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Abstract
The thermal sensing capability of the Tm3+-doped yttrium orthoaluminate nanoperovskite in the infrared range, synthetized by a sol-gel method, was studied. The temperature dependence of the infrared upconverted emission bands located at around 705 nm (3F2,3→3H6) and 800 nm (3H4→3H6) of YAP: Tm3+ nanoperovskite under excitation at 1210 nm was analyzed from RT up to 425 K. Calibration of the optical sensor has been made using the fluorescence intensity ratio technique, showing a high sensitivity in the near-infrared compared to other trivalent rare-earth based optical sensors working in the same range. In addition, a second calibration procedure of the YAP: Tm3+ optical sensor was performed by using the FIR technique on the emission band associated to the 3H4→3H6 transition in the physiological temperature range (293-333 K), showing a very high relative sensitivity compared with other rare-earth based optical temperature sensors working in the physiological range. Moreover, the main advantage compared with other optical sensors is that the excitation source and the upconverted emissions do not overlap, since they lie in different biological windows, thus allowing its potential use as an optical temperature probe in the near-infrared range for biological applications.
© 2017 Optical Society of America
1. Introduction
It is known that the optical properties of materials depend strongly on the environment conditions, e.g., temperature, pressure, etc [1,2]. Therefore, any changes in these parameters will lead to changes in the optical response of materials. Taking advantage of this behavior, a new researching issue is the development of optical sensors and thus, the searching for novel materials with extraordinary luminescence responses [3] that could be applied as probes that could monitor physical or chemical fluctuations of the environment with respect to their initial conditions.
The main idea of an optical sensor is to record any physical or chemical fluctuation of the environment as a variation of its optical properties. Among the materials that can be employed in the development of optical temperature sensors, those doped with trivalent rare- earth (RE3+) ions turns out to be great candidates to obtain optically active materials, due to the extraordinary optical properties of RE3+, having thermally coupled emitting levels (TCL). The relative population of these TCL depends on the environment temperature, and, therefore, so does their emission [4]. The study of the temperature evolution of the RE3+ emission response implies the use of several optical parameters as calibration tools, e.g., bandwidth, lifetime, band shift, etc. However, the relative intensities between the TCL, known as the fluorescence intensity ratio technique (FIR), is the most used procedure to calibrate an optical temperature sensor [5,6].
From the point of view of the optical sensors, the research on nanomaterials field has become a relevant issue, because these nano-materials present the ability of keeping the properties of their bulk counterpart [7]. On the other hand, these materials, especially RE3+-doped nano-materials, present enormous advantages compared to other materials such as organic dyes and quantum dots. First of all, the RE3+-doped nano-materials can be synthetized through cheap methods that cannot be used in quantum-dots syntheses. Secondly, these materials present extraordinary chemical and photostability properties as well as good biocompatibility, and most importantly, low toxicity, which are very important for biological applications. It is possible to find several works focused on RE3+-doped nano-materials as optical temperature sensors in the literature, for instance, Yb3+/Tm3+-based optical sensor, such as NaNbO3:Yb3+/Tm3+ nanocrystals [8] or Y2O3:Yb3+/Tm3+ and Y2O3:Yb3+/Ho3+ nanopowders [9]. In addition, Tm3+/Yb3+ and Nd3+-based optical sensors has been successfully used due to their potential employment in medical applications such as fluorescence bioimaging, cellular temperature probe or anticancer and photodynamic therapies, for example, NaYF4:Tm3+/Yb3+ [10], LaF3:Nd3+ nanoparticles [11,12] or yttrium aluminum nano-garnets doped with Nd3+ ions (YAG:Nd3+) [13].
Among the RE3+-doped nano-materials, the yttrium orthoaluminate nanoperovskite (YAP) appear to be good candidates due to their mechanical and thermal properties, as well as chemical stability, such as their bulk counterpart [14]. Furthermore, combining the quantum-chemical properties of the YAP nanoperovskite with the optical properties of Tm3+ ions, which show absorption and emission bands that lie in the near-infrared (NIR), the viability of the orthoaluminate nanoperovskite doped with 2.5 mol% of Tm3+ ions as an optical temperature sensor in the NIR was analyzed.
