Exploiting the Yb3+ and Er3+ Codoped Î²-NaYF4 Nanoparticles as

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Exploiting the Yb and Er codoped #-NaYF Nanoparticles as Luminescent Thermometers for White LED Free Thermal Sensing at the Nanoscale Leipeng Li, Feng Qin, Yuan Zhou, Yangdong Zheng, Hua Zhao, and Zhiguo Zhang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00303 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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ACS Applied Nano Materials

Exploiting the Yb3+ and Er3+ codoped β-NaYF4 Nanoparticles as Luminescent Thermometers for White LED Free Thermal Sensing at the Nanoscale Leipeng Li,a Feng Qin,b,* Yuan Zhou,a Yangdong Zheng,b Hua Zhaoc and Zhiguo Zhanga,** a.

Condensed Matter Science and Technology Institute, Harbin Institute of Technology, Harbin,

150001, P.R. China. b.

Department of physics, Harbin Institute of Technology, Harbin, 150001, P.R. China.

c.

School of Materials and Engineering, Harbin Institute of Technology, Harbin, 150001, P.R.

China.

ACS Paragon Plus Environment

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KEYWORDS. Er3+, thermal sensing, upconversion, anti-disturbance, luminescent ratiometric technology.

ABSTRACT. Luminescent ratiometric technology has been regarded as one of the most promising methods for temperature measurement, as it enables noncontact thermal sensing with minimal disturbance to the object of interest. Particularly, it, as expected, is free from the influences of many surrounding factors. However, we demonstrate here that in some cases, this technology is under the influence of white LED that is commonly used in our daily life and industrial processes, considering the fact that these involved emitting lines used for thermal sensing are overlapped severely with the emitting spectrum of white LED. It is found that using the traditional green upconversion (UC) luminescence emanating from Er3+, namely the 2

H11/2/4S3/2-4I15/2 transitions, for thermal sensing leads to a very large temperature error, up to 17

K at 303 K in the case where there is the influence of white LED. While the two violet UC luminescence bands, separately originating from the

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G11/2/2H9/2-4I15/2 transitions of Er3+

embedded in the β-NaYF4:40% Yb3+, 2% Er3+ nanoparticles, are capable of enabling white LED free thermal sensing in that these two emission bands are absent from the emitting spectrum of white LED. Our work is likely to provide a new perspective for the study of luminescent ratiometric technology for thermal sensing. Most importantly, it presents a strategy for white LED free thermal sensing when the influence of white LED cannot be ignored in practical applications.


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INTRODUCTION To detect the temperature accurately and conveniently is a permanent topic as this fundamental parameter is always closely connected with much useful and important information in many fields.1-3 Recent years have witnessed the boom of conventional contact thermometers, as they own several advantages, such as low cost and high performance.4 These sensors, however, are excluded from an increasing number of domains with the development of nanoscience and nano-technology, in that they usually work with an invasive pattern, which means that they must be contacted tightly with the object of interest.5 Therefore, non-invasive temperature

measurement

methods

gain

ever-increasing

attention

nowadays.6-11

The

luminescence ratiometric thermometry is especially attractive for its independence on the excitation source fluctuations, the optics transmission losses, and the luminescent centre concentration variations. Up to now, there are lots of reports concerning on this technology for temperature measurement in various cases. For instance, Er3+, Tm3+, Nd3+, Ho3+, Gd3+, Yb3+ and Sm3+ have shown their abilities to be used as luminescent centres for thermal sensing.11-18 However, quite a number of the involved emitting lines used for thermal sensing, for example, the 2H11/2/4S3/2-4I15/2 transitions of Er3+,19 the 1G4-3H6 transition of Tm3+,20 are localized in the electromagnetic spectrum from 410 to 750 nm, which is just the emitting spectral region of white LED, regardless of the warm or the cool types.21 With the popularity of white LED in our daily life and industrial processes, there emerges a potential problem, i.e., these light sources are likely to have a negative effect on the eventual readings of temperature sensors, which has been neglected for a long time and thus needs to be solved timely and properly. Obviously, a general strategy that can be easily conceived is the use of the emitting bands escaped perfectly from the emission spectrum of white LED for temperature measurement in a real case. It means that the


