Preparation of Dual-Emitting Ln@UiO-66-Hybrid Films via

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Preparation of Dual-emitting Ln@UiO-66-Hybrid Films via Electrophoretic Deposition for Ratiometric Temperature Sensing Ji-fei Feng, Shuiying Gao, Tian-Fu Liu, Jianlin Shi, and Rong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17947 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018

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ACS Applied Materials & Interfaces

Preparation of Dual-emitting Ln@UiO-66-Hybrid Films via Electrophoretic Deposition for Ratiometric Temperature Sensing Ji-fei Feng†,‡,§, ∥, Shui-ying Gao†*, Tian-fu Liu†, Jianlin Shi‡,§,∥ and Rong Cao†§,∥*

†. State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Mater, Chinese Academy of Science, Fuzhou 350002, China. ‡. School of Physical Science and Technology, Shanghai Tech University, Shanghai 201210, China. §. State Key Laboratory of high Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, China. ǁ. University of Chinese Academy of Science, Beijing 100049, China.

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ABSTRACT

Engineering novel dual-emitting MOFs with wide emission ranges for application as ratiometric temperature sensors is still a challenge. In this paper, two novel dual-emitting MOFs with intergrated lanthanide metals and luminescent ligand in a UiO-66-type structure, named Ln@UiO-66-Hybrid, were prepared via the combination of postsynthetic modification (PSM) and postsynthetic exchange (PSE) methods. Subsequently, the as-synthesized MOFs were deposited onto FTO substrates through electrophoretic deposition (EPD) by taking advantage of the charges from the unmodified carboxylic groups of the MOFs. The as-prepared Tb@UiO-66Hybrid and Eu@UiO-66-Hybrid films were applied to detect temperature changes. The resulting Tb@UiO-66-Hybrid film exhibited good temperature sensing properties with the relative sensitivity up to 2.76%·K-1 in the temperature range from 303 K to 353 K. In addition, the Eu@UiO-66-Hybrid film showed excellent temperature sensing performance based on the energy transfer between the luminescent ligand (H2NDC) and Europium ions with a relative sensitivity up to 4.26%·K-1 in the temperature range from 303 K to 403 K. Keywords Metal-organic frameworks, Postsynthetic modification, Postsynthetic exchange, Electrophoretic deposition, Dual-emitting, Ratiometric temperature sensing INTRODUCTION Metal-Organic Frameworks (MOFs) are porous hybrid materials constructed from metal ions or clusters bridged via organic ligands to form one-, two-, or three-dimensional infinite networks. 111

Based on high specific surface area, tunable pore size and shape and multiple coordination


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sites, MOFs can offer various functions such as chemical separation, catalysis.12-17 Luminescent MOFs, a subfamily of MOF materials, have been widely used in various applications, such as optics, photocatalysis, electroluminescent devices, biomedical imaging, and temperature sensing.18,19 Temperature is a critical parameter both in scientific research and industry.14-16 Some mixedlanthanide MOF materials have been used as self-calibrated or ratiometric temperature sensors since 2012.20-31 Compared with thermometers based on the luminescence intensity of only one center, ratiometric thermometers gauge the temperature changes relied upon the ratio of two luminophore centers which are more accurate without being interfered by the luminophore quantity, excitation power or intensity. However, mixed-lanthanide MOF materials, usually based on Tb (λem= 544 nm) and Eu (λem=616 nm), have a fixed level gap (approximately 3300 cm-1) which limits the further application of this material for temperature sensing.28 To overcome this disadvantage, luminescent perylene was encapsulated in ZJU-88 to endow the composite with a broad emission range, as reported by Qian and co-workers in 2015.28 However, it is still a challenge to engineer novel dual-emitting MOFs with wider level gap (or wider emission range) for use as ratiometric temperature sensors. Postsynthetic methods provide a potential alternative for obtaining dual-emitting MOFs. There are three postsynthetic methods usually reported: postsynthetic modification (PSM)31-35, postsynthetic exchange (PSE)36-42, and postsynthetic deprotection (PSD)43. We report here the first example of the synthesis of dual-emitting MOFs from the combination of PSM and PSE methods. Instead of terephthalic acid, 1’2’4’5’benzenetetracarboxylic (H4BTEC) acid was employed to construct a UiO-66 framework to introduce two uncoordinated carboxylic groups decorated on the pore surface 36 The twocomponent UiO-66 MOFs (denoted as UiO-66-Hybrid) were synthesized by PSE with the


