A water-stable europium-MOF sensor for the selective, sensitive ratiometric fluorescence detection of anthrax biomarker

High proton conduction in a series of three-dimensional lanthanide(III)-organic frameworks constructed by 2,5-dihydroxyterephthalic acid

In designing and preparing new proton-conductive materials, using cheap and easily available raw materials to efficiently prepare metal-organic frameworks (MOFs) with high stability and excellent proton conductivity is still a huge challenge. Herein, six lanthanide(III)-MOFs, {[Ln2(DHBDC)3(DMF)4](DMF)2}n [(Ln III = Pr III (1), Nd III (2), Sm III (3), Eu III (4), Gd III (5), Tb III (6))] with high stability were solvothermally synthesized utilizing 2,5-dihydroxy-1,4-benzenedicarboxylic acid (H4-DHBDC) as a bridging ligand. These isostructural MOFs all possess a three-dimensional framework and a dense H-bond network formed by the carbonyl groups in the framework, the non-coordinated hydroxyl groups, and the coordinated and free DMF molecules, which ensure efficient proton conduction. Their good water and thermal stability were verified using various characterization techniques (powder X-ray diffraction, thermogravimetric analysis, and infrared). Then, their proton conductivity was investigated in detail concerning temperature and relative humidity (RH). At 100 °C and 97 % RH, their optimum proton conductivity can reach up to 0.96 × 10-2, 0.67 × 10-2, 0.85 × 10-2, 1.03 × 10-2, 0.53 × 10-2, and 0.93 × 10-2 S/cm for 1-6, respectively. Finally, their proton-transport processes were thoroughly examined through detailed structural analyses, adsorption-property determinations, and activation energy values. Notably, these MOF materials have the advantages of easy preparation and relatively low cost, which paves the way for their practical applications.

A Eu-MOF-Based Fluorescent Sensing Probe for the Detection of Tryptophan and Cu2+ in Aqueous Solutions

Abnormal tryptophan (Trp) metabolism can be used as an important indicator of chronic hepatitis, paranoia, Parkinson's disease and other diseases. Deficiency or excessive accumulation of Cu2+ can cause diseases such as Wilson's disease and Alzheimer's disease. Eu-based metal-organic framework (Eu-MOF) was successfully prepared for fluorescence sensing of Trp and Cu2+ in an aqueous solution (pH = 7.4). Eu-MOF showed high selectivity and sensitivity for Trp and Cu2+ with detection limits of 0.22 µM and 0.09 µM and Ksv of 6.17 × 103 M- 1 and 2.37 × 104 M- 1 respectively. Trp and Cu2+ had overlapped UV absorption spectra with that of Eu-MOF and competed for the excitation light source. Trp also attenuated the antennae effect of organic ligands on Eu-MOF, thus quenching the red fluorescence of Eu-MOF. This study provides insights into the application of MOFs in bioanalysis and diagnostics.

Ratiometric fluorescence sensor for Escherichia coli detection using fluorescein isothiocyanate-labeled metal-organic frameworks

A ratiometric fluorescence sensor for detecting Escherichia coli (E. coli) was fabricated based on the fluorescein isothiocyanate (FITC)-labeled zirconium (Zr)-tetraphenylporphyrin tetrasulfonic acid (TPPS) hydrate metal-organic frameworks (ZTMs@FITC). The ZTMs have strong red fluorescence emission at 683 nm, which can be quenched by Cu2+. E. coli can capture and convert external Cu2+ into Cu+ through its distinctive metabolic activities. To minimize environmental and instrumental influences and enhance detection precision, green FITC with an emission peak at 515 nm was utilized as the fluorescence labeling agent to fabricate the ratiometric fluorescence probe (ZTMs@FITC). The prepared ZTMs@FITC probe showed excellent performance in the detection of E. coli. As the concentration of E. coli increased, the fluorescence intensity at 683 nm (ZTMs, F683) increased considerably, while the fluorescence intensity at 515 nm (FITC, F515) decreased. By monitoring the increase in the ratio of F683 to F515, this sensor achieved rapid and sensitive detection of E. coli within the concentration range from 1.0 × 101 to 5.0 × 105 CFU/mL. The limit of detection was 6 CFU/mL. When observed under a 365 nm ultraviolet lamp, the fluorescence color of the solution changed from yellow to red. Additionally, the dual-signal ratiometric fluorescence method exhibited high selectivity for E. coli and was successfully utilized to detect E. coli in juice samples, demonstrating its practical application potential in food analysis.

