Luminescent Lanthanide MOFs: A Unique Platform for Chemical Sensing

In recent years, lanthanide metal–organic frameworks (LnMOFs) have developed to be an interesting subclass of MOFs. The combination of the characteristic luminescent properties of Ln ions with the intriguing topological structures of MOFs opens up promising possibilities for the design of LnMOF-based chemical sensors. In this review, we present the most recent developments of LnMOFs as chemical sensors by briefly introducing the general luminescence features of LnMOFs, followed by a comprehensive investigation of the applications of LnMOF sensors for cations, anions, small molecules, nitroaromatic explosives, gases, vapors, pH, and temperature, as well as biomolecules.

As a subclass of MOFs, luminescent MOFs possess potential for practical applications because of their explicit environments for luminophores in a crystalline state and characteristic optical performance [20]. Generally, the luminescent properties of MOFs generate from metal components and organic linkers with aromatic or conjugated π systems. The metal-ligand charge transfer (MLCT) related luminescence can extend their luminescence functionalities to another dimension. Moreover, some adsorbed guest molecules within MOFs are able to contribute to the luminescent properties. Until now, research on luminescent MOFs has mainly focused on the fundamental luminescent properties of MOFs, and the rational design of tunable luminescent MOFs for light emitting applications [21]. Recently, luminescent MOFs have been proven to be a unique platform for chemical sensing due to their special features, including (i) easily tunable luminescence that can be used as the appropriate sensing signal; (ii) specific functional groups (e.g., Lewis sites and open metal sites) that are able to promote preferred host-guest binding for selective sensing; and (iii) the permanent MOFs' porosity that could concentrate the guest molecules, thereby enhancing detective sensitivity. Numerous luminescent MOF sensors have been developed and reported in the literature for detecting cations [22,23], anions [24,25], small molecules [26][27][28], biological molecules [29,30], explosive chemicals [31][32][33], vapors [34,35], and pH [36], as well as temperature [37][38][39].
Lanthanide MOFs (LnMOFs) have drawn much attention among the luminescent MOFs because of the unique luminescent properties of lanthanide ions, such as long lifetime, characteristic sharp emissions, large Stokes shifts, and high color purity with high quantum yields in the near-infrared and visible regions [40][41][42][43][44][45]. Additionally, the luminescent properties of lanthanide ions highly depend on the structural details of their coordination environment, offering a unique platform as chemical sensors. The combination of these characteristic luminescent properties of lanthanide ions with the intriguing topological structures of MOFs opens up promising possibilities for developing luminescent materials with special applications.
In this review, we present the most recent developments of LnMOFs as chemical sensors. We begin by briefly introducing the general luminescence features of LnMOFs, followed by a comprehensive investigation of the applications of LnMOF sensors with single or multiple luminescent centers. More specifically, LnMOF sensors for cations, anions, small molecules, nitroaromatic explosives, gases, vapors, pH, and temperature, as well as biomolecules will be discussed in detail in this review.
Typically, the 4f-4f transitions of Ln 3+ are Laporte forbidden due to the 4f orbitals that are well-shielded by the filled 5s 2 5p 6 subshells [51]. Consequently, direct photoexcitation of Ln 3+ ions rarely produces highly luminescent materials due to the low absorption efficiency of the 4f-4f transitions. This problem can be overcome by the "antenna effect" (Figure 1), which commonly uses a strong absorbing chromophore to sensitize Ln 3+ [52,53]. The overall process of antenna sensitization involves the following characteristic steps: (i) the organic ligands can absorb light upon excitation; (ii) the excitation energy is then transferred into Ln 3+ excited states through intramolecular energy transfer; and (iii) Ln 3+ ions undergo a radiative process by characteristic luminescence. This process could effectively increase the luminescence quantum yield of Ln 3+ in normal conditions at room temperature. Furthermore, the solvent quenching and self-quenching of Ln 3+ ions are almost nullified in LnMOFs due to the separation of Ln 3+ ions by organic ligands. Consequently, LnMOFs exhibit strong luminescence and can be utilized as chemical sensors. There are two other types of electronic transitions of Ln 3+ ions: broad charge-transfer transitions (ligand-metal charge transfer (LMCT) and metal-ligand charge transfer (MLCT)) and broad 4f-5d transitions. They usually occur with high energies, resulting in rare observation in coordination compounds. However, the excitation energy of Sm 3+ , Eu 3+ , and Yb 3+ can be transferred from an LMCT state to their 4f levels when the LMCT state lies at a high enough energy level. It is of great importance to investigate the numerous energy transfer processes for well-tuning the luminescent properties of LnMOFs.
