A Multifunctional Dual-Luminescent Polyoxometalate@Metal-Organic Framework EuW10@UiO-67 Composite as Chemical Probe and Temperature Sensor

The luminescent [EuW10O36]9− polyoxometalate has been introduced into the cavities of the highly porous zirconium luminescent metal-organic framework UiO-67 via a direct synthesis approach, affording the EuW10@UiO-67 hybrid. Using a combination of techniques (TGA, BET, elemental analysis, EDX mapping,…) this new material has been fully characterized, evidencing that it contains only 0.25% in europium and that the polyoxometalate units are located inside the octahedral cavities and not at the surface of the UiO-67 crystallites. Despite the low amount of europium, it is shown that EuW10@UiO-67 acts as a solid-state luminescent sensor for the detection of amino-acids, the growth of the emission intensity globally following the growth of the amino-acid pKa. In addition, EuW10@UiO-67 acts as a sensor for the detection of metallic cations, with a high sensitivity for Fe3+. Noticeably, the recyclability of the reported material has been established. Finally, it is shown that the dual-luminescent EuW10@UiO-67 material behave as a self-calibrated-ratiometric thermometer in the physiological range.


INTRODUCTION
Polyoxometalate (POM)-based photosystems are currently undergoing a remarkable development due to their high relevance in photocatalysis or their ability to act as efficient photoswitches or fluorescent probes (Han et al., 2015;Saad et al., 2015;Holmes-Smith et al., 2016;Chen et al., 2017;Natali et al., 2017). Focusing on this last topic, both materials based on purely inorganic POMs incorporating luminescent metallic centers and hybrid organic-inorganic entities bearing grafted organic optically active moieties have been elaborated. For example, fluorescent microspheres prepared from a Lindqvist POM covalently connected to pyrene groups have been used for the detection of foodborne pathogens (Ju et al., 2016). Inorganic tungsten and/or molybdenum POMs incorporating rare earth (RE) centers have also been shown to act as efficient luminescent probes. For instance, cryogenic optical thermal probes made of polyoxomolybdate building blocks incorporating Eu III and Tb III ions have been recently reported (Kaczmarek et al., 2017). The luminescence of such RE-incorporating POMs is also very sensitive to the chemical environment. This can be illustrated considering the works devoted to the interaction between proteins and the seminal [EuW 10 O 36 ] 9− (EuW 10 ) europium decatungstate characterized by narrow emission bands, a large Stokes shift and a long lifetime . The characteristic emission of the europium decatungstate located at ca. 620 nm is strongly enhanced in presence of bovine or human serum albumin without any alteration of the secondary structure of the protein, showing that such simple molecular oxides could act as biological optical labeling agents (Hungerford et al., 2008;Zheng et al., 2010). Accordingly, Wu et al. showed that hybrid nanospheres made of arginine/lysine-rich peptides and EuW 10 are characterized by a large Eu III luminescence enhancement. Using a combination of techniques, it was evidenced that the observed phenomenon was due to the exclusion of the hydration water molecules from the secondary coordination sphere of EuW 10 caused by the strong electrostatic and hydrogen bond interactions between POMs and basic amino-acids (AAs) . Inversely, it was found that acidic AAs quench the luminescence while nonpolar AAs do not significantly influence the optical properties of the POM (Zhang et al., 2016). The processing of EuW 10 by incorporation into films has been achieved, allowing the fabrication of moisture-responsive systems (Clemente-León et al., 2010;Xu et al., 2011;Qiu et al., 2015). Based on all these results, we have undertaken to elaborate a multi-functional hybrid system where the EuW 10 POM is incorporated in the cavities of a metal-organic framework (MOF). This class of 3D coordination networks represents a unique platform for the development of solid-state luminescent materials due to their crystalline nature, permanent porosity, chemical tunability and robustness (Fernando-Soria et al., 2012;Hu et al., 2014;Lustig et al., 2017). To date, POM@MOF materials have been mainly synthesized for catalytic purposes. Two main synthetic strategies have been considered: (i) a onepot strategy, where the precursors allowing the formation of the MOF and the POM are mixed all together, the MOF being built around the POM (direct synthesis) and (ii) the preformed MOF is impregnated with a solution of the POM (twostep strategy). Such materials have shown their efficiency for catalytic reactions ranging from the oxidation of alkylbenzene  to the C-H activation of nitrile (Shi et al., 2016) and water oxidation (Mukhopadhyay et al., 2018;Paille et al., 2018). Interestingly, it has also been shown that a single-molecule magnet iron POM isolated in the cavities of a diamagnetic MOF preserves its magnetic properties (Salomon et al., 2016). Besides, while Ln-MOFs and Ln-doped MOFs have been largely studied, the encapsulation of a well-defined inorganic luminescent compound in MOFs for sensing and detection has been rarely described (Cui et al., 2015;Wu et al., 2017). The white water-stable and highly porous zirconium MOF UiO-67 (Cavka et al., 2008) has been selected, and the ability of the EuW 10 @UiO-67 composite to act as sensor for metallic cations and AAs was investigated. Considering the dualluminescent properties of EuW 10 @UiO-67-arising from both the inserted POMs and the 3D host network-its aptitude to act as a self-calibrated temperature sensor has also been explored.

