Upconverting Nanoparticles Working As Primary Thermometers In

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Upconverting Nanoparticles Working as Primary Thermometers in Different Media Sangeetha Balabhadra, Mengistie L. Debasu, Carlos D.S. Brites, Rute A.S. Ferreira, and Luis D. Carlos J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 09 Jun 2017 Downloaded from http://pubs.acs.org on June 9, 2017

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The Journal of Physical Chemistry

Upconverting Nanoparticles Working As Primary Thermometers In Different Media

Sangeetha Balabhadra†,‡, Mengistie L. Debasu†,‡, Carlos D. S. Brites†, Rute A. S. Ferreira†, and Luís D. Carlos†*

†

M.Sc. S. Balabhadra, Dr. M. L. Debasu, Dr. C. D. S. Brites, Prof. Dr. A. S. Ferreira, Prof. Dr.

L. D. Carlos, Departamento de Física and CICECO-Aveiro Institute of Materials, Universidade de Aveiro, 3810–193 Aveiro (Portugal)

‡

M.Sc. S. Balabhadra, Dr. M. L. Debasu, Departamento de Química and CICECO-Aveiro

Institute of Materials, Universidade de Aveiro, 3810–193 Aveiro (Portugal)

Corresponding author e-mail: [email protected] (L.D.C.)

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ABSTRACT In the last decade, non-invasive luminescent thermometry becomes popular due to the limitations of traditional contact thermometers to operate at scales below 100 µm, as required by current demands in disparate areas. Generally, the calibration procedure requires an independent measurement of the temperature to convert the thermometric parameter (usually an intensity ratio) to temperature. A new calibration procedure is necessary whenever the thermometer operates in a different medium. However, recording multiple calibrations is a time-consuming task, and not always possible to perform, e.g. in living cells and in electronic devices. Typically, a unique calibration relation is assumed to be valid, independently of the medium, which is a bottleneck of the secondary luminescent thermometers developed up to now. Here we report a straightforward method to predict the temperature calibration curve of any upconverting thermometer based on two thermally-coupled electronic levels independently of the medium, demonstrating that these systems are intrinsically primary thermometers. SrF2:Yb/Er powder and water suspended nanoparticles were used as an illustrative example.


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INTRODUCTION Temperature is a central concept of thermodynamics and statistical mechanics affecting the dynamics and viability of virtually all natural and engineered systems, from atomic to macroscopic levels. The well-known limitations of contact thermometry in micro- and nanostructures, in which the required spatial resolution is lower than a micrometer, boosted in the last decade the research on non-invasive, self-referencing and highly sensitive temperature nanoprobes.1 Distinct luminescent phosphors have been enquired for thermometry purposes, such as, organic dyes,2 polymers,3 quantum dots (QDs),4 silver and gold nanostructures,5 DNA or protein conjugated systems,6-7 Cr3+-based materials,8-9 and trivalent lanthanide (Ln3+) ions incorporated in organic-inorganic hybrids,10 multifunctional heater-thermometer nanoplatforms,11-13 upconverting,14-17 down converting18 and downshifting19-20 nanoparticles. Ln3+-based materials are versatile, stable and narrow band emitters with, in general, high emission quantum yields (>50%).21 Implementation of these phosphors as ratiometric thermometers was extensively reviewed in the past five years for diverse applications.22-30 Few of the most recent cutting edge examples are the works of Brites et al.,31 determination of the instantaneous Brownian velocity of nanofluids, Wang et al.,32 production of intracellular thermometers, Rodrigues et al.,33 temperature triggered molecular logical gates, and Antić et al.,34 fabrication of a luminescent thin-film to determine the temperature of an alanine dosimeter in a high-energy radiation field. Three main approaches to determine the absolute temperature were followed in Ln3+based luminescent thermometry: (i) spectral shift of the emission arising from a given transition, (ii) emission intensity measurements, using the integrated intensity of a single transition or of a pair of transitions, and (iii) lifetime and risetime measurements.22, 27 Among them, the most popular approach is to take the intensity ratio of two transitions (so-called thermometric parameter, ∆) originated in two excited states that are in thermal equilibrium 3 ACS Paragon Plus Environment


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(denoted by |1> and |2>). The temperature, T, is based exclusively upon the validity of the Boltzmann distribution between the levels:35

