Investigation on the fluorescence intensity ratio sensing thermometry

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Investigation on the fluorescence intensity ratio sensing thermometry based on non-thermally coupled levels Panpan Li, Mochen Jia, Guofeng Liu, Anqi Zhang, Zhen Sun, and Zuoling Fu ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.9b00115 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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ACS Applied Bio Materials

Investigation on the fluorescence intensity ratio sensing thermometry based on non-thermally coupled levels Panpan Li, Mochen Jia, Guofeng Liu, Anqi Zhang, Zhen Sun, Zuoling Fu* Coherent Light and Atomic and Molecular Spectroscopy Laboratory, Key Laboratory of physics and Technology for Advanced Batteries, College of Physics, Jilin University, Changchun 130012, China, Fax: +86-431-85167966; Tel: +86-431-85167966; E-mail: [email protected] KEYWORDS：Temperature sensing; Fluorescence intensity ratio; Non-thermally coupled levels. ABSTRACT ：Fluorescence intensity ratio (FIR) of rare earth ions has been widely used as realtime and accurate temperature sensing because of its superiority of rapid response, self-reference and non-contact in recent years. However, the energy gap (∆E) restriction of thermally coupled levels (TCLs) has hindered the sensitivity and practical use of such detectors. Herein, we investigate the FIR thermometry based on non-thermally coupled levels (NTCLs) of rare earth ions for fabricating a sensitive, precise temperature detector. Compared with those traditional FIR thermometry based on TCLs (TCL-FIR), the designed NTCL-FIR sensing thermometry exhibits a series of excellent performance including extremely low temperature uncertainty (∼0.27 K), an ultrahigh temperature sensitivity (>10% K-1) and satisfactory signal recognition

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ability. The rise of sensitivity and recognition is attributed to breaking the ∆E restriction of TCLs by using Arrhenius equation. The proposed ideas and methods of NTCL-FIR sensing thermometry can not only improve the performance of temperature sensing devices but more importantly contribute to the practical development of rare earth ions. 1. INTRODUCTION How to accurately measure temperature is an eternal topic, because time, length, current and other basic physical quantities have been able to detect at very accurate magnitude, such as the length, which can be measured at the level of nanoscale; and the time, which can be detected at the level of femtosecond. At present, the accuracy of the temperature measurement is around 0.1 degree, so searching for more accurate methods and instruments has become a challenging frontier. Among the current temperature measuring methods such as thermocouples, thermistors, and resistance temperature detectors (RTDs), FIR thermometer based on the rare earth ions has gained devoted attention because of its superiority of non-invasion, rapid response, self-reference as well as high sensitivity.1-4 The past few decades have witnessed significant efforts in the FIR temperature measurement.5-8 In 1976, Kusama et al. first proposed that the FIR produced by the transition of two energy levels to the same level can be used for temperature measurement.9 In 1983, Shinn et al. gave the theoretical expression of FIR temperature measurement, and pointed out that 2H11/2, 4S3/2 levels of Er3+ are thermally coupled levels, which can be used for temperature measurement.10 Seven years later, Berthou et al. used above thermally coupled levels of Er3+ to achieve temperature measurement in 293-493 K range, with maximum temperature deviation 2 K.11 Since twenty-first century, the FIR temperature measurement technology has been further developed. In 2010, FIR technology was first applied to single cell