Physical Methods
Infrared (IR) spectra were recorded on a Nicolet 30 ATR 6700 FT spectrometer. Powder X-Ray diffraction data were obtained on a Bruker D5000 diffractometer using Cu radiation (1.54059 Å). C, H, N elemental analyses were performed by the Service de microanalyse of CNRS, 91198 Gif-sur-Yvette Cedex France. EDX measurements were performed on a JEOL JSM 5800 LV apparatus. Thermogravimetry analyses (TGA) were performed on a Mettler Toledo TGA/DSC 1, STARe System apparatus under oxygen flow (50 mL min −1 ) at a heating rate of 5 • C min −1 up to 800 • C. N 2 adsorption isotherms were obtained at 77 K using a BELsorp Mini (Bel, Japan). Prior to the analysis, approximately 30 mg of sample were evacuated at 90 • C under primary vacuum overnight. Room-temperature and temperature-dependant photoluminescence spectra were recorded on a Jobin-Yvon Fluorolog 3 fluorometer equipped with a CCD camera (excitation source: 450 W Xe arc lamp). The temperature was controlled by a nitrogen-closed cycle cryostat with vacuum system measuring and an Oxford Instrument ITC503S auto-tuning temperature controller with a resistance heater. The temperature can be adjusted from ca. 77 to 300 K with a maximum accuracy of 0.1 K. The sample temperature was fixed to a particular value using the auto-tuning temperature controller; after waiting 5 min to thermalize the sample, five consecutive steady-state emission spectra were measured for each temperature. The luminescence sensing experiments were carried out by introducing EuW 10 @UiO-67 powder (1.5 mg) into aqueous solutions (3 mL, 10 −2 mol L −1 ) of MCl x (M n+ = Na + , K + , Ni 2+ , Cr 3+ , Cu 2+ , Al 3+ , Mn 2+ , and Fe 3+ ) or into MES/NaOH (MES = 2-(N-morpholino)ethanesulfonic acid) buffer solutions (pH = 6) of amino-acids (lysine, L-glycine, β-alanine, L-histidine, L-tryptophane, γ-aminobutyric acid, Larginine) at room temperature. The mixtures were magnetically stirred during 5 min before collecting the luminescence data.

Chemicals and Reagents
All available chemicals were purchased from major chemical suppliers and used as received. The Lindqvist-type europium decatungstate Na 9 [EuW 10 O 36 ]·32H 2 O (EuW 10 )  and the UiO-67 MOF (Salomon et al., 2015) have been synthesized as previously described.

Synthesis of DODA
The dimethyldioctadecylammonium (DODA) salt of EuW 10 was synthesized according to a procedure developed by L. Wu

Synthesis and Characterization of the EuW 10 @UiO-67 Material
We recently evidenced that the [PW 12 O 40 ] 3− Keggin (Salomon et al., 2015), the [P 2 W 18 O 62 ] 6− Dawson (Salomon et al., 2015) as well as the sandwich [(FeW 9 O 34 ) 2 Fe 4 (H 2 O) 2 ] 10− (Salomon et al., 2016) polyanions can be incorporated into the pores of the Zr(IV) biphenyldicarboxylate UiO-67 MOF. Considering the smallest dimensions of these POMs (ca. 12 Å) and the size of the triangular windows of the microporous MOF (ca. 8 Å), no impregnation of the molecular units into the preformed MOF could be envisaged and the POM@UiO-67 materials were obtained via a direct synthesis approach (Salomon et al., 2015(Salomon et al., , 2016. The same approach has been considered here, as the smallest dimension of EuW 10 is ca. 9 Å. The title compound ( Figure 1A) was thus synthesized by heating at 120 • C in DMF a mixture of the dimethyldioctadecyl ammonium (DODA) salt of the POM and the precursors of UiO-67, affording EuW 10 @UiO-67 in good yield (60% based on Zr) as a white crystalline powder.
