Highly stable Tb(III) metal-organic framework derived from a new benzothiadiazole functionalized ligand for fluorescence recognition of ascorbic acid

Because ascorbic acid (AA) is one of the basic elements to maintain the normal physiological functions of human body, it is urgent to develop a material that can achieve efficient, rapid and in-situ detection for AA. A new fluorescence organic compound 4',4'''-(benzo[c][1,2,5]thiadiazole-4,7-diyl)bis([1,1'-biphenyl]-4-carboxylic acid) (H2BTBC) based on benzothiadiazole group has been synthesized, which can detect Fe3+ ions by fluorescence turn-off effect with a detection limit of 0.015 μM, as well as recognize linear amines by fluorescence turn-on effect. Moreover, a highly stable Tb(III) metal-organic framework has been solvothermally prepared with H2BTBC, namely {[(CH3)2NH2]2[Tb2(BTBC)4]∙solvents}n (JXUST-39), which can selectively detect AA among biological fluids by fluorescence enhancement effect with a detection limit of 0.077 μM. In addition, the mechanism for JXUST-39 detecting AA is possibly the cooperative effect of absorbance-caused enhancement and charge transfer between JXUST-39 and AA. Moreover, LED lamp beads, fluorescent films and fluorescent detection test paper based on JXUST-39 were prepared to achieve portable detection via fluorescence enhancement effect.

Ultrastable Lanthanide Metal-Organic Frameworks for Smartphone-Assisted Ratiometric Fluorescent Sensing of Toluenediamines and Tunable Luminescence

Lanthanide metal-organic frameworks (Ln-MOFs) have excellent optical properties and structural diversity, providing a unique platform for the development of fluorescent sensing and optical materials. In the work described herein, a series of isostructural 3D Ln-MOFs [Ln(L)(H2O)]·2H2O (Ln = Eu (1), Gd (2), Tb (3), H3L = 3,3',3″-[1,3,5-benzenetriyltris(carbonylimino)]tris-benzoate) are fabricated under solvothermal conditions. The good thermal, water, and acid-base stabilities of 3 are prerequisites for fluorescent sensing applications. 3 can be used as a ratiometric broad-spectrum fluorescent sensor for toluenediamines (TDAs) in real urine with the advantages of visualization, ultrasensitivity, and selectivity. Interestingly, a smartphone-assisted intelligent sensing platform manifests promising results for the detection of TDAs, providing a chance for further development of portable diagnostic tools. In addition, by tuning the ratios of Eu3+/Tb3+ and Eu3+/Gd3+/Tb3+, nine bimetallic-doped EuxTb1-x (x = 0.10-0.90, 4-12) and one trimetallic-doped Gd0.95Tb0.015Eu0.035 (13) were obtained. 4-12 exhibit a gradient of luminescent colors from yellow-green to pink with different ratios of Eu3+ and Tb3+ ions. Meanwhile, the trimetallic-doped Gd0.95Tb0.015Eu0.035 (13) shows near-white-light emission with a quantum yield of 8.76%. Interestingly, the inks made with 1-13 are invisible under ambient light but show visual color-tunable luminescence under a 254 nm UV lamp, which may facilitate their anti-counterfeiting applications.

Regulation of the Metal Center in Lanthanide Nanoparticles to Achieve Multifunctional Sensing

Development of a multifunctional sensor is highly desirable. In this work, traces of a carcinoid cancer biomarker of 5-hydroxyindole-3-acetic acid (5-HIAA) in real human urine can be detected by lanthanide nanoparticle Eu-CFC (CFC = 7-chloro-1-cyclopropyl-6-fluoro-4-oxo-1,4-dihydroquinoline-3-carboxylic acid) and the sensing devices of the test paper and agarose gel, achieving an ultralow LOD of 0.8 × 10-3 ppm within a sensing time of 2.0 min. Interestingly, by metal center regulation of Tb and Eu codoping, nanoparticle TbEu2-CFC shows high-sensitivity and low-LOD (0.019% v/v) sensing of water in ethanol. The sensing mechanisms are revealed by both experiments and quantum chemical studies.

