Metal-Organic Frameworks Based Sensor Platforms for Rapid Detection of Contaminants in Wastewater

Metal-organic frameworks (MOFs) are a type of multifunctional material with organic-inorganic doped metal complexes that have a lot of unsaturated metal sites and a consistent network structure. MOFs work has great performance for enhancing the mass transfer, signal, and sensitivity as well as analyte enrichment. This study highlights the recent advancements of MOFs-based sensors for pollutant detection in a water environment and summarizes the effect of various synthetic materials on the performance of MOFs-based sensors. The related challenges and optimization techniques have been discussed. Then the research results of various MOFs sensors in the detection of wastewater pollutants are analyzed. Finally, the challenges facing MOFs-based water sensor development and the outlook for future research are discussed.

Isolation of a Cd-Based Coordination Polymer Containing Mixed Ligands: Time- and Temperature-Dependent Synthesis, Sulfonamide Antibiotics Detection, and Schottky Diode Fabrication

In this study, we present a new cadmium(II)-based two-dimensional coordination polymer [Cd (L)(NA)(H2O)] (L = Iminol form of N-nicotinoyl glycinate, NA = nicotinate), 1, containing two linkers generated from N-nicotinoyl glycine. A comprehensive investigation was carried out during the synthesis of the coordination polymers by varying the reaction time interval and temperature, and it revealed the formation of three distinct phases, of which two phases were previously reported and one was a new compound (1). The structure of compound 1 was determined by single-crystal X-ray diffraction, and it shows a corrugated layer structure with hydrogen bond interactions leading to three-dimensional supramolecular arrangements. Compound 1 exhibited strong emission at 420 nm when excited at 260 nm in an aqueous medium. The emission behavior of this compound was used for the detection of various sulfonamide antibiotics, sulfadiazine, sulfamethazine, sulfachloropyridazine, sulfameter, sulfaquinoxaline, and sulfathiazole, in the presence of common water pollutants. The luminescence quenching response of compound 1 to sulfonamide antibiotics was significant, ranging from 81 to 94%, and the detection sensitivity reached parts per billion (ppb) levels (226-726 ppb). Compound 1 also used for the fabrication of Schottky diode devices with a barrier height of 0.86 eV along with an excellent ideality factor of 1.24.

Water-stable metal-organic frameworks (MOFs): rational construction and carbon dioxide capture

Metal-organic frameworks (MOFs) are considered to be a promising porous material due to their excellent porosity and chemical tailorability. However, due to the relatively weak strength of coordination bonds, the stability (e.g., water stability) of MOFs is usually poor, which severely inhibits their practical applications. To prepare water-stable MOFs, several important strategies such as increasing the bonding strength of building units and introducing hydrophobic units have been proposed, and many MOFs with excellent water stability have been prepared. Carbon dioxide not only causes a range of climate and health problems but also is a by-product of some important chemicals (e.g., natural gas). Due to their excellent adsorption performances, MOFs are considered as a promising adsorbent that can capture carbon dioxide efficiently and energetically, and many water-stable MOFs have been used to capture carbon dioxide in various scenarios, including flue gas decarbonization, direct air capture, and purified crude natural gas. In this review, we first introduce the design and synthesis of water-stable MOFs and then describe their applications in carbon dioxide capture, and finally provide some personal comments on the challenges facing these areas.

Ultrafast Response in Nonenzymatic Electrochemical Glucose Sensing with Ni(II)-MOFs by Dimensional Manipulation

Metal-organic frameworks (MOFs) are emerging as promising candidates for electrochemical glucose sensing owing to their ordered channels, tunable chemistry, and atom-precision metal sites. Herein, the efficient nonenzymatic electrochemical glucose sensing is achieved by taking advantage of Ni(II)-based metal-organic frameworks (Ni(II)-MOFs) and acquiring the ever-reported fastest response time. Three Ni(II)-MOFs ({[Ni6L2(H2O)26]4H2O}n (CTGU-33), {Ni(bib)1/2(H2L)1/2(H2O)3}n (CTGU-34), {Ni(phen)(H2L)1/2(H2O)2}n (CTGU-35)) have been synthesized for the first time, which use benzene-1,2,3,4,5,6-hexacarboxylic acid (H6L) as an organic ligand and introduce 1,4-bis(1-imidazoly)benzene (bib) or 1,10-phenanthroline (phen) as spatially auxiliary ligands. Bib and phen convert the coordination mode of CTGU-33, affording structural dimensions from 2D of CTGU-33 to 3D of CTGU-34 or 1D of CTGU-35. By tuning the dimension of the skeleton, CTGU-34 with 3D interconnected channels exhibits an ultrafast response of less than 0.4 s, which is superior to the existing nonenzymatic electrochemical sensors. Additionally, a low detection limit of 0.12 μM (S/N = 3) and a high sensitivity of 1705 μA mM-1 cm-2 are simultaneously achieved. CTGU-34 further showcases desirable anti-interference and cycling stability, which demonstrates a promising application prospect in the real-time detection of glucose.

