Design of Na+ -Selective Fluorescent Probes: A Systematic Study of the Na+ -Complex Stability and the Na+ /K+ Selectivity in Acetonitrile and Water

Na+ Selective Fluorescent Tools Based on Fluorescence Intensity Enhancements, Lifetime Changes, and on a Ratiometric Response

Over the years, we developed highly selective fluorescent probes for K+ in water, which show K+ -induced fluorescence intensity enhancements, lifetime changes, or a ratiometric behavior at two emission wavelengths (cf. Scheme 1, K1-K4). In this paper, we introduce selective fluorescent probes for Na+ in water, which also show Na+ induced signal changes, which are analyzed by diverse fluorescence techniques. Initially, we synthesized the fluorescent probes 2, 4, 5, 6 and 10 for a fluorescence analysis by intensity enhancements at one wavelength by varying the Na+ responsive ionophore unit and the fluorophore moiety to adjust different Kd values for an intra- or extracellular Na+ analysis. Thus, we found that 2, 4 and 5 are Na+ selective fluorescent tools, which are able to measure physiologically important Na+ levels at wavelengths higher than 500 nm. Secondly, we developed the fluorescent probes 7 and 8 to analyze precise Na+ levels by fluorescence lifetime changes. Herein, only 8 (Kd =106 mm) is a capable fluorescent tool to measure Na+ levels in blood samples by lifetime changes. Finally, the fluorescent probe 9 was designed to show a Na+ induced ratiometric fluorescence behavior at two emission wavelengths. As desired, 9 (Kd =78 mm) showed a ratiometric fluorescence response towards Na+ ions and is a suitable tool to measure physiologically relevant Na+ levels by the intensity change of two emission wavelengths at 404 nm and 492 nm.

Coumarin-Based Small-Molecule Fluorescent Chemosensors

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.

A Ratiometric Fluorescent Probe for K+ in Water Based on a Phenylaza-18-Crown-6 Lariat Ether

This work presents two molecular fluorescent probes 1 and 2 for the selective determination of physiologically relevant K⁺ levels in water based on a highly K⁺ /Na⁺ selective building block, the o-(2-methoxyethoxy)phenylaza-18-crown-6 lariat ether unit. Fluorescent probe 1 showed a high K⁺ -induced fluorescence enhancement (FE) by a factor of 7.7 of the anthracenic emission and a dissociation constant (K_d ) value of 38 mm in water. Further, for 2+K⁺ , we observed a dual emission behavior at 405 and 505 nm. K⁺ increases the fluorescence intensity of 2 at 405 nm by a factor of approximately 4.6 and K⁺ decreases the fluorescence intensity at 505 nm by a factor of about 4.8. Fluorescent probe 2+K⁺ exhibited a K_d value of approximately 8 mm in Na⁺ -free solutions and in combined K⁺ /Na⁺ solution a similar K_d value of about 9 mm was found, reflecting the high K⁺ /Na⁺ selectivity of 2 in water. Therefore, 2 is a promising fluorescent tool to measure ratiometrically and selectively physiologically relevant K⁺ levels.

Lighting-up protein–ligand interactions with fluorescent PET (photoinduced electron transfer) sensor designs

Extending the versatile fluorescent PET sensing/switching system causes ‘off–on’ signalling when a ligand binds to its appropriate protein.

Benzo[g]coumarin-Based Fluorescent Probes for Bioimaging Applications

Benzo[g]coumarins, which consist of coumarins fused with other aromatic units in the linear shape, have recently emerged as an interesting fluorophore in the bioimaging research. The pi-extended skeleton with the presence of electron-donating and electron-withdrawing substituents from the parent coumarins changes the basic photophysical parameters such as absorption and fluorescence emission significantly. Most of the benzo[g]coumarin analogues show red/far-red fluorescence emission with high two-photon absorbing property that can be applicable for the two-photon microscopy (TPM) imaging. In this review, we summarized the recently developed benzo[g]coumarin analogues including photophysical properties, synthesis, and applications for molecular probes that can sense biologically important species such as metal ions, cell organs, reactive species, and disease biomarkers.

Highly K+ -Selective Fluorescent Probes for Lifetime Sensing of K+ in Living Cells

The new K⁺ -selective fluorescent probes 1 and 2 were obtained by Cu^I -catalyzed 1,3-dipolar azide alkyne cycloaddition (CuAAC) reactions of an alkyne-substituted [1,3]dioxolo[4,5-f][1,3]benzodioxole (DBD) ester fluorophore with azido-functionalized N-phenylaza-18-crown-6 ether and N-(o-isopropoxy) phenylaza-18-crown-6 ether, respectively. Probes 1 and 2 allow the detection of K⁺ in the presence of Na⁺ in water by fluorescence enhancement (2.2 for 1 at 2000 mm K⁺ and 2.5 for 2 at 160 mm K⁺ ). Fluorescence lifetime measurements in the absence and presence of K⁺ revealed bi-exponential decay kinetics with similar lifetimes, however with different proportions changing the averaged fluorescence decay times (τ_f(av) ). For 1 a decrease of τ_f(av) from 12.4 to 9.3 ns and for 2 an increase from 17.8 to 21.8 ns was observed. Variation of the substituent in ortho position of the aniline unit of the N-phenylaza-18-crown-6 host permits the modulation of the K_d value for a certain K⁺ concentration. For example, substitution of H in 1 by the isopropoxy group (2) decreased the K_d value from >300 mm to 10 mm. 2 was chosen for studying the efflux of K⁺ from human red blood cells (RBC). Upon addition of the Ca²⁺ ionophor ionomycin to a RBC suspension in a buffer containing Ca²⁺ , the fluorescence of 2 slightly rose within 10 min, however, after 120 min a significant increase was observed.