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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2025 , Vol. 37 >Issue 4: 612 - 620

DOI: https://doi.org/10.7536/PC240526

Review

Application of Eu-Tb Lanthanide Bimetallic Organic Frameworks in Fluorescence Sensing

  • Zongxing Wang ,
  • Yue Zhang , * ,
  • Pengcheng Zhao ,
  • Yifei Wang ,
  • Ce Nan ,
  • Zhiyue Zhang
Expand
  • College of Criminal Science and Technology，Criminal Investigation Police University of China，Shenyang 110035，China
*

Received date: 2024-06-05

  Revised date: 2024-09-27

  Online published: 2025-03-10

Supported by

Ministry of Public Security Science and Technology Strengthening Police Basic Work Project(2022JC03)

Ministry of Public Security Science and Technology Strengthening Police Basic Work Project(2023JC05)

Criminal Investigation Police University of China Scientific Research Project(D2022044)

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Abstract

Eu-Tb lanthanide bimetallic organic frameworks （Ln-BMOFs） are inorganic organic hybrid materials with a periodic network structure and functional diversification，which are composed of lanthanide Eu-Tb as the center and organic ligands. It has unique luminescence characteristics，especially sharp absorption，and large Stokes displacement，which makes it exhibit excellent performance in the field of fluorescence sensing. By adjusting the ratio of Eu and Tb in MOFs，we can obtain a series of EuxTb1-x doped MOFs with different luminous colors，and containing different proportions of Eu and Tb，which have similar or different luminous sensing mechanisms. Since the Eu-Tb lanthanide bimetallic organic frameworks have important research value in the field of fluorescence sensing，this paper will comprehensively and systematically review the research progress of lanthanide bimetallic organic frameworks from the aspects of background，sensing mechanism and application of fluorescence sensing.

Contents

1 Introduction

2 Luminescence and sensing mechanisms

2.1 Energy transfer

2.2 Changes in the coordination environment

3 Luminescence sensing applications

3.1 Detection of organic compounds

3.2 Detection of biomolecules

3.3 Detection of ions

3.4 Sensing of temperature and pH

4 Conclusions and prospects

Key words： lanthanide bimetallic metal-organic frameworks; Eu-Tb bimetal; luminescence mechanism; fluorescence sensing

Cite this article

Zongxing Wang , Yue Zhang , Pengcheng Zhao , Yifei Wang , Ce Nan , Zhiyue Zhang . Application of Eu-Tb Lanthanide Bimetallic Organic Frameworks in Fluorescence Sensing[J]. Progress in Chemistry, 2025 , 37(4) : 612 -620 . DOI: 10.7536/PC240526

1 Introduction

Metal-organic frameworks (MOFs) are a class of emerging multifunctional materials that have attracted widespread attention from researchers over the past few decades. As porous coordination polymers composed of metal ions and organic ligands, MOFs have been extensively applied in areas such as gas storage and capture[1-2], chemical catalysis[3-6], and fluorescent sensing[7-16].
Lanthanide metal-organic frameworks (Ln-MOFs) represent a class of luminescent MOFs composed of organic bridging ligands and Ln3+ ions or clusters. Compared to transition metals, they offer higher coordination numbers, more diverse coordination geometries, excellent porosity, topological structural diversity, high specific surface areas, and highly tunable structures. Particularly notable are their unique luminescent properties, such as sharp absorption bands of lanthanides, intrinsic emission bands, high color purity, narrow emission ranges, large Stokes shifts, and long luminescence lifetimes caused by f-f transitions[17-20]. To achieve precise modulation of the emission color range, lanthanide bimetallic organic frameworks (Ln-BMOFs) can be constructed by varying the types and mixing ratios of lanthanide ions. Compared with the single fluorescence emission from monometallic lanthanide MOFs, Ln-BMOFs materials have the following advantages: (1) The introduction of a second metal center enables enhanced luminescent properties through synergistic effects between the two metals. By adjusting the ratio of different lanthanide elements, the emission color can be effectively tuned; (2) Bimetallic systems often provide improved thermal and chemical stability, enhancing physicochemical performance in various biological and chemical environments for effective analytical detection; (3) Ln-BMOFs can serve as ratiometric fluorescent probes with reduced external interference and certain self-calibration capabilities, providing more pronounced fluorescent intensity ratio changes in response to variations in target analyte concentrations, thereby improving detection sensitivity and yielding more reliable and stable experimental results. Among lanthanide metals, Eu and Tb are excellent luminescent materials that emit red and green light, respectively, under ultraviolet excitation. For instance, MOFs based on Eu-Tb bimetallic centers can produce a series of Ln-MOFs with tunable emission colors by altering the mixing ratios of Eu and Tb. These materials can be utilized as luminescent substances and applied for sensing and detection across numerous fields, including anions, cations, small molecules, biomarkers, temperature, and pH.
Compared with other literatures, this paper mainly reviews recent research progress of Ln-BMOFs in fluorescent sensing, especially focusing on Eu and Tb as two lanthanide metals, while most existing reviews summarize single lanthanide metal organic frameworks, and studies on Ln-BMOFs are rarely reported. Secondly, the luminescent sensing mechanism of Ln-BMOFs is summarized in detail. Finally, we selected Eu and Tb as the metal centers of Ln-BMOFs and summarized their applications in fluorescent sensing, such as detection of organic compounds, biomarkers, ions, temperature and pH, hoping to provide some guidance for future development of Ln-BMOFs.