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luminescent ligands, 1,4-naphthalene dicarboxylic acid (H2NDC), at room temperature, as shown in Scheme 1a. Subsequently, another emitting center, Terbium (Tb) or Europium (Eu) ions, was introduced through the PSM method by taking advantage of the un-coordinated carboxylic groups in the UiO-66-Hybrid. As expected, the Ln@UiO-66-Hybrid MOFs exhibit the redemission of Eu3+ at 616 nm or the green-emission of Tb3+ at 544 nm and the blue-emission of the luminescent ligand (H2NDC) at 430 nm. Recently, some methods have been developed to prepare MOF films, such as in-situ, spincoating, dip-coating, electrochemical deposition and electrophoretic deposition (EPD) methods.44-50 Among these methods, the EPD method is well suited to the facile and rapid fabrication of MOF films on the traditional substrates like indium-tin-oxide (ITO), fluorine-tinoxide (FTO) glass and metal plates. Therefore, Ln@UiO-66-Hybrid films were prepared by the EPD method on unmodified FTO glass (see Scheme 1b). The as-prepared films were then used as ratiometric temperature sensors. The resulting Tb@UiO-66-Hybrid film exhibited good performance with a relative sensitivity up to 2.76%·K-1 in the temperature range from 303 K to 353 K. In addition, the Eu@UiO-66-Hybrid film showed excellent performance, with a relative sensitivity up to 4.26%·K-1 in the temperature range from 303 K to 403 K. To the best of our knowledge, the work reported here is the first preparation of dual-emitting MOFs through the combination of PSE and PSM, followed by deposition on the FTO substrates through EPD to fabricate MOF films.


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Scheme 1. Preparation of Ln@UiO-66-Hybrid composition by postsynthetic exchange and postsynthetic modification (a) and fabrication of composite films via electrophoretic deposition method (b).

EXPERIMENT Materials Zirconium (IV) chloride (ZrCl4, 99.95%) was obtained from the Strem Chemical Industry. 1’2’4’5’- benzenetetracarboxylic (H4btec, 98%) was purchased from Tokyo Chemical Industry Co. Ltd (Japan). 1,4-Naphthalenedicarboxylic acid (H2NDC, 98%) was obtained from Adamas Reagent Co. Ltd (China). Europium nitrate hexahydrate (Eu(NO3)3·6H2O, 99.95%) and Terbium nitrate hexahydrate (Tb(NO3)3·6H2O) were obtained from Energy Chemistry Industry Co. Ltd (China). Benzoic acid (BA), Ethanol, DMF and Acetone were purchased from Sinopharm Chemical Reagent Co. Ltd (China). Zinc plate (99%) was purchased from Tianjin Fu Chen Chemical Reagent (China). Above all chemicals or solvents were used directly without further purification. Measurements