As observed in Figure S1, the experimental powder X-ray diffraction (PXRD) pattern of EuW 10 @UiO-67 and simulated PXRD pattern of UiO-67 are analogous, showing that the formation of the UiO-67 network can be obtained in the presence of the POM in the synthetic medium. In addition, the experimental PXRD pattern of the as-synthesized EuW 10 @UiO-67 and of EuW 10 @UiO-67 stirred for 1 h in water at room FIGURE 1 | (A) Polyhedral representations of EuW 10 and an octahedral cage of EuW 10 @UiO-67; blue octahedra: WO 6 , pink sphere: Eu, orange polyhedra: ZrO 8 , black sphere: C, (B) SEM image of crystals of EuW 10 @UiO-67 and EDS mapping for zirconium and tungsten, (C) N 2 adsorption/desorption isotherms (77 K, P/P 0 = 1 atm.) of UiO-67 (black) and the EuW 10 @UiO-67 composite (red).
temperature are similar, showing that the hybrid is stable in such conditions. EDS mapping evidences both the presence of MOF and POM ( Figure 1B) Figure S2) reveals steps that are attributed to water removal (weight loss 5.0%, calculated 5.5%), linker decomposition and formation of inorganic oxides (weight loss 58.6%, calculated 60.5%). The IR spectrum (Figure S3) confirms that no DODA counterion is present in the MOF, the negative charges introduced by the POMs being compensated by linker deficiencies (Katz et al., 2013). Also, no vibration at ca. 1,680 cm −1 is observed, indicating that no free benzoic acid is present in the cavities of the MOF. In addition, it can be deduced from the formula that only 1/25 of the octahedral cavities are occupied by EuW 10 , this loading being lower than for [(FeW 9 O 34 ) 2 Fe 4 (H 2 O) 2 ] 10− (1/10) (Salomon et al., 2016). The N 2 sorption isotherms experiments were conducted for both UiO-67 and EuW 10 @UiO-67 ( Figure 1C and Table S1). As expected, the surface area is lower for EuW 10 @UiO-67 (S BET = 1,900 m².g −1 ) than for UiO-67 (S BET = 2,400 m².g −1 ), since a heavy compound representing 5% of the sample weight has been added. The value of the normalized specific surface area, taking into account the contribution of the mass of UiO-67 in EuW 10 @UiO-67 (2,400 × 0.95 = 2,280 m².g −1 ), is significantly different from the experimental value of EuW 10 @UiO-67 (1,900 m².g −1 ), suggesting that EuW 10 is located inside the octahedral cavities and not at the surface of the UiO-67 particles (Table S1). In addition, it is observed that the porous distribution ( Figure S4) is not significantly modified, evidencing only one peak since the hexagonal and tetrahedral cavities are connected by the same triangular windows (Barrett et al., 2012).

Photophysical Characterization of EuW 10 @UiO-67
The photophysical properties of EuW 10 @UiO-67 were thoroughly investigated at room temperature. Firstly, the solid-state photoluminescence (PL) properties of the pure EuW 10 and UiO-67 compounds were studied separately before considering the EuW 10 @UiO-67 composite. As expected, the excitation spectrum of EuW 10 ( Figure S5) is composed of a broad band centered at 280 nm, corresponding to the O→W ligand-to-metal charge transfer (LMCT) band, and the sharp lines characteristic of the f-f transitions of the Eu 3+ ion . Under photoexcitation at λ exc = 280 nm, EuW 10 shows a red luminescence (Figures S5 and S6) associated to the typical 5 D 0 → 7 F 0−4 transitions of Eu 3+ ions in the 587-700 nm range, indicating an efficient intramolecular energy transfer from ligand to Eu 3+ . As this latter ion is located in a slightly distorted square antiprismatic site close to D 4d point symmetry, the 5 D 0 → 7 F 2 electric dipole transition (611-620 nm) gets a very weak intensity compared to that of the 5 D 0 → 7 F 1 magnetic dipole transition (Capobianco et al., 1990; Nogami FIGURE 2 | Room-temperature excitation spectra of EuW 10 @UiO-67 monitored at λ em = 471 nm (red line) and at λ em = 700 nm (blue line) and excitation spectrum of EuW 10 (black dotted line) monitored at λ em = 700 nm. Inset: the asterisk represents the 7 F 0 → 5 L 6 excitation band of the Eu 3+ ion. and Abe, 1996). Upon UV excitation at 254 nm, the UiO-67 MOF exhibits a blue emission ( Figure S6). Its PL spectrum under excitation at 336 nm ( Figure S7) displays a broad band in the 450-600 nm range, with a maximum wavelength