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emitting lines that are localized in the electromagnetic spectrum smaller than 410 nm or larger than 750 nm are better choices. Actually, there are several pairs of luminescence bands at these two ideal spectral regions, such as the 4F5/2/4F3/2-4I9/2 transitions of Nd3+,22 and the 4F7/2-4F5/2 transition of Yb3+.23 Regardless of these facts, the corresponding reports concerning on white LED free thermal sensing is still on the way. To make a clear investigation on the anti-white LED ability of the luminescence ratiometric thermometry, Er3+ is studied in this work as a proof-of-concept. The reason lies that there are two pairs of upconversion (UC) emission lines, i.e., the 2H11/2/4S3/2-4I15/2 and 4G11/2/2H9/2-4I15/2 transitions, that are simultaneously owned by Er3+ embedded in one material. The former two emission lines, with peaks at 520 and 540 nm, respectively, are totally overlapped with the emission spectrum of white LED, while there is almost no spectral overlap between the latter two bands, with separate peaks at 390 and 410 nm, and the white LED’s emission spectrum. It means that we are able to make a comparison between these two pairs of emission lines, which represent for two different cases, in one material. Therefore, the 2H11/2/4S3/2-4I15/2 and 4G11/2/2H9/24

I15/2 transitions of Er3+ embedded in the β-NaYF4:40%Yb3+, 2%Er3+ UC nanoparticles (UCNPs)

are investigated against the influence of white LED. It is found that when the UCNPs are exposed to the irradiation of white LED, the 4G11/2/2H9/2-4I15/2 transitions are free from the influence of these disturbance light. In contrast, the 2H11/2/4S3/2-4I15/2 transitions show an obvious dependence on white LED lighting in the same scenario. Specifically, there is a temperature error up to 17 K at 303 K for these two emission lines. It can be concluded that compared with the 520/540 nm emission bands, the 380/409 nm ones are, undoubtedly, better choices when the irradiation from white LED cannot be neglected in a practical case. EXPERIMENTAL SECTION


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Materials and Synthesis. XCl3•6H2O crystals (X=Y/Yb/Er, >99.99%, Sigma-Aldrich), oleic acid and 1-octadecene (OA and ODE, technology grade, >90%, Sigma-Aldrich), NaOH (used as sodium source, >98%) and NH4F (used as fluorine source, >98%) particles were all of analytical purity. All involved chemicals were used as purchased without any further purification. The β-NaYF4:40%Yb3+, 2%Er3+ UCNPs were prepared according to the solvothermal method with minor modification.24 The reaction devices were filled with continuously flowing Ar to prevent O2 from oxidizing the protective solvents OA and ODE. In a typical synthesis of the UCNPs, 0.58 mmol YCl3•6H2O, 0.4 mmol YbCl3•6H2O, 0.02 mmol ErCl3•6H2O, together with 6 mL OA and 15 mL ODE, were added into a 50 mL round-bottom, three-necked bottle at room temperature. Thereafter, the mixture was heated to 433 K with continuous magnetic stirring and maintained at 433 K for 1 h to get a clear and homogeneous solution. After the solution cooled down naturally to room temperature, 10 mL methanol solution which contained 2.5 mmol NaOH and 4 mmol NH4F particles was added into the round-bottom, three-necked bottle. Half an hour later, the solution was heated to 343 K for 10 min to evaporate the methanol, and was further heated to 373 K for another 10 min to evaporate the residual water. Subsequently, the solution was rapidly heated to 573 K and maintained at 573 K for 1 h, followed by the natural cooling of the solution to room temperature. After the addition of much ethanol and centrifugation for 10 min, the white precipitation was obtained. Being washed by ethanol for three times, the desired UCNPs were successfully obtained and then were carefully stored for the following optical tests. Instrumental. Powder X-ray diffraction (XRD) patterns were carried out (Rigaku D/MAX2600/PC with Cu Kα radiation, λ=1.5406 Å) at room temperature. Transmission electron microscopy (TEM) images were obtained (FEI Tecnai TF20) with an accelerating voltage of 200 kV. A continuous wave 980 nm diode laser, controlled by the current adjustor (ITC-4005,


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Thorlabs), was used as the excitation source. The power density was estimated to be about 1.5 W/cm2. A monochromator (SBP-300, Zolix Instruments Co., Ltd) connected with a photomultipler tube (PMTH-S1-CR131, Zolix Instruments Co., Ltd) was used to collect the emission spectra focused by a lens with f=3.8 cm at different temperatures over the range between 303 and 483 K. The sample was heated by a home-made heating chamber with an accuracy of ±0.3 K. The white LED source used in the experiment is commonly used in our daily life and could be purchased easily and cheaply from stores. RESULTS AND DISCUSSION

Scheme 1. Schematic of the experimental set-up for temperature measurement based on the NaYF4:40%Yb3+, 2%Er3+ UCNPs.

