Study on the fluorescent covalent organic framework for selective “turn-off”recognition and detection of Fe3+ ions

A sensitive molecular probe exhibiting significant change in their photophysical and morphological behavior upon interaction with Fe3+ ion

An efficient molecular probe 8 has been designed and synthesized. The photophysical, electrochemical and morphological behavior of the probe has been examined in the absence and presence of different ions. The probe 8 at 90 % water fraction in acetonitrile showed aggregation induced emission (AIE). Probe 8 upon interaction with ions binds with Fe3+ ion selectively in a 1:1 stoichiometry and showed fluorescence "turn-Off" response with good limit of detection (LOD = 92.2 nM). The particle size (DLS method) of probe upon increasing water fraction in acetonitrile showed a gradual increase while upon formation of a stable complex, 8 + Fe3+ particle size decreased along with change in morphology of the probe. SEM and TEM studies showed that in pure acetonitrile probe self-assemble into a sheet like structure of uneven surface. While in aggregated state (fw, 90 %) it changes to a uniform hollow rectangular rod shape structure. Further interaction of the probe with Fe3+ ions in aggregated state acquired a well-defined smooth sheet. Electrochemical (CV) studies suggested that the redox property of the probe incurred a marginal change in band gap upon complexation with Fe3+. The cell imaging studies were performed to detect Fe3+ in HeLa cells. The paper strip test and real water sample analysis showed the potential analytical application of probe to detect Fe3+ with a naked-eye sensitive visible color change. The formation of a complex, 8 + Fe3+ involving N and O atoms of the probe molecule was confirmed by 1HNMR and HRMS data.

Designing fluorescent covalent organic frameworks through regulation of link bond for selective detection of Al3+ and Ce3

The high thermal stability and chemical durability of amide-linked covalent organic frameworks (amide COFs) make them a promising material for a range of new applications. Nevertheless, the low reversibility of the amide bond presents a significant challenge to the direct synthesis of amide-bonded COFs. In this paper, we present a simple method for synthesizing amide COFs. The synthesis of imine-linked COFs was initially achieved through the reaction of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and 2,5-dimethoxybenzene-1,4-dicarboxaldehyde. Subsequently, amide COFs were synthesized via the oxidation of the imine bond into an amide bond, utilizing ammonium persulfate as the oxidizing agent. Due to the difference of link bond, two COFs separately displayed distinct and significant fluorescence enhancement for Al3+ and Ce3+, which was highly sensitive and less affected by environmental factors. The strategy offers a novel approach to the convenient and environmentally benign synthesis of amide COFs, which may facilitate their wider applications.

Novel Olefin-Linked Covalent Organic Framework with Multifunctional Group Modification for the Fluorescence/Smartphone Detection of Uranyl Ion

Monitoring and purification of uranium contamination are of great importance for the rational utilization of uranium resources and maintaining the environment. In this work, an olefin-linked covalent organic framework (GC-TFPB) and its amidoxime-modified product (GC-TFPB-AO) are synthesized with 3-cyano-4,6-dimethyl-2-hydroxypyridine (GC) and 1,3,5-tris(4-formylphenyl) benzene (TFPB) by Knoevenagel condensation. GC-TFPB-AO results in specificity for rapid fluorescent/smartphone uranyl ion (UO22+) detection based on the synergistic effect of multifunctional groups (amidoxime, pyridine, and hydroxyl groups). GC-TFPB-AO features a rapid and highly sensitive detection and adsorption of UO22+ with a detection limit of 21.25 nM. In addition, it has a good recovery (100-111%) for fluorescence detection in real samples, demonstrating an excellent potential of predesigned olefin-linked fluorescent COFs in nuclear contaminated wastewater detection and removal.

Porous organic polymers-based fluorescent chemosensors for Fe(III) ions-a functional mimic of siderophores

Extended organic polymers such as amorphous Covalent Organic Polymers (COPs) and crystalline Covalent Organic Frameworks (COFs) are emerging functional polymeric materials that have recently been shown promises as luminescent materials for chemosensing applications. A wide variety of luminescence COPs and COFs have been synthesized and successfully used as fluorescence-sensing materials for hazardous environmental pollutants and toxic contaminants. This review exemplifies various COPs and COFs-based fluorescence sensors for selective sensing of Fe(III) ions. The fluorescence sensors are sorted according to their structural features and each section provides a detailed discussion on the synthesis and fluorescence sensing ability of different COPs and COFs towards Fe(III) ions. Also, this review highlights the limitations of the existing organic polymer-based chemosensors and future perspectives on translating COPs and COFs-based fluorescence sensors for the practical detection of Fe(III) ions.

