Coumarins are phenolic substances composed of fused pyrones and benzenes with the pyrone carbonyl group at position 2, belonging to a family of benzopyrone (1,2-benzopyranones or 2H-1-benzopyran-2-ones). They are widely distributed in the plant kingdom and are secondary metabolites of higher plants, a few microorganisms and sponges. Coumarins exhibit a wide range of fluorescence emission properties with high fluorescence quantum yields, low extinction coefficient and strong photostability. By introducing various electron-withdrawing or electron-donor groups into coumarin molecules, the excitation and emission wavelengths can be tuned, resulting in blue-shifted or red-shifted fluorescence. Therefore, many substituted coumarin derivatives are capable of emitting sufficient fluorescence in the visible range.
Fig.1 The chemical structure of coumarin
Coumarin fluorophores and their derivatives have been widely applied in fluorescent chemosensors and fluorescent probes.
Coumarin fluorophores have great promise in the field of fluorescence sensing due to their strong and stable fluorescence emission properties. By combining the coumarin moiety with other functional receptors, small molecule fluorescent chemosensors were constructed. These coumarin-derived fluorescent chemosensors play an important role in the fields of biology and medicine science communities and they have been applied to analyze a variety of substances such as inorganic anions, metal cations, reactive oxygen, sulfur or nitrogen substances, and small bioactive molecules. For example, a conjugate of coumarin-malononitrile compound can be used as fluorescent chemosensor, prepared by nucleophilic addition of coumarin and CN-, allowing highly sensitive and selective recognition of CN- in HeLa cells, as shown in Figure 2.
Fig. 2 Nucleophilic addition of coumarin and sensing processes in a coumarin-based fluorescent chemosensors.
Coumarin derivatives can be used as small-molecule fluorescent probes due to their good chromogenic, fluorescent properties, hydrophobicity, etc. These coumarin-based fluorescent probes have been widely used in environmental and medical fields.
(a). Environmental application: Hg2+ has attracted widespread attention in worldwide as one of the most harmful elements. A novel fluorescent probe VC was synthesized by optimizing the elimination reaction conditions using 7-bromoethaneoxy-4-methylcoumarin as a fluorophore and vinyl ether group as the recognition unit, which showed remarkable selectivity and sensitivity to Hg2+ in neat aqueous solution. When the probe VC was added to Hg2+, intense fluorescence was produced due to the fluorescence properties of the coumarin, as shown in Figure 3. Therefore, this fluorescent probe VC can be used to effectively detect Hg2+ in real water sample.
Fig. 3 A novel coumarin-based fluorescent probe VC for Hg2+ detection.
(b). Medical application: 7-Dimethylaminocoumarin-4-acetic acid as a fluorophore has immunoafﬁnity-ﬂuorescent (IAF) properties, enabling the identification and isolation of proteins by co-immunoprecipitation with its own specific antibody partners. The coumarin carboxylic acid derivative reacts with N-Boc-1,8-diamino-3,6-dioxaoctane firstly to generate coumarin-linked derivative, and finally obtains hydrophilic fluorescent probe, as shown in Figure 4. This coumarin-based fluorescent probe as an efﬁcient reagent to label cysteine-bearing biomolecules on peptides or proteins.
Fig. 4 Synthesis of coumarin probe.
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