In the beginning, it is necessary to show how we synthesize the system for practical applications, which is illustrated in Scheme 1. After preparation of the NaYF4:40%Yb3+, 2%Er3+ UCNPs via the solvothermal method (see Experimental Section), they are put in the coolingheating stage, which is used to adjust the temperature of these UCNPs. Following the NIR excitation of a 980 nm diode laser, the UCNPs emit the characteristic green and violet UC


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luminescence. According to the Boltzmann distribution, the luminescence intensity ratio between the two green bands, and that between the two violet bands, which are sensitive to the temperature changes, could be used as the temperature indicators. Based on these two pairs of emission bands, two calibration curves could be obtained with the help of the cooling-heating stage in the temperature range of 303-483 K. It should be noticed that these procedures are obtained in a dark room where white LED is absent, aiming to avoid the influence of white LED lighting. Thereafter, a commonly used white LED was turned on to emit the disturbance light for the temperature measurement, and this is the quite common case no matter in our daily life or various industrial fields. In this case, the luminescence intensity ratio between the two green bands and that between the two violet lines are re-investigated as a function of temperature. Naturally, two calibration curves for each pair of emission bands could be achieved once again. It is easy to know that if the two calibration curves, obtained with and without the influence of white LED lighting, are identical, the corresponding emission lines are thus immune to the white LED lighting. Therefore, compared with another two lines, they are expected to be more suitable for the practical application when the white LED lighting cannot be neglected or prevented. After these analyses, the UCNPs could find their applications in various fields based on the two emission bands that are immune to the white LED lighting. For example, these UCNPs, after being modified by water soluble groups, are capable of being used as the nano-thermometers for thermal mapping of the nanofluid, as Brites et al. demonstrated in the pioneering work.13


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Figure 1. (a) XRD pattern, (b) TEM image and (c) particle size distribution of the NaYF4:40%Yb3+, 2%Er3+ UCNPs.

XRD patterns of the NaYF4:40%Yb3+, 2%Er3+ UCNPs are presented in Figure 1(a), indicated with the blue line. As can be observed, the positions of these diffraction peaks of the samples can be well indexed to the data from the Joint Committee on Powder Diffraction Standards file number 16-0334 which is marked by the red lines. It indicates undoubtedly that the hexagonal phase holds the structure of the NaYF4:40%Yb3+, 2%Er3+ UCNPs,25 being helpful for improving the UC luminescence efficiency of Er3+, by comparison with the cubic phase NaYF4 host.26 These sharp diffraction peaks are the evident indications of well crystallization of the asprepared UCNPs. TEM image of the β-NaYF4:40%Yb3+, 2%Er3+ UCNPs is shown in Figure 1(b). It can be seen that these UCNPs are uniform and round shape, without aggregations. As can be found from Figure 1(c), these UCNPs’ sizes range from 21 to 33 nm, with an average particle diameter of around 27 nm. These encouraging features enable the UCNPs to be used at the nanoscale.


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Figure 2. (a) Room temperature UC emission spectrum of the UCNPs and (b) possible UC mechanism for the emission spectrum shown in (a), following the NIR excitation at 980 nm.

Following the NIR excitation of a 980 nm continuous wave diode laser, the sample exhibits visible UC luminescence at room temperature, which is presented in Figure 2(a). As can be observed, there are four characteristic emission bands over the wavelength range between 350 and 600 nm, peaking at around 380/410/520/540 nm, respectively. According to the previous reports, the four emission bands can be separately assigned to the 4G11/2-4I15/2, 2H9/2-4I15/2, 2H11/24

I15/2, and 4S3/2-4I15/2 transitions of Er3+. The UC mechanism and specific processes responsible

for these four transitions are graphically depicted in Figure 2(b).27 It can be seen clearly that both the 380 and 410 nm emission bands come from the three-photon UC mechanism, while the 520 and 540 nm ones, which have been sufficiently investigated,28,29 stem from the two-photon UC mechanism.


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Figure 3. (a) Normalized temperature dependent UC emission spectra of the β-NaYF4:40Yb3+, 2%Er3+ UCNPs between 303 and 483 K; (b) luminescence intensity ratio between the 380 and 410 nm bands (∆2) as well as that between the 520 and 540 nm bands (∆1) as a function of temperature.