Ionic Covalent Organic Framework as a Dual Functional Sensor for Temperature and Humidity

Visual sensing of humidity and temperature by solids plays an important role in the everyday life and in industrial processes. Due to their hydrophobic nature, most covalent organic framework (COF) sensors often exhibit poor optical response when exposed to moisture. To overcome this challenge, the optical response is set out to improve, to moisture by incorporating H-bonding ionic functionalities into the COF network. A highly sensitive COF, consisting of guanidinium and diformylpyridine linkers (TG-DFP), capable of detecting changes in temperature and moisture content is fabricated. The hydrophilic nature of the framework enables enhanced water uptake, allowing the trapped water molecules to form a large number of hydrogen bonds. Despite the presence of non-emissive building blocks, the H-bonds restrict internal bond rotation within the COF, leading to reversible fluorescence and solid-state optical hydrochromism in response to relative humidity and temperature.

Advances on fluorescence chemosensors for selective detection of water

Water, although an important part of everyday life, is acts as one of the most significant contaminants in various applications such as biomedical monitoring, chemical production, petroleum-based fuel and food processing. In fact, the presence of water in other solvents is a huge concern. For the quantification of trace water content, different methods such as Karl-Fischer, electrochemical, nuclear magnetic resonance, chromatography, and thermogravimetric analysis have been used. Although every technique has its own benefit, each one suffers from several drawbacks that include high detection costs, lengthy procedures and specialized operations. Nowadays, the development of fluorescence-based chemical probes has become an exciting area of research for the quick and accurate estimation of water content in organic solvents. A variety of chemical processes such as hydrolysis reaction, metal ions promoted oxidation reaction, suppression of the -C═N isomerization, protonation and deprotonation reactions, and molecular aggregation have been well researched in the last few years for the fluorescent detection of trace water. These chemical processes eventually lead to different photophysical events such as aggregation-induced emission (AIE), aggregation-induced emission enhancement (AIEE), aggregation-caused quenching (ACQ), fluorescent resonance energy transfer (FRET), charge transfer, photo-induced electron transfer (PET), excited state intramolecular proton transfer (ESIPT) that are responsible for the detection. This review presents a summary of the fluorescence-based chemosensors reported in recent years. The design of water sensors, sensing mechanisms and their potential applications are reviewed and discussed.

Recent Advances of Bimetallic-Metal Organic Frameworks: Preparation, Properties, and Fluorescence-Based Biochemical Sensing Applications

Bimetallic-metal organic frameworks (BiM-MOFs) or bimetallic organic frameworks represent an innovative and promising class of porous materials, distinguished from traditional monometallic MOFs by their incorporation of two metal ions alongside organic linkers. BiM-MOFs, with their unique crystal structure, physicochemical properties, and composition, demonstrate distinct advantages in the realm of biochemical sensing applications, displaying improvements in optical properties, stability, selectivity, and sensitivity. This comprehensive review explores into recent advancements in leveraging BiM-MOFs for fluorescence-based biochemical sensing, providing insights into their design, synthesis, and practical applications in both chemical and biological sensing. Emphasizing fluorescence emission as a transduction mechanism, the review aims to guide researchers in maximizing the potential of BiM-MOFs across a broader spectrum of investigations. Furthermore, it explores prospective research directions and addresses challenges, offering valuable perspectives on the evolving landscape of fluorescence-based probes rooted in BiM-MOFs.

