Dual-modulation of lanthanide valence and MOF phase transition: advancing ratiometric thermometry in Eu-doped ZIF-L films for high-sensitivity thermal sensing

Abstract

Ratiometric optical thermometry has emerged as a vital platform for non-contact temperature monitoring in industrial and scientific applications, demanding sensors with high sensitivity, non-invasiveness, and spatial resolution. Although zeolitic imidazolate framework-8 (ZIF-8) possesses attractive thermal stability and structural tunability, its limited luminescence intensity hinders practical thermometric applications. Meanwhile, ZIF-L, a two-dimensional (2D) polymorph of ZIF-8 with identical ligands, remains unexplored for temperature sensing. Herein, we propose a dual-modulation strategy synergizing lanthanide valence engineering with MOF phase control to develop a multi-emission center Eu³⁺/Eu²⁺ co-doped ZIF-L film (Eu/ZIF-L). By introducing Eu³⁺ into ZIF-8 precursors, the partial reduction of Eu³⁺ to Eu²⁺ under thermal treatment triggers a structural evolution from 3D ZIF-8 to 2D ZIF-L. The resultant Eu/ZIF-L exhibits dual emission peaks at 450 nm (Eu²⁺) and 495 nm (ligand-centered), with their intensity ratio (I₄₉₅/I₄₅₀) demonstrating remarkable linear temperature dependence over 296–373 K and achieving a relative sensitivity of 1.95% K⁻¹. This work pioneers the integration of valence-state manipulation and MOF phase transition for thermometric optimization, providing a novel paradigm to design advanced ratiometric MOF thermometers for industrial thermal management.

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1. Introduction

Temperature (T), as a fundamental physical parameter, is intrinsically linked to virtually all physical and chemical processes. Precise non-contact temperature measurement plays a crucial role in diverse scientific research and industrial production fields, demanding enhanced requirements for temperature sensor accuracy and sensitivity.¹ Luminescent thermometers, empowered by non-contact optical thermometry techniques, have achieved high spatial resolution, superior sensitivity, and remote monitoring capabilities, demonstrating promising applications in biomedical engineering, chemical synthesis, and environmental surveillance.^2,3 Furthermore, ratiometric thermometers exhibit distinctive advantages, including self-calibration capability, rapid response time, high precision, and broad applicability, establishing them as the most extensively studied and applied temperature measurement technology within optical thermometry.^4–6 Current characterization strategies for thermal sensing performance primarily involve tracking spectral shifts, fluorescence lifetime variations, intensity modulations, or fluorescence intensity ratio (FIR) evolution.

To date, diverse materials have been developed as optical ratiometric thermometers, including polymers, rare earth ion (Ln)-based phosphors, quantum dots (or nanocrystals), and metal organic frameworks (MOFs).^7–11 MOFs, with their tunable metal nodes, organic linkers, and guest species, offer vast structural diversity that creates expansive opportunities for fluorescence applications.^12–14 Lanthanide luminescent materials have emerged as ideal guest species in MOFs due to their tunable emission characteristics and versatile energy transfer processes.^15,16 By synergistically combining the luminescence of lanthanide elements (Ln) with the advantages of porous frameworks, dual/multi-emission lanthanide-incorporated MOFs (Ln-MOFs) demonstrate exceptional thermometric characteristics, showing promising potential for ratiometric thermometry applications.^17–19

Zeolite imidazole frameworks (ZIFs), a class of MOFs, are constructed from imidazolate ligands and tetrahedral metal ions (e.g., Zn²⁺ and Co²⁺) through coordination.²⁰ ZIF-8 (Zn(mim)₂) as a typical ZIF is composed of Zn²⁺ coordinated with deprotonated 2-methylimidazole (Hmim) i.e. mim- to form a 3dimensional (3D) sodalite topology (SOD). According to previous reports, ZIF-8 has been paid more attention by researchers due to its large specific surface area, high thermal stability and facile functionalization.^21–23 In addition, ZIF-L (Zn(mim)₂·(Hmim)_1/2·H₂O_3/2) with 2D structures has the same coordination structure unit as ZIF-8, whose structure is stabilized by the intertwining layers of the ligated unsaturated Hmim and free Hmim molecules through hydrogen bonding. As a structural analog of ZIF-8, ZIF-L possesses a unique layered two-dimensional structure and demonstrates excellent chemical and thermal stability, which has been extensively investigated in fields such as catalysis, gas adsorption, and fluorescent probes.²⁴ Compared with three-dimensional architectures, this two-dimensional configuration endows ZIF-L with enhanced superior structural flexibility and exposed active sites, making it an ideal candidate for dynamic photophysical applications.²⁵ Notably, while addressing the critical challenge of ZIF-8's inherent fluorescence weakness in temperature sensing applications, we propose that ZIF-L's larger specific surface area and abundant adsorption sites could significantly enhance temperature detection sensitivity, potentially expanding the practicality of MOF-based luminescent thermometers.

