Fluorescent Covalent Organic Frameworks: A Promising Material Platform for Explosive Sensing

Abstract

Covalent organic frameworks (COFs) are a novel class of porous crystalline organic materials with organic small molecule units connected by strong covalent bonds and extending in two- or three-dimension in an ordered mode. The tunability, porosity, and crystallinity have endowed covalent organic frameworks the capability of multi-faceted functionality. Introduction of fluorophores into their backbones or side-chains creates emissive covalent organic frameworks. Compared with common fluorescent organic solid materials, COFs possess several intrinsic advantages being as a type of irreplaceable fluorescence materials mainly because its highly developed pore structures can accommodate various types of guest analytes by specific or non-specific chemical bonding and non-bonding interaction. Developments in fluorescent COFs have provided opportunities to enhance sensing performance. Moreover, due to its inherent rigidified structures and fixed conformations, the intramolecular rotation, vibration, and motion occurred in common organic small molecules, and organic solid systems can be greatly inhibited. This inhibition decreases the decay of excited-state energy as heat and blocks the non-radiative quenching channel. Thus, fluorescent COFs can be designed, synthesized, and precisely tuned to exhibit optimal luminescence properties in comparison with common homogeneous dissolved organic small molecule dyes and can even compete with the currently mainstream organic solid semiconductor-based luminescence materials. This mini-review discusses the major design principle and the state-of-the-art paragon examples of fluorescent COFs and their typical applications in the detection and monitoring of some key explosive chemicals by fluorescence analysis. The challenges and the future direction of fluorescent COFs are also covered in detail in the concluding section.

Recent Progress of Covalent Organic Frameworks Applied in Electrochemical Sensors

“…Since the first luminescent COF (TP-COF) was reported, many COFs were designed and synthesized for fluorescence detection of explosives, metal ions, biological and biomedical relevant species, and so on. − Qiu and co-workers developed a Kadsura-shaped fluorescent COF for sensitive detection and effective removal of TNP for the first time . The fluorescence of the COF can be quenched by TNP due to IFE (internal filtration effect) with a low detection limit (1.7 nM) and a wide linear range (5.0 × 10 –3 – 2.0 × 10 2 μM).…”

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

“…Luminescent COFs usually possess large π-conjugated building elements, or contain both inherent rigid structure and an extended πconjugated system, which can be further strengthened by forming À C=NÀ and À C=NÀ N=CÀ bonds to result in the high quantum yield and strong intensity. [30][31][32][33][34][35][36][37][38] The extended conjugate in COFs makes their own π-systems under electronic excitations easily produce a slightly long wavelength emission due to its relatively low energy difference.…”

“…The luminescence responsive modes of COFs for chemical sensing have been focused on the luminescence quenching or enhancement or both of them for even ratiometric triggered by analytes. (Figure 3) [1,8,9,14,24,[30][31][32][33][34][35][36][37][38][39][40][41][42]…”

Covalent Organic Framework Nanosheets for Fluorescence Quantification of Peptide

“…The integration of aggregation-induced emission (AIE) functional group into the covalent organic frameworks (COFs) extended π-conjugation skeleton endowed some of the COFs with excellent luminescence activity. , In recent years, detection platforms based on the luminous response of fluorescent COFs to target molecules started to flourish, expanding the application of COFs in sensing of various analytes, including heavy metal ions and other small organic molecules. , Taking advantage of the unique structural robustness and biological compatibility, COFs fluorescent sensors also have great potential in life sciences and clinical applications. However, this field is just getting afoot, and it is mainly focused on the fluorescence detection of some small-molecular disease markers, reactive oxygen species, and antibiotics. − …”