Abstract:

Trace contaminants are toxic and their widespread presence in the environment potentially threatens human health. The levels of these pollutants are often difficult to determine directly using instruments owing to the complexities of environment matrices. Hence, pretreatment steps, such as sample purification and concentration, are key along with various processes that enhance the accuracy and sensitivity of the detection method. To date, researchers have successfully developed a variety of efficient and reliable sample-pretreatment techniques that are based on different principles. Among these, magnetic solid-phase extraction (MSPE) is a rapid and efficient sample-pretreatment technology that is based on the similar solid-phase-extraction (SPE) principle, which mainly enriches target analytes by exploiting their interactions with functional groups on the surfaces of magnetic materials, thereby achieving rapid separation when an external magnetic field is applied. MSPE has been a focus of attention in the environmental-sample-pretreatment, food-analysis, and other fields owing to advantages that include ease of operation, low cost, and high enrichment efficiency. The selection of the magnetic material is key to MSPE process because traditional magnetic materials exhibit certain functionality limitations. Accordingly, designing and synthesizing green and efficient functionalized magnetic materials have become a research focus in this field, with researchers having extensively explored multiple ways of preparing functionalized modified magnetic materials through the introduction of a variety of emerging materials, including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), carbon nanotubes (CNTs), graphene oxide (GO), and other specific functional groups to modify magnetic materials and effectively expanded the applications scope of MSPE. Among these, COFs are porous crystalline materials consisting of light elements (C, N, H, O and B, etc.) linked through covalent bonds. COFs are mainly classified as imine COFs, boronic-acid COFs, triazine COFs, and ketenimine COFs according to bonding type. Moreover, it is worth mentioning that COFs can be synthesized from a number of monomers, and the functional groups exposed on the COF surface can also be used for further modification purposes. COFs are versatile and modifiable; consequently, they have attracted significant research attention, with a series of COF-functionalized magnetic materials having been designed and synthesized. The magnetic COFs (MCOFs) combine the advantages of COFs and magnetic materials. MCOFs are not only endowed with the large specific surface areas, high porosities, and good stabilities that are characteristic of COFs, but also exhibit the rapid separation and reusability characteristics of magnetic material, thereby quickly and efficiently enriching targets through hydrogen bonding, hydrophobicity, π-π stacking, and van der Waals forces, making them ideal sample-pretreatment materials. MCOFs have also been converted into more-versatile functional materials using post-modification strategies. Combining MCOFs with advanced analytical techniques, such as high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) has effectively improved the limits of detection (LODs) for various analytes as well as method accuracy; these techniques have been widely used to enrich and detect trace pollutants. However, some material design and synthesis challenges remain and need to be overcome, despite the promising potential of MCOFs. Future research needs to focus on exploring novel synthetic strategies that reduce experimental costs and improve the functionalities of MCOFs while expanding their applicabilities to broader sample matrices. In this review, we first introduce and discuss the construction and functionalized designs of various MCOF composites, and then summarize their use in applications that include the enrichment and detection of pesticides, endocrine disruptors (EDCs), pharmaceuticals, and personal care products (PPCPs), and finally provide an outlook on future developmental prospects.

Key words: magnetic solid-phase extraction (MSPE), magnetic covalent organic framework (MCOFs), pesticide, endocrine disruptors (EDCs), pharmaceutical and personal care products (PPCPs), review