https://orcid.org/0000-0002-6389-5994
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Abstract
Covalent organic frameworks (COFs) have much potential in the field of analytical separation research due to their distinctive characteristics, including easy modification, low densities, large specific surface areas and permanent porosity. This article provides a historical overview of the synthesis and broad perspectives on the applications of COFs. The use of COF-based membranes in gas separation, water treatment (desalination, heavy metals and dye removal), membrane filtration, photoconduction, sensing and fuel cells is also covered. However, these COFs also demonstrate great promise as solid-phase extraction sorbents and solid-phase microextraction coatings. In addition to various separation applications, this work aims to highlight important advancements in the synthesis of COFs for chiral and isomeric compounds.
Graphical abstract
Covalent organic frameworks (COFs) have emerged as ideal candidate materials for advanced applications in analytical separation science due to their high porosity, large surface areas, well-defined pore structures, tunable pore sizes, adjustable surface properties and excellent stability. The design principles (topologies and linkages, building blocks), synthetic methods and functionalized strategies (bottom-up, in situ and postmodification) of COFs make them versatile for application purposes.
Types of COFs
The 2D or 3D crystalline structures of COF materials are well-defined and predictable due to the generation of stiff covalent interactions through a variety of synthetic organic processes between the structural units.
The production of COFs has been steadily increasing with various internal functionalizations, such as triazine, imine, hydrazone, borazine, azine and so on, as their presence and potential applications have been thoroughly explored.
The insertion of the secondary building units of MOFs into the building units of COFs will result in targeted products with traits from both MOFs and COFs. Those formed components will work in concert to provide multifunctional properties for particular uses such as energy conservation, photodynamic energy conversion, heterogeneous catalysis, adsorption and separation, detecting and semiconducting capabilities.
Applications
The potential applications of functional COFs were highlighted including adsorption and separation, heterogeneous catalysis, fluorescence, conductivity and solar cells.
Pharmaceutical applications: COFs are promising in separation science from sample pretreatment (solid-phase extraction and solid-phase microextraction) to chromatography (gas chromatography, high-performance liquid chromatography, capillary electrochromatography) for diverse targets. The unique properties of COFs make them good candidates as sorbents in solid-phase extraction. Several specific, nonhydrophobic mechanisms including hydrogen bonding, electrostatic attraction and π–π electron donor–acceptor interaction are reasons for its excellent extraction performance. Large surface areas and good thermal and chemical stability make COFs ideal candidates for the stationary phase for gas chromatography. Chiral COF-bound capillary columns provide better resolution and larger separation factors than commercial chiral capillary columns. The chiral microenvironment, π–π interactions, hydrogen bonding and van der Waals interactions play significant roles in chiral gas chromatography. The unique structures, good solvent stability and large surface area also make COFs potentially useful in high-performance liquid chromatography.
Environmental applications: COFs are prominent candidates in the area of water treatment, such as the removal of salts, dyes, metal ions and other organics. The applications of COF-based nanofiltration membranes mainly focus on the removal of dyes and salts from water.
COFs in drug delivery: Combining outstanding thermal and chemical stabilities, structural designability and inherent porosity in one material, COFs have shown their potential in biorelated applications. Recent focus has been devoted to exploring COFs for antibacterial activity and drug delivery. The basic concept is that skeletons have been developed for docking specific functional sites to trigger interplay with cells that cause antibacterial activity or target cancer cells, and the pores can be designed to load specific drugs for delivery into cells.
Financial disclosure
The authors have no financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
Competing interests disclosure
The authors have no competing interests or relevant affiliations with any organization or entity with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, stock ownership or options and expert testimony.
Writing disclosure
No writing assistance was utilized in the production of this manuscript.
Acknowledgments
The authors are grateful to the National Institute of Pharmaceutical Education and Research (NIPER), Hyderabad, India, for its valuable insights in preparing the manuscript. The authors thank the Department of Pharmaceuticals, Ministry of Chemicals & Fertilizers, Government of India, New Delhi, for a NIPER fellowship award. This manuscript bears the NIPER-Hyderabad communication number: NIPER-H/2023/.