∆≡

 − ∆E  I2  = B exp I1  k BT 

( 1)

where I2 and I1 are the intensities of the |2> → |0> and |1> → |0> transitions, respectively (|0> is the lower energy level), kB is the Boltzmann constant, and ∆E is the energy difference between the barycenters of the two transitions. The constant B depends on the degeneracies of the two excited levels, the total spontaneous emission rates, the transitions branching ratio relatively to the ground state, and the angular frequencies of the |2> → |0> and |1> → |0> transitions.27, 35 Up to now, ∆E and B are not determined independently of the temperature and, therefore, an external calibration of the thermal dependence of the thermometric parameter is needed. The usual calibration procedure requires an independent measurement of the temperature (using, for instance, a thermocouple or an infrared camera) to allow the corresponding conversion between relative intensities and temperature. Thus, a new calibration procedure is necessary whenever the thermometer operates in a different medium, as other variables, such as the ionic strength, pH, pressure, Ln3+ local surroundings, or atmosphere composition, impact the thermometric parameter value. However, recording multiple calibrations in different medium is a time-consuming task that is not always possible to be implemented, as, for instance, in living cells and operating electronic devices. In general, a unique calibration relation is assumed to be valid, independently of the medium, which is a central bottleneck of secondary luminescent thermometers. Therefore, there is a great challenge to develop effective luminescent primary nanothermometers in which the intensity


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ratio is related univocally with the absolute temperature through a well-established equation of state. Thermometric systems that must be referred to a well-known temperature for their calibration are classified as secondary thermometers, while primary thermometers are characterized by a well-established equation of state that directly relate a particular measured value to the absolute temperature without the need of calibration.27, 36-37 Primary luminescent thermometers can overwhelmed the above-mentioned limitation of secondary thermometers as the intrinsic calibration parameter is dependent on well-known quantities and, then, it is computable from other measurements not requiring the material calibration. Although several examples of gas, acoustic, noise and radiation primary thermometers have been reported in the literature,36 examples of primary luminescent thermometers are, up to now, very scarce. As far as we know, only three cases can be found in the literature: i) CdSe(ZnS)38 QDs, ii) Si nanoparticles functionalized with 1-dodecene,39 in both cases the thermometric parameter (the emission peak position) is described by the Varshni’s law, and iii) Y2O3:Eu3+ micro- and nanoparticles,40 in which the thermometric parameter is defined as the ratio between the emission intensities of the 5D0→7F4 transition when the 5D0 emitting level is excited through the 7F2 and 7F0 levels (physiological temperatures) or through the 7F1 and 7F0 levels (for temperatures down to 180 K). In this work, we report a straightforward method to predict the temperature calibration curve of any upconverting thermometer based on two thermally-coupled electronic levels independently of the medium, demonstrating that these systems are intrinsically primary thermometers. We use SrF2:Yb/Er upconverting nanoparticles in powder and in water suspensions as an illustrative example, since the Yb/Er ion pair is the most reported pair in Ln3+-luminescent thermometry.16,

27

Moreover, Ln3+-doped SrF2 micro and nanostructures

have attracted extensive attention in the last decade due to their technological importance in photovoltaics (Ln=Pr, Yb),41 as scintillators (Ln=Ce),42 upconverting ultraviolet emitters 5 ACS Paragon Plus Environment


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(Ln=Yb/Tm),43 in in vivo bio imaging (Ln=Nd),44 and for tissue visualization and singleparticle spectroscopy (Ln=Yb/Er45-47 and Ln=Yb/Tm48). The main reasons for this interest are i) the well-controlled size and morphology of SrF2 micro/nano structures; ii) the wide bandgap (10 eV); iii) the low phonon energy (∼350 cm−1); and iv) the clustering of the Ln3+ ions, favoring an enhancement in the upconversion process when the divalent Sr2+ ions are substituted.41