A Zn(II) coordination polymer as a dual sensor for ppb level detection of antibiotics and organo-toxins in a green solvent

Herein, we describe the synthesis of a new fluorescent d¹⁰ coordination polymer, [Zn₂(CFDA)₂(BPEP)]_n·nDMF (CP-1) under solvothermal reaction condition using zinc metal ion. In CP-1, Zn(II) ion along with CFDA and BPED ligand forms a 2-fold self-interpenetrated 3D coordination polymers. This CP-1 is characterized by the single crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), infrared spectra, optical microscope image and thermogravimetric analysis and the framework is found to maintain its structural stability in different solvents. The framework (CP-1) detected antibiotics (NFT (nitrofurantoin) and NZF (nitrofurazone)) and organo-toxin trinitrophenol in aqueous dispersed medium. Apart from the fast responsive (10 s), the detection limit for them was found at ppb level. The detection of these organo-aromatics were also comprehended by the colorimetric response through solid, solution and low cost paper strip technique i.e., triple mode recognition capability. The probe is re-usable without changing in its sensing efficiency and in addition, it has been applied for the detection of these analytes in the real field specimens (soil, river water, human urine and commercial tablet). The sensing ability is established by in-depth experimental analysis and the life time measurement where mechanism such as photo induced electron transfer (PET), fluorescence resonance energy transfer (FRET), inner filter effect (IFE) was recognized. The presence of guest interaction sites on the linker backbone in CP-1 induces diverse supramolecular interaction with the targeted analytes results to bring them in proximity for the occurrence of these sensing mechanism. The Stern-Volmer quenching constant values of CP-1 for the targeted analytes are admirable and the low detection limit (LOD) values for NFT, NZF and TNP are found to be 34.54, 67.79 and 43.93 ppb respectively. Further, the DFT theory is carried out in details to justify the sensing mechanism.

Detection Enhancement of One Multifunctional Cd-Metal-Organic Framework toward Tetracycline Antibiotics by Simply Mixing Eu3+ in Suspension

It is necessary to find more simple methods to improve the detection selectivity and sensitivity of antibiotics. Herein, we constructed a novel three-dimensional (3D) Cd-MOF LCU-117 assembled from p-terphenyl-4,2″,5″,4'-tetracarboxylic acid, which showed a special 3D helical structure with carboxylic acid ligands and nitrogen-containing ligands crossing each other vertically. Luminescence measurements indicated that LCU-117 has high selectivity and sensitivity toward Eu³⁺ through the ratiometric effect. Meanwhile, this complex itself could detect antibiotics oxytetracycline (OTC) through the turn-off mechanism. When Eu³⁺ was added in suspensions of LCU-117 (noted as Eu³⁺@LCU-117), the detection toward OTC was enhanced significantly and visually. The sensing mechanism was investigated in detail by various measurements and theoretical calculations. LCU-117 has a good effect on the logic gate, potential fingerprint detection, and mixed-matrix membranes (MMMs). The practical application for monitoring OTC in water samples also provided a satisfactory result.

Fluorescent Sensing of Ciprofloxacin and Chloramphenicol in Milk Samples via Inner Filter Effect and Photoinduced Electron Transfer Based on Nanosized Rod-Shaped Eu-MOF

Rapid, facile, and accurate detection of antibiotic residues is vital for practical applications. Herein, we designed a sensitive, visual, and rapid analytical method for sensitive detection of ciprofloxacin and chloramphenicol based on a nanosized rod-shaped Europium metal organic framework (Eu-MOF). The fluorescent Eu-MOF was firstly synthesized by a simple synthetic route at room temperature, which displays a red emission. The mechanisms of detecting ciprofloxacin and chloramphenicol were confirmed to be the inner filter effect (IFE) and photoinduced electron transfer (PET). Under the optimized experimental conditions, the detection limits of the developed method for ciprofloxacin and chloramphenicol detection were 0.0136 and 3.16 μM, respectively. Moreover, the sensor was effectively applied for quantitative determination of ciprofloxacin and chloramphenicol milk samples with satisfactory recoveries of 94.5-102% and 97-110%, respectively. This work developed a new method for rapid detection of ciprofloxacin and chloramphenicol residues. In addition, the established method has potential practical application value for on-site safety regulation on antibiotic residues in animal-derived food.