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intracellular temperature measurement.12 In 2015, Carlos et al. used the 4F5/2, 4F3/2 levels of Nd3+ to increase the sensitivity by an order of magnitude, and the sensitivity reached 1.75% K-1.13 In the past 40 years, since the introduction of FIR technology, the fluorescence intensity ratio thermometry based on thermally coupled levels has been gradually improved, but some intrinsic limitations of this method can not be solved.14-16 Firstly, the TCL-FIR thermometry is in the restriction of rapid particle exchange between two luminescent excited levels. Therefore the ∆E of two monitoring energy levels should be located at 200-2000 cm-1. Meanwhile the relative sensitivity expression of TCL-FIR thermometry is Sr= ∆E/KBT2, so the upper limit of Sr is very clear.15 Continuing along this path, the conventional TCL-FIR thermometry may have a relatively low temperature sensitivity (1% or even lower).17-19 Such a low sensitivity will inhibit the accuracy of temperature measurement, therefore, leading to more side effects and not be satisfied with the requirements of precise engineering such as clinical medicine.20-22 Secondly, for TCL-FIR temperature sensing, a narrow ∆E between TCLs would be conductive to the absolute sensitivity (Sa), but go against the relative sensitivity (Sr). Moreover, the narrow ∆E would cause overlap of the two monitoring peaks, which is adverse to signal recognition. In a word, the TCL-FIR temperature sensing can not simultaneously improve Sa, Sr and signal recognition ability.14,16 Besides, this TCL-FIR temperature sensing theory is invalid in the cases of double luminescent centers.23-25 All above intrinsic limitations make the TCL-FIR temperature sensing almost impossible to have a qualitative leap in improving temperature sensitivity. So searching for new temperature measurement method that is more suitable for the needs of the times, has become a challenging frontier. Although some previous works have referred to the temperature measurement by utilizing non-thermal coupling levels.26-29 However, most of them were based on the


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experimental data fitting of one or two kinds of materials, their universality was a bit low. And few of them systematically compared it with the thermal coupling levels temperature measurement method to explore the advantages of the non-thermal coupling levels temperature sensing method. Nor did they apply the non-thermal coupling levels thermometry to biological applications. In this paper, we proposed the NTCL-FIR temperature sensing method with an extremely low temperature uncertainty, an ultrahigh temperature sensitivity, along with satisfactory signal recognition ability. And we applied the NTCL-FIR temperature sensing method to Tm3+/Er3+ and Tm3+/Ho3+ double luminescent centers doping cases. Importantly, the superiorities and universality of NTCL-FIR sensing thermometry were verified experimentally by measuring and analyzing the experimental temperature characteristics of five groups rare earth ions doped situations based on five kinds of matrix materials. In addition, the liquid temperature measurement system with simultaneous upconversion luminescence cell imaging evidenced that the novel NTCL-FIR sensing thermometry has high sensitivity in biological systems, indicating tremendous potential for their subsequent application in vivo temperature monitoring during the course of clinical treatment. 2. RESULTS AND DISCUSSION 2.1 The FIR sensing thermometry based on NTCLs The FIR sensing thermometry based on NTCLs involving two independent luminescent excited levels, each with its distinctive photoluminescence temperature dependence. In terms of kinetics, the two luminescent excited levels are electronically independent, so there is no energy transfer (ET) between them on the time scale of their luminescence (1/τ

16

um≫kET).

To further illustrate


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the temperature sensing mechanism of NTCL-FIR temperature sensing method, Arrhenius equation is applied to analyze the temperature properties of rare earth ions levels. According to Arrhenius equation,30 the relationship between luminescence intensity and temperature can be expressed as:

I（T）

I0 1  a exp(

 Ea

K BT

(1)

)

where I(T) is the emission strength of sample at the temperature of T, I0 is the initial emission strength of sample, a is a pre-exponential constant, Ea is the quenching activation energy, kB is the Boltzmann constant (kB= 8.617x10-5 eVK-1) and T is the absolute temperature. So the luminescence temperature dependence characteristics of the two monitoring levels can be expressed as follows:

I（ 1 T）

I1,0 1  a1 exp(

I（ 2 T）

 E1

K BT

(2) )

I2 ,0 1  a 2 exp(

 E2

K BT

I（T） I 1,0 FIR  1  I（ I 2,0 2 T）

(3) )

1  a2 exp(

 E2

)

 E K BT )     exp(  E1 K BT 1  a1 exp( ) K BT

(4)

Where α, β and ΔE are parameters associated with the I0, a, and E. Specifically, β is approximately related to a2/a1 and α is approximately related to I1,0/I2,0. Furthermore, it is better


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to select two relatively isolated and far away non-thermal coupling levels as monitoring levels, so as to make the monitoring signals without interaction with each other and be more consistent with Equation 2 and (3). And then, the FIR experimental signals will well fit with the Equation 4. Finally, the absolute and relative temperature measurement sensitivities, Sa and Sr, can be further deduced and expressed as follows:

Sa 

E a  E a FIR   exp( ) 2 T K BT K BT

E a FIR  Sr  FIR T K BT 2 1

 exp(

(5)