The luminescence of Ln 3+ ions is only possible from resonance levels, such as 5 D 0 for Eu 3+ , 5 D 4 for Tb 3+ , and 2 F 5/2 for Yb 3+ . The energies of resonance levels of Eu 3+ ( 5 D 0 ), Tb 3+ ( 5 D 4 ), and Yb 3+ ( 2 F 5/2 ) lie at 17,250, 20,430, and 10,200 cm −1 , respectively [54]. If the Ln 3+ ions are excited to a nonresonance level, the excitation energy is dissipated through a nonradiative process until a resonance level is reached. Therefore, the lowest triplet state of the organic ligands in LnMOFs must be located at an energy level nearly equal to or above the resonance level of the Ln 3+ ions. If the energy difference between the organic linkers and Ln 3+ ions is too small, a thermally activated energy back-transfer will occur. On the other hand, large energy differences may lead to slower energy transfer rates. The energy of the triplet state must be elaborately tuned to maximize the transfer and minimize the back-transfer. Thus, the rational design of suitable organic ligands with the appropriate energy level is of great significance for the synthesis of LnMOFs with the desired luminescent properties.

LnMOFs for Chemical Sensing
LnMOFs have been widely studied in various sensor applications owing to their inherent porosity and the particular luminescent properties of Ln 3+ ions. Most of the LnMOF sensors show luminescence intensity changes, including luminescence enhancement (turn-on response) and quenching (turn-off response) upon recognition of the analytes. Eu 3+ and Tb 3+ are commonly used as luminescent centers in LnMOF sensors because of their strong, characteristic red emission at around 614 nm and green emission at around 541 nm, respectively [55]. LnMOFs succeed in sensing ionic species, small molecules, explosive chemicals, and pH, as well as temperature. In addition, the inherent structural and chemical features of LnMOFs make them considerably useful in biosensing and bioimaging applications [56]. In the remainder of this section, recent developments of LnMOFs for chemical sensing will be discussed in detail.

LnMOFs for Cation Sensing
Sensing and detecting metal ions is of great significance in environmental and ecological systems. Some transition-metal cations, such as Cu 2+ , Fe 2+ , Fe 3+ , and Zn 2+ , are essential in biological metabolism. The excess or deficiency of these metal cations can cause various diseases, such as Alzheimer's disease, Wilson's disease, anemia, mental decline, etc. [57][58][59][60]. Hg 2+ , Pb 2+ , and Cd 2+ are well-known toxic metal ions that can give rise to serious damage to the human body and environment [61,62]. Therefore, the design and preparation of efficient and straightforward metal ion probes are urgently needed.
