Lanthanide-Functionalized Metal–Organic Framework Hybrid Systems

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Cite This: Acc. Chem. Res. 2017, 50, 2789-2798

Lanthanide-Functionalized Metal−Organic Framework Hybrid Systems To Create Multiple Luminescent Centers for Chemical Sensing Bing Yan* School of Chemical Science and Engineering, Tongji University, Siping Road 1239, Shanghai 200092, China CONSPECTUS: Metal−organic frameworks (MOFs) possess an important advantage over other candidate classes for chemosensory materials because of their exceptional structural tunability and properties. Luminescent sensing using MOFs is a simple, intuitive, and convenient method to recognize species, but the method has limitations, such as insufficient chemical selectivity and signal loss. MOFs contain versatile building blocks (linkers or ligands) with special chemical reactivity, and postsynthetic modification (PSM) provides an opportunity to exploit and expand their unique properties. The linkers in most MOFs contain aromatic subunits that can readily display luminescence after ultraviolet or visible (typically blue) excitation, and this is the main luminescent nature of most MOFs. The introduction of photoactive lanthanide ions (Ln3+) into the MOF hosts may produce new luminescent signals at different positions from that of the MOF linker, but this depends on the intramolecular energy transfer (antenna effect) from the MOF (linkers) to the Ln3+ ions. Controlling the Ln3+ content in MOF hybrids may create multiple luminescent centers. The nature of the unique luminescent centers may cause different responses to sensing species (i.e., ratiometric sensing), which may provide a new opportunity for luminescence research with applications to chemical sensing. In this Account, recent research progress on using lanthanide-functionalized MOF hybrid materials to create multiple luminescent centers for chemical sensing is described. Here we propose a general strategy to functionalize MOF hosts with lanthanide ions, compounds, or other luminescent species (organic dyes or carbon dots) and to assemble types of photofunctional hybrid systems based on lanthanide-functionalized MOFs. Five main methods were used to functionalize the MOFs and assemble the hybrid materials: in situ composition, ionic doping, ionic exchange, covalent PSM, and coordinated PSM. Through the lanthanide functionalization, multiple (double or triple) luminescent centers were created with different luminescent bands in the visible region. Because of the different luminescent natures of the lanthanide ions, MOF linkers, and other species (organic dyes or carbon dots), they display different responses to sensing species. Currently, using these strategies, we have utilized a dual-response luminescent probe to realize chemical sensing of different types of cations (Fe3+/Fe2+, Hg2+, and Cd2+), anions (Cr2O72−/CrO4− and CO32−), molecules (volatile organic compounds and O2), special air pollutants (formaldehyde), and biomarkers of food spoilage as well as pH and temperature. Additionally, we have achieved tripleluminescence-response sensing of ions (Ag+, Hg2+, and S2−) in complicated aqueous environments, which was developed using a logic operation. comes from five components in MOFs,8 and the luminescence of the linkers and metal ions is the most important and extensive. The luminescence from groups on linkers or ligands can be emitted directly from the linker or can involve charge transfer with coordinated metal ions or clusters. Framework metal ions can cause a pronounced increase in the luminescence intensity through an antenna effect, e.g., lanthanide ion (Ln3+) MOF systems. MOFs offer a unique platform for luminescent materials because they have a degree of structural predictability and well-defined environments for luminophores in a crystalline state.9

1. INTRODUCTION Metal−organic frameworks (MOFs) or porous coordination polymers (PCPs) have caused a renaissance in coordination chemistry because of their simple synthesis and versatility that originates from their organic linkers. The highly porous and crystalline frameworks of MOFs endow them with suitable properties or functions for practical applications.1−5 MOFs can undergo late-stage transformations without compromising the overall framework integrity, and a variety of chemical reactions are available to modify the framework components. Postsynthetic modification (PSM) of MOFs is broadly defined as the chemical derivatization of MOFs after their formation.6,7 With both organic ligands and metal ions, MOFs exhibit a wide range of emissive phenomena. Luminescence mainly © 2017 American Chemical Society

