luminescence spectra of Pr3+:LaF3 ( = 1%) microcrystalline powder and Pr3+:LaF3 ( = 12%) NPs excited by laser beam at 444 nm are presented in Figure 3. The spectra of Pr3+ ions for the powder and the NPs do not differ much between each other as it is commonly expected for rare-earth ions . The luminescent spectra have the emissions bands at about 487, 523, 537, 580, 601, and 672 nm which are interpreted as a result of the transition from the 3Pj ( = 0, 1, 2) excited states to 3H4, 3H5, 3H6, and 3F3 states of Pr3+ ions, respectively . The emission from 1D2 state was not found under the excitation condition and at the studied temperature range. Probably the emission from 1D2 is not observed because of the lack of nonradiative relaxation of 3Pj to 1D2 due to low cutoff phonon frequency in LaF3 (350–400 cm−1), which is 2 times less than one for YAG (700–865 cm−1). Indeed to bridge the 3P0–1D2 energy gap, the 9 phonons are required in case of LaF3 and only 4 or 5 ones for YAG; thus, the multiphonon relaxation is expected negligible [21, 30]. Moreover, the electron phonon coupling constants of YAG are highly different from those of LaF3.
The temperature dependence of the luminescence spectra of the Pr3+:LaF3 microcrystalline powder is shown in Figure 4(a). The emission bands at about ~470–500, ~520–540, ~570–620, ~660–700, and ~690–730 nm corresponding to the transitions from 3P1 and 3P0 to 3H4, 3H5, 3H6, 3F3, and 3F4 states, respectively, are the most temperature sensitive in the range above 160 K. For instance, when the temperature increases from 80 to 160 K, both intensities of emissions from 3P0 state (e.g., at ~487, ~537, ~610, and ~725 nm) and the ones from 3P1 (e.g., at ~523 and about 660–700 nm) demonstrate negligible changes. And the intensities of luminescence from 3P1 state were much less than from the 3P0 one, whereas the emission from 3P0 (at 487 nm) is decreased dramatically in contrast to the emission from 3P1 (523 nm) which is increased at the temperature higher than 160 K (Figure 4(a)). At the same time, the intensity of emission at about 537 nm from the same 3P0 state stays almost constant into the temperature range from 80 to 160 K and decreases smoothly above 200 K probably because of overlapping spectral bands associated with the transitions from both 3P0 and 3P1 states as it is shown on the histogram (Figure 4(b)). Figure 5 also demonstrates temperature dependence of 523 and 532 nm emission bands. Generally, the temperature sensitivity looks similar to the process occurring in Pr3+:β-NaYF4 under excitation to 3P1 state : after excitation of 3P1 state at 15 K, the nonradiative relaxation from 3P1 to 3P0 states takes place and the 3P0-3H4 emission band is clearly observed. At 100 K, additional emission band appeared at 525 nm and 470 nm, which were assigned to transitions from 3P1 state to 3H5 and 3H4 ones, respectively. In our case, decreasing of the intensity of the emission from 3P0 (e.g., at about 487 nm or 537 nm) and simultaneous increasing of the intensity of the emission from 3P1 (at 523 nm) at 80 K can be attributed to the fact that 3P1 and 3P0 states share their electronic populations according to the Boltzmann process . It was shown in  that at 15 K the electrons remain in 3P0 state, which is the only emitting state. Then, the thermalization of 3P1 state begins at around 100 K and operates more and more efficiently upon warming [22, 31]. Also the intensity of the emission bands of rare-earth ions depends on several parameters among which, temperature is the most critical ones [1, 32] and transitions from 3P0 are also temperature dependent. The relationship of the relative emission intensities with temperature can be used to calculate the activation energy () from thermal quenching using the following equation :where and are the intensity of 3P1 emission (at 523 nm) for the Pr3+:LaF3 microcrystalline powder at 80 K and the testing temperature (120–320 K), respectively (Figure 5). Here is a constant, and is the Boltzmann constant. The and were calculated to be = 487 cm−1 ± 40 cm−1 and = −2,27058. This value is in good agreement with value of the energy gap between 3P1 and 3P0 states which is about 500 cm−1 . The as a function of temperature is shown on the inset of Figure 5. Therefore, the local temperature inside bulk crystal can be estimated by (1). The absolute and relative sensitivities ( and ) can be expressed by the following equations [6, 33, 34]:where is the energy difference between thermally coupled levels, is the Boltzmann constant, is the absolute temperature, and is the integrated luminescence ratio between the two thermally coupled levels 3P1 and 3P0. Noteworthy, the maximum value of reaches 0.017 K−1 (at 320 K), while the maximum value of is 40% K−1 (at 443 K) (Figure 6). Figure 7 shows the ratio between intensities of 537 and 523 nm emissions of Pr3+ ions as a function of temperature and the fitting is performed using calculated value. It is clearly seen that rises from 0.01 to 2.3 (exhibiting a 200-time increase), as shown in Figure 6. These results indicate that the Pr3+:LaF3 ( = 1%) microcrystalline powder is a promising candidate for luminescent thermometry application especially in physiological temperature range (10–50°C).