Herein, we developed a novel europium ion (Eu³⁺/Eu²⁺)-doped ZIF-L (Eu/ZIF-L) multi-emissive film via a facile sol–gel approach, demonstrating fluorescence intensity ratio (FIR)-based thermometric capability with exceptional potential for non-contact temperature sensing. The structural transformation and the formation process of the emission centers are illustrated in Scheme 1. The tripartite emission centers arise from the imidazolate ligands (450 nm) in the ZIF-L framework, the intense narrow-band Eu²⁺ emission (495 nm), and the subdued Eu³⁺ red luminescence (618 nm). Surprisingly, a dimensionality transition from 3D ZIF-8 to 2D ZIF-L was triggered by modulating the Eu(NO₃)₃·6H₂O concentration in the composite sol. Thermal activation induces partial reduction of Eu³⁺ to Eu²⁺, with the latter exhibiting sharpened and intensified emission attributed to the ligand-to-metal antenna effect via Eu²⁺–N coordination in the ZIF-L matrix. Crucially, across a wide operational window (296–373 K), the dual emission bands (450 nm and 495 nm) and their intensity ratio display robust linear thermal responsiveness (R² = 0.976), achieving a relative thermometric sensitivity of 1.95% K⁻¹ at 363 K. Following this, the Eu/ZIF-L film with a good reversible fluorescence thermal response performance has application prospects in ratiometric temperature-sensing.

Scheme 1 Diagram of the multi-emission center Eu³⁺/Eu²⁺ co-doped ZIF-L film.

2. Experimental sections

2.1. Materials

Zinc(II) acetate dihydrate (Zn(CH₃COO)₂·2H₂O, ≥99.9%, Shanghai China), 2-methylimidazole (Hmim, C₄H₆N₂, 99%, Aladdin), ethanol (C₂H₅OH, ≥99.7%, Shanghai China), and europium(III) nitrate hexahydrate (Eu(NO₃)₃·6H₂O, 99.9%, Macklin)were used; all reagents were directly used without further purification.

2.2. Preparation of Eu/ZIF-L films

The specific preparation process was as follows. Firstly, Hmim (0.05 mol) and zinc acetate dihydrate (0.01 mol) were mixed in 2 mL of ethanol, and then stirred at room temperature to form transparent ZIF sols. 3 mL of the ZIF sol was taken in a small bottle and 0.4 mmol Eu(NO₃)₃·6H₂O was added into the bottle with continuous stirring to form a transparent composite sol. (The concentration of Eu salts was 0.133 mol L⁻¹ serving the ZIF sol as solvents.) After that, the composite sol was coated on a clear quartz glass through a spin-coating method and the wet Eu/ZIF film was obtained. And the wet film was taken on a hot plate for heat treatment. The specific procedure of heat treatment was to hold at 70 °C for half an hour and raise the temperature to 80 °C firstly, then heat up to 90 °C at a rate of 10 °C h⁻¹ and hold for an hour. After that, the temperature was increased at a rate of 20 °C h⁻¹ to 100 °C and maintained for 5 h. Finally, the Eu/ZIF-L film was obtained. On the other hand, the amount of Eu salts in Eu/ZIF-L films could be controlled by changing the concentration of Eu(NO₃)₃·6H₂O in the composite sol.

2.3. Characterization

The phase structure of the film samples was characterized using an X-ray diffractometer (GIXRD, Empyream) with grazing incidence. Its X-ray source was 4 kW Cu Kα (λ = 1.5418 Å) radiation, with a scanning speed of 0.02° 2θ s⁻¹ and a test range of 5°–60°. A field emission scanning electron microscope equipped with X-Max 50 X-ray energy dispersive spectrometer (FESEM-EDS, Zeiss Ultra Plus) was used to analyze the morphology and microstructure of the prepared films. Its secondary electron resolution was 1.0 nm (15 kV) and 1.2 nm (1 kV), the acceleration voltage was 0.02–30 kV, and the resolution of the energy dispersive spectrometer at Mn Kα was better than that at 127 eV. A Fourier transform infrared spectrometer (FTIR, Nicolet 6700) was used to detect the characteristic infrared vibration peaks in the microstructure of the films to observe the changes of the film structure. The detection wavenumber range was 4000–400 cm⁻¹ (mid-infrared region), with a fast scanning speed of 1 time per second and a maximum resolution of 0.019 cm⁻¹. The changes in the surface structure of the films were tested by X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, ThermoFisher), with an energy scanning range of 0–5000 eV. The UV-visible absorption spectra of the films were tested by UV-visible-NIR spectrophotometer (UV-3600, SHIMADZU), with a test wavelength range of 300–2500 nm and a scanning step of 1 nm. At room temperature, the photoluminescence (PL) and photoluminescence excitation (PLE) spectra of all films were recorded using a fluorescence spectrophotometer (TRFS, QM/TM/NIR, USA). The steady-state part of its light source was a 75 W xenon lamp; the transient part was a tunable picosecond laser, with a signal-to-noise ratio of 6000 : 1 (up to 10 000 : 1). The thermal response fluorescence of the films was detected using thermal sensing accessories, which could record the temperature-dependent fluorescence spectra of the film samples and calculate the corresponding relative sensitivity. The fluorescence decay curves of the samples were measured using a time-resolved fluorescence spectrometer (Newport, USA) with a resolution of 100–400 fs, and the fluorescence lifetime and time resolution were obtained by fitting the test results.