Reaction Time Induced a Two-Step Dissolution and Recrystallization Structural Transformation with Three Eu Metal-Organic Frameworks: Crystal Structures and Multiresponsive Fluorescence Detection

Under solvothermal conditions, three 3D lanthanide metal-organic frameworks (Ln-MOFs): [Eu(H2DHTA)1.5(DMF)2]·DMF (1), [Eu(H2DHTA)0.5(DHTA)0.5(DMF)(H2O)]·2H2O (2), and Eu(HCOO)3 (3) (H4DHTA = 2,5-dihydroxyterephthalic acid) have been synthesized by different reaction times. Interestingly, induced by reaction time, compounds 1-3 underwent a two-step dissolution and recrystallization structural transformation (DRST) reaction. Investigations on the DRST processes were carried out, and the transformation pathway was deduced, which was verified by XRD analyses. Notably, compound 2 demonstrates pronounced luminescence as well as high stability in water and other organic solvents. The fluorescent detection of furan antibiotics can serve as turn-off effects, and glutamic acid (Glu), aspartic acid (Asp), and riboflavin (VB2) can serve as the turn-on effect. To explain the enhancing and quenching mechanisms, XRD, UV-visible absorption spectroscopy, electrochemistry, IR spectra, theoretical calculation, fluorescence lifetimes, and XPS were discussed. Additionally, MOF-coated test strips were utilized to detect these analytes, exhibiting excellent agreement with fluorescence spectroscopy. This work provides an example for more effective designs to employ Ln-MOFs as multiresponsive fluorescent sensors for detection of environmental pollutants in aqueous solution.

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.

Fluorescent Sensors Based on Lanthanide-Based Metal-Organic Frameworks via Devices and pH Response

In light of the escalating industrial and environmental pollution, there is a pressing need for the development of novel materials capable of swiftly detecting pollutants. Here, we report the synthesis of five lanthanide metal-organic frameworks sharing a common structure, prepared via a hydrothermal method and denoted as [Ln2(H2DHBDC)2(phen)(H2O)6]n (where CUST-888 corresponds to Tb, CUST-889 corresponds to Eu, CUST-890 corresponds to Gd, CUST-891 corresponds to Dy, and CUST-892 corresponds to Nd). Notably, CUST-888 and CUST-889 exhibit discernible visual alterations in response to acidic and alkaline conditions. To assess their practical utility, luminescent test strips and light-emitting diode lights based on CUST-888 and CUST-889 were devised, enabling the visual detection of luminescence color changes induced by Hg2+, Cr2O72-, tetracycline, and 2,4,6-trinitrophenol. Furthermore, highlighters derived from CUST-888 and CUST-889 were designed, showcasing robust stability, adjustable color, and substantial potential for application in the realm of anticounterfeiting.

Dual-mode detection of 2,6-pyridinedicarboxylic acid based on the enhanced peroxidase-like activity and fluorescence property of novel Eu-MOFs

2,6-Pyridinedicarboxylic acid (DPA) is a significant biomarker of anthrax, which is a deadly infectious disease for human beings. However, the development of a convenient anthrax detection method is still a challenge. Herein, we report a novel europium metal-organic framework (Eu-MOF) with an enhanced peroxidase-like activity and fluorescence property for DPA detection. The Eu-MOF was one-step synthesized using Eu3+ ions and 2-methylimidazole. In the presence of DPA, the intrinsic fluorescence of Eu3+ ions is sensitized, the fluorescence intensity linearly increases with an increase in DPA concentration, and the fluorescence color changes from blue to purple. Simultaneously, the peroxide-like activity of the Eu-MOF is enhanced by DPA, which can promote the oxidation of TMB to oxTMB. The absorbance values increase linearly with DPA concentrations, and the colorimetric images change from colorless to blue. The dual-mode detection of DPA has good sensitivity with a colorimetric detection limit of 0.67 μM and a fluorescent detection limit of 16.67 nM. Moreover, a simple detection method for DPA was developed using a smartphone with the RGB analysis system. A portable kit with standard color cards was developed using paper test strips. The proposed methods have good practicability for DPA detection in real samples. In conclusion, the developed Eu-MOF biosensor offers a valuable and general platform for anthrax diagnosis.