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Powder X-ray diffraction (PXRD) patterns were recorded on a MiniFlex 600 with Cu Kα (1.5406 angstroms) with a scanning speed of 0.1° per minute for film and 1° per minute for the powders. Transmission electron microscopy (TEM) and mapping images were acquired with JEOL-2010 FEI Tecnai G20 field-emission microscope (JEOL, Tokyo, Japan) operated at 200 kV. Scanning electron microscopy (SEM) patterns were obtained with a Phenom G2 system. Brunauer– Emmett–Teller (BET) specific surface areas were measured by the N2 adsorption method on an ASAP 2020 instrument. Thermogravimetric analysis (TGA) spectra were recorded on an SDT Q600 instrument in N2 flow. The photoluminescence (PL) spectra, temperature-dependent PL spectra and lifetime measurement of the materials were conducted on the FLS 980 luminescence equipment. The inductively coupled plasma (ICP) test was conducted on an Inductively Coupled Plasma OES spectrometer. The EPD process was performed using an Agilent E3 612A DC power. The temperature-dependent PXRD patterns were recorded on a Desktop X-ray Diffractometer. Preparation of UiO-66-(COOH)2 MOFs UiO-66-(COOH)2 MOFs were synthesized according to the previous report.51 In a typical process, 2.3 g of ZrCl4 were added in a 100 mL round bottom flask with 50 mL of deionized water under stirring. Then, 4.3 g of H4BTEC was injected the flask. The mixture was then stirred at 100 °C for 24 h under reflex. After 24 h, the white precipitates were collected by centrifugation and washed with deionized water 3 times. Then, the collected powers were stirred at 100 °C for 16 h to remove the unreacted ligand. The remained powers were washed with deionized water and acetone 3 times each and dried at 70 °C under vacuum. According to the previous report52, UiO-66-(COOH)2-1 and UiO-66-(COOH)2-2 MOFs with bigger particle size were prepared using similar process with UiO-66-(COOH)2 except using 5 g and 10 g of BA.


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Preparation of UiO-66-Hybrid MOFs A total of 432 mg of H2NDC was dissolved in 50 mL of DMF under ultrasonication. Then, 1.0 g of UiO-66-(COOH)2 powders was added into the H2NDC solution. The mixture was stirred at 2000 r/min for 8 h at room temperature (approximately 25 °C). Then, UiO-66-Hybrid were obtained by the centrifugation and washed DMF, Ethanol and acetone 3 times and dried out at 70 °C in vacuum overnight. UiO-66-Hybrid-1 and UiO-66-Hybrid-2 MOFs were synthesized according to the above procedure. Preparation of Ln@UiO-66-Hybrid MOFs A total of 230 mg of Eu(NO3)3·6H2O or Tb(NO3)3·6H2O was dissolved in 25 mL of ethanol under stirring, and 200 mg of the prepared UiO-66-Hybrid was added into the ethanol solution. After stirring for 8 h at 2000 r/min, Eu@UiO-66-Hybrid MOFs

were

collected

by

centrifugation and dried out at 70 °C for overnight. xEu@UiO-66-Hybrid MOFs were named based on the ratio (x) of Eu addition content during the preparation process. 0.5Eu@UiO-66Hybrid and 2Eu@UiO-66-Hybrid MOFs were prepared according to the above procedure except using 115 mg and 460 mg g of Eu(NO3)3·6H2O. Eu@UiO-66-Hybrid-1 and Eu@UiO-66Hybrid-2 MOFs were synthesized according to the above procedure. The particle size of asprepared Eu@UiO-66-Hybrid, Eu@UiO-66-Hybrid-1 and Eu@UiO-66-Hybrid-2 MOFs was approximately 40 nm, 180 nm and 240 nm, respectively. Preparation of Ln@UiO-66-Hybrid films using the EPD method FTO glasses were cut into a rectangle (10×20 mm2) and sequentially washed with deionized water, ethanol, and acetone for 15 min each under ultra-sonication to remove the organic


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residues on the surface. A 10 mg portion of the as-prepared MOF powders was dispersed in 15 mL of CH2Cl2 under ultra-sonication, and then two similar glasses were used as positive and negative electrodes. The distance between the electrodes was 10 mm. Under a DC voltage of 90 V for 5 minutes, the Ln@UiO-66-Hybrid films were prepared on a positive electrode. Then, asprepared film was dried in air. RESULTS AND DISCUSSION

Figure 1. (a) NMR spectrum of destroyed UiO-66-Hybrid with dilute HF in d6-DMSO; (b) Adsorption isotherms of pristine UiO-66-(COOH)2 and UiO-66-Hybrid; (c) IR spectra of Tb@UiO-66-Hybrid and Eu@UiO-66-Hybrid.


