Coumarins are a very large family of compounds containing the unique 2H-chromen-2-one motif, as it is known according to IUPAC nomenclature. Coumarin derivatives are widely found in nature, especially in plants and are constituents of several essential oils. Up to now, thousands of coumarin derivatives have been isolated from nature or produced by chemists. More recently, the coumarin platform has been widely adopted in the design of small-molecule fluorescent chemosensors because of its excellent biocompatibility, strong and stable fluorescence emission, and good structural flexibility. This scaffold has found wide applications in the development of fluorescent chemosensors in the fields of molecular recognition, molecular imaging, bioorganic chemistry, analytical chemistry, materials chemistry, as well as in the biology and medical science communities. This review focuses on the important progress of coumarin-based small-molecule fluorescent chemosensors during the period of 2012-2018. This comprehensive and critical review may facilitate the development of more powerful fluorescent chemosensors for broad and exciting applications in the future.

Coumarin-Based Small-Molecule Fluorescent Chemosensors.

Considerable amount of research has been carried out on designing and improving metal-recognition methodologies in environmental and biological media. The development of fluorescent-based techniques has proven to be an important milestone for non-invasive metal detection and quantification in a multichannel environment. Metals as natural components of the Earth's crust are generally present in trace concentrations in environmental samples, wherein humic substances have a complexation affinity toward them. Iron, zinc and copper are the 1st, 2nd and 3rd most abundant elements that are indispensable to the human body in trace amounts as they play crucial roles in many biological processes. However, unregulated amounts either an excess or deficiency may exacerbate deterioration of the vital organs and trigger the progression of complications. In addition to these three essential elements, mercury is widely considered to be one of the most hazardous pollutants and highly dangerous elements due to its recognized accumulative and toxic effects in the environment and in biological media. In the present study, we attempted to summarize all the recently developed fluorescent signaling materials for the detection of Cu2+, Fe2+/Fe3+, Zn2+ and Hg2+. The spectral shifts in the molecules on metal chelation, the mode of complexation and the stoichiometries of the resulting adducts have been discussed in detail. Furthermore, we highlight molecules that have been reported as an intracellular metal detector via bioimaging, which can be useful for the future design and development of cell-viable and membrane-permeable molecular probes.

Optical sensor: a promising strategy for environmental and biomedical monitoring of ionic species

An unprecedented dinuclear CuII complex, [Cu2(L2)2], derived from a salamo-like chelating ligand H2L2, was produced by the cleavage of a newly synthesized, half-salamo-like ligand HL1 (2-[O-(1-ethyloxyamide)]oxime-3,5-dichloro-phenol). This was synthesized and characterized by elemental analyses, IR, UV–Vis and fluorescent spectra, single crystal X-ray diffraction analysis, and Hirshfeld surface analysis. X-ray crystallographic analysis indicated that the two CuII (Cu1 and Cu2) ions bore different (N2O3 and N2O2) coordination environments, the penta-coordinated Cu1 ion possessed a slightly twisted tetragonal pyramid geometry with the τ value τ = 0.004, and the tetra-coordinated Cu2 ion showed a slightly twisted square planar geometry. Interestingly, one oxime oxygen atom participated in the coordination reported previously. Moreover, an infinite two-dimensional layered supramolecular network was formed. Compared with HL1, the CuII complex possessed the characteristic of fluorescence quenching.

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Unprecedented Dinuclear CuII N,O-Donor Complex: Synthesis, Structural Characterization, Fluorescence Property, and Hirshfeld Analysis

A highly sensitive and selective bis(salamo)-type fluorescent chemosensor for identification of Cu2+ and the continuous recognition of S2-, Arginine and Lysine.

A new homo-trinuclear Ni(II) half-salamo-based complex [Ni3(L)2(μ-OAc)2(OAc)2(CH3OH)2]·2CH3OH was synthesized via the reaction of a tridentate ligand HL (2-[O-(1-ethyloxyamide)]oxime-4-bromophenol) and Ni(OAc)2·4H2O, and characterized using elemental analyses, IR spectra, UV-Vis absorption spectra, X-ray crystallography, and Hirshfeld analysis. Interestingly, single-crystal X-ray analysis showed that the two acetate molecules were bonded simultaneously with the Ni(II) atoms by mono-dentate chelating and bidentate bridging coordination modes, respectively, and the resulting hexa-coordinate geometries were ultimately formed. Furthermore, the Hirshfeld analysis of the complex was studied. Compared with HL, the complex fluorescence intensity was significantly lowered, indicating that the Ni(II) ions have fluorescence quenching characteristics.

