Rare-earth UiO-66 for temperature sensing near room temperature

Abstract

In the last decade, rare-earth metal–organic frameworks (RE-MOFs) have received notable interest in luminescence thermometry, which involves correlating the absolute temperature of an object to an optical emission. Numerous investigations that are already reported deal mainly with mixed Eu/Tb coordination polymers and MOFs, and they are based on the Eu³⁺-to-Tb³⁺ emission ratio between the transitions ⁵D₄→⁷F₅ and ⁵D₀→⁷F₂ of the Tb³⁺ and Eu³⁺ ions, respectively. In this study, we focus our attention on the well-known UiO-66, which can be also built with lanthanoid cations to give RE-UiO-66 analogues. Thus, we investigated the thermometric properties of RE-UiO-66 by synthesizing three different bimetallic Eu/Tb materials, with the aim to develop new sensitive luminescent thermometers.

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Introduction

Temperature is a thermodynamic parameter that is crucial to monitor for various domains in fundamental or industry research.¹ Compared to conventional thermometry with contact detection, luminescence thermometry has unique and distinct advantages including remote detection, fast response times, high accuracy, and non-invasive character while presenting high spatial resolution (typically submicron scale) where traditional methods are lacking or ineffective.^2–4 Among the diverse methods used for RE³⁺-based luminescence thermometry, one of the most robust methodologies relies on the measurement of the intensity of two transitions of distinct RE³⁺ emitting centers.^2,5–7 Metal–organic frameworks (MOFs) are a class of coordination polymers built up from metallic nodes and organic linkers that have attracted great interest due to their wide range of possible applications,^8–12 such as in catalysis¹³ or gas separation.¹⁴ Among the various types of MOFs, rare-earth MOFs (RE-MOFs) are a subclass developed to exploit, among other things, the unique luminescence properties of RE elements, rendering them very attractive for lighting,^15–17 chemical sensing,^18–21 and luminescence thermometry.^{2,5,6,22–25} Using Eu-Tb bimetallic RE-MOFs, the temperature of the surrounding environment can thus be determined from the ratio between the intensity of the ⁵D₄→⁷F₅ and of the ⁵D₀→⁷F₂ transitions of Tb³⁺ and Eu³⁺, respectively.^7,26,27

Zr-UiO-66 has been extensively investigated for its robust structure, high surface area, and porosity.^28–31 Recently, RE analogues of UiO-66 were designed and synthesized,³² comprising RE₆–cluster nodes bridged by 1,4-benzene-dicarboxylate linkers (BDC²⁻). We previously studied the luminescent properties of some mixed-metal RE-UiO-66 analogues, particularly the white light emitting behaviour of the trimetallic Tb:Gd:Eu-UiO-66.¹⁶ Herein, we focus our attention on three different bimetallic Eu-Tb RE-UiO-66 analogues and their potential as luminescent thermometers sensitive near room temperature.

Results and discussion

Discovered in 2008, Zr-UiO-66³³ is one of the most studied MOFs in the literature due to its permanent porosity, high surface area, and thermal and chemical stabilities. The typical UiO-66 MOF is comprised of 12-connected hexanuclear zirconium clusters [Zr₆O₄(OH)₄]¹²⁺ bridged by BDC²⁻ linkers, giving rise to a framework with fcu topology that features accessible pores with apertures of approximately 6 Å. Nearly 13 years after its initial discovery, a method for the synthesis of RE analogues of UiO-66 (Fig. 1) was reported in the literature.³² In this initial study, eight new RE-MOFs (RE = Y³⁺, Yb³⁺, Tm³⁺, Er³⁺, Ho³⁺, Tb³⁺, Gd³⁺, and Eu³⁺) were obtained through a solvothermal method, where N,N-dimethylacetamide (DMA) is used as a solvent and 2,6-difluorobenzoic acid (2,6-dFBA) plays the role of modulator in the reaction. It was proposed that the final material presents a chemical formula of [(CH₃)₂(NH₂)]₂[RE₆X₈(BDC)₆] (X = OH⁻32 and later discovered to also contain F⁻).³⁴ Different from the typical Zr-UiO-66 that is neutral in charge, RE-UiO-66 is anionic in nature, requiring the presence of counterions for charge balancing purposes, dimethylammonium in this case. This, however, does not seem to impact the most prominent properties of the material, as RE-UiO-66 displays permanent porosity, high surface areas (> 1000 m² g⁻¹), and chemical and thermal (> 400 °C) stabilities. More recently, new research involving the RE-UiO-66 platform has expanded our knowledge regarding these materials. In one example, two new RE-UiO-66 analogues were reported (Lu- and Sm-UiO-66) and a series of single crystals were synthesized and analysed to relate structural trends to thermal stability in ten RE-UiO-66 analogues.³⁵ In another study, the RE-UiO-66 platform was modified to generate new bimetallic and trimetallic MOFs containing Tb³⁺, Gd³⁺, and Eu³⁺, giving rise to a material with white-light emitting properties.¹⁶