A PEDOT enhanced covalent organic framework (COF) fluorescent probe for in vivo detection and imaging of Fe3

Simple and accurate in vivo monitoring of Fe3+ is essential for gaining a better understanding of its role in physiological and pathological processes. A novel fluorescent probe was synthesized via in situ solid-state polymerization of 3,4-ethylenedioxythiophene (PEDOT) in the pore channels of a covalent organic framework (COF). The PEDOT@COF fluorescent probe exhibited an absolute quantum yield (QY) 3 times higher than COF. In the presence of Fe3+ the PEDOT@COF 475 nm fluorescence emission, 365 nm excitation, is quenched within 180 s. Fluorescence quenching is linear with Fe3+ in the concentration range of 0-960 μM, with a detection limit of 0.82 μM. The fluorescence quenching mechanism was attributed to inner filter effect (IEF), photoinduced electron transfer (PET) and static quenching (SQE) between PEDOT@COF and Fe3+. A paper strip-based detector was designed to facilitate practical applicability, and the PEDOT@COF probe successfully applied to fluorescence imaging of Fe3+ levels in vivo. This work details a tool of great promise for enabling detailed investigations into the role of Fe3+ in physiological and pathological diseases.

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.

A Ratiometric Fluorescent Probe Based on RhB Functionalized Tb-MOFs for the Continuous Visual Detection of Fe3+ and AA

In this study, a red-green dual-emitting fluorescent composite (RhB@MOFs) was constructed by introducing the red-emitting organic fluorescent dye rhodamine B (RhB) into metal-organic frameworks (Tb-MOFs). The sample can be used as a ratiometric fluorescent probe, which not only avoids errors caused by instrument and environmental instability but also has multiple applications in detection. The results indicated that the RhB@MOFs exhibited a turned-off response toward Fe3+ and a turned-on response for the continuous detection of ascorbic acid (AA). This ratiometric fluorescent probe possessed high sensitivity and excellent selectivity in the continuous determination of Fe3+ and AA. It is worth mentioning that remarkable fluorescence change could be clearly observed by the naked eye under a UV lamp, which is more convenient in applications. In addition, the mechanisms of Fe3+- and AA-induced fluorescence quench and recovery are discussed in detail. This ratiometric probe displayed outstanding recognition of heavy metal ions and biomolecules, providing potential applications for water quality monitoring and biomolecule determination.

A water-soluble naphthalimide fluorescent probe for Cr2O72- and Fe3+ based on inner filter effect

A probe 3 (2-ethoxy-N-(2-(2-(2-hydroxyethoxy)ethyl)-1,3-dioxo-2,3-dihydro-1H-benzo[de] isoquinolin-6-yl)benzamide) that could selectively respond to Cr₂O₇^2- and Fe³⁺ was reported in this paper. The selectivity, pH titration, concentration titration, detection limit, time dependence, quenching constant and recognition mechanism of probe 3 for Cr₂O₇^2- and Fe³⁺ were studied in CH₃CN/HEPES buffer solution. The results showed that Cr₂O₇^2- and Fe³⁺ could rapidly quench the fluorescence of probe 3 through the inner filter effect (IFE). The quenching kept constant after 30 s, and the quenching constants were 7.99 × 10³ L^.mol^-1 and 4.13 × 10³ L^.mol^-1, respectively. The detection limits of probe 3 for Cr₂O₇^2- and Fe³⁺ were 1.15 μmol^.L^-1 and 1.95 μmol^.L^-1, respectively, which were lower than the maximum allowable concentrations in drinking water stipulated by EPA. The determination results of Cr₂O₇^2- and Fe³⁺ in water samples indicated that probe 3 could be used as a potential detection tool in practical applications.

Porous Organic Frameworks Constructed by the Synergetic Effect of Covalent Bonds and Hydrogen Bonds for the Selective Identification and Detection of Explosives

Covalent organic frameworks (COFs) and hydrogen-bonded organic frameworks (HOFs) have emerged as novel platforms for material design and functional explorations. Here, we report an unconventional structural organic framework (named CHOF-1) combined with 1,3,5-tris(4-aminophenyl)benzene (TAPB) and 5-hydroxyisophthalaldehyde (HP) through the synergetic effect of covalent bonds and hydrogen bonds. CHOF-1 contains two different pores and is an eclipsed AA-stacking model with high thermal stability. It exhibits selective "turn-off" and red shift fluorescence responses toward 4-nitrophenol (4-NP) and 2,4,6-trinitrophenol (TNP) in ethanol, respectively, and the low detection limit was 8.78 μM for 4-NP and 11.1 μM for TNP. It has satisfactory recovery in the detection of 4-NP and TNP in practical water samples. The fluorescence quenching of CHOF-1 caused by 4-NP is attributed to the absorption competition quenching (ACQ) mechanism. In the structure of CHOF-1, the hydroxyl of TNP has strong acidity and can form strong hydrogen bonds with the unreacted NH2. Then, the crystalline structure of CHOF-1 decomposed and the electron structure of the photophore changed; thus, the fluorescence spectra red-shifted. This is a novel example of a covalent bond and hydrogen bonds combined organic framework (CHOF). This work not only demonstrates a synthesis strategy of the first novel structural organic framework through the synergetic effect of covalent bonds and hydrogen bonds but also provides a fluorescent sensing material for the detection of explosives 4-NP and TNP with high selectivity and sensitivity.

