Abstract
The metal-free cousins of metal-organic frameworks, covalent organic frameworks (COFs), are a class of pre-designable crystalline polymers composed of light elements and connected by strong covalent bonds. COFs are being given more and more attention in the electrochemical sensor field due to their fascinating properties, such as highly tunable porosity, intrinsic chemical and thermal stability, structural diversity, large specific surface area, and unique adsorption characteristics. However, there are still some key issues regarding COFs that need to be urgently resolved before they can be effectively applied in electrochemical sensing. In this review, we summarized recent achievements in developing novel electrochemical sensors based on COFs, and discussed the key fundamental and challenging issues that need to be addressed, including the mechanisms underlying charge transport, methods to improve electrical conductivity, immobilization methods on different substrates, synthesis strategies for nanoscale COFs, and the application of COFs in different fields. Finally, the challenges and outlooks in this promising field are tentatively proposed.
Highlights
Covalent organic frameworks (COFs), a new class of multifunctional porous crystalline materials, are two- or three-dimensional (2D or 3D) porous crystalline materials built by light elements (C, B, O, Si, and N) via strong covalent bonds (C-N, C=N, C=C-N, B-O) (Xue et al, 2017; Chen et al, 2019; Wang and Zhuang, 2019; Zhu et al, 2019)
This critical review briefly summarized the key properties of covalent organic frameworks (COFs) that influence the electroanalytical performances, and corresponding solutions were discussed in detail, which will serve as a guide for the novel design and fabrication of an electrochemical sensor
To promote the development of electrochemical sensors based on COFs, the following challenges and outlooks should be considered in future work: (1) novel synthesis strategies need to be developed for conductive, nanoscale, and electroactive COFs for enhanced sensitivity and electrocatalytic activity
Summary
Covalent organic frameworks (COFs), a new class of multifunctional porous crystalline materials, are two- or three-dimensional (2D or 3D) porous crystalline materials built by light elements (C, B, O, Si, and N) via strong covalent bonds (C-N, C=N, C=C-N, B-O) (Xue et al, 2017; Chen et al, 2019; Wang and Zhuang, 2019; Zhu et al, 2019). The low intrinsic conductivity of COFs still imposes a great challenge for their applications in electrochemical sensing (Meng et al, 2019; Wu et al, 2019; Xu L. et al, 2019) This problem could be overcome via the following methods and strategies including doping with oxidants and guest molecules, template synthesis, introducing conductive polymers, π-conjugated planar 2D structures, and the metalation of COFs. dopants. In order to further enhance the electric conductivity, metal ions were introduced into the COFs lattice to form conductive metal-covalent organic frameworks (MCOFs), which can be synthesized through either direct synthesis or post-synthetic metalation by using planar and large π-electronic macrocycles as the building and paring units for the metal (Dong et al, 2020; Xie et al, 2020). Tetrakis(4-aminophenyl) methane/terephthalaldehyde Benzene-1,4-diboronic acid/2,3,6,7,10,11-hexahydroxytriphenylene hydrate BDT/ETTA 1,3,5triformylphloroglucinol/p-phenylenediamine 1,3,5triformylphloroglucinol/p-phenylenediamine tetra-(4-anilyl) methane and 4,4′ -biphenyldicarboxaldehyde 1,4-benzenediboronic acid BDBA/1,4-dioxane–mesitylene DAAQ/TFP DAB/TFP
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