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.

Porous organic polymers (POPs) for environmental remediation

Modern global industrialization along with the ever-increasing growth of the population has resulted in continuous enhancement in the discharge and accumulation of various toxic and hazardous chemicals in the environment. These harmful pollutants, including toxic gases, inorganic heavy metal ions, anthropogenic waste, persistent organic pollutants, toxic dyes, pharmaceuticals, volatile organic compounds, etc., are destroying the ecological balance of the environment. Therefore, systematic monitoring and effective remediation of these toxic pollutants either by adsorptive removal or by catalytic degradation are of great significance. From this viewpoint, porous organic polymers (POPs), being two- or three-dimensional polymeric materials, constructed from small organic molecules connected with rigid covalent bonds have come forth as a promising platform toward various leading applications, especially for efficient environmental remediation. Their unique chemical and structural features including high stability, tunable pore functionalization, and large surface area have boosted the transformation of POPs into various macro-physical forms such as thick and thin-film membranes, which led to a new direction in advanced level pollutant removal, separation and catalytic degradation. In this review, our focus is to highlight the recent progress and achievements in the strategic design, synthesis, architectural-engineering and applications of POPs and their composite materials toward environmental remediation. Several strategies to improve the adsorption efficiency and catalytic degradation performance along with the in-depth interaction mechanism of POP-based materials have been systematically summarized. In addition, evolution of POPs from regular powder form application to rapid and more efficient size and chemo-selective, "real-time" applicable membrane-based application has been further highlighted. Finally, we put forward our perspective on the challenges and opportunities of these materials toward real-world implementation and future prospects in next generation remediation technology.

Lanthanide Functionalized Covalent Organic Frameworks Hybrid Materials for Luminescence Responsive Chemical Sensing

Covalent organic frameworks (COFs) possess several unique features of structural and functional chemistry, together with other modular photophysical performance, which make them candidates for luminescence responsive chemical sensing. Lanthanide (Ln3+ ) functionalized COFs hybrid materials still keep the parent COFs' virtues and also embody the abundant multiple luminescence response with both COFs and Ln3+ ions or other guest species. In this review, the summary is highlighted on the lanthanide functionalized COFs hybrid materials and their relevant systems for luminescence responsive chemical sensing. It is subdivided into five sections involving the three main topics. Firstly, the basic knowledges of COFs materials related to the luminescence responsive chemical sensing are introduced (including three sections), involving the chemistry, application and post-synthetic modification (PSM) of COFs, the luminescence and luminescence responsive chemical sensing, and the luminescence responsive chemical sensing of non-lanthanide functionalized COFs hybrids materials. Secondly, the systematic progresses are outlined on the lanthanide functionalized COFs hybrid materials in luminescence responsive chemical sensing, which is the emphasis for this review. Finally, the conclusion and prospect are given.

Porous organic polymers as a platform for sensing applications

Sensing analysis is significantly important for human health and environmental safety, and has gained increasing concern. As a promising material, porous organic polymers (POPs) have drawn widespread attention due to the availability of plentiful building blocks and their tunable structures, porosity and functions. Moreover, the permanent porous nature could provide a micro-environment to interact with guest molecules, rendering POPs attractive for application in the sensing field. In this review, we give a comprehensive overview of POPs as a platform for sensing applications. POP-based sensors are mainly divided into five categories, including fluorescence turn-on sensors, fluorescence turn-off sensors, ratiometric fluorescent sensors, colorimetric sensors and chemiresistive sensors, and their various sensing applications in detecting explosives, metal ions, anions, small molecules, biological molecules, pH changes, enantiomers, latent fingerprints and thermosensation are summarized. The different structure-based POPs and their corresponding synthetic strategies as well as the related sensing mechanisms mainly including energy transfer, donor-acceptor electron transfer, absorption competition quenching and inner filter effect are also involved in the discussion. Finally, the future outlook and perspective are addressed briefly.

