Rational construction of luminescent Eu-doped Y-MOF for ratiometric temperature sensing

Abstract

Introducing lanthanide(III) ions into a MOF structure is one of the most effective strategies to construct luminescent MOFs with multiple emission centers for fluorescent applications. In this work, a functionalized Eu³⁺-doped Y-MOF (Eu@SNNU-325) was constructed by using a cation exchange strategy. The photoluminescence result shows that Eu@SNNU-325 exhibits a unique emission spectrum, namely, the absence of the organic ligand peak and the very strong Y³⁺/Eu³⁺ characteristic peaks. Interestingly, the smart luminescent Eu@SNNU-325 as a ratiometric thermometer for temperature sensing has good self-calibrated ability and a high maximum relative sensitivity (S_m) value (1.2% K⁻¹ at 260 K). This work presents the construction of a smart Eu³⁺-functionalized Y-MOF thermometer through a cation exchange strategy, providing a good idea for the future development and design of Y-MOF thermometers.

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Introduction

Luminescent metal–organic frameworks (LMOFs), as an important class of materials, have advantages for application in light-emitting diodes (LEDs), fluorescent sensors, and bioimaging, etc.^1–15 Generally, most luminescent MOFs with single emission centers exhibit many disadvantages, such as low sensitivity, poor accuracy, and low responsiveness, which limits their widespread application.^16–20 It is well known that the luminescent MOFs with lanthanide ions centers have attracted much research interest owing to their designability, high sensitivity, fast response, and high accuracy.^21–23 Therefore, the rational design of lanthanide ions based-MOFs (Ln-MOFs) is still a great challenge.

In recent years, the luminescent Ln-MOF thermometers have been widely studied due to their distinct characteristic emissions, wide temperature detection range and strong anti-interference abilities.^24–30 On the other hand, when constructing Ln-MOFs, selecting appropriate organic ligands as antenna chromophores plays a key role in sensitizing Ln³⁺ ion emissions. However, MOF materials based on rare earth Y³⁺ ions are still rare.

To our knowledge, a highly ultra-stable yttrium-MOF (SNNU-325, SNNU = Shaanxi Normal University) consists of the Y³⁺ ions, π-conjugated ligands, large octahedral cages, and quadrilateral channels, which can be potentially applied in the field of fluorescence sensing.³¹ This yttrium-MOF (Y-MOF) has been reported by Zhai's group, and its crystal structure, characterization, and gas adsorption/separation performance have been studied. However, the fluorescence performance of this Y-MOF has not yet been studied. Based on the structural characteristics of SNNU-325, we speculate that this MOF constructed from Y³⁺ ions and rich π-conjugated aromatic ligands should exhibit excellent unique fluorescence properties.

Herein, we have successfully synthesized this Y-MOF (SNNU-325) and further studied its structural characteristics. The result of elemental mappings demonstrates that Sc³⁺ ions are uniformly dispersed on the MOF. The presence of Sc³⁺ ions in the pores provides technical support for introducing lanthanide(III) ions with strong luminescent properties via a cation exchange strategy (Scheme 1). More significantly, the luminescent Eu@SNNU-325 as a ratiometric thermometer not only exhibits the Y³⁺/Eu³⁺ characteristic peaks but also shows good fluorescence temperature sensing over a wide range of temperature from 80 K to 260 K.

Scheme 1 Schematics illustrating the cation exchange strategy to construct a functionalized Eu-doped Y-MOF.

Experimental

Synthesis of SNNU-325

A mixture of the ligand H₃TATB (2,4,6-tris(4-carboxyphenyl)-1,3,5-triazine) (12 mg, 0.027 mmol), Y(NO₃)₃·6H₂O (12 mg, 0.03 mmol), Sc(NO₃)₃·6H₂O (16 mg, 0.07 mmol) and 2-FBA (2-fluorobenzoic acid) (173 mg, 1.23 mmol) was ultrasonically dissolved in DMF (N,N-dimethylformamide) (6 mL)/H₂O (0.9 mL) solvent system in a glass vial (20 mL). The mixed solution was placed in an oven preheated at 90 °C for 5 days and then cooled to room temperature. After that, the colorless cubic block crystals were obtained by filtering and washing the fresh DMF and EtOH, and then dried at 50 °C.

Synthesis of Eu@SNNU-325

The above obtained Y-MOF (50 mg) and EuCl₃·6H₂O (50 mg) were dispersed in DMF (5 mL) and then placed in an oven at 75 °C for 24 h. The obtained Eu@SNNU-325 sample was obtained by washing several times with fresh DMF and then dried in an oven at 50 °C.