3. Results and discussion

3.1. Structural characterization

We analyzed the physical structure of the obtained ZIF films before/after Eu-modified, as shown in the XRD pattern of Fig. 1. It can be seen that the film prepared with the ZIF sol without Eu(NO₃)₃·6H₂O was ZIF-8, consistent in the CCDC data base numbered CCDC 602542.^26,27 However, after adding Eu(NO₃)₃·6H₂O into the ZIF sol, the ZIF-L (CCDC 1509273) film was obtained,²⁸ indicating that the addition of Eu ions promoted the transformation from ZIF-8 to ZIF-L. (The chemical formulas of ZIF-8 and ZIF-L are Zn(mim)₂ and Zn(mim)₂·(Hmim)_1/2·H₂O_3/2, respectively, in which mim represents deprotonated 2-methylimidazole.²⁹) The possible reason for this phenomenon was a small amount of Eu ions coordinated with Hmim resulting in changes of the growth direction of ZIFs from three dimensional (3D) ZIF-8 structures to two dimensional (2D) ZIF-L.

Fig. 1 XRD patterns of the ZIF-8 film without Eu and the Eu/ZIF-L film.

Based on the diffraction data in Fig. 1, we calculated the lattice parameters of ZIF-8 (a = b = c = 17.03 Å, β = 90°) and the lattice parameters of ZIF-L with Eu (a = 24.00 Å, b = 17.68 Å, c = 19.8 Å, β = 90°), these calculated results agree well with the standard cards (CCDC 602542, a = b = c = 16.99 Å; CCDC 1509273, a = 24.12 Å, b = 17.06 Å, c = 19.74 Å) respectively.

In order to observe the morphologies of the ZIF-8 film and the Eu/ZIF-L film, FESEM characterization was carried out as shown in Fig. 2. It can be seen from Fig. 2a that there were particles with sizes of ∼10 μm on the surface of the ZIF-8 film and the morphology of particles was cubes without sharp corners. In contrast, the thickness was ∼3.34 μm (Fig. 2c). In Fig. 2b, a lot of well-defined square blocks with a relatively uniform size of ∼4 μm were observed, speculating that the shape was formed due to the stacking of lamellar structures. Numerous lamellar structures were stacked irregularly together and the thickness was up to 4.31 μm (Fig. 2d), it further indicated that the transformation from 3D ZIF-8 to 2D ZIF-L can be achieved through introducing Eu salts into the ZIF sol. As expected, EDS results (Fig. 2e and f) showed that Eu elements existed in the ZIF-L film with an obviously increased relative intensity of O elements compared to the ZIF-8 film, suggesting that the Eu/ZIF-L film was successfully synthesized.

Fig. 2 Top-view (a and b) and side-view (c and d) FESEM images and EDS spectra (e and f) of the ZIF-8 film (a, c and e) and the Eu/ZIF-L film (b, d and f).

To investigate the structural difference between the ZIF-8 film and the Eu/ZIF-L film, we performed FTIR characterization as shown in Fig. 3 and Table S1. Since the coordination unites of ZIF-8 and ZIF-L are almost consistent, most of bonds exhibited the same IR spectral absorptions, as shown in Table S1. It is noted that a new infrared absorption peak centered at 1384 cm⁻¹ appeared in the Eu/ZIF-L film (Fig. 3II), which was attributed to the vibration of Eu–N.^30,31 However, no peaks could be observed in the ZIF-8 film, indicating the formation of Eu–N bonds after introducing Eu salts. Therefore, some Eu ions have been successfully coordinated with the N active site on the imidazole ligands in the ZIF-L skeleton.

Fig. 3 FTIR spectra of ZIF-8 and Eu/ZIF-L films. The enlarged IR spectra in the range from 1200 to 400 cm⁻¹ (I), from 1750 to 1250 cm⁻¹ (II).

In order to further investigate differences in surface structures between the ZIF-8 film and the Eu/ZIF-L film, XPS characterization was performed as shown in Fig. 4. Fig. 4a shows the presence of Eu, C, N, O and Zn elements in the Eu/ZIF-L film and C, N, O and Zn elements existed in the ZIF-8 film. In addition, high-resolution XPS spectra of C 1s both in the ZIF-8 film (Fig. 4d) and the Eu/ZIF-L film (Fig. 4g) were deconvoluted into four peaks centered at ∼284.8, ∼285.6, ∼286.6 and ∼288.5 eV, mainly corresponding to C-sp³, C=C, C–N and C=N in Hmim ligands, respectively^32,33 (there would be some amorphous carbon due to the need for testing). And it can be seen that the relative intensity of C–N and C=N in the Eu/ZIF-L film was obviously higher than that in the ZIF-8 film since more organic ligands participated in forming the ZIF-L framework. Moreover, high resolution N 1s spectra (Fig. 4e and h) show two peaks at ∼399.0 and ∼400.1 eV corresponding to C=N–C and N–H, respectively. Since the layers are connected with each other using free 2-methelimidazole molecules by hydrogen bonding interactions in ZIF-L frameworks,²⁸ as expected, the relative intensity of protonated N (N–H) in Eu/ZIF-L was higher than that in ZIF-8. On the other hand, high-resolution Eu 3d spectrum of the Eu-modified ZIF-L film was deconvoluted into five peaks (Fig. 4b). In the Eu/ZIF-L film, two peaks at ∼1134.5 and ∼1164.3 eV were attributed to Eu³⁺ and another two peaks centered at ∼1124.7 and ∼1155.3 eV were related to Eu²⁺, which was from the surface layer, called the “surface valence transition”.^34–36 However, no peaks can be seen in the ZIF-8 film (Fig. 4b). And XPS high resolution Zn 2p spectra (Fig. 4c) were composed of two peaks centered at ∼1021.8 and ∼1044.8 eV, consistent with binding energies of Zn 2p^3/2 and Zn 2p^1/2, respectively, indicating the existence of Zn²⁺ both in the ZIF-8 film and the Eu/ZIF-L film.³⁷ In Fig. 4f and i, high-resolution O 1s spectra were fitted into two peaks. A peak at ∼531.6 eV was assigned to Zn–OH groups and another peak at ∼532.7 eV was attributed to H₂O molecules, suggesting that there were extra oxygen atoms bonding with unsaturated Zn defects in the form of hydroxyl groups both in the ZIF-8 film and the Eu/ZIF-L film.^38,39 It was noteworthy that unsaturated Eu sites (i.e. Eu–OH) may exist in the Eu/ZIF-L film due to the involvement of Eu ions in the formation of the ZIF-L structure (Fig. 3II). In this sense, the transformation from ZIF-8 to ZIF-L may be related to Eu ions, and further testing and characterization is required to verify the valence of such Eu ions.

