Introduction

Extensive studies on luminescent nanoparticles for their use as functional materials, that exhibit an optical response to specific changes in physical or chemical parameters like temperature, pressure, or pH is the major demand of current nanotechnology1. This results, among others, from the extraordinary potential of such materials for the diagnosis and treatment of various illnesses, especially cancer diseases2,3,4,5,6. Hopes are particularly high for materials that allow remote temperature sensing7,8,9,10,11,12,13. Exploiting the thermally induced changes in spectroscopic properties of the phosphor luminescence thermometry enables the measurement of temperature at the tissue and even cellular level due to the sub-micro spatial resolution14,15,16. Due to their high physical stability, especially extensively explored for this purpose are the inorganic materials doped with lanthanide ions4, 17,18,19,20. Ladder-like energy level diagrams of lanthanide ions together with the relatively long lifetime of their metastable excited states offer the possibility of inducing their Stokes and anti-Stokes emission. As it is well known the reduction of the population of the metastable states of lanthanides at elevated temperatures leads to the quenching of their Stokes emission21. On the other hand, the thermalization of the higher lying excited states according to the Boltzmann distribution leads to the opposite thermal monotonicity of the emission intensity of the anti-Stokes emission13. When the same metastable energy state is emitting state for Stokes emission and the starting point from which thermalization to the higher energy states occurs at elevated temperatures it is possible to achieve the opposite thermal monotonicity of two luminescence signals from one type of dopant ions. Hence the difference in the thermal dependence of those two signals enables the development of the ratiometric luminescence thermometer of high thermal sensitivity. Efficient thermalization with preserving the intense emission intensity from the upper state impose the selection of lanthanide ion of the energy separation between two subsequent energy state lower than 2000 cm−113. One of the most commonly used lanthanide ion in luminescence thermometry, due to is unique energy level diagram, is neodymium (Nd3+)22,23,24,25,26,27,28. As proved recently by Suo et al. the unique energy level configuration of Nd3+ facilitates the development of Stokes/anti-Stokes ratiometric luminescent thermometers29,30,31,32,33. A high concentration of the Nd3+ ions facilitates the thermalization process due to the more efficient absorption of the incident radiation and the more efficient light-to-heat conversion related to the heating of the particle. The light-to-heat conversion process in Nd3+ doped nanoparticles enables to develop a light-induced nanoheaters17, 34,35,36,37. Since the energy difference between metastable 4F3/2 state and upper laying 4F5/2, 2H9/2 and then 4F7/2, 2S3/2 are relatively low ~ 1000 cm−1 the thermalization process itself according to the Boltzmann distribution is more probable to be involved than the energy transfer up-conversion.

Therefore in this work, the influence of Nd3+ dopant concentration on the luminescence thermometer exploiting Stokes to anti-Stokes emission intensity ratio will be systematically investigated. In order to minimize the effect of the nonradiative processes associated with the host material, the NaYF4 nanoparticles were used in these studies, that is well known for their low phonon energies. The influence of the Nd3+ concentration on the thermometric performance of the ratiometric thermometer including relative sensitivity and usable temperature range is analyzed.

Materials and methods

Materials preparation

The materials were synthesized by the solvothermal method in oleic acid as solvent. Neodymium(III) oxide (99.999%), yttrium(III) oxide (99.999%) were purchased from Alfa Aesar, sodium fluoride (99.99%), chloric acid (99%), pure oleic acid were purchased from Sigma Aldrich. Sodium hydroxide (99.8%), ethanol (96% pure p.a.), n-hexane and chloroform were purchased from POCH S.A. (Poland). All of the chemical reagents were used as received without further purification.