Received: August 4, 2017 Published: October 6, 2017 2789

DOI: 10.1021/acs.accounts.7b00387 Acc. Chem. Res. 2017, 50, 2789−2798

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Accounts of Chemical Research These features allow for the development of photofunctional hybrid materials based on Ln3+-functionalized MOFs. When Ln3+ ions are introduced to functionalize a MOF host, they provide multiple (mainly dual) luminescent centers in the hybrid systems that emit at different positions in the visible region. Generally, the luminescence from linkers occurs in the blue or violet region with a narrow wavelength, but photoactive lanthanide ions, especially Eu3+, emit in the red region with a long wavelength, which allows for luminescence tuning and white-light integration for practical applications in lighting or displays. The different luminescent natures of these species (linkers, Ln3+, or other species) also result in different responses, which are useful for chemical sensing, especially ratiometric sensing. In this Account, a general strategy is proposed to create multiple luminescent centers by Ln3+ functionalization of MOF hosts to create hybrid systems for chemical sensing (Figure 1).

Figure 2. Scheme for the five main paths to assemble luminescent lanthanide-functionalized MOF hybrids, with typical MOFs shown in parentheses.

emission at 450 nm. Through selective excitation, up- and downconversion luminescence was obtained. Duan and Yan14 embedded zinc oxide into MOFs (ZnO@Zn(pdc), pdc = 2,5pyridinedicarboxylate). Upon the formation of ZnO, the morphology of the hybrid system changed. Moreover, Eu3+activated MOFs were obtained and displayed white-light emission through a dual-emitting pathway (Eu3+ ion and ZnO), and the fabricated white-light-emitting diode (WLED) had CIE and correlated color temperature (CCT) values of (0.35, 0.41) and 5018 K, respectively.

Figure 1. Scheme for the strategy to assemble lanthanide-functionalized MOF hybrid systems for chemical sensing.

Here dual-luminescent centers are mainly used, and Eu3+ is the most important lanthanide ion because of its bright red emission. For triple-luminescent centers, two different lanthanide ions (e.g., Eu3+ and Tb3+) are both introduced into the MOF units via a simple process. A more extensive method can be used to assemble other photoactive species (organic dyes or carbon dots). Thus, chemical sensing research can be performed by examining the responses of the different luminescent centers to various sensing species.

2.2. Ionic Doping

For lanthanide MOFs, ionic doping is the first choice to assemble lanthanide-ion-functionalized MOF hybrids. Duan and Yan15 prepared MOF-76(Y):Ln hybrids via ionic doping of MOF-76 developed by Yaghi’s group.16 The photoluminescent color of MOF-76(Y):10 mol % Eu3+/10 mol % Tb3+ could be tuned from yellow to yellow-green, warm white, and orange by changing the excitation wavelength of 300−380 nm. The surface of a NMOF-76:Eu3+ polymer film was smooth, continuous, and transparent under sunlight.15 Lian and Yan17 further prepared a series of MOF-76(Ln) compounds using the ionic doping approach and evaluated their adsorption ability for different dyes. The samples soaked in cationic dyes were dyed blue, red, violet, and pink, but the samples soaked in the anionic dyes remained colorless. The selective adsorption of different dyes was due to the interactions between the dye molecules and the frameworks, which could be used to develop an adsorbent to remove cationic dyes from aquatic environments.

2. FUNCTIONALIZATION PATHS Lanthanide-ion-functionalized MOFs as photofunctional hybrid systems can be created via several methods, and the method depends on the MOF host. According to our research and other relevant work, five main methods are used for different interactions between Ln3+ ions and MOFs (Figure 2). 2.1. In Situ Composition

Lanthanide compounds can be assembled with MOF hosts through an in situ solvent dispersion process, which is physically composed to keep the whole luminescent hybrid system stable. Here we select zeolitic imidazolate frameworks (ZIFs) with special imidazole derivative linkers as an example.10−12 Liu and Yan13 assembled surfactant-capped nanoparticles (NaYF4:Yb3+, Er3+/Tm3+) with ZIF-8 through the in situ composition method. Under excitation with a 980 nm diode laser, the NaYF4:Yb3+/Er3+@ZIF-8 and NaYF4:Yb3+/ Tm3+@ZIF-8 nanocomposites emitted yellow-green and lightviolet-white colors, respectively. Under near-ultraviolet (NUV) excitation at 396 nm, both nanocomposites displayed a blue

