English

• 1. Introduction

  Metal-organic frameworks (MOFs) are an emerging class of porous hybrid materials, building from metal ions/clusters and polydentate organic ligands connected by coordination bonds [1-3]. Attracted by the high porosity and the tunable framework structures and functionalities, MOFs have been extensively studied in past two decades, and realized applications in many fields, such as gas storage and separation [4-6], heterogeneous catalysis [7-9], drug delivery [10], and chemical sensing [11].

  Incorporation of luminescent moieties into porous MOFs has generated many unique luminescent materials. Because the photoluminescence (PL) of MOFs is responsive to the encapsulated guest molecules, MOFs have been realized applications in environmental sensing and biology imaging [12-14]. Compared with traditional materials, MOFs exhibit the following features: (1) The PL properties are systematically tunable by simply adjusting the constituent luminescent moieties, such as metal nodes, organic ligands and encapsulated molecules inside the pores; (2) The pore sizes, shapes, functionality, and hydrophobic and hydrophilic pore nature can be deliberately designed and modified to improve the recognition ability; (3) Tiny differences of the interactions between PL moieties and encapsulated guests would significantly affect the emissions.

  There have been a large number of MOFs which were used as PL sensors for probing of metal cations [15, 16], hazard anions [17, 18], volatile organic molecules (VOMs) [19, 20], toxic gases [21, 22] and explosive compounds [23, 24]. However, because the PL signals are based on quenching or enhancing the emissions of single chromophores, the PL responses are often indistinguishable for those analytes with similar structures and chemical and physical properties. To improve the PL sensitivity, secondary luminescent moieties were therefore introduced into MOFs to develop dual-emission sensors [25-27]. Because both PL emissions are responsive to the encapsulated guests in MOFs, the PL signals were significantly amplified by monitoring the emission intensity ratio of two luminescent centers in which one emission was used as an internal standard. Moreover, this internal label strategy has the advantage of self-calibrating the random errors of PL intensities in the solid-state, since the absolute emission intensities of solid materials are affected by many uncontrollable factors. Herein, we will highlight the recent progress on dual-emission MOFs, and their applications as highly differentiable chemical and thermometer sensors.

  2. Mixed-lanthanide luminescent MOFs

  Due to their superior optical properties, such as long luminescent lifetime, narrow emission band, sharp Stokes shift and high color purity, lanthanide (Ln) ions have been employed as metal nodes for the assembly of PL Ln-MOFs [28]. Because the emission energies of organic chromophores are often higher than those of Ln^Ⅲ ions, the emission energy can be transferred from organic chromophores to Ln^Ⅲ ions by "antennae effect". Tuned by the encapsulated guest molecules, Ln-MOFs would emit guest-responsive PL signals. However, most Ln-MOFs, consisting of single luminescent Ln centers, are only highly PL sensitive to few analytes with strong quenching or enhancing effect.

  To improve the PL sensitivity of Ln-MOFs, Wu's group reported a series of isostructural Ln-MOFs, Eu_xTb_1-x-TCM, consisting of mixed Ln³⁺ ions with dual-emission Ln^Ⅲ centers, in which Eu_0.35Tb_0.65-TCM was used as a PL sensor for probing of different VOMs [26]. Ln-TCM is a flexible lamellar network consisting of 1D channels of 12.3 × 12.5 Ų in dimensions (Fig. 1). The structure of Ln-TCM is flexible, which could adapt different induced-fit guest-host interactions, and emit solvent-dependent PL signals by tuning the energy transfer efficiency from organic ligands to Ln³⁺ ions and the energy distribution between Eu³⁺ and Tb³⁺ ions. Even though the VOM-dependent signals of single Ln^Ⅲ ions are not prominent, however, it is interesting that the PL readouts can be significantly amplified by monitoring the emission intensity ratios of ⁵D₀ → ⁷F₂ (Eu³⁺, 614 nm) to ⁵D₄ → ⁷F₅ (Tb³⁺, 545 nm) transitions in the PL spectra. Such a self-referencing PL sensor could selectively probe those VOMs with similar structures, including isomerides of o-, m- and p-xylene, and homologens of benzene, toluene and ethylbenzene, and benzene halides of fluoro-, chloro-, bromo- and iodo-benzene. Moreover, this strategy presents the advantage of self-calibrating the random errors of solid-state PL signals. In the recycling experiments, the absolute emission intensity of single Ln³⁺ is variable at different runs, whereas the ratiometric intensity of I₆₁₄/I₅₄₅ is almost a constant in nine cycles.