Fig. 4 XPS survey spectra (a), high resolution Eu 3d (b), Zn 2p (c), C 1s (d and g), N 1s (e and h), and O 1s (f and i) spectra of the ZIF-8 film (a–f) and the Eu/ZIF-L film (a–c and g–i).

To explore the reason for the phase structure transformation from ZIF-8 to ZIF-L after introducing Eu salts, a series of Eu/ZIF films with different concentrations of Eu salts were synthesized and characterized by XRD as shown in Fig. S1. It can be seen from the figure that the ZIF-8 structure was obtained when the concentration of Eu was not more than 0.017 mol L⁻¹. Obviously, a small bulge appeared at ∼17.2° ((402) facet) as the concentration of Eu salts was 0.033 mol L⁻¹, however, the phase structure was still ZIF-8. When the concentration of Eu salts was up to 0.067 mol L⁻¹, characteristic diffraction peaks originating from ZIF-L was obvious. Additionally, the phase structure of ZIF-L was still maintained as continuing increasing the concentration of Eu salts. Since the 2D layers in the ZIF-L stacking along the c direction are part of the sodalite topology in the 3D structure of ZIF-8.⁴⁰ Moreover, metal Eu ions can serve as a metal source to coordinate with organic imidazole ligands in the form of Eu²⁺.^41,42 In this sense, it was likely due to the phase transformation, that a small amount of Zn²⁺ was replaced by Eu²⁺ forming coordination bonds between Eu²⁺ and the N active site in 2-methylimidazole, leading to the change of the growth direction of the ZIF framework. However, the ability of Eu²⁺ in the system to participate in coordination is closely related to the amount of Eu salts added to the ZIF sol and the amount of Eu²⁺ produced by the “surface valence transition”.

In addition, we mixed Hmim and Eu(NO₃)₃·6H₂O in ethanol in a 2 : 1 molar ratio at room temperature to investigate whether there would be a reaction between them. The experimental phenomenon indicated that the two reagents could react rapidly to form white precipitates, the precipitates were dried and characterized by FTIR and XPS, and the results obtained are shown in Fig. S2. It can be seen in Fig. S2a that there were Eu–O and Eu–N infrared absorption peaks, indicating the reaction can occur at room temperature. Moreover, Eu 3d spectra (Fig. S2c) showed that almost no peaks of Eu²⁺ were present, and the relative intensities of NO₃⁻ and N–H in N 1s (Fig. S2e) were obviously higher than that of C=N–C. These results suggest that Eu(NO₃)₃·6H₂O can react with Hmim at room temperature in the absence of zinc salts, while the reaction is incomplete and almost all metal ions coordinated to N sites from Hmim are Eu³⁺ rather than Eu²⁺.

We propose that the structural transition from ZIF-8 to ZIF-L is not only related to the doped Eu³⁺ but also significantly influenced by the curing temperature. This phenomenon is consistent with relevant research findings, where, for instance, researchers observed that the phase transition of LiYO₂ is closely associated with both Yb³⁺ concentration and temperature when Yb³⁺ is doped into the LiYO₂ matrix.⁴³ In order to explore the source of Eu²⁺ produced in the Eu/ZIF-L film, FTIR was performed on the Eu/ZIF sol and films cured at different temperatures and the results are shown in Fig. S3. In Fig. S3II, no infrared absorption peak of Eu–N was observed in the Eu/ZIF sol in the presence of Zn²⁺, indicating that the Eu³⁺ coordination with Hmim was suppressed. Additionally, the relative intensity of the Eu–N peak at 1384 cm⁻¹ was enhanced as the curing temperature increased. Furthermore, Fig. 4b shows the apparent presence of Eu²⁺ in the well-cured Eu/ZIF-L film, implying that the production of Eu²⁺ was closely related to the curing temperature.