 E a

K BT

   exp(

)

 E a

K BT

)

(6)

Besides temperature sensitivity, temperature accuracy δT is another parameter to characterize the optical thermometer, being estimated as:31, 32

T 

1 R

Sr R



R E a

K BT

exp -

E a

(7)

K BT

where δR/R=0.50% is the relative error of temperature measurement parameter, a typical value of used spectral testing system.31 There are two major advantages of the NTCL-FIR sensing thermometry method. Firstly, the temperature sensitivity of NTCL-FIR method is higher than that of the TCL-FIR method. As shown in Figure 1a, the maximum Sa and Sr of NTCL-FIR sensing thermometry reached 44.90% K−1 and 1.85% K−1 in YbPO4:Tm3+ phosphors, increased by nearly 100 times and 10 times


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compared with those Sa (0.55% K−1) and Sr (0.27% K−1) by using TCL-FIR thermometry method. The above content can be expressed more intuitively in Figure 1b and 1c which shows the corresponding Sa, Sr values calculated by above two kinds of method in same temperature points. It reveals that the Sa and Sr values of optical thermometer using NTCL-FIR method are much higher than that of TCL-FIR method in almost the entire experimental temperature range. The sensitivity superiority of the NTCL-FIR sensing thermometry is also the same in Tm3+ doped different materials. (Table S1)

Figure 1. (a) The temperature sensing characteristics comparison of YbPO4: Tm3+ by using NTCL-FIR and TCL-FIR mehod. The corresponding (b) Sa, (c) Sr and (d) δT values of YbPO4:Tm3+ calculated by NTCL-FIR and TCL-FIR mehod in same temperature points. Secondly, the accuracy by using NTCL-FIR sensing thermometry method is higher than the TCL-FIR method. Figure 1a shows that the temperature uncertainty of YbPO4:Tm3+ is nearly 10 times more accurate, which from 1.85 K to 0.27 K. Moreover, from Figure 1d we can see that the δT values of optical thermometer using NTCL-FIR method are much smaller than that of TCLFIR method in the entire experimental temperature range. And the experimental data of Tm3+


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doped different materials are provided in Table S2. Besides, the two monitoring peaks of NTCLFIR method are farther apart, which is favor of improving signal recognition ability. All these data indicate that the NTCL-FIR thermometry method has better temperature accuracy. As a comparison, we listed temperature sensitivity (Sa and Sr) and temperature accuracy δT of several representative luminescent materials in supporting information (Table S3). In a word, simultaneously improving the sensitivity (Sa and Sr), temperature accuracy δT and signal recognition ability have been realized by introducing NTCL-FIR sensing thermometry method. 2.2 The universality of NTCL-FIR thermometry method In our experiments, five groups of different rare earth ions doped situations, including Tm3+, Er3+, Ho3+ as single luminescent center doping and Tm3+/Er3+, Tm3+/Ho3+ as double luminescent centers doping cases; five kinds of matrix materials, including YbPO4, NaYb(MoO4)2, BaTiO3, LaAlO3, Y2O3 were fabricated and their NTCL-FIR temperature sensing characteristics were discussed. As a result, the NTCL-FIR method is not restricted to single luminescent center for temperature sensing, but is also applicable among the cases of double luminescent centers. We take YbPO4:Tm3+ as example. We select 1G4 and 3F3 as target NTCLs, whose energy separation ∆E is 6258 cm-1. Different temperature dependence characteristics of the luminescent energy levels (1G4 and 3F3 of Tm3+) induce the emission intensity of 700 nm to be strengthened sharply while the emission intensity of 650 nm exhibits a slight decline with the increase of temperature (Figure 2b). Therefore the FIR values (I700/I650) of NTCLs will vary greatly. As shown in Figure 2c the experimental plots of FIR (I700/I650) based on NTCLs versus temperature can be fitted well by Equation 4. The Sa and Sr values that calculated by Equation 5 and 6 are


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Figure 2. (a) The energy level diagram; (b) The contour plots displaying the luminescence intensity at various temperatures; (c) Experimental measured and theoretical fitted FIR (I700I650) plots of YbPO4: Tm3+ versus temperature; (d) The corresponding Sa and Sr versus temperature. Table 1. Summarized temperature sensing characteristics of NTCL-FIR method based on Tm3+ doped different materials in our work. Materials

