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 [EuW₁₀O₃₆]⁹⁻ (EuW₁₀) europium decatungstate characterized by narrow emission bands, a large Stokes shift and a long lifetime (Sugeta and Yamase, 1993). 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 EuW₁₀ 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 EuW₁₀ caused by the strong electrostatic and hydrogen bond interactions between POMs and basic amino-acids (AAs) (Zhang et al., 2015). 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 EuW₁₀ 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 EuW₁₀ 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 one-pot 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 pre-formed MOF is impregnated with a solution of the POM (two-step strategy). Such materials have shown their efficiency for catalytic reactions ranging from the oxidation of alkylbenzene (Sun et al., 2016) 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 EuW₁₀@UiO-67 composite to act as sensor for metallic cations and AAs was investigated. Considering the dual-luminescent properties of EuW₁₀@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.

Experimental Section

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. N₂ 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 EuW₁₀@UiO-67 powder (1.5 mg) into aqueous solutions (3 mL, 10⁻² mol L⁻¹) of MCl_x (Mⁿ⁺ = Na⁺, K⁺, Ni²⁺, Cr³⁺, Cu²⁺, Al³⁺, Mn²⁺, and Fe³⁺) 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, L-arginine) at room temperature. The mixtures were magnetically stirred during 5 min before collecting the luminescence data.

Synthetic Procedures

Chemicals and Reagents

All available chemicals were purchased from major chemical suppliers and used as received. The Lindqvist-type europium decatungstate Na₉[EuW₁₀O₃₆]·32H₂O (EuW₁₀) (Sugeta and Yamase, 1993) and the UiO-67 MOF (Salomon et al., 2015) have been synthesized as previously described.

Synthesis of DODA₉[EuW₁₀O₃₆]·(DODACl)₂ ((DODA)EuW₁₀)

The dimethyldioctadecylammonium (DODA) salt of EuW₁₀ was synthesized according to a procedure developed by L. Wu. et al. for (DODA)₁₃H₂[Eu(BW₁₁O₃₉)₂]·25H₂O (Li et al., 2008). A stoichiometric amount of DODA.Cl (1 equivalent by charge, 3.281 g, 5.6 mmol) was dissolved in 20 mL of chloroform. The solution was then added dropwise to a 10 mL aqueous solution of Na₉[EuW₁₀O₃₆]·32H₂O (2.082 g, 0.62 mmol) under stirring. The mixture was kept under vigorous stirring for 2 h. The organic phase was separated and dried with MgSO₄. The solvent was then evaporated with a rotary evaporator to recover the DODA salt (3.9 g, yield = 72%). Anal. Calc. (found) for (DODA)₉[EuW₁₀O₃₆]·(DODACl)₂ (C₄₁₈H₈₈₀N₁₁EuW₁₀O₃₆Cl₂; M.W.: 8,699 g mol⁻¹): C 57.71 (57.45), H 10.20 (10.51), N 1.77 (1.66). IR (ATR): ν (cm⁻¹) 2,916 (s), 2,849 (s), 1,466 (m), 940 (m), 922 (m), 836 (m), 753 (s),718 (s). EDX measurements: atomic ratio calc. (exp.): W/Eu 10.0 (11.5).

Synthesis of [Zr₆o₄(OH)₄][C₁₄H₈O₄]_5.82[EuW₁₀O₃₆]_0.04·7H₂O (EuW₁₀@UiO-67)