  Figure 1

  

  Figure 1.  (a) The structure of Ln-TCM, consisting of 1D open channels; (b) The integrated emission intensity ratios of ⁵D₀ → ⁷F₂ (Eu³⁺, 614 nm) to ⁵D₄ → ⁷F₅ (Tb³⁺, 545 nm) transitions for Eu_0.35Tb_0.65-TCM after adsorption of nitrobenzene (1), styrene (2), benzyl alcohol (3), aniline (4), benzene (5), methanol (6), none (7), chlorobenzene (8), and DMF (9) molecules excited at 285 nm. Reproduced with permission [26]. Copyright 2014, American Chemical Society.

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  This ratiometric dual-emission strategy can also be used to develop PL thermometers. Qian and coworkers reported the first PL thermometer based on a mixed Ln-MOF, Eu_0.0069Tb_0.9931-DMBDC [29]. Ln-DMBDC is a 3D rod-packing structure, building from infinite Ln^Ⅲ-carboxylate chains inter-connected by the phenylene portion of DMBDC (Fig. 2). When the temperature was increased from 10 K to 300 K, the characteristic emission intensities of Eu-DMBDC and Tb-DMBDC diminished respectively due to the thermal-enhanced nonradiative-decay. Interestingly, the mixed Ln-MOF Eu_0.0069Tb_0.9931-DMBDC exhibits a quite different thermal PL behavior. The emission intensity of Tb³⁺ decreases whereas the intensity of Eu³⁺ gradually increases accompanying increase of temperature, because high temperature would increase the energy transfer from Tb³⁺ to Eu³⁺ ions through Föster energy transfer mechanism. The intensity ratio of ⁵D₄-⁷F₅ (Tb³⁺, 545 nm) to ⁵D₀-⁷F₂ (Eu³⁺, 613 nm) transitions exhibits a linear-relationship in the range of 50–200 K, and the corresponding PL color shifts from greenish yellow to orange which are easily observed by the naked eye. The unique temperature-dependent behavior makes Eu_0.0069-Tb_0.9931-DMBDC be an excellent candidate as self-calibration PL thermometer.

  Figure 2

  

  Figure 2.  (a) Crystal structure of Tb-DMBDC; (b) Temperature dependence of the integrated intensity ratio of Tb³⁺ (545 nm) to Eu³⁺ (613 nm) for Eu_0.0069Tb_0.9931- DMBDC, and temperature dependence of the integrated intensity of Tb³⁺ (545 nm) for Tb-DMBDC (inset shows the fitted curve of the integrated intensity ratios for Eu_0.0069Tb_0.9931-DMBDC). Reproduced with permission [29]. Copyright 2012, American Chemical Society.

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  3. Encapsulation of fluorescent dyes in MOFs

  Fluorescent dyes are a class of organic chromophores with interesting optical and electronic properties. Dyes can be incorporated into porous MOFs to synthesize dual-emission dye@MOF composites by either in situ synthesis or postmodification [25, 27]. The overlaps between the adsorption bands of encapsulated dyes and the emission bands of organic ligands in dye@MOFs let MOFs transfer energy to dyes upon excitation of the organic ligands. Tuned by the encapsulated VOMs inside the pores, the dual-emissions in the PL spectra of dye@MOFs would display guest-dependent signals, and thus to establish internal-reference systems for PL sensing of VOMs.

  The Wu group reported a dye@MOF composite RhB@CZJ-3 by encapsulating Rhodamine B (RhB) in porous CZJ-3 [25]. CZJ-3 is a 3D structure, consisting of 1D hexagonal nanotube channels with diameter of approximate 2.0 nm (Fig. 3). Upon excitation of the organic ligands in CZJ-3 at 340 nm, RhB@CZJ-3 displayed two broad emission bands centered at 420 and 595 nm for organic ligands and encapsulated RhB, respectively. Because the PL emissions of organic ligands and RhB dyes and the ligand-to-dye energy transfer are very sensitive to the structures and properties of encapsulated VOMs, there are fingerprint relationships between the emissions of RhB@CZJ-3 and VOMs by monitoring the ratiometric intensity of organic ligands to dyes. Therefore, different VOMs were unambiguously differentiated by RhB@CZJ-3 sensor, even for homologues and isomerides.