located at 471 nm, attributed to intraligand π → π * transitions centered on the biphenyldicarboxylate linker. For the EuW 10 @UiO-67 composite, the excitation spectrum, monitored within the 5 D 0 → 7 F 4 (700 nm) Eu 3+ transition (Figure 2), shows the LMCT broad band (λ max = 280 nm) and the weakly intense sharp lines of the Eu 3+ f-f transitions of EuW 10 as well as the excitation bands of UiO-67 (λ max = 336 nm). The presence of the LMCT band is a strong indication of the integrity of EuW 10 in the composite, whereas the presence of the excitation bands of UiO-67, within the 5 D 0 → 7 F 4 Eu 3+ transition, quite evidences an intermolecular MOF-to-Eu 3+ energy transfer. Then, upon UV excitation at 336 nm, the emission spectrum of EuW 10 @UiO-67 corresponds to the association of the red emission of EuW 10 together with the UiO-67 blue one (Figure 3), and hence, the composite is a good violet emitter with color coordinates (x,y) equal to x = 0.356 and y = 0.246. Moreover, the intensity ratio of the 5 D 0 → 7 F 2 transition to the 5 D 0 → 7 F 1 transition is significantly increased compared to that observed for pure EuW 10 . In addition, the symmetry-forbidden electric dipole 5 D 0 → 7 F 0 transition (580 nm) which does not appear in pure EuW 10 is distinguishable in the PL spectrum of EuW 10 @UiO-67. These results unambiguously reveal that the site symmetry of the Eu 3+ ion in EuW 10 decreases when the POM is introduced into the MOF pores .
Cr 3+ , Cu 2+ , Al 3+ , Mn 2+ , and Fe 3+ ) present in aqueous solutions (10 −2 M) as chloride salts. The PL properties of suspensions of EuW 10 @UiO-67 into the metallic solutions (8.5 µM in Eu 3+ ) were monitored at λ exc = 336 nm and the luminescence intensity variations of the most intense 5 D 0 → 7 F 2 Eu 3+ transition at 611 nm are depicted in Figure 4 and Figure S8a. The presence of all studied metal ions in solution systematically induces a decrease of the luminescence intensity of the Eu 3+ f-f bands compared to their intensity when EuW 10 @UiO-67 is simply immersed in deionized water. The same effect is also observable on the organic ligand emission at 471 nm. Strikingly, Fe 3+ ions can quench the whole emission of the composite, which becomes non-emissive under UV-light. This high sensitivity regarding Fe 3+ ions offers an interesting perspective to use EuW 10 @UiO-67 as an efficient luminescent chemical probe for Fe 3+ ions which play a relevant role in many biological processes (Xu and Yan, 2015). A study of the dependence of the quenching of the luminescence with the Fe 3+ concentration was then conducted. The quenching effect can be quantitatively rationalized by the Stern-Volmer equation: I 0 /I = 1 + K SV [Fe 3+ ], where I 0 and I are the fluorescence intensities of the 5 D 0 → 7 F 2 transition of the Eu 3+ ion in the EuW 10 @UiO-67 suspension in the absence or presence of Fe 3+ , respectively. K SV is the quenching constant, which is calculated as 2,667 L.mol −1 with a very good linear correlation (R) of 0.999, indicating the strong quenching effect from Fe 3+ (Figure S9). The concentration limit for the detection is estimated to be 37 µM, corresponding to the concentration for which the fluorescence intensity variation is <10%. To assess the robustness of the EuW 10 @UiO-67 composite, we have checked its recyclability after immersion into a Fe 3+ aqueous solution (C = 10 −2 M). The emission spectrum of the powder was monitored in the same conditions ( Figure S10) before, directly after immersion, and after being washed several times with ultrapure water, filtered and dried under air. This study highlights that after washing, the emission of the composite practically recovered its initial intensity, quite evidencing that the EuW 10 @UiO-67 exhibits a robustness enabling to be reusable to detect Fe 3+ ions in an aqueous solution. Nevertheless, while the XRD data of EuW 10 @UiO-67 after immersion in Fe 3+ aqueous solution are in accordance with the simulated UiO-67 pattern, a certain loss of crystallinity is observed (Figure S11). In short, although the amount of the EuW 10 emitter is weak into the MOF, the properties of the composite as a chemical probe is very promising with a recyclability, never investigated in these systems so far.
