MOF@COFs with Strong Multiemission for Differentiation and Ratiometric Fluorescence Detection

Rhodamine/Cucurbit[8]uril Co-assembled supramolecular aggregates realize the precise and enhancive bioimaging of plant active signals

Fluorescent molecular probes are commonly used to detect active substances. However, in variable living microenvironmental systems, most of conventional aromatic fluorescent probes often suffer from aggregation-caused quenching (ACQ) due to π-π stacking, which severely limit their selective recognition and bioimaging functions. To tackle this challenge, we devise an structurally novel small-molecule-adamantane-modified Rhodamine probe (RAA) and employ a predictable cucurbit[8]uril (Q[8])-involved host-guest supramolecular strategy to optimize molecular aggregation behaviors at the molecular level, thereby creating supramolecular aggregates (RAA@Q[8]) as an innovative fluorescent probe. This encouraging result is important for precise and efficient detection of plant signaling molecules such as salicylic acid (SA) in various environments. Experimental investigations found that RAA@Q[8] was 2.2-fold more sensitive than RAA for detecting SA, with high selectivity and anti-interference, and a low detection limit of 3.0 × 10-8 M. Importantly, RAA@Q[8] realizes the enhanced, precise recognition and bioimaging of SA on pea sprouts and HEK-293 cells. This study offers a guidance for developing efficient chemosensors from small-molecule conception to ultrasensitive supramolecular fluorescent probes that are opposite to the intractable ACQ obstacle.

Deep machine learning-assisted MOF@COF fluorescence/colorimetric dual-mode intelligent ratiometric sensing platform for sensitive glutathione detection

Glutathione (GSH) levels have been linked to aging and the pathogenesis of various diseases, highlighting the necessity for the development of sensitive analytical methods for GSH to facilitate disease diagnosis and treatment. In this study, we synthesized a novel core-shell material, UiO@TBTA, by in-situ growing TFPB-TAPA COF on UiO-66-NH2 through a Schiff base reaction. The resulting composite capitalize on the advantages of both materials, demonstrating excellent stability, large specific surface area, and abundant active functional groups while preserving superior crystallinity. Notably, this strategy effectively reduces the occurrence of aggregation-caused quenching (ACQ) in COFs. Due to the inner filter effect and hydrogen bonding interactions between UiO@TBTA and GSH, a specific ratiometric fluorescence detection of GSH was achieved in the range of 0.1-7 μM, with a limit of detection (LOD) of 0.0685 μM. In addition, due to the sensitive color change of the sensing material from orange to black caused by GSH, a proportional colorimetric sensing strategy has also been proposed, enabling the detection of GSH within the range of 1-200 μM. What's more, two intelligent artificial neural networks models were constructed with the help of machine learning that can quickly, accurately, and sensitively determine the concentration of GSH based on fluorescence images and color photographs respectively. Our work represents the first study utilizing MOF@COF composite for the multimodal detection of GSH, thus providing a novel strategy for the multimodal detection of the target analyte. Prospectively, the construction of the fluorescence/colorimetric dual-mode intelligent ratiometric sensing platform using deep machine learning holds great promise for real-time monitoring.

In situ enrichment and ultrasensitive analysis of interstitial fluid miRNA enabled by hydrogel microneedles coupled with DNA-gated metal-organic frameworks

A novel strategy combining GelMA hydrogel microneedles and DNA-gated MOFs for sensitive miRNA detection in skin interstitial fluid (ISF) is reported. GelMA MNs efficiently extract ISF, while DNA-gated MOFs offer dual-mode detection via fluorescence and SERS. In vivo results demonstrate successful miRNA extraction and sensitive biomarker detection, advancing minimally invasive diagnostics and real-time health monitoring.

Converging the Complementary Traits of Metal-Organic Frameworks and Covalent Organic Frameworks

Since their discovery, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) featuring permanent nanopores have transformed the landscape of porous materials, excelling as platforms for catalysis, gas separation, and sensing thanks to their exceptional surface areas, adjustable pore sizes, and modular functionality. However, MOFs, while versatile, face stability challenges due to their coordination bonds, whereas COFs, although robust, lack metal sites, limiting their catalytic activity, redox functionality, and other metal-specific applications. To bridge these gaps, innovative porous materials, such as MCOFs, which incorporate metal ions into COF lattices; covalent cluster frameworks, formed by assembling metal clusters into covalent networks; and MOF-COF composites, which integrate the strengths of both systems, have emerged. This review explores the synthesis and design principles of these advanced materials, showcasing their applications and the unique advantages conferred by their composite nature. It provides insights into future directions and their potential to address key challenges in materials science and beyond.

Fluorescent core-shell SiO2@COF composite for ultra-sensitive detection of cysteine and homocysteine

To improve the fluorescence properties of covalent organic framework (COF) materials, core-shell SiO2@COF composite was prepared, in which SiO2 was used as inner substrate, and BMTH (2,5-bis (2-methoxyethoxy) p-phenylhydrazide) and HB (2-hydroxy-1,3,5-phenyltrialdehyde) acted as precursors to construct COFBMTH-HB layer on the surface of SiO2. Compared with COFBMTH-HB, SiO2@COFBMTH-HB composite showed better dispersion property and improved fluorescence performance under the same experimental conditions. Using SiO2@COFBMTH-HB as a fluorescence probe, the fluorescence intensity was gradually decreased with the addition of Cu2+, while it was restored by introducing Cys or Hcy, realizing "ON-OFF-ON" fluorescence sensing detection. Other amino acids with five times the concentration of Cys showed little effect on the determination of Cys or Hcy. The SiO2@COFBMTH-HB-Cu2+ probe exhibited ultrahigh sensitivity and good selectivity for the detection of Cys and Hcy, with a detection range of 0.23 to 250.0 µM for Cys and 0.31 to 170.0 µM for Hcy. The detection limits (LOD = 3 σ/s) were 75 and 104 nM for Cys and Hcy, respectively. The system also showed a good recovery and low relative standard deviations in actual sample tests for Cys or Hcy, demonstrating its potential for practical applications.