EXPERIMENTAL SECTION Synthesis of sodium citrate capped SrF2 nanoparticles. Sodium citrate capped SrF2 and SrF2:Yb/Er(20/2%) were prepared by a hydrothermal method following the procedure described by Pedroni et al.48, in which 6 hours of reaction time were used to synthesize nanoparticles with sizes of ~10 nm. In order to increase the size of the nanoparticles we have implemented a strategy of increasing the time of the reaction up to 48 hours (details in the Supporting Information). This procedure yields undoped powder SrF2 (denoted SrF2-1) and Yb/Er(20/2%) doped SrF2 nanoparticles with distinct sizes 10±2 nm (SrF2-2), 27±8 nm (SrF23), and 41±10 nm (SrF2-4), Table S1 in the Supporting Information. SrF2-4 suspensions were prepared by dissolving 2.5 mg of nanoparticles in 1 mL of distilled water (volume fraction is 0.59%). The hydrodynamic diameter and the zeta potential are 55±12 nm and –6.4 mV (with a standard deviation of 3.4 mV), Figure S1 in the Supporting Information. Powder X-ray diffraction. The X-ray diffraction (XRD) patterns of the synthesized powder samples were collected on a PANalytical Empyrean X-ray diffractometer operating at 45 kV and 40 mA, with CuKα radiation at 1.5406 Å, in the 2θ range 20o–80o with a 0.02o step size and 40 seconds acquisition time per step, in the reflection scanning mode. Transmission electron microscopy. The morphology of the samples was analyzed on a Jeol JEM-2200FS transmission electron microscope (TEM) operated at 200 kV and on a Hitachi H9000 TEM operated at 300 kV. 6 ACS Paragon Plus Environment

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Photoluminescence and temperature dependent measurements. The upconversion spectra were recorded using a Fluorolog-3 Horiba Scientific (Model FL3-2T) spectrofluorometer, with a TRIAX 320 single-emission monochromator (fitted with a 1200 grooves/mm grating blazed at 500 nm) coupled to a R928 Hamamatsu photomultiplier, using the lateral face acquisition mode. The spectra were corrected for the detection and optical spectral response of the spectrofluorometer. The emission spectral radiant flux, or spectral radiant power, (S(λ), W·nm−1) of powders and suspensions were measured using an integrating sphere (ISP 150L-131, Instrument Systems). All the spectra were acquired with a resolution of 0.1 nm, 200 ms integration time and 5 averaged spectra scans. The integrating sphere (BaSO4 coating) has an internal diameter of 150 mm and was coupled to an array spectrometer (MAS-40, Instrument Systems). The measurements have an accuracy within 5%, according to the manufacturer. The increment of temperature in powder samples was carried out using a Kapton thermofoil heater (Minco) mounted on a Cu holder (2.5×2.5 cm2) and coupled to a temperature controller (IES-RD31). The samples were placed on a smaller Cu plate (1.0×0.5 cm2) attached to the holder by a thermal conductive paste (WLP 500, Fischer Elektronik). The temperature was measured with a Barnant thermocouple 100 (model 600-2820) with a temperature accuracy of 0.1 K, accordingly to the manufacturer. Water suspensions were placed in a quartz cuvette (CV10Q1400, Thorlabs) in which the temperature was measured by an immersed thermocouple (I620-20147, VWR) with an accuracy of 0.1 K, accordingly to the manufacturer.

RESULTS AND DISCUSSION The nominal concentrations of 20.00, 2.00 mol% Yb3+, Er3+ relative to Sr2+ in the final SrF2-2, SrF2-3, and SrF2-4 nanocrystals are presented in Table S1 in the Supporting 7 ACS Paragon Plus Environment


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Information. Figure 1 shows the powder XRD patterns of SrF2:Yb/Er nanoparticles. The samples contain the cubic phase of SrF2 (space group Fm3തm), in agreement with ICDD card (00-06-0262) and references.49-50 The diffraction peaks of the Yb/Er co-doped SrF2 nanoparticles (SrF2-2, SrF2-3 and SrF2-4) show a slight shift to higher angles, in comparison to the undoped SrF2-1 peaks (Figure 1), attributed to the smaller ionic radii of Yb3+ (0.0985 nm) and Er3+(0.1004 nm), compared with that of Sr2+ (0.1260 nm).51 Accordingly, smaller lattice parameters are calculated for SrF2:Yb/Er samples using Rietveld refinement (Table S1 in the Supporting Information), in agreement with previous reports.49, 52 Representative TEM images of the nanoparticles are shown in Figure 2a-c. The nanoparticles are spherical and increasing the reaction time it is observable an increase in the particle’s average size and in its clustering, Figure 2d-f. The measured distances between adjacent planes were determined from the high resolution TEM (HRTEM) images as 0.332±0.002 nm (111) and 0.288±0.005 nm (200) along with the corresponding orientations of the indexed planes by powder XRD (Figure 2g and h). The values are in accord with the corresponding interplanar distances listed in the ICDD database, 0.335 nm and 0.290 nm, respectively. As shown in the size distribution histograms of Figure 2d-f, the diameter values of the nanoparticles range from 5 to 70 nm, with average values of 10±2 nm, 27±8 nm and 41±10 nm, for SrF2-2, SrF2-3, and SrF2-4, respectively. Figure 3a shows the upconversion spectral radiant flux of the SrF2:Yb/Er nanoparticles under 160±16 W⋅cm−2 excitation, whereas Figure 3b depicts a partial energy-level diagram of Yb3+ and Er3+ ions showing the upconversion mechanism responsible for the 2H11/2→4I15/2 (I2, 510−533 nm), 4S3/2→4I15/2 (I1, 533−570 nm) and 4F9/2→4I15/2 (630−690 nm) Er3+ transitions (Figure S2 in the Supporting Information). The spectral radiant flux increases with the particle size and the maximum radiant flux (or radiant power) values measured are 1.8×10−6 W, 7.8×10−6 W and 13.0×10−6 W, for SrF2-2, SrF2-3 and SrF2-4, respectively. The corresponding 8 ACS Paragon Plus Environment