Multifunctional Eu3+-MOF for simultaneous quantification of malachite green and leuco-malachite green and efficient adsorption of malachite green

Multi-target detection combined with in-situ removal of contaminants is a challenging issue difficult to overcome. Herein, a dual-emissive Eu3+-metal organic framework (Eu3+-MOF) was constructed by pre-functionalization with a blue-emissive ligand and post-functionalization with red-emissive Eu3+ ions using a UiO-66 precursor. The fluorescence of the synthesized Eu3+-MOF is highly selective and sensitive toward malachite green (MG) and its metabolite leuco-malachite green (LMG), which are environmentally persistent and highly toxic to humans. The limit of detection of MG and LMG are 34.20 and 1.98 nM, respectively. Interestingly, the fluorescence of this Eu3+-MOF showed ratiometric but different responsive modes toward MG and LMG, which enabled the simultaneous quantification of MG and LMG. Furthermore, a paper-based sensor combined with the smartphone was fabricated, which facilitated not only the dual-channel detection of MG, but also its portable, visual, rapid, and intelligent determination. Furthermore, the high surface area of MOFs, together with the coordinate bonding interaction, π-π stacking, and electrostatic interaction sites, endows Eu3+-MOF with the efficient ability toward MG removal. This multifunctional Eu3+-MOF can be successfully used for trace detection, simultaneous determination of MG and LMG, as well as efficient removal of MG. Thus, it exhibits bright prospects for widespread applications in the field of food and environmental analysis.

Amorphous amEu-NH2BDC and amTb-NH2BDC as ratio fluorescence probes for smartphone-integrated naked eye detection of bacillus anthracis biomarker

The abnormal concentration of anthrax spore biomarker 2,6-pyridinedicarboxylic acid (2,6-DPA) will seriously affect public health. Therefore, a sensitive and rapid assay for 2,6-DPA monitoring is of vital importance. In this work, novel nano-sized amorphous Eu-NH2BDC (amEu-NH2BDC) and amorphous Tb-NH2BDC (amTb-NH2BDC) metal organic frameworks are prepared by adjusting the ratio of metal and ligand, respectively. Both of them exhibit highly sensitive and selective ratiometric fluorescence detection for 2,6-DPA with wider linear range and lower detection limit in aqueous solutions and human serum. Attributed to the coordination effect of 2,6-DPA in triggering the characteristic fluorescence emissions of Eu3+or Tb3+ by replacing coordinated solvent molecules, as evidenced by ultraviolet-visible spectroscopy, the fluorescence lifetimes analysis, thermal gravimetric analysis, Fourier-transform infrared spectroscopy, density functional theory (DFT) simulations and X-ray photoelectron spectroscopy. In addition, the amEu-NH2BDC or amTb-NH2BDC loaded paper-based microsensors are constructed for real-time and sensitive detection of 2,6-DPA and coupled with a smartphone-assisted visual portable device.

Double-emitting lanthanide metal-organic frameworks composed of Eu/Tb doping and ratiometric fluorescence detection of nitrofurazone

Lanthanide metal-organic frameworks (LnMOFs) have substantial potential in luminescence due to their unique antenna effect. Nevertheless, the single emission is susceptible to pseudo-signals caused by external environmental conditions, which significantly threaten the accurate measurement of the concentration. In this case, we prepared a dual-emission fluorescent probe {EuxTb1-x(NH2-BDC)3(DMF)4·2DMF}∞ (NH2-BDC = Diaminoterephthalic acid, DMF = N,N-dimethylformamide). The stable dual-emission signal provides a superior signal output for detecting nitrofurazone (NFZ), which is detected by the probe with excellent fluorescence for 0-10 μM NFZ. In the investigation of the detection mechanism, it is speculated that NFZ incorporates with probe to generate a novel complex. Furthermore, The UV absorption curves of the novel complexes and NFZ overlap extensively with those of the probe. The addition of NFZ attenuates the characteristic luminescence of Eu and Tb by competing for the absorption of the excitation light of the probe. The probe has exhibits rapid response, excellent sensitivity, visual detection and a meagre detection limit (LOD = 0.013 μM) for the detection of NFZ. This work not only broadens the application of LnMOFs in the field of ratiometric detection but also provides a favorable fluorescent probe for the quantitative detection of NFZ.

Unlabelled LRET biosensor based on double-stranded DNA for the detection of anthraquinone anticancer drugs

Based on upconversion nanoparticles (UCNPs) as energy donor and herring sperm DNA (hsDNA) as molecular recognition element, an unlabelled upconversion luminescence (UCL) affinity biosensor was constructed for the detection of anthraquinone (AQ) anticancer drugs in biological fluids. AQ anticancer drugs can insert into the double helix structure of hsDNA on the surface of UCNPs, thereby shortening the distance from UCNPs. Therefore, the luminescence resonance energy transfer (LRET) phenomenon is effectively triggered between UCNPs and AQ anticancer drugs. Hence, AQ anticancer drugs can be quantitatively detected according to the UCL quenching rate. The biosensor showed good sensitivity and stability for the detection of daunorubicin (DNR) and doxorubicin (ADM). For the detection of DNR, the linear range is 1-100 μg·mL-1 with a limit of detection (LOD) of 0.60 μg·mL-1, and for ADM, the linear range is 0.5-100 μg·mL-1 with a LOD of 0.38 μg·mL-1. The proposed biosensor provides a convenient method for monitoring AQ anticancer drugs in clinical biological fluids in the future.