Construction of MOF@COF-derived composites for ratiometric fluorescence detection of water with ultralow background

A ratiometric sensor with ultralow background is highly desired due to its low environmental influence and high sensitivity. Herein, inspired by the solubility difference of carboxylate in aqueous and organic solvents, we prepared a core-shell structure porous zirconia-covalent organic framework (COF) composite through thermal hydrolysis of UiO-66-COF precursors in organic alkali solution. The ligand 2-aminoterephthalic acids (H2BDC-NH2) of UiO-66 were transformed into 2-aminoterephthalate salts (ATA salts) that existed in zirconium-oxo clusters building units. The composites emitted only yellow emission (597 nm) from the COF in organic solvent due to the insolubility of ATA salts that induce aggregation-caused quenching (ACQ) and the protection of the COF shell. Contrarily, when water was added into mixture, the ATA salts were released into solution and its fluorescence recovered at 446 nm, while the fluorescence of COF was quenched due to the blockage of the intramolecular charge transfer (ICT) process by water. Thus, a high-sensitivity ratiometric fluorescence method is obtained with ultralow background signal and fast response (less than 1 min) for sensing water in organic solvent. We believe that the proposed ratiometric fluorescence sensor based on the zirconia-COF composite will provide the guidance for detection with wide applications.

Fluorescent monitoring osteogenic differentiation of osteosarcoma cells with an aggregation-induced emission probe

Osteosarcoma is widely believed to be an osteogenic differentiation disorder. In recent years, to further understand this disease, a lot resources were poured into the potential link between differentiation defects and tumorigenesis. Long-term monitoring of the differentiation progress of osteosarcoma cells is of great importance. In order to better promote the research, we have developed a novel fluorescent probe called PTB-EDTA, which exhibits remarkable bio-compatibility and demonstrates high selectivity towards osteosarcoma cells. Not only is the PTB-EDTA is capable of live cell imaging while conventional histology requires to kill the cells, its fluorescence is also enhanced as the osteogenic differentiation proceeding. These properties make PTB-EDTA a promising tool for monitoring osteosarcoma cells.

Realizing Ultrasensitive and Accurate Point-of-Care Profiling for ATP with a Triple-Mode Strategy Based on the ATP-Induced Reassembly of a Copper Coordination Polymer Nanoflower

New strategies for accurate and reliable detection of adenosine triphosphate (ATP) with portable devices are significant for biochemical analysis, while most recently reported approaches cannot satisfy the detection accuracy and independent of large instruments simultaneously, which are unsuitable for fast, simple, and on-site ATP monitoring. Herein, a unique, convenient, and label-free point-of-care sensing strategy based on novel copper coordination polymer nanoflowers (CuCPNFs) was fabricated for multimode (UV-vis, photothermal, and RGB values) onsite ATP determination with high selectivity, sensitivity, and accuracy. The resulting CuCPNFs with a 3D hierarchical structure exhibit the ATP-triggered decomposition behavior because the competitive coordination between ATP and the copper ions of CuCPNFs can result in the formation of ATP-Cu, which reveals preeminent peroxidase mimics activity and can accelerate the oxidation of 3, 3', 5, 5'-tetramethylbenzidine (TMB) to form oxTMB. During this process, the detection system displayed not only color changes but also a strong NIR laser-driven photothermal effect. Thus, the photothermal and color signal variations are easily monitored by a portable thermometer and a smartphone. This multimode point-of-care platform can meet the requirements of onsite, without bulky equipment, accuracy, and reliability all at once, greatly enhancing its application in practice and paving a new way in ATP analysis.

Recent Advances in Construction Strategies for Fluorescence Sensing Films

A year ago, film-based fluorescent sensors (FFSs) were recognized in the "IUPAC Top Ten Emerging Technologies in Chemistry 2022" due to their extensive application in detecting hidden explosives, illicit drugs, and volatile organic compounds. These sensors offer high sensitivity, specificity, immunity to light scattering, and noninvasiveness. The core of FFSs is the construction of high-performance fluorescent sensing films, which are dependent on the processes of "energy transfer" and "mass transfer" in the active layer and involve complex interactions between sensing molecules and analytes. This Perspective focuses on the latest strategies in constructing these films, emphasizing the design of sensing molecules with various innovative features and structures that enhance the mass transfer efficiency. Additionally, it discusses the ongoing challenges and potential advancements in the field of FFSs.

Eu-MOF based fluorescence probe for ratiometric and visualization detection of Cu2

Water contamination caused by heavy metals represents an urgent global issue. Cu2+, a potential trace heavy metal pollutant, can accumulate in the human body through the food chain, leading to excessive levels that give rise to diverse health complications. Hence, in this investigation, a novel and efficacious fluorescent probe named Eu-BTB was developed for the detection of Cu2+, employing 1,3,5-triphenyl(4-carboxyphenyl) (H3BTB) as the ligand and Eu3+ as the metallic framework. The probe demonstrates exceptional fluorescence characteristics. The interaction between the probe ligand BTB and Eu3+ triggers an antenna effect, heightening the emission efficiency of Eu3+ while preserving its intrinsic emission. The introduction of Cu2+ competes with BTB for binding, thus quelling the antenna effect and inducing a fluorescence alteration. Within the concentration range of 0.05-10 μM, the fluorescence intensity-to-Cu2+ concentration ratio exhibits a robust linear correlation, with a remarkably low detection limit of 10 nM and a rapid response time of 3 min. The fluorescent probe has been effectively deployed for the detection of copper ions in water across diverse environmental conditions, with the obtained outcomes being validated via the conventional approach of inductively coupled plasma mass spectrometry (ICP-MS). The Eu-BTB probe showcases the advantages of simplicity, swiftness, and broad applicability, thus affirming its potential for the prompt and accurate detection of Cu2+ in diverse environmental water samples.

Fluorescent covalent organic frameworks for environmental pollutant detection sensors and enrichment sorbents: a mini-review

Covalent organic frameworks (COFs) are a class of porous crystalline materials based on organic building blocks containing light elements, such as C, H, O, N, and B, interconnected by covalent bonds. Because of their regular crystal structure, high porosity, stable mechanical structure, satisfactory specific surface area, easy functionalization, and high tunability, they have important applications in several fields. Currently, most of the established methods based on COFs can only be used for individual detection or adsorption of the target. Impressively, fluorescent COFs as a special member of the COF family are able to achieve highly selective and sensitive detection of target pollutants by fluorescence enhancement or quenching. The construction of a dual-functional platform for detection and adsorption based on fluorescent COFs can enable the simultaneous realization of visual monitoring and adsorption of target pollutants. Therefore, this paper reviews the research progress of fluorescent COFs as fluorescence sensors and adsorbents. First, the fluorescent COFs were classified according to the different bonding modes between the building blocks, and then the applications of fluorescent COF-based detection and adsorption bifunctional materials for various environmental contaminants were highlighted. Finally, the challenges and future application prospects of fluorescent COFs are discussed.