To test the temperature detection ability of the 380 and 410 nm emission bands, as well as the 520 and 540 nm ones, the temperature dependent spectra of the samples are then studied. Figure 3(a) only shows the spectra recorded at 303/363/423/483 K for representatives. It should be stressed here that for a clear observation, these emitting spectra between 350 and 450 nm, as well as that between 500 and 600 nm, are normalized to the 410 and 540 nm peaks’ emitting intensity, respectively. As can be observed, the 380 and 520 nm emission bands increase markedly with respective to the 410 and 540 nm ones, respectively, upon gradually increasing the temperature from 303 to 483 K. It suggests undoubtedly that the luminescence intensity ratio between the 380 and 410 nm emission lines, and that between the 520 and 540 nm ones, monotonously increase with the temperature. In view of the fact that the 380/410 nm and 520/540 nm emission bands are separately spaced by the gaps of around 1900 and 700 cm-1, both of which are smaller than 2000 cm-1, there should be the thermal effect between the 4G11/2 and 2H9/2 levels, as well as between the 2H11/2 and 4S3/2 ones, according to the Boltzmann distribution.5 Therefore, there is an


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increment in the population of the upper levels, namely the 4G11/2 and 2H11/2 levels, which is at the expense of the depopulation of the adjacent lower levels, i.e., the 2H9/2 and 4S3/2 levels. The intensity ratio between the two emission lines originating from two closely spaced levels is thus expected to increase with increasing the temperature. The integrated emission intensity ratio between the 370-390 nm and 400-420 nm spectral regions, as well as that between the 510-533 nm and 533-570 nm emission bands, marked with ∆2 and ∆1, respectively, are obtained as a function of temperature, which are shown in Figure 3(b). As can be observed, all these ratios, including ∆2 and ∆1, can be well fitted with the use of the following expression5

∆ = A exp(-

∆E )+B , kT

(1)

where A is a pre-exponential constant, ∆E is the gap between two thermally coupled levels, k is the Boltzmann constant, T is the absolute temperature, and B is an offset parameter. From Figure 3(b), it can be seen that Equation (1) describes the relationship between ∆2 and temperature well (R2>0.996), meaning that the 380/410 nm emission bands are the promising candidate for thermal sensing. The fit results are A=(2.7±0.6), ∆E/k=(1296±129) K, and B=(0.140±0.007). It means that ∆E is about 900 cm-1, which is smaller than the practical value that separates the 380 and 410 nm emission bands obtained from the emission spectra presented in Figure 3(a). The reason behind this phenomenon is complicated and needs more investigation, but it is quite common that the fitted gap is smaller than the practical value.5,17,30-32 As Wang et al. concluded in the review article, this mismatch phenomenon has been found for the majority of thermally coupled energy levels.17 Even if the 2H11/2 and 4S3/2 levels of Er3+ that have been researched over forty years are involved without exception.17,31,32 Therefore, the change in the luminescence


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intensity ratio between the 380 and 410 nm emission bands could be mainly attributed to the thermalization, that is, Equation (1). In fact, if the change in the intensity ratio between these two lines cannot be attributed to the thermalization, a lot of temperature dependent physical mechanisms, for example, the multiphonon non-radiative relaxation, are likely to play a key role in determination of the eventual ratio. In this case, the method can also own a good robustness as the corresponding function is definite. It can be noticed that there is an offset parameter, B≠0, for the 380 and 410 emission lines. Wade et al. stated that the spectral overlap between two emission lines is probably the source of this offset parameter, which is, however, inapplicable here for the 380 and 410 nm lines as there is no any overlap between them.5 Furthermore, there is no offset parameter for the 520 and 540 bands although the two bands are partially overlapped, as shown in Figure 3(a). These two facts suggest that the offset parameter for the 380 and 410 nm lines cannot be attributed to the spectral overlap. We proposed recently that the blending of other populating processes on the population of the upper level of two thermally coupled levels is likely to play a key role in determination of the offset parameter.33 Therefore, the fact that B≠0 for the 380 and 410 nm lines indicates that other populating processes are likely to be present. In contrast, ∆1 is also fitted well as a function of temperature with the use of Equation (1), with the fit results A=(7.43±0.04), ∆E/k=(982±2) K, and B=0. It indicates that the fitted gap between the 520 and 540 nm emission bands is 682 cm-1, being close to the practical value of 712 cm-1. This encouraging result suggests that the luminescence intensity ratio between the 520 and 540 nm emission lines can be the promising temperature indicator. As can be observed from Figure 3(b), over the whole temperature range from 303 to 483 K, ∆2 is smaller than ∆1 although the 380/410 nm lines own a much larger gap then the 520/540 ones. In fact, Equation (1) suggests that at the same temperature the luminescence intensity ratio not only depends on the energy gap between