I1/I2

YbPO4

700/ 650 689/ 650 701/ 654 700/ 652 696/ 655

NaYb(MoO4)2 BaTiO3 LaAlO3 Y2O3

Transitions ∆E [cm-1] 3F →3H / 6258 3 6 1G →3F 4 4 3F →3H / 6258 3 6 1G →3F 4 4 3F →3H / 6258 3 6 1G →3F 4 4 3F →3H / 6258 3 6 1G →3F 4 4 3F →3H / 6258 3 6 1G →3F 4 4

Sa [%K-1] 44.90

Sr [%K-1] 1.85

38.80

2.83

25.12

2.12

4.10

1.38

6.65

1.05


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Table 2. Summarized temperature sensing characteristics of NTCL-FIR method based on different rare earth ions doped YbPO4 in our work. Ions

I1/I2

Transitions

Tm3+

700/ 650 663/ 551 652/ 545

3F

Er3+ Ho3+

3H

3→

1G

→3F

4

6/

Sa [%K-1] 44.90

Sr [%K-1] 1.85

3217

4.91

1.04

3093

5.77

0.45

4

4F

9/2/

4S

5 3/2→ I15/2

5F

∆E [cm-1] 6258

5/ 5F →5I 4 8

given in Figure 2d. Noteworthily, the maximum value of Sa is 44.90% K−1 (at 573 K), while that of Sr is 1.85% K−1 (at 423 K). To assess the universality of NTCL-FIR temperature sensing method, we fabricated five kinds of Tm3+ doped matrix materials including YbPO4, NaYb(MoO4)2, BaTiO3, Y2O3 and LaAlO3. We still selected 1G4 and 3F3 as target NTCLs. As shown in Table 1, among the five kinds of Tm3+ doped matrix materials, the maximum value of Sa is 44.90% K−1 (in YbPO4:Tm3+), while the maximum value of Sr is 2.83% K−1 (in NaYb(MoO4)2:Tm3+). Moreover, we also discussed the situation of different rare earth ions (Tm3+, Er3+, Ho3+) doped in the same matrix (YbPO4). We selected 1G4/3F3 of Tm3+, 4F9/2/4S3/2 of Er3+, 5F5/5F4 of Ho3+ as target NTCLs, respectively. As shown in Table 2, the maximum value of Sa is 44.90% K−1 (Tm3+ doped), 4.91% K−1 (Er3+ doped), 5.77% K−1 (Ho3+ doped), while the maximum value of Sr is 1.85% K−1 (Tm3+ doped), 1.04% K−1 (Er3+ doped), 0.45% K−1 (Ho3+ doped), respectively. So, the NTCL-FIR temperature sensing method is widely available in single luminescent center cases. In order to further prove that the NTCL-FIR sensing method is also applicable among the cases of double luminescent centers. We have synthesized Tm3+/Er3+ co-doped and Tm3+/Ho3+ co-doped different matrix materials. For Tm3+/Er3+ double luminescent centers doping cases, we


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utilized energy transfer process 3F3→3H6 (Tm3+)/4S3/2→4I15/2 (Er3+) of Tm3+ and Er3+. And we selected 3F3 and 4S3/ as target NTCLs, whose energy separation ∆E is 3952 cm-1. Table 3 shows the temperature sensing characteristics of NTCL-FIR temperature sensing method based on Tm3+/Er3+ co-doped five materials. The maximum value of Sa is 5.11% K−1 (in NaYb(MoO4)2), while that of Sr is 3.43% K−1 (in BaTiO3). The data fitting results of Tm3+/Ho3+ co-doped five samples are just as ideal as these cases. Detailed information can be found in Table S4. Hence, the NTCL-FIR method is also valid for the cases of double luminescent centers. Table 3. Summarized temperature sensing characteristics of NTCL-FIR method based on Tm3+/Er3+ co-doped different materials in this work. Materials