Reticular Chemistry for Optical Sensing of Anions

In the last few decades, reticular chemistry has grown significantly as a field of porous crystalline molecular materials. Scientists have attempted to create the ideal platform for analyzing distinct anions based on optical sensing techniques (chromogenic and fluorogenic) by assembling different metal-containing units with suitable organic linking molecules and different organic molecules to produce crystalline porous materials. This study presents novel platforms for anion recognition based on reticular chemistry with high selectivity, sensitivity, electronic tunability, structural recognition, strong emission, and thermal and chemical stability. The key materials for reticular chemistry, Metal-Organic Frameworks (MOFs), Zeolitic Imidazolate Frameworks (ZIFs), and Covalent-Organic Frameworks (COFs), and the pre- and post-synthetic modification of the linkers and the metal oxide clusters for the selective detection of the anions, have been discussed. The mechanisms involved in sensing are also discussed.

Interfacial chemistries in metal-organic framework (MOF)/covalent-organic framework (COF) hybrids

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have been attracting tremendous attention in various applications due to their unique structural properties. Recent interest has been focused on their combination as hybrids to enable the engineering of new classes of frameworks with complementary properties. This review gives a comprehensive summary on the interfacial chemistries in MOF/COF hybrids, which play critical roles in their hybridization. The challenges and perspectives in the field of MOF/COF hybrids are also provided to inspire more efforts in diversifying this hybrid family and their cross-disciplinary applications.

Design, Preparation, and High Intrinsic Proton Conductivity of Two Highly Stable Hydrazone-Linked Covalent Organic Frameworks

Assembling crystalline materials with high stability and high proton conductivity as a potential alternative to the Nafion membrane is a challenging topic in the field of energy materials. Herein, we concentrated on the creation and preparation of hydrazone-linked COFs with super-high stability to explore their proton conduction. Fortunately, two hydrazone-linked COFs, TpBth and TaBth, were solvothermally prepared by using benzene-1,3,5-tricarbohydrazide (Bth), 2,4,6-trihydroxy-benzene-1,3,5-tricarbaldehyde (Tp), and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (Ta) as monomers. Their structures were simulated by Material Studio 8.0 software and confirmed by the PXRD pattern, demonstrating a two-dimensional framework with AA packing. The presence of a large number of carbonyl groups as well as -NH-NH₂- groups on the backbone is responsible for their super-high water stability as well as high water absorption capacity. AC impedance tests demonstrated a positive correlation between the water-assisted proton conductivity (σ) of the two COFs and the temperature and humidity. Under 100 °C/98% RH, the highest σ values of TpBth and TaBth can reach 2.11 × 10^-4 and 0.62 × 10^-5 S·cm^-1, which are among the high σ values of the reported COFs. Their proton-conductive mechanisms were highlighted by structural analyses as well as N₂ and H₂O vapor adsorption data and activation energy values. Our systematic research affords ideas for the synthesis of proton-conducting COFs with high σ values.

Highly Efficient Dual-Color Luminophores for Sensitive and Selective Detection of Diclazepam Based on MOF/COF Bi-Mesoporous Composites

Currently, studies on electrochemiluminescence (ECL) mainly focused on the single emission of luminophores while those on multi-color ECL were rarely reported. Here, a bi-mesoporous composite of the metal-organic framework (MOF)/covalent-organic framework (COF) with strong and stable dual-color ECL was prepared to construct a novel ECL sensor for sensitive detecting targets. A PTCA-COF with excellent ECL performance was loaded with a great amount of another ECL emitter Cu₃(HHTP)₂. Remarkably, the integrated composite had both ECL properties of PTCA-COF at 520 nm and Cu₃(HHTP)₂ at 600 nm wavelengths. Furthermore, Cu₃(HHTP)₂ with good electron transfer ability can greatly enhance the electrical conductivity and promote electrochemical activation. Thus, the simultaneous enhanced two-color ECL intensity and the catalytic properties of the conductive MOF exerted a dual enhancement effect on the ECL signal of the composite. Significantly, diclazepam can not only be adsorbed well on the multi-stage porous structure MOF/COF composite by π-π interactions but also selectively quench the ECL signal of the PTCA-COF, realizing the sensitive detection. The ECL sensor showed a wide detection range from 1.0 × 10^-13 to 1.0 × 10^-8 g/L, and the limit of detection (LOD) was as low as 2.6 × 10^-14 g/L (S/N = 3). The proposed ECL sensor preparation method was simple and sensitive, providing a new perspective for the potential application of multi-color ECL in the sensing field.

Composite materials based on covalent organic frameworks for multiple advanced applications

Covalent organic frameworks (COFs) stand for a class of emerging crystalline porous organic materials, which are ingeniously constructed with organic units through strong covalent bonds. Their excellent design capabilities, and uniform and tunable pore structure make them potential materials for various applications. With the continuous development of synthesis technique and nanoscience, COFs have been successfully combined with a variety of functional materials to form COFs-based composites with superior performance than individual components. This paper offers an overview of the development of different types of COFs-based composites reported so far, with particular focus on the applications of COFs-based composites. Moreover, the challenges and future development prospects of COFs-based composites are presented. We anticipate that the review will provide some inspiration for the further development of COFs-based composites.

Ultrasensitive Detection of Multidrug-Resistant Bacteria Based on Boric Acid-Functionalized Fluorescent MOF@COF

The widespread use of antibiotics has made multidrug-resistant bacteria (MDRB) one of the greatest threats toward global health. Current conventional microbial detection methods are usually time-consuming, labor-intensive, expensive, and unable to detect low concentrations of bacteria, which cause great difficulties in clinical diagnosis and treatment. Herein, we constructed a versatile biosensing platform on the basis of boric acid-functionalized porous framework composites (MOF@COF-BA), which were able to realize highly efficient and sensitive label-free MDRB detection via fluorescence. In this design, MDRB were captured using aptamer-coated nanoparticles and the fluorescent probe MOF@COF-BA was tightly anchored onto the surface of MDRB due to interactions between boric acid groups and glycolipids on bacteria cells. Benefitting from the remarkable fluorescence performance of MOF@COF-BA, rapid and specific detection of MDRB, such as methicillin-resistant Staphylococcus aureus (MRSA) and Acinetobacter baumannii (AB), was realized with a detection range of 20-10⁸ CFU/mL (for both) and limits of detection of 7 CFU/mL (MRSA) and 5 CFU/mL (AB). The feasibility of using the developed platform to selectively detect MRSA and AB from complex urine, human serum, and cerebrospinal fluid samples was also demonstrated. This work provides a promising strategy for accurate MDRB diagnosis, avoiding serious infection using rational antibiotic therapy.

Covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) as electrochemical sensors for the efficient detection of pharmaceutical residues

Pharmaceutical residues are the undecomposed remains from drugs used in the medical and food industries. Due to their potential adverse effects on human health and natural ecosystems, they are of increasing worldwide concern. The acute detection of pharmaceutical residues can give a rapid examination of their quantity and then prevent them from further contamination. Herein, this study summarizes and discusses the most recent porous covalent-organic frameworks (COFs) and metal-organic frameworks (MOFs) for the electrochemical detection of various pharmaceutical residues. The review first introduces a brief overview of drug toxicity and its effects on living organisms. Subsequently, different porous materials and drug detection techniques are discussed with materials' properties and applications. Then the development of COFs and MOFs has been addressed with their structural properties and sensing applications. Further, the stability, reusability, and sustainability of MOFs/COFs are reviewed and discussed. Besides, COFs and MOFs' detection limits, linear ranges, the role of functionalities, and immobilized nanoparticles are analyzed and discussed. Lastly, this review summarized and discussed the MOF@COF composite as sensors, the fabrication strategies to enhance detection potential, and the current challenges in this area.

Aptamers dimerization inspired biomimetic clamp assay towards impedimetric SARS-CoV-2 antigen detection

Antigen-detecting rapid diagnostic testing (Ag-RDT) has contributed to containing the spread of SARS-CoV-2 variants of concern (VOCs). In this study, we proposed a biomimetic clamp assay for impedimetric SARS-CoV-2 nucleocapsid protein (Np) detection. The DNA biomimetic clamp (DNA-BC) is formed by a pair of Np aptamers connected via a T₂₀ spacer. The 5'- terminal of the DNA-BC is phosphate-modified and then anchored on the surface of the screen-printed gold electrode, which has been pre-coated with Au@UiO-66-NH₂. The integrated DNA-material sensing biochip is fabricated through the strong Zr-O-P bonds to form a clamp-type impedimetric aptasensor. It is demonstrated that the aptasensor could achieve Np detection in one step within 11 min and shows pronounced sensitivity with a detection limit of 0.31 pg mL^-1. Above all, the aptasensor displays great specificity and stability under physiological conditions as well as various water environments. It is a potentially promising strategy to exploit reliable Ag-RDT products to confront the ongoing epidemic.

Multi-emitter metal-organic frameworks as ratiometric luminescent sensors for food contamination and spoilage detection

Food contamination and spoilage is a worldwide concern considering its adverse effect on public health and food security. Real time monitoring food quality can reduce the risk of foodborne disease to consumers. Particularly, the emergence of multi-emitter luminescent metal-organic frameworks (LMOFs) as ratiometric sensory materials has provided the possibility for food quality and safety detection with high sensitivity and selectivity taking advantage of specific host-guest interactions, pre-concentrating and molecule-sieving effects of MOFs. Furthermore, the excellent sensing performance of multi-emitter MOF-based ratiometric sensors including self-calibration, multi-dimensional recognition and visual signal readout is able to meet the increasing rigor requirement of food safety evaluation. Multi-emitter MOF-based ratiometric sensors have become the focus of food safety detection. This review focuses on design strategies for different multiple emission sources assembly to construct multi-emitter MOFs materials based on at least two emitting centers. The design strategies for creating multi-emitter MOFs can be mainly classified into three categories: (1) multiple emission building blocks assembly in a single MOF phase; (2) single non-luminescent MOF or LMOF phase as a matrix for chromophore guest(s); (3) heterostructured hybrids of LMOF with other luminescent materials. In addition, the sensing signal output modes of multi-emitter MOF-based ratiometric sensors have critically discussed. Next, we highlight the recent progress for the development of multi-emitter MOF as ratiometric sensors in food contamination and spoilage detection. Their future improvement and advancing direction potential for their practical application is finally discussed.

Fabrication of Two Luminescent Imidazolyl Cadmium-Organic Frameworks and Their Sensing Mechanism for 2,6-Dichloro-4-nitroaniline

2,6-Dichloro-4-nitroaniline, alias dicloran (DCN), is a broad-spectrum pesticide that can cause irreversible damage to the human body. Therefore, it is of great significance to develop a technology for the rapid and convenient detection of DCN. Luminescent metal organic frameworks have attracted extensive attention in the field of sensing and detection due to their excellent optical properties. In this study, two kinds of 2D Cd-MOFs (CdMOF-1 and CdMOF-2) were developed for the detection of residual DCN in the environment. Both CdMOFs exhibit excellent solvent and acid-base stability and can respond to DCN quickly and sensitively in a short time (30 s). CdMOFs not only have good selectivity and anti-interference toward DCN but also have good reusability. Under the conditions of DCN concentrations of 1-15 and 0.3-30 μM, the change in fluorescence intensity of CdMOF-1 and CdMOF-2 showed a good linear relationship with DCN concentration (R² = 0.999/0.991), and the detection limits were 0.36 and 0.12 μM, respectively. Through ultraviolet-visible absorption spectroscopy (UV-Vis), X-ray photoelectron spectroscopy, fluorescence lifetime, and density functional theory calculations, it is revealed that the fluorescence quenching mechanisms of DCN for two kinds of Cd-MOFs are competitive absorption and photoinduced electron transfer, and there may be a weak π-π interaction. Finally, it is demonstrated that by using two types of fluorescent CdMOFs to make the fluorescent test paper and detect actual soil, these can be applied to the actual scene and achieve onsite real-time detection.

S,N-rich luminous covalent organic frameworks for Hg2+ detection and removal

The challenge for simultaneous detection and removal of Hg²⁺ is the design of bifunctional materials bearing abundant accessible chelating sites with high affinity. Covalent-organic frameworks (COFs) are attracting more and more attention as potential bifunctional materials for Hg²⁺ detection due to their large specific surface area, ordered pores, and abundant chelating sites. Here, a new luminous S,N-rich COF_BTT-AMPD based on hydrophilic block unit of 2,2'-azobis(2-methylpropionamidine) dihydrochloride (AMPD) was constructed, which improved the solubility and affinity for Hg²⁺ greatly. Another S-rich fused-ring unit of benzotrithiophene tricarbalaldehyde (BTT) enhanced the conjugation of COF_BTT-AMPD, and the methyl-rich chains block unit of AMPD effectively suppressed the aggregation-caused quenching. Thus, the COF_BTT-AMPD emitted strong fluorescence at 546 nm in liquid and solid as well as different solvent with a wide pH range, which was used for the visual detection and removal of Hg²⁺ (detection limit: 2.6 nM, linear range: 8.6 × 10^-3-20 μM, monolayer adsorption capacity: 476.19 mg g^-1) successfully. COF_BTT-AMPD-based fabric and light-emitting diode coatings were further constructed to realize the visual detection of Hg²⁺ vapor. The results reveal the potential of S,N-rich luminous COF_BTT-AMPD for Hg²⁺ detection and remediation in the environment.