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To demonstrate that the ligand (H2NDC) was successfully introduced via the PSE method, nuclear magnetic resonance (NMR) spectroscopy was conducted by digestion of the UiO-66Hybrid MOFs with dilute HF in d6-DMSO. As shown in Figure 1a and Figure S1, the peak at approximately 7.95 ppm (E) belongs to the characteristic peak of H4BTEC. The peaks at 7.71(D), 8.11(C) and 8.78(A) ppm belong to the luminescent ligand (H2NDC). In addition, the content of ligand (H2NDC) in the UiO-66-Hybrid MOFs was approximately 3%. Meanwhile, the results were further supported by N2 adsorption isotherms. The Brunauer–Emmett–Teller (BET) surface area of prototype UiO-66-(COOH)2 is about 459 m2/g which was decreased to 216 m2/g after the bulky ligand H2NDC was introduced by PSE method (Figure 1b). In addition, as shown in Figure S2, compared with the UiO-66-(COOH)2 with a pore size distribution from 10 angstroms to 80 angstroms, UiO-66-Hybrid MOFs have a narrow pore distribution from 10 angstroms to 60 angstroms which indicates that the ligand (H2NDC) was introduced successfully. The existence of mesopores in the as-prepared UiO-66-Hybrid MOFs allows for the lanthanide functionalization in the MOFs via PSM method. Furthermore, the existence of un-coordinated carboxylic groups in the UiO-66-Hybrid was proved by the observed of a peak at 1715 cm-1 that is the typical peak of free –COOH in the IR spectra (Figure S3). In addition, the residual uncoordinated carboxylic groups could chelate lanthanide ions. As a result, the Europium and terbium ions were uniformly distributed in the MOFs without aggregation. This result was further proved by the TEM images, as shown in Figure 2. Moreover, Tb and Eu were evenly distributed throughout the MOF structures. In addition, the molar ratio of Tb3+ ions to Zr4+ ions in the Tb@UiO-66-Hybrid was 0.113, the molar ratio of Eu3+ ions to Zr4+ ions in the Eu@UiO66-Hybrid was 0.149, the molar ratio of Eu3+ ions to Zr4+ ions in the 0.5Eu@UiO-66-Hybrid was 0.105, and the molar ratio of Eu3+ ions to Zr4+ ions in the 2Eu@UiO-66-Hybrid was 0.183


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according to the ICP data. In addition, the molar ratio of Eu3+ ions to Zr4+ ions in the Eu@UiO66-Hybrid-1 and Eu@UiO-66-Hybrid-2 was comparative with that of Eu@UiO-66-Hybrid.

Figure 2. TEM images and EDS mapping images of Tb@UiO-66-Hybrid (a) and Eu@UiO-66Hybrid (b) (Scale bar: 100 nm). The powder X-ray diffraction (PXRD) patterns as shown in Figure S4 demonstrate that the UiO66-Hybrid, Tb@UiO-66-Hybrid and Eu@UiO-66-Hybrid MOFs are isostructural with UiO-66. After lanthanide ions were incorporated, the IR spectra of Tb@UiO-66-Hybrid and Eu@UiO-66Hybrid MOFs were also recorded (Figure 1c). From the results, the un-coordinated carboxylic groups were still existent in the MOFs, with the typical peak of free –COOH observed at 1715 cm-1, these groups can generate negative charges on the surface of the MOFs. Based on the principle of electrophoretic deposition in which charged particles can be deposited on the electrode with opposite charge, these two MOFs could be deposited on the positive electrode. As expected, the films were prepared on the positive electrode during the EPD process.


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Figure 3. SEM images (left) and cross-sectional images (right) of UiO-66-Hybrid (a), Tb@UiO66-Hybrid (b) and Eu@UiO-66-Hybrid (c) (Scale bar: 100 µm) UiO-66-Hybrid, Tb@UiO-66-Hybrid and Eu@UiO-66-Hybrid MOF films were fabricated on the FTO substrate in 5 minutes. From the XRD patterns shown in Figure S5, the films had similar crystalline structures as the powders, which demonstrated that the structure was not destroyed during the EPD process. Moreover, the films were smooth and uniform with a thickness of approximately 50 µm, as shown in Figure 3.