3.3. Luminescent Nanothermometry of Pr3+:LaF3 (CPr = 12%, 20%) Nanoparticles
The intensity of luminescence of the Pr3+:LaF3 NPs was at least one order of magnitude weaker than for the microcrystalline powder. The temperature dependent spectra of the Pr3+:LaF3 ( = 12% and 20%) NPs are shown in Figures 8(a) and 8(b), respectively. The intensities of two emission bands at about 487 and 538 nm corresponding to the 3P0-3H4 and 3P0-3H5 transitions, respectively, decrease significantly when the temperature rises from 80 to 320 K. Whereas unlike the Pr3+:LaF3 bulk crystal ones demonstrate drastic decrease only into the temperature range from 200 to 320 K. Also surprisingly, the Pr3+:LaF3 NPs luminescence at about 523 nm (3P1-3H5 transitions) demonstrates negligible intensity changing within the whole temperature range in contrast to the crystal bulk where it was most temperature dependent (see Figures 9(a) and 9(b)). To be fair, it should be noted that in case of Pr3+:LaF3 ( = 12%) NPs the emission band at 538 nm demonstrates slight increase of intensity from 80 K to 200 K and then remains almost constant within 240–320 K temperature range (Figure 9(a)). On the other hand, in case of Pr3+:LaF3 ( = 20%) NPs the emission peak at 538 nm remains constant within the whole temperature range (80–320 K). However, the emissions stipulated by transitions from 3P0 state (e.g., at 487 and 538 nm) demonstrate significant change into the whole temperature range (80–320 K). It can be concluded that thermalization between 3P0 and 3P1 states dominates only for the Pr3+:LaF3 ( = 12%) NPs into the 80–200 K temperature range and another temperature dependent quenching mechanism of 3P0 state takes place above 200 K. Indeed, the phonon spectrum of nanosized LaF3 is modified and differs from the bulk one [28, 35]. This modified phonon spectrum can affect the interaction between 3P0 and 3P1 states. Besides, there are numerous other possible mechanisms that could affect luminescent transitions of rare-earth ions and which are dependent on the temperature: a multiphonon decay, an energy transfer between rare-earth ions or to quenching centers, appearance of phonon assisted Auger conversion processes, and thermal enhancement of energy transfer processes between rare-earth ions and the host levels or charge transfer states [1, 31, 32].
One of the significant differences between NPs and bulk crystals (including nanosized ones) is the increased role of surface and, as a consequence, interaction of ions with surface ligands . It should be noted that the concentration of Pr3+ ions in the NPs is high; thus, concentration quenching takes place. This quenching can be related to migration of energy to quenching centers (most probably OH groups). This process is also temperature dependent for Pr3+ ions . It is clearly seen from Figure 10 that and values for the NPs are almost two times less then ones for the crystal.