Luminescent Covalent Organic Frameworks for Biosensing and Bioimaging Applications

Luminescent covalent organic frameworks (LCOFs) have attracted significant attention due to their tunability of structures and photophysical properties at molecular level. LCOFs are built to highly ordered and periodic 2D or 3D framework structures through covalently assembling with various luminophore building blocks. Recently, the advantages of LCOFs including predesigned properties of structure, unique photoluminescence, hypotoxicity and good biocompatibility and tumor penetration, broaden their applications in biorelated fields, such as biosensing, bioimaging, and drug delivery. A specific review that analyses the advances of LCOFs in the field of biosensing and bioimaging is thus urged to emerge. Here the construction of LCOFs is reviewed first. The synthetic chemistry of LCOFs highlights the key role of chemical linkages, which not only concrete the building blocks but also affect the optical properties and even can act as the responsive sites for potential sensing applications. How to brighten LCOFs are clarified through description of structure managements. The ability to utilize the luminescence of LCOFs for applications in biosensing and bioimaging is discussed using state-of-the-art examples of varied practical goals. A prospect finally addresses opportunities and challenges the development of LCOFs facing from chemistry, physics to the applications, according to their current progress.

Covalent organic frameworks as multifunctional materials for chemical detection

Sensitive and selective detection of chemical and biological analytes is critical in various scientific and technological fields. As an emerging class of multifunctional materials, covalent organic frameworks (COFs) with their unique properties of chemical modularity, large surface area, high stability, low density, and tunable pore sizes and functionalities, which together define their programmable properties, show promise in advancing chemical detection. This review demonstrates the recent progress in chemical detection where COFs constitute an integral component of the achieved function. This review highlights how the unique properties of COFs can be harnessed to develop different types of chemical detection systems based on the principles of chromism, luminescence, electrical transduction, chromatography, spectrometry, and others to achieve highly sensitive and selective detection of various analytes, ranging from gases, volatiles, ions, to biomolecules. The key parameters of detection performance for target analytes are summarized, compared, and analyzed from the perspective of the detection mechanism and structure-property-performance correlations of COFs. Conclusions summarize the current accomplishments and analyze the challenges and limitations that exist for chemical detection under different mechanisms. Perspectives on how future directions of research can advance the COF-based chemical detection through innovation in novel COF design and synthesis, progress in device fabrication, and exploration of novel modes of detection are also discussed.

Covalent Organic Polymers and Frameworks for Fluorescence-Based Sensors

Following the advancements and diversification in synthetic strategies for porous covalent materials in the literature, the materials science community started to investigate the performance of covalent organic polymers (COPs) and covalent organic frameworks (COFs) in applications that require large surface areas for interaction with other molecules, chemical stability, and insolubility. Sensorics is an area where COPs and COFs have demonstrated immense potential and achieved high levels of sensitivity and selectivity on account of their tunable structures. In this review, we focus on those covalent polymeric systems that use fluorescence spectroscopy as a method of detection. After briefly reviewing the physical basis of fluorescence-based sensors, we delve into various kinds of analytes that have been explored with COPs and COFs, namely, heavy metal ions, explosives, biological molecules, amines, pH, volatile organic compounds and solvents, iodine, enantiomers, gases, and anions. Throughout this work, we discuss the mechanisms involved in each sensing application and aim to quantify the potency of the discussed sensors by providing limits of detection and quenching constants when available. This review concludes with a summary of the surveyed literature and raises a few concerns that should be addressed in the future development of COP and COF fluorescence-based sensors.

Processing nanoporous organic polymers in liquid amines

Rigid network structures of nanoporous organic polymers provide high porosity, which is beneficial for applications such as gas sorption, gas separation, heterogeneous (photo)catalysis, sensing, and (opto)electronics. However, the network structures are practically insoluble. Thus, the processing of nanoporous polymers into nanoparticles or films remains challenging. Herein, we report that nanoporous polymers made via a Knoevenagel-like condensation can be easily processed into nanoparticles (115.7 ± 40.8 nm) or a flawless film by using liquid amines as a solvent at elevated temperatures. FTIR spectra revealed that the carboxyl groups in the nanoporous polymers act as reactive sites for amines, forming new functionalities and spacing the polymeric chains to be dissolved in the liquid amines. The processed film was found to be CO₂-philic despite the low surface area, and further able to be transformed into a fine carbon film by thermal treatment.

Petroleum pitch-based porous aromatic frameworks with phosphonate ligand for efficient separation of uranium from radioactive effluents

Porous aromatic frameworks with structural/pore controllability and rigid skeletons present a series of emerging materials for solid phase extraction. However, the complicated monomers or noble metal catalyst, and cumbersome synthetic strategies result in high-cost engineering application of porous aromatic frameworks. Herein, a simple synthetic strategy of porous aromatic frameworks with phosphonate is reported for efficient separation of uranium from radioactive effluents, and petroleum pitch, a low-cost and widely available material, was used as the building block. 4-Vinylbenzylphosphonic acid diethyl ester monomer is introduced to chelate uranium and to improve the aqueous dispersibility of sorbents. The phosphonate functionalized PPAFs take 40 min to achieve adsorption equilibrium, and the maximum sorption capacity reaches 147 mg U/g at pH 1.0. PPAFs exhibit good selectivity over various competing ions and excellent radioresistance in acidic solution. Besides, PPAFs remain almost 100% sorption efficiency and intact structure over 5 sorption-desorption cycles with alkaline eluent. This work not only applies a low-cost material for uranium extraction, but a new idea for the utilization of waste and recycling of resources.