Results and discussion

Characterizations of SNNU-325 and Eu@SNNU-325

The preparation of SNNU-325 was reported in the literature.³¹ The PXRD peaks of as-synthesized SNNU-325 are in good agreement with the simulated peaks from the crystal structure data (Fig. 1). The author pointed out in the original paper that SNNU-325 cannot be prepared without the addition of Sc(NO₃)₃·6H₂O. After careful analysis of this Y-MOF structure, we speculate that Sc³⁺ ions not only play a structural guiding role in the formation of MOF and but also as cations exist within the MOF framework. To prove our hypothesis, we first soaked the as-synthesized Y-MOF in methanol for 4 days, and then conducted EDS testing on the as-synthesized and the methanol-treated sample, respectively (Fig. S1 and S2†). The EDS results show that the atomic ratios of Sc/Y significantly decrease from 1.99 to 0.35 in the as-synthesized sample and the methanol-treated sample. The PXRD peaks of the methanol-treated sample are well-matched with the as-synthesized sample (Fig. 1). Through the methanol-treated sample for 4 days, it can be clearly observed that the Sc³⁺ ions only decrease in the MOF framework, but cannot be completely removed. This phenomenon indicates that there is a strong interaction force between Sc³⁺ ions within the MOF framework. Therefore, the presence of Sc³⁺ ions within the MOF framework encourages us to design Eu³⁺ doped Y-MOFs by using a cation exchange strategy.

Fig. 1 PXRD patterns of the simulated, as-synthesized SNNU-325, methanol-treated SNNU-325 and Eu@SNNU-325.

The mixture of as-synthesized Y-MOF and EuCl₃·6H₂O under DMF solvent was further employed to construct Eu-functionalized Y-MOFs by post-synthetic modification strategy. By comparing the powder X-ray diffraction (PXRD) peaks (Fig. 1), the peaks of Eu@SNNU-325 remain highly consistent with that of the as-synthesized sample, indicating that this Y-MOF still maintains its crystalline structure after the incorporation of Eu³⁺ ions into the framework. As shown in Fig. 2a and b, the SEM images show that the morphologies of Eu³⁺ remain consistent before and after being introduced into the MOF framework. The elemental mapping images clearly show the homogeneous distribution of C, N, O, F, Cl, Y, Sc and Eu elements in the Eu@SNNU-325 (Fig. S3†), indicating that the Eu³⁺ ions were incorporated into the MOF framework through the Sc³⁺ ions exchange process. Notably, the presence of F or Cl elements in the Eu@SNNU-325 are originated from the raw materials (2-FBA and EuCl₃·6H₂O). Moreover, the FT-IR spectra of SNNU-325 and Eu@SNNU-325 are basically consistent, further indicating that the incorporation of Eu³⁺ ions not change the original structure (Fig. 2c). To demonstrate whether Eu³⁺ has exchanged Y³⁺ ions in the skeleton, X-ray photoelectron spectroscopy (XPS) spectra of SNNU-325 and Eu@SNNU-325 were recorded. The results show that the O 1s XPS spectra of SNNU-325 and Eu@ SNNU-325 remain consistent, indicating that Eu³⁺ ions are not bonded to oxygen atoms in the MOF skeleton, and also indicating that Eu³⁺ ions do not exchange Y³⁺ ions in the skeleton (Fig. 2d).

Fig. 2 SEM images of SNNU-325 (a) and Eu@SNNU-325 (b). (c) FT-IR spectra of SNNU-325 and Eu@ SNNU-325. (d) The O 1s XPS spectra of SNNU-325 and Eu@ SNNU-325.