A novel terbium (III) and aptamer-based probe for label-free detection of three fluoroquinolones in honey and water samples

Fluoroquinolones, a family of synthetic broad-spectrum antibiotics, are widely used in clinical medicine, farm animals and aquaculture. Residues of fluoroquinolones in samples have attracted much attention because of growing food safety and public health concerns. Here, a novel Tb³⁺ ion-enrofloxacin aptamer coordination probe was prepared to develop a sensitive and rapid label-free fluorescence assay for specific detection three fluoroquinolones. In presence of the target, Tb³⁺ ion- enrofloxacin aptamer probe specifically bound with enrofloxacin, norfloxacin and ciprofloxacin, leading to a sharp increase in fluorescence emission of the probe. Under the optimized conditions, fluorescence increased linearly in the 1.0-100.0 ng/mL range for the three fluoroquinolones, with 0.053 ng/mL limit of detection for ciprofloxacin, 0.020 ng/mL limit of detection for norfloxacin and 0.061 ng/mL limit of detection for enrofloxacin. Satisfactory recovery (80.10-102.48%) in spiked honey and water samples were obtained for the three fluoroquinolones with relative standard deviations between 0.21% and 5.44% (n = 3).

Fabrication of Two Isomorphic and Hyperstable Rare Earth-Based Metal-Organic Frameworks with Efficient Ratiometric Probe and Photocatalytic Performances

Two isomorphic lanthanide compounds {[Ln(ddpp)(H₂O)]·CH₃CN}_n (Ln = Eu and Gd, H₄ddpp = 2,5-di(2',4'-dicarboxylphenyl)pyridine) were synthesized. Complex 1-Eu displays ultrahigh acid-base stability and thermal stability. Furthermore, luminescence measurements revealed that 1-Eu could detect quinolone antibiotics with an ultralow limit of detection in aqueous solution. The ratiometric probe properties for sensing antibiotics could be attributed to the incompletely sensitized Eu³⁺ ion of the ligand. Remarkably, it is interesting that 1-Gd exhibits excellent tetracycline degradation properties under visible light. Ultraviolet-visible diffuse reflectance spectroscopy and valence band X-ray photoelectron spectroscopy were carried out to investigate the photodegradation mechanisms. Moreover, a rational explanation for the fluorescent probe and photocatalysis behavior of these two complexes was also discussed with the assistance of density functional theory calculations.

Stimuli-Responsive Naphthalenediimide Cd-MOFs Tuned by Different Aliphatic Dicarboxylic Acids with Extended Spacers

A series of novel Cd metal-organic frameworks (MOFs) (1-9) with different extended spacers with seven kinds of the aliphatic dicarboxylic acids as secondary building linkers based on N,N'-di(4-pyridylacylamino)-1,4,5,8-naphthalenediimide (NDI-A) have been designed and synthesized by changing the volume ratio of solvents under solvothermal conditions. In addition, the secondary building linkers of aliphatic dicarboxylic acids have different spacer lengths, resulting in different structures of complexes 1-9. So, their packing structures are affected by the degree of distortion of the NDI-A ligand, the different aliphatic dicarboxylic acids ligands, and the hydrogen-bonding patterns. Complexes 1-9 showed stimuli-responsive emission tuned by different aliphatic dicarboxylic acids with extended length spacers under UV light irradiation, accompanied by the color change from light orange to dark brown, and achieved reversible photochromic under heating, which indicates that they could serve as secret erasable inks. Moreover, complexes 1-9 exhibited selective vaporchromic behavior to methylamine (MA), and the vaporchromic sample could be recovered after washing with MeOH. It is worth noting that the preparation of poly(vinyl alcohol) (PVA)-NDI-MOF films enables the photochromic and vaporchromic properties of complexes 1-9 to apply in practice. In addition, complexes 1-9 exhibited good fluorescence properties as sensing probes toward 2,4,6-trinitrophenol (TNP) with lower limits of detection. In short, this work provides a broad field to explore the creative NDI-MOF materials with photoactive and luminescent properties.

Construction and application of base-stable MOFs: a critical review

Metal-organic frameworks (MOFs) are a new class of porous crystalline materials constructed from organic ligands and metal ions/clusters. Owing to their unique advantages, they have attracted more and more attention in recent years and numerous studies have revealed their great potential in various applications. Many important applications of MOFs inevitably involve harsh alkaline operational environments. To achieve high performance and long cycling life in these applications, high stability of MOFs against bases is necessary. Therefore, the construction of base-stable MOFs has become a critical research direction in the MOF field. This review gives a historic summary of the development of base-stable MOFs in the last few years. The key factors that can determine the robustness of MOFs under basic conditions are analyzed. We also demonstrate the exciting achievements that have been made by utilizing base-stable MOFs in different applications. In the end, we discuss major challenges for the further development of base-stable MOFs. Some possible methods to address these problems are presented.