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two thermally coupled levels, but also relies on the pre-exponential constant A and the offset parameter B. Furthermore, there are many similar examples indicating that the gap between two thermally coupled levels is not the only factor to determine the corresponding luminescence intensity ratio.33-36 Therefore, although the 380 and 410 nm bands own a larger gap, the fitted pre-exponential constant A for them is smaller than that for the 520 and 540 nm bands, together with the presence of the offset parameter B, thus making ∆2 be smaller than ∆1.

Figure 4. Comparison of the relative sensitivities for the 380/410 nm UC emission bands and the 520/540 nm ones over the temperature range from 303 to 483 K.

In practical applications, the relative sensitivity, indicated with Sr, is a key parameter to evaluate the detection performance of various thermal sensing methods, irrespective of their nature. It can be defined as37,38

Sr =

∂∆ 1 . ∂T ∆

(2)


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On the basis of this expression, the relative thermal sensitivity for the 380/410 emission bands and that for the 520/540 nm ones are achieved, which are presented in Figure 4. For clarity, these two relative thermal sensitivities are marked with S2 and S1, respectively. As can be observed, S1 decreases markedly from 1.13 to 0.44 %K-1 with the increase of the temperature from 303 to 483 K. While S2 presents a different variation law over the same temperature range. It first increases from 0.36 %K-1 at 303 K to 0.43 %K-1 at 384 K, and then decreases monotonically to 0.38 %K-1 at 483 K. It is interesting to notice the detail from Figure 4 that there is an extreme value for S2. Actually, this fact is in good accordance with the conclusion recently proposed.34,39 In short, there are two difference cases. If there is no offset parameter or its value is equal to zero, the relative sensitivity will decrease monotonically with increasing the temperature. If there is a positive offset parameter, which is just the case for the 380/410 emission bands, there must be only one maximum of the relative sensitivity. As can be observed from Figure 4, S1 is always larger than S2 between 303 and 483 K. Over this experimental temperature range, the average relative sensitivity for S1 is about 0.70 %K-1, which is larger than the average value of 0.40 %K-1 for S2. If the relative thermal sensitivity is the only parameter to be taken into consideration when judging the superior or inferior among all kinds of temperature measurement methods, the 520/540 nm emission bands are, undoubtedly, better choices to perform the thermal sensing in a real case. In fact, this pair of emission bands have been investigated intensively and used widely. For example, in 2010, Vetrone et al. used them to acquire the temperature of a single living cell successfully, which is, to the best of our knowledge, the first time for the 520/540 nm emission bands being used to detect the intra temperature of a living cell.40 In another case, Brites et al. also did the pioneering work concerning on the thermal sensing of the nanofluid based on the same pair of emitting levels of Er3+.13 However, the rapid development and wide popularity of


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white LED in recent years put forward higher requirements for noncontact thermal sensing. Although the luminescence intensity thermometry owns strong anti-influence, as mentioned in the beginning, the ability to shield from the influence of white LED for this method is merely investigated. Considering the fact that there indeed are some cases where the white LED’s emitting intensities are strong that cannot be neglected, it is thus of great significance to explore white LED free thermal sensing, which is thus the focus of the following part.

Figure 5. (a)/(b) V and G show the original emission spectra of the UCNPs; V/G+LEDX (X=1-3) shows the emission spectra of the UCNPs at different irradiation cases where white LED is deliberately added; LEDX (X=1-3) shows the emitting spectra of white LED with different intensities.

The 380/410 nm emission bands as well as the 520/540 nm ones are then investigated in respect of their white LED free thermal sensing abilities, as respectively shown in Figure 5 (a) and (b). In the following experiments, the temperature measurement and spectra collection are exposed to the circumstance where white LED is added deliberately with different emitting intensity. The white LED’s emitting spectra with different intensity are shown in Figure 5 (a) and (b), denoted by LEDX (X=1-3). As can be observed from Figure 5 (a), the 380/410 nm emission