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Figure 4. Emission spectra of UiO-66-(COOH)2 (black), UiO-66-Hybrid (red), H2NDC (blue) (a), physical mixture of UiO-66-(COOH)2 and H2NDC (b) excited at 365 nm, Tb@UiO-66-Hybrid (c) and Eu@UiO-66-Hybrid (d) films excited 330 nm. The UiO-66-Hybrid film exhibited blue fluorescence under excitation by the 365 nm UV light excitation. However, the pristine MOFs, UiO-66-(COOH)2, do not show any fluorescence under the same conditions. Moreover, the successful introduction of H2NDC was further demonstrated by the photoluminescence spectra shown in Figure 4. The UiO-66-(COOH)2 MOFs had a very weak emission peak at ～393 nm upon excitation at 365 nm. The luminescent ligand (H2NDC) exhibited strong blue fluorescence with a maximum emission peak at approximately 480 nm


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upon excitation at 365 nm. The UiO-66-Hybrid MOFs also exhibited intense blue fluorescence with a maximum emission peak at 430 nm. However, the maximum emission peak of the UiO66-Hybrid exhibited a blue-shift of approximately 50 nm. Moreover, the peak of H2NDC in the physical mixture of the ligand and MOFs did not change. This result clearly proved that the ligand, H2NDC, was not absorbed by physical interaction, but rather replaced the original ligand (H4BTEC) by coordinating to the Zirconium clusters. In addition, the results were further proved by the solid UV-vis test, as shown in Figure S6. As shown in Figure 4c, the Tb@UiO-Hybrid film features two typical emission peaks upon excitation at 330 nm. The peak at 430 nm is derived from the ligand (H2NDC). In addition, the peaks at 489 nm (5D4-7F6), 544 nm (5D4-7F5), 584 nm (5D4-7F4), and 620 nm (5D4-7F3) correspond to characteristic Tb3+ emissions. Between the two typical emissions, the dominant emission at 544 nm belongs to a Tb-based emission. In addition, the results clearly indicate that the dualemitting MOFs based on both the ligand (H2NDC) and Terbium ions were successfully synthesized. Similarly, the Eu@UiO-66-Hybrid film features two typical emissions based on the ligand (H2NDC) and Europium with characteristic sharp-line at 578 nm (5D0-7F0), 592 nm (5D07

F1), 616 nm (5D0-7F2), 650 nm (5D0-7F3), and 699 nm (5D0-7F4) (Figure 4d). Furthermore, the

results also indicate that the two emitting centers were assembled into the MOFs. To test the thermal stability of the MOFs, thermogravimetric measurements was conducted. As shown in Figure S7, Tb@UiO-66-Hybrid and Eu@UiO-66-Hybrid MOFs were not destroyed under 400 °C, which indicates that the MOFs are stable in the tested temperature range from 303 K to 403 K (130 °C). In addition, the result was further proved by the temperature-dependent PXRD of Eu@UiO-66-Hybrid with temperature increasing from 303 K to 403 K (Figure S8).


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Figure 5. (a) Emission spectra of the Eu@UiO-66-Hybrid film recorded in the temperature range from 273 K to 403 K excited at 330 nm. Inset: the corresponding CIE picture of Eu@UiO-66Hybrid film (b) Emission intensity at 430 nm and 613 nm in the temperature range from 303 K to


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403 K (c) Emission intensity ratio of the Eu@UiO-66-Hybrid film as a function of temperature with the fitting curve (red line, R2=0.9954). To assess the temperature sensing performance of the Tb@UiO-66-Hybrid and Eu@UiO-66Hybrid films, temperature-dependent fluorescence measurements were conducted (Figures 5,6). As-shown in the emission spectra of the Eu@UiO-66-Hybrid film given in Figure 4, the intensity at 430 nm gradually decreased with increasing temperature. In addition, the film exhibited different color fluorescence from pale pink-emitting to red-emitting with temperature increasing from 303 K to 403 K which indicated that the temperature change could be observed by naked eye. The intensity reduced to 15% of pristine intensity with an increase in the temperature from 303 K to 403 K. However, the intensity based on the Europium ions is gradually increased with the escalation of temperature from 303 K to 403 K, and the intensity at 403 K was approximately 2.5 times of pristine intensity at 303 K for the emission at 616 nm. Moreover, there was a good function relationship between the intensity ratio (Y) of 616 nm to 430 nm and temperature (T) in the temperature range from 303 to 403 K, which can be fitted as a function of Y=227.05-1.435×T+0.00228×T2