Multi-analyte fluorescence sensing based on a post-synthetically functionalized two-dimensional Zn-MOF nanosheets featuring excited-state proton transfer process

Covalent post-synthetic modification of metal-organic frameworks (MOFs) represents an underexplored but promising avenue for allowing the addition of specific fluorescent recognition elements to produce the novel MOF-based sensory materials with multiple-analyte detection capability. Here, an excited-state proton transfer (ESPT) active sensor 2D-Zn-NS-P was designed and constructed by covalent post-synthetic incorporation of the excited-state tautomeric 2-hydroxypyridine moiety into the ultrasonically exfoliated amino-tagged 2D Zn-MOF nanosheets (2D-Zn-NS). The water-mediated ESPT process facilitates the highly accessible active sites incorporated on the surface of 2D-Zn-NS-P to specifically respond to the presence of water in common organic solvents via fluorescence turn-on behavior, and accurate quantification of trace amount of water in acetonitrile, acetone and ethanol was established using the as-synthesized nanosheet sensor with the detection sensitivity (<0.01% v/v) superior to the conventional Karl Fischer titration. Upon exposure to Fe3+ or Cr2O72-, the intense blue emission of the aqueous colloidal dispersion of 2D-Zn-NS-P was selectively quenched even in the coexistence of common inorganic interferents. The prohibition of the water-mediated ESPT process and local emission, induced by the coordination of ESPT fluorophore with Fe3+ or by Cr2O72- competitively absorbs the excitation energy, was proposed to responsible for the fluorescence turn-off sensing of the respective analytes. The present study offers the attractive prospect to develop the ESPT-based fluorescent MOF nanosheets by covalent post-synthetic modification strategy as multi-functional sensors for detection of target analytes.

Hg2+ detection and information encryption of new [1+1] lanthanide cluster

The sensing of heavy metal ion and information encryption are two very important research areas. Therefore, developing multi-functional materials capable of sensing heavy metal ions and encrypting information is highly important. In this work, three [1 + 1] lanthanide clusters [Ln(TFBA)3(dmp) (H2O)2]2 (Ln = Tb3+Tb1+1, Eu3+Eu1+1, Gd3+Gd1+1, HTFBA = 2,3,4,5-tetrafluorobenzoic acid, dmp = 4,7-dimethyl-1,10-phenanthroline) were designed and synthesized. Among them, Tb1+1 shows excellent luminescence sensing towards Hg2+ (Ex = 350 nm, Em = 545 nm). Results demonstrates the sensing with high selectivity, strong anti-interference, 20-s response time, high accuracy, excellent linear relationship in 0-20.0 μM, and a very low limit of detection (0.02 ppb). Furthermore, paper strips based on Tb1+1 is fabricated for visual detection of Hg2+ in real samples of tap water, lake water, human urine, and human serum. More interestingly, a new method for confidentiality of information is realized through multi-color anti-counterfeiting patterns with the [1 + 1] lanthanide cluster ink, based on the luminescence "on-off" sensing towards Hg2+.

MOF@Au NPs/aptamer fluorescent probe for the selective and sensitive detection of thiamethoxam

The luminescence performance of fluorescent reagents plays a crucial role in fluorescence analysis. Therefore, in this study, a novel bi-ligand Zn-based metal-organic framework, Au nanoparticle (NP) fluorescent material was synthesized using a hydrothermal method with Zn as the metal source. Simultaneously, a DNA aptamer was introduced as a molecular recognition element to develop a Zn-based MOF@Au NPs/DNA aptamer fluorescent probe for the ultra-trace detection of thiamethoxam residues in agricultural products. The probe captured different concentrations of the target molecule, thiamethoxam, through the DNA aptamer, causing a conformational change in the DNA aptamer and bursting the fluorescence of the probe, therefore establishing a fluorometric method for thiamethoxam detection. This method is highly sensitive due to the excellent luminescence properties of the Zn-based MOF@Au NPs, and the DNA aptamer can specifically recognize thiamethoxam, offering high selectivity. The linear range of the method was 2.5-6000 × 10-11  mol L-1 , with a detection limit of 8.33 × 10-12  mol L-1 . This method was applied to the determination of actual samples, such as bananas, and the spiked recovery rate was found to be in the range 84.05-109.07%. Overall, the proposed probe has high sensitivity, high selectivity, and easy operation for the detection of thiamethoxam residues in actual samples.