2.3. Ionic Exchange

Many anionic MOF systems with dimethylammonium (DMA) cations in their porous structure exist, and other cations, including metal ions, can be introduced into the MOF via ionic exchange with DMA. For example, Bio-MOF-1 (Zn8(ad)4(BPDC)6O·2Me2NH2, ad = adeninate, BPDC = biphenyldicarboxylate) was functionalized via postsynthetic cation exchange of luminescent lanthanide ions by An et al.,18 and visible (Tb3+, Sm3+, and Eu3+) and NIR (Yb3+) emissions 2790

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Figure 3. (a) Temperature-dependent emission spectra of Eu3+@UiO-bpydc. (b) Intensity ratio of bpydc (530 nm) to Eu3+ (614 nm) as a function of temperature. (c) Temperature dependence of the luminescence lifetime of Eu3+. (d) Thermometric response curve plotting the back energy transfer rate (KBEnT) vs temperature. (e) Arrhenius plots for KBEnT of Eu3+@UiO-bpydc. Reproduced with permission from ref 40. Copyright 2015 Royal Society of Chemistry.

within the MOF pores were observed.19 Shen and Yan20 introduced 2-thenoyltrifluoroacetone (TTA) to the Eu3+ (or Eu3+/Tb3+)-exchanged Bio-MOF-1 via the “ship-in-bottle” method. The introduction of the TTA ligand to the Eu3+@ Bio-MOF-1 hybrid system improved the luminescence intensity of Eu3+, and the emission intensity of the Bio-MOF barely changed. The emission spectrum of double lanthanide ion (Eu3+/Tb3+)-functionalized Bio-MOF-1 hybrids showed CIE coordinates close to the white region. Zhao et al.21 presented an anionic MOF, {[Me2NH2]0.125[In0.125(H2L)0.25]·xDMF}n (H4L = 4,8-disulfonyl-2,6-naphthalenedicarboxylic acid), and used Ln3+-modified MOF hybrids as potential luminescent probes for Ln3+ ions.

characteristic luminescence of Ln3+ in both the visible and NIR regions. 2.5. Coordinated Postsynthetic Modification

If the linkers in MOFs possess free coordinating groups during the construction of MOF systems, they provide a position to react with lanthanide ions. Some typical linkers include 2,2′bipyridine-5,5′-dicarboxylic acid (H2bpydc) and multiple carboxylic acids. H2bpydc possesses two free bipyridine N atoms and forms a typical MOF, MOF-253,25 which can be coordinated to Ln3+ to realize the PSM.26 Lu and Yan27 prepared a MOF-based monolayer luminescent thin film by encapsulating lanthanide ions into a fabricated MOF (MOF253) host and then introducing a second ligand, TTA or 1,1,1trifluoropentane-2,4-dione (TAA), to sensitize the incorporated Ln3+ ions. Additionally, they introduced multiple visibleemitting Ln3+ ions (Eu3+, Tb3+, and Sm3+) to MOF-253 via PSM for barcoding.28 The luminescence intensities of two lanthanide ions could be quantitatively controlled and reflected as a unique visible color corresponding to three distinct barcodes. Research work could be extended to other MOFs with similar linkers.29,30 For example, a thin layer of Eu3+@ MOFUiO-67-bpydc was coated onto a commercially available UV-LED chip, and upon illumination, the chip generated bright white light with CIE and CCT values of (0.3481, 0.3292) and 4914 K, respectively. The color rendering index (CRI) was 75, and the luminous efficiency of the fabricated LED was 32 lm W−1.30 Other typical MOFs for PSM were an Al-MIL-53 derivative31 and its analogues with a noncoordinating carboxyl group.32−36 Zhou and Yan32 introduced Ln3+ cations into the pores of Al-MIL-53-COOH. The emission colors were modulated from blue to white to red. They prepared a transparent, continuous, and homogeneous thin film of Eu3+@