Fig. 1 Structure of the RE-UiO-66 MOF demonstrating that it consists of RE₆ clusters and H₂BDC linkers, giving rise to a framework with fcu topology. It should be noted that some of the μ₃-OH ligands in the cluster are likely to be μ₃-F.

In the present study, three new bimetallic RE-UiO-66 analogues (RE = Tb³⁺ and Eu³⁺) are synthesized and named according to the content of Tb³⁺ and Eu³⁺ in the hexanuclear clusters of the MOFs: Tb_5.94Eu_0.06-UiO-66, Tb_5.82Eu_0.18-UiO-66, and Tb_5.58Eu_0.42-UiO-66. As can be seen in Fig. 2a, all Bragg reflections of the new bimetallic MOFs match with the simulated pattern of Tb-UiO-66, confirming the expected UiO-66 structure with fcu topology. N₂ sorption measurements were collected (Fig. 2b) and the Brunauer–Emmett–Teller (BET) surface area of the MOFs calculated. Tb_5.94Eu_0.06-UiO-66, Tb_5.82Eu_0.18-UiO-66, and Tb_5.58Eu_0.42-UiO-66 display the expected reversible type I isotherms with surface areas of 1090, 1050, and 1090 m² g⁻¹, being in accordance with the literature value for Tb-UiO-66.³²

Fig. 2 (a) PXRD patterns of simulated Tb-UiO-66 and bimetallic Tb/Eu-UiO-66 MOFs and (b) N₂ sorption isotherms of the bimetallic Tb/Eu-UiO-66 MOFs.

Scanning electron microscopy (SEM) images (Fig. S1–S3, ESI†) demonstrate that all MOFs have the expected octahedral morphology but in agglomerates of different sizes. Energy dispersive X-ray spectroscopy (EDS) mapping (Fig. S1–S3, ESI†) was performed to gain a better understanding of the distribution of RE metals in the crystals. In all cases, a homogeneous distribution of Tb³⁺ and Eu³⁺ is observed across the crystal agglomerates. Interestingly, the qualitative atomic ratio obtained from EDS data gives almost the exact ratio of Tb:Eu used in the synthesis of these materials (Table S1, ESI†): 98.86 and 1.14% for Tb³⁺ and Eu³⁺, respectively, in Tb_5.94Eu_0.06-UiO-66; 97.09 and 2.91% for Tb³⁺ and Eu³⁺, respectively, in Tb_5.82Eu_0.18-UiO-66; and 93.09 and 6.97% Tb³⁺ and Eu³⁺, respectively, in Tb_5.58Eu_0.42-UiO-66. To confirm these results, inductively coupled plasma mass spectrometry (ICP-MS) measurements were conducted on digested MOF samples. Similar results to the EDS atomic ratio analysis are obtained (Table S1, ESI†), with Tb_5.94Eu_0.06-UiO-66 displaying a ratio of 99.06 : 0.94 ± 0.12% (Tb³⁺:Eu³⁺), Tb_5.82Eu_0.18-UiO-66 displaying a ratio of 97.43 : 2.57 ± 0.04%, and Tb_5.58Eu_0.42-UiO-66 displaying a ratio of 93.99 : 6.01 ± 0.14%.