In a 50-mL autoclave, 0.6 g of NaOH was dissolved into 5 mL of deionized water under stirring. Thereafter, an aqueous solution of rare earth chlorides (0.2 mmol) was added. Then, 10 mL of ethanol and 10 mL of oleic acid were added under vigorous stirring. After stirring at 50 °C for 1 h 0.2 mmol aqueous solution of sodium fluoride was added immediately. Finally, 10 mL of ethanol was added into the autoclave after stirring for another 30 min, and the autoclave was sealed and heated at 180 °C for 8 h. The solution was cooled to room temperature and the nanoparticles were collected by centrifugation and washed three times with hexane/ethanol solution. The final product was redispersed in 5 mL of chloroform or for spectroscopic measurements was prepared by drying precipitates at room temperature.

Methods

Powder diffraction data were obtained using a PANalytical X'Pert Pro diffractometer equipped with an Anton Paar TCU 1000 N Temperature Control Unit using Ni-filtered Cu Kα radiation (V = 40 kV and I = 30 mA)38, 39. Transmission electron microscope (TEM) images were recorded with a Philips CM-20 SuperTwin transmission electron microscope, operating at 160 kV. A drop of the suspension was put on a copper microscope grid covered with carbon. Before the measurement, the sample was dried and purified in a H2/O2 plasma cleaner for 1 min. The excitation spectra and luminescence decay profiles were obtained using an FLS1000 Fluorescence Spectrometer from Edinburgh Instruments equipped with a 450 W xenon lamp and μFlash lamp as an excitation sources and R5509-72 photomultiplier tube from Hamamatsu in a nitrogen-flow cooled housing as a detector. To carry out the temperature measurement, the temperature of the sample was controlled using a THMS 600 heating–cooling stage from Linkam (0.1 °C temperature stability and 0.1 °C set point resolution). The emission spectra were recorded using 808 nm excitation lines from laser diode (LD of 1.1 W/cm2 excitation density) and a Silver-Nova Super Range TEC Spectrometer from Stellarnet (1 nm spectral resolution) as a detector.

Results and discussion

In order to analyze the influence of Nd3+ dopant concentration on the thermometric properties of this type of luminescence thermometer a series of nanoparticles with different concentrations of Nd3+ changes in the range of 0.1–75% in respect to the Y3+ ions. As a result of the proposed simple procedure, β-NaYxNd1−xF4 nanoparticles crystallizing in the hexagonal phase were synthesized. The determination of the symmetry group of these compounds crystallizing in the β-phase is still debatable due to the several possibilities (P6, P63/m or P 2 m) of assigning the symmetry group40. In the case of P6 (Fig. 1a) crystal structure the RE3+ ions (RE3+-rare earths) accommodate the first crystallographic position with the nine-fold coordination. The same coordination number is achieved by RE3+/Na+ ions (in the ratio 3:1) for the P63/m crystallographic position, whereas Na+ ions occupy the third sixfold coordinated position. In turn, for the P63/m group, the positions occupied by the rare-earth metal ion (RE3+ and RE3+/Na+) are symmetrically correlated and mixed with each other. In this study, the β-NaYF4 of the P6 space group was used as a reference pattern. As it can be seen in Fig. 1b all of the diffraction reflections correspond to the reference data, and there are no additional peaks that could indicate the presence of another phase or by-products. However, some differences can be observed due to the change in the intensity of the diffraction peaks. An increase in the Nd3+ concentration results in a broadening of the reflections that suggests a reduction of the particle size (Fig. 1c). However, TEM images reveal that the size of the materials oscillated in the range of over 15 nm, 23 nm, 15 nm to 20 nm for NaYF4:1%Nd3+, NaYF4:5%Nd3+, NaYF4:25%Nd3+ to NaYF4:75%Nd3+, respectively (Fig. 1c–f, see also Supplementary Fig. S1 for particles size distribution). TEM image analysis confirmed the preparation of crystalline nanoparticles with a narrow grain distribution and a non-aggregated form. Only for nanoparticles with a dopant ion concentration of 25% Nd3+ subtle of the aggregation was observed.