To further understand the trend of the content of Eu ions used for the formation of the ZIF-L skeleton in Eu/ZIF-L films, FTIR (Fig. S4) and high-resolution XPS spectra of Eu 3d (Fig. S5a) and N 1s (Fig. S5b) in Eu/ZIF films synthesized with different initial concentrations of Eu(NO₃)₃·6H₂O were carried out. It can be seen that the intensities of imidazole ligand-related IR vibrational peaks in Fig. S4 were decreased with the increase of the content of Eu salts. Moreover, the relative intensity of the Eu–N peak centered at 1384 cm⁻¹ exhibited a trend of first increase then decrease and the maximum intensity was reached when the concentration of Eu(NO₃)₃·6H₂O was 0.133 mol L⁻¹, suggesting that the content of Eu ions involved in the formation of ZIF-L frameworks was up to saturation. Also, it can be seen from Fig. S5a that the relative proportion of Eu²⁺ decreased with the increasing of Eu contents in the synthesized reaction system, indicating that the addition of more Eu salts in the ZIF sol may not be beneficial for the surface valence transition of Eu³⁺. Furthermore, Table S2 shows that the mole ratio of Eu/Zn on the surface of the ZIF@0.133 mol L⁻¹ Eu sample was higher than that of the other samples, demonstrating that the content of Eu²⁺ produced by the Eu³⁺ transition was relatively higher, which would be more favorable for coordination with 2-methylimidazole ligands. Additionally, it can be seen from Fig. S5b that a fitted peak at ∼407.0 eV for the sample (ZIF@0.333 mol L⁻¹ Eu) was obvious, corresponding to NO₃⁻.⁴⁴ And the relative intensity of the peak at ∼532.7 eV fitted from O 1s in Fig. S4d was obviously increased compared to other samples, indicating the existence of more Eu(III) compound, i.e. Eu(NO₃)₃ on the surface of the ZIF@0.333 mol L⁻¹ Eu sample.⁴⁵

In a word, the phenomenon about “surface valence transition” from Eu³⁺ to Eu²⁺ can occur through controlling the curing temperature in Eu/ZIF films, to achieve the transformation from 3D ZIF-8 to 2D ZIF-L, a possible explanation for the result is that, a small amount of Eu²⁺ is involved in the formation of ZIF frameworks and affects the growth direction of ZIF, which eventually forms ZIF-L structures.

3.2. Photoluminescence performances

To study the luminescence properties of the Eu/ZIF-L film, firstly, the UV-Vis absorption spectra about the ZIF-8 film and the Eu/ZIF-L film (i.e. the sample of ZIF@0.133 mol L⁻¹ Eu) were recorded as shown in Fig. S6. It can be seen that both the ZIF-8 film and the Eu/ZIF-L film exhibited an absorption edge of ∼240 nm because of their same coordinated structures, which was consistent with a previous report.²⁹ And the excitation and emission spectra of ZIF-8 and Eu/ZIF-L at room temperature are shown in Fig. 5. When the films were excited at 365 nm, linker-based fluorescence could be observed at ∼450 nm both in ZIF-8 and Eu/ZIF-L.⁴⁶ Additionally, Eu/ZIF-L showed the characteristic emission bands of Eu³⁺ at λ = 595 and 618 nm, which was attributed to ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ of the Eu³⁺ ion, respectively.⁴⁷ However, other characteristic emission peaks of Eu³⁺ were too weak to be observed. Surprisingly, a strong narrow emission peak at 495 nm appeared, corresponding to the fluorescence emission of Eu²⁺ due to a d–f transition, which was affected by the coordination environment,^41,48 which was well consistent with the above analysis in Fig. 4b and S3. As for the formation of strong fluorescence originating from Eu²⁺, the most possible reason was the antenna effect, in which the luminescence from linkers (like the 2-methylimidazole linker) can involve charge transfer with coordinated metal ions (like Eu²⁺) or clusters.⁴⁹

Fig. 5 PL and PLE spectra of the ZIF-8 film (a) and the Eu/ZIF-L film (b).

In order to demonstrate origins of these fluorescence peaks in the Eu/ZIF-L film, PL and PLE spectra of luminescent reactants including Eu(NO₃)₃·6H₂O and Hmim ligands in solid and solution states were performed at room temperature as shown in Fig. S7a–d. It can be seen from the result that the fluorescence of Eu³⁺ and ZIF-L frameworks in the Eu/ZIF-L film was consistent with Eu(NO₃)₃·6H₂O and Hmim, respectively. Nevertheless, the fluorescence peak at 495 nm in the Eu/ZIF-L film was not observed both in Eu(NO₃)₃·6H₂O and Hmim. Additionally, the white precipitate and supernatant obtained from the mixture of Hmim and Eu(NO₃)₃·6H₂O with a mole ratio of 2 : 1 were tested by fluorescence spectroscopy, and the results in Fig. S7e and S7f show that there also the fluorescence peak was absent. These results indicated that the fluorescence peak at 495 nm was generated during the preparation of the Eu/ZIF-L film and did not originate from Eu³⁺.

To further demonstrate that the appearance of the fluorescence peak is related to the curing temperature, PL spectra of Eu/ZIF sols synthesized with different concentrations of Eu salts and the Eu/ZIF-L films (0.133 mol L⁻¹ Eu salts) cured at different temperatures were recorded, as shown in Fig. 5.