Photoluminescence study

Considering the unique Y³⁺ ion characteristic, rich π-conjugated organic ligand, and the very obvious Eu³⁺ emission bands, we measured the solid-state photoluminescence (PL) spectra of SNNU-325 and Eu@SNNU-325 at room temperature. As shown in Fig. 3a, the emission spectrum of SNNU-325 shows two characteristic emission centers (λ_ex = 355 nm), one at 435 nm belonging to ligand emission,³² and the other at 488, 544, 578, 592, 618, 651 and 699 nm belonging to Y³⁺ ion characteristic emission.³³ However, the emission spectrum of Eu@SNNU-325 is consistent with that of SNNU-325, except for the disappearance of the emission peak of the TATB³⁻ ligand (λ_ex = 300 nm) (Fig. 3b). It can be clearly observed that the absence of the emission band (at about 435 nm) arising from the ligand, indicating that the ligand emission process in Eu@SNNU-325 should be suppressed by the energy transfer from the ligand to the Eu³⁺ and Y³⁺ ions.^34,35 Under UV-light irradiation, SNNU-325 displays dark purple visible emission, and Eu@SNNU-325 displays red visible emission (insets in Fig. 3b). The above results demonstrate that the emission spectrum of Eu³⁺ doped Y-MOFs has not changed compared to that of Y-MOF, but affect the energy transfer between the organic ligand and Eu³⁺ and Y³⁺ ions, resulting in Eu@MOF displaying strong red emission of Eu³⁺. This phenomenon is very rare in other Eu³⁺-doped MOF systems.

Fig. 3 Excitation and emission spectra of SNNU-325 (λ_ex = 353 nm) (a) and Eu@SNNU-325 (λ_ex = 300 nm) (b) (insets show the photographs under 254 nm UV light).

Temperature-dependent emission properties

The significant luminescence characteristics of Y³⁺/Eu³⁺ ions in the Eu@SNNU-325 inspired us to evaluate its application in the field of low-temperature sensing. The temperature-dependent fluorescence performance of Eu@SNNU-325 was studied over a wide range of temperature from 80 K to 300 K. As shown in Fig. 4, the luminescence intensities in the Eu@SNNU-325 gradually decrease of both I₆₁₈ and I₅₄₄ with the increasing temperature under excitation at 300 nm, which can be mainly attributed to the thermal deactivation of the Y³⁺/Eu³⁺ ions. As a ratiometric luminescent thermometer, the intensity ratio of I₆₁₈/I₅₄₄ (Δ) of Eu@SNNU-325 was used to evaluate its temperature sensing performance. The Boltzmann equation (Δ = (A₁ − A₂)/(1 + exp((T − T₀)/dT)) + A₂) was used to fit the relationship between Δ and the a wide temperature range (80–260 K). Interestingly, the result shows that Eu@SNNU-325 exhibits a good fit (R² = 0.9946) (Fig. 5a, and Table S1†).

Fig. 4 Temperature-dependent emission spectra of Eu@SNNU-325 recorded in the range of 80–300 K.

Fig. 5 (a) Temperature-dependent emission intensity ratio Δ (I₆₁₈/I₅₄₄) and the fitted curve of Eu@SNNU-325 from 80 to 260 K. (b) Temperature-dependent relative sensitivity (S_r) of Eu@SNNU-325.

To further evaluate the sensing sensitivity, the relative sensitivity (S_r, S_r = |(∂Δ/∂T)/Δ|) was used as an important index. Because S_r represents an intrinsic nature of the thermometer. As a result, the maximum relative sensitivity (S_m) was determined to be 1.2% K⁻¹ at 260 K (Fig. 5b), which was comparable to that of the other reported luminescent MOF thermometers such as Tb_0.80Eu_0.20(bpda) (1.39),³⁶ ZJU-88⊃perylene (1.28)³⁷ and Nd_0.577Yb_0.423BDC-F₄ (1.2).³⁸ The above results show that Eu@SNNU-325 can be used as a smart thermometer for self-calibrating temperature sensing.

Conclusions

In summary, a functionalized Eu-doped Y-MOF (Eu@SNNU-325) has been constructed by the cation exchange strategy. The obtained Eu@SNNU-325 was characterized by combining the PXRD, SEM, XPS, FT-IR, and EDX mapping. Due to the introduction of Eu³⁺ ions into the Y-MOF framework, Eu@SNNU-325 exhibits the characteristic emission centers of Y³⁺/Eu³⁺ ions. Moreover, the smart luminescent Eu@SNNU-325 exhibits highly sensitive temperature sensing over a wide range of temperature from 80 K to 260 K. Most importantly, the maximum relative sensitivity (S_m) of Eu@SNNU-325 reached 1.2% K⁻¹ at 260 K, indicating that it can be applied as a temperature transfer material in practical applications. This work provides a powerful and effective synthesis strategy for constructing Eu-doped Y-MOFs and applying a field of fluorescence temperature sensing.

Data availability

Data are available upon request from the authors.

Author contributions

Wei Wei: experiment instruction, methodology. Xi Li: data curation. Yong-Ya Zhang: software. Jian-Wei Zhang: writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22301172 and 52472222) and the Science and Technology Innovation Talents in Universities of Henan Province (22HASTIT028).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra05796a

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