2.4. Covalent Postsynthetic Modification

The aim of covalent postsynthetic modification of MOFs is to create coordination groups of linkers in MOFs to further coordinate lanthanide ions. Abdelhameed et al.22,23 used the isoreticular MOF IRMOF-3 because of its noncoordinating amino groups on the benzenedicarboxylate linker, which could be easily modified by nucleophilic substitution, nucleophilic addition, or reductive amination and subsequently coordinated with Ln3+ ions. The second aromatic ring on the β-diketonate as an assistant ligand could further enhance the Ln3+ sensitivity. Lian and Yan24 prepared lanthanide hybrids containing both MOFs and SBA-15 (a hexagonal mesoporous silica structure) using the covalent postsynthetic modification path. SBA-15 is easily modified by a cross-linking reagent to form Si-SBA-15, and IRMOF-3 could be covalently grafted to Si-SBA-15 through an addition reaction. Finally, lanthanide ions were introduced into the SBA-15-Si-IRMOF-3 host via a coordination reaction, and the resultant product exhibited the 2791

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Figure 4. (top) Scheme of MOF-253 modified by the Eu3+ complex with TTA, with two types of Eu3+ (Eu1 and Eu2). (bottom) (a, b) Emission spectra of MOF-253-Eu-TTA in aqueous solutions with different pH with (a) λex = 330 nm and (b) λex = 375 nm (b); (c) linear variation of the 5D0 → 7F2 emission intensity ratio (Ir) of the two types of Eu3+. Reproduced with permission from ref 41. Copyright 2014 Royal Society of Chemistry.

Figure 5. (a) PL spectra of Eu3+@MIL-124 dispersed into different aqueous solutions of various metal ions (10−2 mol L−1). The inset shows the spectra of Fe2+ (black line) and Fe3+ (red line). (b) Luminescence intensities of the 4D0 → 7F2 transition of Eu3+@MIL-124 interacting with different metal ions in 10−2 mol L−1 aqueous solutions of MCln (λex = 298 nm). The inset shows the corresponding photographs under UV irradiation at 254 nm. Reproduced from ref 36. Copyright 2014 American Chemical Society.

response luminescence is the most popular, and herein we mainly focus on chemical sensing with dual-luminescent centers.

Al-MIL-53-COOH, which showed white-light emission under 350 nm excitation. Xu and Yan 3 6 used MIL-124 (Ga2(OH)4(C9O6H4)) to construct hybrids with Ln3+ cations, and the hybrids showed the luminescent properties of both the Ln3+ ions and the linker. In addition, Cao et al.37 prepared Ln3+-exchanged MOFs to sensitize Tb3+.

3.1. Thermometer

Ratiometric sensing based on two different luminescent centers can be used as a thermometer, which is based on MOF hosts cofabricated with two lanthanide ions (Eu3+ and Tb3+). As a result of energy transfer from Tb3+ to Eu3+, the MOFs have different luminescence intensities as the temperature changes. In addition, this technique does not require additional calibration of the luminescence intensity. Rao et al.38 first created a mixed lanthanide MOF thermometer, Tb0.9Eu0.1PIA (PIA = 5-(pyridin-4-yl)isophthalate), which exhibited a significantly different temperature-dependent luminescent behavior compared with the emissions of Eu3+ at 615 nm and

3. CHEMICAL SENSING This section focuses on lanthanide-ion-functionalized MOFs as hybrid systems with other photoactive species and the use of their multiple luminescent centers for chemical sensing. Luminescent centers may have different responses to sensing species based on their luminescent nature. For example, the luminescence intensity can show contrary changes (quenching or enhancement), resulting in ratiometric sensing. Dual2792

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Figure 6. (a) Linear relationship between IEu/ICD and CHg2+ (λex = 360 nm). The inset shows photographs of the test plates treated with different concentrations of Hg2+. (b) Comparison of the PL intensity ratio IEu/ICD for Eu3+/CDs@MOF-253 dispersed in aqueous solutions of metal ions. Reproduced with permission from ref 43. Copyright 2016 Royal Society of Chemistry.

Figure 7. (a) Emission spectra and (b) IL/IEu intensity ratios of Eu3+@UiO-bpy after the encapsulation of VOCs. (c) 2D decoded map of the aromatic VOCs based on the emission intensity ratio (IL/IEu) and quantum yield (Φ) responses of the Eu3+@bpy-UiO nanocomposite. Reproduced with permission from ref 46. Copyright 2016 Royal Society of Chemistry.