Dual-ligand lanthanide metal-organic framework probe for ratiometric fluorescence detection of mercury ions in wastewater

By serving dipyridylic acid (DPA) and 2,5-dihydroxyterephthalic acid (DHTA) as the biligands, a novel lanthanide (Eu3+) metal-organic framework (MOF) namely Eu-DHTA/DPA was prepared for specific Hg2+ fluorescence determination. The dual-ligand approach can endows the resulting luminescent MOF with dual emission of ratiometric fluorescence and uniform size. Eu3+ produces intense red fluorescence when activated by the ligand DPA, while the other ligand DHTA produces yellow fluorescence. Under 273 nm excitation, the presence of Hg2+ in the monitoring environment causes an increase in the intensity of the DHTA fluorescence peak at 559 nm and a decrease in the intensity of the Eu3+ fluorescence peak at 616 nm. Hg2+ effectively quenches the fluorescence emission of the central metal Eu3+ in Eu-DHTA/DPA at 616 nm through a dynamic quenching effect. This recognition process occurs due to the coordination of Hg2+ with ligands such as benzene rings, carboxyl groups, and pyridine N in three-dimensional space. Hg2+ was detected by measuring the ratio between two fluorescence peaks (I559 nm/I616 nm) within the range 2-20 μM, achieving a remarkably low detection limit of 40 nM. The established ratiometric fluorescence method has been successfully applied to the determination of Hg2+ in industrial wastewater of complex composition. The method plays a crucial role in the rapid and sensitive monitoring of Hg2+ in real environmental samples. The recoveries ranged from 92.82% to 112.67% (n = 3) with relative standard deviations (RSD) below 4.8%. This study offers a convenient and effective method for constructing probes for Hg2+ monitoring, with practical applications in environmental monitoring.

Dual-mode strategy for 2,6-dipicolinic acid detection based on the fluorescence property and peroxidase-like activity inhibition of Fe-MIL-88NH2

This work designed a fluorometric/colorimetric dual-mode sensor for detecting 2,6-dipicolinic acid (DPA) based on the blue emission property and peroxidase-like activity of Fe-MIL-88NH₂. The fluorescence of Fe-MIL-88NH₂ was obviously turned off by Cu²⁺, but DPA was able to bring it back because it has a strong chelate bond with Cu²⁺. Fe-MIL-88NH₂ also displayed high peroxidase-like activity, which accelerated the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to the blue oxidation product (oxTMB) when H₂O₂ was present. When DPA was added, it efficiently inhibited the peroxidase-like activity of Fe-MIL-88NH₂, causing less oxTMB and less absorbance at 652 nm. The fluorescence recovery of Fe-MIL-88NH₂ and the change in absorbance at 652 nm were used as analytical signals for dual-mode detection of DPA. The linear responses in the range of 10-60 μM and 60-160 μM were achieved for the fluorometric mode, and the limit of detection (LOD) was 1.46 μM. The respective values of linear range and LOD for the colorimetric mode were 5-25 μM and 3.00 μM, respectively. In summary, the dual-mode testing strategy successfully detected DPA in aqueous environmental samples, suggesting great potential in disease prevention and environmental analysis.

Cation-Exchange in Metal-Organic Framework as a Strategy to Obtain New Material for Ascorbic Acid Detection

Ascorbic acid (AA) is an important biomolecule, the deficiency or maladjustment of which is associated with the symptoms of many diseases (e.g., cardiovascular disease or cancer). Therefore, there is a need to develop a fluorescent probe capable of detecting AA in aqueous media. Here, we report the synthesis, structural, and spectroscopic characterization (by means of, e.g., XRD, XPS, IR and Raman spectroscopy, TG, SEM, and EDS analyses), as well as the photoluminescent properties of a metal-organic framework (MOF) based on Cu²⁺ and Eu³⁺ ions. The ion-exchange process of the extraframework cation in anionic Cu-based MOF is proposed as an appropriate strategy to obtain a new material with a nondisturbed structure and a sensitivity to interaction with AA. Accordingly, a novel Eu[Cu₃(μ₃-OH)(μ₃-4-carboxypyrazolato)₃] compound for the selective optical detection of AA with a short detection time of 5 min is described.