It was inferred from Fig. S8a that the coordination degree between Zn²⁺ and Hmim was very low due to the high dispersion of reactants in the Eu/ZIF sol. And combined with the fluorescence peak of the Hmim ligand (Fig. S7c and d), it indicated that the fluorescence peak observed at ∼450 nm in all sols was caused by the formation of metal complexes with low coordination degree and Hmim ligands. Moreover, the emission peak at 495 nm was not observed in all Eu/ZIF sols, indicating that the formation of the peak was not directly related to the concentration of Eu salts in Eu/ZIF sols. At the same time, the data in Fig. S8a were normalized to obtain Fig. S8b. It is noteworthy that the emission peak (618 nm) of Eu³⁺ increased gradually with the increase of the Eu salt concentration in the Eu/ZIF sol, which further proved that Eu³⁺ was hardly reduced to Eu²⁺ under the chemical environment in the sol. On the other hand, PL spectra of Eu/ZIF-L films (0.133 mol L⁻¹ of Eu salts) cured at different temperatures (Fig. S8c and d) show that the emission peak at 495 nm was more and more obvious with the increase of curing temperature, demonstrating that the curing temperature is the key to generate Eu²⁺.

To further verify that the fluorescence peak at 495 nm originated from Eu²⁺, we also calculated the fluorescence lifetimes of ZIF-8 and Eu/ZIF-L films, as shown in Fig. 6. The average fluorescence lifetime of ZIF-8 was shown to be 2.56 ns by biexponential fitting (Fig. 6a), whereas the average fluorescence lifetime of the fluorescence emission peak (450 nm) originating from ZIF-L in the Eu/ZIF-L membrane was 1.97 ns (Fig. 6b), this was shorter than that in ZIF-8, and the cause of this phenomenon might be that the photon energy in the ZIF-L framework (based on the ligand) is partially transferred to Eu²⁺ involved in coordination, which makes its fluorescence lifetime shorter. In addition, the fluorescence lifetimes measured when monitoring the fluorescence peaks at 495 nm and 618 nm were calculated by fitting the average lifetimes to be 1.20 μs (Fig. 6c) and 0.56 ms (Fig. 6d), which were in agreement with the fluorescence lifetimes of Eu²⁺ and Eu³⁺, respectively,^30,50,51 further confirming that the fluorescence peak at 495 nm originated from Eu²⁺.

Fig. 6 The fluorescence decay curve at 450 nm for the ZIF-8 film (a) and fluorescence decay curves for the Eu/ZIF-L film monitored at 450 nm (b), 495 nm (c) and 618 nm (d), respectively.

3.3. Temperature sensing performance

To investigate the dependence of fluorescence of the Eu/ZIF-L film on temperature, temperature (T)-dependent fluorescence was recorded in the range of room temperature (23 °C)–100 °C. Prior to the test, excitation wavelength-dependent fluorescence spectra were recorded to select an appropriate excitation wavelength and the result is shown in Fig. 7.

Fig. 7 Excitation wavelength-dependent fluorescence spectra of the Eu/ZIF-L film.

Considering the signal intensity of the main fluorescence peaks (450 and 495 nm) in the Eu/ZIF-L film, an excitation wavelength of 360 nm was chosen for the high-temperature fluorescence measurement. As shown in Fig. 8a, thermal quenching of the emission peaks at 450 and 495 nm occurred when the temperature was increased from room temperature to 100 °C. Additionally, the intensity of the corresponding fluorescence peaks gradually increased as the temperature decreased from 100 °C to room temperature (Fig. 8b), suggesting the fluorescence reversibility to temperature. Moreover, fluorescence intensities of the peaks at 450 nm (I_{450 nm}) and 495 nm (I_{495 nm}) quenched by 61.5% and 76.1% at 23–100 °C, as shown in Fig. 8c, respectively.

Fig. 8 Fluorescence spectra of the Eu/ZIF-L film over a temperature range of room temperature (23 °C) to 100 °C during processes of increasing temperature (a) and decreasing temperature (b). Experimentally measured and nonlinear-fitted plots (c) and linear-fitted plots (d) of fluorescence intensities of peaks (450 and 495 nm) and relative strength of the two peaks.

In Fig. 8d, a relationship of I_{495 nm}/I_{450 nm}versus temperature (°C) reveals a linear fitting equation: I_{495 nm}/I_{450 nm} = −0.0084 T + 3.6617 at 313–363 K with a correlation coefficient R² = 97.61%, whose sensitivity was calculated as 1.95% K⁻¹ at 363 K. The relationship of emission peaks at 450 and 495 nm versus T also showed linear fitting equations: I_{450 nm} = −11 519.7690 T + 4 793 135.0127 (R² = 98.96%, 296–373 K, sensitivity: 1.26% K⁻¹ at 373 K) and I_{495 nm} = −17 679.1704 T + 6 731 939.4256 (R² = 97.66%, 304–363 K, sensitivity: 1.61% K⁻¹ at 363 K). Furthermore, by consulting relevant literature,⁵² based on the relative sensitivity (Sr), the temperature resolution of this thermometer could be calculated viaeqn (1) and (2) as 0.18 K. These results indicate that both the fluorescence peaks and relative intensities of the two emission peaks have a good linear relationship with temperature over a range of temperatures, suggesting a great potential to be used in fluorescence thermal response applications. Additionally, the Eu/ZIF-L film exhibits sufficient sensitivity to temperature, making it suitable for sensing applications in non-contact temperature measurement. By comparing our current work with previously reported MOF-based temperature sensors (Table 1), it is demonstrated that Eu/ZIF-L developed in this study not only has higher sensitivity but also exhibits superior temperature-sensing performance.^4,7,53–57