Lanthanide Metal-Organic Framework Isomers with Novel Water-Boosting Lanthanide Luminescence Behaviors

Lanthanide metal-organic frameworks (Ln-MOFs) with exceptional optical performance and structural diversity offer a unique platform for the development of luminescent materials. However, Ln-MOFs often suffer from luminescence quenching by high-vibrating oscillators, especially in aqueous solution. Thus, multiple strategies have been adopted to improve the luminescence of Ln3+. Anomalous research about water-induced lanthanide luminescence enhancement of Ln-MOFs is in the primary stage. Here, two Eu-based metal-organic framework (Eu-MOF) isomers named QXBA-Eu-1 and QXBA-Eu-2 were constructed by using the same ligand under different solvent thermal conditions, which exhibited distinctive water- and methanol-boosting emission behaviors. As for QXBA-Eu-1, water and methanol molecules replaced the free N,N-dimethylacetamide (DMA) molecules in the framework, repressed the rotation or libration suppression of the QXBA linker, and formed hydrogen bonds with the coordinated water molecules, which suppressed the O-H high-energy vibrations, reduced nonradiative transitions, stabilized the T1 state, and facilitated the intersystem crossing (ISC) process. For QXBA-Eu-2, water molecules tended to replace the coordinated DMA ligands, which altered the S1 and T1 energy levels of the ligand and facilitated the ligand-to-metal energy transfer (LMET) process and strengthened the luminescence of Eu3+. Importantly, free solvent molecules and the hydroxylation of Eu3+ centers also restrained the rotation or libration of the QXBA linker, by which the nonradiative transition was further inhibited and the lanthanide luminescence enhanced. Thus, this work not only opened an unprecedented path to enhance lanthanide luminescence in aqueous solution but also expanded its application scope.

Tetra-Nuclear Cluster-Based Lanthanide Metal-Organic Frameworks as White Phosphor, Information Encryption, Self-Calibrating Thermometers, and Fe2+ Sensors

The application of one kind of metal-organic framework (MOF) material used in multiple fields is one of the most interesting research topics. In this work, four new tetra-nuclear cluster-based lanthanide metal-organic frameworks (LnMOFs) [Ln₂(BTDB)₃(DMA)(phen)]_n (Ln = Tb TbMOF, Eu EuMOF, Gd GdMOF, Tb_1.830Eu_0.170 Tb,EuMOF, 3,5-bis(trifluoromethyl)-4',4″-dicarboxytriphenylamine = H₂BTDB, 1,10-phenanthroline = phen) are obtained based on the ligand of H₂BTDB that is synthesized in our laboratory, and the precise single-crystal structure of H₂BTDB is obtained for the first time. The white phosphor was obtained by facilely hybridizing two components of the orange-yellow emission phosphor of Tb,EuMOF and the blue luminescence material of triphenylamine according to the trichromatic theory. At the same time, TbMOF, EuMOF, Tb,EuMOF, and the white phosphor can be used for information encryption, demonstrating their potential application in the field of anti-counterfeiting. Tb,EuMOF is also a multi-mode and self-calibrating thermometer within a broad temperature range of 110-300 K. Further studies show that EuMOF is a rapid response sensor for Fe²⁺, with a very low limit of detection of 2.0 nM, which is much lower than the national standards for Fe²⁺ (GB 5749-2005, 5.357 μM). It can achieve strong anti-interference detection of Fe²⁺ in actual samples of tap water and lake water. In addition, EuMOF can also be made into an easy-to-use sensing device of test paper for real-time and visual sensing of Fe²⁺.