2 Luminescence and Sensing Mechanisms

Ln-BMOFs have been extensively studied and applied in the field of fluorescent sensing due to their unique electronic structure and excellent luminescent properties. The photoluminescence of Ln-BMOFs is generated after absorbing radiative excitation energy. Luminescence can be broadly defined as the emission of light from an excited state following energy absorption. In the absorption process, ultraviolet light is the most common energy source. Among several other forms of excitation, luminescence can be classified into fluorescence and phosphorescence based on the pathway involving photon emission during the relaxation of the excited state. Due to the narrow absorption cross-section and limited absorption efficiency of lanthanide ions, it is difficult to directly optically excite highly luminescent lanthanide ions. This issue can be addressed by the “antenna effect”[21]. The "antenna effect" refers to organic ligands acting as antennas that fully absorb light energy, subsequently transferring this energy to the coordinated central rare-earth ions through molecular energy transfer. Typically, the specific process involves chromophoric organic ligands being excited from the ground state (S0) to the first excited singlet state (S1) upon absorbing excitation light of an appropriate wavelength. Subsequently, molecules in the excited state return to the ground state via non-radiative or radiative pathways (Figure 1). The transition from the first excited singlet state (S1) to the triplet excited state (T1) (S1 → T1) is called intersystem crossing (ISC). Depending on the radiative mode, luminescence can be categorized into two types: fluorescence (S1 → S0) or phosphorescence (T1 → S0)[22-23]. If the ligand coordinates effectively with Ln3+ and the energy level of the ligand's T1 state matches well with that of the Ln3+ ion, then the energy of the T1 state will be transferred to the metal ion, resulting in sensitized luminescence of Ln3+, while the ligand emission will be completely quenched. Subsequently, the excited state of Ln3+ ions lowers its energy to the ground state through radiative transitions, emitting strong fluorescence characteristic of lanthanides. If the energy transfer from the ligand to the Ln3+ ion is incomplete, intramolecular transitions of the organic ligand may emit fluorescence in a radiative form (π-π*, n-π*).
图1 配体到Tb3+/Eu3+的能量转移过程[24]

Fig.1 Energy transfer process from ligand to Tb3+/Eu3+ [24] Copyright 2023，Elsevier

The sensing mechanisms of Ln-BMOFs in the field of fluorescent sensing typically involve two major methods: energy transfer and changes in the coordination environment.

2.1 Energy Transfer

Lanthanide metals have abundant electronic energy levels, especially the f-f transitions are very sensitive and can participate in energy transfer. For example, in Ln-MOFs, the sensing mechanism typically involves the organic ligand absorbing specific energy, after which Ln3+ receives energy from the organic ligand. Ln3+ is then excited to a higher energy state and subsequently returns to the ground state, accompanied by the emission of its characteristic light. In Ln-BMOFs, such as Eu-Tb-MOFs, after the organic ligand absorbs specific energy, the energy is first transferred to Tb, and then Tb transfers the energy to Eu. Finally, Eu becomes excited and returns from the excited state to the ground state, emitting characteristic red light. During this process, if there exists any substance that affects the energy transfer, it will influence the efficiency of the energy transfer, thereby affecting the luminescent properties of Ln-BMOFs[24].