ZrCl₄ (116 mg, 0.5 mmol), biphenyl-dicarboxylic acid (121 mg, 0.5 mmol), (DODA)EuW₁₀ (429 mg, 4.93 10⁻⁵ mol) and benzoic acid (1.83 g, 15 mmol) were briefly stirred in 10 mL of dimethylformamide (DMF) inside a 23 mL polytetrafluoroethylene-lined vessel. Hydrochloric acid 37% (83 μL) was added, and the mixture was heated at 120°C for 24 h. The solid was isolated by filtration of the hot mixture and thoroughly washed with DMF, chloroform, dry acetone and dried in an oven at 120°C overnight (yield: 118 mg, 60% based on Zr, 4% based on POM). Anal. Calc. (found) for [Zr₆O₄(OH)₄][C₁₄H₈O₄]_5.82[EuW₁₀O₃₆]_0.04·7H₂O (C_81.2H_64.4O_39.64Zr₆W_0.4Eu_0.04; M.W.: 2,306 g mol⁻¹): C 42.40 (42.50), H 2.82 (3.21). IR (ATR): ν (cm⁻¹) 1,593 (m), 1,545 (m), 1,503 (w), 1,409 (s), 1,180 (w), 770 (m), 754 (w),736 (m), 704 (m), 669 (s), 455 (s). EDX measurements: atomic ratio calc. (exp.): Zr/W 15.0 (15.6), Eu/W 0.10 (0.13).

Results and Discussion

Synthesis and Characterization of the EuW₁₀@UiO-67 Material

We recently evidenced that the [PW₁₂O₄₀]³⁻ Keggin (Salomon et al., 2015), the [P₂W₁₈O₆₂]⁶⁻ Dawson (Salomon et al., 2015) as well as the sandwich [(FeW₉O₃₄)₂Fe₄(H₂O)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, 2016). The same approach has been considered here, as the smallest dimension of EuW₁₀ 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 EuW₁₀@UiO-67 in good yield (60% based on Zr) as a white crystalline powder.

FIGURE 1

Figure 1. (A) Polyhedral representations of EuW₁₀ and an octahedral cage of EuW₁₀@UiO-67; blue octahedra: WO₆, pink sphere: Eu, orange polyhedra: ZrO₈, black sphere: C, (B) SEM image of crystals of EuW₁₀@UiO-67 and EDS mapping for zirconium and tungsten, (C) N₂ adsorption/desorption isotherms (77 K, P/P₀ = 1 atm.) of UiO-67 (black) and the EuW₁₀@UiO-67 composite (red).

As observed in Figure S1, the experimental powder X-ray diffraction (PXRD) pattern of EuW₁₀@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 EuW₁₀@UiO-67 and of EuW₁₀@UiO-67 stirred for 1 h in water at room temperature are similar, showing that the hybrid is stable in such conditions. EDS mapping evidences both the presence of MOF and POM (Figure 1B). EDX measurements combined with elemental analysis allowed to propose for EuW₁₀@UiO-67 the formula [Zr₆O₄(OH)₄][C₁₄H₈O₄]_5.82[EuW₁₀O₃₆]_0.04·7H₂O, indicating that the material only contains ca. 0.25 wt % in europium. The TGA curve (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 EuW₁₀, this loading being lower than for [(FeW₉O₃₄)₂Fe₄(H₂O)₂]¹⁰⁻ (1/10) (Salomon et al., 2016). The N₂ sorption isotherms experiments were conducted for both UiO-67 and EuW₁₀@UiO-67 (Figure 1C and Table S1). As expected, the surface area is lower for EuW₁₀@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 EuW₁₀@UiO-67 (2,400 × 0.95 = 2,280 m2.g⁻¹), is significantly different from the experimental value of EuW₁₀@UiO-67 (1,900 m2.g⁻¹), suggesting that EuW₁₀ 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 EuW₁₀@UiO-67