2. Experimental details
Nano-crystalline YAP perovskite of composition Y0.975Tm0.025AlO3 (YAP: Tm3+) was successfully prepared by the Pechini citrate sol–gel method in an air atmosphere. Stoichiometric molar ratios of high-purity precursor salts of Y(NO3)3·4H2O (ALDRICH, 99.9%), Al(NO3)3·9H2O (ALDRICH, 99.9%) and Tm(NO3)3·5H2O (ALDRICH, 99.9%) materials were mixed and dissolved in 25 ml of 1 M HNO3 under stirring at 353 K for 3 h. Then citric acid, with a molar ratio of metal ions to citric acid of 1:2, was added to the solution, which was stirred and heated at 363 K until reaching the transparency of the solution. Afterwards, 4 mg of polyethylene glycol was added to the solution. This last step created a gel that was fired at 673 K for 6 h in order to remove the residual nitrates and organic compounds and the subsequently obtained powder sample was finally annealed out at 1473 K for 20 h. The second thermal treatment was performed at 1823 K for 12 h. The sample obtained by this synthesis method is chemically stable.
Powder X-ray diffraction data were collected on a PANalytical X’Pert PRO diffractometer (Bragg-Brentano geometry) with an X’Celerator detector employing the Cu Kα1 radiation (λ=1.5405 Å) in the angular range 5° < 2θ < 80°, by continuous scanning with a step size of 0.02°. TEM measurements were performed in a JEOL JEM 2100 equipment operanting at 200 kV. Dynamic light scattering (DLS) measurements were carried out on a Mastersizer 2000/E.
Luminescence measurements from 294 K to 425 K were carried out in a tubular electric furnace (Gero RES-E 230/3) where the sample was placed at its center. Temperature of the sample was controlled with a type K thermocouple in contact with it and connected to a voltmeter (Fluke Calibrator 714). Upconverted emissions of YAP: Tm3+ nanoperovskite were measured by exciting at 1210 nm with a 10 ns pulsed optical parametric oscillator OPO (EKSPLA/NT342/3/UVE). Emissions were focused on the entrance slit of a spectrograph (Andor SR-303i-A) equipped with a cooled CCD (Andor Newton). All spectra were corrected from the spectral response of the equipment.
3. Fluorescence intensity ratio technique (FIR)
FIR seeks the determination of the temperature analyzing the changes in the band shape of RE3+ emissions. This technique studies the relative emission intensities of two nearby energy levels, close enough in energy to allow the thermal redistribution of the population from the lower level (E2) to the upper one (E3) (see Fig. 1). Therefore, the intensity ratio of these levels depends on the temperature T, but at the same time, this ratio is independent of the source power excitation, because the population of each level is proportional to the pump power used. Under low pump power excitation, the intensity ratio between the emitting E2 and E3 levels, R, is described by Boltzmann’s law, given by:
where kB is the Boltzmann constant, ∆E = E3 – E2 is the energy gap between E3 and E2 excited thermalized levels, g3 and g2 are the degeneracies (2J+1) of the levels, and and are the spontaneous emission rates of the E3 and E2 levels to the ground level (E1), respectively.
Fig. 1 Simplified diagram for three levels particularized to Tm3+ ion. ∆E is the energy gap between the two excited levels (E2 and E3), gi is the degeneracy of the i-th-level and is the spontaneous emission rate between the i-th and j-th levels.
The sensor sensitivity S can be defined as the rate at which R changes with temperature:
However, it is necessary to introduce another parameter, the relative sensor sensitivity SREL, which is defined as follows:
that only depends on the temperature. Hence, it allows the comparison with other optical temperature sensors.From the last equation, it is clear that the larger the energy gap between two thermalized levels the larger the sensitivity. Nevertheless, as the energy gap between these levels increases, the population and the intensity from the upper thermalized level decreases.
4. Results and discussion
4.1 Structural characterization
The X-ray diffraction (XRD) pattern of YAP: Tm3+ is depicted in Fig. 2. It was indexed to an orthorhombic structure space group, Pnma. Hence, XRD confirms the perovskite-type structure of YAP: Tm3+. The unit cell of the nanoperovskite is also shown in Fig. 2. The crystal structure parameters have been obtained from the fitting of the profile of the nanoperovskite by the Rietveld method using FULLPROF program [15] (see Table 1).
Fig. 2 XRD pattern of the YAP: Tm3+ nanoperovskite. The unit cell and the Bragg positions (red ticks) allowed for the space group Pnma are also depicted.