As it was mentioned above, the NPs were synthesized via coprecipitation method in water and the surface of the NPs probably covered with adsorbed OH groups . Moreover, it is known that the NPs synthesized via the same method contain water clusters inside the structure . Also, the slight inhomogeneity of the Pr3+:LaF3 ( = 12%) NPs structure shown on Figure 1(b) suggests the presence of water clusters inside the NPs. It means that there are two kinds of surfaces (internal and external) with adsorbed OH groups which can act as main quenching units attributed to nonradiative transitions from rare-earth exited state to a vibrational state of OH molecule [38, 39]. It is that the luminescence emission intensity of the NPs is less than ones for the crystal. As it is clearly seen from Figures 8(a) and 8(b), all the peaks corresponding to transitions from 3P0 state strongly depend on temperature and, in contrast, the peaks corresponding to transitions from 3P1 one do not demonstrate obvious temperature sensing properties. Therefore, in case of Pr3+:LaF3 NPs the temperature dependent nonradiative transitions from rare-earth 3P0 exited state to a vibration state of OH groups dominate over the thermalization process and the populations of the excited 3P0 and 3P1 states decay quasi independently. The 3P1 state lifetime is determined by rare-earth ions intracenter nonradioactive relaxation to 3P0 state and that is why the correspondent emission intensities are weakly temperature sensitive. After analyzing the literature, several systems based on the rare-earth doped LaF3 NPs operating in the physiological temperature range were chosen for the further comparison with the Pr3+ doped LaF3 system. Also, temperature sensing technique of the chosen systems is based on the ratio of the fluorescence intensity of the emission bands corresponding to the suitable transitions. The value of relative temperature sensitivity for Pr3+:LaF3 macropowder and NPs ( = 12%, 20%) into the physiological temperature range at 45°C appeared to be 1.0, 0.5, and 0.3%°C−1, respectively. The two well-known systems Nd3+:LaF3 ( = 10%) and Nd3+, Yb3+:LaF3 ( = 10%, = 10%) NPs demonstrate relative sensitivities around 0.01%°C−1 and 0.1%°C−1, respectively [10, 11]. Also it should be noted that in case of Dy3+:LaF3 transparent glass ceramics , Nd3+:LaF3 ( = 10%) and Nd3+, Yb3+:LaF3 ( = 10%, = 10%) NPs [10, 11], and our systems the relative sensitivity curves decay almost linearly in the physiological temperature range. Probably, in order to increase relative sensitivity of Pr3+:LaF3 NPs, the concentration of Pr3+ ions in LaF3 host matrix should be reduced and addition experiment should be carried out in the future.
The thermometer performance is characterized not only by the relative sensitivity but also by the maximum temperature uncertainty. The minimum temperature uncertainty can be estimated usingwhere is the relative uncertainty in the determination of the thermometric parameter (we use the typical value of a portable detector, 0.5%) . The temperature uncertainty in the physiological temperature range for Pr3+:LaF3 macropowder and NPs ( = 12%, 20%) was 0.5, 1.0, and 1.7°C. These values are comparable with Yb3+/Er3+-doped PbF2 nanocrystal as temperature sensor ( = 1/0°C). The technique presents temperature uncertainty 1.0 K. Also, the repeatability of a thermometer’s readout upon temperature cycling is quantified using the expression :Thermometric parameter is recorded in 10 heating-cooling temperature cycles showing reproducibility higher than 99.1% for all the samples (Pr3+:LaF3 macropowder, and NPs ( = 12%, 20%)).
In summary, the Pr3+ ions doped LaF3 crystal and NPs were demonstrated to be one of the most promising inorganic optical temperature sensing materials in the temperature range from 80 to 320 K, including physiological temperature range (10–50°C). The NPs are nontoxic at micromolar concentrations , which is extremely important for biomedical application. The two thermal sensing peaks at 690 nm and 723 nm corresponding to 3P1-3F4 and 3P0- 3F4 transitions, respectively, situated within the biological window open the way for subtissue thermal sensing. The absolute and relative sensitivities ( and ) reach values of 0.017 K−1 (at 320 K) and 40% K−1 (at 443 K), respectively.
It was revealed that the mechanisms of temperature sensitivity for the crystal and the NPs are different. In the crystal, the 3P1 and 3P0 states share their electronic populations according to the Boltzmann and thermalization of 3P1 state takes place. In the NPs, there are two temperature dependent mechanisms: energy migration within 3P0 state in the temperature range from 80 K to 200 K followed by quenching of 3P0 state by OH groups at temperatures higher 200 K. The latter process dominates over the thermalization between 3P0 and 3P1 states. It is also demonstrated that of the NPs increases when the concentration of Pr3+ ions decreases and can be optimized by choosing suitable concentration of Pr3+ ions. For example, the value of relative temperature sensitivity for Pr3+:LaF3 macropowder and NPs ( = 12%, 20%) into the physiological temperature range appeared to be 1.0, 0.5, and 0.3% °C−1, respectively.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Microscopy studies were carried out at the Interdisciplinary Center of Analytical Microscopy of Kazan Federal University. The microscopy studies were funded by the subsidy of the Russian Government [Agreement no. 02.A03.21.0002] to support the Program of Competitive Growth of Kazan Federal University among World’s Leading Academic Centers. The synthesis of the nanoparticles, crystal growth, and X-ray diffraction experiments were supported by the Russian Foundation of Basic Research (RFBR) Grant no. 15-02-08648 A and the optical spectroscopy experiments were supported by the state assignment in the sphere of scientific activities (Project no. 3.1156.2017/4.6).
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