The photophysical properties of EuW₁₀@UiO-67 were thoroughly investigated at room temperature. Firstly, the solid-state photoluminescence (PL) properties of the pure EuW₁₀ and UiO-67 compounds were studied separately before considering the EuW₁₀@UiO-67 composite. As expected, the excitation spectrum of EuW₁₀ (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 Eu³⁺ ion (Yamase and Sugeta, 1993). Under photoexcitation at λ_exc = 280 nm, EuW₁₀ shows a red luminescence (Figures S5 and S6) associated to the typical ⁵D₀→⁷F₀₋₄ transitions of Eu³⁺ ions in the 587–700 nm range, indicating an efficient intramolecular energy transfer from ligand to Eu³⁺. As this latter ion is located in a slightly distorted square antiprismatic site close to D_4d point symmetry, the ⁵D${}_0{\to }^7$F₂ electric dipole transition (611–620 nm) gets a very weak intensity compared to that of the ⁵D${}_0{\to }^7$F₁ magnetic dipole transition (Capobianco et al., 1990; Nogami 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 EuW₁₀@UiO-67 composite, the excitation spectrum, monitored within the ⁵D₀→⁷F₄ (700 nm) Eu³⁺ transition (Figure 2), shows the LMCT broad band (λ_max = 280 nm) and the weakly intense sharp lines of the Eu³⁺ f-f transitions of EuW₁₀ 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 EuW₁₀ in the composite, whereas the presence of the excitation bands of UiO-67, within the ⁵D₀→⁷F₄ Eu³⁺ transition, quite evidences an intermolecular MOF-to-Eu³⁺ energy transfer. Then, upon UV excitation at 336 nm, the emission spectrum of EuW₁₀@UiO-67 corresponds to the association of the red emission of EuW₁₀ 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 ⁵D${}_0{\to }^7$F₂ transition to the ⁵D${}_0{\to }^7$F₁ transition is significantly increased compared to that observed for pure EuW₁₀. In addition, the symmetry-forbidden electric dipole ⁵D${}_0{\to }^7$F₀ transition (580 nm) which does not appear in pure EuW₁₀ is distinguishable in the PL spectrum of EuW₁₀@UiO-67. These results unambiguously reveal that the site symmetry of the Eu³⁺ ion in EuW₁₀ decreases when the POM is introduced into the MOF pores (Sugeta and Yamase, 1993).

FIGURE 2

Figure 2. Room-temperature excitation spectra of EuW₁₀@UiO-67 monitored at λ_em = 471 nm (red line) and at λ_em = 700 nm (blue line) and excitation spectrum of EuW₁₀ (black dotted line) monitored at λ_em = 700 nm. Inset: the asterisk represents the ⁷F₀→⁵L₆ excitation band of the Eu³⁺ ion.

FIGURE 3

Figure 3. (A) Room-temperature emission spectrum monitored at λ_exc = 336 nm of EuW₁₀@UiO-67. The asterisk highlights the ⁵D${}_0{\to }^7$F₀ transition related to the Eu³⁺ ion; (B) CIE chromaticity diagram for EuW₁₀@UiO-67 excited at 336 nm.

EuW₁₀@UiO-67 as Sensor for Metallic Cations and Amino-Acids

The luminescent sensing properties of EuW₁₀@UiO-67 were first investigated for the detection of metal ions (Na⁺, K⁺, Ni²⁺, Cr³⁺, Cu²⁺, Al³⁺, Mn²⁺, and Fe³⁺) present in aqueous solutions (10⁻² M) as chloride salts. The PL properties of suspensions of EuW₁₀@UiO-67 into the metallic solutions (8.5 μM in Eu³⁺) were monitored at λ_exc = 336 nm and the luminescence intensity variations of the most intense ⁵D${}_0{\to }^7$F₂ Eu³⁺ 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 Eu³⁺ f-f bands compared to their intensity when EuW₁₀@UiO-67 is simply immersed in deionized water. The same effect is also observable on the organic ligand emission at 471 nm. Strikingly, Fe³⁺ ions can quench the whole emission of the composite, which becomes non-emissive under UV-light. This high sensitivity regarding Fe³⁺ ions offers an interesting perspective to use EuW₁₀@UiO-67 as an efficient luminescent chemical probe for Fe³⁺ 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 Fe³⁺ concentration was then conducted. The quenching effect can be quantitatively rationalized by the Stern-Volmer equation: I₀/I = 1 + K_SV[Fe³⁺], where I₀ and I are the fluorescence intensities of the ⁵D₀→⁷F₂ transition of the Eu³⁺ ion in the EuW₁₀@UiO-67 suspension in the absence or presence of Fe³⁺, 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 Fe³⁺ (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 EuW₁₀@UiO-67 composite, we have checked its recyclability after immersion into a Fe³⁺ 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 EuW₁₀@UiO-67 exhibits a robustness enabling to be reusable to detect Fe³⁺ ions in an aqueous solution. Nevertheless, while the XRD data of EuW₁₀@UiO-67 after immersion in Fe³⁺ 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 EuW₁₀ 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.