The photophysical properties of Tb_6−xEu_x-UiO-66 were thoroughly investigated at room temperature. First, their emission spectra (Fig. 3a) display the characteristic green and red luminescence of Tb³⁺ and Eu³⁺ ions, respectively, with the typical lines at 490, 545, 581, and 619 nm attributed to the Tb^{3+ 5}D₄→⁷F₆₋₃ transitions, and at 587, 611, 652, and 695 nm for the Eu^{3+ 5}D₀→⁷F₁₋₄ transitions. Furthermore, the emission bands of the BDC²⁻ linker are totally absent in the luminescence spectra, proving the efficient sensitization of Ln³⁺ ions by the organic linker. In the literature,^16,36 the lowest-lying triplet energy state for BDC²⁻ is estimated to be around 25 600 cm⁻¹, which is perfectly suitable to sensitize Eu³⁺ and Tb³⁺. Increasing the Eu³⁺ content, the emission color slightly shifts from pure green emission (related to pure Tb-UiO-66) to orange emission, as evidenced by the CIE chromaticity diagram (Fig. 3b, Table S2, ESI†). In mixed Eu-Tb-UiO-66, the emission color change is often emphasized by the occurrence of a Tb³⁺-to-Eu³⁺ energy transfer, which can be evidenced by the presence of ⁷F₆→⁵D₄ Tb³⁺ transition (at 483 nm) within the ⁵D₀→⁷F₂ Eu³⁺ transition in the excitation spectra of a mixed compounds (Fig. S4, ESI†). Emission decay times were determined at room temperature for all MOFs. A monoexponential behavior was observed for the ⁵D₄ decay curve monitored at 541 nm (Tb³⁺ transition) and for the ⁵D₀ decay curve monitored at 617 nm (Eu³⁺ transition), the decay curves and the decay times being reported in Fig. S5, and Table S3 (ESI†), respectively. The decay times are in good agreement with those reported by Ajoyan et al.¹⁶

Fig. 3 (a) Room-temperature emission spectra monitored at λ_exc = 296 nm for samples Tb_5.94Eu_0.06-UiO-66 (red line), Tb_5.82Eu_0.18-UiO-66 (green line), and Tb_5.58Eu_0.42-UiO-66 (blue line). (b) CIE chromaticity diagram of the Tb_6−xEu_x-UiO-66 samples calculated from the previous emission spectra at room temperature.

The potential application of the Tb_6−xEu_x-UiO-66 compounds as ratiometric luminescent thermometers was probed using emission spectra collected between 255–295 K. As an illustrative example, Fig. 4a presents the temperature dependent emission spectra of Tb_5.94Eu_0.06-UiO-66 (similar data for the Tb_5.82Eu_0.18-UiO-66 and Tb_5.58Eu_0.42-UiO-66 materials are reported in Fig. S6a and S7a, respectively, ESI†). The temperature dependence of the I_Tb (green) and I_Eu (red) parameters is represented in Fig. 4b. The integrated areas of the ⁵D₄→⁷F₅ (I_Tb) and of the ⁵D₀→⁷F₂ (I_Eu) emissions were used to define the thermometric parameter as Δ = I_Tb/I_Eu, I_Tb and I_Eu, obtained using the emission peaks between 532–565 nm and 605–635 nm intervals, respectively. As previously demonstrated, we assumed that the overlap of the Tb^{3+ 5}D₄→⁷F₃ and the Eu^{3+ 5}D₀→⁷F₂ transitions should not impact substantially the determined temperature dependence of I_Eu.⁶ Increasing the temperature from 255 to 295 K causes a thermal quenching of 10% for I_Tb while the intensity I_Eu increases by 35%. These moderate thermal evolutions slightly affect the colour of emission, which shifts from green to yellow, as displayed on the CIE chromaticity diagram (Fig. S8, ESI†). Similar tendencies are observed with two other samples of Tb-Eu-UiO-66, i.e., Tb_5.82Eu_0.18-UiO-66 and Tb_5.58Eu_0.42-UiO-66, although the I_Eu increase is slightly weaker (around 20%, and 30% for Tb_5.82Eu_0.18-UiO-66 and Tb_5.58Eu_0.42-UiO-66, respectively). The opposite variation of I_Tb and I_Eu indicate that the present materials are expected to be promising ratiometric self-calibrated luminescent thermometers.³⁷ Indeed, the temperature can be readily correlated to the thermometric parameter Δ = I_Tb/I_Eu, as depicted in Fig. 4c, along with Fig. S5c and S6c (ESI†) for other MOFs. A three-order polynomial curve was arbitrarily used to fit the calibration curve Δ = f(T) as the regular semi-empirical Mott–Seitz^7,38 model did not give good reliability factors. All the fitting parameters are reported in ESI† in Table S4, with reliability factors.