In 2009, Chen et al. reported a new LnMOF [Eu(PDC) 1.5 (DMF)](DMF) 0.5 (H 2 O) 0.5 (PDC = pyridine-3,5-dicarboxylate, DMF = N N-dimethylformamide) with Lewis basic pyridyl sites for sensing Cu 2+ ions [63]. The desolated MOF Eu(PDC) 1.5 can selectively detect Co 2+ and especially Cu 2+ among other metal ions via a turn-off response. The authors hypothesized that the antenna efficiency of the PDC organic ligands was reduced by the binding of the pyridyl nitrogen atoms to Cu 2+ or Co 2+ , resulting in luminescence quenching. From then on, many LnMOF sensors with unsaturated Lewis basic sites have been synthesized based on this mechanism for detecting metal ions [64][65][66][67][68]. Recently, Yan and coworkers developed a FAM-ssDNA and Eu 3+ @Bio-MOF-1 for sensing Cu 2+ in aqueous solutions [69]. This luminescent hybrid material can simultaneously exhibit FAM and Eu 3+ emissions by varying the ratio of Eu 3+ @Bio-MOF-1 and FAM-ssDNA. Cu 2+ can quench FAM emission, while enhancing the luminescence intensity of Eu 3+ (Figure 2). The mechanism behind this is possibly based on the interaction of Cu 2+ and ssDNA.  Tan et al. prepared adenine-based lanthanide coordination polymer nanoparticles (CPNPs), consisting of adenine (Ad), a Tb 3+ ion, and dipicolinic acid (DPA). It showed a turn-on luminescence response for Hg 2+ in aqueous solutions [72]. Due to the photoinduced electron transfer (PET) process, the Ad can transfer energy to the DPA and simultaneously prevent intramolecular energy transfer from DPA to Tb 3+ , leading to the luminescence quenching of the CPNPs (Figure 3a). However, significantly enhanced luminescence (approximately fivefold) was observed in the CPNPs because of the suppression of the PET process from Ad to DPA by Hg 2+ , which was further confirmed by Fourier-transform infrared spectroscopy (FTIR) and lifetime study ( Figure 3b). This Hg 2+ nanosensor also showed superior selectivity and exceptionally high sensitivity up to the detection limit of 0.2 nM and can be used in biosensing and imaging. Li (Figure 3c,d). The luminescence enhancement was due to a more efficient energy transfer from organic linkers to Sm 3+ evoked by Ag + (Figure 3e) [75].

LnMOFs for Anion Sensing
Various anions, such as halogen ions SO 4 2− , PO 4 3− , and CN − , are fundamental in environmental and biological systems [76]. Therefore, the sensing of such anions is a remarkably interesting topic to investigate. In recent years, LnMOF-based sensors have been successfully utilized for sensing inorganic anions [77][78][79][80]. Chen and coworkers synthesized a TbMOF [Tb(BTC)·G] (BTC = benzene-1,3,5tricarboxylate, G = guest solvent) with OH groups in the terminal solvents [78]. This TbMOF showed a fourfold luminescence enhancement in the presence of F − , suggesting that this porous luminescent MOF is a promising candidate for sensing F − (Figure 4a). The possible mechanism of luminescence enhancement by F − ions lies in the stronger hydrogen bonding interactions between the F − ion and the terminal methanol molecules that can restrict the stretching of the OH bond and thus reduce its quenching effect. The turn-off detection for F − was achieved by Zhou and coworkers using an isostructural-doped LnMOF, [Eu 2x  This highly stable EuMOF sensor with excellent sensitivity and selectivity can also be utilized in real environmental conditions, such as lake water and sea water, suggesting the possible application of MOF chemical sensors in environmental fields. In another study, Li  The results demonstrate the promising practical applications of this recyclable PO 4 3probe.