Recent Advances and Perspectives on the Sources and Detection of Antibiotics in Aquatic Environments

Water quality and safety are vital to the ecological environment, social development, and ecological susceptibility. The extensive use and continuous discharge of antibiotics have caused serious water pollution; antibiotics are widely found in freshwater, drinking water, and reservoirs; and this pollution has become a common phenomenon and challenge in global water ecosystems, as water polluted by antibiotics poses serious risks to human health and the ecological environment. Therefore, the antibiotic content in water should be identified, monitored, and eliminated. Nevertheless, there is no single method that can detect all different types of antibiotics, so various techniques are often combined to produce reliable results. This review summarizes the sources of antibiotic pollution in water, covering three main aspects: (1) wastewater discharges from domestic sewage, (2) medical wastewater, and (3) animal physiology and aquaculture. The existing analytical techniques, including extraction techniques, conventional detection methods, and biosensors, are reviewed. The electrochemical biosensors have become a research hotspot in recent years because of their rapid detection, high efficiency, and portability, and the use of nanoparticles contributes to these outstanding qualities. Additionally, the comprehensive quality evaluation of various detection methods, including the linear detection range, detection limit (LOD), and recovery rate, is discussed, and the future of this research field is also prospected.

Stable Metal-Organic Frameworks for Fluorescent Detection of Tetracycline Antibiotics

The rapid detection of antibiotics in agricultural products is of great significance. In this work, two stable fluorescent metal-organic frameworks (MOFs), BUT-178 and BUT-179, are synthesized and used to detect tetracycline antibiotics. Among them, BUT-179 exhibits better performance in the detection of different tetracycline antibiotics in water and eggs. The limits of detection of BUT-179 toward tetracycline, aureomycin, oxytetracycline, and doxycycline all reach the nanomolar level. Furthermore, the cycling tests confirm that BUT-179 can be easily recovered and repeatedly used without an obvious performance loss. This work demonstrates the excellent application potential of MOFs for food safety, especially the fluorescence detection of antibiotics in foods.

Lanthanide-Organic Frameworks with Uncoordinated Lewis Base Sites: Tunable Luminescence, Antibiotic Detection, and Anticounterfeiting

Several new isostructural lanthanide metal-organic frameworks (Ln-MOFs), {[Ln₂(L)₃DMA₄]·2DMA}_n (1-Ln, where Ln = Eu, Tb, or Eu_xTb_1-x), were first constructed via the solvothermal reactions of 4,6-di(4-carboxyphenyl)pyrimidine and Ln³⁺ ions. 1-Ln exhibits a 4-connected two-dimensional framework endowed with uncoordinated Lewis base sites. An exploration of luminescence sensing demonstrated 1-Eu can be used for the selectivity detection of dimetridazole and metronidazole antibiotics in other antibiotics, blood plasma, and urine, acting as an exceptional recyclable luminescent probe. More importantly, the luminescent inks of 1-Ln are invisible, color adjustable, and stabilized, which may greatly improve their anticounterfeiting applications.

Recent Progress in Metal-Organic Framework Based Fluorescent Sensors for Hazardous Materials Detection

Population growth and industrial development have exacerbated environmental pollution of both land and aquatic environments with toxic and harmful materials. Luminescence-based chemical sensors crafted for specific hazardous substances operate on host-guest interactions, leading to the detection of target molecules down to the nanomolar range. Particularly, the luminescence-based sensors constructed on the basis of metal-organic frameworks (MOFs) are of increasing interest, as they can not only compensate for the shortcomings of traditional detection techniques, but also can provide more sensitive detection for analytes. Recent years have seen MOFs-based fluorescent sensors show outstanding advantages in the field of hazardous substance identification and detection. Here, we critically discuss the application of MOFs for the detection of a broad scope of hazardous substances, including hazardous gases, heavy metal ions, radioactive ions, antibiotics, pesticides, nitro-explosives, and some harmful solvents as well as luminous and sensing mechanisms of MOF-based fluorescent sensors. The outlook and several crucial issues of this area are also discussed, with the expectation that it may help arouse widespread attention on exploring fluorescent MOFs (LMOFs) in potential sensing applications.

CuII Ion Doping Enhances the Water Stability of Luminescent Metal–Organic Framework, Realizing the Detection of Fe3+ and Antibiotics in Aqueous Solutions

Luminescent metal–organic frameworks (LMOFs) have been widely developed in the field of chemical sensing owing to their outstanding photoluminescence performance, high selectivity, anti-interference, high sensitivity, and fast response, and have become one of the research hotspots of emerging functional materials. However, in practical applications, many tests are carried out in the water environment, and fragile water stability greatly limits the application of MOFs in the field. Therefore, it is important to develop a method to enhance the water stability of MOFs. Herein, a new complex {[Zn(L)]·CH₃CN}_n (Zn-MOF, H₂L = 5-(benzimidazol-1-yl) isophthalic acid) with a superior photophysical property has been synthesized first. Its water stability was highly enhanced by the doping of Cu^II ions by the one-pot method. In addition, the detection performances of doping material Cu_0.1/Zn-MOF for sixteen metal ions and thirteen antibiotics were well studied. It was found that Cu_0.1/Zn-MOF displays high sensitivity, fast response, lower detection limit, and long-term stability for the detection of Fe³⁺, NFT, NFZ, FZD, and TC in the aqueous medium.