I1/I2

Transitions

YbPO4

703/ 526 703/ 550 698/ 528 698/ 550 525/ 477 546/ 477 524/ 472 545/ 472 531/ 483

3F

NaYb(MoO4)2

BaTiO3

LaAlO3

Y2O3

3+ 6 (Tm )/ 2H 4 3+ 11/2→ I15/2 (Er ) 3F →3H (Tm3+)/ 3 6 4S 2→4I 3+ 3/ 15/2 (Er ) 3F →3H (Tm3+)/ 3 6 2H 4I 3+ → 11/2 15/2 (Er ) 3F

3H

3→

3H

(Tm3+)/ 4S 2→4I 3+ 3/ 15/2 (Er ) 2H 4 3+ 11/2→ I15/2 (Er )/ 1G →3H (Tm3+) 4 6 4S →4I 3+ 3/2 15/2 (Er )/ 1G →3H (Tm3+) 4 6 2H 4 3+ 11/2→ I15/2 (Er )/ 1G →3H (Tm3+) 4 6 4S →4I 3+ 3/2 15/2 (Er )/ 1G →3H (Tm3+) 4 6 2H 4 3+ 11/2→ I15/2 (Er )/ 1G →3H (Tm3+) 4 6 3→

6

∆E [cm-1] 4746

Sa [%K-1] 3.22

Sr [%K-1] 0.86

3952

3.69

1.65

4746

0.90

1.11

3952

5.11

1.85

2118

1.19

3.43

2912

0.55

0.85

2118

2.28

0.94

2912

1.05

0.22

2118

2.99

1.22

2.3 An application of NTCL-FIR method


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The NTCL-FIR sensing thermometry method can be used in liquid temperature measurement. We prepared the NaYF4:Tm3+,Yb3+@NaYF4 core-shell with average diameter 77 ± 1.0 nm. As shown in Figure S5, the luminescence intensity is enhanced by four times after introducing coreshell structure. Because the NaYF4 shell reduces the energy loss caused by surface defects. And we selected 1G4 and 3F3 as target NTCLs, which energy separations ∆E is 6258 cm-1. Figure 3a is the upconversion luminescence (UCL) spectrum of NaYF4:Tm3+,Yb3+@NaYF4 dispersed in cyclohexane solutions under excitation of 980 nm at 25 °C. As shown in Figure 3b, the experimental curve of FIR (I470/I695) of NTCLs versus temperature fits well with the Equation 4. The Sa values obtained by NTCL-FIR method and TCL-FIR method are given in Figure 3c. Notably, the maximum Sa value by using NTCL-FIR method is 13.55% K−1 (at 293 K), while the

Figure 3. (a) The UCL spectrum of NaYF4:Tm3+,Yb3+@NaYF4 dispersed in cyclohexane solutions under excitation of 980 nm; (b) Experimental measured and theoretical fitted FIR (I470/I695) plots versus temperature; (c) The corresponding Sa calculated by NTCL-FIR method and TCL-FIR method; (d) Comparison diagram of Sa calculated by NTCL-FIR method and TCL-FIR method in the same coordinate system; (e) Comparison diagram of the situ temperature calculated by spectra and set solution temperature; (f) The HeLa cells viability assessment after


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incubated with different concentrations of NaYF4:Tm3+,Yb3+@NaYF4 for 24 h.

Figure 4. (a) Bright-field image of HeLa cells; (b) Fluorescence image of Hela cells incubated with NaYF4:Tm3+,Yb3+@NaYF4 for 2 h at 37 °C 5% CO2; (c) The overlay of (a) and (b).

Figure 5. Plots of FIR (I470/I695) of NaYF4:Yb/Er@NaYF4:Yb/Tm versus temperature measured in the heating, cooling processes. maximum Sa value by using TCL-FIR method is 0.01% K−1 (at 293 K). The above content can be expressed more intuitively in Figure 3d. In the practical applications, we get the situ temperature values from the FIR of spectra, according to the following calibration curve.1, 31

E T  KB

 FIR -    Ln    

-1

(8)

The solution mean temperature is set by the temperature controller (Figure S1). The situ temperature calculated from spectra is in good agreement with the set temperature (Figure 3e).