2.2 Variation in Coordination Environment

The variation in the coordination environment of Ln-BMOFs significantly influences their sensing mechanism. These changes can affect the energy transfer efficiency and optical properties of the material in various ways, enabling a response to external environmental changes. Among these effects, both solvent effects and ion exchange alter the coordination environment of Ln-BMOFs, thereby influencing their luminescent properties. In Eu-Tb-MOFs, variations in the coordination environment can impact the efficiency of energy transfer from the excited state ligands to the metal ions. For instance, solvent molecules or external molecules entering the pores of the organic framework may change the Eu-Tb coordination environment by directly coordinating with the metal ions or indirectly affecting the electronic structure of the metal ions through hydrogen bonding, van der Waals forces, and other interactions, leading to changes in luminescent properties.
Ln-BMOFs achieve highly sensitive fluorescence sensing of substances through two main mechanisms: efficient regulation of energy transfer and changes in the coordination environment, relying on their unique optical properties. This enables Ln-BMOFs to have broad application prospects in sensing fields such as ion detection, organic compounds, biomolecules, temperature, and pH.

3 Luminescent Sensing Applications

3.1 Detection of Organic Compounds

Antibiotics are widely used to treat infections caused by a broad range of bacterial species. However, the overuse of antibiotics has led to their widespread presence in medical waste, food, and the environment[25]. The pervasive existence of such antimicrobial agents has resulted in the accelerated development and global spread of antibiotic resistance[26], posing a high risk to human health and the environment, and it has been recognized as one of the most serious threats to public health in the 21st century[27].
Wang et al.[28] developed a dual-emitting lanthanide metal-organic framework Tb/Eu(BTC) fluorescent sensor using Tb and Eu as luminescent centers and 1,3,5-benzenetricarboxylic acid (H3BTC) as the ligand, for rapid identification and quantitative detection of fluoroquinolone (FQ) antibiotics. Due to differences in the sensitization effect and inner filter effect of FQ on lanthanides, this sensor enables discriminative detection of various FQs within a range of 0 to 90 µmol/L, with a detection limit as low as 16 nmol/L. Based on the fingerprint correlation between fluorescence spectral color and target quantum dots, a series of color-coded maps were established according to color variations, enabling rapid identification and accurate concentration measurement of unknown fluorescent quantum dots. The visual characteristics of the proposed colorimetric strategy can be integrated into smartphone-based portable detection systems for imaging analysis of antibiotics through novel color signal processing methods (Fig. 2). This study presents a new design strategy for on-site multiplex detection using fluorescent sensors and demonstrates significant potential for application in public health and environmental monitoring fields.
图2 （a）Tb/Eu（BTC）的合成过程；（b）抗生素的检测原理和智能手机成像分析[28]

Fig.2 （a） Synthesis process of Tb/Eu（BTC）；（b）Antibiotic detection principles and smartphone imaging analysis[28]. Copyright 2022，Elsevier