LnMOFs for Small Molecule Sensing
Formaldehyde (HCHO) is widespread in construction, furniture, and particle board, posing an impact on human health, such as watery eyes, asthma, and respiratory irritation [88]. Yu and coworkers developed a ratiometric luminescence HCHO probe through incorporation of Eu 3+ ions into NH 2 -UiO-66 under microwave irradiation conditions [89]. The dual-emitting luminescence originated from the characteristic red emission of Eu 3+ ions (615 nm) and linker-to-cluster (Eu-oxo or Zr-oxo) charge transfer transition-related emission (465 nm). The interaction of the free amino groups with HCHO can drastically enhance emission around 465 nm due to the added electron transfer from the amino group with lone pair electrons to the positively charged HCHO. This is in contrast to the emission of Eu 3+ at 615 nm that was only slightly enhanced. Then, a ratiometric luminescence HCHO probe was performed based on the intensity ratio of two emission bands at 465 nm and 615 nm. The results indicated that the fabrication of a ratiometric luminescence probe based on multiband luminescent MOFs can serve as a common sensing method for organic molecules. Another ratiometric luminescence sensor for HCHO was reported by Yang and coworkers [90]. This self-calibrating luminescent film was fabricated directly by growing Eu-NDC (H 2 NDC = 2,6-naphthalenedicarboxylate) on hydrolyzed polyacrylonitrile (HPAN) via a layer-by-layer strategy (Figure 6a). The Eu-NDC@HPAN thin film can detect HCHO via a ratiometric luminescence approach with a 3.2-fold increase of the relative ratio of luminescence intensities at 453 nm and616 nm. It has been proposed that the Eu-NDC frameworks will decompose after adding HCHO, while the NDC ligands regenerate, resulting in luminescence quenching and enhancing of Eu 3+ ions and NDC, respectively (Figure 6b). The remarkable selectivity, sensitivity, and water stability of this film HCHO probe indicates its potential use in life sciences.  3+ and Gd 3+ , tctpH 3 = tris(p-carboxylato)triphenylphosphine (P(C 6 H 4 -p-CO 2 H) 3 )) has a 3D structure consisting of puckered 2D honeycomb sheets with large hexagonal channels and exhibits the characteristic luminescence of Eu 3+ and Tb 3+ (Figure 6c). This material allows for immediate solvent identification through color changes, which can easily be observed by the naked eye. Interestingly, the sensor can also be employed to quantitatively detect trace H 2 O in D 2 O (Figure 6d,e), as well as acetone, ethanol, and acetonitrile by uncomplicated spectrophotometry. To the best of our knowledge, this codoped LnMOF is the first material-based sensor for detecting H 2 O in D 2 O from 10 to 120,000 ppm. Buschbaum [22]. The luminescence intensity of this EuMOF primarily depends on the organic solvents, particularly in the case of acetone, which exhibited the most significant quenching effect. It has been suggested that the competition of absorbing excited light energy between FBPT and acetone plays an important role in their luminescence diminishment. Guo   Benzene and its homologues, a prime type of toxic pollutant, bring great harm to both the environment and humans. It is therefore of significant importance to develop an efficient and easily processed approach to detect this kind of pollutant. Cheng and coworkers constructed a red luminescence sensor based on {[Eu 2 (L 6 ) 3 (DMF) 2 ]·DMF·MeOH} n (H 2 L 6 = 5-(4H-1,2,4-triazol-4-yl) benzene-1,3-dicarboxylic acid) to effectively detect polychlorinated benzenes [98]. This EuMOF represents a highly efficient quenching effect on detecting polychloriznated benzenes, including 1,2,4-trichlorobenzene, 1,2,3,4-tetrachlorobenzene, 1,2,4,5-tetrachlorobenzene, pentachlorobenzene, and hexachlorobenzene, which can be ascribed to the competition of the absorption of the excitation light between the analytes and ligands. Weng et al. fabricated a dual-emissive hybrid N-GQDs/Eu 3+ @Mg-MOF (N-GQDs = N atom-doped graphene quantum dot, Mg-MOF = {[Mg 3 (ndc) 2.5 (HCO 2 ) 2 (H 2 O)][NH 2 Me 2 ]·2H 2 O·DMF} 1,4-ndc = 1,4-naphthalenedicarboxylate) and employed it as a ratiometric luminescence sensor for decoding benzene homologues [53]. It exhibits dual-emission of N-GQDs and Eu 3+ when excited at 394 nm, while the emission of the ligands and Eu 3+ can be collected when excited at 349 nm. Thus, a 2D decoded map with I L /I Eu as abscissa and I Eu /I N-GQDs as ordinate is established to identify benzene homologues. The results demonstrated that the decoded map can be used for the precise recognition of unknown compounds.