(1)

with a correlation coefficient of 0.9954. Temperature-dependent photoluminescence measurements of the Tb@UiO-66-Hybrid film were also conducted. As shown in Figure 6, the intensity at 430 nm was decreased in quantity with increasing temperature. Unlike the Eu@UiO-66-Hybrid film, the intensity based on Tb3+ did not vary notably with changes in the temperature. In addition, the film exhibited blue-emitting to pale green-emitting with increasing the temperature increasing from 303 K to 353 K. Similarly,


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there was a good linear relationship between the ratio (Y) of the intensity at 544 nm to 430 nm and temperature (T) in the range from 303 K to 353 K, which could be fitted as a function of Y= 0.462-0.758×T

(2)

with a correlation coefficient of 0.99.

Figure 6. (a) Emission spectra of the Tb@UiO-66-Hybrid film recorded in the temperature range from 303 K to 353 K excited at 330 nm. Inset: the corresponding CIE picture of Tb@UiO-66-


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Hybrid film (b) Emission intensity at 430 nm and 544 nm in the temperature range from 303 K to 353 K (c) Emission intensity ratio of the Tb@UiO-66-Hybrid film as a function of temperature with the fitting curve (red line, R2=0.99) and the relative sensitivity curve. Relative sensitivity (S) is another important parameter for evaluating the performance of the temperature sensors, and can be defined according to the following equation:

ܵ=

ୢଢ଼

(3)

ଢ଼ୢ୘

The sensitivity of the Eu@UiO-66-Hybrid film was as high as 4.26%·K-1 based on equations (1) and (3), and the sensitivity of the Tb@UiO-Hybrid film is up to 2.76%·K-1 based on equation (2) and (3) in the tested temperature range. These values are much higher compared with two repored

MOF-based

ratiometric

temperature

sensors,

mixed-lanthanide

MOF

Tb0.99Eu0.01(BDC)1.5(H2O)2 (S=0.31%·K-1) and ZJU-88⊃perylene MOF with a maximum relative sensitivity of 1.28%·K-1. In addition, the values are higher than those of all MOF-based ratiometric temperature sensors reported to date at 363 K. (Figure S9 and Table S1) The accuracy and precision (uncertainty) are other important parameters to evaluate the performance of solid thermometers.53-55 The accuracy is a related factor with the correlation coefficient. For Eu@UiO-66-Hybrid film, the accuracy was 0.0046 and temperature uncertainty was approximately 2 K based the equation (1). And for Tb@UiO-66-Hybrid film, the accuracy was 0.01 and temperature uncertainty was approximately 4 K based the equation (2). To explore the influence of Eu3+ content in MOFs on the temperature sensing performance, temperature-dependent PL measurements of 0.5Eu@UiO-66-Hybrid and 2Eu@UiO-66-Hybrid films were conducted in the temperature range from 303 K to 403 K. As shown in Figures S10