Table 1 Comparison of MOF-based temperature sensing performance

Samples	Material system	Temperature range (K)	S r (K^−1)	Temperature resolution (K)	Ref.
Eu/ZIF-L	Ln-MOF with multiple luminescent centers	296–373	1.95% (363 K)	0.18	This work
Cdots&RB@ZIF-8^2-MMM	MOF–nanocomposite mixed–matrix membrane	293–353	0.74% (293 K)	<0.058	4
Cz-Ant@ZIF-8	Dye emissions within MOF	278–353	1.55% (278 K)	0.39–0.62	7
Eu0.05Y0.95-PTC	Dual-luminescence Ln-MOF	303–383	1.94% (383 K)	—	53
ZJU-28⊃ACF	Dye molecules encapsulated in MOF	293–313	0.025% (313 K)	—	54
EuxTb1−xBPT	Mixed-lanthanide MOFs	293–353	>1.5%	—	55
MOF@UCNPs	Lanthanide-nanoparticles anchoring on MOF	298–316	1.92% (298 K)	—	56
EuW10/GCDs@ZIF-8^2	MOF incorporating polyoxometalates and carbon dots	273–353	1.45% (353 K)	0.29	57

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3.4. Origins of Eu²⁺ and kinetic insights into ZIF-8-to-ZIF-L structural transformation

3.4.1. Discussion on the source of Eu²⁺. The generation mechanism of Eu²⁺ in the research system can be clearly attributed to the valence transition of Eu³⁺ under the thermal activation of curing temperature, which leads to the conversion of part of Eu³⁺ to Eu²⁺. The experimental data show that no characteristic signal of Eu²⁺ is detected in the sol system without heat treatment (Fig. S8a), while the sample treated with 100 °C curing shows an obvious characteristic peak of Eu²⁺ in the XPS spectrum (Fig. 4b, Fig. S5a). Moreover, the characteristic emission peak of Eu²⁺ at 495 nm in the PL spectrum is significantly enhanced with the increase of curing temperature (Fig. S8c and d), which directly confirms that the curing temperature is the core driving force for the conversion of Eu³⁺ to Eu²⁺.

From the perspective of thermal activation mechanism, the energy provided by the curing temperature can break the stability of the electronic configuration of Eu³⁺: when the curing temperature rises to a certain value (such as 100 °C in the experiment), the thermal energy of the system promotes Eu³⁺ to obtain electrons to complete the valence band transition, and part of Eu³⁺ is converted into Eu²⁺. At the same time, during the curing process of the Eu/ZIF-L wet film, Eu²⁺ coordinates with the protonated nitrogen active sites in the ZIF framework, occupies the Zn²⁺ sites, and causes the change of the growth direction of the ZIF structure. The XRD results show that the characteristic peak of ZIF-L appears along with the generation of Eu²⁺ (Fig. 1), which also verifies that there is a close relationship between the valence transition induced by thermal activation of curing temperature and the structural phase transition.

3.4.2. Effect of Eu²⁺ concentration on the kinetics of ZIF-8-to-ZIF-L structural transformation. Eu²⁺ concentration plays a crucial role in the structural transformation from ZIF-8 to ZIF-L. Based on the XRD results of a series of Eu concentrations (Fig. S1), when the concentration of Eu(NO₃)₃·6H₂O is ≤0.033 mol L⁻¹, ZIF-8 is the main phase, with only a weak characteristic peak of ZIF-L appearing at 17.2° (402). When the concentration is ≥0.067 mol L⁻¹, the characteristic peak of ZIF-L is significantly enhanced. This indicates that when the concentration of Eu²⁺ in the system is low, its ability to compete with Zn²⁺ for 2-methylimidazole ligands is limited, and the three-dimensional cross-linked structure of ZIF-8 dominates.

With the increase of Eu²⁺ concentration, synchronous XPS analysis shows that the proportion of Eu²⁺ reaches the highest at 0.133 mol L⁻¹ (Fig. S5a). More Eu²⁺ forms coordination with protonated N active sites in part of the ZIF structure, and the relative strength of Eu–N bonds also reaches a peak (Fig. S4). Since the ionic radius of Eu²⁺ is larger than that of Zn²⁺ and it tends to have a higher coordination number, this causes a change in the arrangement of ligands. More ligands are arranged in a manner of “intra-layer bridging and inter-layer interaction”, gradually inducing the structural transformation from 3D ZIF-8 to 2D ZIF-L.