Multifunctional Cd-CP for fluorescence sensing of Cr(VI), MnO4-, acetylacetone and ascorbic acid in aqueous solutions

The development of multifunctional fluorescent chemosensors for the detection of multiple targets remains challenging but of great importance. In this paper, one novel coordination polymer (CP), denoted as [Cd₂(edda)(phen)₂]∙H₂O (compound 1, H₄edda = 5,5' (ethane-1,2-diylbis(oxy)) diisophthalic acid, phen = 1,10-phenanthroline) is successfully designed and prepared under hydrothermal conditions. Structural analysis indicates that compound 1 possesses a one-dimensional (1D) double chain structure, then self-assembles into a three-dimensional (3D) supramolecular framework via π…π interactions between phen molecules. Interestingly, compound 1 is found to be tolerant in wide range of acidic to alkaline aqueous solutions (pH = 2-13). Fluorescent spectral investigations reveal that compound 1 exhibits highly selective and sensitive fluorescence responses toward MnO₄^-, Cr(VI) ions, acetylacetone (acac) and ascorbic acid (AA) by fluorescence quenching in the aqueous phase. The detection limits are in the very low range, reaching μM level for the detection of MnO₄^-, Cr(VI) ions, nM for AA and ppm for acac detection. The distinguished multi-responsive performance suggests compound 1 to be a potential multifunctional probe. Furthermore, the possible quenching mechanisms have also been systematically investigated in this work.

Visual and Real-Time Monitoring of Cd2+ in Water, Rice, and Rice Soil with Test Paper Based on [2 + 2] Lanthanide Clusters

Cadmium ions (Cd²⁺) are highly toxic to animal and human health, especially through the drinking of Cd²⁺-contaminated water and eating Cd²⁺-contaminated rice. Therefore, accurate detection of Cd²⁺ in water, rice, and rice soil is urgent. In this work, two [2 + 2] lanthanide clusters of Tb₂Tb₂ and Eu₂Eu₂ were synthesized and characterized in detail. Interestingly, Tb2Tb2 is a rapid sensor for Cd²⁺ through luminescence "turn-off". Further studies show that Tb2Tb2 is a highly sensitive and selective sensor toward Cd²⁺ in water, rice supernatants, and rice soil supernatants, with a very short response time of 20 s. The limit of detection (LOD) in the above three real samples is as low as 0.0112, 1.1240, and 0.1124 ppb, respectively, which is lower than the national standards for food safety in China (GB 2762-2022). More interestingly, a portable sensing device of test paper based on Tb2Tb2 is developed with a facile method, which shows visible, highly sensitive, and selective sensing toward Cd²⁺ in real samples of water, rice supernatants, and rice soil supernatants. Tb2Tb2 and its sensing device of test paper are an on-site analysis sensor for potentially non-expert users, especially for people in remote rural areas.

Controllable Synthesis of TbIII Metal-Organic Frameworks with Reversible Luminescence Sensing for Benzaldehyde Vapor

Two novel lanthanide metal-organic frameworks (MOFs) with the formulas [Tb(bidc)(Hbidc)(H₂O)]_n (JXUST-20) and {[Tb₃(bidc)₄(HCOO)(DMF)]·solvents}_n (JXUST-21) were synthesized based on 2,1,3-benzothiadiazole-4,7-dicarboxylic acid (H₂BTDC) under solvothermal conditions. Interestingly, benzimidazole-4,7-dicarboxylic acid (H₂bidc) was formed in situ using H₂BTDC as the starting material. The self-assembly process of the targeted MOFs with different topological structures can be controlled by the solvents and concentration of the reactants. Luminescence experiments show that JXUST-20 and JXUST-21 exhibit strong yellow-green emission. JXUST-20 and JXUST-21 can selectively sense benzaldehyde (BzH) via a luminescence quenching effect with detection limits of 15.3 and 1.44 ppm, respectively. In order to expand the practical application of MOF materials, mixed-matrix membranes (MMMs) have been constructed by mixing targeted MOFs and poly(methyl methacrylate) in a N,N-dimethylformamide (DMF) solution, which can also be used for BzH vapor sensing. Therefore, the first case of MMMs derived from Tb^III MOFs has been developed for the reversible detection of BzH vapor, providing a simple and efficient platform for the future detection of volatile organic compounds.