In addition, recently Wang et al. [29] synthesized a dual lanthanide metal-organic framework Eu-Tb (BDC) using Eu and Tb as centers and 1,4-phthalic acid (H2BDC) as a ligand through a co-precipitation method. The crystal structure is stable with tunable fluorescence color. Eu-Tb (BDC) demonstrates rapid sensing and quantitative detection capabilities for fluoroquinolone antibiotics in urine. Experimental and theoretical results indicate that Eu-Tb (BDC) exhibits high selectivity toward norfloxacin (NFX), is unaffected by urinary components, has a short response time, wide detection concentration range, and low detection limit. This study proposes a novel and accurate method for detecting fluoroquinolone antibiotics and establishes a foundation for further theoretical investigations.
Nitro compounds are hazardous substances that are widely used in civilian and industrial explosives[30-31], endangering residents' health and causing environmental pollution[32-33]. Among them, 4-nitrophenol (4-NP) is an important raw material for organic synthesis and is commonly used as an intermediate in dye production, as well as in pharmaceuticals and pesticides[34]. However, its misuse has caused serious harm to the environment and human health. Currently, extensive research has been conducted on how to detect these molecules, such as gas chromatography (GC), atomic absorption spectroscopy (AAS), and mass spectrometry (MS)[35-36], but many limitations still exist, such as inconvenience and difficulty in implementation.
Cheng et al.[37] successfully synthesized a Eu0.5Tb0.5-MOFs (Figure 3) through a solvothermal reaction using 1,3-bis(3,5-dicarboxyphenyl)imidazolium chloride as an organic ligand. It exhibited strong fluorescence when placed in N,N-dimethylformamide (DMF), MeOH, and EtOH, and weaker fluorescence in acetone. Therefore, a DMF solution of the complex was prepared to detect various nitro compounds (NP). The results showed that 2-nitrophenol (2-NT) and 2,4,6-trinitrotoluene (TNT) caused slight fluorescence quenching of Eu0.5Tb0.5-MOFs, while the fluorescence intensity significantly decreased in 4-nitrotoluene (4-NT). Detection results indicated better quenching efficiency of 4-NP on the fluorescence intensity of Eu0.5Tb0.5-MOFs. Additionally, the recyclability performance of Eu0.5Tb0.5-MOFs was studied, showing no significant decrease in fluorescence intensity after five cycles of reuse.
图3 Eu0.5Tb0.5-MOFs的合成及通过荧光猝灭效应检测4-NP[37]

Fig.3 Synthesis of Eu0.5Tb0.5-MOFs and detection of 4-NP by fluorescence quenching effect[37]. Copyright 2020，Elsevier

Phenolic compounds are a class of important chemical reagents that can be used as raw materials and intermediates in the production of dyes, pharmaceuticals, pesticides, and polymers. They are widely used in industry and agriculture and can contaminate soil and groundwater. Inhalation, ingestion, or contact with these compounds may lead to skin lesions, dizziness, severe systemic damage, and even death[38-39]. To the best of our knowledge, there are only a few reported examples of effective detection of phenolic compounds[40-42], but no reports have been published regarding the detection of β-naphthol. Therefore, it is of great significance to develop sensitive and selective methods for detecting phenolic compounds, especially β-naphthol. Zhang et al.[43] synthesized a series of mixed Ln-BMOFs (with 2,5-thiophenedicarboxylic acid as the organic ligand) by simultaneously incorporating Eu and Tb into an isostructural framework under solvothermal conditions. Based on the trichromatic theory, the EuxTb1-x-MOFs exhibited tunable luminescence by rationally adjusting the ratio of Eu to Tb and the excitation wavelength. Notably, the synthesized white-light-emitting material Eu0.00667Tb0.99333-MOFs was successfully excited at 350 nm. Moreover, within the physiological temperature range of 288–353 K, Eu0.00667Tb0.99333-MOFs demonstrated excellent selectivity and sensitivity toward β-naphthol. Furthermore, after five repeated cycles, the material retained almost its initial fluorescence intensity and quenching efficiency, showing good reproducibility and stability.

