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.

Colorimetric metal ion sensors – A comprehensive review of the years 2011–2016

Fluorescence imaging is a powerful approach for noninvasive and real-time visualization and tracking of biomolecules and biological processes in living systems. The fluorescent chemosensors with dual/triple interplaying sensing mechanisms tend to provide diverse fluorescence signals or amplify the response signals, which are propitious to simultaneously track multiple analytes or to improve the selectivity and sensitivity of the chemosensors. Thus, the development of dual/triple sensing mechanism-based chemosensors has attracted great interest recently. This review highlights the representative cases of the fluorescent chemosensors with dual/triple interplaying sensing mechanisms published since 2010, and these chemosensors are classified according to the types of the interplaying sensing mechanisms, including ICT-FRET, PET-FRET, PET-ICT-ESIPT, etc.

Fluorescent chemosensors manipulated by dual/triple interplaying sensing mechanisms

Here, a dual-wavelength ratiometric electrochemiluminescence (ECL) approach is reported based on resonance energy transfer (RET) from graphite-like carbon nitride nanosheet (g-C3N4 NS) to Ru(bpy)32+ for sensitive detection of microRNA (miRNA). In this approach, Au nanoparticles (Au NPs) functionalized g-C3N4 NS nanohybrid (Au-g-C3N4 NH) coated on glassy carbon electrode (GCE) could exhibit strong and stable ECL emissions with emission peak centered at 460 nm. The ECL emission at such wavelength matched well with the absorption peak of Ru(bpy)32+ as well as impeccably stimulating the emission of Ru(bpy)32+ at the wavelength of 620 nm, producing ECL-RET with high efficiency. Thus, based on the ECL signals quenching at 460 nm and increasing at 620 nm, a dual-wavelength ratiometric ECL-RET system was achieved. This system was then utilized for determination of target miRNA. With the attachment of thiol-modified molecular beacon on Au-g-C3N4 NH, target miRNA hybridized with the molecular beacon to form a DNA-R...

Dual-Wavelength Electrochemiluminescence Ratiometry Based on Resonance Energy Transfer between Au Nanoparticles Functionalized g-C3N4 Nanosheet and Ru(bpy)32+ for microRNA Detection

Many of the key questions facing cellular biology concern the location and concentration of chemical species, from signalling molecules to metabolites to exogenous toxins. Fluorescent sensors (probes) have revolutionised the understanding of biological systems through their exquisite sensitivity to specific analytes. Probe design has focussed on selective sensors for individual analytes, but many of the most pertinent biological questions are related to the interaction of more than one chemical species. While it is possible to simultaneously use multiple sensors for such applications, data interpretation will be confounded by the fact that sensors will have different uptake, localisation and metabolism profiles. An alternative solution is to instead use a single probe that responds to two analytes, termed a dual-responsive probe. Recent progress in this field has yielded exciting probes, some of which have demonstrated biological application. Here we review work that has been carried out to date, and suggest future research directions that will harness the considerable potential of dual-responsive fluorescent probes.

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Fluorescent probes for the simultaneous detection of multiple analytes in biology

A new fluorescent probe, which contains rhodamine and chromone moieties, was designed and synthesized for selective recognition Al 3+ and Zn 2+ . The sensor showed immediate responses toward Al 3+ and Zn 2+ in different systems. The sensor exhibited highly selective and sensitive “off-on” fluorescent responses toward Al 3+ based on the ring-opening mechanism of the rhodamine spirolactam in an acetonitrile:water solution (excitation 500 nm and emission 550 nm). Whereas, in an ethanol:water solution, the sensor showed a significant fluorescent enhancement for Zn 2+ over a wide range of tested metal ions (excitation 424 nm and emission 496 nm), due to chelation-enhanced fluorescence process generated along with the photoinduced electron transfer process suppressed simultaneously. Moreover, the prepared zinc complex of the sensor could detect Al 3+ on the basis of fluorescence resonance energy transfer mechanism and the detection limit for Al 3+ was as low as 1.83 × 10 −7  M.

A novel rhodamine chromone-based “Off–On” chemosensor for the differential detection of Al(III) and Zn(II) in aqueous solutions

Fluorescent sensor for selective detection of Al3+ based on quinoline–coumarin conjugate

In this study, a simple one-photon and two-photon probe, 7-(4'-(diethylamino)-2'hydroxybenzylideneimino)-4-methyl coumarin, for Zn2+ is designed and synthesized. The sensor shows high selectivity towards Zn2+ ions with significant fluorescence intensity enhanced at 500 nm, which might be attributed to the formation of 2:1 stoichiometric L-Zn complex resulting in the restricted C=N isomerization and the inhibited photo-induced electron transfer. More importantly, the reversibility of the recognition process of HL is performed by adding a Zn2+ bonding agent Na(2)EDTA. (C) 2016 Elsevier B.V. All rights reserved.

A small-molecule and resumable two-photon fluorescent probe for Zn2+ based on a coumarin Schiff-base

In this study, a simple fluorescent sensor 2-hydroxybenzcarbaldehyde-(2-methylquinoline-4-formyl) hydrazone (HL) has been designed, synthesized and characterized by 1H-NMR, IR, ESI-MS. Upon addition of Al3+, HL shows a large fluorescence enhancement (220-fold) at 484 nm. The reasons for this phenomenon are attributed to formation of a 1:1 complex (Ka = 5.6 × 104), which inhibits the excited-state intramolecular proton transfer (ESIPT) process and photo-induced electron transfer (PET) process. Other metal ions including Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Hg2+, K+, Mg2+, Mn2+, Na+, Ni2+, Pb2+ and Zn2+, have almost no influence on the fluorescence. The lowest detection limit for Al3+ is calculated to be 7.2 × 10−7 M in ethanol.

Jing-can Qin

Papers

A novel rhodamine chromone-based “Off–On” chemosensor for the differential detection of Al(III) and Zn(II) in aqueous solutions

Fluorescent sensor for selective detection of Al3+ based on quinoline–coumarin conjugate

A small-molecule and resumable two-photon fluorescent probe for Zn2+ based on a coumarin Schiff-base

Design and synthesis of a chemosensor for the detection of Al3+ based on ESIPT

FRET-based rhodamine–coumarin conjugate as a Fe3+ selective ratiometric fluorescent sensor in aqueous media