Reversible Luminescent Switching Induced by Heat/Water Treatment in a Zero-Dimensional Hybrid Antimony(Ⅲ) Chloride

Recently zero-dimensional (0-D) inorganic-organic metal halides (IOMHs) have become a promising class of optoelectronic materials. Herein, we report a new photoluminescent (PL) 0-D antimony(III)-based IOMH single crystal, namely [H2BPZ][SbCl5]·H2O (BPZ = benzylpiperazine). Photophysical characterizations indicate that [H2BPZ][SbCl5]·H2O exhibits singlet/triplet dual-band emission. Density functional theory (DFT) calculations suggest that [H2BPZ][SbCl5]·H2O has the large energy difference between singlet and triplet states, which might induce the dual emission in this compound. Temperature-dependent PL spectra analyses suggest the soft lattice and strong electron-phonon coupling in this compound. Thermogravimetric analysis shows that the water molecules in the lattice of the title crystal could be removed by thermal treatment, giving rise to a dehydrated phase of [H2BPZ][SbCl5]. Interestingly, such structural transformation is accompanied by a reversible PL emission transition between red light (630 nm, dehydrated phase) and yellow light (595 nm, water-containing phase). When being exposed to an environment with 77% relative humidity, the emission color of the dehydrated phase was able to change from red to yellow within 20 s, and the red emission could be restored after reheating. The red to yellow emission switching could be achieved in acetone with water concentration as low as 0.2 vol%. The reversible PL transition phenomenon makes [H2BPZ][SbCl5]·H2O a potential material for luminescent water-sensing.

High-Performance Turn-On Fluorescent Metal-Organic Framework for Detecting Trace Water in Organic Solvents Based on the Excited-State Intramolecular Proton Transfer Mechanism

Simple, fast, and sensitive detection of trace water in organic solvents is an urgent requirement for chemical industries. Herein, combining the unusual excited-state intramolecular proton transfer (ESIPT) mechanism with the effective strategy of pore space partition, for the first time, we construct a powerful fluorescent metal-organic framework (SNNU-301) probe with excellent water stability. The SNNU-301 probe shows a remarkable performance for turn-on ESIPT-based fluorescence response to water in nine common organic solvents, exhibiting wide linear ranges, low limit of detection values, and ultrafast response, especially in dimethyl sulfoxide (0-5.2%; 0.011%, v/v; 110 s). The typical ESIPT-sensitive linker 2,5-dihydroxyterephthalate (DHBDC) imparts it with discriminative detection properties via enol-keto tautomerism, and light-responsive triangular tri(pyridin-4-yl)-amine (TPA) realizes pore space partition. The theoretical calculation gives an in-depth explanation about the proton transfer mechanism. Comparative experiments and GCMC simulation provide evidence that the synergy of the ESIPT process and TPA of the framework further boosts its performance effectively. Definitely, this work not only offers a promising candidate with fast detection speed, high sensitivity, excellent universality, and visual observation for the determination of water in organic solvents but also provides valuable guidance for the design of high-performance fluorescent probes.

A 2-fold interpenetrated zinc–organic framework with Lewis basic triazole sites: luminescence sensing of Fe3+ and Cr2O72−, and warm white-light emission by encapsulated Ln3+ ions

A 2-fold interpenetrated Zn-MOF with Lewis basic triazole sites shows selective luminescence sensing of Fe3+ and Cr2O72− and tunable white-light emission by encapsulated Ln3+ ions.

Smart intercalation and coordination strategy to construct stable ratiometric fluorescence nanoprobes for the detection of anthrax biomarker

L@Mg-Al-Ln-LDHs (Ln = Tb, Eu) constructed by the intercalation coordination strategy exhibited a strong and stable fluorescence reference signal and achieved reliable ratiometric detection of DPA in complex environments and actual spores.