Dandelion-like covalent organic frameworks with high-efficiency fluorescence for ratiometric sensing and visual tracking-by-detection of Fe3

Iron ions, one of the most common heavy metal pollutants in industrial waste materials, are continuously actively or passively delivered to the environment. Meanwhile, the importance of Fe³⁺ in biological processes in vivo can not be neglected due to its crucial role in maintaining normal physiological function. Therefore, a ratiometric fluorescence covalent organic framework (TD-COF) was constructed for tracking-by-detection of Fe³⁺. Alkynes-extended 1,3,6,8-tetrakis(4-ethynyl benzaldehyde)-pyrene (TEBPY) with complete planar structure and 2,5-dihydroxyterephthalohydrazide (DHTH) with functional group -OH were selected as the building blocks. The ratiometric fluorescence TD-COF with a dandelion-like structure exhibited its dual emission peaked at 510 nm and 630 nm. It displayed an obvious fluorescence color variation of yellow-red-black in the presence of Fe³⁺. Benefiting from the high luminescent efficiency (QY of 36.4%) and multiple identical binding sites, TD-COF exhibited a wide linear range to Fe³⁺ (0.005-50 μM) with a detection limit of 10.9 nM. Additionally, a smartphone visual sensing platform integrated with TD-COF was developed based on the color transformation and successfully applied to visual smart real-time monitoring Fe³⁺. More surprisingly, the maximum adsorption capacity of TD-COF towards Fe³⁺ was 833.3 mg/g due to the coordination interaction and cationic π-effect. The practicability of the smartphone-integrated ratiometric sensing platform for visual tracking-by-detection of Fe³⁺ was verified by choosing tap water as the actual sample, and the recoveries were calculated to be 98.71-100.88%. This work thus developed COF-based ratiometric sensing of Fe³⁺, which is an attractive candidate for further application in fluorescent sensing and visual monitoring.

A self-standing three-dimensional covalent organic framework film

Covalent crystals such as diamonds are a class of fascinating materials that are challenging to fabricate in the form of thin films. This is because spatial kinetic control of bond formation is required to create covalently bonded crystal films. Directional crystal growth is commonly achieved by chemical vapor deposition, an approach that is hampered by technical complexity and associated high cost. Here we report on a liquid-liquid interfacial approach based on physical-organic considerations to synthesize an ultrathin covalent crystal film. By distributing reactants into separate phases using hydrophobicity, the chemical reaction is confined to an interface that orients the crystal growth. A molecular-smooth interface combined with in-plane isotropic conditions enables the synthesis of films on a centimeter size scale with a uniform thickness of 13 nm. The film exhibits considerable mechanical robustness enabling a free-standing length of 37 µm, as well as a clearly anisotropic chemical structure and crystal lattice alignment.

A new NCDs@ZIF-90-based sensor: fluorescent "turn-on" detection of Al3+ ions with high selectivity and sensitivity

A new NCDs@ZIF-90 composite as fluorescence sensor was designed and prepared by encapsulation of N-doped carbon dots (NCDs) in metal-organic framework (MOF) ZIF-90 in one-pot synthesis. NCDs@ZIF-90 retained the crystal structure and high thermal stability of ZIF-90; meanwhile, it also displayed the good chemical stability. NCDs@ZIF-90 dispersion in ethanol exhibited selective "turn-on" fluorescence response towards Al³⁺. The other coexisting competing metal ions had no obvious influence on the sensing performance of NCDs@ZIF-90 for Al³⁺. The fluorescence intensity at 447 nm of NCDs@ZIF-90 dispersion in ethanol had good linear relation with the concentrations of Al³⁺ with a low detection limit of 3.196 μM. The fluorescence enhancement after the addition of Al³⁺ was attributed to the release of NCDs from the inside of ZIF-90 to ethanol solution. In addition, NCDs@ZIF-90 displayed good recovery in detection of Al³⁺ in water samples indicating its practical application capability. The high selectivity and sensitivity indicate that NCDs@ZIF-90 is a good candidate as a "turn-on" fluorescence chemosensor to identify and detect Al³⁺.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

Covalent organic frameworks (COFs) are a class of organic crystalline porous materials discovered in the early 21st century that have become an attractive class of emerging materials due to their high crystallinity, intrinsic porosity, structural regularity, diverse functionality, design flexibility, and outstanding stability. However, many chemical and physical properties strongly depend on the presence of metal ions in materials for advanced applications, but metal-free COFs do not have these properties and are therefore excluded from such applications. Metalated COFs formed by combining COFs with metal ions, while retaining the advantages of COFs, have additional intriguing properties and applications, and have attracted considerable attention over the past decade. This review presents all aspects of metalated COFs, from synthetic strategies to various applications, in the hope of promoting the continued development of this young field.