3.2 Biomolecular Detection

The tricarboxylic acid (TCA) metabolites in cancer cells exhibit significant differences compared to those in normal cells. Metabolites within the TCA cycle demonstrate varying degrees of alteration in the metabolic characteristics of numerous physiological diseases, especially cancers[44-45]. Therefore, detection of metabolites in the TCA cycle can be utilized for diagnostic purposes. Histopathological examination is the primary clinical method for cancer diagnosis; however, its application is limited due to low sensitivity and high invasiveness[46]. Among various materials, Eu/Tb-MOFs serve as a targeted detection material for TCA cycle metabolites, enabling non-invasive, real-time, and rapid discrimination of cancer cells. Li et al.[47] constructed Eu/Tb-MOFs with multiple luminescent signals using Tb, Eu, and the organic ligand 1,3,5-tris(4-carboxyphenyl)benzene. In the presence of TCA metabolites, six characteristic peaks of Eu/Tb-MOFs exhibited significant changes due to host-guest interactions. During qualitative detection testing, the sensor array accurately distinguished 18 TCA metabolites at four concentrations (50, 100, 200, and 300 μM) through linear discriminant analysis. Pattern recognition and labeling experiments demonstrated both quantitative and qualitative detection capabilities of the sensor array. This novel sensor array enables rapid and accurate detection of TCA cycle metabolites and differentiation of cancer cells via host-guest interactions, showing great potential for clinical cancer diagnosis.
Bacillus anthracis is a hazardous bacterium with high mortality and morbidity rates in humans and animals, and has been widely used as a biological weapon[48]. Inhalation of more than 104 B. anthracis spores can lead to death if not treated promptly. 2,6-Pyridinedicarboxylic acid (DPA) is a unique biomarker of B. anthracis spores, accounting for 5%–15% of the spore dry weight, and has attracted extensive attention in recent years[49]. Wang et al.[50] synthesized Tb/Eu mixed Ln-MOFs with different molar ratios using a solvothermal method, specifically TbxEu1-x-cppa (cppa = 5-(5-carboxypyridin-3-yl)isophthalic acid). The results indicated that the fluorescent probe [Tb0.533Eu0.467-(Hcppa)1.5(H2O)(DMF)]·3H2O exhibited excellent water and pH stability, good sensitivity (LOD = 2.286 μmol/L), high selectivity, and rapid response (< 2 min) toward DPA detection (Fig. 4). Due to the hindered energy transfer from Tb to Eu combined with the inner filter effect of DPA, the fluorescent probe showed an evident color change from orange-red to green (Fig. 5). Moreover, visual detection of DPA was achieved by recognizing RGB values of MOF-based agarose hydrogel membranes using a smartphone, highlighting the practical application value of the fluorescent probe for detecting DPA under aqueous conditions.
图4 Tb0.533Eu0.467-cppa中的能量传递和DPA的比率传感[50]

Fig.4 Tb0.533Eu0.467-cppa of ratio sensing of energy transfer and DPA[50]. Copyright 2023，American Chemical Society

图5 BHM-COOH的结构和Ln-MOFs的结构（Eu0.24Tb0.76-BHM-COOH）[54]

Fig.5 Structure of BHM-COOH and Ln-MOFs（Eu0.24Tb0.76-BHM-COOH）[54]. Copyright 2020，Elsevier

3.3 Ion Detection

As is well known, iron, an essential trace element in living organisms, is one of the necessary trace elements for the human body. It binds to hemoglobin in blood and participates in oxygen transport, playing an extremely important role in many physiological processes[51]. Therefore, even slight changes in iron content within the human body can have significant impacts on health. For example, iron deficiency leads to anemia[52-53]. In recent years, iron detection has attracted increasing attention. Chemical sensing methods are excellent approaches for detecting iron, offering advantages such as good selectivity, high sensitivity, and low detection limits. Currently, many researchers worldwide are investigating how to design luminescent MOFs as chemical sensors for detecting metal ions. Jia et al.[54] synthesized a series of luminescent metal-organic frameworks (MOFs) materials based on Eu/Tb using a novel flexible ligand (BHM-COOH) containing twelve carboxyl groups (see Fig. 6). A reliable and convenient luminescence detection platform was constructed by combining polylactic acid (PLA) film with Eu0.24Tb0.76-BHM-COOH. More importantly, this luminescent platform exhibits high sensitivity toward Fe3+ through fluorescence quenching (the Stern-Volmer constant Ksv of Fe(NO3)3 reaches 1.27×104 L/mol), with a detection limit as low as 4.47 μmol/L. The sensing mechanism of this sensor is attributed to fluorescence quenching caused by competitive absorption between Eu0.24Tb0.76-BHM-COOH and Fe3+ ions. Meanwhile, the sensor can be reused multiple times. These results indicate that Eu0.24Tb0.76-BHM-COOH film can serve as a multi-responsive luminescent sensor for monitoring environmental pollutants.
图6 二维Tb0.6Eu0.4-bop纳米片的合成及其对Hg2+的检测机理[57]