Citrate Functionalized Zirconium-Based Metal Organic Framework for the Fluorescent Detection of Ciprofloxacin in Aqueous Media

Ciprofloxacin (CIP) is a commonly used antibiotic for the treatment of infectious diseases in humans and as a prophylactic agent in the livestock industry, leading to the environmental discharge of significant amounts of CIP. CIP is stable in aquatic systems leading to its pseudo-persistence. Constant exposure to these antibiotics results in the generation of antibiotic-resistant pathogens and potential toxicity/hypersensitivity in humans. Therefore, it is necessary to develop a convenient, rapid, and cost-effective method for the monitoring of ciprofloxacin in environmental samples. Rhodamine-based fluorescent receptors have the limitation of aqueous solubility. Therefore, in order to overcome this drawback, we designed a novel fluorescent receptor based on a zirconium-based metal organic framework (MOF-808). The precursor, MOF-808, was synthesized and functionalized by using sodium citrate to obtain a receptor called C-MOF-808. The C-MOF-808 was structurally characterized by XRD and spectroscopic analyses. Thus, this synthesized receptor can be used for the fluorescent detection of CIP in aqueous media with a detection limit of 9.4 µM. The detection phenomena of the receptor were studied by absorption as well as fluorescent spectra. The binding behavior of CIP with the receptor was studied by FT-IR and ¹H-NMR analyses, and a binding mechanism is proposed.

An Overview of the Design of Metal-Organic Frameworks-Based Fluorescent Chemosensors and Biosensors

Taking advantage of high porosity, large surface area, tunable nanostructures and ease of functionalization, metal-organic frameworks (MOFs) have been popularly applied in different fields, including adsorption and separation, heterogeneous catalysis, drug delivery, light harvesting, and chemical/biological sensing. The abundant active sites for specific recognition and adjustable optical and electrical characteristics allow for the design of various sensing platforms with MOFs as promising candidates. In this review, we systematically introduce the recent advancements of MOFs-based fluorescent chemosensors and biosensors, mainly focusing on the sensing mechanisms and analytes, including inorganic ions, small organic molecules and biomarkers (e.g., small biomolecules, nucleic acids, proteins, enzymes, and tumor cells). This review may provide valuable references for the development of novel MOFs-based sensing platforms to meet the requirements of environment monitoring and clinical diagnosis.

Tale of COF-on-MOF Composites with Structural Regulation and Stepwise Luminescence Enhancement

Integrating metal-organic framework (MOF)-covalent organic framework (COF) allows versatile engineering of hybrid materials with properties superior to pristine components, especially COFs suffered from aggregation-caused quenching (ACQ), unlocking more possibilities to improve the luminescence of COFs. In this work, we prepared various MOF@COF composites with different COF layer thicknesses, in which stable UiO-66-NH₂ served as the inner substrate and 1,3,5-benzenetricarboxaldehyde (BT), and 3,3'-dihydroxybenzidine (DH) were used to construct a COF layer. In addition to the conventional preparation method, we increased the ratio of BT and DH to be 1:2.5, and impressively, the morphologies of acquired UC (1:2.5) materials were quite different from the previous reticular structure and gradually extended from the spherical structure to the prickly structure with the increase of COF monomers. Remarkably, all of the UC materials possessed better luminescence properties than individual COF due to the limited COF layers. Meanwhile, UC-1 materials with an optimal COF layer displayed the strongest emission. In comparison with a single COF, the quantum yields of UC-1 and UC-1 (1:2.5) were increased nearly 7 times and 5 times, respectively. Moreover, the fluorescence of UC-1 materials was progressively enhanced via selective F^- sensing. This work is expected to shed light on the potential hybridization of MOF-COF with structural adjustment, morphological design, and luminescence enhancement.

Progress in Hybridization of Covalent Organic Frameworks and Metal-Organic Frameworks

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) hybrid materials are a class of porous crystalline materials that integrate MOFs and COFs with hierarchical pore structures. As an emerging porous frame material platform, MOF/COF hybrid materials have attracted tremendous attention, and the field is advancing rapidly and extending into more diverse fields. Extensive studies have shown that a broad variety of MOF/COF hybrid materials with different structures and specific properties can be synthesized from diverse building blocks via different chemical reactions, driving the rapid growth of the field. The allowed complementary utilization of π-conjugated skeletons and nanopores for functional exploration has endowed these hybrid materials with great potential in challenging energy and environmental issues. It is necessary to prepare a "family tree" to accurately trace the developments in the study of MOF/COF hybrid materials. This review comprehensively summarizes the latest achievements and advancements in the design and synthesis of MOF/COF hybrid materials, including COFs covalently bonded to the surface functional groups of MOFs (MOF@COF), MOFs grown on the surface of COFs (COF@MOF), bridge reaction between COF and MOF (MOF+COF), and their various applications in catalysis, energy storage, pollutant adsorption, gas separation, chemical sensing, and biomedicine. It concludes with remarks concerning the trend from the structural design to functional exploration and potential applications of MOF/COF hybrid materials.

Ratiometric Fluorescent pH Sensor Based on a Tunable Multivariate Covalent Organic Framework

Ratiometric detection of pH is always significant in environmental regulation, medical diagnosis, synthetic chemistry, and beyond. The construction of practical ratiometric pH sensors with reusability is still challenging. Herein, by exploiting a multivariate strategy, we first synthesized and reported a series of novel three-component covalent organic frameworks (COF-COOH_X, X = 33, 50, and 67) through Schiff base reaction between 2-hydroxybenzene-1,3,5-tricarbaldehyde (HTA), 4,4'-diamino-3,3'-biphenyldicarboxylic acid (DBA), and 5,5'-diamino-2,2'-bipyridine (BPY) at various molar ratios (X = [DBA]/([BPY] + [DBA]) × 100 = 33, 50, and 67). COF-COOH_X (X = 33, 50, and 67) displayed ratiometric pH sensing performance in acidic conditions with selectivity and repeatability. By tuning the molar ratio of DBA and BPY, the fluorescent properties, linear pH responsive ranges, and pK_a values of COF-COOH_X (X = 33, 50, and 67) can be regulated. Meanwhile, the two-component COF-COOH₀ and COF-COOH₁₀₀ did not exhibit ratiometric pH detection ability. Moreover, the constructed three ratiometric sensors can be applied to detect pH in drug solutions and carbonated drinks with satisfactory results. This work sheds new light on the design and fabrication of innovative ratiometric fluorescent sensors using COFs.