From a kinetic perspective, the increase in Eu²⁺ concentration is equivalent to increasing the “driving force” for structural transformation in the reaction system. Analogous to the effect of concentration on the reaction rate in chemical reaction kinetics, a higher Eu²⁺ concentration increases the number of effective collisions in the structural transformation, thereby accelerating the rate of transformation from ZIF-8 to ZIF-L. In our experimental observations, as the concentration of Eu salt increases, the intensity of the characteristic diffraction peak of ZIF-L gradually enhances, which indirectly confirms that a higher Eu²⁺ concentration promotes the phase transformation process.

3.4.3. Effect of curing temperature on the kinetics of ZIF-8-to-ZIF-L structural transformation. Curing temperature is also an important factor affecting the kinetics of ZIF-8-to-ZIF-L structural transformation. From the experimental results, only the Eu/ZIF film subjected to 100 °C heat treatment exhibits the ZIF-L phase, while ZIF-L is not detected in the Eu/ZIF sol without heat treatment (Fig. 1, Fig. S8a). This indicates that the curing temperature has a significant impact on the structural transformation from ZIF-8 to ZIF-L.

The increase in curing temperature, on one hand, accelerates the thermal motion of molecules in the system, thereby increasing the reaction rates between Eu³⁺ and ligands, Eu²⁺ and ligands, and Zn²⁺ and ligands. On the other hand, the increase in temperature may affect the reduction rate of Eu³⁺ to Eu²⁺. The PL spectra (Fig. S8c and d) show that the intensity at 495 nm (characteristic peak of Eu²⁺) significantly enhances with the increase of curing temperature, indicating that high temperature promotes the reduction of Eu³⁺ → Eu²⁺, thereby increasing the generation amount of Eu²⁺ and providing more “power” for the structural transformation.

In addition, temperature also has a significant impact on the activity of ligands and the stability of metal–ligand coordination bonds. Appropriately increasing the temperature may weaken the strength of Zn–ligand coordination bonds in the original ZIF-8 structure, while enhancing the ability of Eu²⁺ to form coordination bonds with ligands, making the structure more prone to transformation to ZIF-L.

3.5. Structure formation and luminescence mechanism

Following the above test results, we propose possible structure formation mechanisms and luminescence mechanism explanations for the structure and fluorescence properties of the Eu/ZIF-L film, as shown in Fig. 9. Eu(NO₃)₃·6H₂O was introduced into the ZIF sol, and europium mainly existed in the sol as Eu(III). The Eu/ZIF sol was spin-coated to form a film, and the wet film was transformed into Eu²⁺ due to the valence band transition of Eu³⁺ in the curing process affected by the temperature, and the Eu²⁺ coordinated with the N-active sites in the ZIF framework occupied some of the protonation sites that had been partially coordinated with Zn²⁺. On the other hand, the coordination of Eu²⁺ with some of the N active sites in the ZIF-L framework produces an antenna effect, i.e., the electrons in ZIFL absorb photon energy and jump from the ground state (S₀) to the excited state , the electrons in the excited state traversed to the single-line triplet state via intersystem crossing, and then part of the energy was transferred to Eu²⁺, which enhanced the fluorescence of Eu²⁺ (495 nm) and formed a strong narrow-band fluorescence emission.

Fig. 9 Schematic illustration of the structure formation (a) and luminescence mechanism (b) for the Eu/ZIF-L film.

4. Conclusions

Eu/ZIF-L films with multiple emissions were successfully synthesized through introducing small amount of Eu(NO₃)₃·6H₂O into a ZIF sol. Controlling the curing temperature of the Eu/ZIF-L film promotes the valence conversion of Eu³⁺ to Eu²⁺, which actively participates in the formation of the ZIF framework, hence changing the growth direction of the Eu/ZIF film and achieving the phase transformation from 3D ZIF-8 to 2D ZIF-L. The fluorescence peak of Eu²⁺ becomes narrow and strong due to the antenna effect originating from the coordination of Eu²⁺ with N active sites in the ZIF-L skeleton. Under 360 nm excitation, the dual-emission peaks at 450 and 495 nm demonstrate temperature-dependent fluorescence quenching with 61.5% and 76.1% intensity reduction over a broad temperature range from room temperature to 373 K, respectively. Notably, FIR (I₄₉₅/I₄₅₀) shows excellent linear correlation with temperature, accompanied by a relative sensitivity of 1.95% K⁻¹. In this sense, the Eu/ZIF-L film with a good reversible fluorescence thermal response performance has great potential for temperature-sensing. In this work, the composite MOF materials prepared by us with the Eu³⁺ ion as a multi-luminescent center show intriguing structural transformations and performance enhancements, which provide new insights for the innovative design of luminescent composites for applications with ratiometric temperature sensing.

Author contributions

Qiufen Liu and Longxiao Zhou conceived experimental ideas and conducted experimental investigations. Jiayu Zheng assisted with experimental investigations and manuscript writing. Senwei Wu and Yuting Peng were responsible for testing and characterization and experimental data analysis. Shouqin Tian and Xiujian Zhao were both involved in the process of supervision and extensive revision of the manuscript.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data supporting this article have been included as part of the SI. See DOI: https://doi.org/10.1039/d5qi01248a.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51772229), the National Natural Science Foundation of Hubei Province (No. 2023AFD055) and the National Innovation and Entrepreneurship Training Program for College Students (No. S202410497018). We thank the Analytical and Testing Center of WUT for the help with carrying out XRD, XPS, FESEM and FTIR analyses.

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