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luminous flux values are 0.57×10−3, 1.3×10−3 and 1.3×10−3 lm (Table S2 in the Supporting Information). As upconversion is a nonlinear process, the upconversion quantum yields are strongly dependent on the excitation laser power density corresponding the maximum value to the onset of the saturation regime of the power dependence.53-56 This is exactly what we observed for SrF2-2, SrF2-3 and SrF2-4, with the maximum emission quantum yield (at the onset of the saturation regime) of 0.00036±0.00002% (at 162±16 W⋅cm−2), 0.0019±0.0001% (at 250±25 W⋅cm−2), and 0.0057±0.0006% (at 388±39 W⋅cm−2), respectively (Figure S3 in the Supporting Information). Figure 4a shows the temperature dependence of the emission spectra of SrF2-4 powder nanoparticles in the range 303−373 K (Figure S4 in the Supporting Information for SrF2-2 and SrF2-3). The temperature values were measured using a thermocouple (I620-20147, VWR) positioned in contact with the powder sample holder. Increasing the temperature between 303 and 373 K results in a significant variation in the thermometric parameter ∆, while the intensity of the I1 transition decreases approximately 50% that of I2 is nearly constant (Figure 4b). The figures of merit usually used to compare the performance of the thermometers, independent of their nature, are the thermal sensitivity Sr, the temperature uncertainty δT, and the repeatability.23, 27 The relative sensitivity of the SrF2-2, SrF2-3, and SrF2-4 nanoparticles is calculated by:57

Sr =

1 ∂∆ ∆E = ∆ ∂T kBT 2

( 2)

in which the energy gap ∆E is inferred by fitting the envelope of the I1 and I2 transitions (Figure S5 and Table S3 in the Supporting Information). Maximum Sr values are 1.207±0.016 9 ACS Paragon Plus Environment


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%⋅K−1 (298.2 K), 1.195±0.016 %⋅K−1 (300.2 K) and 1.169±0.016 %⋅K−1 (303.2 K), for SrF2-2, SrF2-3 and SrF2-4, respectively, Figure S6 and Table S4 in the Supporting Information. The similarity between these values is expected as the ∆E values are similar for all the nanothermometers within the corresponding uncertainties. The temperature uncertainty of the nanothermometers δT is defined as:27

δT =

1 δ∆ Sr ∆

( 3)

where δ∆ is the uncertainty in the determination of ∆ estimated through the errors in I1 and I2. The calculated temperature uncertainties are 0.265−0.438 K (298−383 K), for SrF2-2, 0.268−0.414 K (300−373 K), for SrF2-3, and 0.274−0.415 K (303−373 K), for SrF2-4 (Figure S7 in the Supporting Information). The repeatability of the nanothermometers was measured in ten consecutive temperature cycles of laser irradiation between 0.81±0.08 and 36±4 W⋅cm−2, corresponding to average temperature values of 310 and 393 K (SrF2-2), 303 and 337 K (SrF2-3) and 300 and 316 K (SrF2-4), respectively. The computed repeatability in ∆ is >99%, indicating a highly reversibility without significant changes induced by the exposure to high laser power densities (Figure S8 in the Supporting Information). What we propose in this work is to demonstrate a straightforward method to predict the temperature calibration curve of any upconverting thermometer based on two thermallycoupled electronic levels independently of the medium, using SrF2 nanoparticles as an illustrative example. Generally in upconverting thermometers based on two thermally-coupled electronic levels ∆ increases linearly with the laser excitation power, for Yb/Er-based thermometers, for 10 ACS Paragon Plus Environment

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instance, up to 240 W⋅cm−2.58 In the limit of zero pump power (Figure S9 and Table S5 in the Supporting Information), the temperature, T0, corresponds to no laser-induced heating and the thermometric parameter ∆0 is:

 ∆E   . ∆ 0 = B exp  −  k BT0 

( 4)

Although the Judd-Ofelt theory can be used to calculate the constant B,35, 54, 59 here this is unnecessary as the absolute temperature is directly determined by the ∆/∆0 ratio (calculated through the ratio between Eq. 1 and Eq. 4) as:

1 1 kB = − ln T T0 ∆E

 ∆   ∆0

  . 