  Figure 3

  

  Figure 3.  (a) The 3D structure of CZJ-3 with 1D chiral nanotube channels; (b) The solvent-dependent emission peak-height ratios of organic ligand to dye moieties in the PL spectra of RhB@CZJ excited at 340 nm.

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  The porosity might not be the essential factor to realize dye@MOF sensing, which was proved by a nonporous RhB@[Zn₂L₂] PL sensor [27]. To embed dye molecules in nonporous MOFs, the Wu group developed an in-situ encapsulation strategy for the synthesis of RhB@[Zn₂L₂] composite by confining RhB dye molecules in the crystal matrix of a layered MOF, [Zn₂L₂], building from Zn²⁺ metal nodes and amino acid derivatives (Fig. 4). Upon excitation at 310 nm, the emission bands for organic ligands and RhB in RhB@[Zn₂L₂] are centered at 416 and 583 nm, respectively. Even though VOMs cannot directly contact with the imbedded dyes, RhB@[Zn₂L₂] remained highly sensitive to different VOMs. For example, the PL could be apparently quenched by nitrobenzene at low concentration (0.01 wt%). The PL signals were significantly amplified by self-referencing the emission ratio of organic ligands to dyes to improve the sensing capability of different VOMs. This work demonstrates that nonporous MOFs could also be used to establish dye@MOF platform for dual-emission chemical sensing.

  Figure 4

  

  Figure 4.  (a) The lamellar structure of nonporous [Zn₂L₂]; (b) The solvent-dependent emission peak-height ratios of organic ligand to dye moieties in the PL spectra of RhB@[Zn₂L₂] excited at 310 nm. Reproduced with permission [27]. Copyright 2016, Wiley-VCH.

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  4. Surface modification of MOFs with dyes

  Alternatively, fluorescent dyes can be attached on the surfaces of MOF nanocrystals to realize dual-emissions. Zhang et al. designed a PA (picric acid) sensor RGH-Eu(BTC) by covalently attaching rhodamine 6G hydrazide (RGH) onto the surfaces of Eu (BTC) nanocrystals (Fig. 5) [30]. Upon excitation at 285 nm, RGH-Eu (BTC) presents the characteristic PL signals centered at 594 nm, 616 nm and 692 nm for Eu(BTC) and a weak peak at 543 nm for RGH. When PA was added to the suspension of RGH-Eu(BTC) in ethanol, the weak emission peak of RGH gradually increased and shifted to 555 nm due to the formation of rhodamine derivative through pH-dependent structural ring-opening transformation, whereas the emission of Eu(BTC) was gradually quenched ascribed to the charge transfer between Eu(BTC) and PA. It is interesting that the intensity ratio of I₅₅₅/I₆₁₆ exhibited an excellent linear relationship to PA concentration (2–200 mmol/L) with the detection limit of 0.45 mmol/L. Even though the electrophilic nitroaromatic compounds can quench both the emission of RGH-Eu(BTC), the ratio of I₅₅₅/I₆₁₆ remained unchanged.

  Figure 5

  

  Figure 5.  (a) Schematic representation of sensing mechanism of RGH–Eu(BTC) for PA; (b) Plot of the relationship between the integrated intensity ratio of I₅₅₅/I₆₁₆ and PA concentration (0–600 mmol/L) excited at 285 nm (inset shows the PL images of RGH–Eu(BTC) and RGH–Eu(BTC) in the presence of PA under 254 nm UV light). Reproduced with permission [30]. Copyright 2017, Royal Society of Chemistry.