Figure 8. (left) Values of the luminescence intensity ratio (IEu/IL) for coated NMOFs@O-PP films under repeated cycles of 1 atm O2 and 1 atm N2 conditions with λex = 395 nm. (right) Photographs of a 395 nm LED modified by coating with NMOFs@O-PP and emission spectra of the coated NMOFs@O-PP under (a) 100% N2 and (b) 100% O2. Reproduced with permission from ref 47. Copyright 2016 Royal Society of Chemistry.

Tb3+ at 546 nm. From 100 to 300 K, the emission intensity of Tb3+ decreased while that of Eu3+ increased, and the temperature-dependent luminescence colors could be tuned from green to yellow. Zhou et al.39 discovered the temperature sensing potential of Ln3+@In(OH)(bpydc) hybrids obtained by Eu3+/Tb3+ coordinated PSM. The ratiometric thermometric parameter of Eu3+/Tb3+@In(OH)(bpydc) was defined as the ratio of the intensities of the 5D4 → 7F5 (Tb3+, 545 nm) and 5 D0 → 7F2 (Eu3+, 613 nm) transitions (ITb/IEu). The thermal

sensitivity of Eu3+/Tb3+@In(OH)(bpydc) in MOF nanothermometers was high (4.97% °C−1). Zhou and Yan40 also examined the feasibility of Eu3+@UiObpydc for ratiometric thermometry (Figure 3a,b). When the temperature increased, the Eu3+ emission intensity decreased while the ligand-centered emission significantly increased, which was attributed to the back energy transfer (BEnT) from Eu3+ cations to bpydc linkers. The temperature-dependent emission lifetimes (Eu3+) of the MOF hybrids were recorded to 2793

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Figure 9. (a) Fluorescence intensities (IEu, IL, and IEu/IL) of Ag+/Eu3+@UiO-66 films after treatment with various indoor polluting gases. (b) Concentration-dependent intensities IEu and IL. (c) Dependence of IEu/IL on the FA concentration. Reproduced with permission from ref 51. Copyright 2016 Royal Society of Chemistry.

Figure 11. (a, c) Column diagrams of the normalized fluorescence intensities FMR and FEu toward HI at concentrations of (a) 250 μM and (c) (0−100 μM). (b, d) Truth tables for (b) the one-to-two decoder logic gate and (d) the logic analytical device for HI monitoring. Reproduced with permission from ref 53. Copyright 2017 Wiley-VCH.

Figure 10. (top) Schematic diagrams of MOF-to-ZnO energy transfer in ZnO@UiO-MOF heterostructures and the sensing mechanism toward aldehydes over ZnO. (bottom) PL intensities (I614 and I470) of Eu3+@ZUM test paper after exposure to various polluting gases in vehicles (λex = 365 nm). Reproduced with permission from ref 52. Copyright 2017 Royal Society of Chemistry.

energy absorption via TTA. The weakened sensitization caused the excitation wavelength of Eu2 to be at 375 nm and not 330 nm (Figure 4, bottom). The sensor had a linear response in the pH range from 5.0 to 7.2, which was required for work with biological fluids such as blood and cell culture media.

determine KBEnT and the activation energy, Ea (Figure 3c). The calculated KBEnT increased with increasing temperature (Figure 3d). The slope of the Arrhenius plots for KBEnT was 1.98, which resulted in a value of 44 kJ mol−1 for Ea (Figure 3e). To quantitatively determine the temperature sensing function of Eu3+@UiO-bpydc, the ratiometric parameter was defined as the ratio of the intensities of the dual 530 nm (bpydc) emission to the 614 nm (Eu3+) emission (I530/I614).

3.3. Cations

Biological iron is most commonly found in the +2 (ferrous) and +3 (ferric) oxidation states. Luminescence probes to detect Fe3+ in environmental and biological systems mainly depend on the quenching effect that metal cations have on the luminescence of MOFs.42 Xu and Yan36 encapsulated Eu3+ cations into MIL-124 (Ga2(OH)4(C9O6H4)) and used it to detect Fe3+/Fe2+ ions via the fluorescence quenching of the dual-emission centers (Eu3+ and the MOF). Although both Fe2+ and Fe3+ ions significantly quenched the Eu3+ emission, the emission colors of the two ions were different and easy to distinguished under UV light. Therefore, Eu3+@MIL-124 could selectively sense Fe2+ and Fe2+ ions via the different quenching effects on Eu3+ and the MOF. To detect Fe2+ and Fe3+ in water, a test paper was immersed in aqueous solutions of the metal ions, and the fluorescent color changed from red to dark red, faint dark red, and black with increasing Fe3+ (Figure 5a). When the test paper was immersed in aqueous solutions with