Design of "turn-off" luminescent Ln-MOFs for sensitive detection of cyanide anions

Two novel 2D lanthanide metal-organic frameworks (Ln-MOFs), namely {[Eu₂(DBTA)₃(DMF)₂]·DMF}_n (1) and {[Tb₂(DBTA)₃(DMF)₂]·DMF}_n (2) (H₂DBTA = 2,5-dibromoterephthalic acid), have been successfully synthesized by the solvothermal method. Single-crystal X-ray diffraction results proved that the complexes possess the same topological structure of a (4²·6)₂(4²·8⁴)(4⁷·6³)₂-connected net. The recognition of CN^- from interfering anions with a low detection limit by "turn-off" luminescence makes them promising candidates for the highly selective and sensitive detection of the cyanide ion. The Ln-MOFs 1 and 2 exhibit excellent chemical sensing properties for CN^- with efficiency, selectivity, and excellent performance in various mixed anions. The evaluation parameters, including the quenching constant and detection limit, have been investigated to obtain the detection performance for CN^-.

A Highly Stable Eu−MOF Multifunctional Luminescent Sensor for the Effective Detection of Fe3+, Cr2O72−/CrO42− and Aspartic Acid in Aqueous Systems

Heavy metal ions were common pollutants in water pollution. Amino acids, as important substances in organisms, participate in many life activities. The detection of heavy metal ions and amino acids...

An europium(III) metal-organic framework as a multi-responsive luminescent sensor for highly sensitive and selective detection of 4-nitrophenol and I- and Fe3+ ions in water

A luminescence sensor based on an europium(III)-based lanthanide-organic framework, [Eu(BCB)(DMF)]·(DMF)_1.5(H₂O)₂ (1), was synthesized via a solvothermal method using 4,4',4''-benzenetricarbonyltribenzoic acid (H₃BCB) as a bridging ligand. Single-crystal X-ray diffraction indicates that Eu centers are eight-coordinated with a trigonal dodecahedron and a square antiprismatic configuration, and adjacent Eu atoms are bridged by BCB organic linkers to form a 3D rod-packing structure. Photoluminescence studies show that compound 1 emits bright red luminescence and behaves as a multi-responsive luminescent sensor toward 4-nitrophenol (4-NP) and I^- and Fe³⁺ ions in water with high sensitivity, selectivity and low detection limits. Furthermore, the possible luminescence sensing mechanisms were also investigated by PXRD analysis, UV-vis spectroscopy and X-ray photoelectron spectroscopy (XPS). The recognition mechanism for 4-NP and I^- ions can be attributed to the competition absorption and that for Fe³⁺ ions is considered to be a multi-quenching mechanism dominated by competition absorption. This study demonstrates that the lanthanide-based MOF might be a promising candidate for the detection of 4-NP and I^- and Fe³⁺ ions in aqueous medium.

A hydrostable samarium(III)-MOF sensor for the sensitive and selective detection of tryptophan based on a "dual antenna effect"

Tryptophan (Trp) is one of the essential amino acids, which plays important roles in biological systems and the normal growth of human beings, and it is of great significance to be able to detect Trp in a rapid, efficient, and sensitive way. Herein, a 3D network metal-organic framework ([Sm₂(BTEC)_1.5(H₂O)₈]·6H₂O) with excellent thermal and water stability was synthesized by a hydrothermal method. Interestingly, it could discriminate Trp from other natural amino acids in aqueous solution through a significant fluorescence enhancement effect, and showed high detection sensitivity (LOD = 330 nM) and outstanding anti-interference ability. The sensor system was successfully applied to the detection of Trp in practical samples, so it was expected to be a sensitive and efficient Trp sensor. In addition, the sensing mechanism was explained in detail by a series of characterization methods combined with density functional theory (DFT). There were many coordination water molecules in the crystal structure of the complex. Based on the small steric hindrance and molecular structure of water molecules, it provided the possibility for coordination interaction between Trp and Sm³⁺. On the other hand, the triplet energy level (T1) of Trp matched with the ⁴G_5/2 vibrational energy level of Sm³⁺, so Trp could be used as the second "antenna molecule" besides 1,2,4,5-benzenetetracarboxylic acid (H₄BTEC). Therefore, it effectively broadened the way for Sm-MOF to absorb excitation light.