LnMOFs for Nitroaromatic Explosive Sensing
It is of great importance to selectively and rapidly detect nitroaromatic explosives in environmental monitoring, civilian safety, and homeland security [99]. The current methods for explosive detection are limited by their equipment demands and cost drawbacks [100]. However, luminescence sensing has proven to be an excellent detection technique for explosives owing to its speed and cost effectiveness, as well as to the fact that it is easily portable [101].
The luminescence quenching efficiency of LnMOFs towards nitroaromatic explosives was analyzed using a quenching constant K sv (M −1 ) and detection limits. The quenching constant K sv (M −1 ) is calculated by using the Stern-Volmer (SV) equation, (I 0 /I) = K sv [A] + 1, where I 0 and I are the luminescence intensities before and after the addition of the analyte, respectively, and where [A] is the molar concentration of the analyte. The detection limit was calculated by K sv values and the standard deviation (S b ), defined as nS b /K sv [108].

LnMOFs for Gas and Vapor Sensing
The luminescent MOF films, CPM-5⊃ Tb 3+ and MIL-100(In)⊃ Tb 3+ , were designed by Qian and coworkers as a fast-response oxygen probe (Figure 9a,b) [109]. The luminescence intensities of the activated CPM-5⊃ Tb 3+ and MIL-100(In)⊃ Tb 3+ decreased gradually with increasing O 2 pressure. MIL-100(In)⊃ Tb 3+ showed higher quenching efficiencies (88%) than did CPM-5⊃ Tb 3+ (47%) at 1 atm of O 2 (Figure 9c,d). This is because the exposed carboxylate acids in MIL-100(In) can form Tb-O bonds with Tb 3+ ions, leading to the intramolecular energy transfer, whilst Tb 3+ merely balances cations in the pores of CPM-5, leading to intermolecular energy transfer. The high-oxygen sensitivity and short response/recovery time of MIL-100(In)⊃ Tb 3+ indicate their potential in sensing gases or vapors.  [110]. A water-exchanged framework was formed by submerging the EuMOF in distilled water for 3 days and consequently showed much weaker Eu 3+ -based emission due to the quenching effect of the water molecules. The Eu 3+ luminescence intensity exhibits a more than eightfold increase in the presence of DMF vapor. This is primarily due to partial replacement of the channel water by DMF molecules that reduce the quenching effect of the water molecules. This explanation was further confirmed by the fluorescence decay of deuteroxide-and water-exchanged samples. Moreover, DMF molecules within the channels of the compound can also modulate the energy levels of the ligands, thus promoting the LMCT process, all confirmed by NMR and XRD studies.
Besides the distinct rotten egg smell for which this toxic gas is commonly known, hydrogen sulfide (H 2 S) is of great importance in biological systems, as well as the cause of acid rain and other environmental problems [111]. Tan and coauthors developed a ratiometric sensor for H 2 S based on Cu 2+ -mediated fluorescence of LnCPs doped with carbon dots (CDs) (CDs@ZIF-8@GMP/Tb) [112]. GMP/Tb on the surface of ZIF-8 (zeolitic imidazolate framework-8) displays a typical ON-OFF-ON behavior upon the sequential addition of Cu 2+ and H 2 S, an observation that can be put to use in response signaling (Figure 10a,b). The fluorescence of the CDs of CDs@ZIF-8@GMP/Tb remains unchanged in the presence of Cu 2+ or/and H 2 S, empowering CDs to be of good reference. As a result, a ratiometric fluorescence sensor based on CDs@ZIF-8@GMP/Tb for sensing H 2 S was fabricated (Figure 10c). The high selectivity towards H 2 S against other anions (e.g., thiols and biological species) and the distinct feature of reversible sensing of this ratiometric sensor will promote the development of more sensitive ratiometric sensors based on LnMOFs. Another H 2 S probe was reported by Yang and coworkers based on the postsynthetic modification of Tb 3+ @Cu-MOF [113]. The Tb 3+ @Cu-MOF (Cu-MOF: [Cu(HCPOC) 2 ] n H 2 CPOC = 5-(4 -carboxylphenoxy) nicotinic acid) exhibits a typically weak emission of Tb 3+ yet a strong ligand-centered emission. The Tb 3+ -based emission can be strongly enhanced by H 2 S due to its superior affinity towards Cu 2+ ions. The detection performance of Tb 3+ @Cu-MOF (1.20 µM) is capable of meeting that of biological systems indicating its potential in real-time organismal H 2 S sensing.  [114]. This EuMOF has a robust three-dimensional network with significant hydrophilic open channels filled with water molecules. The luminescence intensity of the EuMOF gradually decreases as the humidity increases. This effective and remarkably reliable humidity sensor also shows good linearity over a broad humidity range from 0% to 100% RH. Moreover, this sensing material was also examined for electrical detection methods. The recovery time of these methods was found to be similar to that in the photoluminescence measurement.