3.2. pH

Eu3+ can be controlled to produce a dual-emission center. Although the emission position is the same for Eu3+, its luminescent performance changes as the surrounding conditions change. Lu and Yan41 reported a new ratiometric pH sensor based on nanoscale PSM MOF-253. Two types of Eu3+ with different excitation wavelengths were simultaneously present in MOF-253 (Figure 4, top). One of the ions was insensitive to the pH, and the other one was sensitive to the pH. In PSM, the Eu3+ ions had two different coordination environments: Eu1 was linked only to the bipyridine, and Eu2 was linked to both bipyridine and TTA. The broad band at 330 nm was assigned to the excitation of Eu1, and the broad band at 375 nm was mostly from the excitation of Eu2 due to the 2794

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Figure 12. Electronic equivalent circuits, truth tables, and column diagrams of the fluorescence intensities of B485, G530 and R614: (a) Hg2+, S2−; (b) Hg2+, S2−; (c) Ag+, S2−. Reproduced with permission from ref 54. Copyright 2017 Wiley-VCH.

various concentrations of Fe2+, the color changed from red to blue, as shown in Figure 5b. The quenching effect of Fe3+ on Eu3+@MIL-124 was ascribed to the partial replacement of Ga3+ by Eu3+, and Eu3+ substitution was the main reason for the fluorescence quenching due to Fe2+. Mercury(II) ions are dangerous because of their high toxicity and carcinogenicity, and qualitative and quantitative detection of mercury is important. Xu and Yan43 reported encapsulation of carbon dots (CDs) with strong fluorescence activity into MOF-253, and the emissions of both the CDs and Eu3+ formed the dual-emissive Eu3+/CDs@MOF-253. In a solution in the absence of Hg2+, it exhibited both characteristic emissions of the CDs and Eu3+ (IEu/ICD = 1.5) with a faint blue light (Figure 6a). In contrast, in the presence of Hg2+ the emission changed from blue to red under UV light, corresponding to quenching of the CD emission. Mercury had less of an influence on the emission of Eu3+ in Eu3+/CDs@MOF-253, which led to a colorimetric and ratiometric fluorescence Hg2+ sensor. The quenching effect of Hg2+ on the CDs in Eu3+/CDs@MOF-253 was attributed to the coordination between Hg2+ and functional

groups in the CDs. Figure 6b shows the relative photoluminescence (PL) intensities (IEu/ICD) versus the concentration of Hg2+. The limit of detection (LOD) was estimated to be 13 nM, close to the maximum level of mercury in drinking water permitted by the U.S. Environmental Protection Agency (2 ppb, 10 nM). 3.4. Anions

As common, toxic anion pollutants, Cr(VI) species (e.g., CrO42− and Cr2O72−) are a concern because they cause serious damage to human health and the environment. Hao and Yan44 investigated the anion recognition of Eu3+@MIL-121 (Al(OH)(H2btec)·H2O, H4btec = pyromellitic acid) through Eu3+ coordinated PSM of MIL-121 with its free carboxylate because its intensity was quenched by most anions. The quenching effects of F− and Cr2O72− were very pronounced, especially for Cr2O72− ion. Although both F− and Cr2O72− significantly quenched the Eu3+ emission, they could be distinguished by Eu3+@MIL-121. F− ion had a quenching effect only on the Eu3+ luminescence and not the ligand-centered (LC) emission, while Cr2O72− quenched the emissions of both Eu3+ and the ligand. 2795

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Accounts of Chemical Research As a result, in the presence of Cr2O72− and F−, the luminescent color of Eu3+@MIL-121 changed from red to blue and dark, respectively. Sun and Yan45 presented a facile, rapid, and selective strategy for ratiometric sensing of carbonate ions based on heterobimetallic Eu/Pt-MOFs with bpydc as the linker, which had dual emissions from the ligand and Eu3+. The interaction with CO32− drastically enhanced the luminescence intensity of Eu3+, and the maximum ratio of the two intensities (IEu(614)/ILigand) increased upon the addition of carbonate ions. This might be an efficient tool for analytical monitoring of trace CO32− in real samples because of the high orientation selectivity for CO32−.