Fig. 4 (a) Temperature-dependent emission spectra in the 255–295 K range of Tb_5.94Eu_0.06-UiO-66 upon 296 nm excitation. (b) Corresponding dependence of the normalized integrated areas of I_Tb (⁵D₄→⁷F₅) and I_Eu (⁵D₀→⁷F₂). (c) Temperature dependence of the thermometric parameter Δ = I_Tb/I_Eu, and (d) Relative thermal sensitivity for the mixed compound. (c) Relative thermal sensitivity for the mixed compound.

The corresponding relative thermal sensitivity, defined as and used as a figure of merit to compare the performance of distinct systems^7,38,39 is plotted in Fig. 4d for Tb_5.94Eu_0.06-UiO-66, along with Fig. S5d and S6d (ESI†) for the other MOFs. Therefore, as luminescent ratiometric thermometers, the compounds Tb_5.94Eu_0.06-UiO-66, and Tb_5.82Eu_0.18-UiO-66 are highly sensitive around room temperature with a maximum relative sensitivity (S_m) equal to 4.9 and 3.4% K⁻¹ at 295 K for Tb_5.94Eu_0.06-UiO-66 and Tb_5.82Eu_0.18-UiO-66, respectively. Consequently, both materials are very competitive with the most sensitive material in the physiological range (293–313 K) so far, as can be see in Table 1. When increasing the Eu³⁺ content, a shift of the maximum temperature, where the thermal sensitivity is maximum, to lower temperatures appears. In fact, for the compound Tb_5.58Eu_0.42-UiO-66, the maximum relative sensitivity S_m is equal to 1.12% K⁻¹ at 255 K, which is 40 degrees lower than others materials. As we have highlighted in previous studies, the Eu/Tb molar ratio has an impact on relative thermal sensitivity and the operating temperature range.^6,40 However, here the increase of Eu content leads to a diminution of the relative thermal sensitivity while we obtained an opposite trend in different Eu–Tb coordination polymers.^6,40 That could be explained by the structure of RE-UiO-66, which exhibits RE₆-clusters with a minimal distance of 11 Å between them, which prevents Tb³⁺-to-Eu³⁺ energy transfer between clusters. Thus, when increasing Eu³⁺ content, the probability to obtain a RE₆-cluster totally composed of Eu³⁺ ions is increased and the Tb³⁺-to-Eu³⁺ energy transfer is limited. As this energy transfer seems to dictate the thermometric performances (as supposed by the opposite trend of I_Tb and I_Eu with temperature), the relative thermal sensitivity decreases with the increase of Eu³⁺ content. Others MOFs with cluster-based nodes should be investigated to understand this phenomenon further. With that said, this investigation demonstrates that the Eu/Tb molar ratio is a crucial parameter to tune the thermometric performance of rare-earth cluster-based MOFs.

Table 1 Comparison of the relative intensity of other reported ratiometric lanthanide Eu–Tb-based thermometers operating in physiological range. S_m corresponding to the maximum relative sensibility at the temperature T_m

Materials	S m (% K^−1) at Tm	Ref.
Eu0.95Tb0.05(BT)	1.46 at 300 K	41
TbMOF@7.3%Eu_tfac	1.33 at 325 K	42
Eu0.50Tb0.50DPA.PMO	1.56 at 360 K	43
Tb0.8Eu0.2BPDA	1.19 at 313 K	44
Tb5.94Eu0.06-UiO-66	4.9 at 295 K	This work
Tb5.82Eu0.18-UiO-66	3.4 at 295 K	This work
Tb5.58Eu0.42-UiO-66	1.12 at 255 K	This work

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Finally, temperature cycling between 255 and 295 K (Fig. S9, ESI†) demonstrates repeatability better than 95% for Tb_5.94Eu_0.06-UiO-66.