Table 1. Cell Parameters and Reliability Factors Obtained From Fitting of XRD Pattern for YAP: Tm3+
The average grains size, D, was determined from Scherrer formula:
where λ= 1.5406 Å, β is the full width at half maximum of the peaks and θ is the angle of diffraction. The average grains size value was around 35 nm. No amorphous phase was detected in the nanoperovskite sample.TEM image shows the homogenous and reduced size of the crystallites. Fig. 3 shows the size of such crystallites at a 20 nm scale.
DLS measurements on the sample were carried out on a MASTERSIZER 2000E instrument and the conglomeration of most of the particles around 1 micrometer was observed (see Fig. 4). The sonication of it allowed the reduction of the conglomeration down to 0.1 micrometer.
Fig. 4 YAP: Tm3+ nanoperovskite DLS scattering spectra performed before and after (inset) sonication.
4.2 Optical sensor calibration
Trivalent Thulium (Tm3+) ion shows absorption and emission bands lying in the well-known “near-infrared biological windows” (NIR-BW). These biological windows are those infrared spectral regions in which the transmission of the light through the human skin is more effective, because in these regions the optical scattering due to the absorption of water and other compounds present in tissues (such as hemoglobin) is lower compared to other spectral regions, such as the visible one [16]. Commonly, NIR-BW is divided in two main regions, the first biological window (I-BW), which ranges between 650 and 950 nm, and the second window (II-BW) between 1000 and 1300 nm [16]. Thus, YAP: Tm3+ nanoparticles are analyzed in order to study their viability as optical temperature probes in the I-BW by exciting in the II-BW.
The upconverted emission spectra of the YAP: Tm3+ at room temperature (RT) up to 425 K exciting resonantly the 3H6→3H5 transition at 1210 nm are shown in Fig. 5. Just one emission band and their corresponding Stark levels associated to the 3H4→3H6 transition at RT are observed. When the temperature increases, the emission associated with the 3F2,3→3H6 transition starts to show up due to thermally-induced population of the 3F2,3 multiplets from the 3H4 lower emitting level. The emission associated to the 3H4→3H6 transition also increases with temperature, suggesting that the upconversion process is favored with the increase of temperature. In addition, cross-relaxation processes between Tm3+ ions favor the population of the 3F2,3 and 3H4 states [17].
Fig. 5 Temperature evolution of upconversion emission spectra of YAP: Tm3+ nanoperovskite from 294 up to 425 K exciting at 1210 nm. Transitions are also indicated. Emission band associated to 3F2,3→3H6 has been magnified ten times for a better observation.
From the upconverted emission spectra, the experimental intensities were calculated by the integration of the emission bands associated with the 3F2,3→3H6 and 3H4→3H6 transitions for the calibration procedure. The ratio of these areas, measured from 324 K up to 425 K, can be described by using Eq. (1). From the fitting procedure, the values of C=4.61 and the energy gap ∆E=1926 cm−1 were obtained. The relative sensor sensitivity SREL as a function of temperature T, defined by the Eq. (3), is also shown in Fig. 6. The relative sensitivity of this sensor reaches its maximum value of 0.026 K−1 at 324 K. According to this result, the calibration procedure of the YAP: Tm3+ optical sensor could be suitable for its employment in the I-BW exciting within the II-BW with a notable sensitivity.
Fig. 6 Experimental intensity area ratio and relative sensitivity of the YAP: Tm3+ of 3F2,3→3H6 and 3H4→3H6 transitions obtained exciting at 1210 nm. The experimental values were fitted to a single exponential function. The fit curve (red line) of the experimental intensity ratio is also shown.
As can be observed (see Table 2), YAP: Tm3+ nanoperovskite shows a notably sensitivity value, which allows its consideration to become a NIR luminescence material for optical temperature sensor based on the FIR technique. Moreover, in YAP this result has been obtained exciting and detecting in different biological windows taking advantage of the non-linear upconversion processes of the Tm3+ ions, which means that the overlapping between excitation and emission is neglected.
Table 2. Thermal Relative Sensitivity Values of Different RE3+-based Temperature Optical Sensors.
Another possibility that can be considered is the calibration of YAP: Tm3+ nanoperovskite in the physiological range. Nonetheless, the first calibration procedure for this range turned out to be ineffective, because the obtained sensitivity was very low, i.e, the ratio of the intensity of the thermally coupled emitting levels was close to zero. Therefore, another optical sensor calibration procedure was considered in the temperature range between 294 K and 325 K. This time, the intensity ratio between the Stark levels at 776.42 nm and 821.50 nm of the emission band associated with 3H4→3H6 transition were employed for the optical calibration procedure by using the FIR technique (see Fig. 7).