Eu3+ to increase the intensity of I614 and realize selective sensing. The open connected channels and high surface area of UiO MOFs could facilitate the effective adsorption of aldehydes, leading to a low detection limit (42 ppb for FA, 58 ppb for AA, and 66 ppb for ACA). The efficient charge transfer from the MOFs to ZnO could lead to reusable detectors for aldehyde gases. In addition, the MOFs showed stable and reliable fluorescent responses for aldehydes at different temperatures (25−65 °C), which made them suitable for practical applications, e.g., vehicle detection. 3.7. Food Spoilage

Unsafe food is a large threat to human health and economies, and early detection of food spoilage is an ongoing and imperative need. Sensors (MR@EuMOFs) were created by covalently attaching methyl red (MR) to NH2-rich EuMOFs. A double stimuli-responsive fluorescent center was produced via energy transfers from the ligands to Eu3+ and MR.53 Portable sensory hydrogels exhibited a color transition upon “smelling” HI vapor. Using the HI concentration as the input signal and the two fluorescent emissions as the output signals, an advanced analytical device based on a one-to-two logic gate was constructed. The four output combinations NOT (0, 1), YES (1, 0), PASS 1 (1, 1), and PASS 0 (0, 0) allowed the direct analysis of HI levels, which could be used for real-time food freshness evaluations. The normalized fluorescence signals of MR (FMR) and Eu3+ (FEu) were used as the dual outputs with a threshold value of 0.5. In the absence of HI, FMR did not increase and FEu did not decrease, generating the output (0, 1). After exposure to HI, FMR turned on and FEu turned off, generating the output (1, 0). Figure 11a,b shows the different responses of the MR@EuMOFs under various input states. The output signals were normalized (Figure 11c). In Figure 11d, the PASS 0 gate was triggered by 40 μM HI, and the YES gate was triggered by 80 μM HI. When the concentration was below 40 μM, the NOT gates appeared.

3.5. Molecules or Solvents

Zhou and Yan46 examined the capability of Eu3+@bpy-UiO nanohybrids for volatile organic compound (VOC) sensing. Figure 7a shows the emission spectra of Eu3+@bpy-UiO after exposure to different VOCs, and the intensity ratio of the ligand-based emission to the Eu3+ emission (IL/IEu) significantly depended on the VOC. Moreover, the ratiometric luminescence of Eu3+@bpy-UiO was highly responsive to different concentrations of VOCs and mixed VOCs. Figure 7b shows the one-to-one correlation between the resulting IL/IEu and the encapsulated VOCs. The ratiometric emission intensity and luminescence quantum yield (Φ) of the VOC-included Eu3+@ bpy-UiO were related to a unique two-dimensional (2D) readout (IL/IEu, Φ), which was arranged in a decoded map (Figure 7c). These readouts showed that one can precisely differentiate the VOCs using this dual-readout process. Xu and Yan47 prepared a portable luminescent LnMOF film by in situ growth of nanoscale SUMOF-6-Eu on an oxidationtreated nonwoven polypropylene surface (NMOFs@O-PP) (Figure 8). By controlling the excitation wavelength, they predicted and tuned the emission of the coated NMOFs@OPP film with dual-emission bands (IL and IEu). As a result of the disturbance of the energy transfer from the ligand to Eu3+ in the NMOFs by O2, the coated NMOFs@O-PP film could be used for ratiometric O2 sensing. The film showed a high O2 sensitivity (Ksv = 6.73, LOD = 0.45%), short response and recovery times (10 and 60 s, respectively), and good reversibility.

