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.

Metal cations and anions are essential for versatile physiological processes. Dysregulation of specific ion levels in living organisms is known to have an adverse effect on normal biological events. Owing to the pathophysiological significance of ions, sensitive and selective methods to detect these species in biological systems are in high demand. Because they can be used in methods for precise and quantitative analysis of ions, organic dye-based ratiometric fluorescent probes have been extensively explored in recent years. In this review, recent advances (2015-2019) made in the development and biological applications of synthetic ratiometric fluorescent probes are described. Particular emphasis is given to organic dye-based ratiometric fluorescent probes that are designed to detect biologically important and relevant ions in cells and living organisms. Also, the fundamental principles associated with the design of ratiometric fluorescent probes and perspectives about how to expand their biological applications are discussed.

Synthetic ratiometric fluorescent probes for detection of ions

Applications of the method of continuous variations (MCV or the Method of Job) to problems of interest to organometallic chemists are described. MCV provides qualitative and quantitative insights into the stoichiometries underlying association of m molecules of A and n molecules of B to form AmBn . Applications to complex ensembles probe associations that form metal clusters and aggregates. Job plots in which reaction rates are monitored provide relative stoichiometries in rate-limiting transition structures. In a specialized variant, ligand- or solvent-dependent reaction rates are dissected into contributions in both the ground states and transition states, which affords insights into the full reaction coordinate from a single Job plot. Gaps in the literature are identified and critiqued.

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Method of Continuous Variations: Applications of Job Plots to the Study of Molecular Associations in Organometallic Chemistry[**]

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

Exposure to mercury ions can damage the human brain, the nervous system, the endocrine system, and other biological systems. Much effort has therefore been made to develop real-time monitoring of mercury variations, and many mercury-ion sensors have been reported recently. In this review, mercury-ion sensors reported since 2008 are described and discussed. The sensors are classified as molecular, nanomaterial based, and others. Molecular sensors are based on chemical and hydrogen bond formation, and the other types are based on changes in the materials used.

Fluorescent and colorimetric sensors for environmental mercury detection

A multi-signal mitochondria-targeted fluorescent probe for real-time visualization of cysteine metabolism in living cells and animals.

A novel rhodamine-cyanine based probe LS1 was designed and synthesized, which can act as a bifunctional NIR ratiometric colorimetric and fluorescent probe for detecting Fe 3+ and Cu 2+ in solvent of MeOH/H 2 O and MeCN/H 2 O respectively. As expected, it exhibited high selectivity and sensitivity for detecting Fe 3+ and Cu 2+ over other commonly coexistent metal ions in their respective systems with a broad pH span. The detection limit was measured to be 0.737 μM for Fe 3+ and 1.019 μM for Cu 2+ . Furthermore, fluorescence imaging experiments of Cu 2+ ions in living SH-SY5Y109 cells demonstrated its value of practical application in biological systems.

A novel solvent-dependently bifunctional NIR absorptive and fluorescent ratiometric probe for detecting Fe3+/Cu2+ and its application in bioimaging

Based on fluorescent resonance energy transfer (FRET), we developed a ratiometric fluorescent probe 1 for sensing Cu2+. As expected, probe 1 exhibited high selectivity and excellent sensitivity in both absorbance and fluorescence detection of Cu2+ in aqueous solution. A significant color change from pale yellow to pink could be observed, enabling naked-eye detection of Cu2+. Furthermore, fluorescence imaging experiment of Cu2+ in living MGC-803 cells demonstrated its value of practical applications in biological systems.

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Highly sensitive and fast responsive ratiometric fluorescent probe for Cu2+ based on a naphthalimide-rhodamine dyad and its application in living cell imaging

Recent advances in multifunctional fluorescent probes for viscosity and analytes

A novel near-infrared (NIR) ratiometric colorimetric ICT-based fluorescence probe for hydrazine has been synthesized and characterized. The probe DCM-LX selectively distinguished hydrazine with other analyses with a wide pH range. Once hydrazine is added to the solution of probe DCM-LX, the color of solution changed to orange from yellow immediately. It means probe can be used for “naked-eye” detection of hydrazine. DCM-LX exhibits high selectivity and excellent sensitivity towards hydrazine with detection limit as low as 7.71 nM. Furthermore, DCM-LX was successfully applied in living cellular imaging with low toxicity and hydrazine detection in H 2 O sample.

Yong Ye

Papers

A multi-signal mitochondria-targeted fluorescent probe for real-time visualization of cysteine metabolism in living cells and animals.

A novel solvent-dependently bifunctional NIR absorptive and fluorescent ratiometric probe for detecting Fe3+/Cu2+ and its application in bioimaging

Highly sensitive and fast responsive ratiometric fluorescent probe for Cu2+ based on a naphthalimide-rhodamine dyad and its application in living cell imaging

Recent advances in multifunctional fluorescent probes for viscosity and analytes

A NIR ratiometric probe for hydrazine “naked eye” detection and its imaging in living cell