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  Zhang and coworkers reported a dual-emission PL nanosensor for probing of peroxynitrite (ONOO^-) [31]. They first prepared Zr-MOF nanocrystals (NMOF) coated with PVA-ABt (PVA = poly (vinyl alcohol); ABt = (E)-4-((2-(2-(benzo[d]thiazol-2-yl)-2-cyanovinyl)-5-(diethylamino)phenoxy)methyl)phenylboronic acid) to form NMOF-PVA-ABt composite (MA). MA exhibits strong PL at 540 nm due to the efficient energy transfer from NMOF to ABt (Fig. 6). Because ABt would react instantaneously with ONOO^-, the addition of ONOO^- into the system could cut off the energy pathway between NMOF and ABt by reacting with the arylboronate group on ABt, and thus to diminish the green emission of ABt and increase the blue emission of NMOF. Interestingly, the ratio of the emission heights at 403 nm and 530 nm displays a linear relationship with the concentration of ONOO^- (0.0–0.1 μmol/L). MA presents the features of fast response and high selectivity for ONOO^-with dual color switching, which showed great promise for imaging in living cells.

  Figure 6

  

  Figure 6.  Schematic representation of the sensing mechanism of MA for ONOO^-. Reproduced with permission [31]. Copyright 2017, Royal Society of Chemistry.

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  5. Summary and prospective

  To improve the sensitivity of MOF sensors, secondary PL moieties, acting as the self-reference centers, were introduced into MOFs. By monitoring their emission intensity ratios, the PL signals are significantly amplified to extend the probing scopes of analytes. The primary results on dual-emission MOF sensors demonstrate that they are the promising candidates for detection of different analytes with unique ratiometric PL signals. Since there are numerous PL moieties, such as dyes, Ln ions and quantum dots, are readily incorporated into MOFs by physical adsorption, in-situ synthesis or surface modification, the dual-emission MOF sensors might be undergone explosive. By deliberately tuning the combination of different PL moieties and MOF structures, it is confident that the detection of target analytes would be more accurate and convenient in the near future.

  Acknowledgments

  We are grateful for the financial support of the National Natural Science Foundation of China (Nos. 21373180 and 21525312), and the Fundamental Research Funds for the Central Universities (Nos. 2017XZZX001-03A and 2017FZA3007).

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• 

  Figure 1  (a) The structure of Ln-TCM, consisting of 1D open channels; (b) The integrated emission intensity ratios of ⁵D₀ → ⁷F₂ (Eu³⁺, 614 nm) to ⁵D₄ → ⁷F₅ (Tb³⁺, 545 nm) transitions for Eu_0.35Tb_0.65-TCM after adsorption of nitrobenzene (1), styrene (2), benzyl alcohol (3), aniline (4), benzene (5), methanol (6), none (7), chlorobenzene (8), and DMF (9) molecules excited at 285 nm. Reproduced with permission [26]. Copyright 2014, American Chemical Society.

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  Figure 2  (a) Crystal structure of Tb-DMBDC; (b) Temperature dependence of the integrated intensity ratio of Tb³⁺ (545 nm) to Eu³⁺ (613 nm) for Eu_0.0069Tb_0.9931- DMBDC, and temperature dependence of the integrated intensity of Tb³⁺ (545 nm) for Tb-DMBDC (inset shows the fitted curve of the integrated intensity ratios for Eu_0.0069Tb_0.9931-DMBDC). Reproduced with permission [29]. Copyright 2012, American Chemical Society.

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  Figure 3  (a) The 3D structure of CZJ-3 with 1D chiral nanotube channels; (b) The solvent-dependent emission peak-height ratios of organic ligand to dye moieties in the PL spectra of RhB@CZJ excited at 340 nm.

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  Figure 4  (a) The lamellar structure of nonporous [Zn₂L₂]; (b) The solvent-dependent emission peak-height ratios of organic ligand to dye moieties in the PL spectra of RhB@[Zn₂L₂] excited at 310 nm. Reproduced with permission [27]. Copyright 2016, Wiley-VCH.

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  Figure 5  (a) Schematic representation of sensing mechanism of RGH–Eu(BTC) for PA; (b) Plot of the relationship between the integrated intensity ratio of I₅₅₅/I₆₁₆ and PA concentration (0–600 mmol/L) excited at 285 nm (inset shows the PL images of RGH–Eu(BTC) and RGH–Eu(BTC) in the presence of PA under 254 nm UV light). Reproduced with permission [30]. Copyright 2017, Royal Society of Chemistry.

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  Figure 6  Schematic representation of the sensing mechanism of MA for ONOO^-. Reproduced with permission [31]. Copyright 2017, Royal Society of Chemistry.

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