LnMOFs for pH Sensing
The need to explore fast pH sensing in industry, biomedicine, and many other environmental fields in order to monitor pH values and changes in biological systems and living cells has recently become of top priority [115]. The advantages of luminescence-based pH probes including quick response and high sensitivity, as well as easy operation, making them particularly desirable [116]. Chen and coworkers designed a pH-sensitive MOF nanoparticle using DMF and 1,10-phenanthroline (Phen) as ligands with Tb 3+ ions based on the intramolecular-charge-transfer (ICT) effect [117]. A DMF molecule contains both an electron-donor and -acceptor part, allowing it to generate ICT [118]. It can furthermore change the Tb 3+ -based luminescence through the antenna effect. Consequently, the protonation of H + could change the charge transfer of DMF and further change the antenna effect for Tb 3+ , in turn resulting in a change of Tb 3+ -based luminescence. The Phen molecule in the nanoparticle was used to improve such a change and reduce the luminescence quenching effect of Tb 3+ by replacing the coordinated water molecules. The emission intensity of DMF-Tb was improved approximately 4 times, while the emission intensity of DMF-Tb-Phen was improved 10 times due to a decrease of the ICT effect and increase of the antenna effect on the Tb 3+ ions upon adding H + . This MOF nanoparticle pH sensor with high specificity and sensitivity could be used in strong acidic conditions, indicating its potential applications in biological systems. Qian and coworkers fabricated a fluorescence pH sensor by encapsulating Eu 3+ ions into the pores of the nanoscale UiO-67-bpydc (bpydc = 2,2 -bipyridine-5,5 -dicarboxylic acid) [119]. The luminescence intensity of Eu 3+ @UiO-67-bpydc shows a significant luminescence turn-off response in acidic solutions while exhibiting florescence enhancement in basic solutions. This is because protonation and deprotonation of the ligands first change the excited-state energy level of the ligands followed by a change in ligand-to-Eu energy transfer efficiency, explaining the different changes in the Eu 3+ -based luminescence. This Eu 3+ @UiO-67-bpydc pH sensor is stable within a wide pH range of 1.06 to 10.99 and can thus be used in physiological environments (pH = 6.80-7.60). The bio-compatibility of Eu 3+ @UiO-67-bpydc was further confirmed by an MTT (MTT = 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide) assay. Cell imaging results demonstrate that the Eu 3+ @UiO-67-bpydc pH probe could be a promising candidate for monitoring pH both in vitro and in vivo. Very recently, the same group reported another luminescence pH sensor based on a nanoscale mixed LnMOF Eu 0.034 Tb 0.966 (fum) 2 (ox)(H 2 O) 4 (fum = fumarate, ox = oxalate) [120]. The Eu 0.034 Tb 0.966 (fum) 2 (ox)(H 2 O) 4 pH sensor shows high stability in aqueous solutions. Moreover, its morphology and size can easily be adjusted by changing the amount of CTAB surfactant. The mixed LnMOF exhibits both Tb 3+ (545 nm) and Eu 3+ (618 nm) emissions, which can be used for sensing pH values ranging from 3.00 to 7.00 in a ratiometric manner (Figure 11a-c). The MTT analysis and optical microscopy assay show that this mixed LnMOF sensor has low cytotoxicity and favorable biocompatibility (Figure 11e-f), indicating its potential to be applied as a pH sensor in physiological environments.

