Application of imine covalent organic frameworks in sample pretreatment

Imine covalent organic frameworks (I-COFs), including imine-linked COFs and hydrazone-linked COFs, are a new type of crystalline porous organic materials constructed by the condensation of organic monomers by the Schiff-base reaction. Because they are composed of lightweight elements linked by strong covalent bonds, I-COF materials possess the advantages of low skeleton density, large surface area, high porosity, abundant monomer species, controllable pore size, functionalized structure, diverse synthetic methods, excellent adsorption performance, outstanding physical and chemical stabilities, etc. In recent years, interest in the field of I-COFs has increased tremendously because of their exceptional performance and broad applications in gas storage, gas separation, catalysis, sensing, photoelectric materials, sample pretreatment, drug delivery, and other fields. To date, imine bonds are one of the most widely used covalent bonds in COFs, and represent one of the most important ways to obtain I-COFs with excellent chemical stabilities. The synthesis methods for I-COFs include solvothermal synthesis, microwave synthesis, mechanochemical grinding synthesis, and room-temperature synthesis methods. Solvothermal synthesis is the most extensively used method for the production of I-COFs with high specific surface areas and good thermal stabilities. The microwave synthesis method is conducive to the rapid synthesis of COFs in industry, providing a more time-saving, simpler, and safer route for large-scale preparation of I-COFs. The mechanochemical grinding synthesis of porous solids has gained importance as an alternative to conventional solvothermal synthesis, because the process is quick, environment-friendly, and potentially scalable. The room-temperature method is characterized by mild reaction conditions and rapid reactions. It is an energy-saving, economic, safe, and green synthesis method, which has emerged as a hot spot in the preparation of I-COFs in recent years. Research progress over the past years on the application of I-COFs in the field of materials science has undoubtedly established the basis of its application in analytical chemistry. Owing to the excellent physical and chemical properties of I-COF materials, they are suitable for use as separation and enrichment media for trace target compounds in complex samples. The high specific surface area and porosity, extended conjugate network skeleton, and π-electron-rich nature of the materials endow it with a high adsorption capacity. These materials are highly enriched in target analytes by π-π interactions, acid-base interactions, donor-acceptor interactions, hydrogen bonding, hydrophobic interactions, and other intermolecular interactions. Precise control of the microporous structure of I-COFs was obtained by controlling the chain length, geometric structure, doping elements, and substituent groups of the organic monomers. Selective enrichment of target trace substances was achieved by modifying the groups of I-COFs based on the principle of host guest adaptation, molecular sieving, or microporous filling effect. At present, research on the synthesis of I-COF materials is in the stage of rapid development, and many I-COFs with excellent properties and great application potential have been synthesized, allowing widespread application of I-COFs in sample pretreatment medium. This review summarizes the current state-of-the-art on the main types and synthetic methods of I-COFs, as well as the applications of I-COFs in solid-phase extraction, magnetic solid-phase extraction, dispersive solid-phase extraction, and solid-phase microextraction. The prospects of I-COFs in sample pretreatment are also presented.