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The NTCL-FIR sensing method can more accurately capture temperature change which is caused by laser irradiation and too weak to appear in solution mean temperature. Hence, the spectral temperature is about 2 K higher than the set temperature at each point, which could be conducive to precise tumor photothermal therapy.1, 33-35 Moreover, no significant toxicity to HeLa cells is observed after exposure to 3.90-62.50 μg/mL of NaYF4:Tm3+,Yb3+@NaYF4 for 24 h. Even when the sample concentration is 62.50 μg/mL, the cell viability remains at 92.60% for HeLa cells (Figure 3f).36-38 In addition, we use the NaYF4:Tm3+,Yb3+@NaYF4 for bio-imaging. Under 980 nm excitation, Figure 4b shows intense blue emission of Tm3+. From the merged images of bright-field image and fluorescence image, we can see that the nanoparticles luminescent center has entered the cells (Figure 4c). So NTCL-FIR temperature sensing based on NaYF4:Tm3+,Yb3+ @NaYF4 is expected to be used for accurate temperature sensing in cells. Figure 5 shows a good repeatability of the temperature-dependent FIR. Together, these results suggest that the NTCLFIR temperature sensing system based on NaYF4:Tm3+,Yb3+@NaYF4 has high sensitivity and low cytotoxicity, which is essential for their subsequent application in vivo temperature monitoring during the course of clinical treatment. 3. CONCLUSIONS In this work, Arrhenius equation was applied to analyze the temperature properties of rare earth ions NTCLs and the NTCL-FIR sensing thermometry method was proposed. The NTCL-FIR method proposed in this work exhibits extremely low temperature uncertainty (∼0.27 K), ultrahigh temperature sensitivity (>10% K-1) and satisfactory signal recognition ability. Compared with the traditional TCL-FIR thermometry method, the maximum Sa using the NTCLFIR method was improved by 2 order of magnitude, reaching 44.90% K−1 at 573 K and the maximum Sr increased by nearly 10 times, reaching 1.85% K−1 in YbPO4:Tm3+ phosphors in this


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work. Besides, the NTCL-FIR sensing thermometry method was not restricted to single luminescent center for temperature sensing, but was also applicable among the cases of double luminescent centers. The rise of detection performance was attributed to breaking the ∆E restriction of TCLs by using the photoluminescence temperature dependence of NTCLs from rare earth ions. In addition, we have applied the NTCL-FIR sensing thermometry method to liquid temperature measurement system based on NaYF4:Tm3+,Yb3+@NaYF4 core-shell structure, which was expected to be used for accurate temperature measurement in vivo. All of these results indicate that the designed NTCL-FIR thermometry method have great potential for future applications. And it not only has significance on temperature detection performance but also provides inspiration and reference for practical development of rare earth ions-based temperature detectors. 4. EXPERIMENTAL SECTION 4.1 Synthesis of NaYF4:Tm3+, Yb3+ We utilized solvothermal method to synthesize NaYF4:Tm3+, Yb3+ nanoparticles.1 Firstly, we mixed 1 mmol lanthanide chloride (81.5% mol Y, 18% mol Yb, 0.5% mol Tm), 7 ml oleic acid (OA) and 15ml 1-octadecene. The mixture was degassed at 70 °C for 0.5 h, then was stirred for 0.5 h at 160 °C in the nitrogen environment. When the temperature dropped below 70 °C, 7 ml methanol solution of NH4F and NaOH (8:5 n/n) was added then the mixture was volatilized at 70 °C for 1h. Secondly, the mixture was degassed at 70 °C for 0.5 h again then was heated to 300 °C kept for 0.5 h in nitrogen environment. When the temperature droped below 25 °C, samples were obtained by centrifugation and washed with excess amount of ethanol and