LnMOFs for Temperature Sensing
Temperature is an important thermodynamic parameter in human life and scientific investigations. Therefore, accurate temperature measurement is essential in both scientific and human development. Among the approaches for temperature determination, luminescence-based measurements have achieved tremendous attention with regards to their prominent advantages, including noninvasiveness, fast response, accuracy, high spatial resolution, and ability to work in strong electro or magnetic fields [121]. However, the most luminescent thermometers depend on a single emission susceptible to errors because of sample concentration changes and drifts of the optoelectronic system.
Qian and coworkers fabricated the first self-calibrated luminescent temperature sensor using a mixed LnMOF Eu 0.0069 Tb 0.9931 -DMBDC (DMBDC = 2,5-dimethoxy-1,4-benzenedicarboxylate) [37]. For Tb-DMBDC and Eu-DMBDC, the characteristic luminescence gradually decreases because of thermal activation of nonradiative decay pathways. However, Eu 0.0069 Tb 0.9931 -DMBDC exhibits a significant temperature-dependent luminescent behavior as the temperature increases from 10 to 300 K. The Tb 3+ -based emission in Eu 0.0069 Tb 0.9931 -DMBDC decreases as the temperature increases, while that of the Eu 3+ ions increases. This can be ascribed to the efficient energy transfer from Tb 3+ to Eu 3+ based on the phonon-assisted Förster transfer mechanism, an effect confirmed by luminescence lifetime measurements. The good linear relationship between the I Tb /I Eu ratio and temperature in the range of 50-200 K suggests that Eu 0.0069 Tb 0.9931 -DMBDC is an excellent temperature thermometer within this temperature range. These results suggest that mixed LnMOFs featuring temperature-dependent luminescence can be ideal candidates for self-referencing temperature sensing. Since then, many mixed LnMOFs have been fabricated for temperature measurement based on similar luminescent behavior [122].

LnMOFs for Biosensing
Nitrofurans are a type of extensively used veterinary antibiotics effective for the treatment of protozoan and bacterial infections in human beings. It is, however, still urgently needed, as ell as very challenging to develop a rapid and effective approach to detect nitrofuran antibiotics (NFAs) [125]. Yang 3 ]·0.5DMF·H 2 O} n , BCA = 2,2 -biquinoline-4,4 -dicarboxylate) thin-film sensor for NFAs by coating a cost-effective stainless-steel wire mesh using the Co 3 O 4 nano-anchor fixation approach. The Eu-BCA thin-film sensor shows significant quenching effect for NFAs owing to the synergistic effect of electron-transfer and the inner-filter effect. It furthermore shows high selectivity and sensitivity to NFAs with detection limits of 0.21 and 0.16 mm for nitrofurantoin (NFT) and nitrofurazone (NFZ), respectively. NFAs were also successfully detected in real samples, indicating the potential of this Eu-BCA thin-film for biosensing [126].