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bands are nearly immune to the white LED lighting. Even if the white LED lighting is adjusted to own the approximate intensity with that of the 380/410 nm emission bands, these two emission bands also keep unchanged. The reason lies that there is no overlap between these two bands and the emission spectra of white LED. In contrast, the 520/540 nm emission bands show an obvious dependence on the white LED lighting. As depicted in Figure 5 (b), with the increase of the white LED emitting intensity, both the 520 and 540 nm emission bands present the marked growth tendency. Obviously, the total overlap between the 520/540 nm emission bands and the white LED’s emission spectra should be responsible for these variations. For a more clear observation, the UC emission spectra at the background of the white LED lighting with different intensity are presented in the inset in Figure 5(b), which have been normalized to the 540 nm emission intensity. It can be seen that the 520 nm emission band, and thus the intensity ratio between the 520/540 nm emission bands, increase markedly with the increased emitting intensity of white LED, being an indication that the luminescence intensity ratio between the 520/540 nm emission bands is sensitive to the white LED lighting.

Figure 6. (a) ∆2 and (b) ∆1 as a function of the normalized white LED emitting intensity (here it is defined as the integrated spectral intensity) at room temperature.


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At 303 K, we then investigate the influence of the white LED lighting on the intensity ratio between the 380/410 nm emission bands, as well as that between the 520/540 nm ones, which are presented in Figure 6(a) and (b), respectively. Note that when there is no disturbance from white LED, the integrated 510-533 and 533-570 nm spectral regions are taken as the intensities for the 2

H11/2→4I15/2 and 4S3/2→4I15/2 transitions of Er3+, respectively. Furthermore, the integrated 370-

390 and 400-420 nm spectral regions are separately regarded as the intensities for the 4G11/2→ 4

I15/2 and 2H9/2→4I15/2 transitions of Er3+. Therefore, for a proper investigation of the influence of

the white LED lighting on the thermal sensing ability of the 2H11/2/4S3/2→4I15/2 and 4G11/2/2H9/2→ 4

I15/2 transitions of Er3+, these methods are also used when there is the disturbance of white LED.

The two intensity ratios, obtained in the same scenario where white LED is not triggered, are used as the references (the left most squares in Figure 6(a) and (b)). As can be observed, with the gradual increase of the emitting intensity of white LED, ∆2 and ∆1 show two totally different variation laws. Specifically, ∆2 presents a slight fluctuation around the reference value, as shown in Figure 6(a). It can be seen that ∆2 obtained under the irradiation of white LED with different emitting intensity are in the range of the experimental error. It is an indication that the 380 and 410 nm emission bands can be used for thermal sensing free from the influence of white LED. In contrast, ∆1 presents an obvious dependence on the white LED lighting in the same irradiation conditions, as shown in Figure 6(b). It can be seen that ∆1 increases markedly with the rise of the emitting intensity of white LED, from the reference value 0.257 to 0.307 with an increment up to 19%. It is necessary to quantitate the temperature errors introduced by the white LED irradiation, which can be estimated by using37

δT =

δ∆/∆ , Sr

(3)


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where δT refers to the temperature resolution or temperature uncertainty, and δ∆ is the increment of ∆ introduced by exposing the sample to the irradiation of white LED. Based on Equation (3) and Figure 6(b), the conclusion can be reached easily that δT for the 520/540 nm emission lines is up to 17 K when the emitting intensity of white LED (here it refers to the integrated spectral region that is overlapped with the 520/540 nm emitting lines) is an order of magnitude smaller than that of the 520/540 nm emitting lines. It is known that the temperature uncertainty depends mainly on the experimental setup. In our experiments, δ∆/∆ is estimated to be 0.33%, which is the typical value for the photomultipler tube based spectral acquisition system, thus causing a temperature uncertainty 0.29 K at 303 K. Notably, the temperature error 17 K, introduced by the background light from white LED for the 520/540 nm emitting lines, is much larger than the temperature uncertainty 0.29 K. In contrast, δT for the 380/410 nm emission bands is, as mentioned above, free from the influence of white LED.

Figure 7. (a) ∆2 and (b) ∆1 as a function of temperature without and with the stable influence of white LED. Here the emitting intensity of white LED is same with ‘LED1’ presented in Figure 5(a) and (b).