Fig.6 Synthesis of two-dimensional Tb0.6Eu0.4-bop nanosheets and detection mechanism for Hg2+ [57] Copyright 2022，Elsevier

Mercury (Hg) is a transition metal and the only metal that remains in liquid state at room temperature. Hg pollution has drawn global attention due to its severe ecological destruction and human health risks[55], and even at low concentrations, Hg2+ can bioaccumulate in the food chain and cause various diseases[56]. Wang et al.[57] developed three fluorescence sensors based on 2D Ln-BMOFs nanosheets (2D Tb-Eu-bop) for rapid and effective detection of Hg2+. These were prepared by simply mixing Tb-Eu salts with 5-boronate isophthalic acid (5-bop) reaction mixtures at room temperature in the presence of triethylamine (TEA). The two-dimensional Tb0.6Eu0.4-bop nanosheets exhibited ratiometric fluorescence response toward Hg2+. Mechanistic studies revealed that the Hg2+-induced metal transfer reaction enhanced energy transfer between the ligand and rare-earth ions, increasing the quantum yield of Tb and thereby improving the energy transfer efficiency between Tb and Eu nodes, resulting in a ratiometric fluorescence response to Hg2+, which further enhanced the sensitivity of Hg2+ sensing (Figure 7). This work not only provides a simple strategy for the preparation of two-dimensional MOFs nanosheets, but also lays the foundation for the rational design of high-performance fluorescent probes.
图7 Tb/Eu-MOFs对磷酸盐（Ps）的检测过程[64]

Fig.7 Detection process of phosphate （Ps） by Tb/Eu-MOFs[64]. Copyright 2023，Elsevier

Phosphate (PO43-) is an important anion in aquatic environments and biological systems. In water systems, PO43- serves as a crucial nutrient that promotes the growth of aquatic flora and fauna[58-59]. However, excessive phosphate can lead to eutrophication, resulting in abnormal growth of algae and other plankton, as well as mass mortality of fish and other organisms[60-62]. Deficiency or excess of PO43- is harmful to human health and can induce serious diseases such as cardiovascular disorders and Parkinson's disease[63]. Zhao et al.[64] prepared Tb/Eu-MOFs with a core-shell structure using a solution-mediated epitaxial growth method. These MOFs exhibit excellent heterostructural characteristics, enabling efficient electronic modulation and multiple energy transfer, thereby significantly enhancing the recognition and sensing capability for PO43-. After coupling with PO43-, the fluorescence intensity of the Ln3+ center decreases, interrupting the fluorescence intensity ratio of Tb/Eu and forming a ratiometric fluorescent sensor (Fig. 8). The Tb/Eu-MOFs with an optimized Tb/Eu molar ratio of 1:1 showed good linear response towards PO43-, with a detection limit of 84.1 nM. Additionally, interference and toxicity experiments confirmed its high selectivity and biocompatibility. This probe can be applied for the detection of PO43- in real environmental water samples.
图8 （a）Tb0.94Eu0.06-HS的发光颜色[75]；（b，c）在50~300 K范围内Eu0.01Tb0.99-BDC-F4的发射光谱和CIE图[76]；（d）Eu0.05Tb0.95-OBA在pH为1~13的HCl或NaOH溶液中的发射光谱[79]

Fig.8 （a） Luminous color of Tb0.94Eu0.06-HS[75]. Copyright 2024，American Chemical Society. （b，c）Emission spectrum and CIE diagram of Eu0.01Tb0.99-BDC-F4 in the range of 50-300 K[76]. Copyright 2020，Elsevier. （d）Eu0.05Tb0.95-OBA in HCl or NaOH solutions with pH 1-13 of emission spectra[79]. Copyright 2021，Royal Society of Chemistry