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ethanol/cyclohexane (1:1 v/v), respectively. The obtained nanoparticles were dispersed in 10 ml cyclohexane for further use. 4.2 Synthesis of NaYF4:Tm3+, Yb3+@ NaYF4 The NaYF4:Tm3+,Yb3+@NaYF4 core-shell samples were synthesize by epitaxial growth on NaYF4:Tm3+, Yb3+ core. Firstly, we mixed 0.5 mmol YCl3, 7 ml oleic acid and 15 ml 1octadecene. Similar to the synthesis process of NaYF4:Tm3+, Yb3+, the mixture was degassed at 70 °C for 0.5 h, and stirred for 0.5 h at 160 °C in the nitrogen environment. When the temperature dropped below 70 °C, 5ml as-prepared NaYF4 cyclohexane solution was added and then the mixture was volatilized at 70 °C for 0.5 h. After that, 4 ml methanol solution of NH4F and NaOH (8:5 n/n) was dripped, then the mixture was kept at 70 °C for 1h. Secondly, the mixture was degassed at 70 °C for 0.5 h again then was heated to 310 °C kept for 1 h under nitrogen environment. The method of collecting NaYF4:Tm3+, Yb3+@NaYF4 was the same as NaYF4. The obtained products were dispersed in 10 ml cyclohexane for use. 4.3 Synthesis of YbPO4, NaYb(MoO4)2, BaTiO3, LaAlO3 and Y2O3 The YbPO4 was synthesized by the method of co-precipitation.39 NaYb(MoO4)2 was synthesized by a hydrothermal method.40 BaTiO3, LaAlO3 and Y2O3 were synthesized by sol-gel method. Detailed information can be found in supplementary information and references.41, 42 4.4 Spectral measurement The UCL spectra were obtained using fluorescence spectrometer (Andor Shamrock SR-750) with a 980 nm pump source which derive from a diode coupled to a fiber laser. The signals of


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samples were collected by a CCD detector that combined with a monochromator and the spectra were registered by Andor SR-500i spectrometer (Andor Technology Co, Belfast, UK) . 4.5 Biocompatibility evaluation of the NaYF4:Tm3+, Yb3+@NaYF4 The MTT (3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide) cell assay of Hela cells can reflect the biocompatibility of NaYF4:Tm3+,Yb3+@NaYF4. First, Hela cells were seeded in a 96-well plate and they would attach to the walls after incubated in 5% CO2 at 37 °C for 24 h. Then the NaYF4:Tm3+,Yb3+@NaYF4 nanoparticles with different concentrations (3.9 μg/ml, 7.8 μg/ml, 15.6 μg/ml, 31.2 μg/ml and 62.5 μg/ml) were placed in and incubated for 24 h again. After that, the MTT was dissolved in phosphoric acidic buffer solution (PBS) to form 5 mM MTT solution. And every well was added in MTT solution and incubated for another 4 h under the same conditions. Finally, we removed supernatant and added 150 μL DMSO to every well. Then the microplate was placed at room temperature for 2 h. 4.6 Cell imaging First, Hela cells (3×105 per well) were seeded in a 6-well plate with 5% CO2 at 37 °C overnight. Then the NaYF4:Tm3+,Yb3+@NaYF4 nanoparticles were incubated with cells for 2 h. After that, the Hela cells were washed with PBS for three times and fixed with 4% formaldehyde (3–5 mL) at 37 °C for 10 min, and washed with PBS again. Finally, the cell imaging was obtained by using confocal laser scanning microscope and 980 nm laser. ASSOCIATED CONTENT Supporting Information. General information, samples synthesis, fluorescence spectra and temperature sensing characteristics of NTCL-FIR method.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Zuoling Fu) Author Contributions . All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank the Project of the National Natural Science Foundation of China (No.11874182), Science and Technology Project of the 13th Five-Year Plan of Jilin Provincial Department of Education (No.JJKH20190179KJ), Special funds for provincial industrial innovation in Jilin Province (No. 2018C043-4) and Jilin University Ph. D. Interdisciplinary Research Funding Project (No.10183201814) for financial support. REFERENCES (1) Zhu, X.; Feng, W.; Chang, J.; Tan, Y. W.; Li, J.; Chen, M.; Sun, Y.; Li, F. TemperatureFeedback Upconversion Nanocomposite for Accurate Photothermal Therapy at Facile Temperature. Nat. Commun. 2016, 7, 10437. (2) Okabe, K.; Inada, N.; Gota, C.; Harada, Y.; Funatsu, T.; Uchiyama, S. Intracellular Temperature Mapping with a Fluorescent Polymeric Thermometer and Fluorescence Lifetime Imaging Microscopy. Nat. Commun. 2012, 3, 705.


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Investigation on the fluorescence intensity ratio sensing thermometry

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