EuW 10 @UiO-67 was also tested as chemical probe for basic amino-acids (AAs), such as glycine, β-alanine, L-histidine, Ltryptophan, L-arginine, and γ-aminobutyric acid ( Table S2). EuW 10 @UiO-67 (8.5 µM in Eu 3+ ) was dispersed in MES/NaOH (pH = 6) buffer solutions of AAs (10 −2 M) to further monitor the luminescence properties at λ exc = 336 nm. The emission spectra are depicted in Figure S8b. As already noticed for the pure [Eu(SiW 10 MoO 39 ) 2 ] 13− POM in homogeneous aqueous solutions , distinct luminescence enhancements are observed when the EuW 10 @UiO-67 material is in contact with the various AAs molecules (Figure 4). The arginine, which is the most basic AA of the investigated series, induces more than 2.5-fold enhancement of emission intensity. Moreover, the increase of the emission intensity globally follows the increase of the AA pKa and isoelectric point (Table S2). This is in line with the fact that the luminescence of Eu-based POMs is quenched by the surrounding water molecules, which are displaced in presence of protonated AAs due to electrostatic interactions and C-H· · ·O contacts with the POM surface . The recyclability of the EuW 10 @UiO-67 composite after exposure to the arginine solution has been also investigated. The emission spectrum of the powder was monitored in the same conditions ( Figure S12) before, directly after immersion, and after being washed several times with ultrapure water, filtered and dried under air. This study highlights that after washing, the emission of the composite practically recovered its initial intensity. However, the XRD data of EuW 10 @UiO-67 after immersion in arginine aqueous solution also reveal a certain loss of crystallinity.
EuW 10 @UiO-67 as Self-Calibrated-Ratiometric Luminescent Thermometer As both the biphenyldicarboxylate linker of UiO-67 and EuW 10 act as emitters, we further evaluated the capability of EuW 10 @UiO-67 to be used as a self-calibrated ratiometric luminescent thermometer. For that, the integrated areas of the ligand (I Ligand ) and 5 D 0 → 7 F 2 Eu 3+ (I Eu ) emissions were used to define the thermometric parameter = I Ligand /I Eu permitting the conversion of emission intensities into absolute temperature. The temperature dependence of the EuW 10 @UiO-67 emission is presented in Figure 5 for the 150-300 K range. Five consecutive emission spectra were collected for each temperature and used to determine subsequent average data. The I Ligand and I Eu parameters have been obtained by integrating the emission spectra in the 438-575 nm and 604-630 nm wavelength intervals, respectively. The temperature dependence of the defined thermometric parameters in the 200-300 K range is presented in Figure S13, where the solid line represents the temperature calibration curve. The parameter decreases with the temperature according to the following empirical linear relationship: = 1.985 -0.0029 T with a correlation coefficient R² of 0.998. The corresponding relative sensitivity, defined as s r = |∂ /∂T|/ , and used as a figure of merit to compare the performance of distinct systems (Brites et al., 2016) is plotted in Figure S14, showing that EuW 10 @UiO-67 exhibits a maximum relative sensitivity S m of ca. 0.26% K −1 for the physiological temperature range.

CONCLUSION
In conclusion, we have reported herein an easy to prepare luminescent multifunctional material made of a metalorganic framework incorporating a europium-containing polyoxotungstate. Despite the very low amount of POM inserted in the UiO-67 matrix, it has been shown that EuW 10 @UiO-67 is able to act as a sensor ([Eu 3+ ] = 8.5 µM) for metallic ions and amino-acids in water and in heterogeneous conditions. Among a series of cationic species, a good sensitivity has been found for Fe 3+ , while an enhancement of the EuW 10 @UiO-67 luminescence is observed in presence of amino-acids, the enhancement of the emission intensity globally following the increase of the amino-acid pKa. Moreover, the recyclability of EuW 10 @UiO-67 has been demonstrated, even if a certain loss of crystallinity is observed. In addition, the dual-luminescent properties-arising both from the POM and from the MOF -lead the reported material to behave as a self-calibrated ratiometric luminescent thermometer. Considering the huge number of luminescent POMs and MOFs already available, this first report of a dual-luminescent POM@MOF material thus opens the way to the development of such optical sensors.