Another pharmaceutical sensor was designed by Wang and coworkers based on a luminescent mixed-crystal LnMOF (MLMOF-3 = Eu 0.1 Tb 0.9 -BTC) thin film [127]. The uniform and continuous thin film was prepared by coating the monodisperse nanoscale MLMOF-3 on indium-tin-oxide (ITO) glass (Figure 14a,b). The luminescence intensity ratios of Eu 3+ at 619 nm to Tb 3+ at 547 nm of the MLMOF-3 film were used to calculate the intensity ratio change by (R-R 0 )/R 0 , where R 0 is the initial intensity ratio without the analyte, and R is the intensity ratio upon the addition of the analyte (Figure 14c). The luminescence intensity depended significantly on several pharmaceutical molecules (such as antipyrine, benzafibrate, caffeine, clofibrate, clotetracycline, coumarin, diclofenac, fluorouracil, nalidixic acid, naproxen, sulfachinoxalin, and tetracycline) Moreover, the MLMOF-3 thin film shows different guest-dependent colors that can intuitively be distinguished by the naked eye (Figure 14d,e). The authors presumed that the different functional groups and structures of these pharmaceutical molecules may not only modulate the antenna effect between organic linkers and Ln 3+ ions but also affect energy transfer between Tb 3+ and Eu 3+ , causing the different luminescent changes in the MLMOF-3 thin film. These results demonstrate that the mixed LnMOF film can be used as luminescence sensors for different pharmaceutical molecules. Yan and coworkers were the first to design a diagnosis platform for vinyl chloride carcinogen based on a 3d-4f-4d heterometallic MOF (Eu 3+ /Cu 2+ -Zr 6 O 4 (OH) 4 (O 2 C-C 6 H 2 -CO 2 (CO 2 H) 2 ) 6 ·xH 2 O) [128]. The nanoprobe exhibits high selectivity to thiodiglycolic acid (TDGA) with a luminescence enhancement of about 27.5-fold, the main metabolite of vinyl chloride monomer (VCM) in human urine. It further shows a fast response to TDGA within 4 min and impressive sensitivity with a detection limit of 89 ng·mL −1 without interference of other coexisting species in urine. Such excellent sensing performance enables it to monitor TDGA levels in human urine. Furthermore, a portable urine dipstick based on the sensor has been developed to conveniently evaluate individual's intoxication degree of VCM.

Conclusions and Outlook
This comprehensive review covers the recent research progress on luminescent lanthanide MOFs and their applications in sensing cations, anions, small molecules, nitroaromatic explosives, gases, vapors, pH, temperature, and biomolecules. The sensing functionality of LnMOF probes is based on their luminescence changes in response to different analytes, all recognizable by means of spectrofluorometry or the naked eye. Most of the luminescent LnMOF sensors operate by a turn-off mechanism when detecting electron acceptors in which luminescence is quenched through both the electron and the energy transfer between LnMOF sensors and analytes. However, the turn-on detection mode with higher sensitivity and lower detection limits has also been implemented in luminescence-based LnMOF sensors resulting in luminescence enhancement or wavelength shifts. Furthermore, rational incorporation of the functional sites (e.g., Lewis acidic or basic sites and open metal sites) on the pores of the LnMOFs has made them very promising sensors to detect target compounds. Moreover, the ratiometric sensing approach has easily been achieved by embedding multi-luminescent motifs onto the frameworks, which can overcome the main drawbacks of the intensity-based measurements with only one transition.
Although the sensing behavior of LnMOFs has been studied comprehensively, some problems remain. While many investigations have shown excellent results for sensing hazardous materials, fast detection of nitroexplosives with a handheld device in public places, such as the airport and railway station, stays challenging. Furthermore, nanoscale luminescent MOFs with controllable size and morphology are very promising in applications for sensing in living cells. More efforts should be devoted to integrating different functionalities such as cellular sensing and imaging and molecular targeting, as well as drug delivery for practical applications in theranostic nanomedicine. Moreover, in-depth studies on the relationships between structure and luminescent behavior must be conducted using theoretical methods. In addition, the stabilities of recycling, material cost, and portability for practical applications need further improvement. With constant efforts being made to handle these challenges, we believe that the LnMOFs definitely hold a bright future in the field of luminescence sensing. H 3 L 4 p-terphenyl-3,4",5-tricarboxylic acid H 3 L 5 4-(2-carboxyphenoxy)benzene-1,3-dioic acid H 2 L 6 5-(4H-1,2,4-triazol-4-yl)benzene-1,3-dicarboxylic acid L 7 2 ,5 -bis(methoxymethyl)- [