Tunable fluorescence emission based on multi-layered MOF-on-MOF

Luminescent metal-organic frameworks (MOFs) have garnered considerable attention in various fields. Herein, we proposed a hierarchical confinement strategy based on MOF-on-MOF to tune luminescence emission ranging from blue to red including white light in a flexible way. The easily available ZIF-8 MOF was used as a host for the confinement of two kinds of size-matching dyes (perylene and rhodamine B) to obtain a layered ZIF-8@dye@ZIF-8@dye via in situ encapsulation and seed-mediated synthesis. ZIF-8@dye@ZIF-8@dye materials with different fluorescence emission in dispersed and solid states were both obtained by tuning the initial encapsulation concentration of dye and changing the structure of the inner and outer ZIF-8@dye layers. To our delight, ZIF-8@0.125perylene@ZIF-8@25RhB with white light emission in the dispersed state was obtained; meanwhile, ZIF-8@0.125perylene + 25RhB and mechanically mixed ZIF-8@0.125perylene + ZIF-8@25RhB could not realize white light emission under the same conditions, indicating that the proposed hierarchical confinement strategy facilitated white light regulation. Similarly, the emission of ZIF-8@dye@ZIF-8@dye in the solid state has also been investigated; ZIF-8@perylene@ZIF-8@3RhB with white light emission was obtained, while white light emission could not be achieved in ZIF-8@perylene + 3RhB and ZIF-8@perylene + ZIF-8@3RhB, which further indicated the importance of the hierarchical confinement strategy based on MOF-on-MOF. The proposed hierarchical confinement strategy may also inspire the development of other functional optical MOF materials.

Tetraphenylethylene-Functionalized Metal-Organic Frameworks with Strong Aggregation-Induced Electrochemiluminescence for Ultrasensitive Analysis through a Multiple Convertible Resonance Energy Transfer System

Since aggregation-induced electrochemiluminescence (AIECL) combined the merits of aggregation-induced emission (AIE) and electrochemiluminescence (ECL), it has become a research hotspot recently. Herein, novel kinds of functional metal-organic frameworks (MOFs) with strong AIECL were reported through doping tetraphenylethylene (TPE) into UiO-66. Due to the porosity and highly ordered topological structure that caused the confinement effect of MOFs, the molecular motion of TPE was effectively limited within UiO-66, resulting in strong AIE. Meanwhile, the large specific surface area and porous structure of UiO-66 allowed TPE to react with coreactants more effectively, which was beneficial to ECL. Thus, the TPE-functionalized UiO-66 (TPE-UiO-66) showed excellent AIECL performance surprisingly. Inspired by this, a multiple convertible ECL resonance energy transfer (ECL-RET) system was constructed through a DNA Y structure that regulated the distance between the energy donor (TPE-UiO-66) and different energy acceptors (gold nanoparticles and Adriamycin). Furthermore, an ultrasensitive ECL biosensor for the detection of Mucin 1 (MUC1) was developed through the introduction of the novel ECL-RET system. In the presence of MUC1, the DNA Y structure was constructed, keeping the gold nanoparticles (AuNPs) away from TPE-UiO-66. Then, Adriamycin (Dox) could be embedded in the DNA Y structure and act as an energy acceptor to receive the energy of TPE-UiO-66, which made the biosensor produce a strong ECL response. As expected, the developed ECL biosensor exhibited superior detection performance for MUC1. This work provided a novel way to realize AIECL and board the application of AIECL in analytical chemistry.

Metal@COFs Possess High Proton Conductivity with Mixed Conducting Mechanisms

The inherent porous structures and aligned functional units inside the skeleton of covalent organic frameworks (COFs) provide an extraordinary promise for post-modification and deservedly expand their application in the field of proton conduction. Herein, we tactfully introduced copper ions into a two-dimensional COF (TpTta) furnished with ample N,O-chelating sites by a post-modification strategy to achieve two copper(II)-modified products, namely, CuCl₂@TpTta-3 and CuCl₂@TpTta-10. Inspiringly, the two modified COFs demonstrated the higher conductivities of 1.77 × 10^-3 and 8.81 × 10^-3 S cm^-1 under 100 °C and 98% relative humidity, respectively, among the previously reported COFs with higher σ values. In comparison to the pristine COFs, the σ values of CuCl₂@TpTta-3 and CuCl₂@TpTta-10 are boosted by 2 orders of magnitude. On the basis of structural analyses, nitrogen and water vapor adsorption tests, and proton conduction mechanism analysis, we deeply analyzed the reason why the conductivity of the modified COFs was significantly increased. To the best of our knowledge, it is the first time to employ the CuCl₂-modified strategy to boost the conductivity of COFs, which offers a wise idea for the fabrication of highly conductive materials in the field of fuel cells.

Characteristic Synthesis of a Covalent Organic Framework and Its Application in Multifunctional Tumor Therapy

For decades, covalent organic frameworks (COFs) have attracted wide biomedical interest due to their unique properties including ease of synthesis, porosity, and adjustable biocompatibility. Versatile COFs can easily encapsulate various therapeutic drugs due to their extremely high payload and porosity. COFs with abundant functional groups can be surface-modified to achieve active targeting and enhance biocompatibility. In this paper, the latest developments of COFs in the biomedical field are summarized. First, the classification and synthesis of COFs are discussed. Cancer diagnosis and treatment based on COFs are studied, and the advantages and limitations of each method are discussed. Second, the specific preparation methods to obtain specific therapeutic properties are summarized. Finally, based on the combination and modification of COFs with various components, this review system summarizes different combination therapies. In addition, the main challenges faced in COF research and prospects for applying COFs to cancer diagnosis and treatment are evaluated. This review provides enlightening insights into the interdisciplinary research on COFs and applications in biomedicine, which highlight the great expectations for their further clinical transformation.

In Situ Fabrication of Porous MOF/COF Hybrid Photocatalysts for Visible-Light-Driven Hydrogen Evolution

Construction of porous metal-organic framework (MOF)/covalent organic framework (COF) hybrid photocatalysts for enriched structures and unprecedented properties is still a great challenge but highly desirable. Herein, a new series of Cu₃(HHTP)₂-MOF/Tp-Pa-1-COF hybrids with different MOF content are successfully fabricated. The as-prepared MOF/COF hybrids exhibit intimate interaction based on the coordination of Cu ions with the carbonyl oxygen and enamine nitrogen groups in Tp-Pa-1. The integrated conductive Cu₃(HHTP)₂ is able to act as an excellent electron extractor instead of noble metal cocatalysts to significantly promote the charge transfer and inhibit the recombination of photogenerated electron-hole pairs. As a results, the optimized photocatalyst with Cu₃(HHTP)₂:Tp-Pa-1 ratio of 1:15 achieves the highest hydrogen evolution rate of 1.76 mmol·h^-1·g^-1 under visible-light irradiation, which is about 93 times higher than that of the pure Tp-Pa-1 and even slightly higher than that of the Tp-Pa-1 with Pt (3 wt %) as a cocatalyst.