Highly efficient removal of quinolones by using the easily reusable MOF derived-carbon

As anthropogenic antibiotics, quinolones, e.g., ofloxacin have adverse impacts on ecological systems and human heaths. The removal of quinolones is of great importance, and adsorption techniques have been widely used to remove this hazardous contaminant. However, a robust and easy-operating adsorbent is still emergently required due to the complex chemical structure of quinolones. In this study, we successfully synthesized the promising metallic carbons (MCs) containing carbon nanotubes and cobalt nanoparticles by carbonizing Zn/Co-ZIF at 900 °C. Three different molar ratios of Co and Zn were applied to optimize the adsorption capacity on ofloxacin (OFL). Results showed MC with molar ratio of Co and Zn at 3:1 (Co-CNT/NPC_3/1) achieved the maximal adsorption capacity to 118.3 mg g^-1. Its adsorption performance was satisfied in the pH range from 5 to 9 and ionic strengths at 0.01 M. The main mechanisms for these adsorptions were identified as electrostatic attraction, metal coordination and π-π EDA. Removal efficiencies of quinolones higher than 68 mg g^-1 indicated the strong feasibility of this adsorbent for wastewater treatments. The regeneration of Co-CNT/NPC_3/1 at 600 °C allowed its at least 4-time reusability and its magnetic property enabled external magnets to recycle it from real environments.

Multiple fluorescence response behaviors towards antibiotics and bacteria based on a highly stable Cd-MOF

The abuse of antibiotics has triggered the rise of drug-resistance bacteria, which has seriously threatened public health globally. As a result, carrying out efficient and accurate antibiotic and bacteria identification are quite significant but challenge. Herein, an unprecedented Cd-MOF-based sensor, [CdL]_n [1, H₂L = 4-(2-methyl-1H-benzo[d]imidazol-1-yl) isophthalic acid] with multiple fluorescence response behaviours towards antibiotics and bacteria was developed. Single-crystal X-ray diffraction revealed that 1 is a mesomeric 2D bilayer, which is comprised of two opposite chiral mono-layers, each assembled by left-handed or right-handed helixes. More interestingly, 1 represented multiplex detection capability towards antibiotics and bacteria through two detection behaviors: toward nitro-antibiotics and chlortetracycline (CTC) via fluorescent quenching, while toward Staphylococcus albus (S. albus) via fluorescent enhancement. Remarkably, 1 showed a low limit of detection (LOD, 47 CFU/mL) accompanied with specificity in the detection of S. albus compared to other bacteria, such as Staphylococcus aureus, Acinetobacter baumannii, Klebsiella pneumonia, Pseudomonas aeruginosa and Escherichia coli. In addition, the LOD could reach to ppm level for nitro-antibiotics and CTC. Moreover, the practical application of 1 was further reinforced through the detection of nitro-antibiotics and CTC, as well as S. albus in fetal calf serum and river water.

Rapid Cs+ Capture via Multiple Supramolecular Interactions in Anionic Metal-Organic Framework Isomers

Metal-organic frameworks (MOFs) provide an ideal platform for ion exchange due to their high porosity and structural designability; however, developing MOFs that have the essential characteristics for ion exchange remains a challenge. These crucial features include fast kinetics, selectivity, and stability. We present two anionic isomers, DGIST-2 (2D) and DGIST-3 (3D), comprising distinctly arranged 5-(1,8-naphthalimido)isophthalate ligands and In³⁺ cations. Interestingly, in protic solvents, DGIST-2 transforms into a hydrolytically stable crystalline phase, DGIST-2'. DGIST-2' and DGIST-3 exhibit rapid Cs⁺ adsorption kinetics, as well as high Cs⁺ affinity in the presence of competing cations. The mechanism for rapid and selective sorption is explored based on the results of single-crystal X-ray diffraction analysis of Cs⁺-incorporated DGIST-3. In Cs⁺-containing solutions, the loosely incorporated dimethylammonium countercation of the anionic framework is replaced by Cs⁺, which is held in the hydrophobic cavity by supramolecular ion-ion and cation-π interactions.

Ultrasensitive fluorescence detection of norfloxacin in aqueous medium employing a 2D Zn(ii)-based coordination polymer

A 2D fluorescent coordination polymer, {[Zn(L)0.5(mip)] 1.75H2O}n (1), was successfully assembled. 1 was developed as an ultrasensitive fluorescent probe for the sensing of norfloxacin (NOR) in water.