A New Half-Salamo-Based Homo-Trinuclear Nickel(II) Complex: Crystal Structure, Hirshfeld Surface Analysis, and Fluorescence Properties

A simple chemosensor L bearing an acylhydrazone group as binding site and naphthalene group as the fluorescence signal group was described and synthesized, which showed a fluorescence turn-on response for Zn 2+ over other metals ions such as Fe 3+ , Hg 2+ , Ag + , Ca 2+ , Cu 2+ , Co 2+ , Ni 2+ , Cd 2+ , Pb 2+ , Cr 3+ and Mg 2+ in aqueous solutions with specific selectivity and high sensitivity When zinc is added, the probe L shows an immediate visible change of fluorescence color in aqueous solution; while other cations do not cause obvious color change In addition, further study demonstrates the detection limit on fluorescence response of the sensor to Zn 2+ is down to 93 × 10 −8  M Moreover, test strips based on the sensor L were fabricated, which could act as a convenient and efficient Zn 2+ test kit Therefore, the probe should be potential application in both the environment and biological systems for the monitoring of zinc

Acylhydrazone based fluorescent chemosensor for zinc in aqueous solution with high selectivity and sensitivity

A new unsymmetrical azine derivative based on coumarin group as dual-modal sensor for CN− and fluorescent “OFF–ON” for Zn2 +

A simple, selective colorimetric and fluorimetric cyanide chemosensor L was designed and synthesized, which showed both colorimetric and fluorescence turn-on responses for cyanide ions in aqueous solution with specific selectivity and high sensitivity. The probe shows an immediate visible change in color from colorless to yellow only after the addition of CN− in aqueous solution; these color changes can be readily observed visually. Moreover, the detection limit on fluorescence response of the sensor to CN− is down to 1.20 × 10−9 M, which is far lower than the maximum level of 1.9 × 10−6 M for cyanide in drinking water according to WHO guidelines. Test strips based on L were fabricated, which could be used as a convenient and efficient CN− test kit to detect CN− in aqueous solution for “in-the-field” measurement.

Selective colorimetric and “turn-on” fluorimetric detection of cyanide using an acylhydrazone sensor in aqueous media

A highly selective colorimetric and "turn-on" fluorimetric chemosensor for detecting CN(-) based on unsymmetrical azine derivatives in aqueous media.

A novel highly selective isonicotiamide-based chemosensor HY for cyanide was successfully designed and synthesized, which showed both colorimetric and fluorescent turn-on responses for cyanide ions in aqueous. This structurally simple chemosensor could detect CN− anion over other anions in aqueous solution DMSO/H2O (v/v = 7 : 3) undergo deprotonation reaction. Results showed that the chemosensor HY exhibited 14-fold enhancement in fluorescence at 552 nm and showed yellow fluorescence under the UV-lamp after the addition of CN− in aqueous solution. An obvious change in color from pale yellow to yellow that could be detected by naked eye. We have successfully utilized the above CN− and H+ for the fabrication of INHIBIT molecular logic gates, using CN− and H+ as chemical inputs and the fluorescence intensity signal as the output. Moreover, the detection limit on fluorescence response of the sensor to CN− is down to 5.12 × 10−8 M by titration method. Test strips based on HY were obtained, which could be used as a convenient and efficient CN− test kit to detect CN− in aqueous solution. Furthermore, the sensor could detect the CN− in food sample as a sample and effective method.

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Papers

Acylhydrazone based fluorescent chemosensor for zinc in aqueous solution with high selectivity and sensitivity

A new unsymmetrical azine derivative based on coumarin group as dual-modal sensor for CN− and fluorescent “OFF–ON” for Zn2 +

Selective colorimetric and “turn-on” fluorimetric detection of cyanide using an acylhydrazone sensor in aqueous media

A highly selective colorimetric and "turn-on" fluorimetric chemosensor for detecting CN(-) based on unsymmetrical azine derivatives in aqueous media.

A colorimetric and “turn-on” fluorimetric chemosensor for the selective detection of cyanide and its application in food samples