( 5)

Replacing ∆E (Table S3), T0, ∆0 (Table S5), and the experimental ∆ values, the calculated temperatures are in excellent agreement with the measured values (Figure 4c,d for SrF2-4 and Figure S10 in the Supporting Information for SrF2-2 and SrF2-3), validating, therefore, the method proposed here to calculate the absolute temperature. Furthermore, the method is reproducible as Figure S11 in the Supporting Information show for the illustrative case of SrF2-2. The relative Er3+ upconversion emission intensity at a given laser power density was much stronger for SrF2-4 than for SrF2-2 and SrF2-3 nanoparticles, both in powder and water suspension. For instance, in powders, the spectral radiant power is 7.2 and 1.6 times higher, respectively (Figure 3a and Table S2 in the Supporting Information). Therefore, in what follows, the SrF2-4 nanoparticles will be used to proof the concept of an Yb/Er-based primary thermometry operating in two distinct mediums (air and water). 11 ACS Paragon Plus Environment


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The emission spectra of SrF2-4 were recorded in a 0.59% aqueous suspension at a fixed laser power density (5.0±0.5 W⋅cm−2) in the 300–365 K range (Figure S12 in the Supporting Information). For this volume fraction and at this laser power density there is no noticeable laser induced local heating, in agreement with previous reports in pure water (local temperature increment around 1 degree31). Substituting in Eq. 5 T0=299.9±0.1 K, ∆E (Table S3), ∆0 (Table S5) and the experimental ∆ values (Figure 4c,d), we obtain calculated temperatures in excellent agreement with the measured values (using the thermocouple immersed in the suspension). Moreover, the calculated temperatures are independently of the nanoparticles medium (air or water), demonstrating that a new calibration procedure is unnecessary and no other variables apart temperature, such as the ionic strength, pH, pressure, Ln3+ local surroundings, or atmosphere composition, impact the thermometric parameter value. Therefore, the SrF2:Yb/Er nanoparticles are, indeed, primary thermometers based on the Boltzmann distribution between the 2H11/2 and 4S3/2 thermally-coupled electronic Er3+ electronic levels.

CONCLUSIONS In summary, cubic phase SrF2:Yb/Er upconverting nanoparticles have been successfully synthesized by a simple hydrothermal route at mild temperature and ambient pressure. The samples were characterized by ICP-OES, DLS, powder XRD, TEM and photoluminescence spectroscopy. The performance of SrF2:Yb/Er nanoparticles as intensity-based ratiometric nanothermometers was evaluated yielding to a maximum relative thermal sensitivity up to 1.169±0.016%⋅K−1 (at ca. 300 K) in two distinct mediums (powder and water suspension) at a fixed minimum laser power density (1.5±0.2 and 5.0±0.5 W⋅cm−2, respectively). The repeatability and the minimum temperature uncertainty of the nanothermometers were determined to be >99% and 0.265 K, respectively. Furthermore, the SrF2:Yb/Er nanoparticles 12 ACS Paragon Plus Environment

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were used here as an illustrative example of a primary Yb/Er co-doped luminescent nanothermometers. Despite the numerous works on Yb/Er co-doped luminescent nanothermometers reported in the past decade (the most reported systems in Ln3+-luminescent thermometry), this is the first time that the temperature calibration curve of such thermometers is predicted independently of the medium. The example of the primary thermometers demonstrated here would open the door to the general implementation of luminescent thermometry overcoming one of its main limitations: the requirement of a new calibration procedure whenever the thermometer operates in a different medium than that in which it was calibrated (or, when not possible, the ad hoc assumption that a single calibration is valid independently of the medium).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding author * Luís D. Carlos: [email protected] Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS Financial support of Fundação para a Ciência e a Tecnologia (FCT) (PTDC/CTMNAN/4647/2014 and POCI-01-0145-FEDER-016687), EC Marie Curie Initial Training Network LUMINET (316906) and COST-CM1403 Action is acknowledged. This work is 13 ACS Paragon Plus Environment