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and S11, the films exhibited good temperature sensing performances. 0.5Eu@UiO-66-Hybrid film had a maximum sensitivity of 3.17%·K-1. 2Eu@UiO-66-Hybrid film had a maximum sensitivity of 3.24%·K-1. Although these two films had high sensitivity, Eu@UiO-66-Hybrid film exhibited better temperature sensing properties with a relative sensitivity up to 4.26%·K-1 (Figure S12). These results indicated that the sensitivity of the MOFs film can be tuned via changing the Eu3+ content in MOFs. To explore the influence of MOFs size on the temperature sensing performance, temperaturedependent PL measurements of Eu@UiO-66-Hybrid-1 with particle size of approximately 180 nm and Eu@UiO-66-Hybrid-2 with particle size of approximately 240 nm films were conducted in the temperature range from 303 K to 403 K (Figure S13). As shown in Figures S14 and S15, these two films exhibited a good temperature sensing performance. Eu@UiO-66-Hybrid-1 films had a maximum relative sensitivity of 3.73%·K-1 and Eu@UiO-66-Hybrid-2 film had a maximum sensitivity of 3.27%·K-1. Compared with these two films, the Eu@UiO-66-Hybrid with particle size of 40 nm exhibited well temperature sensing properties with a relative sensitivity up to 4.26%·K-1 (Figure S16). Therefore, the size of MOFs can influence the sensitivity of sensors. Among these three materials, the material with smaller size had a higher sensitivity. In addition, the performance of Eu@UiO-66-Hybrid film was superior to Tb@UiO-66-Hybrid film due to the bigger level gap. As shown in Scheme S1 (Supporting information), the level gap in Eu@UiO-66-hybrid between Eu ions and ligand was approximately 6056 cm-1, however, the gap in Tb@UiO-66-hybrid was approximately 2756 cm-1.


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To confirm that energy transfer between the two luminescent centers, lifetime measurements of Eu@UiO-66-Hybrid film were conducted. The lifetime of free ligand (H2NDC) at 430 nm was approximately 17.62 ns at 303 K. However, the lifetime of ligand in Eu@UiO-66-Hybrid film was approximately 6.93 ns at 303 K. The lifetime of the Eu emission at 616 nm was approximately 196.95 µs without the introduction of the luminescent ligand at 303 K. However, the lifetime of the Europium emission at 616 nm in Eu@UiO-66-Hybrid was approximately 233.81 µs at 303 K, corresponding to an improvement of approximately 18.7% over that of the former. Moreover, the lifetime based on the Eu3+ emission at 616 nm in the Eu@UiO-66-Hybrid was 390.7 µs at 403 K and was enhanced by 67.1 % upon increasing the temperature from 303 K to 403 K. The results clearly indicate that energy transfer occurred from the luminescent ligand to the Europium ions (Figure S17). To test the stability of the MOF films, the photoluminescence measurements of the Eu@UiO-66Hybrid film were conducted from 303 K to 403 K for 3 cycles, as shown in Figure S18. The emission intensity did not change during the process. The structural stability of the Eu@UiO-66Hybrid film was also confirmed by the PXRD data (Figure S19), indicating the good recyclability of the Eu@UiO-66-Hybrid film. In order to further prove the superiority of the films, temperature-dependent PL spectra of Eu@UiO-66-Hybrid powders were conducted in the temperature range from 303 K to 403 K. As shown in Figure S20, although the intensity of Eu@UiO-66-Hybrid film and powders at 430 nm and 613 nm changed obviously with increasing temperature from 303 K to 403 K, the change tendency of film was superior to that of Eu@UiO-66-Hybrid powder which indicated that the film had a better temperature sensing performance than powders.


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Conclusions In summary, two novel dual-emitting MOF films with wide emission ranges were easily prepared by stepwise post-synthetic exchange (PSE) and post-synthetic modification (PSM) followed by electrophoretic deposition on the FTO substrate, and this method is for the first to prepare dual-emitting MOF materials. The as-synthesized films were used as ratiometric thermometers with a high relative sensitivity (4.26%·K-1) in the temperature range from 303 K to 403 K. In addition， the sensitivity was much higher than those of other MOFs reported to date in this temperature range. We believe that this strategy holds great promise for the synthesis of dual-emitting MOF films and their use in optics, sensors and other fields. Associated Contents Supporting information Characterization of the Ln@UiO-66-Hybrid powers and films and the fluorescence spectra of films. Author Information Corresponding Author *Email: [email protected] [email protected] Acknowledgements The authors acknowledge the financial support of the 973 Program (2014CB845605), NSFC (21331006, 21521061, 21520102001 and 51572260), Strategic Priority Research Program of the


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Table of Content/ Abstract Graphic


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Preparation of Dual-Emitting Ln@UiO-66-Hybrid Films via

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