Conclusions

In conclusion, the first luminescent thermometers based on a Eu³⁺/Tb³⁺ mixed UiO-66 are reported. These compounds present a very high thermal sensitivity around room temperature with a maximum relative sensitivity of 4.9% K⁻¹ at 295 K for the most sensitive one. All reported MOFs are very competitive with Eu-Tb hybrid materials already reported in the literature.

Experimental

Synthetic procedures

All materials were synthesized following an optimized procedure previously reported by our group.³² Three different MOFs were obtained and named according to the initial amounts of Tb³⁺ and Eu³⁺ used in the synthesis. In that way, MOFs containing initial molar amounts of 1, 3, and 7% Eu³⁺ were named Tb_5.94Eu_0.06-UiO-66, Tb_5.82Eu_0.18-UiO-66, and Tb_5.58Eu_0.42-UiO-66, respectively. For that, Tb(NO₃)₃·6H₂O (0.17226, 0.16878, and 0.16182 mmol), Eu(NO₃)₃·6H₂O (0.00174, 0.00522, and 0.01218 mmol), terephthalic acid (0.171 mmol, 28.5 mg), and 2,6-difluorobenzoic acid (2.78 mmol, 440 mg) were solubilized in 8 mL of N,N-dimethylacetamide in 6-dram vials. This mixture was sonicated until a homogeneous solution was obtained. The reaction vial was then placed on a pre-heated oven at 120 °C and left undisturbed for 72 hours (3 days). The obtained precipitates were collected and washed three times with N,N-dimethylformamide over the course of 24 hours. Then, the samples were washed three times with acetone over the course of 24 hours. Finally, the materials were air dried and activated at 120 °C for 24 hours under vacuum.

Materials and methods

Powder X-ray diffraction (PXRD) patterns were obtained on a Rigaku MiniFlex 6G equipped with a Cu-target (λ = 1.54 Å) sealed-tube X-ray source operating at 40 kV/15 mA and a D/tex Ultra detector operating in a range of 3–40° (2θ) at a scan rate of 5° min⁻¹. MOF samples were activated using a Micromeritics SmartVacPrep instrument equipped with a hybrid turbo vacuum pump. Brunauer–Emmett–Teller (BET) apparent surface area measurements were determined by N₂ adsorption–desorption isotherms collected at 77 K on a Micromeritics TriStar II Plus equipment. Scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS) elemental mapping were recorded on a Phenom ProX Desktop instrument with acceleration voltage of 15 kV and using a ThermoScientific^TM Phenom charge reduction sample holder. Inductively coupled plasma mass spectrometry (ICP-MS) measurements were obtained on a 7500 Agilent 7500ce equipment. Prior to the analysis, approximately 1.0 mg of MOF sample was digested in 1.0 mL of concentrated H₂SO₄ at 100 °C in a sand bath for 3 hours. Every hour the sample was sonicated for approximately 5 minutes. The volumes of the digested samples were first adjusted to 10 mL using MilliQ water, and then further diluted 30 times. Room-temperature photoluminescence measurements (emission, excitation and lifetimes) were recorded on a Jobin-Yvon Fluorolog 3 fluorimeter equipped with a CCD camera (excitation source: 450 W Xe arc lamp). Emission spectra were corrected for detection and optical spectral response of the spectrofluorimeter and the excitation spectra were weighed for the spectral distribution of the lamp intensity using a photodiode reference detector. The temperature-dependent photoluminescence measurements were recorded on the same spectrometer, controlling the temperature by a Linkam heating stage coupled with the Fluorolog 3 by fibers.

Author contributions

Elias Djanffar – conceptualization, methodology, investigation, validation, visualization, writing-original draft; Hudson A. Bicalho – methodology, investigation, validation, visualization, writing-original draft; Zvart Ajoyan – investigation, validation; Ashlee J. Howarth – conceptualization, funding acquisition, project administration, resources, supervision, writing-review & editing; Hélène Serier-Brault – conceptualization, investigation, validation, funding acquisition, project administration, resources, supervision, writing-review & editing.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: SEM images, EDX mapping, luminescence (emission and excitation) spectra. See DOI: https://doi.org/10.1039/d4tc00781f

‡ E. D and H. A. B contributed equally to this work.

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