3.4 Temperature, pH Sensing

As is well known, temperature plays a crucial role in evaluating the vitality and physical dynamics of all natural and engineering systems in daily life, industrial production, and pathological research. Particularly under physiological temperature conditions, precise monitoring and accurate measurement of physiological temperatures are essential for understanding physiological or pathological processes and treating diseases. Traditional contact thermometers are unsuitable for measuring the temperature of rapidly moving objects or at submicron scales because they require equilibrium between the thermal probe and the sample[65-68]. Many scientists have devoted efforts to designing noninvasive, precise thermometers, particularly luminescence-based thermometers (Ln-BMOFs), which have attracted increasing attention due to their simplicity, speed, high sensitivity, and high resolution, even enabling operation in strong electromagnetic fields and on rapidly moving objects[69-72].
Feng et al.[73] synthesized a series of binary co-doped Ln-CPs [Tb1-xEux-tcptpy] (H3tcptpy=4-(2,4,6-tricarboxylphenyl)-4,2':6',4''-terpyridine). Among them, Tb0.897Eu0.103-tcptpy acts as a ratiometric luminescent thermometer, achieving a relative sensitivity of up to 8.41% K-1 within the temperature range of 305–340 K. The temperature-dependent photoluminescence properties of the representative Tb0.9Eu0.1-tcptpy were investigated to evaluate its potential as a luminescent thermometer. Tb0.897Eu0.103-tcptpy exhibits significantly different temperature-dependent luminescent behaviors between 10 and 400 K. Initially, when the temperature increases from 10 K to 150 K, the emission intensities of both Tb and Eu ions decrease, commonly attributed to thermally activated non-radiative decay transitions[74]. However, within the temperature range of 300–340 K, the emission intensity of Tb ions decreases while that of Eu species gradually increases. This phenomenon is rationalized via a thermal-driven energy transfer mechanism, further described as energy conversion between Tb and Eu ions. Therefore, Tb0.897Eu0.103-tcptpy serves as an excellent luminescent thermometer within the physiological temperature range of 305–340 K in the visible light region.
Liu et al.[75] selected NaH2SIP as an organic ligand (NaH2SIP = sodium 5-sulfoisophthalate) and obtained three different lanthanide mixed coordination polymers, Tb0.96Eu0.04-HS, Tb0.95Eu0.05-HS, and Tb0.94Eu0.06-HS, by doping with varying content ratios of Eu/Tb ions. The codoped lanthanide coordination polymers with different Eu/Tb ratios exhibited high sensitivity within the physiological temperature range, being 16.8, 7.0, and 14.5% K-1, respectively. Tb0.95Eu0.05-HS and Tb0.94Eu0.06-HS displayed luminescence color transitions from green to yellow and then to orange, achieving visual luminescence within a narrow temperature range (Fig. 8a). The results indicated that they possess high sensitivity and good linear correlation within the physiological temperature range, making them excellent ratiometric luminescent thermometers for measuring physiological temperatures.
Zhou et al.[76] designed and synthesized a series of rare earth luminescent compounds using 2,3,5,6-tetrafluoro-1,4-benzenedicarboxylate (H2BDC-F4) as an organic ligand through a solvothermal reaction. In the mixed lanthanide compound TbxEu1-x-BDC-F4 (italic x = 0.3%, 0.5%, 0.7%, 1%, and 2%), the luminescence color of the mixed rare earth compound can be tuned from green to red by controlling the Tb/Eu ratio, which is attributed to energy transfer from Tb to Eu. The fluorescence intensity ratio between the green emission of Tb (~544 nm) and the red emission of Eu (~619 nm) enables self-referenced optical thermometry, making these compounds excellent candidates for luminescent thermometers and color-tunable emissive materials. Moreover, Eu0.01Tb0.99-BDC-F4 can be tuned from green to orange within the temperature range of 50–300 K (see Figure 8b, c). This feature allows the mixed Eu/Tb-BDC-F4 compounds to exhibit significant temperature sensitivity, rendering them promising candidates for ratiometric luminescent thermometers with potential applications as sensitive luminescent materials.
pH is one of the most important physicochemical parameters in aqueous solutions. Many natural phenomena, chemical reactions and industrial processes involving aqueous solutions are closely related to pH values. Abnormal pH levels in the human body, such as intracellular acidification (pH=4.5~6.0), are often associated with various pathological processes including inflammation, tumor development, and cystic fibrosis[77]. Therefore, accurate monitoring of pH values holds significant value across numerous fields including industry, agriculture, medicine, environmental protection, and scientific research. Recently, optical methods have gained widespread attention due to their high sensitivity, rapid response, ease of miniaturization, non-invasive nature, and high throughput[78]. In particular, fluorescent pH sensors have been extensively applied, among which Ln-BMOFs show great promise in ratiometric fluorescent pH sensing. Li et al.[79] synthesized Eu0.05Tb0.95-OBA through an ion-exchange reaction between 4,4'-dihydroxybenzophenone dicarboxylic acid (H2OBA) and Eu and Tb ions. The emission from combined Eu and Tb transitions is affected by pH and moisture levels, resulting in emission color changes depending on pH variations. When pH=1~2, the acid-base equilibrium of OBA2-→H2OBA occurs, emitting blue light. At pH=3~11, the luminescence color shifts from green to orange. When pH=12~13, the fluorescence is quenched due to OH- disrupting its delocalized conjugated system. Thus, Eu0.05Tb0.95-OBA can serve as a ratiometric fluorescent sensor for pH detection (Figure 8d).