An yttrium-organic framework based on a hexagonal prism second building unit for luminescent sensing of antibiotics and highly effective CO2 fixation

An yttrium-organic framework based on a hexagonal prism second building unit was constructed from nonanuclear yttrium(iii) and 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine for the luminescent sensing of antibiotics and highly effective CO2 fixation.

UiO-67-derived bithiophene and bithiazole MIXMOFs for luminescence sensing and removal of contaminants of emerging concern in wastewater

The UiO-67-type MIXMOF [Zr6O4(OH)4(PhPh)5(TzTz)], containing a blue-emitting bithiazole linker, is a potential multifunctional material for environmental remediation, being both a luminescent sensor and an adsorbent for diclofenac in aqueous solutions.

Pyrazine Functionalization to Boost Antenna Effect in Rare-Earth Metal-Organic Frameworks for Tetracyclines Detection

Here we report a generalizable strategy for the synthesis of rare-earth metal-organic frameworks (RE-MOFs) with 12-connected RE9 clusters and shp-topology. A total of 26 isostructural RE-MOFs (JNU-205-RE and JNU-206-RE) were...

Metal–organic frameworks (MOFs) as fluorescence sensors: principles, development and prospects

This review classifies the latest developments of MOF-based fluorescence sensors according to the analytes, and discusses the challenges faced by MOF-based fluorescence sensors and promotes some directions for future research.

Fabrication of a Stable Europium-Based Luminescent Sensor for Fast Detection of Urinary 1-Hydroxypyrene Constructed from a Tetracarboxylate Ligand

A novel europium-centered metal-organic framework fabricated from a symmetric and rigid ligand with tetracarboxylate groups, 2,6-di(2',5'-dicarboxylphenyl)pyridine (H₄ddpp), has been synthesized solvothermally. Characterized by single-crystal X-ray diffraction, compound 1 features a 3D microporous structure with a butterfly-shaped trinuclear Eu₃(COO)₆ secondary building unit. Interestingly, three kinds of 1D open channels viewed in different directions in compound 1 are discovered, and the void ratio is calculated to be 47.5% by PLATON software. Solid-state luminescent experiments at 298 K reveal that compound 1 displays naked-eye characteristic red emission of Eu³⁺ ions monitoring the typical ⁵D₀ → ⁷F₂ transition. The exploration of luminescent sensing tests discloses that compound 1 has an outstanding capacity for recognizing urinary 1-hydroxypyrene (1-HP) with a quite fast response and high sensitivity, giving the quenching efficiency of 98.2% after the immersion time for just 1 min and 73.2% with the amount of 1-HP only 0.05 mg/mL. To our knowledge, it is the first reported Eu-MOF as an extremely fast-responsive and highly sensitive luminescent sensor for 1-HP which is interference-free from other urinary components. Furthermore, the successful preparation of the luminescent test papers makes compound 1 convenient, easy, and real-time in the application for sensing 1-HP in biomedical and biological fields.

Multifunctional luminescence sensing and white light adjustment of lanthanide metal-organic frameworks constructed from the flexible cyclotriphosphazene-derived hexacarboxylic acid ligand

Considering that cyclotriphosphazene polycarboxylic acid is a kind of organic ligand with fantastic structures and performances and the unique luminescence characteristics of rare earth ions, a series of porous lanthanide metal-organic frameworks (Ln-MOFs) (CH3)2NH2[Ln3(HCPCP)1.5(CH3COO)]·6DMA (Ln = Ce (1), Sm (2), Eu (3), Tb (4), HCPCP = hexa(4-carboxyphenoxy)cyclotriphosphazene, and DMA = N,N-dimethylacetamide) were synthesized with novel topological network structures. Compound 4 exhibited a sensitive recognition of -NO2, and had a fluorescence quenching phenomenon for seven kinds of nitro aromatic compounds (NACs). In particular, it showed the best fluorescence response to 2,4,6-trinitrophenol (TNP) and 2,4-dinitrophenol (DNP), and the KSV values were 2.86 × 105 M-1 and 8.21 × 104 M-1, and the limit of detection (LOD) values were 0.20 μM and 0.71 μM, respectively. At the same time, we successfully doped different concentrations of Eu3+ into compound 4 to obtain a series of doped Ln-MOF materials x%Eu3+@4 (x = 0.5, 2.5, 5, 7.5, 10, 15 and 20). With the increase of Eu3+ doping ratios, the characteristic peaks of Tb3+ and Eu3+ changed regularly, and the energy transfer from Tb3+ to Eu3+ ions occurred. By changing the excitation wavelength of the samples with different Eu3+ doping concentrations, a higher quality white light emitting material 7.5%Eu3+@4 (λex = 340 nm) was finally obtained, with a CIE coordinate of (0.3268, 0.3212).