Figure 1
figure 1

The visualization of the NaYF4 crystal unit cell (a); X-ray diffraction patterns of NaYF4 doped with different concentration of Nd3+ (b); the representative TEM images of NaYF4 nanoparticles doped with 1% (c), 5% (d), 25% (e), 75% (f) of Nd3+-ions.

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Despite the small difference in the particle size was observed when the Nd3+ concentration was changed NaYF4:x%Nd3+ (x = 0.1, 1, 2, 5, 25, 50, 75) this effect should not affect significantly the spectroscopic properties of the analyzed nanoparticles, as shown recently by Trejgis et al.41. Therefore the change in their luminescence properties can be discussed in terms of the Nd3+ concentration effect. To understand this effect the simplified energy level diagram of Nd3+ ions is presented in Fig. 2a. Upon 808 nm excitation, the electrons from the ground 4I9/2 to the 4F5/2, 2H9/2 state are transferred, followed by the nonradiative depopulation to the metastable 4F3/2 state. The radiative relaxation of this state to the 4I9/2, 4I11/2, 4I13/2 energy levels results in the occurrence of emissions bands centered around 890 nm, 1060 nm, and 1325 nm, respectively. An increase in the temperature results in the thermalization of the upper 4F5/2, 2H9/2 and 4F7/2, 2S3/2 and 4F9/2 states. This process enables the generation of the 4F5/2, 2H9/2 → 4I9/2, 4F7/2, 2S3/2 → 4I9/2 and 4F9/2 → 4I9/2 electronic transitions corresponding to the emission bands at 800 nm, 740 nm and 690 nm, respectively. A shortening of the average distance between Nd3+ ions associated with the increase in dopant concentration increases the probability of the {4F3/2, 4I9/2} ↔ {4I15/2, 4I15/2} cross relaxation that leads to the quenching of the emission intensity and shortening of the lifetime of the 4F3/2 state42,43,44,45,46. Both anti-Stokes and Stokes emission bands can be observed simultaneously (Fig. 2b,c). However, the anti-Stokes luminescence is less intense. Therefore to analyze the shape of the emission spectra both parts of the spectrum were presented separately. Although the spectral position of the emission band is independent of dopant concentration the shape of the 4F3/2 → 4I9/2 band changes significantly (see also Supplementary Figs. S2 and S3). At elevated Nd3+ concentration the intensity of the emission lines corresponding to the R1 and R2 Stark levels of the 4F3/2 state to the Z5 Stark component of the 4I9/2 level decreases due to the energy reabsorption (Supplementary Fig. S4). The metastable 4F3/2 state plays a crucial role in the generation of both Stokes (radiative depopulation of 4F3/2 state) and anti-Stokes (as a platform for thermalization of higher laying states) luminescence of Nd3+ ions. Therefore it is important to analyze the influence of the Nd3+ ions concentration on the lifetime of the 4F3/2 state. As shown in Fig. 2d the exponential luminescence decay profile can be found for low dopant concentration and an increase in Nd3+ amount results in a deviation from exponential shape due to the cross relaxation process. Therefore to perform a qualitative analysis the average lifetime was calculated as follows:

$$\tau_{avr} = \frac{{A_{1} \tau_{1}^{2} + A_{2} \tau_{2}^{2} }}{{A_{1} \tau_{1} + A_{2} \tau_{2} }},$$
(1)

where A1, A2, τ1 and τ2 are the parameters determined from the fitting of the decay profiles with bi-exponential functions:

$$I\left( t \right) = I_{0} + A_{1} e^{{ - \frac{t}{{\tau_{1} }}}} + A_{2} e^{{ - \frac{t}{{\tau_{2} }}}} ,$$
(2)