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also financed by Portugal 2020 through European Regional Development Fund (FEDER) in the frame of Operational Competitiveness and Internationalization Programme (POCI) in the scope of the project (SGH-Smart Green Homes) - POCI-01-0247-FEDER-07678, and was partially developed in the scope of the project CICECO − Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (Ref. FCT UID /CTM /50011/2013), financed by national funds through the FCT/MEC and when applicable co-financed by FEDER under the PT2020 Partnership Agreement. SB acknowledges Professor Marco Bettinelli from University of Verona,

Italy,

for

his

support

during

her

LUMINET

secondment.

MLD

(SFRH/BPD/93884/2013) and CDSB (SFRH/BPD/89003/2012) thank FCT for the postdoctoral grants.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:##### •

Additional experimental procedures concerning i) the synthesis and the structural characterization of the nanoparticles and ii) the upconversion measurements, including luminous flux and quantum yield; emission spectra and calibration curves of SrF2-2 and SrF2-3 in powders; method of energy gap determination; parameters characterizing the thermometers’ performance; thermometric parameter in the limit of low laser power density; calculated temperature for SrF2-2 and SrF2-3 in powders; upconversion emission spectra of SrF2-4 in water suspension.

The authors declare no competing financial interest.


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FIGURES

Figure 1. Structural characterization of SrF2 nanoparticles. Powder XRD diffraction patterns of undoped SrF2 (SrF2-1) and Yb/Er doped SrF2 (SrF2-2, SrF2-3and SrF2-4) 22 ACS Paragon Plus Environment

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nanoparticles of distinct diameters. The reflections of cubic SrF2 are also depicted (ICDD Card No 00-06-0262).



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Figure 2. Morphological characterization of SrF2:Yb/Er nanoparticles. HRTEM images of SrF2:Yb/Er nanoparticles and their size distribution histograms (over 100 nanoparticles measured): (a,d) SrF2-2, (b,e) SrF2-3, and (c,f) SrF2-4. The solid lines are the best fit of the experimental data to log-normal distributions (r2 > 0.922). HRTEM images of SrF2-2 nanoparticles showing the (g) (111) and (h) (200) crystallographic planes and the corresponding interplanar distances.


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Figure 3. Radiant flux and energy level diagram of upconversion emission. (a) Upconversion emission spectral radiant flux of SrF2-2 (blue), SrF2-3 (red) and SrF2-4 (black) powder nanoparticles. The inset shows a photograph of the bright green emission of Er3+ with a luminous flux of ∼13×10−3 lm. (b) Partial energy-level diagram of Yb3+/Er3+ ions highlighting the Yb3+ absorption at 980 nm, the Yb3+-to-Er3+ energy transfer pathway and the 2

H11/2→4I15/2,

4

S3/2→4I15/2, and

4

F9/2→4I15/2 Er3+ emissions. The expansion depicts the

thermally coupled 2H11/2 and 4S3/2 Er3+ levels.



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Figure 4. Temperature dependent emission spectra and thermometric performance of SrF2-4 nanoparticles. (a) Upconversion emission spectra of SrF2-4 powder particles measured in the 303−373 K range at a power density of 1.5±0.2 W⋅cm−2. (b) Integrated emission intensities of the spectral regions depicted by I2 (510−533 nm, green) and I1 (533−570 nm, red) measured in the same temperature range. (c) Temperature dependence of the experimental ∆ values for SrF2-4 nanoparticles in powder (up triangles) and water suspension (down triangles). The solid line is the theoretical predication of temperature using Eq. 5, marking the shadowed area the error in the determination of temperature (Supporting 26 ACS Paragon Plus Environment

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Information). The horizontal error bars represent the uncertainty in ∆, whereas the vertical error bars represent the uncertainty of the temperature considering the thermocouple accuracy (0.1 K). (d) Calculated temperature (Eq. 5, y) versus temperature reading using a thermocouple (experimental temperature, x) for SrF2-4 in powder (circles) and water suspension (squares). The dashed line is a guide for the eyes corresponding to y=x. The horizontal error bars represent the thermocouple accuracy and the vertical ones the error in the calculated temperature (Eq. S11 in the Supporting Information).



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TOC Graphic


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Upconverting Nanoparticles Working As Primary Thermometers In

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