A ratiometric fluorescent probe based on carbon dots and gold nanocluster encapsulated metal-organic framework for detection of cephalexin residues in milk

Herein, we report a ratiometric fluorescent probe based on in situ incorporation of both Gold nanoclusters (AuNCs) and Green emitting carbon dots (gCDs) into zeolitic imidazolate framework-8 (ZIF-8) to analysis of Cephalexin (CFX). Under a single excitation wavelength of 400 nm, the sensor exhibits dual-emissions centered at 520 and 630 nm. The fluorescence of AuNCs (630 nm) is selectively quenched by CFX, whereas the fluorescence of gCDs (520 nm) remainsalmostconstant. The ratiometric fluorescence signal (F₅₂₀/F₆₃₀) of the prepared composite (gCDc/AuNCs @ ZIF-8) is linearly proportional to the concentration of CFX from 0.1 to 6 ng/mL with a low detection limit (LOD) of 0.04 ng/mL, which is below the maximum residues limit (MRL) of 100 ng/mL set by the Food and Drug Administration (FDA). Moreover, the designed sensing platform was successfully applied to detect CFX in the milk samples.

MOF@COF Heterostructure Hybrid for Dual-Mode Photoelectrochemical-Electrochemical HIV-1 DNA Sensing

We developed a novel metal-organic framework (MOF)@covalent-organic framework (COF) hybrid with a hierarchical nanostructure and excellent photoactivity, which further acted as the bifunctional platform of a dual-mode photoelectrochemical (PEC) and electrochemical (EC) biosensor for detecting HIV-1 DNA via immobilizing the HIV-1 DNA probe. First, the presynthesized Cu-MOF nanoellipsoids were used as the template for the in situ growth of the COF network, which was synthesized using copper-phthalocyanine tetra-amine (CoPc-TA) and 2,9-bis[p-(formyl)phenyl]-1,10-phenanthroline as building blocks through the Schiff base condensation. In view of the large specific surface area, abundant reserved amino group, excellent electrochemical activity, and high photoactivity, the obtained Cu-MOF@CuPc-TA-COF heterostructure not only can serve as the sensitive platform for anchoring the HIV-1 DNA probe strands but also can be utilized as the signal transducers for PEC and EC biosensors. Thereby, the constructed biosensor shows the sensitive and selective analysis ability toward the HIV-1 target DNA via the complementary hybridization between probe and target DNA strands. The dual-mode PEC and EC measurements revealed that the Cu-MOF@CuPc-TA-COF-based biosensor displayed a wide linear detection range from 1 fM to 1 nM and an extremely low limit of detection (LOD) of 0.07 and 0.18 fM, respectively. In addition, the dual-mode PEC-EC biosensor also demonstrated remarkable selectivity, high stability, good reproducibility, and preferable regeneration ability, as well as acceptable applicability, for which the detected HIV-1 DNA in human serum showed good consistency with real concentrations. Thereby, the present work can open a new dual-mode PEC-EC platform for detecting HIV-1 DNA based on the porous-organic framework heterostructure.

Stable dual-emissive fluorescin@UiO-67 metal-organic frameworks for visual and ratiometric sensing of Al3+ and ascorbic acid

Encapsulation of fluorophore in metal organic framework (MOF) is an effective method to construct multi-emissive composites. Unfortunately, the small molecules loaded in MOF pores are easy to leak. To overcome this difficulty, fluorescin (FL) is proposed to be encapsulated tightly in the cage of the small tetrahedron of UiO-67, as one of the organic ligands coordinated with the central ion Zr. Finally, stable multi-emission fluorescence was successfully achieved, and Förster resonance energy transfer (FRET) occurred between FL and UiO-67. Ascorbic acid (AA) can dynamically quench the fluorescence of FL@UiO-67 nanoclusters (NCs) through internal filtering effect, photoinduced electron transfer (PET). The detection limit of the probe for AA was as low as 0.20 μM, and the detection range was 0.67 μM-0.36 mM. The probe was further employed to detect Al³⁺ due to the coordination between Al³⁺ and the carboxyl group in the FL@UiO-67 NCs. The detection limit for Al³⁺ was 3.3 nM, and the linear range was 11 nM-5 μM agarose film and test paper were both prepared successfully for visual detection of AA and Al³⁺. This work provides new ideas for low-cost and convenient real-time detection method.

Capture, Detection, and Simultaneous Identification of Rare Circulating Tumor Cells Based on a Rhodamine 6G-Loaded Metal-Organic Framework

Circulating tumor cells (CTCs) play a key role in the development of tumor metastasis. It will be a big step forward for CTC application as a reliable clinical liquid biopsy marker to be able to identify the captured CTCs while achieving a high capture efficiency within one analytical system. Herein, in this work, a stimuli-responsive and rhodamine 6G (Rho 6G)-entrapped fluorescent metal-organic framework (MOF) probe, named MOF-Rho 6G-DNA, was designed to capture, detect, and subsequently identify CTCs from blood samples of cancer patients. The probe was fabricated by modifying the epithelial cell adhesion molecule (EpCAM) hairpin DNA aptamer with Rho 6G enclosed and an Arm-DNA-attached UiO-66-NH2 MOF by sequence complementation. CTCs could be captured by the EpCAM hairpin DNA on the probe; as a result, Rho 6G loaded in the probe was released, and the number of CTCs was positively related to the concentration of released Rho 6G. An excellent correlation of fluorescence intensities with CTC numbers was obtained from 2 to 500 cells/mL. More importantly, the MOF-Rho 6G-DNA probe simultaneously realized rapid identification of the captured cells within 30 min by only relying on the residue Rho 6G in the MOF cavity. The captured target cells can be conveniently released from the probe using the complementary DNA sequence. These performance features of the probe were further verified by blood samples from patients of various types of tumor.

Construction of Core-Shell MOF@COF Hybrids with Controllable Morphology Adjustment of COF Shell as a Novel Platform for Photocatalytic Cascade Reactions

Recently, novel core-shell MOF@COF hybrids display excellent performance in various fields because of their inherited advantages from their parent MOFs and/or COFs. However, it is still a grand challenge to adjust the morphology of MOFs and/or COFs for consequent performance improvement. Herein, a Ti-MOF@TpTt hybrid coated with ultra-thin COF nanobelt, which is different from the fibrillar-like parent COF, is successfully synthesized through a sequential growth strategy. The as-obtained Pd decorated Ti-MOF@TpTt catalyst exhibits much higher photocatalytic performance than those of Ti-MOF, TpTt-COF, and Ti-MOF@TpTt hybrids with fibrillar-like COF shell for the photocatalytic cascade reactions of ammonia borane (AB) hydrolysis and nitroarenes hydrogenation. These can be attributed to its high BET surface area, core-shell structure, and type II heterojunction, which offers more accessible active sites and improves the separation efficiency of photo-generated carriers. Finally, the possible mechanisms of the cascade reaction are also proposed to well explain the improved performance of this photocatalytic system. This work presents a constructive route for designing core-shell MOF@COF hybrids with controllable morphology adjustment of COF shell, leading to the improved photocatalytic ability to broaden the applications of MOF/COF hybrid materials.