here I0 represents the initial emission intensity. For the nanoparticles doped with 0.1% Nd3+ ions the longest τavr = 0.430 ms was observed, which shortens with Nd3+ to τavr = 0.336 ms, 0.217 ms, 0.141 ms, 0.026 ms, 0.017 ms and 0.014 ms for 1%, 2%, 5%, 25%, 50% and 75% of Nd3+, respectively (Fig. 2e). The lack of change in the number of components in the excitation spectra of Nd3+ ions in NaYF4:Nd3+ for the 4I9/2 → 2P1/2 electronic transition proves that Nd3+ ions consequently occupy only one crystallographic position (Y3+ site) in NaYF4 structure (Fig. 2f, see also Supplementary Fig. S5). Deeper insight into the change of the local crystallographic surrounding of the Nd3+ ions with an increase of dopant concentration can be provided by the analysis of the intensities ratio of 4I9/2 → 4G5,7/2 band (hypersensitive band) to the 4I9/2 → 4D1/2 bands. In the NaYF4:Nd3+ nanoparticles the ratio changes from 0.13 for 0.5% Nd3+ up to 0.40 for 75% Nd3+ confirming the decrease in local symmetry and an increase in covalency associated with the enlargement of the dopant amount (Supplementary Fig. S6).

Figure 2
figure 2

Simplified energy diagram of Nd3+ ions (a); the representative Stokes (b) and anti-Stokes (c) emission spectra upon 808 nm excitation of the NaYF4:1%Nd3+ nanoparticles; the luminescence decay profiles of the 4F3/2 state (measured for 4F3/2 → 4I11/2 transition at 123 K) (d); the τavr monitored at 123 K as a function of Nd3+ concentration (e); and the representative excitation spectrum of NaYF4:1%Nd3+ nanoparticles measured at 123 K (f).

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To understand how the temperature changes affect the spectroscopic properties of the NaYF4:Nd3+ nanoparticles their emission spectra in both anti-Stokes (Fig. 3a) and Stokes (Fig. 3b) part of spectra were analyzed as a function of temperature in the range of 83–423 K (Supplementary Fig. S7). The representative spectra presented in Fig. 3a reveal that the intensity of the 4F7/2,4S3/2 → 4I9/2 band increases gradually at elevated temperatures which can be understood since 4F7/2,4S3/2 state is thermally coupled with 4F3/2 and its population increases with temperature. On the other hand, the emission intensity of the 4F3/2 → 4I9/2 emission band decreases as a consequence of the reduction of the 4F3/2 state population via two effects: (i) its nonradiative depopulation and (ii) thermalization of the upper laying 4F5/2, 2H9/2 state (followed by thermalization of the 4F7/2,4S3/2 state). The analysis of the thermal dependence of the integrated emission intensity of this band for different concentrations of dopant ions reveals that it is strongly affected by the Nd3+ amount (Fig. 3c). In the case of low Nd3+ concentration an increase in temperature results in almost threefold enhancement of the integrated intensity of this band. However when the concentration increases the rate of thermal enhancement gradually reduces up to NaYF4:5%Nd3+ for which only a bare change in emission intensity was observed. For higher Nd3+ amounts the opposite thermal dependence was found and the strongest thermal quenching of 4F3/2 → 4I9/2 emission intensity was found for NaYF4:75%Nd3+. As stated above the Stokes emission of Nd3+ is expected to be quenched at elevated temperature. The thermal enhancement of the intensity of this band is a consequence of the excitation wavelength used (λexc = 808 nm). Although, this is commonly used optical excitation for Nd3+ doped phosphors in the case NaYF4:Nd3+ the maximum of the 4I9/2 → 4F5/2, 2H9/2 absorption band is slightly shifted toward blue with the maxima around 796 nm (Supplementary Fig. S2). Hence the λexc = 808 nm reached the sideband of this band. An increase in temperature results in a broadening of the absorption band and thus more efficient absorption of the incident light, resulting in an increase in emission intensity (Supplementary Fig. S8). When the concentration of the Nd3+ increases the spectral broadening of this absorption band can be found (Supplementary Fig. S9) and excitation wavelength is efficiently absorbed already at low temperature. Thus the thermal broadening of the absorption band does not affect so strongly the thermal dependence of integral emission intensity. It is worth noticing that the optimization of dopant concentration enables the counteraction of these two processes and achieves almost thermally stable 4F3/2 → 4I9/2 luminescence of Nd3+ ions in NaYF4:Nd3+ nanoparticles. In the case of the 4F7/2,4S3/2 → 4I9/2 luminescence less spectacular dopant effects were observed (Fig. 3d). Independently of dopant concentration and enhancement in the emission intensity was found. However, for a higher Nd3+ amount, the enhancement was slightly lower due to the previously described depopulation of the 4F3/2 state. The difference in the thermal change of the anti-Stokes and Stokes part of the spectrum enables the development of the ratiometric luminescence thermometer in which the luminescence intensities ratio (LIR) is considered as a thermometric parameter:

$$LIR = \frac{{\int_{{720\,{\text{nm}}}}^{{740\,{\text{nm}}}} {Nd^{3 + } } \{^{4} F_{7/2} ,^{4} S_{3/2} \to^{4} I_{9/2} \} d\lambda }}{{\int_{{870\,{\text{nm}}}}^{{900\,{\text{nm}}}} {Nd^{3 + } } \{^{4} F_{3/2} \to^{4} I_{9/2} \} d\lambda }}.$$
(3)
Figure 3
figure 3

The anti-Stokes (a); and Stokes (b) parts of emission of NaYF4:1%Nd3+ nanoparticles upon 808 nm excitation; the impact of the concentration of Nd3+ ions on thermal evolution of normalized integral emission intensities of 4F3/2 → 4I9/2 (c) and 4F7/2,4S3/2 → 4I9/2 (d) emission bands of NaYF4: Nd3+ nanoparticles; the temperature dependent LIR values of NaYF4:Nd3+ nanoparticles (e); the corresponding SR (f).

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Due to the meaningful impact of dopant concentration on the thermal dependence of 4F3/2 → 4I9/2 and 4F7/2,4S3/2 → 4I9/2 emission bands the LIR is strongly affected by the Nd3+ amount (Fig. 3e). Only above 25% Nd3+ the monotonic thermal dependence of LIR in the whole analyzed temperature range can be found, whereas for lower dopant amount increase of LIR is followed by its reduction at elevated temperature. Only in the case of the NaYF4:1%Nd3+ the opposite thermal dependence was found. The change in the thermal monotonicity of the thermometric parameters reduces the temperature range in which a given thermometer can be applied. This is due to the fact that a reliable temperature readout can be provided when the given value of the parameter LIR can be unequivocally assigned to a given temperature. The quantification of the observed thermal changes in LIR can be performed by the relative sensitivity calculation using the following equation:

$$S_{R} = \frac{1}{LIR}\frac{\Delta LIR}{{\Delta T}}100\% ,$$
(4)

where ΔLIR represents the change of LIR corresponding change of temperature by ΔT. The SR was calculated in the temperature range in which an increase of LIR was observed (Fig. 3f). As can be seen the higher values of the NaYF4:2%Nd3+ reaching around SR = 1.1%/K at 410 K. For a higher dopant amount the reduction of the maximal SR was observed. However, it should be noticed here that in the case of the nanoparticles with a high Nd3+ amount (> 5%) the SR reached higher values at temperatures below 250 K comparing to a low dopant counterpart. The repeatability of the LIR readout within a several heating–cooling cycles was also confirms high accuracy of temperature readout (Supplementary Fig. S10). It should be also mentioned that the particle size may affect the thermometric properties of luminescence thermometers. However, as shown in the previously published studies38, the dopant concentration plays a far more important role than particle size. The clear correlation between the dopant concentration and the thermometric parameters of NaYF4:Nd3+ is a clear confirmation of this hypothesis.