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We then investigate the influence of the stable white LED lighting (here the emitting intensity of white LED is same with ‘LED1’ presented in Figure 5(a) and (b)) on ∆2 and ∆1, and the results are presented in Figure 7(a) and (b), respectively. As can be observed from Figure 7(a), the two fit functions obtained with and without the stable white LED lighting are nearly identical for ∆2. It means undoubtedly that from 303 to 483 K, ∆2 is immune to the influence of the stable white LED lighting. In contrast, it can be seen from Figure 7(b) that under the same stable white LED lighting, the calibration curve has been totally changed for ∆1. It means that ∆1 is sensitive to the influence of the stable white LED lighting. It also can be concluded from Figure 7(b) that under the stable white LED lighting, using the 520 and 540 nm lines also seems to be a feasible method for thermal sensing. However, it is difficult to meet the applied conditions as all test factors should be kept strictly unchanged in this case. The relative sensitivities for the 380/410 and 520/540 nm lines are also studied under the stable white LED lighting. From Figure 7(a), we know that ∆2 keeps nearly unchanged over the whole temperature range. Therefore, the relative sensitivity for the 380/410 nm lines should also keep unchanged, that is, S2 in Figure 4. For the 520/540 nm lines, their ratio varies largely under the white LED lighting. It means that the relative sensitivity for these two bands will change accordingly (see Supporting Information Figure S1). As can be observed, the relative sensitivity for the 520/540 nm lines is smaller than that achieved without the disturbance of the white LED lighting. Based on Figure 7(a) and (b), the temperature errors on the basis of the 380/410 and the 520/540 nm lines are shown in Figure 8. As can be observed, at almost every temperature, the error based on the 380/410 nm emission bands is much smaller than that based on the 520/540 nm ones. And over the most temperature range, the error could be smaller than 0.5 K for the 380/410 nm emission bands. By comparison, the average error is larger than 7 K on the basis of the 520/540 nm lines, and at 483 K, the


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temperature error is up to 18 K. It suggests once again that compared with the 520/540 nm lines that are overlapped with the emitting spectrum of white LED, the 380/410 bands could be the promising candidate for white LED free thermal sensing.

Figure 8. Comparison of the temperature errors based on ∆2 and ∆1 without and with the stable white LED lighting. Here the emitting intensity of white LED is same with ‘LED1’ presented in Figure 5(a) and (b).

Generally speaking, every sensor has its specific application range, so does the temperature sensor based on the 380 and 410 nm emission bands. In this work, we state that in the case where the integrated emitting intensity of white LED is equivalent with or several times larger than that of the 380 and 410 nm emission bands, there would be no obvious temperature error by using the two bands for luminescent ratiometric thermometry. Therefore, it is interesting to know that if the white LED emitting intensity is significantly stronger than the two violet emission bands, what is the temperature error by using these two bands as the temperature indicator? Considering the fact that the maximum count of the spectrometer used in the experiments is 30000, the maximum intensity of white LED is thus adjusted to be 30000 (see Supporting Information


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Figure S2). The integrated emitting intensity of white LED is about two orders of magnitude larger than that of the 380 and 410 nm emission bands. In such a case, the results are presented (see Supporting Information Figure S3). As can be observed, if the white LED emitting intensity is significantly stronger than the two violet emission bands at room temperature, the luminescence intensity ratio between the 380 and 410 nm emission bands deviates from the fit curve obviously. It was calculated that there is a -20 K temperature error at room temperature when the integrated emitting intensity of white LED is about two orders of magnitude larger than that of the 380 and 410 nm emission bands. The reason is also depicted (see Supporting Information Figure S2). It can be seen that there is some spectral overlap between the 410 nm emission band and the emitting spectrum of white LED. It causes an increase of the integrated 410 nm emission band, thus making the luminescence intensity ratio between the 380 and 410 nm emission bands decrease accordingly. Therefore, the violet emission bands are not suitable for the case where the white LED lighting is significantly stronger than the intensity of these two violet emission bands. As stated in the beginning, white light is nearly the most popular source, no matter in our daily life or in industrial processes, meaning that this light is probably to pose a threat on the luminescent ratiometric thermometry in a real case. Therefore, in this work, we choose white LED as the disturbance light source to investigate the anti-influence ability of the luminescent ratiometric thermometry. However, white LED might be absent but other light sources might be present in some practical applications. For example, if we need to detect the temperature of a building’s outer wall, which is coated with the UCNPs prepared in this work and is exposed to the direct sunlight, it must be careful to use the 380/410 and 520/540 nm emission bands for thermal sensing. The reason is that these four emission bands, located in the wavelength range