Hybrid Porous Crystalline Materials from Metal Organic Frameworks and Covalent Organic Frameworks

Two frontier crystalline porous framework materials, namely, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), have been widely explored owing to their outstanding physicochemical properties. While each type of framework has its own intrinsic advantages and shortcomings for specific applications, combining the complementary properties of the two materials allows the engineering of new classes of hybrid porous crystalline materials with properties superior to the individual components. Since the first report of MOF/COF hybrid in 2016, it has rapidly evolved as a novel platform for diverse applications. The state-of-art advances in the various synthetic approaches of MOF/COF hybrids are hereby summarized, together with their applications in different areas. Perspectives on the main challenges and future opportunities are also offered in order to inspire a multidisciplinary effort toward the further development of chemically diverse, multi-functional hybrid porous crystalline materials.

Rapid Removal of Mercury from Water by Novel MOF/PP Hybrid Membrane

Mercury is one of the most toxic heavy metals that can cause terrible disease for human beings. Among different absorption materials, MOF (metal–organic framework) materials show potential as very attractive materials for the rapid removal of mercury. However, the instability and difficulty for regeneration of MOF crystals limit their applications. Here, a continuous sulfur-modified MOF (UiO-66-NHC(S)NHMe) layer was synthesized in situ on polymeric membranes (PP non-woven fabrics) by post-synthetic modification and used for rapid mercury removal. The MOF-based membrane (US-N) showed high selectivity for mercury in different aqueous systems, which is better than sulfur-modified MOF powders. A thinner MOF layer on US-N showed a much better mercury ion removal performance. US-N with a 59.3 nm MOF layer could remove more than 85% of mercury in 20 min from an aqueous solution. In addition, the US-N can simply regenerate several times for mercury removal and maintain the initial performance (removal ratio > 98%), exhibiting excellent durability and stability. This work promotes the application of MOF materials in the rapid removal of hazardous heavy metal ions from practical environments.

Hotpots and trends of covalent organic frameworks (COFs) in the environmental and energy field: Bibliometric analysis

Covalent organic frameworks (COFs) have attracted extensive attention due to their low density, adjustable structure, functionalization, and good stability. This paper systematically and comprehensively describes to qualitatively and quantitatively the progress, trends, and hotspots of COFs in the environmental and energy fields from the perspective of bibliometrics. Herein, based on the Web of Science database, a total of 2589 articles from 2005 to October 6, 2020, were collected. Thereafter, co-occurrence, co-citation analysis, and cluster analysis were conducted using CiteSpace and VOSviewer software. The results indicated that COFs research shows the characteristics of rapid growth. The active countries were mainly USA, Germany, Japan, China, and India. More than half of the top 20 active institutions were from China. The research hotspots in this field were systematically elaborated, including synthesis, adsorption, catalysis, membrane, sensor, and energy storage. Research has shown that various COFs are reasonably designed, synthesized, and used in different applications. For example, when COFs are used for photocatalysis, groups containing photocatalytic active sites are integrated into COFs to improve photocatalytic activity. Finally, some challenges were proposed, that are beneficial to the rapid and balanced development of the COFs field. For instance, the preparation methods still need to be further improved for mass production and there is an imbalance in environmental applications such as fewer sensor and membrane applications. We believe that this study provides a comprehensive and systematic overview of the environmental and energy applications of COFs for future investigations.

Dynamic Coordination between a Triphenylamine-Functionalized Salicylaldehyde Schiff Base and a Copper(II) Ion

The coordination between a ligand and a metal is a spontaneous and uncontrollable process. In this Article, we successfully observe the formation of metal coordination in a triphenylamine-functionalized salicylaldehyde Schiff base with a copper(II) ion. The ligand TPA-Py first reacts with Cu²⁺ in a stepwise process to afford the dynamic complex TPA-Py@Cu²⁺ ([ligand]:[Cu²⁺] = 1:1), which further reacts with an extra copper(II) ion to afford 2TPA-Py@4Cu²⁺ with the following stepwise (or cumulative) stability constants: K₁ = 4.0694 × 10³ and K₂ = 1.0761 × 10⁶, respectively. The entire metal coordination process can be visualized, and the coordination mode of the probe toward copper was further evaluated by ultraviolet-visible/fluorescence spectra, single-crystal X-ray diffraction, density functional theory calculations, high-resolution mass spectra, and nuclear magnetic resonance spectroscopic titrations. Compound TPA-Py exhibited excellent sensitivity and specificity toward copper(II) ions in THF/water media with a low limit of detection of 2.687 × 10^-7 mol L^-1. In addition, TPI-An-Py can be applied to the detection of Cu²⁺ in real samples with satisfactory recoveries in the range of 100-112% in lake water and 98-101% in tap water. This Article not only reports an excellent fluorescence probe for copper(II) ion detection but also presents an instance for more fully understanding the metal coordination process.

Zn-based metal organic framework-covalent organic framework composites for trace lead extraction and fluorescence detection of TNP

A novel dual functional composite (MOF_L-TpBD) was prepared through solvothermal methods, with excellent Pb²⁺ ions separation and stable 2,4,6-Trinitrophenol (TNP) fluorescence detection performance. MOF_L-TpBD was characterized by FTIR, XRD, XPS, SEM and TGA et al. The prepared material was used to extract Pb²⁺ ions, with an adsorption capacity of 21.74 mg g^-1 calculated by Langmuir isotherm model. The limit of detection was 0.32 μg L^-1, along with a linear range from 0.7 to 12 μg L^-1 and a precision of 5.4% (1 μg L^-1, n = 9), respectively, where MOF_L-TpBD was adopted as adsorbent for Pb²⁺ ions preconcentration. The practical samples and reference water sample were measured by the provided method, with the satisfactory recoveries (91-110%) and reliable analytical results. MOF_L-TpBD was capable of fluorescent sensing of TNP, with a linear range from 0.01 to 1 mM and a limit of detection of 3.52 μM, respectively, and a precision of 3.29% was obtained (0.2 mM, n = 11). Meanwhile, the recoveries ranged from 91% to 108% in analysis of TNP for the practical samples. The designed material provided a potential candidate material for the detection of heavy metal ions and explosives in environmental water samples.