Depending on the requirement of the particular application different thermometric parameters should be considered. To facilitate this the maximal SR, usable temperature range (UTR) and the temperature determination uncertainty (δT) of the luminescence thermometer based on anti-Stokes to Stokes LIR in NaYF4:Nd3+ with different concentrations of Nd3+ ions were analyzed (Fig. 4). The SR max increases monotonically with dopant concentration up to NaYF4:2%Nd3+ followed by the gradual reduction of its value (Fig. 4a). Above 25%Nd3+ the SR remains almost independent of dopant concentration at around SR = 0.4%/K. Actually, this value of the SR is relatively high comparing the ratiometric thermometer based on R1 and R2 emission lines of Nd3+ (SR ~ 0.1–0.2%/K)17, 27, 47, 48, however, lower than the SR for the LIR of the 4F7/2,4S3/2 → 4I9/2 to 4F5/2, 2H9/2 → 4I9/2 emission bands29,30,31,32,33, 49, 50. It is evident that although the SR decreases with dopant concentration the usable temperature range simultaneously is extended. While the NaYF4:0.1%Nd3+ can be applied only in the 290–423 K temperature range the increase of Nd3+ above 25% results enables to extend of the UTR to 123–423 K. Therefore depending on the requirement of application these two parameters should be appropriately balanced. It should be noted that although some of the luminescent thermometers reveal high relative sensitivity their low emission intensity results in a low signal-to-noise ratio and thus in the high uncertainty of LIR determination (δLIR/LIR). The calculations of temperature determination uncertainty (δT) performed for NaYF4:Nd3+ as follows:

$$\delta T=\frac{ 1}{{S}_{R}}\frac{\delta LIR}{\text{LIR}}100\%,$$
(4)

reveal that the lowest δT ~ 2 K in the whole analyzed temperature range was found for NaYF4:25%Nd3+. Although for a higher concentration of Nd3+ very similar SR was achieved the reduced emission intensity, especially of the Stokes emission significantly affects the δT.

Figure 4
figure 4

The maximum relative sensitivity (a); an usable temperature range (UTR) (b); and temperature determination uncertainty (c) for different Nd3+ concentration in NaYF4:Nd3+ nanoparticles.

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Conclusions

In this work, the development of a ratiometric luminescence thermometer based on the intensities ratio of Stokes to anti-Stokes emission in NaYF4:Nd3+ was described. For this purpose, the effect of Nd3+ ion concentration on the temperature variation of 4F3/2 → 4I9/2 and 4F7/2, 4S3/2 → 4I9/2 band luminescence intensity was investigated. As shown due to the spectrally narrow 4I9/2 → 4F5/2, 2H9/2 absorption band, an increase in 4F3/2 → 4I9/2 emission intensity was observed for low concentrations of dopant ions when using the commercially used 808 nm excitation. An increase in the concentration of Nd3+ ions and the associated broadening of the absorption band caused compensation for this effect, and above 5%Nd3+ the intensity of this band decreased with increasing temperature. On the other hand, the temperature dependence of the 4F7/2,4S3/2 → 4I9/2 band reveals only slight dopant effect indicating that this emission generation process is mainly single-ion in nature and related to the thermalization of the 4F/2,4S3/2 level from the 4F5/2,2H9/2 and 4F3/2 levels. As a result, the highest relative sensitivity of SR = 1.1%/K was recorded for NaYF4:0.1%Nd3+. Above 25%Nd3+ SR remains almost independent of dopant concentration SR ~ 0.4%/K. However, in contrast to what was observed for low concentrations of Nd3+, for high concentrations of Nd3+, the LIR increased monotonically over the full range of temperatures analyzed, significantly widening the useful temperature range over which such a thermometer can be used. In summary, luminescence thermometers based on LIR of Stokes to anti-Stokes emission in NaYF4:Nd3+ nanoparticles are characterized by attractive thermometric properties whose thermometric performance can be modulated by the concentration of Nd3+ ions.