A New Triazine-Based Covalent Organic Framework for High-Performance Capacitive Energy Storage

The new covalent organic framework material TDFP-1 was prepared through a solvothermal Schiff base condensation reaction of the monomers 1,3,5-tris-(4-aminophenyl)triazine and 2,6-diformyl-4-methylphenol. Owing to its high specific surface area of 651 m²  g^-1 , extended π conjugation, and inherent microporosity, TDFP-1 exhibited an excellent energy-storage capacity with a maximum specific capacitance of 354 F g^-1 at a scan rate of 2 mV s^-1 and good cyclic stability with 95 % retention of its initial specific capacitance after 1000 cycles at 10 A g^-1 . The π-conjugated polymeric framework as well as ionic conductivity owing to the possibility of ion conduction inside the micropores of approximately 1.5 nm make polymeric TDFP-1 a favorable candidate as a supercapacitor electrode material. The electrochemical properties of this electrode material were measured through cyclic voltammetry, galvanic charge-discharge, and electrochemical impedance spectroscopy, and the results indicate its potential for application in energy-storage devices.

Trends and challenges for microporous polymers

Recent trends and challenges for the emerging materials class of microporous polymers are reviewed. See the main article for graphical abstract image credits.

Semi-Interpenetrating Polymer Networks with Predefined Architecture for Metal Ion Fluorescence Monitoring

The development of new synthetic approaches for the preparation of efficient 3D luminescent chemosensors for transition metal ions receives considerable attention nowadays, owing to the key role of the latter as elements in biological systems and their harmful environmental effects when present in aquatic media. In this work, we describe an easy and versatile synthetic methodology that leads to the generation of nonconjugated 3D luminescent semi-interpenetrating amphiphilic networks (semi-IPN) with structure-defined characteristics. More precisely, the synthesis involves the encapsulation of well-defined poly(9-anthrylmethyl methacrylate) (pAnMMA) (hydrophobic, luminescent) linear polymer chains within a covalent poly(2-(dimethylamino)ethyl methacrylate) (pDMAEMA) hydrophilic polymer network, derived via the 1,2-bis-(2-iodoethoxy)ethane (BIEE)-induced crosslinking process of well-defined pDMAEMA linear chains. Characterization of their fluorescence properties demonstrated that these materials act as strong blue emitters when exposed to UV irradiation. This, combined with the presence of the metal-binding tertiary amino functionalities of the pDMAEMA segments, allowed for their applicability as sorbents and fluorescence chemosensors for transition metal ions (Fe3+, Cu2+) in solution via a chelation-enhanced fluorescence-quenching effect promoted within the semi-IPN network architecture. Ethylenediaminetetraacetic acid (EDTA)-induced metal ion desorption and thus material recyclability has been also demonstrated.

Robust C-C bonded porous networks with chemically designed functionalities for improved CO2 capture from flue gas

Effective carbon dioxide (CO₂) capture requires solid, porous sorbents with chemically and thermally stable frameworks. Herein, we report two new carbon-carbon bonded porous networks that were synthesized through metal-free Knoevenagel nitrile-aldol condensation, namely the covalent organic polymer, COP-156 and 157. COP-156, due to high specific surface area (650 m²/g) and easily interchangeable nitrile groups, was modified post-synthetically into free amine- or amidoxime-containing networks. The modified COP-156-amine showed fast and increased CO₂ uptake under simulated moist flue gas conditions compared to the starting network and usual industrial CO₂ solvents, reaching up to 7.8 wt % uptake at 40 °C.

A new triazine functionalized luminescent covalent organic framework for nitroaromatic sensing and CO2 storage

A hexagonal COF has been designed via Schiff-base condensation reaction between 1,3,5-tris-(4-aminophenyl)triazine and 1,3,5-triformylphloroglucinol. It showed excellent sensing behavior towards nitroaromatic compounds through fluorescence quenching and excellent CO₂ uptake.