Applications of MOFs as Luminescent Sensors for Environmental Pollutants

The environmental pollution has become a serious issue because the pollutants can cause permanent damage to the DNA, nervous system, and circulating system, resulting in various incurable diseases, such as organ failure, malformation, angiocardiopathy, and cancer. The effective detection of environmental pollutants is urgently needed to keep them far away from daily life. Among the reported pollutant sensors, luminescent metal-organic frameworks (LMOFs) with tunable structures have attracted remarkable attention to detect the pollutants because of their excellent selectivity, sensitivity, and recyclability. Although lots of metal-organic framework (MOF)-based luminescent sensors have been summarized and discussed in previous reviews, the detection of environmental pollutants, especially radioactive ions and heavy metal ions, still have not been systematically presented. Here, the sensing mechanisms and construction principles of luminescent MOFs are discussed, and the state-of-the-art MOF-based luminescent sensors of environmental pollutants, including pesticides, antibiotics, explosives, VOCs, toxic gas, toxic small molecules, radioactive ions, and heavy metal ions are highlighted. This comprehensive review may further guide the development of luminescent MOFs and promote their practical applications for sensing environmental pollutants.

Highly Emissive Metal-Organic Frameworks for Sensitive and Selective Detection of Nitrofuran and Quinolone Antibiotics

The overuse of antibiotics makes its detection very significant for human health. New facile methods and high-performance sensory materials will be urgently needed for detection of antibiotics. Unfortunately, there are few reports on fluorescence enhancement of antibiotics detection. Herein, based on the modulability of the coordination mode, we proposed two MOFs with different coordination modes based on different metal ions: Zn-MOF (1) and Cd-MOF (2). The fluorescence of 1 and 2 can be efficiently and selectively quenched by nitrofuran antibiotics (nitrofurazone, NFZ and furazolidone, FZD) and chloramphenicol (CAP), respectively. Particularly, the matched energy levels between 2 and enrofloxacin (ENR) enables 2 with turn-on sensing for ENR. Moreover, apart from the sensitivity and selectivity, 1 and 2 also have strong recyclable ability, fast response time and anti-interference ability, which make them great potential sensory materials to detect antibiotics.

A water stable Eu(III)-organic framework as a recyclable multi-responsive luminescent sensor for efficient detection of p-aminophenol in simulated urine, and MnVII and CrVI anions in aqueous solutions

A novel 3D Eu(iii) metal-organic framework (Eu-MOF-1) formulated as [Eu(L)(H2O)(DMA)] (L = 2-(2-nitro-4-carboxylphenyl)terephthalic acid) has been successfully synthesized under solvothermal conditions and characterized by structural analyses. Eu-MOF-1 displays a new 3D framework containing EuIII ions, ligand L, and coordinated DMA molecules and water molecules. The fluorescence investigations indicate that Eu-MOF-1 emits bright red luminescence, and shows relatively high water stability and outstanding chemical stability under a relatively wide range of pH conditions. It is noteworthy that Eu-MOF-1 can quantitatively detect p-aminophenol (PAP) which is a metabolite of phenylamine in human urine. More significantly, Eu-MOF-1 is the first reported multi-responsive luminescent sensor for detecting the biomarker PAP, and MnVII and CrVI anions with high selectivity, sensitivity, recyclability and relatively low detection limits in aqueous solutions. Furthermore, the possible sensing mechanisms of Eu-MOF-1 for selective sensing have also been explored in detail. Eu-MOF-1 could be an ideal candidate as a multi-responsive luminescent sensor in biological and environmental areas.

Construction of Zn(II)/Cd(II)-Yb(III) Schiff Base Complexes for the NIR Luminescent Sensing of Fluoroquinolone Antibiotics

Two NIR luminescent Zn(II)/Cd(II)-Yb(III) complexes were obtained by the use of a Schiff base ligand with a binaphthyl backbone. Cd(II)-Yb(III) complex 2 has a triangular structure and exhibits interesting luminescent sensing activity to antibiotics, in particular to ciprofloxacin (CPFX) and norfloxacin (NFX) due to the inner filter effect. The limits of the detection of 2 to CPFX and NFX are 0.18 and 0.36 μM, respectively, and the fluorescence sensitivity is not changed with the existence of other antibiotics tested in this study.