Fig. 7 Temperature upconversion emission spectra of YAP: Tm3+ nanoperovskite from 294 up to 325 K exciting at 1210 nm.
The experimental maximum intensities of these Stark peaks were considered in order to calculate the ratio between them as shown in Fig. 8. The ratio between these intensities follows the Boltzmann distribution law, according to the Eq. (1). This equation was evaluated by recording the emission spectra with the temperature, starting at RT up to 325 K. From the fitting procedure, the values of C=0.15 and the energy gap ∆E=784.10 cm−1 were obtained. The relative sensor sensitivity SREL as a function of temperature, defined by the Eq. (3) is also shown in Fig. 8. The relative sensitivity of this sensor reaches its maximum value of 0.013 K−1 at 294 K. The sensitivity of this optical sensor is very high if it is compared with others RE3+-based optical sensors working in the physiological range (see Table 3). Thus, YAP: Tm3+ nanoperovskite can be considered as a potential candidate as temperature sensor based on the FIR technique in the physiological range with a high sensitivity and, furthermore, for biological applications.
Fig. 8 Experimental intensity area ratio and relative sensitivity of the YAP: Tm3+ Stark levels centered at 776.42 and 821.50 nm associated to the 3H4→3H6 transition. The experimental values were fitted to a single exponential function. The fit curve (red line) of the experimental intensity ratio is also shown.
Table 3. Thermal Relative Sensitivity Values of Different RE3+-based Temperature Optical Sensors Working in the NIR and Physiological Rangea
It is relevant to point out that most of relative sensitivity data presented in Table 2 and Table 3 have been obtained in different conditions (materials dispersed in fluids) compared to YAP: Tm3+. Even though it is true that the overall emission efficiency can vary notably between having or not having the sample dispersed in fluids, the intensity ratio between two emission bands should not change significantly and thus, the sensitivity neither.
It is worth mentioning that the intensity ratio of the YAP:Tm3+ nanoperovskite in the temperature range between 294 and 325 K changes around 43% (twice higher than the one found in NaNbO3: Tm3+ nanoparticles [26]), which is notably higher compared with others RE3+-based optical sensors that have been tested as thermal probes for biological applications. For instance, LaF3 doped with Nd3+ ions, which has been successfully used for subtissue thermal sensing and photothermal treatments [11,12], shows a change of around 4%.
Another advantage of the YAP: Tm3+ nanoperovskite optical sensor compared with those showed in Table 2 and Table 3 (except Ref [26]. that has lower sensitivity), is that the used excitation source lies in the II-BW, in which the optical scattering due to the absorption of water and other compounds present in tissues (such as hemoglobin) is further decreased when is compared to the I-BW, due to the use of longer wavelengths. Thus, with these results and features, the viability of the YAP: Tm3+ nanoperovskite as optical temperature sensor working in the NIR and in the physiological ranges is evidenced and its application in biological systems could be considered.
5. Conclusion
The thermal sensing capability of the YAP: Tm3+ nanoperovskite in the infrared range NIR, synthetized by sol-gel method was studied. The temperature dependence of the infrared upconverted emission bands located around 705 nm (3F2,3→3H6) and 800 nm (3H4→3H6) respectively of YAP: Tm3+ nanoperovskite under excitation at 1210 nm was analyzed from RT up to 425 K, allowing the calibration of the optical sensor by using the FIR technique. The YAP: Tm3+ nanoperovskite shows a high sensitivity in the NIR compared to other RE3+-based optical sensors working in the same range. In addition, a second calibration procedure of the YAP: Tm3+ optical sensor was carried out by using the FIR technique on the upconverted emission band associated to the 3H4→3H6 transition in the physiological range, showing a very relative sensitivity compared with others RE3+-based optical sensors. The intensity ratio shows large changes that turned out to be very high compared with other optical sensors. These features allow the potential utility of YAP: Tm3+ nanoperovskite as optical thermal probe in the NIR and physical range for biological applications with the advantage that the excitation (at 1210 nm) lies in the II-BW while the emission does it in the I-BW, avoiding by this way, the overlap between the excitation and the emission.
Funding
MINECO (MAT2013-46649-C4-4-P, MAT2015-71070-REDC, and MAT2016-75586-C4-4-P), EU-FEDER funds, and FPI grant (BES-2014-068666).
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