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from 350 to 600 nm, are totally overlapped with the spectrum of sunlight. Therefore, if the emitting intensity of sunlight collected by the spectrometer cannot be neglected, it is expected that sunlight would affect the temperature reading on the basis of the 380/410 and 520/540 nm emission bands. This is also an interesting topic that deserves for further investigation in the future as it will provide significant guidance on the practical applications of the luminescent ratiometric thermometry. At this point, it must be stressed here that although we successfully propose a strategy for white LED free thermal sensing by using the violet UC luminescence emanating from Er3+ embedded in the β-NaYF4:40%Yb3+, 2%Er3+ UCNPs, the 520/540 nm emission bands also own its superiority in the aspects of the thermal sensitivity and emission intensity. In other words, in a case where there is no irradiation from white LED or this irradiation intensity can be ignored, for example, in a single living cell, the 520/540 nm emission bands are also recommended to be used for thermal mapping. But for the working place where the influence of white LED cannot be neglected, for instance, thermal mapping of a white LED’s surface, using the strategy proposed in this work is, undoubtedly, far superior to the method using the green UC luminescence of Er3+, as there is no overlap between the violet UC luminescence bands of Er3+ and the emission spectrum of white LED, thus facilitating a more accurate temperature measurement in some cases. CONCLUSIONS In summary, the β-NaYF4:40%Yb3+, 2%Er3+ UCNPs emitted characteristic violet and green UC luminescence peaking at 380/410/520/540 nm, respectively, following the NIR irradiation. Both the 380/410 nm emission bands and the 520/540 nm ones were confirmed to be thermally coupled, the luminescence intensity ratios between which thus could be used for thermal sensing


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between 303 and 483 K. Due to the fact that there was almost no overlap between the 380/410 nm emission bands and the emission spectrum of white LED, using these two violet emission bands could achieve a more accurate temperature measurement free from the influence of the white LED lighting. In contrast, there would be a temperature error up to 17 K at 303 K on the basis of the 520/540 nm emission bands under the same irradiation circumstance. Serious overlapping between the white LED’s emitting spectrum and the 520/540 nm emission bands was responsible for this large temperature detection error. Our work provides a feasible solution for thermal sensing free from the influence of the white LED lighting by using the violet UC luminescence emanating from Er3+. Most importantly, it is also likely to point out a universal strategy to carry out white LED free thermal sensing if necessary, namely using the emission bands which perfectly escape from the emission spectra of white LED. ASSOCIATED CONTENT Supporting Information The following files are available free of charge. Influence of the white LED lighting on S1; emission spectra of the UCNPs and white LED; influence of the strong white LED lighting on ∆2 (PDF) AUTHOR INFORMATION Corresponding author *E-mail: [email protected] (F.Q.). **E-mail: [email protected] (Z.Z.). ORCID Feng Qin: 0000-0003-4696-5142


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC) (Grant No. 81571720 & 61505045). REFERENCES (1) Jaque, D.; Vetrone, F. Flourescence nanothermometry. Nanoscale 2012, 4, 4301-4326. (2) Wang, X. D.; Wolfbeis, O. S.; Meier, R. J. Luminescent probes and sensors for temperature. Chem. Soc. Rev. 2013, 42, 7834-7869. (3) Brites, C. D. S.; Lima, P. P.; Silva, N. J. O.; Millán, A.; Amaral, V. S.; Palacio, F.; Carlos, L. D. Thermometry at the nanoscale. Nanoscale 2012, 4, 4799-4829. (4) Childs, P. R. N.; Greenwood, J. R.; Long, C. A. Review of temperature measurement. Rev. Sci. Instrum. 2000, 71, 2959. (5) Wade, S. A.; Collins, S. F.; Baxter, G. W. Luminescence intensity ratio technique for optical fiber point temperature sensing. J. Appl. Phys. 2003, 94, 4743. (6) Marciniak, L.; Bednarkiewicz, A.; Kowalska, D.; Strek, W. A new generation of highly sensitive luminescent thermometers operating in the optical window of biological tissues. J. Mater. Chem. C 2016, 4, 5559-5563. (7) Marciniak, L.; Pilch, A.; Arabasz, S.; Jin, D.; Bednarkiewicz, A. Heterogeneously Nd3+ doped single nanoparticles for NIR-induced heat conversion, luminescence, and thermometry. Nanoscale 2017, 9, 8288-8297. (8) McLaurin, E. J.; Bradshaw, L. R.; Gamelin, D. R. Dual-emitting nanoscale temperature sensors. Chem. Mater. 2013, 25, 1283-1292.


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Table of Contents graphic


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Exploiting the Yb3+ and Er3+ Codoped Î²-NaYF4 Nanoparticles as

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