Designed Eu3+ functionalized Zr-MOF-808 probe for highly sensitive monitoring multiple dyes

Dyes detection remains a serious task because of their high toxicity. In present work, designed Eu³⁺ functionalized Zr-metal-organic framework (Eu³⁺@Zr-MOF-808) as fluorescent probe was constructed via post-synthetic modification (PSM) for rapid monitoring four most commonly used dyes (malachite green (MG), brilliant green (BG), alizarin red S (ARS), indigo red (IDR)). Systematic exploring on the sensing mechanism reveals that fluorescence resonance energy transfer (FRET) for BG, MG and IDR and inner filter effect (IFE) for ARS contribute to the realization of the fluorescence quenching process. It exhibits excellent sensing performances with low limit of detection (LOD) of 32, 58, 77 and 133 nM for BG, IDR, MG and ARS, respectively. The as-constructed Eu³⁺@Zr-MOF-808 was demonstrated to be a highly sensitive probe for screening of MG in fish pond and IDR in printing wastewater with satisfying results. Moreover, a portable test reagent bottle has been developed for visual on-site screening of sample containing dyes with naked eyes under UV light. This is the first attempt to construct the Eu³⁺@Zr-MOF-808 probe for sensingmultiple dyes in real samples and demonstrates promising applications in water quality monitoring.

Stable Europium-based Metal-Organic Frameworks for Naked-eye Ultrasensitive Detecting Fluoroquinolones Antibiotics

Fluoroquinolone antibiotic (FQ) residues, such as ciprofloxacin (CIP) and ofloxacin (OFLX), have aroused public concerns owing to their serious impact in environmental water or food fields which influence human health. A facile and high-performance sensory method for detecting FQs is highly desirable for practical requirements. Herein, we have presented a luminescent Eu-MOF with unique 2D (4-c) {4⁴.6²}-connected topology, which holds the outstanding fluorescent property and excellent chemical stability in aqueous solution for 15 days. Thus, Eu-MOF can be considered as a highly sensitive chemo-sensor for sensing CIP and OFLX with different fluorescent color conversion (red changes to green for OFLX and to blue for CIP) and a low detection limit of 0.693 and 0.802 ppb, respectively. Furthermore, the mechanism of sensing CIP and OFLX was exposed to the photoinduced electron transfer (PET) and dynamic quenching process, as evaluated by DFT calculations and fluorescence lifetime decay measurements. Our work first reports a simple and efficient strategy for recognizing CIP and OFLX with a special luminescence color-change phenomenon based on MOF materials, serving as a meaningful guide for researchers in beneficial applications.

Water-stable Cd(ii)/Zn(ii) coordination polymers as recyclable luminescent sensors for detecting hippuric acid in simulated urine for indexing toluene exposure with high selectivity, sensitivity and fast response

Three novel Cd(ii)/Zn(ii) coordination polymers (CPs), namely [Cd(L)(BPDC)0.5H2O]·0.5H2O (1), [Zn2(L)2(BPDC)]·2H2O (2) and [Cd2(L)(BTC)H2O]·3H2O (3) (L = 4-(tetrazol-5-yl)phenyl-4,2':6',4''-terpyridine, H2BPDC = 4,4'-biphenyldicarboxylic acid, and H3BTC = 1,3,5-benzenetricarboxylic acid), have been successfully synthesized and characterized. CP 1 and CP 2 display new two-dimensional double-layered honeycomb frameworks containing uncoordinated nitrogen atoms from pyridine and tetrazole rings, which can easily form hydrogen bonds with various analytes. CP 3 exhibits a 3D framework also with uncoordinated nitrogen atoms from pyridine and tetrazole rings. The fluorescence explorations indicate that CPs 1-3 exhibit strong blue luminescence and excellent chemical stability under a relatively wide range of pH conditions. It is worth noting that CPs 1-3 can quantitatively detect hippuric acid (HA), which is a metabolite of toluene in human urine, with high selectivity, sensitivity, fast response and relatively low detection limits. Moreover, the sensing mechanism of CPs 1-3 for HA can mainly be ascribed to fluorescence resonance energy transfer (FRET). CPs 1-3 could be ideal candidates as HA sensors in human urine samples for practical applications. Notably, to the best of our knowledge, we report for the first time Cd(ii)/Zn(ii)-based luminescent sensors for detecting HA in simulated urine.

Two Cd(ii)-based metal–organic frameworks for the highly effective detection of Fe3+ ions and levofloxacin in aqueous media

Two new Cd(ii)-MOFs are hydrothermally synthesized and can be applied as dual-response sensors to identify Fe3+ and levofloxacin (LVX) with high selectivity, sensitivity and excellent reusability.

Metal–organic frameworks as photoluminescent biosensing platforms: mechanisms and applications

Recent progress of MOF-based photoluminescent platforms: a comprehensive overview of their applications in biosensing and underlying mechanisms.

Series of highly stable Cd(ii)-based MOFs as sensitive and selective sensors for detection of nitrofuran antibiotic

The oxygen atom of the MOF ether-bridging group acts as a Lewis base site, improving the connection and allowing the detection of 10 antibiotics through the fluorescence quenching effect.