4 Conclusion and Prospect

This review summarizes the luminescent sensing mechanisms of Ln-BMOFs and their applications in the field of fluorescence sensing, such as the detection of organic compounds, biomolecules, ions, temperature, and pH. Ln-BMOFs offer unique advantages in fluorescence sensing, including: (1) excellent luminescent properties, where Ln-BMOFs can emit strong fluorescence, and their emission wavelengths can be adjusted by varying the doping ratio of Tb and Eu within the compound; (2) stability as ratiometric fluorescence probes, where measuring the ratio of fluorescence signals effectively reduces interference caused by external factors such as probe concentration and light source intensity, thereby improving detection accuracy; and (3) structural diversity, where the chemical structures of Ln-BMOFs can be tuned through various synthesis methods—for example, hydrothermal/solvothemal methods, microwave-assisted synthesis, ultrasonic methods, mechanochemical grinding, and coprecipitation—enabling the production of compounds with different properties and functionalities to meet diverse sensing requirements.
With the continuous deepening of research on Ln-MOFs, more and more reports have emerged regarding their fluorescent sensing and detection applications. Although Ln-MOFs exhibit excellent performance in fluorescence sensing and other fields, they also face certain difficulties and challenges. For example: (1) Single Ln-MOFs contain only one type of metal center, resulting in relatively monochromatic emission colors that cannot achieve tunable luminescence; (2) During analytical detection, single Ln-MOFs are significantly affected by external environmental interferences and lack self-calibration capabilities, often leading to experimental data with certain deviations; (3) Compared with Ln-BMOFs, single Ln-MOFs exhibit poorer thermal and chemical stability, making them unsuitable for effective analytical detection in certain chemical and biological environments. In contrast, Ln-BMOFs can overcome these shortcomings in some aspects and demonstrate additional advantages.
Although Ln-BMOFs generally exhibit high thermal and chemical stability, their stability may be affected under certain extreme conditions (such as strong acidic or alkaline environments, high temperature, and high humidity); the synthesis process of Ln-BMOFs involves complex chemical reaction conditions and precise control of metal ratios, where undesirable crystal structures or uneven metal distributions might occur during synthesis; in luminescent sensing applications, the energy transfer efficiency between Eu and Tb is a critical factor, and insufficiently efficient energy transfer may lead to reduced luminescence efficiency. To meet diverse application requirements, it is necessary to design various types of Ln-BMOFs with tunable properties to adapt to multiple application scenarios, which remains a challenging issue to address in the future. The future development trends include: (1) developing new synthetic techniques and strategies to enhance synthesis efficiency and product purity, while exploring cost-effective and high-yield methods for industrial-scale production; (2) optimizing the structural design of Ln-BMOFs by combining computational simulations with experimental approaches to improve luminescent sensing performance through controlled metal ratios and ligand structures; (3) striving to enhance the energy transfer efficiency between Eu and Tb to optimize luminescent properties. Although Ln-BMOFs face several challenges in their future development, their excellent properties and structural diversity make them hold tremendous potential in the field of fluorescent sensing and represent a valuable resource for fluorescent sensing materials.

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