FIGURE 4

Figure 4. Luminescence intensity variations of the ⁴D${}_0{\to }^7$F₂ transition (λ_exc = 336 nm) of EuW₁₀@UiO-67 interacting with amino-acids MES/NaOH buffer solutions (glyc., glycine; ala., β-alanine; hist, L-histidine; trypt., L-tryptophan; buty., γ-aminobutyric acid; arg., L-arginine) and different metal ions aqueous solutions (C = 10⁻² mol.L⁻¹).

EuW₁₀@UiO-67 was also tested as chemical probe for basic amino-acids (AAs), such as glycine, β-alanine, L-histidine, L-tryptophan, L-arginine, and γ-aminobutyric acid (Table S2). EuW₁₀@UiO-67 (8.5 μM in Eu³⁺) 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(SiW₁₀MoO₃₉)₂]¹³⁻ POM in homogeneous aqueous solutions (Li et al., 2014), distinct luminescence enhancements are observed when the EuW₁₀@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 (Li et al., 2014). The recyclability of the EuW₁₀@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 EuW₁₀@UiO-67 after immersion in arginine aqueous solution also reveal a certain loss of crystallinity.

EuW₁₀@UiO-67 as Self-Calibrated-Ratiometric Luminescent Thermometer

As both the biphenyldicarboxylate linker of UiO-67 and EuW₁₀ act as emitters, we further evaluated the capability of EuW₁₀@UiO-67 to be used as a self-calibrated ratiometric luminescent thermometer. For that, the integrated areas of the ligand (I_Ligand) and ⁵D₀→⁷F₂ Eu³⁺ (I_Eu) emissions were used to define the thermometric parameter Δ = I_Ligand/I_Eu permitting the conversion of emission intensities into absolute temperature. The temperature dependence of the EuW₁₀@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 Δ in the 200–300 K range is presented in Figure S13, where the solid line represents the temperature calibration curve. The parameter Δ decreases with the temperature according to the following empirical linear relationship:

$\Delta =1.985-0.0029T$

FIGURE 5

Figure 5. Emission spectra of compound EuW₁₀@UiO-67 in the range 150–300 K with the excitation fixed at 336 nm.

with a correlation coefficient R2 of 0.998. The corresponding relative sensitivity, defined as s_r = |∂Δ/∂T|/Δ, 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 EuW₁₀@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 metal-organic framework incorporating a europium-containing polyoxotungstate. Despite the very low amount of POM inserted in the UiO-67 matrix, it has been shown that EuW₁₀@UiO-67 is able to act as a sensor ([Eu³⁺] = 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 Fe³⁺, while an enhancement of the EuW₁₀@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 EuW₁₀@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.

Author Contributions

WS, AD, CR-M, PM and GP have performed the synthesis and the structural characterizations of the title compound. RD and HS-B have performed the photophysical characterizations. The manuscript was written by PM and HS-B with valuable contributions and corrections of CR-M and RD.

Funding

This work was supported by CNRS, UVSQ, University of Nantes and the French National Research Agency (ANR) as part of the Investissements d'Avenir program n°ANR-11-IDEX-0003-02 and CHARMMMAT ANR-11-LABX-0039.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2018.00425/full#supplementary-material

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