3.8. Triple-Luminescent Center for Logic Operations

Xu et al.54 constructed a fluorescent system based on Eu3+functionalized UMOFs to effectively combine ion recognition and logic computing. The Eu3+@UMOFs and ions implemented a Boolean logic network system connecting the elementary logic operations (NOR, INH, and IMP) and integrative logic operations (OR + INH). To address uncertain information in an analogue region with a nonlinear response (fluorescence and concentration), soft computations via the formulation of fuzzy logic operations were constructed. On the basis of Boolean logic and fuzzy logic, an intelligent molecule search could be realized by using the chemical events (Hg2+, Ag+, and S2−) as programmable words and the chemical interactions as the syntax. In the logic operations, Eu3+@ UMOFs served as the gates, and the polluting ions (Hg2+, Ag+, or S2−) and fluorescent emission changing at 485 nm (B485), 530 nm (G530), and 614 nm (R614) (triple-luminescent center) were used as the chemical inputs and outputs, respectively. (Figure 12)

3.6. Air Pollutants

Indoor formaldehyde (FA) detection is important for indoor air pollution (IAP).48−50 Hao and Yan51 fabricated a dual-emissive Ag+- and Eu3+-functionalized MOF nanohybrid (Ag+/Eu3+@ UiO-66) and utilized it as a self-calibrated ratiometric luminescent sensor for detection of indoor FA. The two luminescent centers (centered Eu3+ and ligands) were selfcalibrated. FA interacted with Ag+ and changed the luminescent behavior of the nanohybrid by weakening the influence of Ag+ ions on the energy transfer process. The IEu/IL data from the above sensors are shown as bar diagrams in Figure 9a, and the Ag+-loaded film sensors exhibited excellent selectivity for FA. The fluorescent responses (IEu/IL) of this sensor to FA in the presence of various gases were similar to those with only FA (Figure 9b), which indicated that the effect of FA on the IEu/IL was not influenced by coexisting gas pollutants. More recently, Xu and Yan52 fabricated ZnO-doped UiO MOF heterostructures (ZnO@UiO-MOFs) and turned them into a dual-emitting material (I614 and I470) by introducing Eu3+ to the UiO-MOFs through coordinated PSM and controlling the excitation wavelength (Figure 10). Reduced aldehyde molecules could build a bridge via charge transfer from ZnO to

4. CONCLUSIONS AND OUTLOOK In this Account, recent research progress on lanthanidefunctionalized metal−organic framework (MOF) hybrid materials is summarized. Lanthanide-ion functionalization of MOF hosts creates multiple luminescent centers (especially dual-luminescent centers), and these MOFs can be utilized for chemical sensing based on the different luminescent responses. 2796

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Accounts of Chemical Research Herein, five methods for lanthanide functionalization of MOFs are outlined according to the interactions between the lanthanide ions and the MOF host: (1) in situ composition for MOFs such as ZIF; (2) ionic substitution for lanthanide MOFs; (3) ionic exchange for anionic MOFs such as BioMOF-1; (4) coordinated postsynthetic modification for MOFs with free groups in their linkers; and (5) covalent postsynthetic modification to achieve coordinated groups of MOFs. Lanthanide functionalization creates multiple luminescent centers (mainly dual centers) for chemical sensing. Currently, dual-luminescent-response sensing is being successfully applied for species in chemistry, biology, and the environment, including cations, anions, molecules, solvents, and special biomarkers, as well as pH and temperature. Moreover, the triple-luminescent response can be used to sense environmental species, and the logic operation can be further developed. These luminescent probes can be expected to have potential applications in chemical sensing for chemical, biological, or environmental fields, etc. However, some problems still exist in the field of lanthanide-functionalized MOF hybrids. The photostability of these hybrid systems needs to be enhanced, especially in aqueous environments. The fluorescence sensing mechanism still requires further investigation, and theoretical calculations on the sensing process are needed. The disadvantages of fluorescence sensing should be avoided to improve the low detection limit of this technique compared with those of other chemical sensing approaches. In addition, real applications should be explored to prepare practical sensors to solve problems in complicated systems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bing Yan: 0000-0002-0216-9454 Notes

The author declares no competing financial interest. Biography Bing Yan was born in Changchun, China. He received his Bachelor’s degree (1992) and Master’s degree (1995) in Applied Chemistry from Harbin Institute of Technology in China and his Ph.D. (1998) degree from the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, in China. He joined Tongji University as a full professor in 2001 after his postdoctoral studies at City University of Hong Kong and Peking University (1998−2001).

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21571142).

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REFERENCES

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